LetsLearn3Gin10Days_KamalVij.pdf

Let’s Learn

3G in 10 Days

So Much to Learn... So Little Time

EXPLOIT THE EVOLUTION

Written by Kamal Vij Wires and Waves Solutions Let’s take out 10 days and learn the essential concepts about 3G technology.

Let’s Learn 3G in 10 Days written by

Kamal Vij

FOREWORD

The success story of GSM has generated a lot of motivation for businessmen, educational institutes, private consultants, legacy telecom operators, mobile operators, equipment vendors and many more to master the fundamentals of 3G. It has been 13 years since the publication of the first 3G specification. It is no secret that 3G (UMTS along with HSPA) has established itself as a successful and commercially profitable mobile standard. Telecom professionals keep on trying to search for the latest updates on the emerging technologies. There are a lot of white papers, blogs, posters, and e-books available on the Internet which help them in the learning but their busy schedule hardly allows them to spend even a single hour per day to learn the updates of technology. This was my motivation to write this book. In this book, the emphasis is on keeping the language simple and focus on the essential concepts only. This book is not about radio planning or RF optimization. Its sole purpose is to introduce the readers to the 3G technology. To get the maximum benefit out of this 10 days crash course with me, I recommend you to follow the following plan: DAY 1 History and Standardization: On the first day of reading, we will have an ultra quick look at the history and very brief preview of the future. This module or chapter will give us an overview about the legacy systems and their migratory path to 3G and beyond. An the same time, we will also learn about 3GPP releases and their features. 2

DAY 2 Network Elements and Functionalities: The second day is planned for learning about the network elements, interfaces, and to have a look at the combined network architecture of 2G & 3G network. DAY 3 WCDMA Air Interface: On the third day, we will focus on the radio technology used in 3G system. In the third chapter, the principles of spreading and code multiplexing are explained. We will also see the series of physical layer procedures that take place at layer 1 of UE and Node B. DAY 4 Logical, Transport & Physical Channels: On the fourth day, some more physical layer aspects will be discussed when we will learn about the channels of UMTS. In this module, we will focus on the UMTS channels only. For HSDPA & HSUPA, separate modules are planned. DAY 5 Radio Resource Management: The fifth day of our reading should focus on the RRM module, which discusses several features that work in parallel to optimize the radio resource utilization. DAY 6 Protocols & Interfaces: On the sixth day, topic of discussion will be the protocols of UMTS. Along with radio protocols, we will also learn about the control plane & user plane protocols on various UTRAN interfaces. DAY 7 HSDPA: Release 5 onwards, the downlink speeds can be pushed beyond 2 Mbps. The seventh day is reserved for understanding the basics of HSDPA. DAY 8 HSUPA: The project HSPA is completed only when both Uplink and Downlink are High Speed. The eighth module of this book will discuss how HSUPA differs from the conventional UMTS uplink. DAY 9 Signalling: Towards the end of our journey, Module 9 is planned to discuss a few signalling scenarios. Here, we will discuss how a CS AMR call and a PS data session gets established on UMTS & HSPA. Mobility related signalling will also be illustrated, which plays an important role in service continuity and improves call success ratio.

3

DAY 10 Self Test: On the last day, I request all the readers to put themselves to a self-evaluation and evaluate whether they have learnt something from this book. 5 to 8 questions/exercises from each module are planned.

Please visit www.3gin10days.com to watch some video lessons related to these topics and to download the e-book.

4

ABOUT THE AUTHOR Kamal Vij received a B.Engg. degree in Electronics & Communication from Kuvempu University, India in 2001. In 2005, he received a M.Sc. degree in Communication Technology from University of Bremen, Germany. While pursuing his M.Sc., he took special interest in semiconductor simulation and worked as a research assistant in the power electronics department of University of Bremen. In 2006, he started his career as a trainer for telecommunication. Since then, he has been delivering trainings on WCDMA, HSPA & LTE Radio Access Network technologies across the world.

Kamal Vij is now a technical trainer and private consultant. He has keen interest in emerging technologies and following the market trends. His skills are signalling, parameter optimization, radio planning and optimization. More about him can be found at http://www.wiresandwaves.net and he can be reached at: [email protected].

5

ACKNOWLEDGEMENTS

I, Kamal Vij, the author of this book, would like to thank 3GPP for being so kind and allowing me to use their graphics, tables and text pieces in this book. I also want to express my thankful regards to my friends working in the telecom sector who have helped me in writing this book by giving me tips and ideas. My biggest teachers are those telecom professionals whom I met while delivering the classroom training. The discussions I had with them have enhanced my knowledge and motivated me to work more passionately. I cannot mention all the names here but a few colleagues and friends this group are Andreas Annen, Ashok Joshi, Andrey Yaroshenko, Ilya Andreev, Jeetendra Ghare, Karl Hofmann, Kapil Bhutani, Michael Oestreicher, Silviu Mihailescu, Jan Berglund, Ronald Fabian, Ravindra Mawale, Saikat Nandi. I also want to show my appreciation towards the authors of the following three books. These books have been an excellent source of information for me. The Ideas for many sections of this book were inspired from these books. • H.Holma and A. Toskala, ‘WCDMA for UMTS’ , 5th Edition, John Wiley & Sons. • H.Holma and A. Toskala, ‘HSDPA/HSUPA for UMTS’ , 1st Edition, John Wiley & Sons. • Chris Johnson, ‘Radio Access Networks For UMTS; Principles And Practice’ , John Wiley & Sons. I would like to thank ‘Zorba Publishers Pvt. Ltd.’ (www.zorbapublishers.com) 6

for meticulously proof reading this book and removing hundreds of typographic and grammatical errors. Above all, I want to thank my family, who supported and encouraged me in spite of all the time it took me away from them. It was a long and difficult journey for them. I apologize to all those who have been with me during my journey as a telecom trainer and whose names I have failed to mention.

7

DISCLAIMER

The information contained in ‘Let’s Learn 3G in 10 Days’ is for general information purposes only. The author has tried to simplify the explanation, and in that process few complicated equations and rules have been dropped while maintaining the overall correctness to the best of his knowledge. Every effort is made to keep the information accurate. However, the author takes no responsibility for any damage caused by the information obtained by this book. Every care has been taken to mention the references and sources wherever needed. This process has taken several months because references were added after the book was written. Afterwards, it is a time-consuming and laborious task to recall all the sources. The author has tried his best but at few places, he might have forgotten to mention the source/reference due to human limitation. Author asks for forgiveness if he has failed to declare the references in any part of the book.

8

CONTENTS

Preface

2

Preface

5

Acknowledgements

6

1 History and Standardization

2

1.1

Mobile Telecom Market . . . . . . . . . . . . . . . . . . . . . . . . . .

2

1.2

History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3

1.2.1

0G Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4

1.2.2

1G Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4

1.2.3

2G Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5

1.2.4

3G Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

1.3

3GPP and 3GPP2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 1.3.1

3rd Generation Partnership Project (3GPP) . . . . . . . . . . 14

1.3.2

3rd Generation Partnership Project 2 (3GPP2) . . . . . . . . 15

1.3.3

WiMAX as IMT-2000 System . . . . . . . . . . . . . . . . . . 17 9

1.4

WCDMA FDD - Releases . . . . . . . . . . . . . . . . . . . . . . . . 18

1.5

WCDMA FDD - Releases and Features . . . . . . . . . . . . . . . . . 19

2 Network Elements and Functionalities 2.1

2.2

Architecture of the GSM Network . . . . . . . . . . . . . . . . . . . . 26 2.1.1

The Mobile Station MS . . . . . . . . . . . . . . . . . . . . . . 27

2.1.2

Base Station Subsystem BSS . . . . . . . . . . . . . . . . . . . 27

2.1.3

Switching Subsystem . . . . . . . . . . . . . . . . . . . . . . . 28

Improvements of GSM Standard 2.2.1

2.3

25

. . . . . . . . . . . . . . . . . . . . 32

CAMEL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

GPRS Network Architecture . . . . . . . . . . . . . . . . . . . . . . . 35 2.3.1

GPRS Mobile Terminals . . . . . . . . . . . . . . . . . . . . . 36

2.3.2

GPRS Base Station Subsystem . . . . . . . . . . . . . . . . . 37

2.3.3

New Elements in the Core Network . . . . . . . . . . . . . . . 38

2.3.4

Other Changes . . . . . . . . . . . . . . . . . . . . . . . . . . 40

2.3.5

GPRS Roaming Scenario . . . . . . . . . . . . . . . . . . . . . 41

2.4

Migration to 3G Network Architecture . . . . . . . . . . . . . . . . . 42

2.5

UTRAN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 2.5.1

Node B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

2.5.2

RNC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

2.6

Logical roles of RNC: S-RNC and D-RNC . . . . . . . . . . . . . . . 47

2.7

Release 4 Modifications . . . . . . . . . . . . . . . . . . . . . . . . . . 50

2.8

2.9

2.7.1

Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

2.7.2

New Interfaces . . . . . . . . . . . . . . . . . . . . . . . . . . 53

Release 5 Modifications . . . . . . . . . . . . . . . . . . . . . . . . . . 54 2.8.1

IP Transport . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

2.8.2

IP Multimedia Subsystem (IMS) . . . . . . . . . . . . . . . . 55

Release 6 Modifications . . . . . . . . . . . . . . . . . . . . . . . . . . 56 2.9.1

IMS for IP-CAN or IMS phase 2

. . . . . . . . . . . . . . . . 57

2.10 Rel-7 & Rel-8 Modifications . . . . . . . . . . . . . . . . . . . . . . . 58 10

3 WCDMA Air Interface

62

3.1

Duplex Methods

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

3.2

Multiple Access Technologies . . . . . . . . . . . . . . . . . . . . . . . 63 3.2.1

Frequency Division Multiple Access . . . . . . . . . . . . . . . 64

3.2.2

Time Division Multiple Access . . . . . . . . . . . . . . . . . . 65

3.2.3

Code Division Multiple Access . . . . . . . . . . . . . . . . . . 65

3.2.4

Orthogonal Frequency Division Multiple Access . . . . . . . . 65

3.3

UMTS operating Bands and Spectrum . . . . . . . . . . . . . . . . . 65

3.4

Timing in WCDMA

3.5

Spreading Principles . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

3.6

Codes in UMTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

3.7

. . . . . . . . . . . . . . . . . . . . . . . . . . . 66

3.6.1

Channelization Code . . . . . . . . . . . . . . . . . . . . . . . 73

3.6.2

Scrambling Code . . . . . . . . . . . . . . . . . . . . . . . . . 74

3.6.3

Summary of Scrambling Codes

3.6.4

Summary of Codes in UMTS . . . . . . . . . . . . . . . . . . 78

. . . . . . . . . . . . . . . . . 78

Modulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

4 Logical, Transport & Physical Channels

82

4.1

Chronology: First 3G and then 3.5G . . . . . . . . . . . . . . . . . . 83

4.2

Logical Channels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83

4.3

4.4

4.2.1

Logical Channels for Control Plane Information . . . . . . . . 84

4.2.2

Logical Channels for User Plane Information

. . . . . . . . . 85

Transport Channels . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 4.3.1

Common Transport Channels . . . . . . . . . . . . . . . . . . 87

4.3.2

Dedicated transport channels . . . . . . . . . . . . . . . . . . 88

Physical Channels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88 4.4.1

UL Common Channel . . . . . . . . . . . . . . . . . . . . . . 91

4.4.2

DL common Channel . . . . . . . . . . . . . . . . . . . . . . . 94

4.4.3

UL Dedicated Channels . . . . . . . . . . . . . . . . . . . . . 105

4.4.4

DL Dedicated Channels . . . . . . . . . . . . . . . . . . . . . 106 11

4.4.5

Summary of DCH Channels . . . . . . . . . . . . . . . . . . . 108

4.5

Cell Search Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . 109

4.6

HSDPA Channels in Short . . . . . . . . . . . . . . . . . . . . . . . . 111

4.7

HSUPA Channels in Short . . . . . . . . . . . . . . . . . . . . . . . . 113

5 Radio Resource Management 5.1

5.2

117

Inputs for RRM Functionality . . . . . . . . . . . . . . . . . . . . . . 119 5.1.1

RNC Parameter Database . . . . . . . . . . . . . . . . . . . . 120

5.1.2

Node B Measurements . . . . . . . . . . . . . . . . . . . . . . 121

5.1.3

UE Measurements

5.1.4

Internal RNC Measurements . . . . . . . . . . . . . . . . . . . 125

. . . . . . . . . . . . . . . . . . . . . . . . 123

Load Estimation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126 5.2.1

Uplink Load Estimation . . . . . . . . . . . . . . . . . . . . . 126

5.2.2

Downlink Load Estimation . . . . . . . . . . . . . . . . . . . . 128

5.3

Radio Resource Management Strategies . . . . . . . . . . . . . . . . . 129

5.4

Admission Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129

5.5

Code Allocation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132 5.5.1

5.6

5.7

5.8

Code Tree Optimization . . . . . . . . . . . . . . . . . . . . . 134

Packet Scheduler . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134 5.6.1

RRC States . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137

5.6.2

RRC States Transitions . . . . . . . . . . . . . . . . . . . . . 141

Power Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144 5.7.1

Open Loop Power Control . . . . . . . . . . . . . . . . . . . . 145

5.7.2

Inner Loop Power Control . . . . . . . . . . . . . . . . . . . . 146

5.7.3

Outer Loop Power Control . . . . . . . . . . . . . . . . . . . . 152

Handover Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154 5.8.1

Active, Monitored and Detected cells . . . . . . . . . . . . . . 156

5.8.2

Soft/Softer Handover . . . . . . . . . . . . . . . . . . . . . . . 157

5.8.3

ISHO and IFHO Triggering . . . . . . . . . . . . . . . . . . . 162

5.8.4

Inter-Frequency Measurements . . . . . . . . . . . . . . . . . . 163 12

5.8.5

Inter-System Measurements . . . . . . . . . . . . . . . . . . . 164

5.8.6

Compressed Mode . . . . . . . . . . . . . . . . . . . . . . . . 165

5.8.7

Inter System HO Signalling . . . . . . . . . . . . . . . . . . . 166

6 Protocols & Interfaces 6.1

6.2

171

Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172 6.1.1

Horizontal Layers . . . . . . . . . . . . . . . . . . . . . . . . . 173

6.1.2

Vertical Planes . . . . . . . . . . . . . . . . . . . . . . . . . . 173

QoS and Bearer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175 6.2.1

UMTS QoS Classes . . . . . . . . . . . . . . . . . . . . . . . . 177

6.3

Access Stratum and Non-Access Stratum . . . . . . . . . . . . . . . . 178

6.4

Radio Protocols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180

6.5

6.6

6.7

6.8

6.4.1

Control Plane . . . . . . . . . . . . . . . . . . . . . . . . . . . 180

6.4.2

User Plane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181

6.4.3

RRC-layer Functions . . . . . . . . . . . . . . . . . . . . . . . 182

6.4.4

RLC-layer Functions . . . . . . . . . . . . . . . . . . . . . . . 183

6.4.5

MAC-layer Functions . . . . . . . . . . . . . . . . . . . . . . . 185

6.4.6

PDCP-layer Functions . . . . . . . . . . . . . . . . . . . . . . 186

Iu-CS Interface Protocols . . . . . . . . . . . . . . . . . . . . . . . . . 187 6.5.1

Control Plane - Iu-CS . . . . . . . . . . . . . . . . . . . . . . 187

6.5.2

User Plane - Iu-CS . . . . . . . . . . . . . . . . . . . . . . . . 187

6.5.3

RANAP Functions . . . . . . . . . . . . . . . . . . . . . . . . 189

Iu-PS Interface Protocols . . . . . . . . . . . . . . . . . . . . . . . . . 190 6.6.1

Control Plane - Iu-PS . . . . . . . . . . . . . . . . . . . . . . 190

6.6.2

User Plane - Iu-PS . . . . . . . . . . . . . . . . . . . . . . . . 190

Iub Interface Protocols . . . . . . . . . . . . . . . . . . . . . . . . . . 192 6.7.1

Control Plane - Iub CP . . . . . . . . . . . . . . . . . . . . . . 192

6.7.2

User Plane - Iub UP . . . . . . . . . . . . . . . . . . . . . . . 193

6.7.3

NBAP Functions . . . . . . . . . . . . . . . . . . . . . . . . . 193

Iur Interface Protocols . . . . . . . . . . . . . . . . . . . . . . . . . . 195 13

6.9

6.8.1

Control Plane - Iur CP . . . . . . . . . . . . . . . . . . . . . . 195

6.8.2

User Plane - Iur UP . . . . . . . . . . . . . . . . . . . . . . . 195

6.8.3

RNSAP functions . . . . . . . . . . . . . . . . . . . . . . . . . 195

Non-Access Stratum Protocols . . . . . . . . . . . . . . . . . . . . . . 198

7 High Speed Downlink Packet Access

204

7.1

Why HSDPA? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204

7.2

HSDPA Standardization, 3GPP Releases and Evolution . . . . . . . . 205

7.3

7.4

7.5

7.6

7.2.1

Release 99 & Rel-4 . . . . . . . . . . . . . . . . . . . . . . . . 206

7.2.2

Release 5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 206

7.2.3

Release 6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207

7.2.4

Release 7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207

7.2.5

Release 8 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207

HSDPA Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208 7.3.1

HSDPA Operation: Between UE and RNC . . . . . . . . . . . 208

7.3.2

HSDPA Operation: Between Node B and RNC . . . . . . . . 209

What’s new in HSDPA? . . . . . . . . . . . . . . . . . . . . . . . . . 211 7.4.1

Adaptive Modulation & Coding . . . . . . . . . . . . . . . . . 211

7.4.2

Shorter and Fixed TTI . . . . . . . . . . . . . . . . . . . . . . 211

7.4.3

Node B-based Packet Scheduling . . . . . . . . . . . . . . . . 212

7.4.4

Multi-code Operation . . . . . . . . . . . . . . . . . . . . . . . 213

7.4.5

L1 H-ARQ Retransmission . . . . . . . . . . . . . . . . . . . . 217

7.4.6

MAC-hs Protocol in Node B and UE . . . . . . . . . . . . . . 218

7.4.7

Serving Cell Change Instead of Soft HO . . . . . . . . . . . . 219

HSDPA Protocol Architecture . . . . . . . . . . . . . . . . . . . . . . 221 7.5.1

MAC-hs entity - UE Side . . . . . . . . . . . . . . . . . . . . . 221

7.5.2

MAC-hs entity - UTRAN Side . . . . . . . . . . . . . . . . . . 224

Channels and Physical Layer . . . . . . . . . . . . . . . . . . . . . . . 226 7.6.1

HS-DPCCH . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227

7.6.2

HS-SCCH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229 14

7.6.3

HS-PDSCH . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231

7.6.4

Associated DCH . . . . . . . . . . . . . . . . . . . . . . . . . 233

7.6.5

Fractional-DPCH . . . . . . . . . . . . . . . . . . . . . . . . . 234

7.7

Timing of HSDPA Channels . . . . . . . . . . . . . . . . . . . . . . . 235

7.8

HSDPA UE Categories . . . . . . . . . . . . . . . . . . . . . . . . . . 236

7.9

HSDPA Peak Bitrate Calculation . . . . . . . . . . . . . . . . . . . . 237

7.10 Serving HS-DSCH Cell Change . . . . . . . . . . . . . . . . . . . . . 239 7.11 Summary: HSDPA Operation in Short . . . . . . . . . . . . . . . . . 241 8 High Speed Uplink Packet Access

245

8.1

Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245

8.2

Comparison with HSDPA . . . . . . . . . . . . . . . . . . . . . . . . 247 8.2.1

Commonalities with HSDPA . . . . . . . . . . . . . . . . . . . 247

8.2.2

Differences from HSDPA . . . . . . . . . . . . . . . . . . . . . 248

8.3

HSUPA User Plane Protocols . . . . . . . . . . . . . . . . . . . . . . 248

8.4

HSUPA Configuration Options . . . . . . . . . . . . . . . . . . . . . . 250

8.5

E-DCH UE Categories and Bit Rates . . . . . . . . . . . . . . . . . . 251

8.6

Starting of HSUPA Operation . . . . . . . . . . . . . . . . . . . . . . 253

8.7

HSUPA Protocol Architecture . . . . . . . . . . . . . . . . . . . . . . 254

8.8

Channels and Physical Layer . . . . . . . . . . . . . . . . . . . . . . . 260

8.9

8.8.1

E-DPDCH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 261

8.8.2

E-DPCCH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 262

8.8.3

E-AGCH

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264

8.8.4

E-RGCH

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 266

8.8.5

E-HICH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 270

8.8.6

F-DPCH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 272

Summary: Serving and Non-serving RLS . . . . . . . . . . . . . . . . 273

8.10 E-TFC Selection Procedure . . . . . . . . . . . . . . . . . . . . . . . 276 8.10.1 Step 1: UE sends Scheduling Requests to Node B . . . . . . . 276 8.10.2 Step 2: Serving Grant Value . . . . . . . . . . . . . . . . . . . 277 15

8.10.3 Step 3: Find Power Offset . . . . . . . . . . . . . . . . . . . . 278 8.10.4 Step 4: “Reference E-TFCI & Power Offset” Curve . . . . . . 278 8.10.5 Step 5: Calculate E-TFCI Allowed by Grant Value . . . . . . 278 8.10.6 Step 6: Calculate TB Size . . . . . . . . . . . . . . . . . . . . 278 8.10.7 Step 7: Select Channelization Code & L1 Parameters . . . . . 279 8.10.8 Step 8: UL Transmission on E-DCH . . . . . . . . . . . . . . 280 8.10.9 Step 9: Feedback from Node B on E-HICH

. . . . . . . . . . 281

8.10.10 Step 10: Feedback from Node B on E-RGCH . . . . . . . . . . 282 8.11 Summary: HSUPA Operation in Short . . . . . . . . . . . . . . . . . 283 8.12 UL Channelization Codes . . . . . . . . . . . . . . . . . . . . . . . . 288 8.13 DL Channelization Codes . . . . . . . . . . . . . . . . . . . . . . . . 291 8.13.1 R99 DL Channels . . . . . . . . . . . . . . . . . . . . . . . . . 291 8.13.2 HSDPA-related DL Channels . . . . . . . . . . . . . . . . . . 292 8.13.3 HSUPA Related DL Channels . . . . . . . . . . . . . . . . . . 292 9 Signalling 9.1

9.2

295

Building Blocks of 3G Signalling . . . . . . . . . . . . . . . . . . . . . 296 9.1.1

RRC Connection . . . . . . . . . . . . . . . . . . . . . . . . . 296

9.1.2

Radio Access Bearer (RAB) . . . . . . . . . . . . . . . . . . . 298

9.1.3

Radio Bearer . . . . . . . . . . . . . . . . . . . . . . . . . . . 298

9.1.4

Radio Link . . . . . . . . . . . . . . . . . . . . . . . . . . . . 299

9.1.5

Non-Access Stratum (NAS) Signalling Connection . . . . . . . 301

RRC Connection Establishment . . . . . . . . . . . . . . . . . . . . . 302 9.2.1

RRC Connection on Dedicated Channels - DCH . . . . . . . . 302

9.2.2

RRC Connection on Common Channels - FACH/RACH . . . 304

9.3

Mobile Originated Voice Call Establishment . . . . . . . . . . . . . . 306

9.4

Mobile Terminated Voice Call Establishment . . . . . . . . . . . . . . 310

9.5

PS Data Session Setup . . . . . . . . . . . . . . . . . . . . . . . . . . 314

9.6

Soft Handover . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 320

9.7

Inter-RNC Handover with Iur Interface . . . . . . . . . . . . . . . . . 323 16

1 9.8

Inter-RNC Handover without Iur Interface . . . . . . . . . . . . . . . 326

9.9

CS Inter-System Handover (3G to 2G) . . . . . . . . . . . . . . . . . 329

9.10 PS Inter-System Handover (3G to 2G) . . . . . . . . . . . . . . . . . 335 9.11 HSDPA Mobility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 338 9.11.1 Serving HS-DSCH Cell Change . . . . . . . . . . . . . . . . . 338 9.11.2 HS-DSCH Channel Type Switch . . . . . . . . . . . . . . . . . 342 9.11.3 HS-DSCH IFHO and ISHO . . . . . . . . . . . . . . . . . . . 343 9.12 HSUPA Mobility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 345 9.12.1 E-DCH Soft Handover . . . . . . . . . . . . . . . . . . . . . . 345 9.12.2 E-DCH Serving Cell Change . . . . . . . . . . . . . . . . . . . 345 9.12.3 E-DCH Channel Type Switch . . . . . . . . . . . . . . . . . . 345 9.12.4 E-DCH IFHO and ISHO . . . . . . . . . . . . . . . . . . . . . 347 10 Self Test

351

10.1 Module 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 351 10.2 Module 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 354 10.3 Module 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 357 10.4 Module 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 360 10.5 Module 5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 363 10.6 Module 6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 367 10.7 Module 7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 369 10.8 Module 8 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 372 10.9 Module 9 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 376

CHAPTER

1 HISTORY AND STANDARDIZATION

We have come a long way. GSM made it possible to leave the office and yet answer the phone calls, 3G did the same for the e-mails, and SMS still creates wonders despite its tiny size.

If you have decided to read this book, you are definitely involved with the mobile communication industry and it is quite possible that at least once someone has asked you “What is 3G ? ”. Especially after hearing the words iPhone 3G & 3GS, even non-technical people have started wondering what exactly is 3G. In this chapter, we will try to find the answer to this question and have a look at the evolutionary path taken by 3G.

1.1

Mobile Telecom Market Source: Informa Telecoms & Media http://www.4gamericas.org/

The statistics from the 4th Quarter 20121 show that 90% of the mobile subscriptions in the world are using GSM, UMTS & HSPA Systems. Figure 1.1 is taken from 1

Source: Informa Telecoms & Media & http://www.4gamericas.org/

2

1.2. HISTORY

3

www.4gamericas.com, which contains a lot of interesting details about the present telecom market. Among all the cellular systems, GSM has been the most successful technology and due to affordable handsets and the importance of voice service, it will continue to remain the leading technology for the coming years as well. When UMTS was developed as the 3G system, the 3G-2G interworking was planned so that the investments of GSM could be reused and 3G cost of ownership could be reduced. Even today, 2G provides a coverage safety belt to 3G. When subscribers run into 3G coverage holes, then they can use the 2G systems. Similarly, when the 2G load increases, 3G can provide some load-sharing possibility for the smooth operation of 2G cell.

Figure 1.1: Mobile market in Q4 2012; source: Informa Telecoms & Media

1.2

History

Nowadays, the word mobile phone is a synonym of cellular phones. In some countries2 , the mobile phones are called handy. But there was a time when mobile phones were neither handy nor cellular. Those commercial mobile systems can be called as 0G or Single Cell Systems. 2

e.g., in Germany

4

CHAPTER 1. HISTORY AND STANDARDIZATION

1.2.1

0G Systems

0G Systems are called so because they were the predecessors of the first generation (1G) modern cellular communication systems. They were planned to serve a very large geographical area using a very high base station. Due to this huge distance, mobile was required to transmit at a very high transmit power, and it needed a bulky battery to make it possible. Therefore, the tranmitters/receivers of these phones were typically mounted on the top of vehicles and the handset was fitted close to the driver’s seat. Because of the huge cost of equipment and service operation, this could be used only by very few groups of people, e.g., celebrities, politicians and construction managers. For operators: These systems were not a profitable technology because each frequency could be used only once in a very large geographical area. For Users: Cost of service and user equipment was so high that the common man did not feel the need of using this service. Both of the points listed above stopped the growth of 0G systems and these remained as low-subscriber system.

1.2.2

1G Systems

First-generation mobile systems were a big revolution. In 1G systems, for the first time, we divided the coverage areas into cells and hence started the history of the modern cellular mobile communication system. 1G systems could also be called as analog3 mobile phone systems because they used analog transmission for speech services. In 1979, the first cellular system in the world became operational by Nippon Telephone and Telegraph (NTT) in Tokyo, Japan. Two years later, the cellular epoch reached Europe. The two most popular analogue systems were Nordic Mobile Telephones (NMT) and Total Access Communication Systems (TACS). In 1981, the NMT-450 system was commercialized by NMT in Scandinavia. The system operated in the 450 MHz and 900 MHz band with a total bandwidth of 10 MHz. Advanced Mobile Phone System (AMPS) was an analog mobile phone system standard developed by Bell Labs, and officially introduced in the Americas in 1983. TACS, launched in the United Kingdom in 1982, operated at 900 MHz with a band of 25 MHz for each path and a channel bandwidth of 25 KHz. Other than NMT 3

Analog: Amplitude Modulation, Frequency Modulation, Phase Modulation

1.2. HISTORY

5

and TACS, some other analogue systems were also introduced in the 1980s across Europe. For example, in Germany, the C-450 cellular system, operating at 450 MHz and 900 MHz (later) was deployed in September in 1985. All of these systems offered handover and roaming capabilities but the cellular networks were unable to inter-operate between countries. This was one of the inevitable disadvantages of the first-generation mobile networks. Other than being just the local & region-specific systems, these systems also had some other disadvantages: Security: Analog speech was transmitted without any encryption. Quality: Transmission errors were not so easy to correct because it was very difficult to reconstruct exactly the same analog signal. Capacity: Analog speech cannot be compressed. Therefore, 1G radio transmission requires a lot of bandwidth. This causes network congestion for the operator and expensive operation for the subscriber. With the introduction of 1G phones, the mobile market showed some growth and the number of subscribers reached 20 million by 1990. But still, to own a mobile phone was a luxury and only a few people could afford it.

1.2.3

2G Systems

Just like 1G, every region had its own local 2G standards as well. For example, in the USA, 1G AMPS was upgraded to digital AMPS (D-AMPS), in the USA itself, a CDMA based IS-95 2G network was launched. Japan developed its own 2G system called Personal Digital Cellular (PDC). PDC is a TDMA-based technology that is deployed only in Japan. Among all 2G systems, one system that made the largest impact on our lives is Global System for Mobile communication (GSM). The secondgeneration (2G) mobile systems were introduced in the early 1990s. Low bit rate data services were supported as well as the traditional speech service. Development of GSM in Europe But in Europe, there was a growing demand of a common standard which should work in the major part of Europe. To fulfil this need, in 1982 a group was formed, which is knows as Groupe Speciale Mobile (GSM)4 . This group was formed by 4

This was the original name of GSM. Later it was changed to Global System of Mobile Communications

6

CHAPTER 1. HISTORY AND STANDARDIZATION

the Confederation of European Posts and Telecommunications (CEPT) to design a pan-European mobile technology. In coming years, the European Commission and European Union heads of states endorsed the GSM project. The GSM project was started as a European initiative but soon the non-European operators also started endorsing it. In fact, the GSM has been more successful than the most optimistic forecast ever made for it. For example, the number of GSM subscribers reached the 1 Million mark in 1994, 10 million in 1995, 50 million in 1996 and in 1998 the number reached 100 million mark. In 1998, there were more than 100 countries which were using GSM. A more detailed description of the events can be found at http://www.gsma.com/aboutus/history/. On technical aspects also, the second-generation mobile systems were a big enhancement which fulfilled many shortcomings of 1G analog systems. One remarkable aspect of 2G systems is that they all utilize digital 5 modulation. In a digital system, the analog speech signal passes through an analog-to-digital converter and the digitized signal is fed to the modulator. When compared to the first-generation systems, 2G systems are able to achieve: • higher spectrum efficiency due to modern speech codecs • (encrypted) secure radio transmission • better quality by using an error-correction scheme (channel coding) • better data services • more advanced roaming

Three 2G Standards of USA In the United States, there were three lines of development in the second-generation digital cellular systems. 1. The first digital system, introduced in 1991, was the IS-54 (North America TDMA Digital Cellular) of which a new version supporting additional services (IS-136) was introduced in 1996. 2. Meanwhile, IS-95 (cdmaOne) was deployed in 1993. In many regions of the world, it is called 2G CDMA. 5

Digital: Amplitude Shift Keying, Frequency Shift Keying, Phase Shift Keying

1.2. HISTORY

7

Figure 1.2: Commercially deployed Mobile Communication Systems of the 3GPP Family 3. The US Federal Communications Commission (FCC) also auctioned a new block of spectrum in the 1900 MHz band (PCS), allowing GSM1900 to enter the US market. In Japan, the Personal Digital Cellular (PDC) system, originally known as JDC (Japanese Digital Cellular) was initially defined in 1990. Commercial service was started by NTT in 1993 in the 800 MHz band and in 1994 in the 1.5 GHz band. GSM and its Evolution GSM has emerged as a single, unified 2G standard operating in more than 200 countries. This enabled seamless services throughout Europe by means of international roaming. The earliest GSM system operated in the 900 MHz frequency band. Later, GSM specifications for 1800 & 1900 MHz bands were released. During development over more than 20 years, the GSM technology has been continuously improved to offer better services in the market. New technologies have been developed based on the original GSM system, leading to some more advanced systems known as 2.5

8

CHAPTER 1. HISTORY AND STANDARDIZATION

Generation (2.5G) systems. HSCSD, GPRS and EDGE are all based on the original GSM system. High Speed Circuit Switched Data (HSCSD) HSCSD is the first enhancement of the GSM air interface. The new features included in HSCSD are illustrated in figure 1.3, These 2 features are: 1. The new channel coding used in HSCSD yields 14.4 kbps user data per time slot. 2. For high bit rates, several time slots could be bundled and allocated to the same user. HSCSD bundles GSM time slots to give a theoretical maximum data rate of 57.6 kbit/s (bundling 4 X 14.4 kbit/s full rate time slots). HSCSD provides both symmetric and asymmetric services and it is relatively easy to deploy. However, HSCSD is not easy to price competitively since each time slot is effectively a GSM channel.

Figure 1.3: HSCSD compared with GSM

1.2. HISTORY

9

General Packet Radio Service Following HSCSD, GPRS is the next step of the evolution of the GSM air interface. Other than bundling timeslots, 4 new channel coding schemes are proposed. GPRS provides always-on packet-switched services with bandwidth only being used when needed. Therefore, GPRS enables GSM with Internet access at high spectrum efficiency by sharing time slots between different users. Theoretically, GPRS can support data rate up to 160 kbit/s (current commercial GPRS provides 40 kbit/s). Deploying GPRS is not as simple as HSCSD because the core network needs to be upgraded as well. General Packet Radio Service (GPRS) provides packet data radio access for GSM mobile phones. On a general level, GPRS connections use the resources only for a short period of time when sending or receiving data: • In a circuit-switched system, the line is occupied even when no data is transferred. • In a packet-switched system, the resources are released so they can be used by other subscribers. GPRS is, therefore, well adapted to the bursty nature of data applications. GPRS has minimal effects on the handling of circuit switched calls but the inter-operability of existing circuit switched functionalities needs to be taken into account. An investment in the GPRS infrastructure is an investment in future services. GPRS paves the way and is already part of the third generation (3G) network infrastructure. Migration to 3G comprises deployment of the new WCDMA radio interface served by the GSM and GPRS core networks. Many of the 3G services are based on IP, and the GPRS Core network is the key step of introducing the IP service platform into the present GSM networks. When migrating to 3G services, preserving the Core Network investments is a top priority. Introducing UMTS will complement the GSM network, not replace it. Enhanced Data Rates for GSM Evolution Enhanced Data rates for GSM Evolution (EDGE) boosts GSM/GPRS network capacity and data rates to meet the demands of wireless multimedia applications and mass market deployment. EDGE uses the GSM radio structure but with a new modulation scheme, 8-PSK, instead of GMSK, thereby increasing by three times the GSM throughput using the same bandwidth. A quick summary of EDGE technology can be found in figure 1.5.

10

CHAPTER 1. HISTORY AND STANDARDIZATION

Figure 1.4: GPRS improvements for higher bit rates With the new modulation, EDGE increases the radio interface data throughput, as per 3GPP standardization, three-fold compared to today’s GSM and boosts both circuit switched and packet switched services. The maximum standardized data rate per timeslot will triple, and the peak throughput, with eight time slots in the radio interface, can be up to 473 kbit/s. Since it is fully based on GSM, introducing EDGE to the existing network requires relatively small changes to the network hardware and software. EDGE does not entail any new network elements6 . The operators need not make any changes to the network structure or invest in new regulatory licenses. EDGE, in combination with GPRS, will deliver single user data rates of up to 384 kbit/s. EDGE or E-GPRS supports higher data rates compared to basic GPRS, using several Modulation and Coding Schemes (MCSs) varying from 8.8 kbit/s to 59.2 kbit/s in the radio interface. Nine different MCS schemes are designed for EGPRS. When an RLC data block is sent, the information is encoded using one of the MCSs to resist 6

Compared to GPRS network architecture, the EDGE network does not need any additional network element.

1.2. HISTORY

11

Figure 1.5: Summary of EDGE Technology

Modulation Symbol rate Payload/burst Gross rate/time slot

8-PSK 8-PSK, 3 bit/sym 270.833 ksps 346 bits 69.6 kbit/s

GMSK GMSK, 1 bit/sym 270.833 ksps 116 bits 23.2 kbit/s

Table 1.1: Comparison of 8-PSK and GMSK modulation schemes channel degradation and modulated before transmission over the radio interface. Since the resources are limited, the higher the level of protection for information, the less information is sent. The protection that best fits the channel condition is chosen for a maximum throughput. The GMSK modulation provides a robust mode for wide area coverage while 8PSK provides higher data rates.

1.2.4

3G Systems

The idea of next generation mobile network was first conceived by ITU and was called IMT-2000. IMT-2000 is the result of collaboration of many entities, inside the ITU (ITU-R and ITU-T), and outside the ITU (3GPP, 3GPP2, UWCC and so

12

CHAPTER 1. HISTORY AND STANDARDIZATION MCS MCS-1 MCS-2 MCS-3 MCS-4 MCS-5 MCS-6 MCS-7 MCS-8 MCS-9

Modulation GMSK

8-PSK

Code Rate 0.53 0.66 0.80 1.0 0.37 0.49 0.76 0.92 1.0

User Rate 8.8 kbps 11.2 kbps 14.8 kbps 17.6 kbps 22.4 kbps 29.6 kbps 44.8 kbps 54.4 kbps 59.2 kbps

Table 1.2: Peak data rates for E-GPRS Modulation and Coding Schemes on). As shown in figure 1.6, the vision of ITU for its next generation system was something like this. Quoted word-by-word, Source: http://www.itu.int/osg/spu/imt-2000/technology.html IMT-2000 offers the capability of providing value-added services and applications on the basis of a single standard. The system envisages a platform for distributing converged fixed, mobile, voice, data, Internet and multimedia services. One of its key visions is to provide seamless global roaming, enabling users to move across borders while using the same number and handset. IMT2000 also aims to provide seamless delivery of services, over a number of media (satellite, fixed, etc). It is expected that IMT-2000 will provide higher transmission rates: a minimum speed of 2 Mbit/s for stationary or walking users, and 348 kbit/s in a moving vehicle. Second-generation systems only provide speeds ranging from 9.6 kbit/s to 28.8 kbit/s.

When ITU defined the requirements of its next generation mobile system, several standards development organizations started working on finding solutions to fulfill those requirements in the easiest and most cost-effective manner. A few well known organizations in this race were European Telecommunications Standards Institute (ETSI), Telecommunications Technology Association, Korea, Association of Radio Industries and Businesses, Japan etc. As a part of the roadmap, July 1998 was accepted as the deadline for submission of proposals for IMT-2000 by the regional standardization development organizations.

1.3. 3GPP AND 3GPP2

13

Third-generation (3G) systems promise faster communications services, including voice, fax and Internet, anytime and anywhere with seamless global roaming. ITU’s International Telecommunication Union IMT-2000 global standard for 3G has opened the way to enabling innovative applications and services (e.g. multimedia entertainment, infotainment and location-based services, among others).

Figure 1.6: ITU’s vision of IMT-2000 The terrestrial radio transmission technologies proposed to ITU in July 1998 included a number of different Wideband CDMA (WCDMA) based radio access technologies, which can be grouped into two types. Synchronous: These type of proposals requires synchronized base stations. These proposals were built on the IS-95 2G radio transmission technology (e.g., TD-SCDMA). Non-Synchronous: The other group of concepts does not rely on base station synchronization (e.g., WCDMA).

1.3

3GPP and 3GPP2

By the end of 1998, two specification development projects were founded by the regional standardization organizations, 3GPP (3rd Generation Partnership Project)

14

CHAPTER 1. HISTORY AND STANDARDIZATION

and 3GPP2. The goal of both 3GPP and 3GPP2 was to merge a number of the IMT-2000 proposals into a single one, see figure 1.7. 3GPP: 3GPP includes the organizations which are working on evolving GSM towards 3G standards and beyond. 3GPP is responsible for the standardization of GSM, GPRS, UMTS, HSPA & LTE. 3GPP2: 3GPP2 includes the organizations which are working on evolving IS-95 (2G CDMA) towards 3G standards. 3GPP2 is responsible for the standardization of CDMA2000 1X, CDMA200 EV-DO Rev. A/B/C.

1.3.1

3rd Generation Partnership Project (3GPP)

Source: http://3gpp.org/Partners http://3gpp.org/About-3GPP The 3rd Generation Partnership Project (3GPP) unites six telecommunications standards bodies, known as Organizational Partners and provides their members with a stable environment to produce the highly successful Reports and Specifications that define 3GPP technologies. The six 3GPP Organizational Partners - from Asia, Europe and North America determine the general policy and strategy of 3GPP. ARIB The Association of Radio Industries and Businesses, Japan CCSA China Communications Standards Association, China TTA Telecommunications Technology Association, Korea TTC Telecommunication Technology Committee, Japan ETSI The European Telecommunications Standards Institute ATIS The Alliance for Telecommunications Industry Solutions, USA Other than these, 3GPP currently has three Observers. Observers are Standards Development Organizations (SDOs) who have the qualifications to become future Organizational Partners. TIA Telecommunications Industries Association, USA

1.3. 3GPP AND 3GPP2

15

ISACC ICT Standards Advisory Council of Canada, Canada ACIF Communications Alliance - former Australian Communications Industry Forum, Australia Additionally, 3GPP has also members who have the ability to offer market advice to 3GPP and to bring into 3GPP a consensus view of market requirements falling within the 3GPP scope; but does not have the capability and authority to define, publish and set standards within the 3GPP scope. These partners are called ‘Market Representation Partners’. 3GPP has several market representative partners, which are IMS Forum, TDForum, GSA, GSM Association, IPV6 Forum, UMTS Forum, 4G Americas, TD SCDMA Industry Alliance, InfoCommunication Union, Small Cell Forum, CDMA Development Group, Cellular Operators Association of India (COAI) and NGMN Alliance.

1.3.2

3rd Generation Partnership Project 2 (3GPP2) Source:

http://www.3gpp2.com/Public html/Misc/AboutHome.cfm

The Third Generation Partnership Project 2 (3GPP2) is also a collaborative effort among 5 North American and Asian standards development organizations whose aim is to develop the specifications for ANSI/TIA/EIA-41 Cellular Radio telecommunication Intersystem Operations network evolution to 3G. 3GPP2 is responsible for standardization of cdma2000 and its future evolutions. Similar to 3GPP, 3GPP2 also has organizational partners, which are: ARIB The Association of Radio Industries and Businesses, Japan7 CCSA China Communications Standards Association, China TTA Telecommunications Technology Association, Korea TTC Telecommunication Technology Committee, Japan TIA Telecommunications Industry Association, USA 7

ARIB, CCSA, TTA & TTC organizations are partners in both 3GPP and 3GPP2.

16

CHAPTER 1. HISTORY AND STANDARDIZATION

3GPP2 also has market representation partners which have the ability to offer market advice to 3GPP2 and to bring into 3GPP2 a consensus view of market requirements falling within the 3GPP2 scope. There are 4 market representatives of 3GPP2: CDMA Development Group (CDG), IPv6 Forum, Small Cell Forum, CDMA Certification Forum.

Figure 1.7: Standards Development Organizations responsible for forming 3GPP & 3GPP2 List of all IMT-2000 Systems The purpose of this section is to show the complete picture of 3G. In the whole book, we will discuss only WCDMA FDD (officially called as IMT-2000 CDMA Direct Spectrum).

For someone living in Europe, 3G, WCDMA and UMTS are synonyms. But if we analyze carefully, it is not correct. According to ITU’s Recommendation ITU-R M.1457, there were 5 systems recommended for terrestrial radio interfaces of IMT2000. Source: ITU’s Recommendation ITU-R M.1457

1. IMT-2000 CDMA Direct Spread: The IMT-2000 radio-interface specifications for CDMA Direct Spread technology are developed by 3GPP. This radio

1.3. 3GPP AND 3GPP2

17

interface is called Universal Terrestrial Radio Access (UTRA) FDD or Wideband CDMA (WCDMA). In the development of this radio interface, the CN specifications are based on an evolved GSM-MAP. However, the specifications include the necessary capabilities for operation with an evolved ANSI-41-based CN. 2. IMT-2000 CDMA Multi-Carrier: The IMT-2000 radio interface specifications for CDMA multi-carrier (MC) technology are developed by 3GPP2. This radio interface is called cdma2000. In the development of this radio interface the CN specifications are based on an evolved ANSI-41 and IP network, but the specifications include the necessary capabilities for operation with an evolved GSM-MAP based CN, a CN based on IETF protocols, or the 3GPP Evolved Packet Core (EPC). 3. IMT-2000 CDMA TDD: The IMT-2000 radio interface specifications for CDMA TDD technology are developed by 3GPP. This radio interface is called the Universal Terrestrial Radio Access (UTRA) time division duplex (TDD), where three options are defined: • 1.28 Mchip/s TDD (TD-SCDMA) • 3.84 Mchip/s TDD • 7.68 Mchip/s TDD 4. IMT-2000 TDMA Single-Carrier: The IMT-2000 TDMA Single-Carrier radio interface specifications contain two variations depending on: • whether a TIA/EIA-41 circuit switched network component is used, or • a GSM evolved UMTS circuit switched network component is used. In either case, a common enhanced GSM General Packet Radio Service (GPRS) packet switched network component is used. The initial focus of the following sections has been to provide an evolution path for the TIA/EIA-136 pre-IMT2000 radio interface to evolve to IMT-2000. 5. IMT-2000 FDMA/TDMA: The IMT-2000 radio interface specifications for FDMA/TDMA technology are defined by a set of ETSI standards. This radio interface is called digital enhanced cordless telecommunications (DECT).

1.3.3

WiMAX as IMT-2000 System

In 2007, WiMAX was also accepted as a new member of IMT-2000 family with the official name of IMT-2000 OFDMA TDD WMAN. WiMAX (designated as IEEE

18

CHAPTER 1. HISTORY AND STANDARDIZATION

Std 802.16) is developed and maintained by the IEEE 802.16 Working Group on Broadband Wireless Access. It is published by the IEEE Standards Association (IEEE-SA) of the Institute of Electrical and Electronics Engineers (IEEE).

1.4

WCDMA FDD - Releases Source:

http://3gpp.org/Releases

Before 3G, the standardization work of GSM and GPRS was done by ETSI. From R99 onwards, ETSI is one of the bodies in the collaboration of 3GPP. Hence, after R99, we always talk about 3GPP releases instead of ETSI releases. Table 1.3 shows the dates on which various 3GPP releases were standardized and frozen. According to 3GPP, after freezing, a release can have no further additional functions added. However, detailed protocol specifications (stage 3) may not yet be complete. Release Ph 1 Ph 2 R96 R97 R98 R99 Rel-4 Rel-5 Rel-6 Rel-7 Rel-8 Rel-9 Rel-10

Freezing Date 1992 1995 Early 1997 Early 1998 Early 1999 March 2000 March 2001 March - June 2002 December 2004 - March 2005 Stage 3 freeze December 2007 Stage 3 freeze December 2008 Stage 3 freeze December 2009 Stage 3 freeze March 2011

Table 1.3: 3GPP releases of WCDMA and their freezing dates In some of the releases, dates are mentioned according to the stages (stage 1, 2, 3). The term stage has following meaning: “Stage 1” refers to the service description from a service-users point of view. “Stage 2” is a logical analysis, breaking the problem down into functional elements and the information flows amongst them across reference points between functional entities.

1.5. WCDMA FDD - RELEASES AND FEATURES

19

“Stage 3” is the concrete implementation of the protocols appearing at physical interfaces between physical elements onto which the functional elements have been mapped.

1.5

WCDMA FDD - Releases and Features

Before we discuss the details of each 3GPP release and the corresponding features, let us have a quick look at the bit rates offered by various 2G and 3G technologies. Table 1.4 shows both numbers, one which are mentioned in the technology description and the others which are commercially used in practice. It is expected that HSDPA will bridge the gap between theoretical and practical numbers and hence, the mobile operators will feel transparency in the system operation and description. System Typical Max Bitrate(Kbps) Theoretical Max Bitrate(Kbps)

GSM 9.6 9.6

GPRS 50 171

EDGE 150 384

UMTS 384 2048

HSDPA 14400 14400

Table 1.4: Maximum bit rates of various technologies

Figure 1.8: Mobile Evolution from 2G to 4G Release 99 or Rel-99: In December 1999, the first UMTS Release, the so-called ‘Release 99’ was frozen. UMTS Rel-99 is based on the large experience of GSM,GPRS standardization, taking over many principles of the matured GSM, GPRS network, protocol and service architecture. • Mature GSM/GPRS Core network

20

CHAPTER 1. HISTORY AND STANDARDIZATION • New Air IF technology (WCDMA) • New radio network (UTRAN) • bit rates up to 384 kbps8 Rel-99 was the start of 3G. The highlights of R99 was a CDMA based radio access network (UTRAN) and the new interfaces which connect UTRAN to the existing GSM/GPRS core network. Rel. 99 specifies that transmission technology on these interfaces should be ATM9 .

REL-4 Continuing the 3GPP evolution, Release 4 enhanced UMTS via several features, e.g.: • Bearer independent CS Core Network • CAMEL CAMEL Phase 4 • Low chip rate TDD mode • Transcoder Free Operation There was no major enhancement to the UTRAN in Rel-4. Therefore, the bit rates of R99 and Rel-4 are identical. In CS Core Network, MSC functionality was split into 2 separate Units: MSC Server (MSS) and Media Gateway (MGW). This type of Core Network architecture is also called as Split Architecture. The next chapter explains these issues. REL-5 UMTS Release 5 was finalized at the end of 2002, including several Core Network and Radio Interface enhancements such as: • High Speed Downlink Packet Access; peak DL bitrate up to 14.4 Mbps • All IP Core Network • IP Multimedia Subsystem • Wide Band AMR Release 5 is a very significant release which affected all areas of UMTS. For radio network, HSDPA improved the peak bit rates of DL beyond the 2 Mbps limit. For transmission network, IP based Iub, Iur and Iu interfaces were defined. In core network, IP multimedia subsystem (IMS) was defined, which uses SIP based signalling to setup, maintain and tear down the packet sessions. 8

Theoretically 2 Mbps but in practice, we do not get more than 384 kbps using conventional 3G DCH resources. 9 There were 3 options available: TDM, ATM or IP. ATM was chosen because of its flexibility and QoS provisioning.

1.5. WCDMA FDD - RELEASES AND FEATURES

21

REL-6 UMTS Release 6 was frozen 09/2005, containing features such as: • FDD Enhanced Uplink (HSUPA ); peak UL bitrate up to 5.76 Mbps • WLAN-UMTS Interworking • IP Multimedia Subsystem Phase 2 Release 6 was the point where both UL and DL could be High Speed. The common name for HSDPA and HSUPA is HSPA. It was specified that HSUPA cannot work without HSDPA in DL. REL-7 UMTS Release 7 has been closed end of 2007, including important UMTS & HSPA enhancements, improving the UMTS peak rates and spectral efficiency: • Higher order Modulation: 64QAM for the DL (up to 21 Mbps), 16QAM for the UL (up to 11.5 Mbps) • 2x2 MIMO (up to 28 Mbps) for downlink (HSDPA) • Continuous Packet Connectivity CPC • Flexible RLC • Enhanced Cell FACH The development of HSPA in Rel-7 was focussed on 3 different objectives, which are: • To increase the peak bit rate of HSDPA towards higher limits. This could be achieved by – Higher Order Modulation (64QAM in DL & 16QAM in UL), or – Multiple Input Multiple Output (MIMO) in DL. • To decrease the battery consumption so that UEs could stay connected for a longer period even if they are inactive. • Reduced latency (Round trip time) for better support of RT services on HSPA. REL-8 3GPP Release 8 was frozen in 03/2009, containing further HSPA improvements as well as the UMTS Long Term Evolution LTE and the Evolved Packet System EPS: • HSDPA further improvement using DC-HSDPA (42 Mbps) and simultaneous operation of MIMO & 64QAM in DL. • DC-HSUPA (23 Mbps)

22

CHAPTER 1. HISTORY AND STANDARDIZATION • New radio access technology: E-UTRAN /Long Term Evolution (LTE) • New purely packet switched core network Evolved Packet Core (EPC) Release 8 pushed HSDPA peak bit rate to even higher values by using the combined operation of 64-QAM and 2X2 MIMO. Additionally, an OFDMAbased new radio network (E-UTRAN) and a pure IP-based new core network was defined. This new system os commonly known as Evolved Packet System (EPS) or Long Term Evolution (LTE).

REL-9 3GPP Release 9 has been closed end of 2009, including HSPA+ enhancements and initial LTE-Advanced (LTE-A) definitions. • Dual-Cell HSDPA, 2x2 MIMO & 64QAM (up to 84 Mbps). REL-10 3GPP Release 10 was finished in early 2011; central focus will be on LTEAdvanced10 Few highlights of Rel-10 are described below. • Furthermore, definition of Multi-Carrier HSPA for UL & DL is expected. • LTE-Advanced (up to 1 Gbps DL & 500 Mbps UL for low mobility/Indoor) as IMT-Advanced proposal (4G). • Multi-Carrier HSDPA (DL: up to 3 or 4 carrier delivering up to 126 & 168 Mbps respectively). • Dual-Carrier HSUPA (UL: up to 23 Mbps).

10

The term 4G is quite often used without much care. The ITU guidelines and requirements show that only in Rel 10, LTE is able to fulfil the requirements of the 4G system. Hence, it is technically wrong to call REL-8 LTE as 4G system.

1.5. WCDMA FDD - RELEASES AND FEATURES

23

Copyright Notices Main reference material for this book has been technical specifications (TSs) and technical reports (TRs) of 3rd Generation Partnership Project (3GPP). Information has been interpreted and presented in a simplified manner. In the first chapter no copyrighted material has been used. But the information available on the official website of 3GPP has been the main source of information. Text on page 16 Text on page 16 Table 1.3 Figure 1.1 on page 3

http://3gpp.org/Partners http://3gpp.org/About-3GPP http://3gpp.org/Releases source: Informa Telecoms & Media

BIBLIOGRAPHY [1] General information about http://www.3gpp.org/About-3GPP

3GPP;

Available

at

[2] Information about 3GPP Organizational partners and market representatives ; Available at http://www.3gpp.org/Partners [3] Information about 3GPP Releases ; Available at http://3gpp.org/Releases [4] Information about mobile technology and IMT-2000; http://www.itu.int/osg/spu/imt-2000/technology.html

Available

at

[5] General information about 3GPP2; Available at http://www.3gpp2.com/. [6] Information about 3GPP Organizational partners and market representatives ; Available at http://www.3gpp2.com/Public html/Misc/PartnersHome.cfm [7] http://www.4gamericas.org/ [8] http://www.gsma.com/aboutus/history/ [9] M.1457-11 Detailed specifications of the terrestrial radio interfaces of International Mobile Telecommunications-2000 (IMT-2000) [10] H.Holma and A. Toskala, ‘WCDMA for UMTS’ , 5th Edition, John Wiley & Sons.

24

CHAPTER

2 NETWORK ELEMENTS AND FUNCTIONALITIES

To understand the network architecture and interfaces of UMTS, we must go through the evolution of GSM and GPRS. UMTS can be understood as a combination of the existing pre-Rel-99 GSM/GPRS core network and a new type of radio access network (UTRAN). Hence, the core networks of 2G and 3G are the same. This aspect significantly reduced the 3G cost of ownership and inspired the mobile operators to invest in UMTS. The following sections will explain the architecture, network elements and interfaces of an UMTS network. Figure 2.1 shows the GSM network architecture according to its basic release (GSM Phase 1 and phase 2). The services offered by such a purely circuit switched network can be categorized in 2 categories. 1. CS Speech: Back in the early 90’s, the voice service was the main objective while buying a mobile phone. Even today, voice is the most important service for the majority of the subscribers and operators. 2. CS Data: The CS Data service can be compared to “Internet access using dial-up modem”. Compared to the modern day’s Internet access, it is different because in the early days of GSM, the traffic was carried by switches instead of routers. The switches are equipped with an interworking functionality that 25

26

CHAPTER 2. NETWORK ELEMENTS AND FUNCTIONALITIES can convert data into a form which can be accepted at the external packet data network.

Figure 2.1: GSM Network Architecture

2.1

Architecture of the GSM Network

A GSM network is composed of several functional entities whose functions and interfaces are depicted in figure 2.1. It shows the layout of a generic GSM network. The GSM network can be divided into three broad subsystems or parts. 1. The Mobile Station (MS): MS is carried by the subscriber. 2. The Base Station Subsystem (BSS): BSS controls the radio link with the Mobile Station. 3. The Switching Subsystem (SS): SS is also known as core network. Its main part is the Mobile services Switching Center (MSC) which performs the switching of calls between the mobile and other fixed or mobile network users as well as mobility management.

2.1. ARCHITECTURE OF THE GSM NETWORK

27

Not shown is the Operations and Maintenance Center (OMC) which oversees the proper operation and setup of the network. The MS and the BSS communicate across the Um interface1 . The Base Station Subsystem communicates with the Mobile services Switching Center (MSC) across the A interface.

2.1.1

The Mobile Station MS

MS consists of the mobile equipment (the handset) and a smart card called the Subscriber Identity Module SIM. The SIM provides personal mobility so that the user can have access to subscribed services irrespective of a specific terminal. By inserting the SIM card into another GSM terminal, the user is able to receive calls at that terminal, make calls from that terminal and receive other subscribed services. • The mobile equipment is uniquely identified by the International Mobile Equipment Identity (IMEI). • The SIM card contains the International Mobile Subscriber Identity(IMSI), which is used to identify the subscriber. SIM also contains a secret key which is required for authentication of the user and encryption of information. The IMEI and the IMSI are independent thereby allowing personal mobility. The SIM card may be protected against unauthorized use by a password or personal identity number. IMSI is a GSM-specific number which is used for internal signalling between the nodes of GSM. It is a secret information which is typically unknown to the subscriber. For making calls and SMS, we use another number known as MSISDN number2 or simply phone number. MSISDN looks similar to the phone numbers of fixed line telephones. In HLR, the mapping between MSISDN and IMSI can be performed.

2.1.2

Base Station Subsystem BSS

The Base Station Subsystem is composed of two types of network elements, the Base Transceiver Station (BTS) and the Base Station Controller (BSC). These two communicate across the standardized Abis interface3 . The openness is desired to allow the interconnection of BTS from one vendor to the BSC of another vendor. 1

also known as the air interface or radio link Mobile Station ISDN Number 3 Although Abis is planned to be an open interface but typically BTS and BSC belong to the same vendor. 2

28

CHAPTER 2. NETWORK ELEMENTS AND FUNCTIONALITIES

BTS The Base Transceiver Station houses the radio transceivers that define a cell and handle the radio link protocols with the Mobile Stations In a large urban area, there will potentially be a large number of BTSs deployed. Thus the requirements for a BTS are ruggedness, reliability, portability and minimum cost. The most important functions of BTS are: • Physical layer processing (channel coding, interleaving, puncturing etc.) • Uplink physical layer measurements • Radio transmission and reception BSC The Base Station Controller is the most important network element in BSS. It is responsible for radio channel assignment, allotment, maintenance and release. One BSC can control the operations of hundreds of BTS. BSC also controls the handover procedures for connected mode mobility. BSC is the connection between the mobile station and the Mobile service Switching Center MSC.

2.1.3

Switching Subsystem

Network Switching Subsystem (NSS) or Switching Subsystem (SS) is responsible for switching the traffic from one mobile operator to another operator or to PSTN. MSC and VLR The central component of the this subsystem is the Mobile services Switching Center (MSC). It acts like a normal switching node of the PSTN or ISDN and additionally provides all the functionality needed to handle a mobile subscriber such as registration, authentication, location updating, handovers4 and call routing to a roaming subscriber. These services are provided in conjunction with several functional entities which together form the switching subsystem. The MSC provides the connection to the fixed networks such as the PSTN or ISDN. Signalling between functional entities in the this subsystem uses Signalling System Number 7 SS7 used for trunk signalling in ISDN and widely used in current public networks. 4

in the case of Inter-BSC handovers

2.1. ARCHITECTURE OF THE GSM NETWORK

29

The Visitor Location Register (VLR) contains selected subscriber’s information from the HLR necessary for call control and provision of the subscribed services for each subscriber currently located in the geographical area controlled by the VLR. Although each functional entity can be implemented as an independent unit, all manufacturers of switching equipment implement the VLR together with the MSC so that the geographical area controlled by the MSC corresponds to that controlled by the VLR thus simplifying the signalling required. Note that the MSC contains no information about particular mobile stations. This information is stored in the location registers. The following steps take place when a MS tries to register itself with an MSC/VLR. Step 1: A subscriber sends its request to register with an MSC/VLR (Using IMSI). Step 2: MSC analyzes the IMSI and finds out the home operator and the HLR’s address. Step 3: MSC/VLR contacts the HLR and requests for subscriber’s information. Step 4: Using this information, the serving MSC/VLR authenticates the subscriber. Step 5: After successful authentication, VLR informs the HLR about the successful registration. In future, if any incoming call or SMS arrives for this subscriber, this MSC/VLR will be contacted for setting up the connection. Gateway MSC GMSC From basic operation and functionality, GMSC is in fact the same as MSC but its logical role is different. GMSC is that MSC which is at the border of the PLMN and interconnects one network to another. The main function of GMSC is HLRInterrogation. This procedure takes place when a Mobile Terminated Call (incoming call) request comes to GMSC, and GMSC interrogates HLR regarding the current location of a mobile subscriber. To understand the role of GMSC, let us take a look at the sequence of events which take place when an incoming5 call comes. Step 1: Mobile terminated call setup request arrives at GMSC (using MSISDN of called party). Step 2: GMSC queries HLR regarding the current location of subscriber (using MSISDN number). 5

also known as Mobile Terminated Call (MTC)

30

CHAPTER 2. NETWORK ELEMENTS AND FUNCTIONALITIES

Step 3: HLR maps MSISDN to IMSI and finds the VLR address where UE was last reported. Step 4: HLR contacts VLR (using IMSI). Step 5: VLR assigns a temporary Mobile Station Roaming Number (MSRN) to this IMSI and sends it back to HLR. Step 6: HLR sends MSRN to GMSC and hence the call can be forwarded to the serving-MSC where the user is currently located. The acceptance of an interrogation to an HLR is the decision of the operator. The choice of which MSCs can act as Gateway MSCs is for the operator to decide (i.e. all MSCs or some designated MSCs). Home Location Register HLR Home Location Register (HLR) is a central database that stores the information about the subscribers. When a SIM card is issued to a mobile user, it gets registered in HLR. Afterwards, wherever the user goes, it gets registered with local MSC/VLR and that MSC/VLR contacts HLR to get the administrative information of the subscriber. In this way, HLR always keeps track of the user’s location. This information is stored in the form of signalling address of VLR. HLR and VLR communicate using MAP protocol of the SS7 signalling suite. For each subscriber HLR contains a lot of information. Some of that is shown below: • IMSI of the subscriber • MSISDN of the subscriber • VLR-address which is currently serving this subscriber • List of Subscribed services • GPRS related data • Quality-of-service (QoS) profile • Subscribed supplementary services (e.g., Call Waiting, Call Forwarding, etc.) There is logically one HLR per GSM operator but as the number of subscribers grows, there can be more than one HLR in the network. It is also better to have

2.1. ARCHITECTURE OF THE GSM NETWORK

31

another HLR node for redundancy purpose. The second node can be used for backup and for redundancy purpose. This arrangement can be used to guarantee the service continuity in case of technical failure with the first node. The information about HLR-address can be found from the IMSI. Equipment Identity Register EIR The Equipment Identity Register (EIR) is a database that contains a list of all valid mobile equipment on the network where each mobile station is identified by its International Mobile Equipment Identity IMEI. An IMEI is marked as invalid if it has been reported stolen or is not type-approved. An EIR maintains three lists: 1. Black: The IMEI numbers of the mobile handsets which have been reported stolen or inappropriate are stored in black list. 2. White: The while list contains the few digits of IMEI number that identify the handset type. In white list, there is no need to have the full IMEI number. If a handset model has been approved by 3GPP standards, then its ”handset type” is stored in the white list. 3. Grey: Under the grey list of EIR, one can find the IMEI numbers of phones which are under surveillance. Every time, this handset is used to access the network services, a log will be generated. In the criminal investigation, IMEI number proves to be quite helpful. Sometimes, criminals steal the phones and start using them with their own SIM cards. Fortunately, the IMEI number of the handset can help the law enforcement agencies to track the mobile equipment. Authentication Center AuC The Authentication Center (AuC) is a protected database that stores a copy of the secret key stored in each subscriber’s SIM card. The AuC always resides in the HPLMN. This network element is the most secured node in the whole PLMN. The AuC generates security data which is used for authentication of users and encryption of data over the radio channel. The secret master key (K) never leaves the authentication center. The AuC feeds random number (RAND) and master key (K) to a standards algorithm and generate security data. This security data is then forwarded to the serving MSC/VLR in VPLMN.

32

CHAPTER 2. NETWORK ELEMENTS AND FUNCTIONALITIES

As described, MSC/VLR perform an authentication procedure to verify the identity of the user at the time of registration. The relationship between MSC/VLR and the AuC can be illustrated in figure 2.2.

Figure 2.2: Authentication of user during roaming

2.2

Improvements of GSM Standard

Compared to the mobile systems which were available till the 90s, GSM was a magical invention and hence, a huge success in the commercial market. The services offered by GSM could be summarized as: • Incoming and outgoing speech calls • Short message services • Supplementary services e.g., call forwarding, calling line identification presentation etc. • Data services e.g., email, fax, web surfing In the next few sections, we will discuss the developments which improved the GSM system in terms of efficiency and user experience.

2.2. IMPROVEMENTS OF GSM STANDARD

2.2.1

33

CAMEL

Source: (CAP)

3GPP TS 29.002 (MAP) & 3GPP TS 22.078, 23.078, 29.078

CAMEL (Customized Applications for Mobile network Enhanced Logic) is an IN6 architecture within the GSM based on 3GPP standardization. CAMEL provides mechanisms to support services independently of the serving network. With CAMEL, it is possible to offer operator-specific services (OSS), that is, intelligent network services, for the subscriber while roaming outside the home PLMN. In order to support Intelligent-Network applications: • MSCs are often upgraded with a Service Switching Point (SSP) which resides in VPLMN. • Operator-specific services can be generated in the Service Control Point (SCP) which lies in HPLMN. • Inter-operator communication is guaranteed by using an open interface and common protocol. The Interface between SSP-SCP is well-defined open interface and the protocol used is called CAMEL Application Part (CAP). CAMEL was developed under the framework of Virtual Home Environment, which means that the subscriber should get the same ‘look & feel’ of the services, independent of the serving network, type of handset etc. CAMEL within Home PLMN Within Home PLMN (or Home Network), 2 functional entities are involved. HLR: The HLR stores subscriber-related data, which also includes the information whether the subscriber is a CAMEL subscriber. The HLR transfers the CAMEL Subscription Information (CSI) to the network elements which need it to be able to provide CAMEL services. These network elements have to support CAMEL, and sending the CSI to these elements has to be allowed. gsmSCF: The operator-specific services are executed in the gsmSCF, which contains the service logics invoked during originating and terminating CAMEL calls, originating SMSs, etc. The CAMEL standard does not specify the implementation of operator-specific services. 6

Intelligent Network

34

CHAPTER 2. NETWORK ELEMENTS AND FUNCTIONALITIES

Figure 2.3: CAMEL Service Architecture (Phase 1) CAMEL within visited PLMN The PLMN where the CAMEL subscriber is roaming is called the visited network (VPLMN). The VMSC, VLR & gsmSSF handle the processing of originating CAMEL calls and forwarded calls, and terminating CAMEL calls. Visited MSC: The VMSC sets up calls from and towards the visiting subscriber. When handling service setup, the VMSC detects whether the subscriber has CAMEL services. If the VMSC receives CSI from VLR and the triggering criteria are met, an initial contact to the gsmSCF takes place. During the CAMEL call, the gsmSCF may request the VMSC to monitor and report certain call events. gsmSSF: The gsmSSF acts as an interface from the MSC/GMSC towards the gsmSCF. The gsmSSF initiates the dialogue with the gsmSCF to get instructions for the CAMEL call handling. VLR: The VLR stores the CAMEL subscriber data received from the HLR of the home network as part of the subscriber data of the subscriber roaming in the VLR area. The VLR sends the subscriber data to the VMSC during originating or forwarded call processing, and terminating call processing. CAMEL standard has been gradually improved in phases. Currently, there are 4 phases: • CAMEL Phase 1

2.3. GPRS NETWORK ARCHITECTURE

35

• CAMEL Phase 2 • CAMEL Phase 3 • CAMEL Phase 4 In order to identify the CAMEL phase, we must check which version of MAP & CAP protocols are supported by gsmSSF, gsmSCF & HLR.

2.3

GPRS Network Architecture

In the list of services offered by GSM, there is a circuit switched data service but it has 2 basic problems: • Low Bit rate (only 9.6 kbps) • Circuit switching & time-based charging As explained in the previous module, HSCSD was introduced to improve the GSM bit rates by roughly 5 times by allocating multiple time slots to the same subscriber. But as the name suggests, HSCSD is also a circuit switched technology which relies on time-slot management and time-dependent charging. In a data session, typically we never use the channel 100% of allocation time. Hence, the operator has to allocate the resources unnecessarily and the user has to pay for this channel for the whole duration of the connection. This was the main motivation to develop the concept of GPRS. GPRS is a packet switched technology where the packets are carried using routers and not using switches. Figure 2.4 clearly shows the combined GSM & GPRS network architecture. On a general level, GPRS connections use the resources only for a short period of time when sending or receiving data: • In a circuit-switched system, the line is occupied even when no data is transferred. • In a packet-switched system, the resources are released so they can be used by other subscribers.

36

CHAPTER 2. NETWORK ELEMENTS AND FUNCTIONALITIES

Figure 2.4: GPRS Network Architecture The General Packet Radio System (GPRS) is a new service that provides actual packet radio access for mobile GSM and time-division multiple access (TDMA) users. The main benefits of GPRS are that it reserves radio resources only when there is data to send and it reduces reliance on traditional circuit-switched network elements. The increased functionality of GPRS decreases the incremental cost to provide data services and increases the penetration of data services among consumer and business users. In addition to providing new services for today’s mobile user, GPRS is an important migration step toward third-generation (3G) networks. GPRS will allow network operators to implement an IP-based core architecture for data applications, which will continue to be used and expanded upon for 3G services for integrated voice and data applications. Today, everyone knows that packets are transported using routers. Prior to GPRS, the packets were carried via switches (MSCs) which is very inefficient if the nature of traffic is bursty.

2.3.1

GPRS Mobile Terminals

New mobile terminals are required because existing GSM phones do not handle the enhanced air interface nor do they have the ability to packetize traffic directly. All these terminals will be backward compatible with GSM for making voice calls using GSM.

2.3. GPRS NETWORK ARCHITECTURE

37

Class A terminals: Class A terminals support simultaneous circuit-switched (CS) and packet-switched (PS) traffic. Class B terminals: Class B terminals attach to the network as both CS and PS clients but only support traffic from one service at a time. They can monitor GSM and GPRS channels simultaneously. In other words, a Class B terminal can support simultaneous attach, activation, and monitor, but not simultaneous traffic. Class C terminals: Class C terminals support attach to only one type of network (either CS or PS). The user must manually select the service to which it wants to connect. Therefore, a Class C terminal can make or receive calls from only the manually (or default) selected service. The service that is not selected is not reachable. The three modes of operation are defined in 3GPP TS 22.060.

2.3.2

GPRS Base Station Subsystem

Impact of GPRS on BSC Each BSC will require the installation of one or more Packet Control Unit (PCUs) and a software upgrade. The PCU provides a physical and logical data interface out of the base station system (BSS) for packet data traffic. When either voice or data traffic is originated at the subscriber terminal, it is transported over the air interface to the BTS, and from the BTS to the BSC in the same way as a standard GSM call. However, at the output of the BSC the traffic is separated; voice is sent to the mobile switching center (MSC) per standard GSM, and data is sent to a new device called the SGSN, via the PCU over a Frame Relay interface. Impact of GPRS on BTS The BTS may also require a software upgrade but typically will not require hardware enhancements. The upgrade in BTS is called Channel Control Unit (CCU) which is responsible for adaptive coding (CS-1 , 2 , 3 and 4). The CCU (Channel Coding Unit) in the BTS performs channel coding whose rate is adapted to the radio transmission conditions: • CS-1 (Channel Coding Scheme 1) - 9.05 kbps • CS-2 (Channel Coding Scheme 2) - 13.4 kbps

38

CHAPTER 2. NETWORK ELEMENTS AND FUNCTIONALITIES • CS-3 (Channel Coding Scheme 3) - 15.6 kbps • CS-4 (Channel Coding Scheme 4) - 21.4 kbps

2.3.3

New Elements in the Core Network Source :

3GPP TS 23.060

SGSN At the hierarchical level, SGSN can be considered as an “MSC with in-built VLR for PS users”. In other words, SGSN can be viewed as a “packet-switched MSC”. Very similar to the CS registration with MSC/VLR, a GPRS mobile station has to register with SGSN. This registration process is called ‘GPRS attach’. After entering a new area, GPRS user reports its location to the corresponding SGSN. Thus SGSN is responsible for the registration of new mobile subscribers, and keep a record of their location inside a given area. In other words, SGSN performs mobility management functions such as mobile subscriber attach/detach and location management. The SGSN is connected to the base-station subsystem via a Frame Relay connection to the PCU in the BSC. For a better understanding, the following section breaks down the attach process of GRPS into several steps and explains it. Step 1: A subscriber sends its request to register with an SGSN (Using IMSI). Step 2: SGSN analyzes the IMSI and finds out the home operator and HLR’s address. Step 3: SGSN contacts HLR and requests for subscriber’s information (e.g., Subscriber’s service profile, QoS agreement, max bit rate etc., authentication related data). Step 4: Using this information, the serving SGSN authenticates the subscriber. Step 5: After successful authentication, SGSN informs HLR about the successful registration. At this moment, a so-called ‘MM-Context’ is generated at MS and SGSN. In other words, a logical-connection has been established between MS and SGSN. This connection is not enough to receive/transmit IP packets because the MS does not have an IP address yet. The main functions of SGSN can be summarized as:

2.3. GPRS NETWORK ARCHITECTURE

39

• Mobility Management • Subscriber’s registration and authentication • Charging Data Records (CDR) collection • Ciphering7 • Packet routing Gateway GPRS Support Node When we observe the GPRS network from the outside world’s viewpoint, it appears that GPRS is nothing but a private IP network owned by the mobile operator. The access to this IP network is allowed only via a gateway router known as GGSN. Hence GGSNs are used as interfaces to external IP networks such as the public Internet, other mobile service providers GPRS services, or enterprise intranets. UE establishes an IP connectivity with GGSN via a procedure known as ‘PDP Context Activation’. This procedure takes place in following sequence: Step 1: MS sends PDP activation requests by specifying the Access Point Name (APN) Access Point Name (APN) and requested Quality-of-Service (QoS). Step 2: SGSN translates APN into the IP address of GGSN with the help of a DNS system. Step 3: SGSN forwards the request to GGSN and GGSN allocates an IP address to the mobile user. Step 4: GGSN informs SGSN and SGSN informs MS about the successful PDP context activation. From this moment, MS is known in the IP world because it has been allocated a valid IP address. At this point, a so-called PDP Context is generated at MS, SGSN and GGSN side. The main functions of GGSN can be summarized as: • Packet routing • Charging Data Records (CDR) generation • Firewall functionality 7

(only in 2G but not in 3G SGSN)

40

CHAPTER 2. NETWORK ELEMENTS AND FUNCTIONALITIES • IP-address allocation • QoS negotiation • Session management (e.g., PDP context activation)

GGSNs maintain routing information that is necessary to tunnel the protocol data units (PDUs) to the SGSNs that service particular mobile stations. A more detailed information about each parameter of MM-context and PDP-context goes beyond the scope of this book. 3GPP TS 23.0608 contains tables which show the information storage structures required for GPRS. There, we can also find details about subscription data stored in the HLR. For proper GPRS operation, MM-context must be stored in UE & SGSN; PDP-context must be stored in UE, SGSN & GGSN. Please refer to 3GPP TS 23.060 to get more details.

2.3.4

Other Changes

The databases in the GSM network, such as the Home Location Register (HLR) that handle the mobility management in the network also require software updates to be able to handle the GPRS functions. Other than these standard nodes, the GPRS network also requires the following network functionalities: Border Gateway (BG) The BG is a special firewall which connects SGSN or GGSN to GPRS Roaming Exchange (GRX) which is used for inter-PLMN connectivity. To understand the role of BG, please refer to figure 2.5. Charging Gateway (CG) The Charging Gateway or charging gateway functionality (CGF) is used for collecting the CDRs from SGSN and GGSN. Quite often, the format of CDRs is not standardized and varies strongly from one vendor to another. Charging gateway functionality is used for transforming the CDRs into a standard form and forward them to the billing center. It helps the telecom operators to implement different services with little billing and charging effort as well as protecting the subscriber 8

General Packet Radio Service (GPRS); Service description

2.3. GPRS NETWORK ARCHITECTURE

41

and operator from wrong charging. During PDP context activation the GGSN sends a ‘charging ID (CID)’ to the SGSN. During the billing process, CDRs are regularly sent from each network node to a central billing centre. The CID is used to merge the records from different network nodes which apply to the same subscriber. Domain Name System (DNS) While activating the PDP context, UE sends Access Pint Name (APN) to SGSN. APN is a user-friendly name which is designed for the comfort of human beings. But routers cannot work with these names. GSGN uses DNS to translate the APNs into the IP-Address of GGSN9 . Lawful Interception Gateway (LIG) LIG is used for law enforcement agencies (like police) to monitor the activities of some suspicious subscriber. Firewall Firewall is used to filter the malicious packets and keep GPRS networks safe from unwanted virus and other threats. Quite often, Firewall functionality is implemented in the GGSN itself.

2.3.5

GPRS Roaming Scenario

Till now, we have observed that a packet in GPRS network takes the following path: Radio Network (BSS)  SGSN  GGSN The path shown above is depicted in the upper half of figure 2.5. This is true as long as the SGSN and GGSN both reside in the same network. In other words, when the user is not roaming. However, in roaming scenarios, the most popular implementation is to use the SGSN in Visited PLMN and GGSN in the Home PLMN. This inter-PLMN connection is made using a private IP-backbone owned by one of the operators or by a third party. This scenario is depicted by the lower half of figure 2.5. We hereby like to briefly mention the two scenarios: 9

The same principle is used in internet for translating URLs into an IP address.

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CHAPTER 2. NETWORK ELEMENTS AND FUNCTIONALITIES

Figure 2.5: Data Access during roaming and non-roaming scenario 1. In non-roaming scenario: In non-roaming scenario, both SGSN and GGSN are located in the same PLMN using intra-PLMN backbone commonly known as the Gn interface. 2. In roaming scenario: In roaming scenario, SGSN resides in the VPLMN whereas GGSN resides in the HPLMN. The two GPRS nodes are connected to each other using inter-PLMN backbone using Gp interface via a secure border gateway functionality. Gp is the name of the interface between “SGSN and Border Gateway” and “GGSN and Border Gateway”.

2.4

Migration to 3G Network Architecture

In order to re-use the investments of GSM and to minimize the rollout cost of UMTS, it was decided that the existing GSM & GPRS core network will be slightly modified but the very same nodes will be utilized to provide access to both10 the radio access networks. There were some minor modifications defined for 3G MSC and 3G SGSN. Many authors like to show these changes as interworking functionality IWF and 10

TDMA based 2G radio network BSS and WCDMA based 3G radio network UTRAN

2.5. UTRAN

43

reuse the same conventional MSC and SGSN.

Figure 2.6: Block Diagram of Combined GSM & UMTS Network Architecture As a starting point, we should consider combined GSM, GPRS & UMTS the network architecture as a combination of the following subsystems: 1. GSM Base station Subsystem: BTS and BSC 2. GSM CS core network: MSC and GMSC 3. GPRS CN: SGSN and GGSN 4. UTRAN: newly developed WCDMA based radio access network 5. Common units: Databases, registers, application servers and billing system From this list, all objects except UTRAN have been already discussed in this chapter. Therefore, in the following section, we will concentrate on the details of UTRAN.

2.5

UTRAN

UMTS Terrestrial Radio Access Network (UTRAN) is the brand new Wideband CDMA-based access network defined for 3G UMTS networks. UTRAN is divided

44

CHAPTER 2. NETWORK ELEMENTS AND FUNCTIONALITIES

into several Radio Network Subsystems where each RNS is managed by one RNC. A RNS typically consists of hundreds of base stations known as Node B. The Radio Network System (RNS) is the system of base station equipments (transceivers, controllers, etc. . . . ) which is viewed by the MSC through a single Iu-interface as being the entity responsible for communicating with Mobile Stations in a certain area. Similarly, in PLMNs supporting GPRS, the RNS is viewed by the SGSN through a single Iu-PS interface. In short,the RNS consists of one Radio Network Controller (RNC) and one or more Node B.

Figure 2.7: UMTS Network Architecture

2.5.1

Node B

Node B can be simply considered as the “BTS of 3G”. The main functions of Node B is to establish the physical implementation of Uu interface and Iub interface. The realization of Uu interface means that Node B implements WCDMA physical channels and converts the information coming from transport channels to the physical channels under control of RNC. For the Iub interface, Node B performs the inverse functionality. It should be noted here that Node B owns only physical channels’ resources whereas transport channels are completely managed by RNC. • Spreading

2.5. UTRAN

45

Figure 2.8: New Interfaces defined in UTRAN Iub, Iur and Iu • Scrambling • Modulation • Channel Coding • Interleaving • Power Control • Synchronization • Measurement reporting

2.5.2

RNC

Radio Network Controller (RNC) is the main controlling element in UTRAN, since it owns all the logical resources of Radio Network Subsystem. It is responsible for controlling the use and integrity of all 3G radio resources by the means of performing Radio Resource Management (RRM) procedures. This includes functions such as handover and admission control, power control and code allocation, radio resource control (RRC) and macro diversity combining (MDC).

46

CHAPTER 2. NETWORK ELEMENTS AND FUNCTIONALITIES

RNC is the central unit in 3G RAN. It also plays an important role in configuration management because the radio related parameters for the whole RNS are stored in RNC. For performance management, RNC updates counters, which are later used to calculate the key performance indicators (KPIs) for RAN. RNC also provided support in fault management by keeping track of the alarms in any Node B controlled by that particular RNC. • Radio Resource Management • Management of System information • Alarms handling • Interworking node Iub and Iu interfaces • Operation and Maintenance • Performance Measurement RNC works as the intermediate node which connects Core Network (CN) to RAN. It is possible that the transport technologies in RAN and Core are different (e.g., One side is using ATM and the other side IP). In that case, RNC performs the protocol conversion required for interworking.

2.6. LOGICAL ROLES OF RNC: S-RNC AND D-RNC

2.6

47

Logical roles of RNC: S-RNC and D-RNC

Figure 2.9: Logical Roles of RNC: SRNC and DRNC In UMTS, while the user is moving from one cell to another, radio links are added and deleted without breaking the connection. This is called Soft Handover. If the two cells belong to 2 different RNCs, then SHO is possible only if the Iur interface between the two RNCs exists. Otherwise, a hard handover takes place which can be compared to Inter-BSC handover of GSM. As shown in figure 2.9, the logical role played by RNC can earn it 3 different titles. Controlling RNC (CRNC): CRNC of Cell # 1 is the RNC which controls the operation of cell by defining its load and other parameters. For all the tele-

48

CHAPTER 2. NETWORK ELEMENTS AND FUNCTIONALITIES com procedures happening in cell #1, RNC #1 is responsible. Therefore, the CRNC of cell #1 is RNC#1.

Serving RNC (SRNC): SRNC of UE is the RNC, which has a signalling connection (RRC Connection) with UE. From Core Network viewpoint, UE is connected to only this RNC because Core Network (MSC or SGSN) receives/sends data from/to this RNC only. UE always has only one SRNC. Serving RNC owns all logical resources belonging to the user connection. Serving RNC is the place where the Macro Diversity Combining (MDC) is executed. Drift RNC (DRNC): DRNC of UE is the RNC which is involved in soft handover but is not the SRNC. The DRNC is CRNC of one of the cells which are involved in Soft Handover. DRNC comes into play only if we have Inter-RNC Soft HO. Please refer to figure 2.9 for clear understanding. Whenever we talk about Soft HO, there is always a discussion of Micro or Macrodiversity. Micro-diversity: Micro-diversity comes into play when UE is involved in Soft handover with 2 cells which belong to the same Node B. This special case of soft HO is called Softer HO. Micro-diversity combining takes place in Node B. Macro-diversity: Macro-diversity comes into play when UE is involved in soft handover with 2 or more cells which belong to 2 (or 3) different Node Bs. Macro-diversity combining takes place in RNC.

2.6. LOGICAL ROLES OF RNC: S-RNC AND D-RNC

Summary of R99 Network Architecture Source:

3G TS 23.002 V3.1.0; Network architecture

Figure 2.10: Configuration of a PLMN and Interfaces (Source TS 23.002)

49

50

2.7

CHAPTER 2. NETWORK ELEMENTS AND FUNCTIONALITIES

Release 4 Modifications Source: • Overview of 3GPP Release 4 V1.1.2 (2010-02) • 3GPP TS 23.205 V4.0.0; Bearer Independent CS Core Network • 3GPP TS 23.002 V4.8.0; Network architecture

Figure 2.11: BICC Network Architecture or Split Architecture As shown in chapter 1, after Rel-99, 3GPP stopped naming the releases after the year as they did in Rel-95, 96, etc. The first set of modifications introduced were called 3GPP Rel-4. In short, the history and future follows this path: Rel-96 → Rel-97 → Rel-98 → R99 → Rel-4 → Rel-5 → Rel-6 → and so on. 3GPP Rel-4 specifications were frozen in March 2001. One of the highlights of Rel-4 is known as “Bearer Independent Call Control”. In order to understand this feature, please refer to the CS part of core network in figure 2.11. The objective of this feature is to separate the user plane (traffic) and control plane (signalling) in the Circuit Switched (CS) domain. This new scheme offers a better

2.7. RELEASE 4 MODIFICATIONS

51

transport resource efficiency and a convergence with the Packet Switched (PS) domain transport. Also, this enables to use one single set of layer 3 protocols on top of different transport resources as ATM, IP, STM, or even new ones. The users shall not notice whether they are connected to a “bearer independent CS network” or to a classical CS domain. Also, none of the protocols used on the radio interface is modified by this feature. This means, for example, there is no need for the terminals to support IP even if IP is the transport protocol used in the network. Using BICC, the traffic is switched using CS-MGW which is capable of handling all modern codecs (e.g., 4.75 kbps AMR ). Thus the speech can be transported from one CS-MGW to another CS-MGW without the need of transcoding. This feature has at least three direct benefits for the operator: Reduced cost of transmission: The utilization of ATM/IP resources reduces significantly compared to the conventional MSCs. Because speech must be transcoded into 64 kbps TDM format so that MSCs can handle it. Using BICC, the need of transcoding arises just before the speech ‘packet’ leaves UMTS domain and enters PSTN. In conventional CS core network, the transcoding is performed as soon as the first MSC is encountered, whereas in BICC, transcoding is performed by the last CS-MGW in the chain. Improved capacity: By avoiding the unnecessary transcoding, the speech quality gets improved. In CDMA networks, this turns out to be a gain in network capacity. Reduced delay: If transcoding is not performed, then the delay caused by transcoding is also avoided. This reduces the transmission delay and improved user’s perception.

2.7.1

Architecture

The basic principle is that the MSC is split into an MSC server and a (CircuitSwitched) Media Gateway (CS-MGW), the external interfaces remaining the same as much as possible as for a monolithic MSC. According to 23.002, “When needed, the MSC can be implemented in two different entities: the MSC Server, handling only signalling, and the CS-MGW, handling user’s data. A MSC Server and a CS-MGW make up the full functionality of a MSC.”

• The MSC server provides the call control and mobility management functions, and

52

CHAPTER 2. NETWORK ELEMENTS AND FUNCTIONALITIES • The CS-MGW provides the stream manipulating functions, i.e. bearer control and transmission resource functions.

The same applies to the GMSC, split into a GMSC server and a CS-MGW.

Figure 2.12: BICC Network Architecture and Interworking with PSTN

MSC Server The MSC Server comprises all the call control and mobility control parts of an MSC. As such, it is responsible for the control of mobile originated and mobile terminated CS domain calls. It terminates the user to network signalling and translates it into the relevant network to network signalling. It also contains a VLR. The MSC Server controls the parts of the call state that pertain to connection control for media channels in a CS-MGW. A GMSC Server is to a GMSC as an MSC Server is to an MSC. CS-MGW The CS-MGW interfaces the transport part of the UTRAN/BSC with the one of the core network over Iu or the A interface. It interacts with the (G)MSC server for resource control.

2.7. RELEASE 4 MODIFICATIONS

53

A CS-MGW may also terminate bearer channels from a circuit switched network and media streams from a packet network (e.g., RTP streams in an IP network). As the entity interfacing the access and the core network, the CS-MGW operates the requested media conversion (it contains e.g., the TRAU), the bearer control and the payload processing (e.g. codec, echo canceller, conference bridge). It supports the different Iu options for CS services (AAL2/ATM based as well as RTP/UDP/IP based).

2.7.2

New Interfaces

CS-MGW to MSC Server (Mc) The Mc reference point describes the interfaces between the MSC Server and CSMGW, and between the GMSC Server and CS-MGW. It supports a separation of call control entities from bearer control entities, and a separation of bearer control entities from transport entities. It uses the H.248/IETF Megaco protocol, jointly developed by ITU-T and IETF. MSC-Server to MSC-Server (Nc) Over the Nc reference point, the Network-Network based call control is performed. Examples of this are ISUP or an evolvement of ISUP for Bearer Independent Call Control (BICC). CS-MGW to CS-MGW (Nb) Over the Nb reference point, the bearer control and transport are performed. Different options are possible for user data transport and bearer control.

54

CHAPTER 2. NETWORK ELEMENTS AND FUNCTIONALITIES

2.8

Release 5 Modifications Source: Overview of 3GPP Release 5 V0.1.1 (2010-02) 3GPP TS 23.002 V5.12.0; Network Architecture

Release 5 is a very important release because HSDPA was introduced in it. From the architecture perspective too, there were very significant improvements suggested by 3GPP in REL-5 specifications. The IMS11 is standardized by 3GPP in Rel-5. Usage of IP on the transport plane was another improvement which was introduced in this release. These features are briefly illustrated below.

2.8.1

IP Transport

In Rel-99 and Rel-4, only ATM can be used at the transport layer in the various interfaces. Rel-5, introduces the possibility to use IP at the transport layer in the Iub, Iur, Iu-Ps and Iu-Cs interfaces, as an alternative to ATM. However, the use of ATM at the link layer under IP is not precluded. For more detailed description of the protocol stacks, please refer to chapter 6. The introduction of IP as a transport protocol in the radio network does not imply an end to end IP network; the UE may be given an IP address by the higher layers, but it will not be part of the UTRAN IP network (which is private), and packets will be encapsulated in the corresponding User Plane protocol. 3GPP has made a choice for the protocols to transport the Radio and Signalling bearers over IP. Different solutions are adopted. UDP is used in the user plane in the three interfaces, and SCTP with additional protocols is used for the signalling bearers. Additionally, 3GPP resulted in the following decisions on QoS and interworking with ATM transport networks: • Diffserv is the mechanism to provide different service levels, and several alternatives are allowed for the traffic flow classification. It is also allowed that the QoS differentiation can be provided either on a hop-by-hop basis or on a edge-to-edge basis; • Interworking with Rel-99/Rel-4 and Rel-5 ATM nodes is required, and it can be accomplished via a dual stack, a logical interworking function or a separate Interworking unit. 11

Now a days, IMS is a very hot topic because in LTE, there is a provision of supporting IMSbased Voice service over PS-domain.

2.8. RELEASE 5 MODIFICATIONS

2.8.2

55

IP Multimedia Subsystem (IMS)

Figure 2.13: Overview of IMS architecture

Home Subscriber Server Home Subscriber Server (HSS) is an evolved version of HLR. From Rel-5 onwards, the name of HLR is changed into “HSS” to emphasize that this database contains not only location-related data but also subscription-related data, like the list of services the user is able to get and the associated parameters. It keeps track of which subscribers belong to the network and their service capabilities. The CSCF consults with the HSS before initiating SIP connections.

Media Gateway Control Function (MGCF) MGCF performs translation of SIP signalling messages into ISUP messages which can be understood by the PSTN switches. As the name suggests, MGCF controls the functions of IM-MGW.

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CHAPTER 2. NETWORK ELEMENTS AND FUNCTIONALITIES

IP Multimedia Media Gateway Function (IM-MGW) An IM-MGW is used to terminate bearer channels from circuit switched infrastructures and media streams from packet data networks. For instance, when it interfaces an ISDN network, it takes the data of voice from i-law PCM call, processes the user data bits with a voice codec (e.g. AMR), before forwarding the voice information via RTP/UDP/IP over a packet network. To do so, an IM-MGW requires its own resources, such as codecs, echo cancellers, and conference bridges. The IM-MGW communications with the Media Gateway Control Function (MGCF) for resource control via the interface Mc. For this interface, the media gateway control protocol H.248 is applied.

Proxy-Call State Control Function (P-CSCF) P-CSCF is the “first contact point” of IMS. It is located in the same network as the GGSN (visited or home network). Its main task is to select the I-CSCF of the Home Network of the user. It also performs some local analysis (e.g. number translation, QoS policing,..).

Interrogating-CSCF (I-CSCF) I-CSCF is the “main entrance point” of the home network: it selects (with the help of HSS) the appropriate S-CSCF.

Serving-CSCF (S-CSCF) S-CSCF performs the actual Session Control. It handles the SIP requests, performs the appropriate actions (e.g. requests the home and visited networks to establish the bearers), and forwards the requests to the S-CSCF /external IP network of other end users, as applicable. The S-CSCF might be specialized for the provisioning of a (set of) service(s).

2.9

Release 6 Modifications Source: Overview of 3GPP Release 6 V0.1.1 (2010-02)

2.9. RELEASE 6 MODIFICATIONS

2.9.1

57

IMS for IP-CAN or IMS phase 2

IMS was primarily designed in Rel-5 to work on top of UMTS/GPRS using SIP signalling but in 3GPP Rel-6, it was extended to work on top on non-GPRS based access and SIP terminal equipments. ETSI TISPAN12 has worked very hard to adapt the IMS for requirements of fixed networks.

Figure 2.14: Usage of IMS expanded for any IP access network As shown in figure 2.14, the access network for using IMS services is no more restricted to GPRS & UMTS. The name chosen for this generic access network is “IP-Connectivity Access Network (IP-CAN)”. Some of the examples of IP-CAN are DSL, Cable, Wired and Wireless LAN, LTE etc. IP-CAN of 3GPP Rel-6 also addresses the issues like: • Policy Control: “Policy Decision Function” in the IMS (e.g., in P-CSCF) and “Policy Enforcement Function” in the IP-CAN (e.g., in GGSN) • Security • User and service profile • UE and ISIM/USIM • IP version issues 12

TISPAN: Telecommunications and Internet converged Services and Protocols for Advanced Networking

58

CHAPTER 2. NETWORK ELEMENTS AND FUNCTIONALITIES • SIP Compression • Emergency calls

2.10

Rel-7 & Rel-8 Modifications

3GPP REL-7 & 8 have introduced a lot of improvements of HSDPA & HSUPA of REL-6. This bundle-of-enhancements is collectively called as evolved-HSPA or HSPA+. Since, we have not discussed the details of HSPA yet, it makes no sense to talk about HSPA+ in this module. In nutshell, we can say that HSPA+ is trying to: • Push the peak bit rates of HSDPA & HSUPA higher • Reduce the battery consumption for continuous connectivity • Reduce the latency (transfer delay) 3GPP REL-7 & 8 have also introduced some changes in the core network to optimize the network performance towards lower latency. These changes in the PS core network are popularly called as “one tunnel solution”. Figure 2.15 shows the changes introduced in Rel. 7 & 8. 1. As in conventional GPRS, EDGE, UMTS & HSPA (R6), there are two tunnels: One between GGSN & SGSN and the other one between SGSN & RNC. That means, SGSN takes out the IP packets from one tunnel and packs it into another tunnel. This procedure certainly takes some time. 2. In Rel-7, there is a mechanism for “One- Tunnel- Solution”. This allows SGSN to be involved with only a control plane, e.g., connection setup, mobility management, authentication etc. SGSN does not appear in the chain for user plane traffic flow. User data can be directly tunneled from GGSN to RNC. Although this solution reduced the round trip time but there are some complications with this scheme. (a) Volume-dependent charging at SGSN will not be possible with this solution. (b) For Lawful Interception also, SGSN will not be able to provide any details about the packet transmitted during the session.

2.10. REL-7 & REL-8 MODIFICATIONS

59

3. In Rel-8, the concept of one tunnel can be extended by one more step where the user data is directly tunneled from GGSN to Node B. But we must not forget that this Node B is a special one. The Node B has in-built capability of RNC.

Figure 2.15: Direct Tunnel Solution of REL-7 & REL-8

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CHAPTER 2. NETWORK ELEMENTS AND FUNCTIONALITIES

Copyright Notices In order to create some figures, tables and text-sections, the following reference material has been used. Information has been interpreted and presented in a simplified manner. The original references are provided here. Main reference material for this book has been technical specifications (TSs) and technical reports (TRs) of 3rd Generation Partnership Project (3GPP). Figure 2.10 on page 49 Figure 1 of TS 23.002 v 3.1.0 c ⃝1999. 3GPPTM TSs and TRs are the property of ARIB, ATIS, ETSI, CCSA, TTA, and TTC who jointly own the copyright for them. They are subject to further modifications and are therefore provided to you “as is” for information purposes only. Further use is strictly prohibited. Text on page 51

Section 4.1.1 of “Overview of 3GPP Release 4 v 1.1.2” Text about MSC Server on page Section 4.1.2.1 of “Overview of 3GPP Re52 lease 4 v 1.1.2” Text about CS-MGW on page 52 Section 4.1.2.2 of “Overview of 3GPP Release 4 v 1.1.2” Figure 2.11 on page 50 Figure: BICC Network Architecture of “Overview of 3GPP Release 4 v 1.1.2” Figure 2.12 on page 52 Figure: Bearer Independent Core Network with A- and Iu-Interfaces of “Overview of 3GPP Release 4 v 1.1.2” Text in section 2.7.2 on page 53 Section 4.1.3.1, 4.1.3.2 & 4.1.3.3 of “Overview of 3GPP Release 4 v 1.1.2” Text in section 2.8.1 on page 54 Section 6.1 of “Overview of 3GPP Release 5 v 0.1.1” Text about CSCF on page 56 Section 12.2 of “Overview of 3GPP Release 5 v 0.1.1” TM c ⃝2010. 3GPP TSs and TRs are the property of ARIB, ATIS, ETSI, CCSA, TTA, and TTC who jointly own the copyright for them. They are subject to further modifications and are therefore provided to you “as is” for information purposes only. Further use is strictly prohibited.

BIBLIOGRAPHY [1] 3GPP TS 25.401 Ver. 7.0.0 ;‘UTRAN overall description’ [2] 3GPP TS 23.002 ver. 3.1.0 ;‘Network architecture’ [3] 3GPP TS 23.002 ver. 4.0.0 ;‘Network architecture’ [4] 3GPP TS 23.002 ver. 5.0.0 ;‘Network architecture’ [5] 3GPP TS 29.002 ver. 3.0.0 ;‘Mobile Application Part (MAP) specification’ [6] 3GPP TS 22.078 ver. 9.0.0 ;‘Customised Applications for Mobile network Enhanced Logic (CAMEL) Service description’ [7] 3GPP TS 23.078 ver. 5.0.0 ;‘Customised Applications for Mobile network Enhanced Logic (CAMEL) Phase 4’ [8] 3GPP TS 29.078 ver. 5.0.0 ;‘CAMEL Application Part (CAP) specification’ [9] 3GPP TS 23.205 ver. 4.0.0;‘Bearer Independent CS Core Network’ [10] 3GPP TS 23.060 ver. 6.0.0 ;‘General Packet Radio Service (GPRS); Service description’ [11] 3GPP TS 22.060 ver. 6.0.0 ;‘General Packet Radio Service (GPRS); Service description’ [12] Overview of 3GPP Release 4 v 1.1.2 ; Available at http://www.3gpp.org/ftp/Information/WORK PLAN/Description Releases/. [13] Overview of 3GPP Release 5/ 6 / 7 /8 . . . ; Available at http://www.3gpp.org/ftp/Information/WORK PLAN/Description Releases/. [14] H.Holma and A. Toskala, ‘WCDMA for UMTS’ , 5th Edition, John Wiley & Sons. 61

CHAPTER

3 WCDMA AIR INTERFACE

The main difference between GSM of second generation and UMTS of third generation is the air interface technology. The switches, routers and databases in the core network behave in the the same manner in both the technologies. Therefore, understanding the concepts of UMTS air interface is a very important part in learning 3G fundamentals. This module tries to cover the basic principles about spreading, code multiplexing and physical layer processing of the data in UMTS.

3.1

Duplex Methods

Because commercial mobile networks are used to send as well as receive data from UE, they are duplex systems. This is different from simplex transmission as in TV or radio broadcast where the user only receives but does not send data. Hence, in mobile communication we use full-duplex. There are two popular methods which can be used to separate the signals from UE to Node B, Uplink and Node B to UE, Downlink. They are: FDD: As shown in figure 3.1, in FDD scheme, user sends his data on one frequency and receives on another one. These 2 frequencies must be separated by several MHz. FDD can only operate in paired band. For every uplink frequency, there 62

3.2. MULTIPLE ACCESS TECHNOLOGIES

63

Figure 3.1: Popular duplexing methods - FDD & TDD is a downlink frequency. This pair of frequencies forms a carrier. For example, GSM is a FDD system. TDD: In contrast with FDD, TDD operates in an unpaired spectrum. The same frequency is used for both uplink and downlink. This is achieved by organizing the time into time slots and assigning some lots to uplink and remaining slots for downlink. According to 3GPP, UMTS can operate both in FDD and TDD mode. But FDD has been a more popular choice among the commercial telecom operators. According to common practice and usage, when someone speaks of “UMTS”, we undoubtedly assume that UMTS-FDD is referred to. But for TDD, it must be explicitly mentioned that we are referring to the UMTS-TDD version. Both technologies have their advantages and disadvantages but we will investigate only the FDD part in this book due to its popularity.

3.2

Multiple Access Technologies

In the previous section, we saw 2 methods to separate the uplink and downlink streams. Now, if we imagine a cell with several users simultaneously accessing the

64

CHAPTER 3. WCDMA AIR INTERFACE

Figure 3.2: Popular multiple-access methods services, their individual signals will interfere with each other and cause disturbance in the transmission and reception. In order to avoid or minimize the effect of interference from other users, several multiple access schemes have been designed. In other words, multiple access technique is a method by which multiple users can share the same radio resources. This sharing can be in time domain, frequency domain or in power domain. Hence, we have several options for multiple access schemes. Some of them are briefly1 mentioned here:

3.2.1

Frequency Division Multiple Access

FDMA is a multiple access scheme where the total frequency spectrum is divided into small radio channels and each user is allocated one radio channel. This is one of the oldest radio techniques which was used in 1G cellular systems like NMT, C-Nets, AMPS etc. Figure 3.2 shows FDMA principle in the left upper subfigure. In FDMA, several users use the radio resources at their allocated section of frequency spectrum. 1

The discussion is kept very short because generally these topics are well known.

3.3. UMTS OPERATING BANDS AND SPECTRUM

3.2.2

65

Time Division Multiple Access

In the TDMA multiple access scheme, the time resource is structured into TDMA frames and each frame is further divided into time slots. Each user is allocated one time slot every TDMA frame. Therefore, in TDMA we can accommodate only as many users as the number of time slots per TDMA frame. In GSM, such a scheme with 8 slots per frame is used. In TDMA, several users use the radio resources at their scheduled time intervals.

3.2.3

Code Division Multiple Access

CDMA is the main topic to be discussed in this chapter because air interface technology used in UMTS air interface is based on Wideband CDMA. In CDMA, several users are allowed to use the same frequency resource at the same time but their signals are separated by codes. Theoretically, we can accommodate as many simultaneous users as the number of codes. It sounds like magic but this scheme has its limitation. The communication with acceptable quality can be maintained as long as the interference at the receiver is within allowed limits. If the interference rises, the transmitter should increase the power to fight against the disturbance. But the power of UE and Node B are finite. Therefore, CDMA systems are also called as interference limited systems.

3.2.4

Orthogonal Frequency Division Multiple Access

Orthogonal FDMA is a relatively new technology where different frequencies are allocated to different users. Therefore, it is a frequency division multiple access scheme but with one basic difference. In OFDMA, the allocated frequency is further divided into smaller sub-carriers which makes it very robust against inter-symbol interference, multipath fading and other radio disturbances. Radio access technology used in E-UTRAN (LTE), WiMAX and WLAN is based on OFDMA principles.

3.3

UMTS operating Bands and Spectrum Source: 3GPP TS 25.104 ; Base Station (BS) radio transmission and reception (FDD)

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The spectrum allocated for IMT-2000 deployment was declared in WRC-92 which included the bands shown in figure 3.3. This is a country-independent data which might suit one geographical region but may be conflicting with other radio systems in another part of world. The allocated spectrum had separate frequency regions of terrestrial communication systems and mobile satellite communication systems.

Figure 3.3: Operating frequency bands for IMT-2000 System (BAND I) In the same figure (3.3), the core band of UMTS has also been illustrated which is chosen by 3GPP for the UMTS deployment in Europe. This figure shows both the TDD and FDD regions of the UMTS core band. Other than the Core Band or BAND I, UTRA/FDD is designed to operate in the paired bands shown in table 3.1. Nominal channel spacing is 5 MHz. The channel raster is 200 kHz for all bands, which means that the center frequency must be an integer multiple of 200 kHz. This rule is generally true but for some specific bands, the center frequencies are shifted by 100 kHz relative to the channel raster. These frequencies are explicitly listed in table 5.0 & 5.1 in 3GPP TS 25.104. In UTRAN FDD Band I, there is 2 × 60 MHz. Hence, there can be 12 FDD carriers in this band. For example, the center of the first carrier will be 1922.4 MHz for uplink and 2112.4 MHz for DL. Similarly, the center of the last carrier in this band is at 1977.6 MHz for uplink and 2167.6 MHz for downlink.

3.4

Timing in WCDMA

In UMTS, the smallest time unit is called TChip which is equal to 3.84 1Mcps = 0.26 µs. Quite often, other important time units are specified as multiples of TChip . The three

3.4. TIMING IN WCDMA

67

Operating Band

Uplink Freq.

Downlink Freq.

I II III IV V VI VII VIII IX X XI XII XIII XIV XV - XVIII XIX XX XXI

1920-1980 MHz 1850 -1910 MHz 1710-1785 MHz 1710-1755 MHz 824 - 849MHz 830-840 MHz 2500 - 2570 MHz 880 - 915 MHz 1749.9 - 1784.9 MHz 1710-1770 MHz 1427.9 - 1447.9 MHz 699 - 716 MHz 777 - 787 MHz 788 - 798 MHz Reserved 830 845 MHz 832 - 862 MHz 1447.9 - 1462.9 MHz

2110 -2170 1930 -1990 MHz 1805-1880 MHz 2110-2155 MHz 869-894 MHz 875-885 MHz 2620 - 2690 MHz 925 - 960 MHz 1844.9 - 1879.9 MHz 2110-2170 MHz 1475.9 - 1495.9 MHz 729 - 746 MHz 746 - 756 MHz 758 - 768 MHz Reserved 875 -890 MHz 791 - 821 MHz 1495.9 - 1510.9 MHz

TX-RX Freq. separation 190 MHz 80 MHz 95 MHz 400 MHz 45 MHz 45 120 MHz 45 MHz 95 MHz 400 MHz 48 MHz 30 MHz 31 MHz 30 MHz 45 MHz 41 MHz 48 MHz

Table 3.1: UTRAN FDD Bands, reproduced from 3GPP TS 25.104 most important time units discussed in UMTS & HSPA are illustrated in figure 3.4.

Figure 3.4: Time units used in WCDMA air interface

1. Radio Frame: A radio frame is a processing duration which consists of 15 slots.

68

CHAPTER 3. WCDMA AIR INTERFACE The length of a radio frame corresponds to 38400 chips. In other words, a radio frame is 10 ms long and can accommodate 38,400 chips.

2. Slot: A slot is a duration which consists of fields containing bits. The length of a slot corresponds to 2560 chips. Compared to the 2G combination of TDMA frame & Time Slot, 3G uses a combination of Radio Frame & Slot. One Radio Frame of 3G is further divided into 15 Slots but all the times slots are allocated to the same users. The main purpose of having Slots in 3G is so that control information can be sent to the UE at a regular and very fast interval. One slot is 2/3 ms long. 3. Sub-frame: A sub-frame is the basic time interval for E-DCH and HS-DSCH transmission and E-DCH and HS-DSCH-related signalling at the physical layer2 . The length of a sub-frame corresponds to 3 slots (7680 chips).

3.5

Spreading Principles

UMTS air interface is based on code division multiple access scheme where the bandwidth after spreading is 5 MHz wide. The narrowband signal is converted to wideband with the help of a spreading code. The exact details of the codes will be shown in the next section but for the current discussion, they are simply called code. Figure 3.5 shows 4 users using the same 5 MHz wideband carrier. The time is organized in 10 ms radio frame. Each user is allowed to transmit or receive in the entire 10 ms period. Therefore, the users are using the same frequency & time domain resources. It is natural for them to interfere with each other. In order to keep the interference to a minimum level, it is desirable that each users uses as little power as possible. This reduction in power3 is achieved by spreading the whole energy over a wide frequency band. The spreading technique allows the operator to simultaneously allocate the same time and frequency resources to many users. There are two resources in CDMA world, which are (1) code and (2) power. Let us analyze them one by one: 1. Codes: In general, the users must be identified by codes. A new user cannot be admitted until there is a code available for him. Hence, the number of active users can be limited by unavailability of codes. 2

Please note: Prior to the introduction of HSDPA in 3GPP Rel-5, there was no discussion about sub-frame. Therefore, for R99 channels (DCH, FACH, RACH etc.) the term ‘sub-frame’ has no significance. 3 Power spectral density

3.5. SPREADING PRINCIPLES

69

Figure 3.5: Principle of Spreading 2. Power: Code itself is not enough to allow a radio connection in CDMA. 2.a In Uplink: In Uplink, the received power at the Node B’s receiver should be under the manageable limits. If the interference at the receiver becomes very high, then the desired signal cannot be reconstructed from the received signal. This is a very common reason for blocking in CDMA networks. In other words, CDMA systems are interference limited systems. 2.b In Downlink: In Downlink, the transmitted power of Node B is the resource which limits the number of subscribers. With each connected user, Node B needs to spend aome finite power for each active user. Therefore, in DL Node B transmit power is the shared resource. Figure 3.6 illustrates how the spreading & despreading mechanism can be used to suppress interference. After spreading, when the wideband signal is transmitted, it gets interfered by both narrowband and wideband interference. In the receiver, during despreading, the narrow band interference gets spread and its power spectral density gets reduced. After despreading, when the output is passed through a lowpass filter then despreaded data signal can be derived. The received data signal can be used to regenerate the actual data only if the received Eb bit energy is greater than the overall noise energy by at least N [dB]. o If CDMA is successfully used in commercial networks, it should be robust against the interference from the other user interference. This principle is illustrated in figure 3.7. For example, imagine that the transmitter shown in this picture depicts the transmitter in Node B, which spreads the data for user 1 with code # 1 and data for user 2 with code # 2. As expected, the spread signal for both the users will interfere at the radio interface. Now, the User 1 will try to despread the received

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CHAPTER 3. WCDMA AIR INTERFACE

Figure 3.6: Spreading and despreading to suppress the interference signal with code # 1. UE has the knowledge about the codes by prior signalling with RNC. As a result of despreading, the data of user 1 can be reconstructed. In the same figure, we can see that spread signal of user 2 does not get despreaded at the receiver of UE 1. This is possible if: • Code # 1 and code # 2 are orthogonal to each other. [AND] • Codes at the transmitter and receiver are synchronized to each other. In commercial cellular networks, operators want that one cell should cover a large geographical area. In other words, communication should be possible between base station and a distant user equipment. This can be achieved if the sensitivity of the base station and user equipment is good. While performing despreading, the receiver can manifold amplify the received signal. This gain is called Processing gain. Processing gain can be mathematically expressed as: Processing Gain = 10 · log

3.84 Mcps [dB] Rbit

3.5. SPREADING PRINCIPLES

71

Figure 3.7: Multiple access using different codes for 2 users

Figure 3.8: Processing gain at the receiver side

Figure 3.9 shows a fast code sequence whose symbol duration is fixed by 3GPP

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CHAPTER 3. WCDMA AIR INTERFACE

specifications. This small symbol is called a chip. According to 3GPP, in one second, there can be 3.84 million such chips. Hence, each chip is 0.26 µs long. The channelization codes used for spreading always use this fast code. As a result, the product is always 3.84 Mcps. The bandwidth required to transmit this fast waveform is also very large. In UMTS, the licensed bandwidth is 5 MHz but the effective transmission takes place in 3.84 MHz. Figure 3.9 also illustrates that for various services, the symbol rate can vary but the chip rate is always constant and fixed. • For a high bit rate service, the symbol duration is short. Therefore, the SF is also small. • For a low bit rate service, the symbol duration is very large and therefore, the SF is also very high. 1 • In other words Bit Rate ∝ SF

Figure 3.9: Effect of SF on bitrate and symbol duration

3.6

Codes in UMTS Source: 3GPP 25.213 Spreading and Modulation (FDD)

3.6. CODES IN UMTS

73

According to the definition used by 3GPP TS 25.213, spreading consists of two operations. The first is the channelization operation, which transforms every data symbol into a number of chips, thus increasing the bandwidth of the signal. The second operation is the scrambling operation, where a scrambling code is applied to the spread signal. Hence, there are two types of codes used in UMTS. 1. Scrambling Codes: Scrambling codes do not perform any spreading of bandwidth. These codes are used to super-impose the identity of transmitter on the physical layer signals. Scrambling codes are not orthogonal. They are derived using sequence generators consisting of shift registers. 2. Channelization Codes: According to section 4.3.1.1 of 3GPP TS 25.213, the Channelization codes are the codes which perform spreading of bandwidth. Therefore, sometimes, these codes are also called as spreading code. Channelization codes define the user bit rate. These codes are Orthogonal Variable Spreading Factor (OVSF) codes that preserve the orthogonality between a user’s different physical channels. The OVSF codes can be defined using the code tree shown in figure 3.10. The number of chips per data symbol is called the Spreading Factor (SF).

3.6.1

Channelization Code

Channelization Codes have similar properties in DL and UL. As stated earlier, 1 Bit Rate ∝ . Therefore, the user bit rate is defined by the channelization code. SF • In UL, Channelization codes are used to separate control and data channels from the same UE (DPDCH and DPCCH). • In DL, Channelization codes are is used to separate the users within a cell.

In order to calculate the L1 data rate for each spreading factor, we use following formula: Symbol Rate =

Rchip 3.84 Mcps = SF SF

It will be explained in the next section that UL modulation is BPSK and DL modulation is QPSK. Therefore, for the same SF, UL & DL bit rates are different. For QPSK, one symbol corresponds to two bits whereas in BPSK one symbol equals one bit only. This concept is illustrated in Table 3.2.

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CHAPTER 3. WCDMA AIR INTERFACE

SF

Symbol (ksps) [ ]Rate [ 3.84 Mcps ] Rchip = SF = SF

bit rate (kbps) on DL DPCH

bit rate (kbps) on UL DPDCH

512 256 128 64 32 16 8 4

7.5 15 30 60 120 240 480 960

15 30 60 120 240 480 960 1920

15 30 60 120 240 480 960

Table 3.2: SF and the corresponding L1 bitrate

Figure 3.10: Code Tree for generation of Orthogonal codes

3.6.2

Scrambling Code

As stated earlier in the introduction part, the scrambling codes do not perform any spreading of bandwidth. These codes are used to super-impose the identity of transmitter on the physical layer signals. As a result of scrambling, some ‘0’s become ‘1’s and some ‘1’s become ‘0’s, but the time-duration of each chip does not alter. Hence, the scrambling procedure does not affect the bandwidth of the transmitted signal. Spreading is achieved by Channelization code alone. The following sections tries to investigate the usage of scrambling codes in UL and DL.

3.6. CODES IN UMTS

75

UL Scrambling Codes UL Scrambling codes are used as user identity in Uplink. All uplink physical channels are scrambled with a complex-valued Scrambling code. The dedicated physical channels may be scrambled by either a long or a short scrambling code. There are 224 long and 224 short uplink scrambling codes. The usage of long scrambling codes in UL is very popular. Therefore, in this book we will only discuss the long codes. The sequence generator used to generate the long UL scrambling codes is shown in figure 3.11. The shift registers with 24 bit delay capability and can be used to create 224 − 1 or 16.7 Million UL scrambling codes. Uplink scrambling codes are assigned by RRC signalling.

Figure 3.11: Configuration of uplink scrambling sequence generator

DL Scrambling Codes DL (primary) scrambling codes are used as a physical layer cell-id in UMTS. The DL scrambling codes are generated using the sequence generator shown in figure 3.12. There are 18 shift registers in the sequence generator. Hence, we can get a total of 218 − 1 = 262, 143 scrambling codes. But not all of the SC are used. Only 8192 DL scrambling codes are allowed in UMTS which are further divided into 512 groups. Each group contains one primary Scrambling code and 15 secondary scrambling codes. Figure 3.14 illustrates this arrangement. The Scrambling code sequences are constructed by combining two real sequences into

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CHAPTER 3. WCDMA AIR INTERFACE

Figure 3.12: Configuration of downlink scrambling sequence generator a complex sequence. Each of the two real sequences are constructed as the position wise modulo 2 sum of 38400 chip segments of two binary m-sequences generated by means of two generator polynomials of degree 18. The resulting sequences thus constitute segments of a set of Gold sequences. The scrambling codes are repeated for every 10 ms radio frame.

The primary scrambling codes n : The i:th set of secondary scrambling codes:

n = 16 ∗ i where i = 0 . . . 511 16 ∗ i + k, where k = 1 . . . 15

Each cell is allocated one and only one primary Scrambling code. The primary CCPCH, primary CPICH, PICH, AICH and S-CCPCH carrying PCH shall always be transmitted using the primary scrambling code. The other downlink physical channels may be4 transmitted with either the primary scrambling code or a secondary scrambling code from the set associated with the primary scrambling code of the cell. The set of primary scrambling codes is further divided into 64 Scrambling code groups, each consisting of 8 primary scrambling codes. The j:th scrambling code group consists of primary scrambling codes 16*8*j+16*k, where j=0..63 and k=0..7. 4

Use of secondary scrambling code is not very popular in practice. Therefore, this book will further assume that in DL only primary scrambling code is used.

3.6. CODES IN UMTS

Figure 3.13: Total 8192 DL Scrambling Codes and 512 Primary SC

Figure 3.14: 512 SC divided into 64 Groups of 8 codes each

77

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3.6.3

CHAPTER 3. WCDMA AIR INTERFACE

Summary of Scrambling Codes

In the last few sections, the details about channelization and scrambling codes were given. Table 3.3 shows a brief summary of the codes in UMTS. • In UL, the users are separated by using different UL scrambling codes. There are 224 - 1 = 16.7 Million UL scrambling codes. RNC allocates one SC to one user at connection setup. SC are unique within one RNC area. The number of SC available in one RNC are defined by the hardware capacity of RNC. • In DL, the cells are separated by DL scrambling codes. There are 512 primary scrambling codes which are organized in 64 code groups having 8 codes per group ( 64 × 8 = 512)). DL SC are planned by radio planners.

3.6.4

Summary of Codes in UMTS

At this point, it is quite normal for the readers to feel confused and lost in details. Therefore, table 3.3 shows the summary of the previous section by highlighting the main aspects of the codes used in UMTS. Scrambling Code UL: User Id

Usage

DL: Cell Id Does not perform spreading UL : 16.7 Million

Spreading # of codes

DL : 8192 (512 Primary SC and the rest are secondary SC) Orthogonality Non-orthogonal UL : There are 2 options Length • Option 1: chips

Channelization Code UL: To separate control and data channels from UE DL: User Id Performs spreading UL: Depends on the SF ( 256 ,. . . , 4) DL: Depends on the SF (512,. . . , 4) Orthogonal UL: 4 to 256 chips

long Codes 38400

• Option 2: Short codes 256 chips

Code Generation

DL: 10 ms (38400 Chips) Using Shift Register based sequence generator

DL: 4 to 512 chips Using Walsh Code Tree matrix

Table 3.3: Main aspects about the codes used in UMTS

3.7. MODULATION

3.7

79

Modulation

The modulation schemes used in UMTS uplink and downlink are illustrated in figure 3.15. The same figure also shows the codes used in spreading and scrambling. For example, in UL, we use user-specific scrambling codes and in DL, cell-specific scrambling code.

Figure 3.15: Spreading and Modulation in UL & DL • The modulation used in downlink is Quadrature Phase Shift Keying (QPSK) which involves 2 bits per symbol. For example, SF = 256 ⇒ 15 ksps = 30 kbps • The modulation used in uplink is Binary Phase Shift Keying (BPSK ) which involves 1 bits per symbol. For example, SF = 256 ⇒ 15 ksps = 15 kbps Figure 3.15 illustrates the spreading and modulation for the uplink dedicated physical channels & DL dedicated channel. In Uplink, data modulation is dual branch QPSK, that is, the I and Q channels are used as two independent BPSK channels.

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Copyright Notices In order to create some figures, tables and text-sections, the following reference material has been used. Information has been interpreted and presented in a simplified manner. The original references are provided here. Main reference material for this book has been technical specifications (TSs) and technical reports (TRs) of 3rd Generation Partnership Project (3GPP). Text about UMTS operating Section 5.4.1 & 5.4.2 of 3GPP TS 25.104 Bands on page 66 v9.6.0 Table 3.1 on page 67 Table 5.0 of 3GPP TS 25.104 v9.6.0 TM c ⃝2011. 3GPP TSs and TRs are the property of ARIB, ATIS, ETSI, CCSA, TTA, and TTC who jointly own the copyright for them. They are subject to further modifications and are therefore provided to you “as is” for information purposes only. Further use is strictly prohibited. Figure 3.11 on page 75 Figure 5 of 3GPP TS 25.213 v 8.4.0 Figure 3.12 on page 76 Figure 10 of 3GPP TS 25.213 v 8.4.0 Text in section 3.6 on page 72 Section 4.1 of 3GPP TS 25.213 v 8.4.0 Text in section 3.6 on page 73 Section 4.3.1.1 of 3GPP TS 25.213 v 8.4.0 Text about UL Scrambling Codes Section 4.3.2.1 of 3GPP TS 25.213 v 8.4.0 on page 75 Text about DL Scrambling Codes Section 4.1 of 3GPP TS 25.213 v 8.4.0 on page 75 Text in section 3.4 on page 66 Section 5 of 3GPP TS 25.211 v 9.1.0 TM c ⃝2009. 3GPP TSs and TRs are the property of ARIB, ATIS, ETSI, CCSA, TTA, and TTC who jointly own the copyright for them. They are subject to further modifications and are therefore provided to you “as is” for information purposes only. Further use is strictly prohibited.

BIBLIOGRAPHY [1] 3GPP TS 25.201 ver. 6.0.0 ;‘Physical layer - General description’ [2] 3GPP TS 25.211 ver. 6.0.0 ;‘Physical channels and mapping of transport channels onto physical channels (FDD)’ [3] 3GPP TS 25.212 ver. 6.0.0 ;‘Multiplexing and Channel Coding (FDD)’ [4] 3GPP TS 25.213 ver. 6.0.0 ;‘Spreading and Modulation (FDD)’ [5] 3GPP TS 25.214 ver. 6.0.0 ;‘Physical Layer Procedures (FDD)’ [6] 3GPP TS 25.104 ver. 6.0.0 ;‘Base Station (BS) radio transmission and reception (FDD)’ [7] H.Holma and A. Toskala, ‘WCDMA for UMTS’ , 5th Edition, John Wiley & Sons. [8] Chris Johnson, ‘Radio Access Networks For UMTS ; Principles And Practice’ , John Wiley & Sons.

81

CHAPTER

4 LOGICAL, TRANSPORT & PHYSICAL CHANNELS

In order to master the fundamentals of 3G radio transmission and reception, it is essential to get acquainted with the channels used in UMTS and HSPA. Channels are simply a method to organize the information into some categories depending on some common aspects. This chapter is written to provide the most essential details about them. According to UTRAN specifications, there are three hierarchies of channels. • Logical channels • Transport channels • Physical channels The concept of channel was used in GSM as well. In 2G, there are logical and physical channels. The concept of Transport channel is new in UMTS. This page tries to illustrate the difference between the three types of channels and later in this chapter more details can be found about each of them. A Logical channel is used to describe what type of information is being transported by it (e.g., control signalling or user data).

82

4.1. CHRONOLOGY: FIRST 3G AND THEN 3.5G

83

A Transport channel is used to describe the characteristics with which it transports the information carried by it (e.g., using common channels of the cell or dedicated channels especially allocated to one user). A Physical channel is used to describe the physical aspects of it (e.g., frequency, scrambling code, channelization code and slot format).

4.1

Chronology: First 3G and then 3.5G

As we all know, 3G is constantly evolving and getting better with each 3GPP release. Therefore, we should study the changes in chronological order. It is highly recommended that readers must try to learn channels in the same order. The learning becomes much easier if we break the whole process in three steps. Step 1, R99 Channels: R99 channels are the topic of this particular chapter. Here, we will discuss common control channels and dedicated channels of UMTS. Step 2, HSDPA Channels: HSDPA channels will be discussed in chapter 7. All the channels which have something to do with HSDPA, start with HS-. There are only 3 new channels introduced in Rel-5 for HSDPA operation. Step 3, HSUPA Channels: HSUPA channels will be discussed in chapter 8. All the channels which have something to do with HSUPA, start with E-. There are only 5 new channels introduced in Rel-6 for HSUPA operation. As we can see, there will be a lot of channels to learn and discuss. Therefore, we will start building our knowledge on the strong foundation of R99 UMTS channels.

4.2

Logical Channels Source:

3GPP TS 25.301, 25.211, 25.212, 25.213

As explained in the first section of this chapter, Logical channels are used to describe what is being transported. According to 3GPP TS 25.301, logical channels are divided into two groups: 1. Control channels: for the transfer of control plane information, &

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CHAPTER 4. LOGICAL, TRANSPORT & PHYSICAL CHANNELS

2. Traffic channels: for the transfer of user plane information. Figure 4.1 illustrates the distribution of uplink and downlink logical channels. It can be seen that there are 6 logical channels in UMTS, 2 for traffic and 4 for control plane. Some of these channels are only in DL e.g., (BCCH, PCCH and CTCH) whereas the other 3 channels are bidrectional (CCCH, DCCH and DTCH). Splitting the analysis in DL and UL makes it much easier to understand. While describing the logical channels, we do not discuss the issues about power, bit rates, bit error rate, block error rates, etc. At this level, we only consider the nature of data being transported.

Figure 4.1: UL & DL Logical channels

4.2.1

Logical Channels for Control Plane Information

In this section, all 6 logical channels are described. 1. BCCH, Downlink only ↓ BCCH channel is used for system control information broadcasting. It exists only in the downlink.

4.2. LOGICAL CHANNELS

85

2. PCCH, Downlink only ↓ PCCH channel is used to transmit the paging messages. RNC can generate paging after getting the paging requests from core network or generate the paging by itself to page the packet-switched users who are in RRC power saving stand-by states. 3. CCCH, Uplink and Downlink ↕ CCCH is a bi-directional channel. It carries control information between the network and the UE. This channel is used by those UEs which access a new cell after cell re-selection as well as by UEs which do not have a RRC connection. 4.DCCH, Uplink and Downlink ↕ This is a point-to-point bi-directional channel which is set up in the RRC connection establishment procedure. It carries dedicated control information between RNC and the UE.

4.2.2

Logical Channels for User Plane Information

5. CTCH, Downlink only ↓ This point-to-multipoint channel is used to carry dedicated information in the downlink to all or a group of UEs. For example, stock market updates, sports results, weather updates, business promotions and service area broadcast. 6. DTCH, Uplink and Downlink ↕ DTCH is a dedicated point-to-point channel which can be used in the uplink as well as in the downlink. This channel carries user traffic like CS speech, video, streaming video, emails with and without attachments, file transfer etc. As a quick summary, table 4.1 lists all the logical channels. In this table, we can see which channels are used to carry control data and which channels for traffic. The same table also shown whether the channels are unidirectional or bidirectional. Logical 1. 2. 3. 4. Logical 5. 6.

Channels for Control Plane BCCH Broadcast Control Channel PCCH Paging Control Channel CCCH Common Control Channel DCCH Dedicated Control Channel Channels for User Plane CTCH Common Traffic Channel DTCH Dedicated Traffic Channel

DL DL UL UL

only only & DL & DL

DL only UL & DL

Table 4.1: List of all Logical channels in UMTS

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CHAPTER 4. LOGICAL, TRANSPORT & PHYSICAL CHANNELS

4.3

Transport Channels Source:

3GPP TS 25.301, 25.302, 25.321

The main task of a transport channel is to describe the characteristics with which the data will be transported. At this moment, it is quite normal for the readers to doubt why do we need transport channels? As we have seen in the previous section, there is only one logical channel DTCH for describing the one-to-one user traffic, for example, voice, video, streaming and NRT data. We cannot expect the transport conditions of CS voice and FTP file transfer be same. Typically, there are the following preferences: • CS Voice needs low but constant bit rate with strict delay requirements. Voice service is insensitive to bit error rates and we do not re-transmit the speech frame in case of errors. • File transfer requires high bit rate which can tolerate the bit-rate fluctuations. The end-to-end delay can also be flexible but file transfer is very strict about bit errors which are achieved using negative acknowledgements and retransmissions. Furthermore, the packets sessions can be of various natures: 1. Packet session with small amount of infrequent data transmission. 2. Packet session with high amount of data transmission. 3. Packet session with small amount of packets but very frequently transmitted. Medium Access Control Layer (MAC) in RNC & UE is responsible for mapping logical channels to transport channels. This procedure is illustrated in figure 4.2.

Hence, one can argue that UMTS needs different types of transport channels to fulfill the different types of needs. There are 2 types of transport channels defined for UMTS. • 1. Common transport channels • 2. Dedicated transport channels

4.3. TRANSPORT CHANNELS

87

Figure 4.2: UL & DL Transport channels ; Logical  Transport channel mapping Common Transport Channels BCH Broadcast Channel PCH Paging Channel FACH Forward Access Channel RACH Random Access Channel Dedicated Transport Channels DCH Dedicated Channel

DL DL DL UL

only only only only

UL & DL

Table 4.2: List of all Transport channels in UMTS 1

4.3.1

Common Transport Channels

In the common transport channels, if UE addressing is required, then explicit addressing is used. BCH BCCH V BCH. (Logical channel BCCH is mapped on the transport channel BCH.) The Broadcast Channel (BCH) is a downlink transport channel that is used to broadcast system- and cell-specific information. The BCH is always

88

CHAPTER 4. LOGICAL, TRANSPORT & PHYSICAL CHANNELS transmitted over the entire cell and has a single transport format.

PCH PCCH V PCH. (Logical channel PCCH is mapped on the transport channel PCH.) The Paging Channel (PCH) is a downlink transport channel. The PCH is always transmitted over the entire cell. The transmission of the PCH is associated with the transmission of physical-layer generated Paging Indicators, to support efficient sleep-mode procedures. RACH RACH is a well-known name for people who are familiar with 2G. In GSM, RACH is used to make the initial access to the network and ask for dedicated signalling resources. Here in 3G also, the same functions are performed by RACH channel. But in UMTS, RACH can also be used to transmit small amount of NRT PS data in uplink2 . Hence, RACH can generate some revenue for the operator. FACH In 2G, the answer to RACH is received on Access Grant Channel AGCH. In UMTS, the same task has been given to Forward Access Channel (FACH). Hence, FACH is used to inform the users about allocated dedicated signalling resources in response to the RACH request. But in UMTS, FACH can also be used to transmit small amount of NRT PS data in downlink3 . Hence, FACH can generate some revenue for the operator.

4.3.2

Dedicated transport channels

Dedicated Channel DCH is a transport channel allocated to one UE. It can be used either for uplink or downlink. This channel is controlled through the inner power control. DCH bit rate is variable depending on the channel conditions and the allocated bearer. Bit rate variations can be performed every 10 ms.

4.4

Physical Channels

According to 3GPP TS 25.211, a physical channel is defined by: • a specific carrier frequency, • scrambling code, • channelization code, 2 3

3G RACH = 2G RACH + Small amount of UL NRT PS traffic. 3G FACH = 2G AGCH + Small amount of DL NRT PS traffic.

4.4. PHYSICAL CHANNELS

89

• time start & stop (giving a duration) & • on the uplink, relative phase (0 or π/2). Scrambling and channelization codes are specified in chapter 3. Time durations are defined by start and stop instants, measured in integer multiples of chips. Suitable multiples of chips also used in specification are:

Figure 4.3: Slot, Subframe and Radio Frame as used in WCDMA

1. Radio frame: A radio frame is a processing duration which consists of 15 slots. The length of a radio frame corresponds to 38400 chips or 10 ms. 2. Slot: A slot is a duration which consists of fields containing bits. The length of a slot corresponds to 2560 chips or 2/3 ms. 3. Sub-frame: A sub-frame is the basic time interval for E-DCH and HS-DSCH transmission and E-DCH and HS-DSCH-related signalling at the physical layer. The length of a sub-frame corresponds to 3 slots (7680 chips) or 2 ms. Physical layer (L1) in Node B & UE is responsible for mapping transport channels to physical channels. This procedure is illustrated in figure 4.4.

As shown in figure 4.4, there are some physical channels which do not have any corresponding transport or logical channels. These channels are, in fact, physical signals which are generated by physical layer of transmitter (e.g., Node B) and used by the physical layer of the receiver (e.g., UE). The scope of these channels are restricted to only physical layer. These physical channels exist to support some special functions of physical layer e.g., synchronization, channel estimation etc.

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The default time duration for a physical channel is continuous from the instant when it is started to the instant when it is stopped. Physical channels that are not continuous will be explicitly described. UMTS has been designed in such a way that physical layer maps various transport channels to physical channels. When more than one transport channel is multiplexed, this composite channel is called composite coded transport channel (CCTrCH). This composite transport channel (CCTrCH) is mapped to the data part of a physical channel. In addition to data parts, there also exist control parts which are locally generated and inserted by the physical layer.

Figure 4.4: UL & DL Physical channels ; Transport  Physical channel mapping In chapter 3, the basics about channelization and scrambling were discussed. The unique combinations of these codes works as the identity of various physical channels. Example: Let us consider two physical channels and investigate their physical layer attributes. First channel is the Primarycommon pilot channel (P-CPICH) of the cell and the second one a DL dedicated physical channel (DPCH) allocated to a specific user. • P-CPICH is a physical channel that uses

4.4. PHYSICAL CHANNELS

91

– Frequency FDL , that has been assigned to the cell by radio planner, – Scrambling code SCCell , assigned by the planner while radio planning, – & the Channelization Code4 , CC256, 0 . • Downlink DPCH allocated to a particular user is a physical channel that uses – Frequency FDL , that has been assigned to the cell by radio planner, – Scrambling code SCCell , assigned by the planner while radio planning, – & the Channelization Code5 , CCSF, Code Number which is allocated by RNC at the time of call or session setup. There are a lot of physical channels defined for UMTS. In order to make the understanding easier, we will discuss them in four groups: 1. UL Common Channels: PRACH 2. UL Dedicated Channels: DPDCH and DPCCH 3. DL Common Channels: P-SCH, S-SCH, P-CPICH, P-CCPCH, S-CCPCH, AICH and PICH 4. DL Dedicated Channels: DPCH Please refer to figure 4.5 and table 4.3 to keep an overview about the physical channels. There is only one UL common channel and there are 2 UL dedicated channels. Similarly in DL, there are 7 common channels and one dedicated channels.

4.4.1

UL Common Channel

There is only one UL common physical channel PRACH. The next section explains more details about it. 4

This is a standard code which is used in all UMTS networks for primary-common pilot channel (P-CPICH). 5 Here SF is decided based on the allocated bit rate while code number is a random choice based on the codes availability.

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DL Common Channels P-SCH Primary Synchronization Ch. S-SCH Secondary Synchronization Ch. P-CPICH Primary Common Pilot Ch. P-CCPCH Pri. Common Control Physical Ch. S-CCPCH Sec. Common Control Physical Ch. PICH Paging Indication Channel Ch. AICH Acquisition Indication Channel Ch. DL Dedicated Channels DPCH

Dedicated Physical Channel

UL Common Channels

PRACH Physical Random Access Ch.

UL Dedicated Channels DPDCH Dedicated Phy. Data Ch. DPCCH Dedicated Phy. Control Ch.

Table 4.3: List of all R99 Physical channels

Figure 4.5: Summary of all R99 Channels Physical Random Access Channel (PRACH) In section 4.3, we discussed about an UL transport channel RACH. Physical layer of UE maps this transport channel to a physical channel called ‘Physical Random

4.4. PHYSICAL CHANNELS

93

Access CHannel (PRACH)’. Therefore, PRACH is used by user for making initial contact with the UTRAN and also to transmit some small amount of non-real time (NRT) data. PRACH physical channel can be used to carry transport channel RACH, which in turn, carries logical channel DTCH and CCCH. Logical Ch. V Transport Ch. V Physical Ch. CCCH

V

RACH

V PRACH

DTCH

V

RACH

V PRACH

While making the initial access to UTRAN, UE has no idea about the amount the transmitted power which is sufficient to reach Node B. Therefore, the UE uses a mechanism called Open Loop Power Control. This mechanism is explained in full details in chapter 5 in section 5.7.1. This procedure is illustrated in figure 4.6.

Figure 4.6: PRACH procedure in UMTS In short, this procedure can be summarized as following. Step 1: UE transmits a PRACH preamble with a very small power which is calculated by UE, based on path loss calculations and some system parameters. Step 2: UE waits for the response to this preamble on a DL channel called ‘Acquisition Indication Channel’ (AICH). At this point, 3 scenarios can take place which are explained in the step 3-a, 3-b & 3-c.

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Step 3-a: If there is a positive response from Node B on AICH, UE sends the PRACH message part. Using this message, UE informs RNC about its intentions and asks for dedicated resources. Step 3-b: If there is no response from Node B on AICH, then UE ramps up the transmission power and sends another preamble. UE keeps on ramping its preamble power until it hears a reply from Node B. Step 3-c: If there is a negative response from Node B on AICH, UE aborts the random access procedures. The preamble is a sequence of 16 chips which are repeated 256 times. Hence, the length of a PRACH preamble is 256 × 16 = 4096 chips. Table 4.4 shows all the 16 preamble signature sequences defined by 3GPP. This table can be found in 3GPP TS 25.211. Operators can define how many and which preambles are allowed to be used in a cell and broadcast this information using system information. UE randomly selects one of the allowed preamble signatures and forms its preamble. Preamble Signature P0 P1 P2 P3 P4 P5 P6 ... P14 P15

Sequence 1 1 1 1 1 1 1

1 -1 1 -1 1 -1 1

1 1 -1 -1 1 1 -1

1 -1 -1 1 1 -1 -1

1 1 1 1 -1 -1 -1

1 -1 1 -1 -1 1 -1

1 1 -1 -1 -1 -1 1

1 1

1 -1

-1 -1

-1 1

-1 -1

-1 1

1 1

1 1 -1 1 -1 1 1 1 -1 1 1 1 1 1 ... 1 -1 -1 -1

1 -1 1 -1 1 -1 1

1 1 -1 -1 1 1 -1

1 -1 -1 1 1 -1 -1

1 1 1 1 -1 -1 -1

1 -1 1 -1 -1 1 -1

1 1 -1 -1 -1 -1 1

1 -1 -1 1 -1 1 1

-1 1

1 1

1 -1

1 1

1 -1

-1 -1

-1 1

Table 4.4: PRACH preamble signatures

4.4.2

DL common Channel

There are 7 DL common channels which are referred to as R99 common channels. The next sections discuss some essential details of these channels. Each section is numbered from one to seven for convenience.

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95

1. P-SCH At switch-on, UE looks for P-SCH and tries to identify the beginning of a time slot by using a globally unique code called primary synchronization code. Hence, P-SCH is the starting point of all UMTS activities.

The Synchronization Channel (SCH) is a downlink signal used for cell search. The SCH consists of two sub channels, the Primary and Secondary SCH. The 10 ms radio frames of the Primary and Secondary SCH are divided into 15 slots, each of length 2560 chips. Figure 4.8 illustrates the structure of the SCH radio frame. P-SCH is transmitted for only first 10% of each slot. One slot corresponds to 2/3 ms or 2560 chip. Therefore, P-SCH consists of a unique code, Primary Synchronization Code (PSC) which is modulated and transmitted at the beginning of every slot. This is illustrated in figure 4.8. The PSC is the same for every cell in the UMTS system irrespective of the country or operator. The value of code itself goes beyond the scope of our discussion. If you are interested in knowing more about the PSC, please refer to section 5.3.3.5 of 3GPP TS 25.213.

Figure 4.7: Timing of Synch. Channels; sent on the first 10 % of every slot At the beginning, UE is not synchronized to the Node B timing. Therefore, it is impossible to perform spreading and scrambling. Hence, P-SCH is sent without any spreading. In other words, P-SCH does not consume any channelization code. 2. S-SCH After finding the beginning of Slot using P-SCH, UE searches for S-SCH and tries to identify the beginning of radio frame by using a sequence of secondary synchronization code. This sequence is shared by all cells belonging to that scrambling code group.

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SC Group 0 1 2 3 4 5

0 1 1 1 1 1 1

1 1 1 2 2 2 3

2 2 5 1 3 16 4

3 8 16 15 1 6 7

4 9 7 5 8 6 4

5 10 3 5 6 11 1

Group 61 Group 62 Group 63

9 9 9

10 11 12

13 12 10

10 15 15

11 12 13

15 9 14

Group Group Group Group Group Group .. .

Slot # 6 7 8 15 8 10 14 16 3 12 16 6 5 2 5 15 5 12 5 5 3 ... ... ... 15 9 16 13 13 11 9 14 15

9 16 10 11 8 1 6

10 2 5 2 4 15 2

11 7 12 16 4 12 8

12 15 14 11 6 16 7

13 7 12 15 3 11 6

14 16 10 12 7 2 8

12 14 11

14 10 11

13 16 13

16 15 12

14 14 16

11 16 10

Table 4.5: Table 4: Allocation of SSCs for secondary SCH (from TS 25.213) Just like P-SCH, the Secondary SCH is also transmitted in the first 10 % of each time slot only. The information transmitted on S-SCH repeats after every 15 slots. Therefore, S-SCH is transmitting a unique sequence of secondary synchronization codes. These sequences are well defined in 3GPP TS 25.2136 . SSC is a 256 chip long sequence and there are only 16 Secondary Synchronization Codes (SSC). By arranging them in different order, different sequences could be formed. As we know, there are 64 primary scrambling code groups. Therefore, there are only 64 sequences defined for secondary synchronization codes, as shown in table 4.5. From table 4.5, one can say “if a cell belongs to scrambling code group # 0, the it will broadcast SSC # 1 on slot #0, SSC # 1 on slot # 1, . . . , SSC # 16 on slot # 14 of S-SCH channel.” Similarly for a cell belonging to scrambling code group # 63, S-SCH will broadcast SSC # 9 on slot # 0, , SSC # 12 on slot # 1, . . . , SSC # 10 on slot # 14. This principle is illustrated in figure 4.8.

Just like P-SCH, S-SCH is also transmitted without any spreading. Hence, S-SCH also does not consume any channelization code. This sequence on the Secondary SCH indicates which of the code groups the cell’s downlink Scrambling code belongs to. In the cell-search procedure, from 512 options 6

3GPP TS 25.213, section ’Code allocation of SSC’

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97

Figure 4.8: Content of Primary and Secondary SCH ; PSC and SSC

UE has narrowed down to 8. There are 8 cells which belong to one SC group. Therefore, the information on S-SCH is the same in all the cells of one SC Group. 4.5 is copied from 3GPP TS 25.213. As shown in figure 4.9, the radio planners have two choices while allocating SC to a new cell.

Minimize the # of SC Groups: In this scheme, the neighbouring cells are allocated the SC from the same group. Once the SC in that group are all used, then they start with the next group. This scheme is shown as Option 1 in figure 4.9.

Minimize the # of SC Groups: In this scheme, the neighboring cells are allocated the SC from two different groups. When all 64 Groups have been used, then they go to the first group again and pick the next SC from that group, and so on. This is shown as Option 2 in figure 4.9.

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Figure 4.9: Cells belonging to the same SC group can be adjacent or distant 3. Primary Common Pilot Channel (P-CPICH) P-CPICH, or simply ‘Pilot channel’, is probably the most commonly discussed channel by radio planners and optimizers. P-CPICH is used for SC identification and channel estimation. If this channel’s received level is not satisfactory, UE tries for cell reselection or handover. P-CPICH is measured by 2 quantities, CPICH RSCP [dBm] & CPICH Ec/No [dB].

The Primary Common Pilot Channel (P-CPICH) has the following characteristics: • The same channelization code is always used for the P-CPICH (Cch,256,0)7 , • The P-CPICH is scrambled by the primary scrambling code of the cell, • There is one and only one P-CPICH per cell, and • The P-CPICH is broadcasted over the entire coverage area of the cell. Due to its fixed SF (256), the bit rate of this DL control channel is also fixed. P-CPICH can carry 30 kbps information. 7

Cch,256,0 = [1 1 1 1 1 1 1 1 1 1 . . . 1 1 1 1], or 256 chips long sequence of all 1’s

4.4. PHYSICAL CHANNELS

99

Figure 4.10: Primary Common Pilot Channel The Primary CPICH is a phase reference for the following downlink channels: SCH, Primary-CCPCH, AICH, PICH and the Secondary-CCPCH. By default, the Primary CPICH is also a phase reference for downlink DPCH. 4. P-CCPCH Although P-CCPCH is a complicated name but it is simply a physical channel which is used to bring system information from UTRAN to UE.

CCCH

V

BCH

V

P-CCPCH

The Primary CCPCH is a DL control channel which has a fixed SF=256 & a fixed rate 30 kbps. This downlink physical channels used to carry the BCH transport channel (system information). As shown in figure 4.11, the Primary CCPCH is not transmitted during the first 256 chips of each slot. Instead, Primary SCH and Secondary SCH are transmitted during this period. Hence, P-CCPCH has an activity factor of 90% which reduces the effective bit rate to 27 kbps. System information is organized in blocks known as System Information Block or SIB # N where N = 1, 2, 3,. . . . 5. S-CCPCH S-CCPCH can be compared to Swiss Army Knife which is one tool that perform several functions. BCCH

V

FACH

V

S-CCPCH

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Figure 4.11: Primary-CCPCH Structure: ON & OFF periods DCCH

V

FACH

V

S-CCPCH

DTCH

V

FACH

V

S-CCPCH

CTCH

V

FACH

V

S-CCPCH

CCCH

V

FACH

V

S-CCPCH

V

PCH

and PCCH

V

S-CCPCH

S-CCPCH physical channel carries FACH and PCH transport channels. There could be 1, 2 or more S-CCPCH per cell.

The Secondary CCPCH is used to carry the FACH and PCH. The frame structure of the Secondary CCPCH is shown in the figure 4.12 above. S-CCPCH can have a spreading factor in a range from 256 to 4. As usual, the used spreading factor decides the total number of bits per downlink Secondary CCPCH slot. The FACH and PCH can be mapped to the same or to separate Secondary CCPCHs. By having separate S-CCPCHs for FACH & PCH, the physical layer overhead increases but the paging & FACH capacity can be increased. • The main difference between a S-CCPCH and a downlink dedicated physical channel (DPCH) is that an S-CCPCH is not inner-loop power controlled. • The main difference between the Primary and Secondary CCPCH is that the transport channel mapped to the Primary CCPCH (BCH) can only have a fixed predefined transport format combination, while the Secondary CCPCH support multiple transport format combinations using TFCI.

4.4. PHYSICAL CHANNELS

101

Figure 4.12: Secondary Common Control Physical Channel

Figure 4.13: Paging Process in UMTS; First Paging Indication & then Paging 6. PICH PICH is a wake-up call which carries either ‘1’ or ‘0’. Each idle mode UE keeps on monitoring PICH on periodic intervals. If PICH = ‘1’: UE wakes up and reads the PCH on S-SCCPCH which follows 3 slots after PICH. If PICH = ‘0’: UE stays idle and checks PICH on the next PICH occasion.

Figure 4.13 illustrates the two step paging process in UMTS. First the UEs in sleeping mode wake up and read the paging indicator channel (PICH). If there is a paging indicator, they read the S-SCCPH and decode the paging message which carries UE identity (e.g., IMSI). The Paging Indicator Channel (PICH) is a fixed rate (SF=256) physical channel used to carry the paging indicators. The PICH is always associated with an S-CCPCH to which a PCH transport channel is mapped. Figure 4.14 illustrates the frame structure of the PICH. One PICH radio frame of length 10 ms consists of 300 bits (b0, b1, . . . ,b299). Of these, 288 bits (b0, b1, . . . ,

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Figure 4.14: Slot format of PICH for Np = 18, 36, 72 and 144 b287) are used to carry paging indicators. The remaining 12 bits are not formally part of the PICH and shall not be transmitted. In each PICH frame, Np paging indicators, where Np =18, 36, 72, or 144. Further, the PI calculated by higher layers is associated with the value of the paging indicator Pq. If a paging indicator in a certain frame is set to “1” it is an indication that UEs associated with this paging indicator and PI should read the corresponding frame of the associated S-CCPCH. 7. AICH AICH channel is used to inform UE that its PRACH preamble has been acquired by Node B. From this, UE concludes that the currently used transmission power is sufficient to communicate with Node B. At this point, Open Loop Power Control is finished.

AICH is a common DL channel whose operation is closely related to UL PRACH channel. As shown in figure 4.6, the response to the successful PRACH preamble is sent on the AICH channel. Hence, AICH is a channel that carries Acquisition Indicators (AI). The Acquisition Indicator channel (AICH) is spreaded by a fixed SF = 256. Acquisition Indicator AIs corresponds to signature ‘s’ on the PRACH. AICH is aligned with Primary CPICH for the phase reference and timing. Figure 4.15 illustrates the structure of the AICH. The AICH consists of a repeated sequence of 15 consecutive access slots (AS), each of length 5120 chips. Each access slot consists of two parts:

4.4. PHYSICAL CHANNELS

103

Figure 4.15: Acquisition Indication Channel Acquisition-Indicator (AI) part: an Acquisition-Indicator (AI) part consisting of 32 real-valued symbols a0 , . . . , a31 . No Transmission part: For last 1024 chips AICH is switched off8 . According to 3GPP TS 25.211, the real-valued symbols, aj are given by

aj =

15 ∑

AIs · bs,j

(4.1)

s=0

In equation 4.1, there are two terms on right hand side, AIs and bs,j . Let us investigate more about them step-by-step. 1. AIs : AI can have 3 values: • If an Acquisition Indicator is set to +1, it represents a positive acknowledgement. • If an Acquisition Indicator is set to -1, it represents a negative acknowledgement. • 0 2. bs : bs is chosen depending on the signature used by UE on PRACH preamble using table 4.6 which has been defined by 3GPP9 . The real-valued symbols, aj , are spread and modulated in the same fashion as bits when represented in +1, -1 form. 8 9

PRACH Preamble is also 256 × 16 = 4096 chips 3GPP TS 25.211

CHAPTER 4. LOGICAL, TRANSPORT & PHYSICAL CHANNELS 104

S 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1

1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1

1 1 -1 -1 1 1 -1 -1 1 1 -1 -1 1 1 -1 -1

1 1 -1 -1 1 1 -1 -1 1 1 -1 -1 1 1 -1 -1

1 -1 -1 1 1 -1 -1 1 1 -1 -1 1 1 -1 -1 1

1 -1 -1 1 1 -1 -1 1 1 -1 -1 1 1 -1 -1 1

1 1 1 1 -1 -1 -1 -1 1 1 1 1 -1 -1 -1 -1

1 1 1 1 -1 -1 -1 -1 1 1 1 1 -1 -1 -1 -1

1 -1 1 -1 -1 1 -1 1 1 -1 1 -1 -1 1 -1 1

1 -1 1 -1 -1 1 -1 1 1 -1 1 -1 -1 1 -1 1

bs,j , where 1 1 1 1 1 -1 -1 -1 -1 -1 -1 1 -1 -1 -1 -1 -1 1 1 1 1 1 1 -1 1 1 1 1 1 -1 -1 -1 -1 -1 -1 1 -1 -1 -1 -1 -1 1 1 1 1 1 1 -1

j= 1 -1 -1 1 -1 1 1 -1 1 -1 -1 1 -1 1 1 -1

0, 1, 1 1 1 1 1 1 1 1 -1 -1 -1 -1 -1 -1 -1 -1

2, . . . , 31 1 1 1 1 -1 -1 1 1 1 1 -1 -1 1 1 1 1 -1 -1 1 1 1 1 -1 -1 -1 -1 -1 -1 1 1 -1 -1 -1 -1 1 1 -1 -1 -1 -1 1 1 -1 -1 -1 -1 1 1

Table 4.6: AICH Signature patterns

1 1 -1 -1 1 1 -1 -1 -1 -1 1 1 -1 -1 1 1

1 1 -1 -1 1 1 -1 -1 -1 -1 1 1 -1 -1 1 1

1 -1 -1 1 1 -1 -1 1 -1 1 1 -1 -1 1 1 -1

1 -1 -1 1 1 -1 -1 1 -1 1 1 -1 -1 1 1 -1

1 1 1 1 -1 -1 -1 -1 -1 -1 -1 -1 1 1 1 1

1 1 1 1 -1 -1 -1 -1 -1 -1 -1 -1 1 1 1 1

1 -1 1 -1 -1 1 -1 1 -1 1 -1 1 1 -1 1 -1

1 -1 1 -1 -1 1 -1 1 -1 1 -1 1 1 -1 1 -1

1 1 -1 -1 -1 -1 1 1 -1 -1 1 1 1 1 -1 -1

1 1 -1 -1 -1 -1 1 1 -1 -1 1 1 1 1 -1 -1

1 -1 -1 1 -1 1 1 -1 -1 1 1 -1 1 -1 -1 1

1 -1 -1 1 -1 1 1 -1 -1 1 1 -1 1 -1 -1 1

4.4. PHYSICAL CHANNELS

4.4.3

105

UL Dedicated Channels

In the uplink, there are only two dedicated physical channels, the uplink ‘Dedicated Physical Data Channel (uplink DPDCH)’ and the uplink ‘Dedicated Physical Control Channel (uplink DPCCH)’. Uplink Dedicated Physical Channel As stated above, there are 2 dedicated physical channels in UL, the DPDCH and the uplink DPCCH. The DPDCH and the DPCCH are I/Q code multiplexed which means they are modulated by carrier waves which have 90 degree phase difference.

Figure 4.16: Slot format of UL DPDCH and DPCCH Channel 1. DPDCH: The uplink DPDCH is used to carry the DCH transport channel. In other words, DPDCH carries user data and L3 control signalling10 . According to 3GPP specifications, there could be several DPDCHs per radio link, but in practice however, we use only one DPDCH per radio link (per user). This channel has a variable spreading factor which can assume any value from 256 to 4. 2. DPCCH: As shown in figure 4.16, the uplink DPCCH is carries control information which is added by Layer 1. Layer 1 control information contains following fields: 1. Pre-known pilot bits for channel estimation for coherent detection, 10

Note! It is a common misunderstanding that L3 messages are sent using DPCCH. In fact DPCCH is physical channel which does not have any corresponding transport or logical channel. Therefore, DPCCH can be called as L1 control channel.

106

CHAPTER 4. LOGICAL, TRANSPORT & PHYSICAL CHANNELS 2. Transmit power-control (TPC) commands, 3. Feedback information (FBI) (used only if DL transmit diversity is used on DL DPCH channel) and 4. A Transport-format combination indicator (TFCI) which is optional. The transport-format combination indicator (TFCI) serves the duty of informing the receiver receiver about the current transport format combination of the transport channels mapped to the uplink DPDCH radio frame sent in the same slot. It is not allowed to have less than or more than one DPCCH channel per radio link. Due to a fixed SF = 256, the DPCCH bit rate equals 15 kbps in UL.

Figure 4.16 shows the frame structure of the uplink DPDCH and the uplink DPCCH. Each radio frame of length 10 ms is split into 15 slots, each of length Tslot = 2560 chips, corresponding to one power-control period. The DPDCH and DPCCH are always frame-aligned with each other. Having one power control command per time slot means 15 power control commands per radio frame (or 10 ms). This simply implies that in UMTS, when UE is using dedicated physical channels, its power can be modified 1500 times per second. Since DPDCH is our main UL data channel, it is crucial to know about the possible bit rates that can be achieved. DPDCH can have SF = 256, 128, 64, 32, 16, 8 and 4 which corresponds to 15, 30, 60, 120, 240, 480 and 960 kbps respectively. Hence, variable bit rate services can be achieved using variable spreading factors. Please refer to table 4.7 for more details. The modulation used in UL is BPSK.

4.4.4

DL Dedicated Channels

There is only one type of downlink dedicated physical channel, the Downlink Dedicated Physical Channel (downlink DPCH). Downlink Dedicated Physical Channel In uplink, L1 control and data are transmitted on two separate physical channels (DPDCH and DPCCH) but in downlink both L1 control and data is carried by the same physical channel known as DPCH. Here DPCH is a combination of DPDCH & DPCCH.

4.4. PHYSICAL CHANNELS

107

SF

Symbol (ksps) [ ]Rate [ 3.84 Mcps ] Rchip = SF = SF

bit rate (kbps) on DL DPCH

bit rate (kbps) on UL DPDCH

512 256 128 64 32 16 8 4

7.5 15 30 60 120 240 480 960

15 30 60 120 240 480 960 1920

15 30 60 120 240 480 960

Table 4.7: SF and the corresponding Channel bitrate

Figure 4.17: Slot format of DL DPCH Channel As stated above, there is only one type of downlink dedicated physical channel, the downlink DPCH. Within one downlink DPCH, the following information is transmitted: User Data, DTCH logical Channel L3 Control Information, DCCH logical channel L1 Control Information, Physical signals, generated by L1 The physical control information consistes of (1) pilot bits, (2) TPC commands, and (3) an optional TFCI ). Therefore, DL DPCH can be considered as time multiplex of a downlink DPDCH and a downlink DPCCH. As usual, the timing is organized into 10 ms radio frames which equals 15 time slots. If we carefully examine the conents of a slot in figure 4.17, various fields of DPCH

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can be identified. Since there is one TPC command every 2/3 ms, the DL power control also happens at 1500 times per second (just like uplink). DL DPCH can have SF = 512,11 256, 128, 64, 32, 16, 8 and 4 which corresponds to 30, 60, 120, 240, 480, 960 and 1920 kbps respectively. Hence, variable bit rate services can be achieved using variable spreading factors. Please refer to table 4.7 for more details. The modulation used in DL is QPSK.

4.4.5

Summary of DCH Channels

DCH is the main channel used for transferring user data in UMTS. Therefore, it is better to understand the slot format of UL and DL DCH channel. In figure 4.18, the slot format of both UL and DL DCH channels is shown. In order to understand the fast inner loop power control of UMTS, this section is very important.

Figure 4.18: Slot format of UL and DL DPCH Channel In figure 4.19, an example is illustrated where 3 different UEs are using 3G services in a cell which is operating at UL frequency fUL and DL frequency fDL . The DL primary scrambling of the cell is 511 and the UL scrambling codes allocated to the 3 UEs are 1,000,111, 1,000,222 & 1,000,333. 11

Some vendors do not support SF 512

4.5. CELL SEARCH PROCEDURE

109

Figure 4.19: Example of DCH usage, 3 DCH users shown here This example has been specially included to explain the usage of channelization code in UL and DL. • In uplink, the control and data channel from the same UE is identified by UL channelization codes. • In downlink, channelization code is used to identify the UEs because frequency and DL Scrambling code is the same for all users in that cell.

4.5

Cell Search Procedure Source:

3GPP TS 25.214, Annexure C (quoted Word-by-word)

During the cell search, the UE searches for a cell and determines the downlink Scrambling code and frame synchronization of that cell. The cell search is typically carried out in three steps: Step 1: Slot synchronization During the first step of the cell search procedure the UE uses the SCHs primary synchronization code to acquire slot synchronization to a cell. This is typically done with a single matched filter (or any similar device) matched to the primary synchronization code which is common to all cells. The slot timing of the cell can be obtained by detecting peaks in the matched filter output.

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Step 2: Frame synchronization and code-group identification During the second step of the cell search procedure, the UE uses the SCHs secondary synchronization code to find frame synchronization and identify the code group of the cell found in the first step. This is done by correlating the received signal with all possible secondary synchronization code sequences, and identifying the maximum correlation value. Since the cyclic shifts of the sequences are unique the code group as well as the frame synchronization is determined. Step 3: Scrambling code identification During the third and last step of the cell search procedure, the UE determines the exact primary Scrambling code used by the found cell. The primary Scrambling code is typically identified through symbol-by-symbol correlation over the CPICH with all codes within the code group identified in the second step. After the primary Scrambling code has been identified, the Primary CCPCH can be detected and the system- and cell specific BCH information can be read. If the UE has received information about which scrambling codes to search for, steps 2 and 3 above can be simplified.

4.6. HSDPA CHANNELS IN SHORT

4.6

111

HSDPA Channels in Short

Although the channels and other details about HSDPA will be discussed in chapter 7, a list of all HSDPA related physical channels is included in this chapter. There are 3 new physical channels which are illustrated in figure 4.20.

Figure 4.20: HSDPA operation explained using the physical channels.

HS-PDSCH: HS-PDSCH is a shared channel; it is shared between all active HSDPA users in the cell. Each radio frame is divided into 2 ms sub-frames in HSDPA. There are 3 timeslots within one HSDPA sub-frame. The main features about HS-PDSCH are listed below: • High Speed Physical Downlink Shared Channel • Only in DL • Carries User data and scheduled by Node B • SF is fixed, SF = 16 • Uses adaptive modulation, QPSK and 16QAM • No Soft Handover, no fast power control • Shorter transmission time interval (TTI), TTI = 2ms HS-SCCH: The High Speed Shared Control Channel (HS-SCCH) is a downlink control channel that is specially designed to inform UE about the scheduling decisions made by Node B. The information on HS-SCCH is absolutely essential for HS-PDSCH reception. This channel indicates when there is data on the HS-PDSCH that is addressed to this UE.

112

CHAPTER 4. LOGICAL, TRANSPORT & PHYSICAL CHANNELS • High Speed Shared Control Channel • Carries control information for HS-PDSCH: – – – – – – –

Channelization code set information Modulation scheme Transport block size Hybrid ARQ process id Redundancy and constellation version New data indicator UE identity = H-RNTI

HS-DPCCH: In the uplink direction, there is the High Speed Dedicated Physical Control Channel (HS-DPCCH) that is used for sending feedback information to Node B. • High Speed Dedicated Physical Control Channel • Carries L1 feedback information from a UE: – L1 H-ARQ NACK/ACK – Channel Quality Indicator (CQI)

4.7. HSUPA CHANNELS IN SHORT

4.7

113

HSUPA Channels in Short

A detailed description of HSUPA can be found in chapter 8. In this section a very brief introduction to HSUPA channels is given.

Figure 4.21: HSUPA operation explained using the physical channels. As shown in figure 8.23, there are 2 UL channels and 3 DL channels in relation to the HSUPA operations. E-DPDCH: E-DPDCH is the main data channel of HSUPA. In UL, UE can have 1, 2 or 4 E-DPDCHs. The main parameters about E-DPDCH are: • E-DCH Dedicated Physical Data Channel • Carries UL user data up to 5.76 Mbps • Variable SF; SF = 256, 128, 64, 32, 16, 8, 4 & 2 • Uses same modulation as the UL DCH, BPSK • Uses soft Handover & fast power control • Shorter transmission time interval (TTI) , TTI = 10 ms & 2ms (optional) E-DPCCH: E-DPCCH is used to carry L1 control information related to E-DPDCH. The E-DPCCH is time-aligned with the uplink DPCCH. • E-DCH Dedicated Physical Control Channel • Carries control information for E-DPDCH:H: – Retransmission sequence number (RSN), 2 bits – E-TFCI, 7 bits – Happy bit, 1 bit

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E-AGCH: The E-AGCH is a downlink physical channel used to transmit absolute grants to a UE or a group of UEs. The absolute grant consists of a 5 bit grant value which is between 0 and 31. The definition of grant is the ratio between the transmit powers of E-DPDCH and DPCCH. Grant represents the maximum E-DPDCH to DPCCH power ratio the UE may use in the next transmission.

Grant =

Ptx,E-DPDCH Ptx,DPCCH

E-RGCH: The E-RGCH carries relative grants that are used in the scheduling process to gradually increment or decrement the allowed UE grant. • E-DCH Relative Grant Channel • Carries relative grants for uplink E-DCH scheduling • Relative grants transmitted with signature sequences E-HICH: The E-HICH carries the HARQ acknowledgement indicator, ACK or NACK. • E-DCH Hybrid ARQ Indicator Channel • Carries hybrid ARQ ACK/NACK indicator • HARQ acknowledgment indicators transmitted with signature sequences

4.7. HSUPA CHANNELS IN SHORT

115

Copyright Notices In order to create some figures, tables and text-sections, the following reference material has been used. Information has been interpreted and presented in a simplified manner. The original references are provided here. Main reference material for this book has been technical specifications (TSs) and technical reports (TRs) of 3rd Generation Partnership Project (3GPP). Text in section 4.2 on page 83 Section 5.3.1.1.1 of 3GPP TS 25.301 v 7.0.0 Text in section 4.2.1 on page 84 Section 5.3.1.1.1 of 3GPP TS 25.301 v 7.0.0 Text in section 4.2.2 on page 85 Section 5.3.1.1.1 of 3GPP TS 25.301 v 7.0.0 TM c ⃝2006. 3GPP TSs and TRs are the property of ARIB, ATIS, ETSI, CCSA, TTA, and TTC who jointly own the copyright for them. They are subject to further modifications and are therefore provided to you “as is” for information purposes only. Further use is strictly prohibited. Text in section 4.4 on page 88 Section 5 of 3GPP TS 25.211 v 9.1.0. Text about P-CPICH on page 98 Section 5.3.3.1.1 of 3GPP TS 25.211 v 9.1.0. Text about S-CCPCH on page Section 5.3.3.4 of 3GPP TS 25.211 v 9.1.0. 100 Text about PICH on page 101 Section 5.3.3.10 of 3GPP TS 25.211 v 9.1.0. Text about P-SCH on 95 Section 5.3.3.5 of 3GPP TS 25.211 v 9.1.0. Text about AICH on 103 Section 5.3.3.7 of 3GPP TS 25.211 v 9.1.0. Text about Cell Search Procedure Quoted word-by-word from Annex C of in section 4.5 on page 109 3GPP TS 25.214 v 9.1.0. Figure 4.10 on page 99 Figure 13 of 3GPP TS 25.211 v 9.1.0. Figure 4.11 on page 100 Figure 15 of 3GPP TS 25.211 v 9.1.0. Figure 4.12 on page 101 Figure 17 of 3GPP TS 25.211 v 9.1.0. Figure 4.15 on page 103 Figure 21 of 3GPP TS 25.211 v 9.1.0. Figure 4.16 on page 105 Figure 1 of 3GPP TS 25.211 v 9.1.0. Figure 4.17 on page 107 Figure 9 of 3GPP TS 25.211 v 9.1.0. Table 4.4 on page 94 Table 3 of 3GPP TS 25.213 v 8.4.0. Table 4.5 on page 96 Table 4 of 3GPP TS 25.213 v 8.4.0. Table 4.6 on page 104 Table 22 of 3GPP TS 25.211 v 9.1.0. TM c ⃝2009. 3GPP TSs and TRs are the property of ARIB, ATIS, ETSI, CCSA, TTA, and TTC who jointly own the copyright for them. They are subject to further modifications and are therefore provided to you “as is” for information purposes only. Further use is strictly prohibited.

BIBLIOGRAPHY [1] 3GPP TS 25.201 ver. 6.0.0 ;‘Physical layer - General description’ [2] 3GPP TS 25.211 ver. 6.0.0 ;‘Physical channels and mapping of transport channels onto physical channels (FDD)’ [3] 3GPP TS 25.212 ver. 6.0.0 ;‘Multiplexing and Channel Coding (FDD)’ [4] 3GPP TS 25.213 ver. 6.0.0 ;‘Spreading and Modulation (FDD)’ [5] 3GPP TS 25.214 ver. 6.0.0 ;‘Physical Layer Procedures (FDD)’ [6] 3GPP TS 25.104 ver. 6.0.0 ;‘Base Station (BS) radio transmission and reception (FDD)’ [7] 3GPP TS 25.301 ver. 6.0.0 ;‘Radio Interface Protocol Architecture’ [8] H.Holma and A. Toskala, ‘WCDMA for UMTS’ , 5th Edition, John Wiley & Sons. [9] Chris Johnson, ‘Radio Access Networks For UMTS ; Principles And Practice’ , John Wiley & Sons.

For HSDPA-secific details, the version of these specs should be 5.0.0 or higher & for HSUPA-specific details, it shold be 6.0.0 or higher.

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CHAPTER

5 RADIO RESOURCE MANAGEMENT

Source: • 3GPP TR 25.922 ver. 7.0.0 ; ‘Radio resource management strategies’ • H.Holma and A. Toskala, ‘WCDMA for UMTS’ , 5th Edition, John Wiley & Sons. Radio Resource Management or RRM is a collective term used for the algorithms and features designed for the optimized operation of radio networks. Radio Resource is a generic term which is applicable to all radio access technologies. In a TDMA based system, radio resource is a time slot, whereas, in a FDMA system it is a frequency channel. Similarly in our CDMA-based cellular systems, UMTS & HSPA, radio resource can be identified as a combination of: • frequency, • Scrambling code, • channelization code & • power. Radio resources are valuable resources which directly contribute to the revenue of the service provider. Therefore, it is very crucial to have a well-tuned radio resource 117

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management working in the network. The tuning is generally performed by various parameters defined at RNC, Node B or cell level. Other than these, for proper handover and mobility, there are parameters which can separately affect the mobility from one source cell to different target cells. The most common approach for RRM implementation is to take decisions based on cell load where the word load refers to as received power at Node B receiver for Uplink and transmitted power from Node B transmitter for downlink. In short, the functions of Radio Resource Management can be summarized as: 1. Maximize Capacity: If the current load in a cell is less than the planned target load, there should be a mechanism to increase the resources of Non-real Time (NRT) data users and utilize the total cell resources. This feature will have at least two advantages: the increased cell throughput from the operator perspective and improved user experience from end-user perspective. This can be achieved by smart packet scheduling in RNC. With the introduction of HSDPA & HSUPA, there are new schedulers introduced in Node B which can respond much quicker to the variations in radio conditions and adapt the throughput according. This well-known concept is called link adaptation. 2. Guarantee the Planned Coverage: In CDMA-based networks, if a cell experiences overload, then the interference becomes higher than usual. To overcome this, UEs are asked to increase the transmit power. At this moment, the UEs at cell edge, which are already transmitting with maximum power, cannot increase their power and find themselves out-of-coverage area due to this overload mechanism. This well-known mechanism is called cell breathing. Therefore, there should be some mechanism, which will prevent the cell to go into overload. This is achieved by an effective admission control algorithm in RNC. If admission control algorithm is too relaxed, then there will be admission of new users even if the cell is close to overload, which will cause instability. On the other hand, if admission control algorithm is over-protective, then there will be very high blocking although there are some free resources left in the cell. Therefore, proper parameter setting to tune the admission control is very crucial to the network performance. 3. Provide good Quality of Service: In order to achieve an acceptable bit error rate (BER), there must be sufficient quality at the physical link. There are three important QoS attributes which are quite often used in RRM, in order to guarantee a good link quality. They are:

5.1. INPUTS FOR RRM FUNCTIONALITY

119

1. Signal to Interference Ratio (SIR) [in dB] 2. Bit Energy to Noise Energy Ratio (Eb/No) [in dB] 3. Block Error Rate (BLER) [in %] While admitting a new bearer, admission control already takes into account the Planned Required EbNo, BLER target and SIR Target values. Based on these values, admission control makes an estimate of the increment in the cell load caused by this particular bearer. Once the bearer is admitted, the power control mechanism tries to maintain the power level at an absolute minimum level which will be enough to meet the quality criterion1 . 4. Priority Handling: The services can be broadly categorized into 4 traffic classes namely: (1 = Highest priority) 1. Conversational 2. Streaming 3. Background 4. Interactive Generally, Conversational class has the highest priority followed by Streaming, Interactive and then Background. Therefore, Conversational class users are given the highest importance while admitting the service. (e.g., Voice or video calling). Rest 3 classes are subjected to buffering and scheduling according to their relative priorities.

5.1

Inputs for RRM Functionality

In this section, we will discuss the inputs which are used by RRM functions. There are 4 main inputs which are illustrated in figure 5.1. 1. Radio parameters stored in RNC’s database 2. Node B measurements (a) Common Measurements (b) Dedicated Measurements 3. UE measurements 4. RNC’s internal calculation and measurements

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Figure 5.1: Inputs for RRM functionality Let’s us discuss them one by one.

5.1.1

RNC Parameter Database

RNC is responsible for storing the radio parameters for the whole radio network subsystem. Among other parameters, it also stores the cell specific uplink load target and downlink load target. For example, if the target DL load is 75% and the current load is 65%, RRM can easily decide about the next strategy. Therefore, the parameters stored in RNC’s database are an important input for RRM functionality. Figure 5.2 shows three regions according to the cell load status. • The planned area is the safe operation area where the load is under controllable limits and neither coverage nor the quality of active connections gets affected. The threshold which defines the upper limit of planned area is decided in co-ordination with radio network planning strategy. Generally in this situation, admission control is advised to allow more RABs and packet scheduler is advised to schedule higher bit rates. 1

Transmit power should be much as required, as little as possible.

5.1. INPUTS FOR RRM FUNCTIONALITY

121

• The marginal area is the safety window between ‘normal’ and ‘overload’ states. In this situation, the new real-time calls are generally denied. Ongoing packet sessions continue but their bit rates are neither throttled not increased. The threshold which defines the upper limit of marginal area is decided by the planner and defined relative to the threshold for the planned area, for example, 1 dB above the threshold for planned area. • The overlaod area is the area where the cell load is beyond the controllable limits. This can severely affect the quality and coverage of the cell-edge users. Generally, in this state, the admission control stops allowing more real time RABs in the cell and packet scheduler tries to reduce the load by scheduling less bit rates.

Figure 5.2: Load Regions used in Radio Resource Management

5.1.2

Node B Measurements

Source: 3GPP TS 25.433, UTRAN Iub interface Node B Application Part (NBAP) signalling One central difference between the RRM of 2G family of systems (GSM, GPRS & EDGE) and 3G family of systems (WCDMA & HSPA) is the exact knowledge about current actual load.

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• In 2G, the RRM is located at BSC/PCU and it knows exactly how many times slots have been allocated. The decisions about resource allocation is purely in the hand of BSC. BTS does not have the authority to modify the resources autonomously. Thus, there is no confusion about the current load in the cell. • In 3G, the RRM is located at the RNC site. But the exact knowledge about the cell load (UL received power and DL transmitted power) is available at the Node B. RNC makes decisions about the initial, minimum and maximum power of each connection but the instantaneous power can be modified by Node B using power control feature. RNC has to completely depend on the measurements performed by Node B and reported to RNC. The interface connecting Node B & RNC is Iub. The signalling protocol on Iub is called Node B Application Part (NBAP). There are two types of measurement reports, common measurements and dedicated measurements.

Figure 5.3: Common NBAP measurement management, source: 3GPP TS 25.433

Common Measurement Report These reports are handled by C-NBAP protocol. The word common here means “common to the cell”. Hence we have one such report at scheduled intervals decided by operator specific parameters2 . Typical values reported in this report are: • Total Carrier Power (TCP) 2

Typically one such report is sent from Node B to RNC at several 100 ms, e.g., 400 ms.

5.1. INPUTS FOR RRM FUNCTIONALITY

123

• Transmitted carrier power of all codes not used for HS transmission • Received Total Wideband Power (RTWP) • Acknowledged PRACH Preambles • HS-DSCH Required Power • E-DCH Provided Bit Rate Dedicated Measurement Report These reports are handled by D-NBAP protocol. The word dedicated here means “dedicated to one radio link”. Using this measurement report, Node B can inform RNC about the transmit power of a particular radio link in downlink. Typical values reported in this report are: • SIR • SIR Error • Transmitted Code Power NBAP protocol uses 2 special identifiers for this purpose. They are called Node B UE context ID and CRNC Communication Context ID. These IDs are like ‘nicknames’ that were chosen by Node B and RNC at the time of initial radio link establishment. Due to multitude of dedicated measurements, these reports are sent at lower frequency compared to common measurement reports3 .

5.1.3

UE Measurements

According to 3GPP, the measurements performed by UE can be either periodic or event-triggered. Event triggered option requires parameters to be set to clearly define an event.

Source:

3GPP TS 25.215, Physical layer - Measurements (FDD)

Other than the measurements performed by Node B, UE physical layer also performs various measurements which are reported back to RNC for optimum functionality of RRM functions e.g., handover mobility, bit-rate modification etc. According to 3GPP TS 25.215, some of the crucial UE measurements are: 3

Typically every few seconds. e.g., 3s.

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Figure 5.4: Dedicated NBAP measurement management, source: 3GPP TS 25.433 • P-CPICH RSCP • P-CPICH Ec /N0 • UTRA carrier RSSI • GSM carrier RSSI (BCCH RxLev of GSM) • Transport channel BLER • UE transmitted power • UE Rx-Tx time difference • SFN-SFN Observed time difference • ... In order to read more about the definition of these quantities, please refer to section 5.1 & 5.2 in 3GPP TS 25.215. Section 5.1 describes the UE measurement abilities and section 5.2 explains UTRAN measurement abilities. Please note that not all the measurements are performed periodically. According to 3GPP specifications4 , the measurements can be either periodic, or on demand or event-based. This simply implies that there is a great deal of freedom which can be used by infrastructure vendors for controlling the UE reporting whereas the UEs must be capable of measuring these quantities. 4

TS 25.302, section 9.2 and TS 25.215

5.1. INPUTS FOR RRM FUNCTIONALITY

125

It has been observed, that the vendors have opted for event-based triggering of measurements. Therefore, the UE looks out for some special scenarios to take place. For example, UE monitors a new target cell whose CPICH signal is almostequally-strong as the serving cell. When such a scenario happens, UE sends a RRC: Measurement Report Message with the details of the target cell scrambling code and signal strength. Such a scenario is called Event 1A. In the similar fashion, various events have been defined by 3GPP which will be discussed later in this module.

5.1.4

Internal RNC Measurements

RNC is the central controlling unit of the RAN. Therefore, RNC keeps on performing certain calculations, estimations and measurements to guarantee the stability of cells within its controlling area. For example, • For packet users, RNC keeps on performing measurement on ‘DL Transport Channel Traffic Volume’. If the data volume exceeds a given threshold, it can notify the packet scheduler to: – Perform state transition from CELL FACH to CELL DCH or – Upgrade the bit rate of allocated DCH • Once a DCH channel is established for a UE, RNC keeps on measuring the ‘actual’ throughput in UL and DL. Based on these measurements, packet scheduler can: – Decrease the allocated bit rate in next scheduling decision, or – Release the allocated bit rate in next scheduling decision, or – Increase the allocated bit rate if throughput measurements indicate a very high utilization. • Another example is when admission control admits a new real-time (RT) RAB. Admission control informs the load estimation entity of RRM about this ‘inactive’ bearer. This procedure makes sure that the load-calculation entity always has the knowledge about load which is as close to reality as possible. • Similarly, after scheduling a PS bearer, packet scheduler informs the loadcalculation entity about its estimate of the load caused by PS bearers. • ...

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5.2

Load Estimation Source: • 3GPP TR 25.922 ver. 7.0.0 ; ‘Radio resource management strategies’ • H.Holma and A. Toskala, ‘WCDMA for UMTS’ , 5th Edition, John Wiley & Sons. The following section is inspired from the book ‘WCDMA for UMTS’ by H.Holma and A. Toskala, where these topics are explained with a stepby-step mathematical analysis and description. In ‘Let’s Learn 3G in 10 Days’, the author has tried to summarize the final result of the analysis. The advanced readers should refer to the above mentioned reference to get more details.

For the proper functionality of RRM, the RNC must periodically estimate the UL & DL load in order to decide the actions for admission control and packet scheduler. The following section explains the procedure of ‘current cell load estimation’, in both uplink and downlink. Let us start the discussion with uplink cell load estimation.

5.2.1

Uplink Load Estimation

RNC can estimate the current uplink load in 2 different ways. 1. Uplink cell load estimation based on ‘Received Total Wideband Power (RTWP)’, and 2. Uplink cell load estimation based on ‘Total Uplink Throughput’ Option 1: Received Total Wideband Power-based UL load Estimation In all CDMA-based systems, UL capacity is directly affected by the noise rise generated by users in the uplink. Typically, an operator restricts the acceptable uplink load to a certain UL noise rise. The noise rise in the UL is the increase in noise compared to the noise floor of the Node B: Noise Rise, NRUL =

Prx,total Pnoise

(5.1)

5.2. LOAD ESTIMATION

127

Without going into the mathematical derivation, we will write the final relation between Noise Rise (NR) and the cell load ηUL : NR =

1 1 − ηUL

or

ηUL = 1 −

1 NR

(5.2)

Received Total power (RTWP) consists of three components: 1. System and equipment noise (or background noise), 2. Interference caused by ‘OWN CELL USERS’ & 3. Interference caused by ‘OTHER CELL USERS’ Or, RTWP = PNoise + IntOwn cell + IntOther

cell

(5.3)

As explained earlier, Node B keeps on reporting the current Received Total Wideband Power (RTWP) to RNC. RNC uses this RTWP measurements and compares it with the Pnoise . This indicates the amount of noise which has risen. This scheme has a limitation, because: • RTWP does not differentiate between own cell interference and other cell interference. RTWP is simply the measured received power at Node B receiver which might be caused by users in the own cell or neighbouring cells. • RTWP also includes PN oise , as depicted in equation 5.3. If the noise level itself fluctuates, then the RTWP cannot indicate the UL loading in an accurate manner. In order to overcome the problems listed above, it is better to combine the powerbased load estimation with another scheme described below. Option 2: Throughput-based UL Load Estimation Throughput-based UL load estimation utilizes the concepts of fractional load caused by one user and summing the load of all the users to calculate the total cell load. Throughput based UL load can be depicted by LT hr,Cell where LT hr,Cell =

∑ i∈All DCH in Cell

LDCH,i

(5.4)

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where the individual DCH of bit rate Ri kbps, causes a load LDCH,i which is calculated by:

LDCH,i =

1 W/Ri 1 1+ · (Eb/N o)i AFi

(5.5)

where Eb/No is the signal energy per bit divided by noise spectral density that is required to meet a predefined block error rate, W is WCDMA chip rate, Ri is the bit rate of user i & AFi is the activity factor5 for uses i.

5.2.2

Downlink Load Estimation

Transmitted Power-based DL load Estimation In the section related to Node B measurements, it was shown that Node B keeps on reporting the current Total Transmitted Carrier Power (TCP) or Ptx,total to RNC. RNC uses this Ptx,total measurements and compares it with the PBT S,M ax . This indicates what percentage of the Node B Power Amplifier’s power has been utilized in the current measurement period.

ηDL =

Ptx,total PBT S,M ax

(5.6)

The operator must define the DL load target. DL load target is defined as a ratio of maximum Node B power amplifier value. For example, in a 20 W carrier, if we plan to use ηDL = 75%, then the cell is in normal state up to Ptx,total < 15W .

Throughput-based DL Load Estimation In principle, the throughput-based DL load estimation can be utilized in the same manner as discussed in the previous section for Uplink. But in DL, the power based measurements are quite reliable. Therefore, there is not much need for a separate estimation of DL load based on throughput. The implementation is vendor specific. 5

For voice 0.5-0.7 & for Data service 1.0

5.3. RADIO RESOURCE MANAGEMENT STRATEGIES

5.3

129

Radio Resource Management Strategies Source: ‘Section 12: Congestion Control’ of 3GPP TR 25.922, ‘Radio resource management strategies’

In UMTS, congestion control mechanism takes care of the situations where system has reached a congestion state and therefore the QoS guarantees can be at at risk. This feature is implemented in the packet scheduler of RNC. 3GPP only gives rough guidelines about these feature. The exact rules of this feature are decided by the equipment vendors. Some of these features are optional. Therefore, it is possible for the operators to enable only those feature, which sound useful to them. But in principle, the congestion control mechanism should perform the following tasks: 1. Congestion detection: Periodically Node B reports the cell load to RNC and RNC compared the reported load with the target load. After this comparison, RNC declares whether congestion has been detected. 2. Congestion resolution. Various steps can be taken by RNC’s packet scheduler to resolve the state of congestion. • Prioritization: Ordering the different users from lower to higher priority (e.g., from those that expect a lower grade of service to those with more stringent QoS requirements). • Load reduction: Two main actions could be taken: (a) Selective blocking of new connections while in congestion (b) Reducing the maximum transmission rate • Load check: Load reduction actions can be carried on until the considered load factor is below a given threshold for a certain amount of time (i.e., the system can enter the congestion recovery status). 3. Congestion recovery: It is possible to attempt to restore the transmission parameters used before the congestion was triggered, by using a “time scheduling” on a user by user basis.

5.4

Admission Control Imagine an empty party hall. The first two guests arrive and start their smalltalk at a comfortable volume. As few more guests arrive, they also start talking in pairs or groups. Later, when the room is full and everyone is busy in

130

CHAPTER 5. RADIO RESOURCE MANAGEMENT the conversation with their partner, the first two guests realize that they are actually speaking much louder compared to the situation when they were all alone in the hall.

From this small story, one can understand that every subscriber that gets admitted in UMTS cell, adds its contribution to the overall interference. If we want to keep the interference within a controlled limit, the admission control must play an active role and stop admitting new users after a certain limit. For admission control, the following strategies must be used: • Admission control should be performed according to the Quality of Service. Typically, the admission control is very strict while admitting guaranteed bit rate (GBR) services like voice, because once admitted, RNC has no authority to drop the connection if resource congestion is detected. On the contrary, for non-GBR services like email, web-browsing, FTP, etc., admission control is quite relaxed. These bearers are allowed to setup because later if congestion is detected, the resource allocation can be reduced or eliminated. • Admission control should take the decision after considering the current system load and the required service. The quality can be defined in terms of required Block Error Rate (BLER) or required Eb/No. Admission control should use these quality parameters and estimate the increment in load that will be caused if this bearer is admitted. Imagine a room with 10 chairs where already 8 chairs are occupied. 3 more persons want to enter this room. Should admission control allow them to enter?

I am sure your answer is No. From this simple example, we can learn that admission control not only considers the existing load but also the hypothetical or simulated load for the connections for which admission control is deciding. This clearly shows that admission control algorithm prepares for the worst-case scenario before saying yes to a new bearer. There are various scenarios where admission control must step in and make the decisions. The following section describes these situations. 1. AC at the time of RRC Connection Setup. This signalling is depicted in figure 5.5

5.4. ADMISSION CONTROL

131

Figure 5.5: Admission Control in RNC at RRC Connection Establishment In RRC Connection request, UE specifies the cause of establishment. For example, mobile originated call, mobile terminated call, interactive session, emergency call, registration and so on. At this moment, admission control can prioritize the RRC connection for certain causes. It is quite obvious that RRC connection to set up an emergency call must be treated with the highest priority. 2. AC at the time of RAB Setup. This signalling is depicted in figure 5.6. The procedure of RAB establishment is started by core network. Either MSC or SGSN requests RNC to establish a RAB with certain QoS parameters. For example, traffic class, max bit rate, guaranteed bit rate, SDU error ratio, traffic handling priority, allocation retention priority, etc. From these QoS parameters, RNC finds out the radio bearer attributes like Eb/No target, Signal-to-Interference target & block error rate (BLER) target. RNC typically uses some look-up table to do so. Using these attibutes, admission control can estimate the increment in load caused by the RAB in question. (a) For RT RAB setup, AC works independently. RT RABs have certain guaranteed bit rates. Therefore, if the resources for these GBR service are not available, the RT RABs are denied. (b) For NRT RAB setup, AC works in close co-ordination with packet scheduler (PS). Therefore, NRT RAB admission is performed by AC but the subsequent scheduling of resources is performed by PS. 3. AC at the time of SHO diversity branch addition. This signalling is depicted in figure 5.7. The decision about handover is taken in SRNC

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Figure 5.6: Admission Control in RNC at RAB Establishment whereas the admission control takes place in the target cell’s CRNC. In general, admission control is a little bit relaxed for the handover decisions. Admission control allows handover to take place up to a higher load limit compared to the admission control for a new RAB setup.

5.5

Code Allocation

As discussed in the chapter related to CDMA, Spreading and air interface technology, we have learnt that there are 4 types of codes used in WCDMA which are: 1. DL Scrambling Code 2. DL Channelization Code 3. UL Scrambling code 4. UL Channelization Code 1. DL Scrambling Code: Used as the physical cell id. There are totally 512 Primary-Scrambling codes in DL, which are used as L1 identity of any WCDMA

5.5. CODE ALLOCATION

133

Figure 5.7: Admission Control in RNC at Inter-RNC SHO cell. After 512, these codes can be repeated. Therefore, we never face congestion or blocking in DL Scrambling. Therefore, RRM has not much role to play in DL SC. 2. DL Channelization Code: The channelization codes used for spreading are Orthogonal Variable Spreading Factor (OVSF) codes that preserve the orthogonality between physical channels. In DL, Channelization codes are used to differentiate among the individual users. Hence, as the need for capacity increases, the DL Channelization codes face congestion. Code-tree optimization procedure is explained in section 5.5.1. 3. UL Scrambling Code: UL Scrambling codes are used as user Id for Uplink signal separation. According to 3GPP specification, more than 16 Million UL SC have been defined. Out of which, one RNC can utilize a subset of those based on the hardware limitation of RNC (for example 50,000 codes). Therefore, shortage of UL SC is also not a very common cause for congestion. 4. UL Channelization Code: UL channelization code is used to differentiate among the various channels transmitted from the same UE. For example: • In Rel-99 Configuration: DPDCH & DPDCH

134

CHAPTER 5. RADIO RESOURCE MANAGEMENT • In Rel-5 configuration: DPDCH, DPDCH & HS-DPCCH • In Rel-6 configuration: DPDCH, DPDCH, HS-DPCCH, E-DPCCH & E-DPDCH In Uplink, every user uses a different Scrambling code. Therefore, every UE has its own code tree. Hence, the same UL CC can be re-used within the same cell. This is the reason why we never observe congestion in the UL channelization code tree. Summary: DL channelization code is a rare radio resource and must be used very efficiently. In most of 3G cellular networks, 3G and HSDPA are deployed. HSDPA makes use of multiple code allocation to one user. This further increases the code utilization. In order to reduce the code blocking, a technique called code tree optimization is used by RNC (see section 5.5.1).

Code allocation deals with the problem how different codes are allocated to different connections. The channelization codes used for spreading are Orthogonal Variable Spreading Factor (OVSF) codes that preserve the orthogonality between physical channels. The OVSF code is shown in figure 5.8.

5.5.1

Code Tree Optimization

RNC has a smart algorithm which either periodically or based on some threshold, re-organizes the DL channelization code tree. The main purpose of this feature is to avoid the fragmentation of a code tree. Figure 5.9 depicts the same with example where CC16,2 code was allocated to user 1 and CC16,10 is allocated to another user. As a result, CC8,1 and CC8,5 are forbidden to be used in the same cell. This happens due to the fragmented nature of code allocation. In RRM framework, there should be a smart code allocation algorithm that can release CC16,10 and re-assign CC16,3 to the same UE. Because the SF remains unchanged, the net bit rate does not get affected. Therefore, this procedure happens only at physical layer, without disturbing the higher6 protocols layers.

5.6

Packet Scheduler

With the introduction of GPRS into the mobile world, it became clear that packet switched IP-based data services are going to be an important part of the services 6

MAC, RLC and PDCP

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Figure 5.8: OVSF code Tree offered by future mobile systems. That is why Packet Scheduler is introduced in RNC to handle the packet traffic more efficiently. The main function of packet scheduler are: • Transport Channel Type Selection: Logical channel DTCH can be mapped on either common channels, dedicated channels or on shared channels. PS is responsible to select the channel type and later on, if needed, channel type switching7 . • Transport Format Combination Set (TFCS) construction: At the time 7

HS-DSCH ,→ DCH channel type switching can be understood as HSDPA to R99 handover.

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Figure 5.9: DL Channelization Code Tree Optimization to avoid code congestion of radio bearer setup, the PS decides about the possible transport formats (transport block size, TB set size, TTI, channel coding scheme, coding rate, etc.). • RRC state handling: PS is responsible to handle the UE state. This concept is explained later in section 5.6.1. • Priority handling: All the PS bearers to be scheduled are put into queue and PS picks the bearer according to their relative priorities. • Overload Control: If the cell load goes to overload, it is PS which reduced the bit rates and thereby tries to bring back the load to normal state. • Bit Rate Adaptation: Other than these, PS also keeps an eye on the resource allocation & utilization. For example, if allocated bit rate is higher and actual throughput is not high, then the bit rate can be reduced to avoid wastage of resources.

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In order to transmit data in uplink, UE can use RACH, DCH or E-DCH transport channels. Similarly UE can receive downlink data on FACH, DCH or HS-DSCH transport channels. This mapping between logical channels and transport channels is performed by the MAC layer which is implemented in RNC’s packet scheduler algorithm. With the introduction of HSDPA & HSUPA, the packet scheduling function is distributed, which is described below: RACH (↑), FACH (↓) & DCH (↕): For these transport channels, the packet scheduling is performed by RNC. The main input for the scheduler decision are: the amount of data to be transmitted, actual throughput measured in past few TTIs, cell load status, priorities of the bearers etc8 . HS-DSCH (↓) HS-DSCH is the DL transport channel used by HSDPA system. The scheduler for this channel is located at Node B and known as MAC-hs scheduler. CQI plays a central role in selecting which HSDPA user will be served in the next TTI and what transport block size will be selected. E-DCH (↑) Finally, E-DCH is the UL transport channel used by HSUPA. The scheduler for this channel is also located at Node B and known as MAC-e scheduler. The main input to these schedulers are the feedback reports from UE. Each UE keeps on reporting the status of its buffer, power control headroom and the priority of the logical channel whose data is to be transmitted. Scheduling request happens periodically, e.g., every 100 ms. Meanwhile UE keeps on reporting one bit information called ‘Happy Bit’ which indicated the UE’s wish for an upgrade in UL resources.

5.6.1

RRC States Source :

3GPP TS 25.331 RRC Protocol Specification9

RRC is the central L3 protocol in UTRAN. RRC protocol is implemented in UE and RNC. Therefore, whenever UE & RNC want to communicate, they use RRC protocol. Some important procedures of RRC protocols are RRC connection establishment, Radio Bearer Management, Measurement control and reporting, System information transfer, Paging etc. The RRC states introduced in 3G are a compromise of the following aspects: 8

Please note that Radio conditions (CPICH Ec/No or CQI) is not a factor for RNC based packet scheduler. This happens because UE reporting to RNC is so slow that RNC cannot keep track of radio condition of all the users. 9 TS 25.331 is perhaps the most bulky specification of UTRAN. Developments in HSDPA, HSUPA & HSPA+ domain have further increased the details available in this this document.

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• Which physical channels that are allocated to the UE, and thus which transport channels that can be used. This factor affects the effective utilization of UTRAN resources. • Which type of RRC connection mobility procedures that are used. For example, in one state UE performs handovers whereas in another one Cell Reselection. • The level of UE activity, e.g. whether it is known on cell or URA level and whether or not it uses DRX. This is a deciding factor for UE battery consumption and longer standby time. In principle, UE should be in a power saving state, if inactive. At the same time, it should be possibly to quickly make a state transition from stand by state to active state10 . In nutshell, the UE behaviour is broadly decided by the state in which it currently is. Therefore, the knowledge about RRC state is very crucial in 3G understanding. There are two modes: idle mode and connected mode. When UE is in connected mode, its behaviour is decided by the sub-state in which it is. There are 4 sub-states defined. They are, Cell DCH, Cell FACH, Cell PCH & URA PCH. • [1. IDLE Mode] When a UE is in Idle Mode, there is no RRC connection between the UE and the RNC. In other words, RNC does not even know that this UE exists in its area. In such a situation, UE keeps on listening to system information of the cell and periodically reads paging channel. After being paged, UE can establish an RRC connection. Similarly, to initiate a call, UE can establish and RRC connection with RNC and use it to perform call control signalling. • [2. Connected mode] 2.a Cell DCH: The CELL DCH state is characterized by: – A dedicated physical channel is allocated to the UE in uplink and downlink. – The UE is known on cell level according to its current active set. – UE shall use the connected mode measurement control information received in other states until new measurement control information has been assigned to the UE. 10

The words standby and active mentioned above are used as English words rather than telecom specific technical words.

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– It shall perform measurements and transmit measurement reports according to the measurement control information. – UE performs handover in this state - soft, softer or hard handover. – Battery consumption is the highest in Cell DCH state (≈ 300 -400 mA11 ). – From Operator’s perspective, this state is very expensive because certain dedicated radio resources have to be reserved for one subscriber. 2.b Cell FACH: The Cell FACH state is characterized by: – Neither an uplink nor a downlink dedicated physical channel is allocated to the UE. – The UE continuously monitors FACH tranport channel in the downlink (mapped on S-CCPCH physical channel). – The UE is assigned a default common transport channel in the uplink (e.g. RACH) that it can use anytime according to the access procedure defined by system information. – The UE is known on cell level according to the cell where the UE last made a cell update. It performs cell reselection and upon selecting a new UTRAN cell, initiates a cell update procedure. – UE is identified by a C-RNTI on common transport channels. The scope of C-RNTI is limited to a cell. If a new cell is selected, a new C-RNTI must be allocated. – UE must monitor a FACH to receive signalling messages or user data addressed to the UE or any broadcast messages. – UE performs measurements and transmits measurement reports according to the measurement control information. – Battery consumption in this state is lower than that in Cell DCH state but yet very high(≈ 150-200 mA). Therefore, Cell FACH should not be considered as standby state. It is an active state. The difference compared to Cell DCH is the usage of common channel rather than a dedicated channel.

2.c Cell PCH: The Cell PCH state is characterized by: – Neither an uplink nor a downlink dedicated physical channel is allocated to the UE. – The UE uses DRX for monitoring a PCH via an allocated PICH. 11

UE battery consumption strongly depends on the handset’s hardware, features and configuration. Therefore, the number mentioned here should be used as approximate value for understanding purpose.

140

CHAPTER 5. RADIO RESOURCE MANAGEMENT – No uplink activity is possible. If the UE wants to make an uplink access, it autonomously shall enter the Cell FACH state and inform RNC about it using cell Update signalling message. – The UE is known on cell level according to the cell where the UE last made a cell update in CELL FACH state. – In this state, UE shall monitor the paging occasions according to the DRX cycle and receive paging information on the PCH. – UE shall acquire system information on the BCH and use the measurement control information according to that system information when no dedicated measurement control information has been assigned to the UE. – Perform cell reselection and upon selecting a new UTRA cell, enter the CELL FACH state and initiate a cell update procedure. – Perform measurements according to the measurement control information. Consequently, when needed, enter CELL FACH state and transmit measurement reports. – In Cell PCH state, the UE battery consumption is very small (≈ 5-15 mA). 2.d URA PCH: The URA PCH state is very similar to the CELL PCH state. Therefore, a [X] sign has been printed in front of those points which differentiate between these two states. Other points are common in both the states. The URA PCH state is characterized by: – Neither an uplink nor a downlink dedicated physical channel is allocated to the UE: – The UE uses DRX for monitoring a PCH via an allocated PICH. – No uplink activity is possible. If the UE wants to make an uplink access it autonomously enters the Cell FACH state. X The UE is known on URA level according to the URA assigned to the UE during the last URA update in CELL FACH state. – In this state, the UE shall monitor the paging occasions according to the DRX cycle and receive paging information on the PCH. – Acquire system information on the BCH and use the measurement control information according to that system information when no dedicated measurement control information has been assigned to the UE. X Perform cell reselection and upon selecting a new UTRA cell that does not match the URA assigned to the UE, enter the CELL FACH state and initiate a URA update procedure.

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– Perform measurements according to the measurement control information when needed according to the measurement control information, enter CELL FACH state and transmit measurement reports. – Just like the CELL PCH state, in URA PCH state also, the UE battery consumption is very small (≈ 5-15 mA). Some advanced readers might notice that there are a lot of details about RRC states that could be added in the previous section. Their thoughts are absolutely right. I wanted to keep is simple and short. For more details, readers are advised to refer to TS 25.331.

5.6.2

RRC States Transitions

For the discussion about RRC State transition, URA PCH will not be discussed. This will simplify our learning. Afterwards, the same concepts can be extended by involving URA PCH as well. From inactive to active transitions This subsection will mainly treat the transition, which results in Idle to connected mode transition, DCH allocation or DCH bit rate upgrade. 1. RRC Idle to CELL DCH Transition: From RRC IDLE, UE can directly enter CELL DCH or Cell FACH state depending on the establishment cause specified by UE in the RRC Connection Request message. The complete signalling flow is shown in figure 5.5. 2. CELL FACH to CELL DCH Transition: This transition takes place when UE has no DCH allocated. In this scenario, it can use RACH in UL & FACH in DL. But if the UE requires higher bitrates in either DL or UL, a request is received at RNC either from UE or from within RNC. This is typically called as Capacity Request. This capacity request is generated when the UE or RNC buffer contains data [in Bytes] which exceeds a certain threshold. In UL, the capacity request is officially known as Event 4A. 3. CELL DCH to CELL DCH Transition: This special case is not a state transition but we should still discuss it. When a UE has been allocated some bit rates in UL & DL and yet the amount of data [in Bytes] exceeds a certain threshold, then DCH is upgraded to a higher bit rate, if allowed by the cell load condition.

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Figure 5.10: RRC State Transition: From Inactive  Active behaviour 4. Cell PCH to Cell FACH Transition: If a packet session remains inactive for several seconds or minutes, the UE will enter Cell PCH state. In this state, there is no possibility to transmit or receive any data. If RNC receives data from SGSN for the UE in DL: RNC will page the UE in the Cell where it last performed a Cell Update. UE in return responds to this paging with another Cell Update message where the cause will be explicitly specified as Paging Response. This response will go to RNC using RACH and UE will enter Cell FACH state where once again the data transmission can take place. If UE has some data to send in UL: On the contrary, if the UE has some data to send, it autonomously enters into the CELL FACH state. Once again, on RACH it sends a Cell Update message to RNC where the cause is specified as Uplink Data transmission. From Active to Inactive Transitions In contrast to the discussion in the previous section, this subsection will mainly treat the transition which happens, if the UE becomes inactive. We will start by

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imagining the UE has been allocated some DCH channel with N kbps in UL and M kbps in DL. Now let us discuss the UE behaviour if it becomes inactive for a few seconds.

Figure 5.11: RRC State Transition: From Active  Inactive behaviour

1. CELL DCH to Cell FACH Transition: In CELL DCH state, UE has a dedicated code and power allocation. If RNC’s Packet scheduler detects inactivity in UL & DL, the dedicated resources are taken back from UE and it can be sent to CELL FACH state. The timer value used in figure 5.11 is to give a rough idea about the range which this parameter should take. In practice, network optimizers can change these values to control the wastage of resources in cell. 2. CELL FACH to Cell PCH Transition: As it was shown in section 5.6.1, CELL FACH state is a state where UE constantly monitors FACH channel. This causes very high battery consumption in UE. Therefore, if inactivity is detected in this state, the UE moves to the real power saving state known as Cell PCH. 3. Cell PCH to RRC IDLE Transition: In Cell PCH, UE can generally stay inctive for a longer period because neither it is holding any network resource nor it is wasting its battery power.

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5.7

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Power Control

As we have seen in the sections 5.2.1 & 5.2.2, transmit power in any CDMA-based system is directly connected to the capacity of a cell. Therefore, it is desired to keep the transmit power level at a minimum level which will be just enough to meet the quality target but not exceed the desired quality. In UMTS, this task is accomplished by 3 types of power control algorithms, which are explained in this section. DL Common P-SCH S-SCH P-CPICH P-CCPCH S-CCPCH PICH AICH

Channels Primary Synchronization Ch. Secondary Synchronization Ch. Primary Common Pilot Ch. Pri. Common Control Physical Ch. Sec. Common Control Physical Ch. Paging Indication Channel Ch. Acquisition Indication Channel Ch.

DL Dedicated Channels DPCH

Dedicated Physical Channel

UL Common Channels

PRACH

Physical Random Access Ch.

UL Dedicated Channels DPDCH Dedicated Phy. Data Ch. DPCCH Dedicated Phy. Control Ch.

Table 5.1: List of all R99 Physical channels Table 5.1 shows a list of all UL & DL physical channels of R99 UMTS. Among these: • The DL common channels do not undergo any power control. The power of these channels is decided by radio network planners and remains fixed throughout the operation. Their power values can be changed as an optimization effort by the optimization engineers but RRM plays no role in dynamically changing the power of DL common channels. • UL Common channel PRACH is used for making initial access to the network. Additionally, in UMTS, PRACH can carry small amount of UL NRT data traffic. The power control on PRACH is known as Open Loop Power Control. • From the table 5.1, the only remaining channels are UL & DL dedicated channels. These are the main traffic channels in 3G. These channels undergo two power control mechanisms in parallel, known as Inner Loop Power Control and Outer Loop Power Control.

5.7. POWER CONTROL

5.7.1

145

Open Loop Power Control Source :

3GPP TS 25.211, 25.214, 25.331

According to Open Loop Power Control of PRACH channel, UE transmits a PRACH preamble with a certain initial power (see equation 5.7). If it does not receive any response in downlink on the Aquisition Indication channel (AICH), it ramps up the power and sends the next preamble with a higher power. UE keeps on doing it until it receives the response from Node B. According to the procedure defined by 3GPP TS 25.331 (section 8.5.7), UE calculates the power for the first preamble as:

Preamble Initial Power = − + +

Primary CPICH TX power CPICH RSCP UL interference Constant Value

(5.7)

• “Primary CPICH Tx power” and “Constant value” are broadcasted by system information in System Information Block type 5; • “UL interference” is broadcasted by system information in System Information Block type 7; • and the CPICH RSCP is measured by UE; As expressed by equation 5.7, the initial preamble’s strength can be controlled by a constant value. This parameter can be in the range of [-35 . . . -10] dB. Once, the value of this parameter is fixed, then the equation can be simplified as

Preamble Initial Power ∝ Primary CPICH TX power − CPICH RSCP (5.8) ∝ Path Loss From equation 5.8, we can conclude that transmission power of first preamble is directly proportional to the path loss experienced by the UE. Hence, further away the UE is, stronger will be the initial preamble.

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After transmitting the initial preamble UE will wait for a certain time12 . Within this period if there is no response from the Node B, UE will send the next preamble with an increased power. This power ramping is called Open Loop power Control. The word Open Loop means that this power control works autonomously in the transmitter (UE) without any feedback from the receiver (Node B). The moment UE receives a feedback from Node B, open loop PC is finished because its purpose was only to calculate the minimum initial UL power which will allow UE to communicate with Node B. As defined in 3GPP TS 25.214 (section 6.1), before the physical random-access procedure can be initiated, Layer 1 shall receive the following information from the higher layers (RRC): (The parameters related to Open Loop Power Control are indicated by [X].) • The message length in time, either 10 or 20 ms. X The AICH Transmission Timing parameter [0 or 1]. • The set of available signatures and the set of available RACH sub-channels for each Access Service Class (ASC). X The power-ramping factor Power Ramp Step [integer > 0]. X The parameter Preamble Retrans Max [integer > 0]. X The Power offset P p-m = (Pmessage-control Ppreamble ), measured in dB, between the power of the last transmitted preamble and the control part of the random-access message. In order to know more about the RACH procedure in UMTS, advanced readers are advised to refer to 3GPP TS 25.214, (section: ‘Physical random access procedure’). PRACH procedure has also been discussed in chapter 4 of this book in section ULcomCH. As a quick summary, please refer to figure 5.12.

5.7.2

Inner Loop Power Control

In order to understand the fast inner loop power control of UMTS, let us review our knowledge about UL and DL dedicated channel. As shown in figure 5.13, there are two physical channels in UL (DPDCH and DPCCH) and only one physical channel in DL (DPCH). 12

The exact time can be calculated by reading the “AICH transmission timing” from system information.

5.7. POWER CONTROL

Figure 5.12: Open loop Power Control on PRACH physical Channel

Figure 5.13: Slot format of UL and DL DPCH Channel • On UL DPCCH, UE sends Pilot bits whose quality is measured by Node B. In response, Node B sends TPC Command on DL DPCH. Based on this TPC Command, UE can increase or decrease its transmission power. In figure 5.13, this phenomenon is highlighted by oval shapes. • Similarly, on DL DPCH, Node B sends Pilot bits whose quality is measured by UE. In response, UE sends TPC Command on UL DPCCH. Based on this TPC Command, Node B can increase or decrease its transmission power used on that particular radio link. 5.13, this phenomenon is highlighted by triangle shapes.

147

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CHAPTER 5. RADIO RESOURCE MANAGEMENT Source : 3GPP TS 25.214; Section 5.1 Uplink power control, Section 5.2 Downlink power control

The concept of power control mechanism is very easy but to understand the mathematical description available in TS 25.214, we require some acquaintance with these procedures. In the following section, we will try to simplify the explanation. The exact details should be studied from the reference mentioned above. Let us discuss the uplink and downlink power control procedures one by one. First we will start with uplink.

Uplink Inner Loop PC 3GPP TS 25.214 (section 5.1.2.2.1) provides the general description of uplink inner-loop power control. UL inner loop PC adjusts the UE transmit power in order to keep the received uplink signal-to-interference ratio (SIR) at a given SIR target, SIRTarget . The serving cells (cells in the active set) should estimate signal-to-interference ratio SIREst of the received uplink using the Pilot Bits in UL DPCCH. On Node B side, this decision has to be taken: • if SIREst > SIRTarget then the TPC command to transmit is “0”. • if SIREst < SIRTarget then the TPC command to transmit is “1”. If UE is in soft handover with 2 or more cells, it is possible that it received different TPC Commands from different cells. For example, in 2 cells TPC Command = 1 and one cell TPC command = 0. UE must combine the multiple TPC commands and derive one final TPC command that will be effective in that slot. TPC Command is “0” if at least one cell is sending TPC command = “0” and TPC Command is “1” only if all the cells are sending TPC command =“1”. To convert the binary values (0 or 1) to the power step (+1 dB, +2 dB, -1 dB, 0 dB or any other value), UE uses following guidelines: • If the received TPC command is equal to 0, then TPC cmd for that slot is -1.

∆DPCCH = ∆TPC · TPC cmd (or) Ptx,DPCCH (n + 1) = Ptx,DPCCH (n) − ∆TPC

(5.9)

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• If the received TPC command is equal to 1, then TPC cmd for that slot is +1.

∆DPCCH = ∆TPC · TPC cmd (or) Ptx,DPCCH (n + 1) = Ptx,DPCCH (n) + ∆TPC

(5.10)

On UE side, the TPC command is interpreted according to the power control algorithm selected by operator. [PCA 1 Power Control Algorithm 1 (3GPP TS 25.214 section 5.1.2.2.2)] • When PCA 1 is selected, UE responds to TPC commands every time slot. In other words, Power control happens at a rate of 1500 times per second. The rules for TPC cmd calculation are explained in equations 5.10 & 5.9. • ∆TPC = 1 dB or 2 dB, which is derived from the UE-specific higher-layer parameter “TPC-StepSize”. This parameter value is signalled to UE by RNC using L3 RRC signalling at the time of DCH allocation. [PCA 2 Power Control Algorithm 2 (3GPP TS 25.214 section 5.1.2.2.3)] • When the PCA 2 is selected, then UE responds to TPC command every 5th time slot. This reduces the frequency of power control from 1500 to 300 times per second. – For first four slots in set, TPC cmd = 0. – For the 5th time slot, UE follows following rule: ∗ If all 5 TPC commands within a set are 1 (i.e., 11111) then TPC cmd = +1 in the 5th slot. ∗ If all 5 TPC commands within a set are 0 (i.e., 00000) then TPC cmd = −1 in the 5th slot. ∗ Otherwise, TPC cmd = 0 in the 5th time slot. • ∆TPC = 1 dB. For Algorithm 2, ∆TPC shall always take the value 1 dB. After doing all this analysis, UE knows TPC cmd = ‘0’ or ‘1’ in every slot. Two algorithms shall be supported by the UE for deriving a TPC cmd. Which of these two algorithms is used is determined by a UE-specific higher-layer parameter, “PowerControlAlgorithm”, and is signalled to UE by RNC using L3 RRC signalling at the time of DCH allocation. Summary of uplink fast power control algorithms:

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CHAPTER 5. RADIO RESOURCE MANAGEMENT Power Control Algorithm 1: If the power control algorithm is PCA 1, then the UE responds of TPC commands as following: If TPC bit = ‘1’

⇒

Increase the power by 1 or 2 dB

If TPC bit = ‘0’

⇒

Decrease the power by 1 or 2 dB

Power Control Algorithm 2: PCA 2 means: If 5 consecutive TPC bits are = ‘11111’

⇒

Increase the power by 1 dB

If 5 consecutive TPC bits are = ‘00000’

⇒

Decrease the power by 1 dB

If 5 consecutive TPC bits are = ‘01101’

⇒

Ignore the commands

If 5 consecutive TPC bits are = ‘10110’

⇒

Ignore the commands

If 5 consecutive TPC bits are = ‘. . . ’

⇒

Ignore the commands

Please refer to section 5.1 Uplink power control in 3GPP TS 25.214 for the exact mathematical analysis and more details about the UL inner loop power control. Now let us focus on the DL power control mechanism.

Downlink Inner Loop PC We must remember, Node B is transmitting several physical channels simultaneously. The power control explained in this section works independently on all the DL DPCHs. In other words, if there are 10 users using speech service in a cell, the power used for each user is calculated separately and independently. As explained in the introductory remarks about power control, the DL inner loop PC adjusts the Node B transmit power to maintain the received Downlink signal-to-interference ratio (SIR) at a given SIR target, SIRTarget . Node B adjusts it transmit power according to the TPC commands received from UE in UL DPCCH. UE calculates the value of TPC command by comparing the desired Target SIR and actually measured SIR. Figure 5.14, shows that DPDCH and DPCCH are time-multiplexed to form DPCH. The DL power control algorithm controls the DL transmit power of the ’pilot bits’ field of DPCCH. From this figure, one can notice the power offsets as following: P01: The power offset between TFCI fields of DPCCH and the DPDCH. P02: The power offset between TPC fields of DPCCH and the DPDCH. PO3: The power offset between PILOT BITS fields of DPCCH and the DPDCH.

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Figure 5.14: Slot format of DL DPCH Channel As power control takes place, the relative power offsets between the DPCCH and DPDCH are not changed. According to 3GPP TS 25.214 (section 5.2.1.2.1 UE behaviour), the UE shall generate TPC commands to control the network transmit power and send them in the TPC field of the uplink DPCCH. The UE shall check the downlink power control mode (DPCMode ) before generating the TPC command. The DPC MODE parameter is a UE specific parameter controlled by the UTRAN. • If DPCMode = 0: the UE sends a unique TPC command in each slot and the TPC command generated is transmitted in the first available TPC field in the uplink DPCCH; • If DPCMode = 1: the UE repeats the same TPC command over 3 slots and the new TPC command is transmitted such that there is a new command at the beginning of the frame. According to 3GPP TS 25.214 (section 5.2.1.2.2 UTRAN behaviour), upon receiving the TPC commands, UTRAN shall adjust its downlink DPCCH/DPDCH power accordingly. For DPCMode = 0, UTRAN shall estimate the transmitted TPC command TPCest to be 0 or 1, and shall update the power every slot. If DPCMode = 1, UTRAN shall estimate the transmitted TPC command TPCest over three slots to be 0 or 1, and shall update the power every three slots. According to 3GPP TS 25.214, the power control step size can take four values: 0.5, 1, 1.5 or 2 dB. It is mandatory for UTRAN to support the step size of 1 dB, while support of other step sizes is optional. Summary of downlink fast power control algorithms:

152

CHAPTER 5. RADIO RESOURCE MANAGEMENT DPCMode = 0: If the power control algorithm is DPC Mode=0, the then Node B responds of TPC commands as following:

If TPC bit = ‘1’

⇒

Increase the power by a fixed step size

If TPC bit = ‘0’

⇒

Decrease the power by a fixed step size

DPCMode = 1: If the power control algorithm is DPC Mode=1, the then Node B responds of TPC commands as following:

5.7.3

If 3 consecutive TPC bits are = ‘111’

⇒

Increase the power by a fixed step size

If 3 consecutive TPC bits are = ‘000’

⇒

Decrease the power a fixed step size

If 3 consecutive TPC bits are = ‘011’

⇒

Ignore the commands

If 3 consecutive TPC bits are = ‘101’

⇒

Ignore the commands

If 3 consecutive TPC bits are = ‘. . . ’

⇒

Ignore the commands

Outer Loop Power Control

Outer Loop Power Control adjusts the SIR target (SIRTarget ), in order to achieve a desired Target Block Error Rate (BLERTarget ). Therefore, the decisions to increase or decrease the SIR Target are made based on the comparison of estimated (measured) BLER with Target BLER.

DL Outer Loop PC DL outer loop power control is mainly implemented within the user equipment. At the beginning of connection setup, RNC informs UE about the desired value of block error rate (BLERTarget ). When UE receives the data, it calculates the actual value of BLER received in the current TTI. This procedure is illustrated in figure 5.15. • If Estimated BLER is < Target BLER then the DL Target SIR is reduced. • If Estimated BLER is > Target BLER then the DL Target SIR is increased. • If Estimated BLER is = Target BLER then the DL Target SIR is not modified. Figure 5.15 illustrates both DL innerloop and DL outerloop power control mechanisms. The DL outerloop function appears to be an autonomous algorithm which tries to reach the BLERTarget as informed by the RNC at the beginning of connection setup. Hence, the UE handset vendors have some degree of freedom while implementing the DL outer loop PC.

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153

Figure 5.15: Functionality of DL Outer Loop & Inner Loop PC UL Outer Loop PC

Figure 5.16: Functionality of UL Outer Loop & Inner Loop PC In uplink, the Outer Loop Power Control takes place in RNC. The whole procedure is illustrated in figure 5.16. The same sequence of steps are described in the following text:

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1. At connection setup, (RRC or RAB), RNC’s admission control decides the UL BLERTarget . 2. Admission control also decides the initial, minimum and maximum SIRTarget . 3. RNC informs Node B about the initial SIRTarget . 4. On physical layer between UE and Node B, UL inner-loop power control tries to achieve this target value of SIR. The process is briefly described below. (a) UE transmits pilot bits on UL DPCCH channel. (b) Node B estimates the Signal-to-interference-ratio (SIREst ) & decides the polarity of TPC bits. • if SIREst > SIRTarget then the TPC command to transmit is ‘0’ • if SIREst < SIRTarget then the TPC command to transmit is ‘1’ (c) This communication between UE and Node B happens once every slot (once every 2/3 ms). 5. After receiving the data from UE, the Node B forms a frame protocol frame. This frame has 2 parts, header and payload. Payload is for the received data from UE, but the header contains some control information. Among other things, the header field contains frame reliability information. 6. (In case of Soft handover), UE is connected to more than one cell or Node B. RNC receives the frames from all the Node Bs and looks into the frame reliability information. Based on this information, RNC decides, which frame should be forwarded to the core network. 7. After combining the frames from all the Node Bs, RNC estimates the BLEREst and compares it with BLERTarget . Based on the result from this last step, the SIR target is either reduced, increased or kept unchanged. 8. RNC informs Node B about the modified target of SIR and the whole process repeats once again (steps 3, 4, 5, 6 & 7).

5.8

Handover Control In early 90s, people were amazed to know that while talking they can move from one cell to another, without disconnecting the call. Now a days, we treat it as a basic functionality of cellular networks. We have certainly come a long way.

Handover is a mechanism where a UE in connected mode can move from one WCDMA cell to another cell. The target cell can be of the same radio access technology or a different one e.g., GSM. This brings us to the point where we should classify the type of handovers

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in WCDMA. In RRM framework, the handover control makes decisions that will be made based on the measurement results reported primarily by the UE but also by measurements in the network or various parameters set for each cell. In general, the handovers in all the systems can be categorized into two families, namely Soft HO & Hard HO. A brief introduction to both is given below. (a) Soft Handover: Soft Handover is a handover in which the mobile station adds and removes radio links in such a manner that the UE always keeps at least one radio link to UTRAN. This can be performed on the same carrier frequency only. For this reason, Soft Handover allows easily the provision of macro diversity transmission. As a result of this definition, there are areas of the UE operation in which the UE is simultaneously communicating via a number of radio links towards different cells. With reference to Soft Handover, the Active Set is defined as the set of radio links simultaneously involved in the communication between the UE and UTRAN (i.e., the UTRA cells currently assigning a downlink DPCH to the UE constitute the active set). Typically, max Active Set Size = 3. (b) Hard Handover: A Hard handover is a handover in which the mobile station has to remove all the active radio links before establishing a new radio link with the target cell. A need of hard handover arises when: • The target cell is a WCDMA cell but operating at a frequency other than the frequency used in the source cell. • The target cell belongs to a different radio access technology. • The source and target cell are both operating at same frequency but a SHO is not possible13 . Another way of classification of handover is based on the Radio frequency and technology used in the source cell and the same used in the target cell. Based on this criterion, the handover in WCDMA can be categorized in 3 groups: 1. Intra Frequency Handover: This scenario happens when the source cell is a WCDMA cell with operating frequency f1 & the target cell is also a WCDMA cell with the same operating frequency. These kinds of handovers are typically: • Soft HO: Inter Node B soft HO • Softer HO: Intra Node B soft HO • Hard HO: Inter-RNC HO but no Iur interface between the two RNC’s. 13

This happens in the case of Inter-RNC Handover without Iur interface support.

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2. Inter Frequency Handover (IFHO): In GSM, the neighbouring cells generally operate on different frequency. Therefore, while moving from one cell to another is simply an Inter Frequency HO, but there is a big difference between TDMA-based 2G system and CDMA-based 3G system. In CDMA systems, UE is constantly receiving & transmitting on its serving frequency. Therefore, UE cannot measure another carrier without interrupting its reception on the serving UTRAN frequency. Hence we need some kind of scheme where some well-defined gaps are created in which UE can perform measurements of signal strength of P-CPICH of the interfrequency target cell. This concept is called Compressed Mode and will be discussed later in this section. Concept of compressed measurement is also needed for 3G to 2G Handover or ISHO. 3. Inter System Handover (ISHO): In the early days of UMTS deployment, it can be anticipated that the service area will not be as contiguous and extensive as existing second generation systems. It is also anticipated that UMTS network will be an overlay on the 2nd generation network and utilize the latter, in the minimum case, as a fall back to ensure continuity of service and maintain a good QoS as perceived by the user. Therefore, the majority of 3G mobile devices will be a multimode equipment, capable of using both 2G & 3G. This concept is beneficial for both the technologies. Where 3G gets some kind of coverage safety belt from the underlying legacy 2G network, at the same time, 2G investments can be reused in the modern 3G technology. This backward compatibility of 3G to 2G is a major driving force in the success of UMTS.

5.8.1

Active, Monitored and Detected cells

According to 3GPP TS 25.331 (section 8.4.0 ‘Measurement related definitions’), cells that the UE is monitoring are grouped in the three mutually exclusive categories: Active Set Cells: Cells, which belong to the active set. User information is sent from all these cells. The cells in the active set are involved in soft handover. The UE shall only consider active set cells included in the variable CELL INFO LIST for measurement; i.e., active set cells not included in the CELL INFO LIST shall not be considered in any event evaluation and measurement reporting. Monitored Cells: Cells, which are not included in the active set, but are included in the CELL INFO LIST belong to the monitored set. In common man’s language, we can call these cells as defined neighbours. Detected Cells: Cells detected by the UE, which are neither in the CELL INFO LIST nor in the active set belong to the detected set. Reporting of measurements of the detected set is only applicable to intra-frequency measurements made by UEs in the CELL DCH state. These cells can be understood as missing neighbours.

5.8. HANDOVER CONTROL

5.8.2

157

Soft/Softer Handover

The only difference between soft and softer handover is: • In Soft Handover, the cells taking part in HO are served by two different Node Bs, whereas, in Softer handover, they belong to the same Node B. • In Soft Handover, RNC receives the data from two (or more) Node Bs. Both of these data flow can have different block error rates (BLER). RNC can select the data with less BLER and ignore the other one. This procedure in called Macro Diversity Combining (MDC). An example of this was shown in the UL outer loop PC section (see figure 5.16). In Softer HO, there is no MDC because it is Node B which performs the combining of two uplink radio links. • Another difference between the Soft & Softer HO is in terms of Iub utilization. In Softer Handover, the data is sent/received on Iub only on one link, where as in Sot handover at least two Iub links are used and in worst case, even an Iur link is required if the two Node Bs are controlled by two different RNCs. Otherwise from the RF perspective, Soft and Softer HO are very similar. Therefore, in the next sections the word Soft Ho will be used for both types of HO. Soft handovers are Mobile Evaluated Handovers, MEHO. Therefore, it is UE which initiates the handover procedure. As defined in section 5.1.3, UE can inform RNC about the need for handover either periodically or based on some events. According to 3GPP TS 25.331 (section 14.1.1 ‘Intra-frequency measurement quantities’), a measurement quantity is used to evaluate whether an intra-frequency event has occurred or not. It can be: 1. Downlink Ec/No. 2. Downlink path loss. For FDD: Pathloss in dB = Primary CPICH Tx power − CPICH RSCP For Primary CPICH Tx power, the IE “Primary CPICH Tx power” shall be used which is signalled to UE in system information (SIB 5). The unit is dBm. CPICH RSCP is the result of the CPICH RSCP measurement. The unit is dBm. 3. Downlink received signal code power (RSCP) after despreading. In practice, most commonly, CPICH Ec/No is chosen as a measurement quantity for Soft HO decisions. For Inter-frequency and Inter-System handover, both CPICH RSCP and CPICH Ec/No are used to trigger the handover measurements.

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For Soft handover, there are three main events defined in the specifications 3GPP TS 25.331. Within Measurement Control message, the UTRAN notifies the UE which events should trigger a measurement report. The listed events are the toolbox from which the UTRAN can choose the reporting events that are needed for the implemented handover evaluation function, or other radio network functions. In the description about the SHO related events, we will assume that Intrafrequency measurement quantity is CPICH Ec/No. The explanation is a simplified version of the complicated (and complete) procedure explained in 3GPP TS 25.331. • [Event 1A:] A Primary CPICH enters the reporting range.

Figure 5.17: Event 1A triggered Commonly network planners and optimizers define event 1A as Event 1A is used to ADD a cell to the active set. As shown in figure 5.17, event 1A can take place when the UE has an active set = 1 or 2. The threshold value of CPICH Ec/No is calculated with reference to the best active set cell. Therefore, if a neighbour cell is to be added to the active set, its CPICH Ec/No should be greater than the threshold shown in the figure. The threshold does not have an absolute value but relative to the best active set cell. In right sub-figure of figure 5.17, there are 2 cells in AS but the threshold for handover evaluation is calculated with reference to the cell with SC ‘a’ because it is the strongest cell in AS. (CPICH Ec/No)Neighbour Cell > (CPICH Ec/No)Best, AS Cell − Add Window (5.11)

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159

– UE  RNC: Measurement Report After event 1A gets triggered, UE reports this to RNC by sending a L3 RRC: Measurement Report massage. In this, UE specifies the DL SC of the neighbour cell along with the CPICH Ec/No value. This signalling scenario was illustrated in figure 5.7 in admission control section. – RNC  UE: Active Set Update At this moment, RNC performs admission control for the target cell. On successful addition decision, RNC informs UE by sending a L3 RRC: Active Set Update message. – UE  RNC: Active Set Update Complete In response, UE finally replies with RRC: Active Set Update Complete. Adding another cell to the active set makes the neighbours of the added cell also the neighbours for UE. Therefore, RNC performs neighbour list combining and informs UE about its decision using RRC: Measurement Control message. • [Event 1B:] A primary CPICH leaves the reporting range .

Figure 5.18: Event 1B triggered Commonly network planners and optimizers define event 1B as Event 1B is used to DELETE a cell from the active set. As shown in figure 5.18, e1B takes place when the UE has an active set = 2 or 3. Just like e1A, here also the threshold value of CPICH Ec/No is calculated with reference to the best active set cell. Therefore, if a neighbour cell is to be deleted or removed from the the active set, its CPICH Ec/No should be weaker than the

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CHAPTER 5. RADIO RESOURCE MANAGEMENT threshold shown in the figure. The threshold does not have an absolute value but relative to the best active set cell. In the left sub-figure of figure 5.18, there are 2 cells in the AS and in the right sub-figure there are 3 cells in AS. In both the scenarios, the threshold for handover evaluation is calculated with reference to the cell with SC ‘a’ because it is the strongest cell in AS. (CPICH Ec/No)AS,Cell < (CPICH Ec/No)Best, AS Cell − Drop Window

(5.12)

The signalling procedures explained in the case of event 1A are also valid in this case. The name of messages are the same. In short: – UE  RNC: Measurement Report – RNC  UE: Active Set Update – UE  RNC: Active Set Update Complete After deleting an AS cell, RNC performs neighbour list combining and informs UE about its decision using RRC: Measurement Control message. • [Event 1C:] A non-active primary CPICH becomes better than an active primary CPICH.

Figure 5.19: Event 1C triggered Commonly network planners and optimizers define event 1C as Event 1C is used to REPLACE a ‘weak AS Cell’ with a ‘Stronger one outside the AS’.

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161

As shown in figure 5.19, e1C can only take place when the UE has an active set = 3. In other words, when the AS is full. In contrast to e1A & e1B, where the threshold value of CPICH Ec/No is calculated with reference to the best active set cell, for e1C, the threshold is calculated with reference to the Weakest active set cell. Therefore, if a neighbour cell is to be replaced with one of the AS cells, its CPICH Ec/No should be stronger than the threshold shown in figure 5.19. In figure 5.18, there are 3 cells in AS. The threshold for handover evaluation is calculated with reference to the cell with SC ‘c’ because it is the weakest cell in AS.

(CPICH Ec/No)Neighbour Cell > (CPICH Ec/No)Weakest, AS Cell + Replacement Window (5.13) The signalling procedures explained in the case of event 1A & 1B. As a summary, the SHO mechanism can be summarized by figure 5.20. This figure has been copied from ‘Figure 5-1: Example of Soft Handover Algorithm’ of 3GPP TR 25.922 V7.0.0 which explains Radio resource management (RRM) strategies. Advanced readers who might be interested in more details, are advised to refer to section 5.1.4.2 in TR 25.922.

Figure 5.20: Summary of Soft Handover Mechanism (from TR 25.922)

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5.8.3

ISHO and IFHO Triggering

In CDMA, the UEs with only one receiver are only monitoring the DL frequency used by the active set cells. Therefore, to start the inter-frequency or inter-system measurements, certain events must take place. In general, there are several reasons to start an IFHO or ISHO. The following is a non-exhaustive list for causes that could be used for the initiation of a handover process. • Uplink quality (e.g.BLER) • Downlink quality (e.g. Transport channel BLER) • Downlink signal measurements (e.g. CPICH RCSP, CPICH Ec/No, Pathloss) • UE transmit power • Node B radio link Power • Traffic load (or Load Based HO) • Pre-emption • Change of service (service based HO) • ... • ... The exact strategies implemented in the RAN depends on infrastructure vendors. From those strategies, the network optimizers can enable only a subset (or all the strategies) that will control inter-system and inter-frequency handover. In this book, we will discuss the ISHO/IFHO due to downlink pilot channel measurements (e.g. CPICH RCSP, CPICH Ec/No). As depicted by the left sub-figure of figure 5.21, the downlink signal of the active set cell has become very weak. According to 3GPP TS 25.331, there are specific events described for these scenarios. Event 1F: A Primary CPICH becomes worse than an absolute threshold. The strength of P-CPICH can be measured in terms of CPICH RCSP, CPICH Ec/No. In order to trigger an event 1F, either of the two quantities has to fall below a certain threshold. In figure 5.21(see left sub-figure), these thresholds are depicted as ‘N’ dB for Ec/No and ‘M’ dBm for RSCP. Please note: A UE can be in SHO with 2 or 3 cells. If 1F is triggered for one of the AS cells, UE reports this to RNC but RNC does not start the measurement mechanism because there are still other AS cells, which can maintain the service with adequate quality. Only when the e1F is triggered for the last AS cell, the measurements procedure is started.

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Figure 5.21: Event 1E & 1F triggered Event 1E: A Primary CPICH becomes better than an absolute threshold. In order to trigger an event 1E, either of the two quantities has to rise above a certain threshold. In figure 5.21 (see right sub-figure), these thresholds are depicted as ‘N + ∆1 ’ dB for Ec/No and ‘M + ∆2 ’ dBm for RSCP. In simple words, event 1E can be called as Cancel previously reported event 1F. Summary: Event 1F is a method by which UE informs RNC about its poor 3G coverage and the need for an IFHO or ISHO and Event 1E is a method by which UE informs RNC about its 3G reception with acceptable signal quality. For a more detailed information, readers should refer to 3GPP TS 25.331. Section 14.1.2.5 describes the details of Event 1E & 14.1.2.6 illustrates Event 1F.

5.8.4

Inter-Frequency Measurements

Source: 3GPP TS 25.331, section 14.2.0a Inter-frequency measurement quantities Within the measurement reporting criteria field in the MEASUREMENT CONTROL message, UTRAN notifies the UE which events should trigger the UE to send a MEASUREMENT REPORT message. The listed events are the toolbox from which the UTRAN

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can choose the reporting events that are needed for the implemented handover evaluation function or other radio network functions. The measurement quantities are measured on the monitored primary common pilot channels (CPICH) in the FDD mode. In order to understand the events of IF measurements, we need to define 2 terms: 1. Non-used Frequency: A “non-used frequency” is a frequency that the UE has been ordered to measure upon but is not used for the connection. 2. Used Frequency: A “used frequency” is a frequency that the UE has been ordered to measure upon and is also currently used for the connection. The following events are described in section 10.3.7.19 of 3GPP TS 25.331. This section is about Inter-frequency measurement reporting criteria. 1. Event 2a: Change of best frequency. 2. Event 2b: Event 2b is triggered when following 2 conditions are fulfilled: • The estimated quality of the currently used frequency is below a certain threshold, and • the estimated quality of a non-used frequency is above a certain threshold. 3. Event 2c: The estimated quality of a non-used frequency is above a certain threshold. 4. Event 2d: The estimated quality of the currently used frequency is below a certain threshold. 5. Event 2e: The estimated quality of a non-used frequency is below a certain threshold. 6. Event 2f: The estimated quality of the currently used frequency is above a certain threshold.

5.8.5

Inter-System Measurements Source:

3GPP TS 25.331, section 14.3.0a Inter-RAT measurement quantities

At the time of writing of this book, the commonly used inter-system handover from an UTRAN cell are towards a GERAN cell (2G) or a E-UTRAN cell (LTE). We will discuss only the handovers from 3G to 2G. A measurement quantity is used by the UE to evaluate whether an inter-RAT measurement event has occurred or not is described below: Measurement quantity for UTRAN: The measurement quantity for UTRAN is used to compute the frequency quality estimate for the active set, as described in the next subclause, and can be:

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165

• Downlink Ec/No. • Downlink received signal code power (RSCP) after despreading. Measurement quantity for GSM: The measurement quantity for GSM can be: • GSM Carrier RSSI Within the measurement reporting criteria field in the MEASUREMENT CONTROL message, UTRAN notifies the UE which events should trigger the UE to send a MEASUREMENT REPORT message. The listed events are the toolbox from which the UTRAN can choose the reporting events that are needed for the implemented handover evaluation function or other radio network functions. The measurement quantities are measured on the monitored primary common pilot channels (CPICH) in the FDD mode. In order to understand the events of inter-system measurements, we need to define 2 terms: 1. Other System: “Other system” is e.g., GSM or E-UTRA14 . But in this book, we will discuss on the GSM case. 2. Used Frequency: A “used UTRAN frequency” is a frequency that the UE have been ordered to measure upon and is also currently used for the connection to UTRAN. Following events are described in section 10.3.7.30 of 3GPP TS 25.331. This section is about Inter-RAT measurement reporting criteria. 1. Event 3a: Event 3a is triggered when the following two conditions are fulfilled: • The estimated quality of the currently used UTRAN frequency is below a certain threshold, and • The estimated quality of the other system is above a certain threshold. 2. Event 3b: The estimated quality of the other system is below a certain threshold. 3. Event 3c: The estimated quality of the other system is above a certain threshold. 4. Event 3d: Change of the best cell in the other system.

5.8.6

Compressed Mode

In the previous section, we saw how a UE informs RNC about the need for handover to another frequency UTRAN cell or a cell with another RAT (e.g. GSM). In this section, we will try to investigate the method by which the UE can perform measurements on 14

LTE

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another frequency while resuming the service on its serving frequency. This method is called Compressed Mode15 . According to 3GPP TR 25.922, Compressed Mode can be avoided if the device supports dual-receiver. UE can signal this capability to RNC using at the time of RRC establishment. But the majority of UEs, which are commercially available, have only one receiver, therefore, the radio planners cannot rely on this option. It is assumed that UEs do not support dual-receivers and therefore, compressed mode is very much needed.

Methods of Compressed Mode Spreading Factor by 2 or SF/2: This method has advantages and also disadvantages: Adv: This method allows us to achieve the same bitrate in compressed frames as in the normal frame. Disadv: In compressed frames, the SF becomes half, therefore, the power requirement becomes double. This causes problems in terms of coverage and capacity. Higher Layer Scheduling: This method also has advantages and disadvantages: Adv: This method allows us to transmit with the same power in compressed frames and normal frames. Disadv: The bit rate in compressed mode is reduced because “higher” layers have scheduled less data in compressed frame.

5.8.7

Inter System HO Signalling

The signalling procedures involved with Inter-system HO is explained in chapter section 9.9. In short, the steps involved in this procedure are: 1. Phase 1: ISHO triggers 2. Phase 2: Compressed Mode measurements for BCCH RSSI 3. Phase 3: Measurement Reports (UE to RNC) 4. Phase 4: Compressed Mode measurements for BSIC verification 5. Phase 5: Measurement Reports (UE to RNC) 6. Phase 6: HO decision 7. Phase 7: Signalling between SRNC & BSC 15

This has nothing to do with data compression as we know from our computer and IP knowledge.

5.8. HANDOVER CONTROL 8. Phase 8: Communication between UE and GERAN 9. Phase 9: Confirmation about successful HO to RNC Please refer to section 9.9 for the signalling flow and more explanation.

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Copyright Notices In order to create some figures, tables and text-sections, the following reference material has been used. Information has been interpreted and presented in a simplified manner. The original references are provided here. Main reference material for this book has been technical specifications (TSs) and technical reports (TRs) of 3rd Generation Partnership Project (3GPP).

Figure 5.3 on page 122 Figure 13 of 3GPP TS 25.433 v 7.0.0. Figure 5.4 on page 124 Figure 40 of 3GPP TS 25.433 v 7.0.0. Figure 5.20 on page 161 Figure 5-1 of 3GPP TS 25.922 v 7.0.0. Text about RRM Strategies in Section 12 of 3GPP TS 25.922 v 7.0.0. section 5.3 on page 129 Text about Common-NBAP mea- Section 8.2.9.2 of 3GPP TS 25.433 v 7.0.0. surements on page 122 Text about Dedicated-NBAP Section 8.2.9.2 of 3GPP TS 25.433 v 7.0.0. measurements on page 123 Text about UE measurements on Section 5.1 & 5.2 of 3GPP TS 25.215 v 7.0.0. page 123 Text about Initial PRACH Section 8.5.7 of 3GPP TS 25.331 v 6.9.0. Preamble on page 145 Text about Active, Monitored Section 8.4.0 of 3GPP TS 25.331 v 6.9.0. and Detected cells on page 156 Text about Intra-frequency mea- Section 14.1.1. of 3GPP TS 25.331 v 6.9.0. surement quantities on page 157 Text about IF measurement Section 14.2.0a of 3GPP TS 25.331 v 6.9.0. quantities on page 164 Text about Event 2A to 2F on Section 10.3.7.19 of 3GPP TS 25.331 v 6.9.0. page 164 Text about IS measurement Section 14.3.0a of 3GPP TS 25.331 v 6.9.0. quantities on page 164 Text about Event 3A to 3D on Section 10.3.7.30 of 3GPP TS 25.331 v 6.9.0. page 165 c ⃝2006. 3GPPTM TSs and TRs are the property of ARIB, ATIS, ETSI, CCSA, TTA, and TTC who jointly own the copyright for them. They are subject to further modifications and are therefore provided to you “as is” for information purposes only. Further use is strictly prohibited.

5.8. HANDOVER CONTROL

Text about Open Loop PC pa- Section 6.1 3GPP TS 25.214 v 6.9.0. rameters on page 146 Text about UL Inner Loop PC on Section 5.1.2.2.1 3GPP TS 25.214 v 6.9.0. page 148 Text about UL PC Algorithm 1 Section 5.1.2.2.2 3GPP TS 25.214 v 6.9.0. on page 149 Text about UL PC Algorithm 2 Section 5.1.2.2.3 3GPP TS 25.214 v 6.9.0. on page 149 Text about DL PC (UE be- Section 5.2.1.2.1 3GPP TS 25.214 v 6.9.0. haviour) on page 151 Text about DL PC (UTRAN be- Section 5.2.1.2.2 3GPP TS 25.214 v 6.9.0. haviour) on page 151 Figure 5.12 on page 147 Figure 31 of 3GPP TS 25.211 v 9.1.0. c ⃝2009. 3GPPTM TSs and TRs are the property of ARIB, ATIS, ETSI, CCSA, TTA, and TTC who jointly own the copyright for them. They are subject to further modifications and are therefore provided to you “as is” for information purposes only. Further use is strictly prohibited.

169

BIBLIOGRAPHY [1] 3GPP TS 25.211 ver. 6.0.0 ;‘Physical channels and mapping of transport channels onto physical channels (FDD)’ [2] 3GPP TS 25.212 ver. 7.0.0 ;‘Multiplexing and Channel Coding (FDD)’ [3] 3GPP TS 25.213 ver. 6.0.0 ;‘Spreading and Modulation (FDD)’ [4] 3GPP TS 25.214 ver. 6.0.0 ;‘Physical Layer Procedures (FDD)’ [5] 3GPP TS 25.214 ver. 6.0.0 ;‘3GPP TS 25.215, Physical layer - Measurements (FDD)’ [6] 3GPP TS 25.301 ver. 7.0.0 ;‘Radio Interface Protocol Architecture’ [7] 3GPP TS 25.401 Ver. 7.0.0 ;‘UTRAN overall description’ [8] 3GPP TS 25.413 ver. 6.0.0 ;‘UTRAN Iu interface RANAP signalling’ [9] 3GPP TS 25.433 ver. 6.0.0 ;‘UTRAN Iub interface Node B Application Part (NBAP) signalling’ [10] 3GPP TS 25.331 ver. 7.0.0 ;‘Radio Resource Control (RRC) protocol specification’ [11] 3GPP TR 25.922 ver. 7.0.0 ;‘Radio resource management strategies’ [12] 3GPP TR 25.931 ver. 8.0.0 ;‘UTRAN functions, examples on signalling procedures’ [13] H.Holma and A. Toskala, ‘WCDMA for UMTS’ , 5th Edition, John Wiley & Sons. [14] Chris Johnson, ‘Radio Access Networks For UMTS ; Principles And Practice’ , John Wiley & Sons.

For HSDPA-specific details, the version of these specs should be 5.0.0 or higher & for HSUPA-specific details, it should be 6.0.0 or higher.

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6 PROTOCOLS & INTERFACES

Abbreviations In this module, a lot of abbreviation will be used. Therefore, it is better to introduce a list of all the abbreviations used in the coming sections. • AAL2/5: ATM adaptation Layer type 2 / Type 5 • ATM: Asynchronous Transfer Mode • BMC: Broadcast/Multicast Control Protocol • BSSAP: Base Station System Application Part Protocol • FP: Frame Protocol • GMM: GPRS Mobility Management • SM: (GPRS) Session Management • GTP: GPRS Tunneling Protocol • luUP: Iu User Plane Protocol • MAC: Medium Access Control

171

172

CHAPTER 6. PROTOCOLS & INTERFACES • MAP: Mobile Application Part • MM: Mobility Management • MTP-3B: Message Transfer Part Level 3B • NBAP: NBAP Node B Application Part • PDCP: Packet Data Convergence Protocol • ALCAP: Access Link Control Application Part • RANAP: Radio Access Network Application Protocol • RLC: Radio Link Control Protocol • RNSAP: Radio Network Subsystem Application Part • RRC: Radio Resource Control • RTCP: Real Time Control Protocol • RTP: Real Time Protocol • SCCP: Signalling Connection Control Part • SCTP: Stream Control Transmission Protocol • SMS: SMS Short Message Service • SS: Supplementary Services • SSCOP: Service Specific Connection-Oriented Protocol • SSCF-NNI: Service Specific Coordination Function - Network-Network Interface • SSCF-UNI: Service Specific Coordination Function - User-Network Interface • UDP: User Datagram Protocol

6.1

Overview Source:

3GPP TS 25.401; UTRAN overall description

Figure 6.1 shows the general protocol model for UTRAN interfaces. While designing this structure, it was planned to keep the layers and planes logically independent of each other. This strategy was designed so that protocol stacks and planes can be modified according to the future requirements.

6.1. OVERVIEW

173

Figure 6.1: General Protocol Model for UTRAN Interfaces (TS 25.401)

6.1.1

Horizontal Layers

The Protocol Structure consists of two main layers, Radio Network Layer, and Transport Network Layer. A description for both is available in 3GPP TS 25.401 (section 11.1.2). 1. Radio Network Layer: All UTRAN related issues are visible only in the Radio Network Layer. It defines the procedures related to the operation of Node B. The radio network layer consists of a radio network control plane and a radio network user plane. 2. Transport Network Layer: Transport Layer defines the procedures for establishing physical connections between Node B and the RNC. It represents standard transport technology that is selected to be used for UTRAN, but without any UTRAN-specific requirements.

6.1.2

Vertical Planes

Similarly, the UTRAN protocol structure is vertically divided into 3 planes. The description is available in section 11.1.3 of 3GPP TS 25.401.

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Control Plane NBAP RNSAP RANAP RANAP

User Plane FP FP GTP-U

Table 6.1: Main Protocols used on UTRAN Terrestrial Interfaces 1. Control Plane: The Control Plane consists of protocols which have functionality purely designed for the UMTS operation. On the Iu-CS & Iu-PS interface, the control plane protocol is RANAP. On the Iur interface it is RNSAP and on Iub interface it is NBAP. In addition, the control plane also includes the Signalling Bearer for transporting the Application Protocol messages. Application Protocol is used for setting up bearers. The Signalling Bearer for the Application Protocol may or may not be of the same type as the Signalling Protocol for the ALCAP. The Signalling Bearer is always set up by O & M actions. 2. User Plane: The User Plane Includes the Data Stream(s) and the Data Bearer(s) for the Data Stream(s). The Data Stream(s) is/are characterized by one or more frame protocols specified for that interface. 3. Transport Network Control Plane: The Transport Network Control Plane does not include any Radio Network Layer information, and is completely in the Transport Layer. It includes the ALCAP protocol(s) that is/are needed to set up the transport bearers (Data Bearer) for the User Plane. It also includes the appropriate Signalling Bearer(s) needed for the ALCAP protocol(s). 4. Transport Network User Plane: The Data Bearer(s) in the User Plane, and the Signalling Bearer(s) for Application Protocol also belong to Transport Network User Plane. As described in the previous section, the Data Bearers in the Transport Network User Plane are directly controlled by the Transport Network Control Plane during real time operation but the control actions required for setting up the Signalling Bearer(s) for Application Protocol are considered O & M actions. Figure 6.2 shows the UMTS network architecture with only the most essential network elements. Here, various network elements are connected using well-defined standard interfaces called Iub, Iur & Iu, whre Iu itself has 2 versions. One towards CS core network, called as Iu-CS and the other one towards the Packet core network, called as Iu-PS. These four are also called as UTRAN interfaces. All of these interfaces are used to carry signalling as well as the traffic which is depicted by dashed and solid lines respectively in the figure 6.2. On each interface, a shaded box is drawn to indicate the name of Interface, protocol used for control plane and the protocol used for user plane data transfer.

6.2. QOS AND BEARER

175

Other than the UTRAN interfaces, the figure 6.2 also illustrated the UTRAN Radio Interface protocols. The network element which controls the whole radio network is RNC. Therefore, UE & RNC need to communicate very often in UMTS. This communication happens using the radio protocols. Physical realization of this signalling transfer happens using the Uu Interface ( UE  Node B) and Iub Interface (Node B  RNC).

Figure 6.2: Overview of all UTRAN Interfaces and Protocols

6.2

QoS and Bearer Source :

3GPP TS 23.107 ; Quality of Service (QoS) concept and architecture

End-to-End Service: End-to-end service means the service as perceived by the end user. For example, the end-to-end service from one Terminal Equipment (TE) to another TE, or from laptop to web server. In order to provide a certain QoS to a user, there must be a bearer with well-defined characteristics and functionality. End-to-end service is like a chain of several smaller links (or bearers) and it is a well-known fact that a chain is never stronger than the weakest list. Therefore, the weakest bearer in the chain will define the QoS of end-to-end service. End-to-end service = UMTS bearer “ + ” External Bearer. External bearer is beyond the scope of UMTS technology. Therefore, the operator has to rely on the QoS provided by the external bearer. If the external bearer is between GMSC

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Figure 6.3: UMTS QoS Architecture and Bearer Concept (3GPP TS 23.107) and external PSTN exchange, then these links can be the PCM lines which have excellent QoS with guaranteed bit rate. On the other hand, if these external bearers are between GGSN and some web server, then the external bearer is implemented on the IP link. The QoS in IP is a configurable thing. But we will not discuss it here and restrict ourself to the UMTS bearer. The UMTS bearer can be understood as a chain of three smaller bearers. UMTS Bearer = [Radio Bearer] “ + ” [Iu Bearer] “ + ” [Core Network Bearer]. where , [Radio Bearer “ + ” Iu Bearer] is often called as Radio Access Bearer (RAB). Radio Access Bearer can be considered as a service provided by lower layers to higher layers. Using RAB, the information is transferred between UE and core network (MSC or SGSN). In order to have a RAB, UE must have a radio bearer and Iu bearer. Radio bearers are managed by RNC. Therefore, while RAB setup, core network requests RNC and after successful response from RNC, the RAB is established. Please note! RB Reconfiguration and RAB Reconfiguration sound very similar and quite often people mix them up.

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177

We should remember that Radio Bearer (RB) Reconfiguration is a local signalling procedure between UE and RNC, whereas, RAB Reconfiguration happens with the involvement of core network. RB reconfiguration happens very often and can be seen from L3 radio messages, but to analyze the signalling of RAB reconfiguration we must use the signllinng traces on Iu-CS or Iu-PS interface. Please refer to section 6.1 of TS 23.107 for more details.

6.2.1

UMTS QoS Classes

The QoS is simply a phrase. For implementation, we define it using a list of parameters. One of these parameters is the Traffic Class. According to 3GPP TS 23.107, all the services can be classified into 4 groups: • Conversational class • Streaming class • Interactive class • Background class The delay sensitivity of traffic is the main criteria for this classification. Conversational class traffic is affected very badly the bearer is lost for few hundred ms where as the bearer background class will not be affected so badly even if the bearer is unavailable for few seconds. Other than this classification, we can also group the services in two groups: Real-Time (RT) and Non-Real-Time (NRT) services. Conversational and Streaming classes are mainly used for carrying real-time traffic flows whereas the Interactive and Background traffic classes are suitable for carrying Non-Real-Time traffic.

Conversational Class The most well-known use of this scheme is telephony speech (e.g. GSM). But with Internet and multimedia, a number of new applications will require this scheme, for example, voice over IP and video conferencing tools. Real time conversation is always performed between peers (or groups) of live (human) end-users. This is the only scheme where the required characteristics are strictly given by human perception. Real time conversation fundamental characteristics for QoS: • Preserve time relation (variation) between information entities of the stream; • Conversational pattern (stringent and low delay).

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Streaming Class When the user is looking at (listening to) real time video (audio), the scheme of real time streams applies. The real time data flow is always aiming at a live (human) destination. It is a one way transport. Real time streams - fundamental characteristics for QoS: • Preserve time relation (variation) between information entities of the stream.

Interactive Class When the end-user, that is either a machine or a human, is on line requesting data from remote equipment (e.g. a server), this scheme applies. Examples of human interaction with the remote equipment are: web browsing, data base retrieval, server access. Interactive traffic - fundamental characteristics for QoS: • Request response pattern; • Preserve payload content.

Background Class When the end-user, that typically is a computer, sends and receives data files in the background, this scheme applies. Examples are background delivery of E-mails, SMS, download of databases and reception of measurement records. Background traffic - fundamental characteristics for QoS: • The destination is not expecting the data within a certain time; • Preserve payload content.

6.3

Access Stratum and Non-Access Stratum

According to 3GPP TR 21.905 ‘Vocabulary for 3GPP Specifications’ , the definition of Stratum is as follows: Stratum: Grouping of protocols related to one aspect of the services provided by one or several domains. In simple words, Stratum is similar to ‘a stack of protocols’. There are two types of stratums that are often discussed. They are Access Stratum (AS) & Non-Access-Stratum (NAS). The same concept is illustrated in figure 6.4.

6.3. ACCESS STRATUM AND NON-ACCESS STRATUM

179

Figure 6.4: Access Stratum & Non-access Stratum Signalling

Access Stratum: Access Stratum protocols are defined in close co-ordination with the technology and medium of transport. AS protocol in radio interface is RRC, which clearly defines the communication between UE and RNC. Similarly, the AS protocol in Iu Interface is RANAP. RANAP is used to control the communication between RNC and Core network. AS also works like a delivery service for NAS messages. For example, PAGING REQUEST is a NAS signalling message that must be delivered from MSC to UE. Higher layers (NAS) use the services of access stratum protocols RANAP and RRC to deliver this signalling message to UE. This mechanism is called Direct Transfer (DT). Please note that in UMTS, the paging procedure between RNC and UE is different from the paging procedure in GSM between BSC and MS. Therefore, the AS protocols are access-aware protocols.

Non-access Stratum: Non-access stratum is a set of protocols which are access-agnostic. In other words, these protocols are higher layer end-to-end protocols which do not depend on the underlying access network. One example of such protocol is Mobility Management protocol of UMTS. The same MM is used in GSM for procedures like location update, authentication, paging etc. NAS protocol messages can be carried over a TDMA-based 2G access network or CDMA-based 3G access network. The structure of these protocols remain unchanged. There are totally 6 NAS protocols defined which will be discussed in section 6.9.

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6.4

Radio Protocols Source: 3GPP TS 25.301: 3GPP TS 25.321: 3GPP TS 25.322: 3GPP TS 25.323: 3GPP TS 25.324: 3GPP TS 25.331:

Radio Interface Protocol Architecture. MAC Protocol Specification. RLC Protocol Specification. PDCP Protocol Specification. BMC Protocol Specification. RRC Protocol Specification.

These specifications contain the details of UMTS, HSDPA as well as HSUPA. But we should pay attention to the version. For HSDPA, the version number should be 5.0.0 or higher. Similarly, for HSUPA-specific information, the version number has to be 6.0.0 or higher. Radio Protocols are the set of protocols which control the communication between UE and RNC. This section will investigate those set of protocols. As usual, we will focus on control plane and user plane separately.

6.4.1

Control Plane

The main signalling protocol in 3G is RRC protocol but RRC is a higher layer protocol, which uses the services of underlying layers L2 (MAC and RLC) and L1 (PHY layer). The complete protocol stack is shown in figure 6.5. The functions of each individual protocol layer is explained in the coming sections. This figure also shows the protocol termination point. The physical layer is implemented by UE and Node B. Similarly, it can be seen that MAC, RLC and RRC protocols are implemented in UE and RNC.

Figure 6.5: Protocol Termination for DCH, control plane(from TS 25.301)

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181

As seen in figure 6.5, downlink signalling messages can be either generated by RNC or they might arrive from the core network which must be forwarded to the user(s). Similarly, in uplink, the signalling messages from UE can be either processed by RNC or forwarded to core network. • Signalling message coming from/going to Core Network: for example, Paging request/response, authentication request/response, etc. NAS Signaling from CN → RRC Signalling → RLC → MAC → PHY Hence, we can identify the first function of RRC layer which is NAS message transport in the uplink and downlink. • Signalling message generated from/terminated within RNC: for example measurement control/report, handover commands, system information broadcast, etc. RRC Signalling → RLC → MAC → PHY This category of RRC messages are used by RNC to control the behaviour of UE. Similarly, UE can contact RNC and inform about some event that took place. Note! The details shown in figures 6.5 & 6.6 are applicable to DCH transport channel only. In order to keep this book at an overview level, the protocol termination for transport channels RACH & FACH is not shown here. Readers are advised to refer to section 5.6.2 of 3GPP TS 25.301 to learn more. Details about of HS-DSCH and E-DCH will be shown in their respective module.

6.4.2

User Plane

There are many similarities between the control plane and user plane protocols on the radio interface. By comparing the figures 6.5 & 6.6, we observe that the RRC protocol is only for CP whereas in UP we have 2 new protocols: PDCP for packet switched UP and BMC for the broadcast services. As we know, in UMTS, the same transport channel (DCH) is used for voice, video, text, data, streaming and more. Therefore, depending on the service carried by it, the user plane protocol stack gets slightly modified. In this section, we will learn the set of protocols in the path of CS service, PS services and broadcast & multicast service.

CS Services On the transmitter side, the protocol stack for UP is as follows: CS data streams → RLC → MAC → PHY

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Figure 6.6: Protocol Termination for DCH, User Plane(from TS 25.301) PS Services On the transmitter side, the protocol stack for UP is as follows: IP Data flow → PDCP → RLC → MAC → PHY

Broadcast & Multicast Services On the transmitter side, the protocol stack for BC services is as follows: Common Traffic or CBS → BMC → RLC → MAC → PHY

6.4.3

RRC-layer Functions

3GPP TS 25.331 is a bulky document with more than 1200 pages (in Rel-6). The details about all the RRC procedures, RRC messages and their parameters can be found in it. According to Section 5.1 of TS 25.331, RRC layer performs following functions: • Non-access stratum message broadcast • Access stratum related information broadcast • RRC Connection Management • Radio Bearer Management

6.4. RADIO PROTOCOLS

183

• Management of radio resources for RRC Connection and radio bearers • Connected mode mobility functions (handover, cell update, URA update, etc.) • Paging • Control of requested QoS • Control of UE measurement reporting • Outer loop power control • Control of ciphering • Initial cell selection and re-selection in idle mode • Initial Configuration for CBS • ...

6.4.4

RLC-layer Functions

This section provides an overview on services and functions provided by the RLC sublayer. The RLC sublayer is a part of L2 in the Radio interface protocol stack. The detailed description of RLC is available in 3GPP TS 25.322. Depending on the type of information carried in the RLC SDU, the RLC layer can be configured in 3 modes: 1. Transparent Mode (TM): In this mode, RLC layer processing is very minimal. The name transparent mode shows that it appears as if the RLC layer is not present in the processing chain. This mode is generally used for real time user plane services like voice or video telephony. In this mode, there is no feedback from the receiver and there is no re-transmission mechanism. The service provided by RLC layer in TM are: • Segmentation and reassembly, • Transfer of user data, and • SDU discard. Please note! Ciphering is an important function of the RLC layer. But in the list above, ciphering is missing. Does it mean that there is no ciphering for the services which use RLC transparent mode? In other words, is our voice sent without encryption in UMTS? Answer is No. When the RLC-sublayer is configured in the Transparent mode, the ciphering is performed by the MAC sublayer.

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2. Unacknowledged Mode (UM): In the unacknowledged mode, there is no guarantee of delivery because there is no retransmission mechanism. This mode can be used for Voice over IP which is possible with HSPA solution. The following functions are needed to support unacknowledged data transfer: • Segmentation and reassembly. • Concatenation. • Padding. • Transfer of user data. • Ciphering. • Sequence number check. • SDU discard. • Out of sequence SDU delivery. • Duplicate avoidance and reordering. • Provisioning of sequence number. Unacknowledged mode of RLC can be compared to UDP transport, which does not provide guarantee of delivery but is still a popular transport method due to its reduced protocol overhead compared to more expensive alternative, i.e., TCP. 3. Acknowledged Mode: The third mode of RLC configuration uses a ACK/NACK feedback from the receiver and performs re-transmission. Therefore, it is the most reliable mode which provides some guarantee of delivery. But at the same time, this mode is most expensive one if we compare the size of the RLC header and the processing delay. The following functions are needed to support acknowledged data transfer: • Segmentation and reassembly. • Concatenation. • Padding. • Transfer of user data. • Error correction. • In-sequence delivery of upper layer PDUs. • Duplicate detection. • Flow Control. • Protocol error detection and recovery. • Ciphering. • SDU discard.

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185

Other than this, RLC layer also performs the following functions: • Maintenance of QoS as defined by upper layers. • Notification of unrecoverable errors.

6.4.5

MAC-layer Functions

Source: 3GPP TS 25.321; Medium Access Control (MAC) protocol specification It can be said that the MAC sublayer is the brain of modern communication systems like UMTS, HSDPA, HSUPA & LTE. It is the MAC layer who takes decisions about scheduling and bit rate adjustments. Without the MAC layer’s priority handling capability, we would not be discussing QoS concept in modern telecom systems. The functions of MAC include: • Mapping between logical channels and transport channels; • Selection of appropriate Transport Format for each Transport Channel depending on instantaneous source rate; • Priority handling between data flows of one UE; • Priority handling between UEs by means of dynamic scheduling; • Identification of UEs on common transport channels; • Identification of MBMS services on common transport channels; • Multiplexing/demultiplexing of upper layer PDUs into/from transport blocks delivered to/from the physical layer on common transport channels; • Multiplexing/demultiplexing of upper layer PDUs into/from transport block sets delivered to/from the physical layer on dedicated transport channels; • Segmentation and reassembly of upper layer PDUs; • Traffic volume measurement; • Transport Channel type switching and • Ciphering for transparent mode RLC. HSDPA-specific MAC (MAC-hs) and HSUPA-specific MAC(MAC-e/es) will be discussed in their respective modules.

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6.4.6

PDCP-layer Functions

Source: 3GPP TS 25.323; Packet Data Convergence Protocol (PDCP) specification This section provides an overview on services and functions provided by the Packet Data Convergence Protocol (PDCP). Header compression and decompression: Header compression and decompression of IP data streams (e.g., TCP/IP and RTP/UDP/IP headers) at the transmitting and receiving entity, respectively. The header compression method is specific to the particular network layer, transport layer or upper layer protocol combinations e.g. TCP/IP and RTP/UDP/IP. Transfer of user data: Transmission of user data means that PDCP receives PDCP SDU from the NAS and forwards it to the RLC layer and vice versa. Support for lossless SRNS relocation or lossless DL RLC PDU size change: Maintenance of PDCP sequence numbers for radio bearers that are configured to support lossless SRNS relocation or lossless DL RLC PDU size change.

6.5. IU-CS INTERFACE PROTOCOLS

187

Till now, we were focussing on the radio interface protocols. Now, we will draw our attention towards the protocols used on the UTRAN interfaces Iu-PS, Iu-CS, Iub and Iur. Due to various options available in transport (IP, ATM, IP over ATM) and then separate protocol stacks for control plane and user plane, it is very difficult to keep an overview of the protocol stacks. Therefore, instead of going into the details of every protocol, we will aim at getting a big picture about the protocols used on every interface. The interested readers are advised to refer to the 3GPP specs mentioned in the following sections1 .

6.5

Iu-CS Interface Protocols

From protocol description description, Iu-CS and IU-PS are simply referred to as Iu interface. The following section is written with the reference from following specifications. Source : 3GPP TS 25.410: 3GPP TS 25.411: 3GPP TS 25.412: 3GPP TS 25.413: 3GPP TS 25.414: Signalling’’ 3GPP TS 25.415:

6.5.1

‘‘UTRAN ‘‘UTRAN ‘‘UTRAN ‘‘UTRAN ‘‘UTRAN

Iu Iu Iu Iu Iu

Interface: general aspects and principles’’ Interface Layer 1’’ Interface Signalling Transport’’. Interface RANAP Signalling’’. Interface Data Transport and Transport

‘‘UTRAN Iu Interface User Plane Protocols’’.

Control Plane - Iu-CS

Iu-CS interface connects RNC to the CS-domain of the core network. Therefore, the protocols stack shown here is implemented in RNC and MSC. The protocol stacks for the Iu-CS Domain are shown in figure 6.7. As shown in figure 6.7, there are two options for the realization of transport network, they are: • ATM-based transport, & • IP-based transport

6.5.2

User Plane - Iu-CS

User plane protocols on Iu-CS are shown in figure 6.8. Both ATM-based transport option and the IP-based transport options are shown. 1

TS 25.41x for Iu, 25.42x for Iur and 25.43x for Iub.

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Figure 6.7: Iu-CS control plane protocol architecture (TS 25.410)

Figure 6.8: Iu-CS user plane protocol architecture (TS 25.410)

Figures 6.7 & 6.8 are drawn with the help of Figure 6.1 in 3GPP TS 25.410.

6.5. IU-CS INTERFACE PROTOCOLS

6.5.3

189

RANAP Functions

RANAP provides the signalling service between UTRAN and CN, which are described in 3GPP TS25.413. In this book, we will not dig into the details of each function. A list of the functions performed by RANAP lyer are listed below: Relocating serving RNC, Overall RAB management, Queuing the setup of RAB, Requesting RAB release, Release of all Iu connection resources, SRNS context forwarding function, Controlling overload in the Iu interface, Resetting the Iu, Sending the UE Common ID to the RNC, Paging the user, Transport of NAS information between UE and CN. This function has two subclasses: • 1. Transport of the initial NAS signalling message from the UE to CN. • 2. Transport of subsequent NAS signalling messages between UE and CN. Controlling the security mode in the UTRAN, Controlling location reporting, Location reporting, Data volume reporting function, Reporting general error situations, Location related data, and Information Transfer.

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6.6

Iu-PS Interface Protocols

Iu-PS interface connects RNC to the PS-domain of core network. Therefore, the protocols stack shown here is implemented in RNC and SGSN.

6.6.1

Control Plane - Iu-PS

The protocol stacks for the Iu PS Domain is shown in figure 6.9. The standard allows operators to choose one out of the three standardized protocol suites for transport of SCCP messages.

Figure 6.9: Iu-PS control plane protocol architecture (TS 25.410)

• ATM-based transport, • IP-based transport & • IP over ATM-based transport

6.6.2

User Plane - Iu-PS

There are two options for the transport layer for data streams over Iu-PS.

6.6. IU-PS INTERFACE PROTOCOLS • ATM-based Transport (ATM transport option) • IP-based Transport (IP transport option)

Figure 6.10: Iu-PS user plane protocol architecture (TS 25.410) Figures 6.9 & 6.10 are drawn with the help of Figure 6.3 in 3GPP TS 25.410.

191

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6.7

Iub Interface Protocols Source : 3GPP TS 25.430: ‘‘UTRAN Iub Interface: general aspects and principles’’. 3GPP TS 25.431: ‘‘UTRAN Iub Interface Layer 1’’. 3GPP TS 25.432: ‘‘UTRAN Iub Interface Signalling Transport’’. 3GPP TS 25.433: ‘‘UTRAN NBAP Specification’’. 3GPP TS 25.434: ‘‘UTRAN Iub Interface: Data Transport & Transport Signalling for Common Transport Channel Data Streams’’. 3GPP TS 25.435: ‘‘UTRAN Iub Interface: User Plane Protocols for Common Transport Channel Data Streams’’.

Iub interface is used to connect Node B and RNC. For network operation, they must communicate with each other at regular periods. Whenever a radio link is established, NBAP protocol is used. Similarly, Node reports the measurements about current UL interference and DL transmission power to RNC. Based on these reports, RNC performs Radio Resource Management.

Figure 6.11: Iub control plane protocol architecture (TS 25.430)

6.7.1

Control Plane - Iub CP

The Signalling Bearer for NBAP is a point-to-point protocol. There may be multiple point-to-point links between an RNC and a Node B. As shown in figure 6.11, the standard allows operators to choose one out of two protocol suites for transporting the NBAP messages.

6.7. IUB INTERFACE PROTOCOLS

193

• ATM-based transport, & • IP-based transport

6.7.2

User Plane - Iub UP

This section specifies the transport layers that support Common Transport Channel (FACH, RACH, PCH, DSCH, HS-DSCH) data streams. As usual, there are two options for protocol suites for transport of RACH, FACH, DSCH and HS-DSCH Iub data streams, which are shown in figure 6.12. • ATM-based transport, & • IP-based transport.

Figure 6.12: Iub user plane protocol architecture (TS 25.430)

6.7.3

NBAP Functions

The functions performed by NBAP protocol layer are specified in section 7 of 3GPP TS 25.433. NBAP procedures are divided into common procedures and dedicated procedures. • NBAP common procedures or C-NBAP are procedures that are not related to one particular subscriber or radio link. C-NBAP procedures are common to a cell.

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CHAPTER 6. PROTOCOLS & INTERFACES • NBAP dedicated procedures or D-NBAP are procedures that are related to a specific subscriber who is identified by Node B Communication Context in Node B.

The full NBAP specifications are available in 3GPP TS 25.433. From the same specification, the functions performed by NBAP are listed below: Cell Configuration Management, Common Transport Channel Management, System Information Management, Resource Event Management, Measurements on Common Resources, Radio Link Management, Radio Link Supervision, Compressed Mode Control, Measurements on Dedicated Resources, Reporting of General Error Situations, Physical Shared Channel Management, Information Exchange, Bearer Rearrangement, and MBMS Notification.

6.8. IUR INTERFACE PROTOCOLS

6.8

195

Iur Interface Protocols Source : 3GPP TS 25.420: ‘‘UTRAN Iur interface general aspects and principles’’ 3GPP TS 25.421: ‘‘UTRAN Iur Interface: Layer 1’’ 3GPP TS 25.422: ‘‘UTRAN Iur Interface: Signalling Transport’’ 3GPP TS 25.423: ‘‘UTRAN Iur Interface: RNSAP Signalling’’ 3GPP TS 25.424: ‘‘UTRAN Iur Interface: Data Transport & Transport Signalling’’ 3GPP TS 25.426: ‘‘UTRAN Iur & Iub Interface: Data Transport & Transport Signalling for DCH Data Streams’’.

Iur interface is the link between any two RNCs within the UTRAN. Its main purpose is to handle Inter-RNC mobility within UTRAN and hide this mobility from the core network. If Iur is not present between the two RNCs, then the Inter-RNC soft handover cannot take place. In this case, a hard handover will be performed instead. For multi-vendor operability, it is recommended that Iur should be an open interface. Iur interface is not only used for signalling but also for carrying data streams. RNC-to-RNC interface is a logical description. It can be implemented even if there is no direct physical connection between two RNCs.

6.8.1

Control Plane - Iur CP

Due to the similarity between the control plane protocol stack of Iur and Iu-PS, the description is not given in order to avoid repetition. The protocol stack of control plane signalling over Iur is shown in figure 6.13. The main control plane protocol on Iur interface is RNSAP. The word RNS in UMTS means one RNC and several Node B controlled by it. All three transport options are shown in figure 6.13.

6.8.2

User Plane - Iur UP

The user plane protocol stack on Iur interface is illustrated in figure 6.14. As we can easily identify, the protocol stack resembles the user plane protocol stack on Iub. Therefore, the description can be avoided here as well.

6.8.3

RNSAP functions

The full RNSAP specifications are available in section 7 of 3GPP TS 25.423. The functions performed by RNSAP are listed below:

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Figure 6.13: Iur Interface Protocol Architecture for Control Plane

Figure 6.14: Iur Interface Protocol Architecture for User Plane Radio Link Management, Physical Channel Reconfiguration, Radio Link Supervision, Compressed Mode Control,

6.8. IUR INTERFACE PROTOCOLS Measurements on Dedicated Resources, DCH Rate Control, CCCH Signalling Transfer, Paging, Common Transport Channel Resources Management, Relocation Execution, Reporting of General Error Situations, Measurements on Common Resources, Information Exchange, and Resetting the Iur.

197

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Non-Access Stratum Protocols

Till now, in this chapter we have discussed the access stratum. Now it is time for some NAS signalling. The term access stratum and non-access stratum was explained in section 6.3 at the beginning of this chapter. The fact, that NAS protocols are access-agnostic, is illustrated on figure 6.15. In this figure, there are 2 access technologies, TDMA-based BSS (2G) and CDMA-based UTRAN (3G). The NAS messages are depicted with the bidirectional arrows which flow between UE and core network. The structure of NAS message does not depend on the underlying access network.

Figure 6.15: Principle of NAS signalling In figure 6.16, we can see that there are three sublayers in the overall protocol architecture. These sublayers are: The Access Stratum (AS) sublayer: The AS sublayer performs the duties of a postman and transports NAS signalling messages between UE & core network. The Mobility Management sublayer: The MM sublayer provides its services to CM. The MM sublayer contains two protocol entities: • The MM protocol for mobility related signalling towards the CS core network domain, and • The GMM protocol for mobility related signalling towards the PS core network domain. The Connection Management sublayer: The CM sublayer consists of four basic protocol entities: CC, SM, SMS and SS. If we ignore the AS sublayer and focus on only NAS sublayers, we can conclude that there are following protocol entities which together constitute the NAS domain. Those six entities are identified by their protocol discriminator field as shown in table 6.2. In LTE/EPS, the concept of AS and NAS protocols is reused and the definitions are also not changed. The protocols which carry signalling messages between UE and Evolved

6.9. NON-ACCESS STRATUM PROTOCOLS

199

Figure 6.16: NAS protocols in UMTS Name of NAS protocol Call Control (CC) Mobility Management GPRS Mobility Management SMS Session Management Supplementary Services

Protocol Discriminator 3 5 8 9 10 11

Table 6.2: NAS protocols and the protocol discriminator values Packet Core (EPC) are called NAS protocols and they include 2 protocols: EMM and ESM. The words MM and SM are already known from 2G & 3G, ‘E’ stands for EPS or Evolved packet System.

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Copyright Notices In order to create some figures, tables and text-sections, the following reference material has been used. Information has been interpreted and presented in a simplified manner. The original references are provided here. Main reference material for this book has been technical specifications (TSs) and technical reports (TRs) of 3rd Generation Partnership Project (3GPP).

Text in section 6.5.3 on page 189 Section 7 of 3GPP TS 25.413 v 7.0.0. c ⃝2005. 3GPPTM TSs and TRs are the property of ARIB, ATIS, ETSI, CCSA, TTA, and TTC who jointly own the copyright for them. They are subject to further modifications and are therefore provided to you “as is” for information purposes only. Further use is strictly prohibited. Figure 6.1 on page 173 Figure 10 of 3GPP TS 25.401 v 7.0.0. Figure 6.5 on page 180 Figure 11 of 3GPP TS 25.301 v 7.0.0. Figure 6.6 on page 182 Figure 12 of 3GPP TS 25.301 v 7.0.0. Figure 6.7 on page 188 Figure 6.1 of 3GPP TS 25.410 v 7.0.0. Figure 6.8 on page 188 Figure 6.1 of 3GPP TS 25.410 v 7.0.0. Figure 6.9 on page 190 Figure 6.3 of 3GPP TS 25.410 v 7.0.0. Figure 6.10 on page 191 Figure 6.3 of 3GPP TS 25.410 v 7.0.0. Figure 6.11 on page 192 Figure 7 of 3GPP TS 25.430 v 7.0.0. Figure 6.12 on page 193 Figure 7 of 3GPP TS 25.430 v 7.0.0. Figure 6.13 on page 196 Figure 4 of 3GPP TS 25.420 v 7.0.0. Figure 6.14 on page 196 Figure 4 of 3GPP TS 25.420 v 7.0.0. Text about RRC Protocol func- Section 5.1 of 3GPP TS 25.331 v 6.9.0. tions on page 182 Text about Protocol Layers on Section 11.1.2 of 3GPP TS 25.401 v 7.0.0. page 173 Text about Protocol Planes on Section 11.1.3 of 3GPP TS 25.401 v 7.0.0. page 173 Text in section 6.7.3 on page 193 Section 7 of 3GPP TS 25.433 v 7.0.0. Text in section 6.8.3 on page 195 Section 7 of 3GPP TS 25.423 v 7.0.0. c ⃝2006. 3GPPTM TSs and TRs are the property of ARIB, ATIS, ETSI, CCSA, TTA, and TTC who jointly own the copyright for them. They are subject to further modifications and are therefore provided to you “as is” for information purposes only. Further use is strictly prohibited.

6.9. NON-ACCESS STRATUM PROTOCOLS

Figure 6.3 on page 176 Figure 1 of 3GPP TS 23.107 v 7.0.0. Text in section 6.2.1 on page 177 Section 6.3 of 3GPP TS 23.107 v 7.0.0. c ⃝2007. 3GPPTM TSs and TRs are the property of ARIB, ATIS, ETSI, CCSA, TTA, and TTC who jointly own the copyright for them. They are subject to further modifications and are therefore provided to you “as is” for information purposes only. Further use is strictly prohibited.

Text in section 6.4.4 on page 183 Section 6 of 3GPP TS 25.322 v 9.0.0. Text in section 6.4.6 on page 186 Section 5 of 3GPP TS 25.323 v 9.0.0. c ⃝2010. 3GPPTM TSs and TRs are the property of ARIB, ATIS, ETSI, CCSA, TTA, and TTC who jointly own the copyright for them. They are subject to further modifications and are therefore provided to you “as is” for information purposes only. Further use is strictly prohibited.

201

BIBLIOGRAPHY [1] 3GPP TS 23.107 ver. 7.0.0 ;‘Quality of Service (QoS) concept and architecture’ [2] 3GPP TS 25.301 ver. 7.0.0 ;‘Radio Interface Protocol Architecture’ [3] 3GPP TS 25.321 ver. 7.0.0 ;‘MAC protocol specification’ [4] 3GPP TS 25.322 ver. 7.0.0 ;‘RLC protocol specification’ [5] 3GPP TS 25.323 ver. 7.0.0 ;‘PDCP protocol specification’ [6] 3GPP TS 25.324 ver. 7.0.0 ;‘BMC protocol specification’ [7] 3GPP TS 25.331 ver. 7.0.0 ;‘Radio Resource Control (RRC) protocol specification’ [8] 3GPP TS 25.401 Ver. 7.0.0 ;‘UTRAN overall description’ [9] 3GPP TS 25.410 Ver. 7.0.0 ;‘UTRAN Iu Interface: general aspects and principles’ [10] 3GPP TS 25.411 Ver. 7.0.0 ;‘UTRAN Iu Interface: Layer 1’ [11] 3GPP TS 25.412 Ver. 7.0.0 ;‘UTRAN Iu Interface: Signalling Transport’ [12] 3GPP TS 25.413 Ver. 7.0.0 ;‘UTRAN Iu Interface: RANAP Signalling’ [13] 3GPP TS 25.414 Ver. 7.0.0 ;‘UTRAN Iu Interface: Data Transport and Transport Signalling’ [14] 3GPP TS 25.415 Ver. 7.0.0 ;‘UTRAN Iu Interface: User Plane Protocols’ [15] 3GPP TS 25.420 Ver. 7.0.0 ;‘UTRAN Iur Interface: general aspects and principles’ [16] 3GPP TS 25.421 Ver. 7.0.0 ;‘UTRAN Iur Interface: Layer 1’ [17] 3GPP TS 25.422 Ver. 7.0.0 ;‘UTRAN Iur Interface: Signalling Transport’ [18] 3GPP TS 25.423 Ver. 7.0.0 ;‘UTRAN Iur Interface: RNSAP Signalling’

202

BIBLIOGRAPHY

203

[19] 3GPP TS 25.424 Ver. 7.0.0 ;‘UTRAN Iur Interface: Data Transport and Transport Signalling’ [20] 3GPP TS 25.426 Ver. 7.0.0 ;‘UTRAN Iur & Iub Interface: Data Transport & Transport Signalling for DCH Data Streams’ [21] 3GPP TS 25.430 Ver. 7.0.0 ;‘UTRAN Iub Interface: general aspects and principles’ [22] 3GPP TS 25.431 Ver. 7.0.0 ;‘UTRAN Iub Interface: Layer 1’ [23] 3GPP TS 25.432 Ver. 7.0.0 ;‘UTRAN Iub Interface: Signalling Transport’ [24] 3GPP TS 25.433 Ver. 7.0.0 ;‘UTRAN Iub Interface: NBAP Signalling’ [25] 3GPP TS 25.434 Ver. 7.0.0 ;‘UTRAN Iub Interface: Data Transport & Transport Signalling for Common Transport Channel Data Streams’ [26] 3GPP TS 25.435 Ver. 7.0.0 ;‘UTRAN Iub Interface: User Plane Protocols for Common Transport Channel Data Streams’ [27] H.Holma and A. Toskala, ‘WCDMA for UMTS’ , 5th Edition, John Wiley & Sons. [28] Chris Johnson, ‘Radio Access Networks For UMTS ; Principles And Practice’ , John Wiley & Sons.

For HSDPA-specific details, the version of these specs should be 5.0.0 or higher & for HSUPA-specific details, is should be 6.0.0 or higher.

CHAPTER

7 HIGH SPEED DOWNLINK PACKET ACCESS

Source: 3GPP TS 25.308, High Speed Downlink Packet Access (HSDPA); Overall description; Overview of 3GPP Release 5; available at: http://www.3gpp.org/ftp/Information/WORK PLAN/Description Releases/

7.1

Why HSDPA?

Actually the question should be Why HSPA? HSDPA (and later HSUPA) was designed to overcome the limitations of the Rel-99 WCDMA air interface. If an operator disables HSDPA services from any cell, the maximum bit rate drops from Mbps to kbps range. UMTS in its basic form (Rel-99 and Rel-4) can theoretically achieve 2 Mbps, both in uplink and downlink. But these theoretical numbers are very far from the popular implementation. In most common deployments across the globe, Rel-99 UMTS is able to show the peak bit rates of only 384 kbps and that too with a very limited coverage. Due to this, the end-user experience is very poor. From an operator’s perspective, in order to get high cell throughput, it should be possible to have several simultaneous users with high bit rates. But due to high fractional load of

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7.2. HSDPA STANDARDIZATION, 3GPP RELEASES AND EVOLUTION 205 data bearers, unfortunately only a few simultaneous users are possible. This indirectly also affects the handset manufacturers because all the smart phones, pads and tablets are useless if they cannot provide fast wireless internet access to the subscribers. Let us discuss some limitations of Rel-99 UMTS. End user experience: Due to limited practical bit rates, the end user does not experience good throughput. Poor coverage for data bearers: In WCDMA, coverage is separately calculated for each service. As the service bit rate increases, the coverage area decreases. User must be in excellent radio condition and the cell load should be quite low, only then the user can experience the bit rates of several hundred kbps. Cell capacity: Although the load in cell depends on various factors, but it has been observed that only a few users of 384 kbps bearer can block the entire cell capacity. This is very bad for the operator’s revenue and also network accessibility KPI. Cost of usage: 3G was expected to fulfill the dream which was started by GPRS. Everyone expected that affordable “unlimited data usage” plans will become popular. But unfortunately due to the high cost of operation, it did not happen. Hence, one of the requirements while designing HSDPA was to reduce the cost-per-bit from the operator’s perspective so that more affordable data plans could be introduced. Latency: UMTS experiences very high control plane and user plane latency. Revenue vs. Investment: Due to high cost of spectrum licences, mobile operators expected a huge revenue which unfortunately did not happen. 3G or no 3G?: People often described 3G as “a system with 2 new services – Video telephony and broadband data access”. Video telephony never became popular and data rates in 3G were quite limited. Therefore, the GSM operators were unable to decide whether they should go for 3G or just settle down with EDGE1 . At the same time operators started comparing 3G with Mobile-WiMAX solution.

7.2

HSDPA Standardization, 3GPP Releases and Evolution Source: 3GPP TS 25.306 ; UE Radio Access capabilities

HSDPA is just a milestone in the journey of High Speed Packet Access (HSPA). Due to the urgency and demand of higher bit rates in DL, the HSDPA standard was released and 1

EDGE can offer > 200 kbps (practically) & operators do not need to wait/pay for new 3G licences.

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in the next 3GPP release, its counterpart HSUPA was standardized. The terminology of 3GPP specifications & releases is quite complicated. Therefore, we will try to explain only the most important features of each 3GPP release in terms of PS NRT data access. Table 7.1 describes the new categories that were defined when HSDPA was standardized in Rel. 5. In later releases, newer devices with more advanced features were introduced. The following section will take us through this journey in a few steps.

Release

UE Category

Mod

MIMO

DCHSDPA

Peak Bitrate

Release 5

1 to 12

QPSK, 16QAM

No

No

14.4 Mbps

2X2 with 16QAM

No

28 or 21 Mbps

2X2 with 64QAM

2 Carriers

42 Mbps

Release 6

Same as Rel-5

Release 7

13 to 18

Release 8

19 to 24

QPSK, 16QAM, 64QAM QPSK, 16QAM, 64QAM

Table 7.1: HSDPA features in REL-5, REL-7 & REL-8 (Source TS 25.306, Table 5.1a)

7.2.1

Release 99 & Rel-4

• Basic 3G • Downlink data services available on FACH and DCH transport channels • Uplink data services available on RACH and DCH transport channels • RACH, FACH & DCH are scheduled by RNC’s packet scheduler • Peak bit rates UL: 384 kbps & DL: 384 kbps • No concept of CQI reporting, UE categories etc. • DCH uses a fast power control but no link adaptation mechanism

7.2.2

Release 5

• Commonly called as 3.5G • DL HSDPA operation without UL HSUPA

7.2. HSDPA STANDARDIZATION, 3GPP RELEASES AND EVOLUTION 207 • DL HSDPA and UL R99 DCH • DL packet scheduling is done by Node B based on CQI feedback from the UE • Supported UE categories: 1 to 12 • Peak bit rates in UL: 384 kbps & DL: 14.4 Mbps

7.2.3

Release 6

• HSDPA + HSUPA = HSPA • Just like HSDPA R5, HSPA is also commonly called as 3.5G • DL HSDPA operation is a must for UL HSUPA. Hence, HSPA is a synonym for HSUPA. • Channel scheduling is done by Node B based on feedback from the UE (e.g., data buffer status, power headroom, etc.) • Supported HSDPA UE categories: 1 to 12 (no change compared to R5) • Supported HSUPA UE categories: 1 to 6 • Peak bit rates in UL: 5.76 Mbps & DL: 14.4 Mbps

7.2.4

Release 7

• Commonly called as evolved HSPA or HSPA+ • Supported HSDPA UE categories: 1 to 12 & 13 to 18 • Support of 2 X 2 MIMO or 64QAM modulation in DL • Supported HSUPA UE categories: 1 to 6 & 7 • Supported 16QAM Modulation in UL • Peak bit rates UL: 11.2 Mbps & DL: 28 Mbps

7.2.5

Release 8

• Commonly called as evolved HSPA or HSPA+ (just like Rel-7) • Supported HSDPA UE categories: 1 to 12 & 13 to 18 & 19 to 24 • Support of Simultaneous “64QAM with MIMO operation” or DC-HSDPA

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CHAPTER 7. HIGH SPEED DOWNLINK PACKET ACCESS • Supported HSUPA UE categories: 1 to 6 & 7 • Peak bit rates UL: 11.2 Mbps & DL: 42 Mbps

The points listed in previous section are summarized in table 7.1.

7.3

HSDPA Operation

This section explains the operation of HSDPA from an higher-layer perspective. There will be a detailed discussion about L1 physical channels and L2 protocols in the later sections. In this section, we are trying to find an answer to the question ‘ ‘how does HSDPA operation start?” We will analyze in this process by breaking it into two steps. 1. HSDPA Operation: Between UE and RNC 2. HSDPA Operation: Between Node B and RNC

7.3.1

HSDPA Operation: Between UE and RNC

Figure 7.1: Signalling to initiate an HSDPA session

7.3. HSDPA OPERATION

209

The HSDPA-capable user equipment (mobile phone, smart phone, USB stick modem, tablet etc.) starts the connection setup in the same manner as a R99 device. Therefore, on higher layers (L3 and NAS protocols), there are no HSDPA specific messages and procedures. In other words, the call flow of packet switched connection setup of Rel-99 and Rel-5 are the same which is illustrated in figure 7.1. At the time of transport channel type selection, if the DL transport channel is HS-DSCH, in uplink, RNC can choose either DCH or E-DCH based on the UE device capability. UE is informed about this channel selection by RRC: Radio Bearer Setup or RRC: Radio Bearer Reconfiguration messages. Using this message, UE comes to know about its HSDPAspecific id H-RNTI and the HSDPA configuration of the cell. HSDPA without HSUPA: HS-DSCH in DL and DCH in UL, or HSDPA with HSUPA: HS-DSCH in DL and E-DCH in UL. This option is available only for Rel-6 or newer UEs.

7.3.2

HSDPA Operation: Between Node B and RNC

The most remarkable difference between UMTS and HSDPA is the presence of an additional scheduler in Node B for scheduling the resources to HSDPA users. If the transmitted packets are not acknowledged, then Node B performs re-transmission. Therefore, it is required to buffer the user data at Node B. The transfer of data from RNC to Node B takes place in such a way that the buffer at Node B does not over flow. This procedure is called Iub flow control and accomplished by the two messages illustrated in figure 7.2.

Figure 7.2: Iub Flow Control

Step 1: RNC asks Node B, “How much can I send for a particular UE”? As shown in the first message in figure 7.2, the ‘Capacity Request’ message provides the

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CHAPTER 7. HIGH SPEED DOWNLINK PACKET ACCESS Node B with information regarding the RNC buffer occupancy for a specific priority queue belonging to a specific MAC-d flow.

Step 2: Node B informs RNC about the suitable amount. As shown in the second message in figure 7.2, the ‘Capacity Allocation’ message is sent from the serving Node B to the controlling RNC. Its primary purpose is to provide the RNC with permission to transfer MAC-d PDU belonging to a specific MAC-d flow priority queue towards the Node B at a specific maximum rate. This message has 3 important parameters: 1. number of credits, 2. time interval, and 3. repetition period. For example, if the number of credits is 50, the time interval is 20 ms and the repetition period is 10, then the RNC is permitted to transfer 50 MAC-d PDU every 20 ms during the next 200 ms. A repetition interval of ‘0’ is interpreted as unlimited repetition, i.e., if the repetition period in the previous example was ‘0’, the RNC would be permitted to transfer 50 MAC-d PDU every 20 ms for an indefinite period.

7.4. WHAT’S NEW IN HSDPA?

7.4

211

What’s new in HSDPA?

HSDPA can be better understood by comparing it with the Rel-99 DCH transport channel. In other words, we can focus on the new features introduced for HS-DSCH transport channel. • Adaptive modulation and coding • Shorter and fixed TTI (2 ms) • Node B based packet scheduling • Multi-code Operation • L1 H-ARQ retransmission • MAC-hs protocol in Node B • Serving Cell Change instead of Soft Handover

7.4.1

Adaptive Modulation & Coding

The Node B selects the modulation and the coding for each TTI for each user based on an estimate of the downlink. Each UE reports an indicator of the DL channel quality in the uplink signalling. One of the main drawbacks of R99 DCH channel is its inflexibility. If the UE comes close to Node B, power control decreases the transmission power but the bit rate remains the same. In DCH, bit rate modification is not very easy because the scheduler is located at RNC and it does not know anything about the current radio conditions. In contrast to this, in HSDPA, the transport block size for HS-DSCH channel can be changed every TTI. In other words, 500 times in one second, the bit rates can be adjusted to match the radio conditions. Table 7.2 illustrates the effect of modulation and coding on the net user throughput. The number of codes is also a deciding factor in determining the net bit rates.

7.4.2

Shorter and Fixed TTI

Transmission Time Interval is defined as the inter-arrival time of Transport Block Sets, i.e. the time it should take to transmit a Transport Block Set. In general, if this time is big, then the information bits from higher layer will be buffered at MAC layer before being delivered to the lower layers for transmission.

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CHAPTER 7. HIGH SPEED DOWNLINK PACKET ACCESS Modulation

Coding Rate

# codes

QPSK QPSK QPSK QPSK QPSK QPSK 16QAM 16QAM 16QAM 16QAM 16QAM 16QAM

1/4 1/2 3/4 1/4 1/2 3/4 1/2 3/4 4/4 1/2 3/4 4/4

5 5 5 10 10 10 10 10 10 15 15 15

Gross Bit Rate 2.4 Mbps 2.4 Mbps 2.4 Mbps 4.8 Mbps 4.8 Mbps 4.8 Mbps 9.6 Mbps 9.6 Mbps 9.6 Mbps 14.4 Mbps 14.4 Mbps 14.4 Mbps

Net Bit Rate 600 kbps 1.2 Mbps 1.8 Mbps 1.2 Mbps 2.4 Mbps 3.6 Mbps 4.8 Mbps 7.2 Mbps 9.6 Mbps 7.2 Mbps 10.8 Mbps 14.4 Mbps

Table 7.2: Effect of Modulation and Coding scheme on net bit rate For the DCH Transport channel, TTI can be either 10, 20, 40 or 80 ms. For HS-DSCH, TTI has been fixed and its value is 2 ms. In simple words, every 2ms one2 MAC-hs transport block can be delivered to the physical layer for transmission. Shorter TTI interval helps in reducing the round trip time (RT) for the user plane. If HS-DSCH is used for L3 signalling, then the control plane latency can also be reduced.

7.4.3

Node B-based Packet Scheduling

In R99, the packet scheduling is purely handled by RNC. Due to the dynamic nature of the radio conditions, it is impossible to inform the scheduler about the user’s radio channel’s quality. Therefore, the bit rate upgrade/downgrade is only possible by RNC. To illustrate this, two methods are briefly explained below: High Traffic Volume Measurement: User data might be buffered at UE for Uplink transmission or in RNC for DL transmission: • If the amount of data (in Bytes) buffered in user-specific buffer at RNC’s side exceeds a certain threshold, then RNC automatically tries to upgrade the DL DCH bitrate (For example, DCH128 to DCH256). 2

From R7 onwards, more than one TB can also be transmitted but that is possible only with MIMO. From R8 onwards, DC-HSDPA operation can also deliver 2 TABS per TTI.

7.4. WHAT’S NEW IN HSDPA?

213

• If the amount of data (in Bytes) buffered in user equipment’s buffer exceeds a certain threshold, then UE sends a measurement report to RNC and informs about this event3 . After receiving this measurement report, RNC automatically tries to upgrade the UL DCH bitrate. High Throughput Measurement: If a DCH has been allocated to a user in UL & DL, RNC constantly keeps on measuring the actual throughput in terms of kbps. If the throughput in UL or/and DL drops/exceeds some operator specific thresholds, then the allocated bitrates in that direction can be reduced or increased. This mechanism is called Throughput Based Bitrate Adaptation. Although the two methods explained above are very effective in adjusting the bitrate allocation to the UE’s requirements but this mechanism is very slow and it takes several hundred ms before the bit rate modification takes place. These delays are caused because the scheduler is residing in RNC and the signalling between UE & RNC is not very frequent. By introducing a MAC-hs scheduler at Node B and CQI reporting mechanism, it is possible to look into the instantaneous channel quality and select the scheduled user in current TTI. Furthermore, the TB size in that TTI can also be adapted to the current radio conditions. This is explained in more details in CQI section. In fact, the dynamic sharing of HS-PDSCH among users is only possible if the decisions are made by Node B-based scheduler. This changed behaviour is beneficial for both end-user and the operator. The end-user benefits by always getting the suitable bitrate and reduced number of retransmission. On the other hand, the operator can more often allocate resources to the users in favorable conditions and improve the cell throughput.

7.4.4

Multi-code Operation

• In Rel-99 DCH, the flexibility in bit rates (8, 16, 32, . . ., 384) is achieved by using variable spreading factor from SF4 to SF256. • Whereas, in HSDPA, the SF is fixed to 16. Therefore, the flexibility in bit rates comes from: – varying the number of SF16 codes simultaneously allocated to a user, – varying the modulation scheme, and – varying the channel coding scheme. CQI reporting is a mechanism where UE suggests the Node B about the suitable modulation, number of codes and suitable transport block size. 3

Commonly known as Capacity Request or Event 4a

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A user can be allocated up to 15 HS-PDSCH channel codes. But the instantaneous actual multicode allocation is decided by UE handset category, instantaneous CQI and the current load in the cell. CQI reports one value at a time from the CQI report definition. CQI report definition is a table containing 31 values, each of which is defined with N parameters. These parameters shall consist of one or more of the following: • the transport block size, • the coding rate, • the number of HS-PDSCH codes, • modulation, • power offsets, etc. For every UE category, there is a specifications are readily available Instead, we will use only two table will take a low-end device category

CQI table defined in 3GPP TS 25.214. Since these on 3GPP website, we will not show all the tables. and try to understand its fields. In the examples, we 6 & a high-end device category 14.

CQI table for UE Category 6 HS-DSCH Category 6 UE has following features Modulation: QPSK & 16QAM only Max. # of codes: 5 Category 6 HSDPA device uses CQI table A which is shown in table 7.3

CQI table for UE Category 14 Category 14 has following features Modulation: QPSK, 16QAM & 64QAM Max. # of codes: 15 For example, category 14 device uses CQI table D (from 3GPP TS 25.214, not included in this book), if 64QAM is not configured and table G if 64QAM is configured. CQI table G is shown in table 7.4.

7.4. WHAT’S NEW IN HSDPA? CQI value 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Transport Block Size N/A 137 173 233 317 377 461 650 792 931 1262 1483 1742 2279 2583 3319 3565 4189 4664 5287 5887 6554 7168 7168 7168 7168 7168 7168 7168 7168 7168

215 Number of HS-PDSCH 1 1 1 1 1 1 2 2 2 3 3 3 4 4 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5

Modulation

Reference power adjustment ∆

Out of range QPSK QPSK QPSK QPSK QPSK QPSK QPSK QPSK QPSK QPSK QPSK QPSK QPSK QPSK QPSK 16QAM 16QAM 16QAM 16QAM 16QAM 16QAM 16QAM 16QAM 16QAM 16QAM 16QAM 16QAM 16QAM 16QAM 16QAM

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 -1 -2 -3 -4 -5 -6 -7 -8

Table 7.3: CQI Table A, taken from Table 7A of TS 25.214 Observations from the CQI tables By carefully analyzing the information available in CQI tables and comparing the same for two different device categories, we can make following observations:

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CQI value 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Transport Block Size N/A 136 176 232 320 376 464 648 792 928 1264 1488 1744 2288 2592 3328 3576 4200 4672 5296 5896 6568 7184 9736 11432 14424 15776 21768 26504 32264 38576

Number of HS-PDSCH 1 1 1 1 1 1 2 2 2 3 3 3 4 4 5 5 5 5 5 5 5 5 7 8 10 10 12 13 14 15

Modulation

Reference power adjustment ∆

Out of range QPSK QPSK QPSK QPSK QPSK QPSK QPSK QPSK QPSK QPSK QPSK QPSK QPSK QPSK QPSK 16QAM 16QAM 16QAM 16QAM 16QAM 16QAM 16QAM 16QAM 16QAM 16QAM 64QAM 64QAM 64QAM 64QAM 64QAM

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

Table 7.4: CQI Table G, taken from Table 7G TS 25.214 Observation # 1. TB Size divided by 2ms TTI length gives the MAC-hs throughput. Observation # 2. As the CQI becomes better, TB size, # of HS-PDSCH codes and

7.4. WHAT’S NEW IN HSDPA?

217

modulation scheme is improved. Observation # 3. Cat 6 & Cat 14 UEs have a similar TB size for poor & medium CQIs. Therefore, a high-end device experiences better throughput only in the excellent radio conditions. Observation # 4. 64QAM modulation of Cat. 14 is only available at CQI ≥ 26. Observation # 5. In table 7.3, the maximum TB size, max # of codes and the best modulation is already used at CQI = 15. Therefore, as the CQI becomes better, there is a reference power adjustment factor. This factor is a negative factor whose absolute value increases as the radio channel becomes better.

7.4.5

L1 H-ARQ Retransmission

Here the word ARQ stands for Automatic Retransmission Query. In other words, if the packet received by receiver is erroneous, it will send a negative-acknowledgement which will trigger the retransmission of the same packet. Some books also call it backward error correction (BEC) because we take the action after the errors have occurred. In contrast to backward error correction, there is another scheme called forward error correction (FEC) or channel coding. In FEC, we add some extra redundant bits to improve the channel conditions and to fight against the errors. FEC is called so because FEC steps are performed at the transmitter end before the errors have actually occurred. The scheme used in HSDPA is a mixture of both FEC and BEC. Therefore, it is called hybrid -ARQ. The specialty of HSDPA is that this H-ARQ happens at MAC-hs layer between Node B and UE. UE also needs to play an active part in this scheme. If UE receives a packet with a lot of errors, it sends a negative-Ack and stores a copy of this erroneous reception. Later on, after receiving the retransmitted packet, UE has to softly combine these two versions. This soft combining capacity is decided by the number of soft channel bits in the handset. Retransmission on negative acknowledgement can be done in many ways, e.g., • Stop-and-Wait • Go Back ‘N’ • Selective Repeat • ... The method chosen for HSDPA retransmission is Stop-and-Wait (SAW) procedure, where the transmitter sends a transport block and waits for the receiver’s response before sending a new block or retransmitting the erroneous one.

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In its basic form, stop-and-wait mechanism is not very efficient, since the transmitter is inactive until it gets a response. This eventually reduces the throughput. Therefore, in HSDPA, 3GPP chose a smart way to improve the existing stop-and-wait algorithm. In HSDPA, Node B can configure up to six parallel H-ARQ processes active for one user. While Node B is waiting for a feedback from UE for process # 1, data can be transmitted from process # 2, 3, 4, 5 or 6. Hence, Node B can transmit without interrupting the data flow.

Figure 7.3: Ilustration of parallel processes of HSDPA H-ARQ scheme

7.4.6

MAC-hs Protocol in Node B and UE

It won’t be any exaggeration if we say that MAC-hs is the brain of HSDPA. Prior to HSDPA, MAC layer was implemented at RNC. Therefore, only RNC had the authority to perform packet scheduling for Rel-99 channels. Please refer to section 7.5 for detailed information about MAC-hs protocols and also its interworking with it others protocols. In short, the MAC-hs protocol is responsible for: • packet scheduling, • transport format combination (TFC) selection, • L1 Hybrid-ARQ, and • Iub flow control etc.

7.4. WHAT’S NEW IN HSDPA?

7.4.7

219

Serving Cell Change Instead of Soft HO

UE in HSDPA connected mode does not perform soft handover. While moving from one HSDPA cell to another HSDPA cell, UE undergoes serving cell change. As a result of serving cell change, UE stops receiving data from one Node B and starts receiving from another one. This topic is explained in more details in carefully in section 7.10. In short, the serving cell change mechanism is divided into three phases (as shown in figure 7.4).

Figure 7.4: HS-DSCH Serving Cell Change; 3 phases The figure 7.4, explains the serving cell change procedure by dividing into three chronological steps. 1. When UE is in Source Cell: UE has no radio link with the target cell. HS-DSCH as well as the associated-DCH channels are only between the UE and the Node B of the source cell. 2. When UE is in overlapping area of the 2 cells: In the overlapping area, UE sends a measurement report to RNC and performs soft handover for the associated-DCH (A-DCH) channel. But the HS-DSCH channel is only received from the source cell. 3. When UE is in Target Cell After leaving the overlapping area, if UE comes to the target cell’s area, it will maintain the radio link only with the Node B of the target cell.

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When the cell change action is triggered, there is always some interruption of HS-DSCH but the end-user sees it as a reduced throughput. During this procedure, the user data buffered at old Node B cannot be transferred to the new Node B. In case of Inter-Node B serving cell change, the old Node B flushes (or discards) the buffered data and the new Node B receives the same data from RNC. In case of Intra-Node B serving cell change, both the source and the target cells are served by the same Node B. Therefore, the same data can be delivered to UE in the target cell.

7.5. HSDPA PROTOCOL ARCHITECTURE

7.5

221

HSDPA Protocol Architecture

Figure 7.5: MAC-hs Protocol in UE and Node B Figure 7.5 shows the user plane protocol stack of HSDPA operation. In conventional Rel99 operation, MAC-d PDUs are transmitted from RNC to Node B using Frame Protocol (FP) for DCH. For HSDPA operation, a new ‘FP for HS-DSCH’ is introduced. MAC-d flows from RNC are buffered at Node B. The data flow on Iub is controlled by MAC-hs protocol and the procedure is known as Iub Flow Control. Figure 7.5 illustrates a FP data frame from RNC to Node B. After getting the CQI reports from UE, Node B can decide the size of MAC-hs transport block, which is used by Node B to calculate the number of MAC-d PDUs that can be multiplexed in MAC-hs PDU. Please note that the size of MAC-hs PDU can change every TTI. Therefore, UE must also be informed about it. The information about the number of MAC-d PDUs and their sizes is signalled to UE using the header field of MAC-hs PDU.

7.5.1

MAC-hs entity - UE Side

According to section 4.2.3.3 of 3GPP TS 25.321, the functions performed by MAC-hs entity on UE side is depicted in figure 7.6. These functions are listed in table 7.5. Let’s discuss them one-by-one. 1. HARQ: The HARQ functional entity handles all the tasks that are required for hybrid ARQ. It is responsible for generating ACKs or NACKs. In the case of retransmission, UE has to perform soft combining of previous erroneous reception and new received transport block. There are two popular algorithms for this: Chase Combining CC and Incremental Redundancy IR. These two schemes are described using figure 7.7.

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On UE side

1. Flow Control

1. HARQ

2. Scheduling/Priority Handling

2. Reordering Queue distribution:

3. HARQ

3. Reordering

4. TFRC selection

4. Disassembly

Table 7.5: Summary of MAC-hs protocol functions

Figure 7.6: MAC-hs Protocol in UE Side (from TS 25.321) Chase Combining: Chase combining is also called as ‘identical retransmission’. As shown in the upper part of figure 7.7, in chase combining, when Node B gets a negative acknowledgement for an MAC-hs transport block, it retransmits exactly the same amount of bits as the original transmission. In other words, the same Redundancy Version (RV) is used for the original transmission and re-transmission. We can also say that the coding schemes used in

7.5. HSDPA PROTOCOL ARCHITECTURE

223

Figure 7.7: Chase Combining (upper) and Incremental Redundancy (lower) schemes for HSDPA HARQ transmission and subsequent re-transmissions are identical. Incremental Redundancy: Incremental redundancy scheme is illustrated in the lower subfigure of figure 7.7. IR has the liberty to change the redundancy version after getting a negative acknowledgement. 2. Reordering Queue distribution: Using the QUEUE ID field of MAC-hs header, UE’s MAC-hs protocol entity forwards the MAC-hs PDUs to the correct reordering buffer. 3. Reordering: On UE side, one reordering entity is configured for each Queue ID. The main purpose of this function is to deliver the MAC-hs PDUs with consecutive Trasnmission Sequence Number (TSN) to the disassembly function. If MAC-hs PDUs with lower TSN are missing, MAC-hs PDUs are not delivered to the disassembly function. 4. Disassembly: A MAC-hs PDU contains three fields: (1) MAC-hs header, (2) MAC-d PDUs & and (3) padding bits. Disassembly function removes the other two parts and extracts the useful part, which is MAC-d PDUs. These MAC-d PDUs are delivered to the MAC-d protocol layer.

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7.5.2

MAC-hs entity - UTRAN Side

In the previous section, we discussed the MAC-hs entity on the UE side. Now we will focus on the same protocol entity on UTRAN side. In UTRAN, MAC-hs is implemented in Node B which is shown in figure 7.8. These functions are described in section 4.2.4.3 of 3GPP TS 25.321 and listed in table 7.5.

Figure 7.8: MAC-hs Protocol in UTRAN Side (from TS 25.321) 1. Flow Control: Flow control mechanism has already been discussed in section 7.3 and figure 7.2. Flow control function in MAC-d provides a controlled data flow between the MACd (RNC) and MAC-hs (Node B), taking the transmission capabilities of the air interface into account in a dynamic manner. In other words, the flow control’s function is to negotiate the numbers of MAC-d PDUs transferred from RNC to Node B. Node B is in a better position to decide this for individual HSDPA user because of the received CQI feedback from each user. The aim of flow control is to limit the layer 2 signalling latency and minimize the discarded and retransmitted data which can happen due to HS-DSCH congestion.

7.5. HSDPA PROTOCOL ARCHITECTURE

225

In case of congestion, Node B can decrease the number of credits which means RNC will send less amount of MAC-d data. This avoids buffer overflow and makes the Iub transmission more effective. Flow control is provided independently by MAC-d flow for a given MAC-hs entity. 2. Scheduling & Priority Handling: Every TTI Node B has to allocate HS-DSCH resources between HARQ entities and data flows according to their priority class. Based on UE’s feedback in uplink, Node B decides whether new transmission or retransmission should be transmitted. 3. HARQ: One HARQ entity is responsible for managing the hybrid ARQ functionality for one user. As explained in an earlier section (see figure 7.3), we can have up to six parallel processes per HARQ entity. These multiple processes are used to avoid the interruption in continuous data flow caused by stop-and-wait HARQ algorithm. There can be only one HARQ process per HS-DSCH per TTI. In HSDPA, up to six parallel HARQ processes can be configured. There can be one HARQ process per TTI, whose identity is signalled to UE using L1 signalling. For more information, please read about the information delivered on HS-SCCH channel in a later part of this chapter (section 7.6.2). 4. TFRC selection: Transport Format and Resource Combination selection for the data to be transmitted on HS-DSCH is very strongly attached to the link adaptation. As discussed earlier, after receiving the feedback from UE, Node B decides: • the size of MAC-hs Transport block, • number of HS codes, • channel coding rate, & • modulation scheme etc.

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Channels and Physical Layer Source: 3GPP TS 25.211 Physical channels and mapping of transport channels onto physical channels

In chapter 4, we discussed about the logical, transport and physical channels and restricted our discussion to only Release 99 channels. With HSDPA development, there is no new logical channel introduced in the system. But a new transport channel is designed which is known as High speed- downlink shared channel (HS-DSCH). This transport channel is specially used to carry DTCH logical channel4 . In R99, the logical channel DTCH5 could be mapped to FACH and DCH transport channels. But with Rel-5, RNC has the options to select from the choices shown below. From Rel-5 onwards, the DL logical channel DTCH is mapped to:  if data volume is very small  FACH DCH if data volume is large but HSDPA not supported =  HS-DSCH if data volume is large and HSDPA is supported Supported means that both UE device and the UTRAN must be capable of HSDPA functionality. The transport channel HS-DSCH is further mapped to HS-PDSCH (High Speed-Physical Downlink Shared Channel). Log. Ch. DTCH → Tra. Ch. HS-DSCH → Phy. Ch. HS-PDSCH In the final section of chapter 4, there was brief overview of the HSDPA related channels in section 4.6. For clarity, the same figure is shown in figure 7.9. Now, we will focus on the functionality of L1 signalling for HSDPA operation. As shown in figure 7.1, the decision to use HS-DSCH is taken by RNC. After this, RNC informs Node B and UE about the HSDPA configuration to kick-start the HSDPA operation. RNC can only select the transport channel type but the actual scheduling is done by Node B. As we know, HS-DSCH is a shared channel which is shared among all the users in a cell. Node B has to notify the UEs about its scheduling decisions. The procedure described in figure 7.9 can be understood in 4 steps. Step 1: Every UE reports its radio conditions in the form of a Channel Quality Indicator (CQI ). The UL channel used for this feedback is HS-DPCCH. 4

Optionally DCCH can also be carried by HS-DSCH. This is called Signalling Radio Bearer (SRB) on HSPA. 5 Dedicated Traffic Channel

7.6. CHANNELS AND PHYSICAL LAYER

227

Figure 7.9: HSDPA related physical channels Step 2: The MAC-hs scheduler at Node B calculates the Priority Metric for all the users and selects the User (or Users)6 , who will get scheduled in the next TTI. Each scheduled user is individually notified using an HS-SCCH channel. Step 3: Exactly 2 slots after the HS-SCCH, Node B transmits data on HS-PDSCH channel to the scheduled users. There can be maximum 15 HS-PDSCH per cell. One user can be allocated 1 to 15 HS-PDSCH codes. Therefore, it is also possible to allocate the whole cell resources to one user. Step 4: After receiving and decoding the data, each scheduled UE transmits ACK or NACK in UL. The uplink channel used for this purpose the same as used in step 1, that is, HS-DPCCH. In the section below, we will try to investigate these 3 physical channels in more depth.

7.6.1

HS-DPCCH

HS-DPCCH is a dedicated UL channel for sending HSDPA related feedback information to Node B.

• HS-DPCCH is a dedicated channel. In simple words, if there are, for example, 50 users in HSDPA active mode, then each user will transmit its own UL feedback channel. • The timing of HS-DPCCH is organized in sub-frame which is 2ms or 3 slots long. 6

The number of scheduled users is decided by the number of HS-SCCH configured in the cell. We can have at least one and at most 4 such channels. The most popular choices are 3 and 4.

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Figure 7.10: Frame structure for uplink HS-DPCCH (TS 25.211) • The HARQ-ACK is carried in the first slot of the HS-DPCCH sub-frame. • The CQI is carried in the second and third slot of a HS-DPCCH sub-frame. • SF for HS-DPCCH is 256. • Bit Rate = 15 kbps. Therefore, 30 bits can be sent in UL every TTI. • 10 bits are used for ACK/NACK and 20 bits for CQI7 . • CQI repetition cycle can be configured by operator. Figure 7.10 shows that there are ‘N’ users in a cell and every UE is sending L1 feedback in uplink using HS-DPCCH channel. The same figure also shows that a 10 ms radio frame is broken down into 5 sub-frames of 2ms. Each sub-frame can accommodate three slots and these three slots of HS-DPCCH sub-frame carry two fields. 1. CQI: An active HSDPA UE is bound to report the DL channel conditions back to the Node B. The network signals the periodicity of channel condition indicator (CQI ) reporting, and whether it is repeated (optionally). The UE measures the received P-CPICH & uses a proprietary algorithm to calculate CQI. CQI value also strongly depends on the ratio of HS-PDSCH Power to Total Carrier power. For example, 7

After channel coding 1 bit of ack/nack becomes 10 bits and 5 bits of CQI become 20 bits

7.6. CHANNELS AND PHYSICAL LAYER

229

6W out of 20W is allocated to HSDPA. Therefore, UE must get this information by higher layer signalling. The reported value indicates the maximum amount of data the UE estimates it could receive given the current channel conditions and UE capabilities. The network can then use this value as a guideline when it schedules the next block of data. Node B can of course, perform some vendor specific compensation to this reported CQI. There are 30 different CQI values for each UE category, so a CQI can be addressed using 5 bits. However, CQI values are coded using a robust (20,5) code, so the channel coder output is 20 bits long and fills completely the two slots allocated for CQI. 2. ACK/NACK: After the UE has received the HS-PDSCH frame and successfully decoded it, it has to send an ACK (or NACK in case of errors) back to Node B using a HS-DPCCH channel. The UE has approx. 7.5 slots (5 ms) to complete this procedure. ACK/NACK channel coding is very robust, because the input consists of only one bit (ACK=1, NACK=0), and the channel coder simply repeats this ten times, so the output is ten bits long.

7.6.2

HS-SCCH

Figure 7.11: Subframe structure for the HS-SCCH (TS 25.211)

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CHAPTER 7. HIGH SPEED DOWNLINK PACKET ACCESS HS-SCCH is a common DL channel which can be read by every HSDPA users. Each user reads this channel (or channels) every 2 ms to find out if he has been scheduled. If not: UE ignores the content of HS-SCCH (or HS-SCCHs). If Yes: UE finds out which codes, how many codes, which modulation &, which Transport Block (TB) Size has been scheduled for him. • HS-SCCH is a common channel. Operator can configure 1, 2, 3 or 4 such channels per cell. • SF for HS-SCCH is 128. • Bit Rate = 60 kbps. Therefore, 120 bits can be sent in DL on each channel every TTI. • It is used to informs all the UEs how and when to receive the HS-PDSCH. • For example, if 3 such channels are configured then 15 HS-PDSCH codes can be divided into 3 ‘blocks’ every TTI ( e.g., ‘5+5+5’ or ‘2+8+5’ or ‘3+10+2’ etc.).

Figure 7.11 shows a cell with several users. In this example, the cell has been configured with only one HS-SCCH channel. In this example, only one UE can be scheduled in one TTI. Therefore, the cell uses pure time-multiplexing principle. It is also allowed to have more than one HS-SCCH in a cell. This alternative, increases the overhead in code and power domain but allows the operator to serve more than one UE in one TTI which allows us to have code multiplexing of resources. Figure 7.11 also shows that HS-SCCH transmission is two slots ahead of actual data transmission of HS-PDSCH. In short, we can say that HS-SCCH informs and prepares the UE to receive HSDPA data on shared resources. HS-SCCH channel carries the following fields: . Channelization Code Set, 7 bits: The CCS field indicates the number of SF16 codes and the code offset that are used for the HS-DSCH during the specific 2 ms TTI. Modulation Type, 1 bit: This bit indicates the modulation type. In Rel-5 & Rel-6, there are only two options. Therefore, one bit is sufficient. But from Release 7, the third option of 64QAM is also available. Hence, if 64QAM is configured in the cell, then 7 bits of CCS and 1 bit modulation type should be considered together to identify the modulation. Transport Block Size, 6 bits: Hybrid-ARQ Process ID, 3 bits: Redundancy and Constellation Version, 3 bits:

7.6. CHANNELS AND PHYSICAL LAYER

231

New Data Indicator, 1 bit: This bit toggles (0 to 1 or 1 to 0) for every new transmission and remains the same in case of retransmission. • 7 bits of ‘Channelization Code Set’ and 1 bit of ‘Modulation Type’ are multiplexed together. These 8 bits are channel coded and the result is sent on the first slot of HS-SCCH sub-frame. • 6 bits of ‘Transport Block Size’, 3 bits of ‘HARQ process ID’, 3 bits of ‘Redundancy and Constellation Version’ & 1 bit of ‘New Data Indicator’ fields are multiplexed together. These 13 bits are channel coded and sent on 2nd and 3rd slot of HS-SCCH sub-frame. Several times, it has been stated that HS-SCCH carries the UE identity. But that identity field is missing from the list shown above. This list is actually copied from section 4.6.2 of 3GPP TS 25.212. Are we missing something? Yes, we are missing the concept of masking UE identity on the CRC field. From the aforementioned 21 bits (8 + 13 bits) of HS-SCCH fields, a 16-bit CRC is calculated by Node B. The CRC is masked with a 16-bit user specific identity called H-RNTI. H-RNTI is allocated by RNC at the time of radio bearer setup or radio bearer reconfiguration, if the HS-DSCH transport channel is selected. Although HS-SCCH transmission is on three slots of a sub-frame, UE can read the UE identity from the first slot itself. UE must monitor all HS-SCCHs in the HS-SCCH set. If the UE did detect control information intended for this UE in the previous subframe, it is sufficient to only monitor the same HS-SCCH used in the previous subframe. If a UE detects that one of the monitored HS-SCCHs carries control information intended for this UE, the UE shall start receiving the HS-PDSCHs indicated by this control information. For more details, readers are advised to refer to 3GPP TS 25.212; Multiplexing and channel coding (FDD).

7.6.3

HS-PDSCH

HS-PDSCH is the main DL channel which carries DL data for the subscribers. • HS-PDSCH is a shared channel. There can be up to 15 such channels per cell • SF for HS-PDSCH is 16 • Bit rate = 240 ksps per code8 • No soft handover 8

To get kbps, multiply by #bits per symbol, 2 for QPSK, 4 for 16QAM and 6 for 64QAM

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Figure 7.12: Subframe structure for the HS-PDSCH (TS 25.211)

• No fast inner-loop power control

The High Speed Physical Downlink Shared Channel (HS- PDSCH) is used to carry the High Speed Downlink Shared Channel (HS-DSCH). A HS-PDSCH corresponds to one channelization code of fixed spreading factor SF=16 from the set of channelization codes reserved for HS-DSCH transmission. Multi-code transmission is allowed, which translates to UE being assigned multiple channelization codes in the same HS-PDSCH subframe, depending on its UE capability. According to the principles of channelization codes, there are 16 codes of SF=16, but one of them CC16,0 is forbidden to use because a SF 256 (CC256,0 ) code from the same branch is used for P-CPICH in the same cell. Therefore, to maintain orthogonality on DL, it is decided to use only 15 codes for HSDPA transmission. This concept is described in figure 7.12. The same figure also shows the subframe and slot structure of HS-PDSCH. An HS-PDSCH may use QPSK, 16QAM or 64QAM modulation symbols. All relevant Layer 1 information is transmitted in the associated HS-SCCH i.e. the HS-PDSCH does not carry any Layer 1 information. The slot formats for HS-PDSCH are shown in table 7.6. The three rows of this table emphasize the effect of modulation on channel bit rate.

7.6. CHANNELS AND PHYSICAL LAYER Slot format #i 0 (QPSK) 1 (16QAM) 2 (64QAM)

Channel Bit Rate (kbps) 480 960 1440

Channel Symbol Rate (ksps) 240 240 240

233 SF 16 16 16

Bits/ HS-DSCH subframe 960 1920 2880

Table 7.6: HS-DSCH fields (TS 25.211)

Figure 7.13: All Channels in REL-5 Configuration (including A-DCH)

7.6.4

Associated DCH

A-DCH or associated DCH is the new name used for the well-known R99 DCH channels when these channels are used in association with HSDPA channels. Uplink: In UL, the Control channel (DPCCH) and Data channel (DPDCH) are code multiplexed. DPCCH is used for carrying L1 Control9 bits & DPDCH is used for carrying for user data and signalling radio bearer (SRB or L3 signalling) Downlink: In DL Control channel and Data channel are time multiplexed. The multiplexed channel is called DPCH. Hence, DPCH is used for L1 control, User Data and SRB. One again, we would emphasize that A-DCHs are dedicated channels. Therefore, if there are 50 active HSDPA users then there will be 50 UL channels and 50 DL channels. Due to this, every active user’s A-DCH will cause additional UL interference and DL code & power congestion. 9

TFCI, Pilot Bits and TPC

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Figure 7.14: Fractional DPCH Channel as reduced version DL DPCH (TS 25.211)

7.6.5

Fractional-DPCH

As the name implies, F-DPCH is a fractional version of the normal DL DPCH channel. For every active HSDPA user, one DL DPCH is needed but one FDPCH can be used by upto 10 users. This solves DL code and power congestion up to some extent. An F-DPCH carries control information generated at layer 1 (TPC commands) for one uplink DPCCH.

As explained in section 7.6.4, every active HSDPA user requires one SF256 from DL channelization code tree. At the time of writing this book (August 2012), vendors are supporting more than 70 active HSDPA users per cell. If conventional A-DCH is used, then for every active user a DL SF256 code will be reserved for DPCH. To solve this problem, 3GPP has introduced DL Fractional-DPCH which can be used as a replacement for DL DPCH. But there are a few prerequisites for using F-DPCH. • F-DPCH is possible for HSDPA+HSUPA. • SRB on HSPA must be configured because DL RRC signalling cannot be conveyed on F-DPCH.

As shown in figure 7.14, Normal DPCH with SF 256 can be used to transmit 20 bits per time slot. But in Fractional DPCH, the transmitter is ‘OFF’ for 18 bits and ‘ON’ for only two bits. These two bits are DL TPC (Transmit Power Control) command. The users are allocated a slot format number (0, 1, 2, . . . , 9). Based on the slot number, UE finds out when TPC bits are transmitted for him. In the remaining 90% of time, other nine users are provided with their respective TPC commands. In the same figure, an example of slot format # 4 is shown. The exact definition of each slot format # can be found in table 7.7.

7.7. TIMING OF HSDPA CHANNELS Slot Format # 0 1 2 3 4 5 6 7 8 9

SF 256 256 256 256 256 256 256 256 256 256

Total Bits/Slot 20 20 20 20 20 20 20 20 20 20

235 NOFF1 Bits/Slot 2 4 6 8 10 12 14 16 18 0

NTPC Bits/Slot 2 2 2 2 2 2 2 2 2 2

NOFF2 Bits/Slot 16 14 12 10 8 6 4 2 0 18

Table 7.7: F-DPCH fields (from 3GPP TS 25.211)

7.7

Timing of HSDPA Channels Source: 3GPP TS 25.211; section 7; Timing relationship between physical channels

A simplified HSDPA operation is depicted in figure 7.15. In the example shown in this figure, we have assumed that there is only one HS-SCCH in the cell and the UEs are expected to send CQI reports every 2 ms. UE # 1 is scheduled in first TTI, UE #2 and UE # 3 in the 2 next TTIs and UE #2 is again scheduled in the 4th TTI. The same figure (fig. 7.15) also shows the behaviour of UE # 1 and UE # 2 from the reception and transmission perspective. x It can be seen that CQI reports are sent periodically. If the HSDPA user gets scheduled, it receives data and sends either positive or negative acknowledgement. A/NACK are sent on the first time slot of HS-DPCCH channel. Timing of HS-SCCH: This downlink channel has the same reference and frame timing as P-SCH, S-SCH, P-CPICH and P-CCPCH. The start of HS-SCCH subframe #0 is aligned with the start of the P-CCPCH frames. Timing of HS-PDSCH: Figure 7.15 illustrates the timing structure for the HS-DSCH control signalling. The fixed time offset between the HS-SCCH information and the start of the corresponding HS-DSCH TTI equals 2 × time slots (2*Tslot=5120chips). Timing of HS-DPCCH: The timing of HS-DPCCH is calculated in relation to the DL HS-PDSCH reception time and UL DPCCH/DPDCH transmission time. The relative timing between DPCCH/DPDCH and HS-DPCCH is kept constant.

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Figure 7.15: Summary of HSDPA operation and timing

• The start of HS-DPCCH subframe which carries Ack/Nack for the received HS-PDSCH data is approx. 7.5 Time slot after the reception of corresponding HS-PDSCH subframe at UE receiver. • The time offset between UL DPCCH/DPDCH and HS-DPCCH is a multiple of 256 chip (n × 256 chips).

7.8

HSDPA UE Categories

Quite often the network performance is limited by the population of low-end HSDPA devices on the network. Therefore, it is quite important to learn about the maximum bit rates that can be achieved by a certain UE category. Every 3GPP release has added new functionalities to HSDPA operation and thereby defined new device categories. According to 3GPP Rel-9, there are 28 HSDPA UE categories, whose details are readily available in 3GPP TS 24.306. The purpose of this book is to make the learning easier. Therefore, we would focus on the device categories according to each release.

7.9. HSDPA PEAK BITRATE CALCULATION Category 11 & 12 1 to 6 7&8 9 & 10

Modulation Only QPSK QPSK & 16QAM

237 Max. Codes 5 5 10 15

Table 7.8: UE categories according to Rel-5 & Rel-6 Category 11 & 12 1 to 6 7&8 9 & 10 13 & 14 15 & 16 17 & 18 17 & 18

Modulation Only QPSK QPSK & 16QAM QPSK & 16QAM QPSK & 16QAM QPSK & 16QAM & 64QAM QPSK & 16QAM QPSK & 16QAM & 64AM QPSK & 16QAM

Max. Codes 5 5 10 15 15 15 15 15

MIMO Support No No No No No Yes No Yes

Table 7.9: UE categories according to Rel-5, Rel-6 & Rel-7

7.9

HSDPA Peak Bitrate Calculation

In this section, we will investigate the maximum bit rates that can be achieved with an HSDPA device of certain category. The word maximum here means the peak instantaneous bit rate for 2ms TTI. In order to calculate the average throughput, we should also consider those TTIs where the user was not scheduled. ] [ Rchip Data Rate per code = Symbols/second SF Bit Rate per code = [Data Rate [ksps] × Bits per Symbol] kbps ,   2 if Modulation is QPSK, 4 if Modulation is 16QAM, Bits per Symbol =  6 if Modulation is 64QAM . Max. Gross Bit Rate = [Bit Rate per code × Max. # of codes supported] kbps Max. Net Bit Rate = [Gross Bit Rate] × [Channel Coding Rate] kbps Let us take examples of device categories 12, 6, 8, 10, 14 & 16 and calculate the peak net bit rates achieved. In this example, we will assume channel coding rate of 3/4. Please refer to table 7.10.

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UE Cat. 12 6 8 10 14

Symbol Rate 240 Ksps 240 Ksps 240 Ksps 240 Ksps 240 Ksps

Best Modulation QPSK 16QAM 16QAM 16QAM 64QAM

480 kbps 960 kbps 960 kbps 960 kbps 1440 kbps

# codes 5 5 10 15 15

16

240 Ksps

16QAM

960 kbps

15

Bit Rate

Rbit (Gross) 2.4 Mbps 4.8 Mbps 9.6 Mbps 14.4 Mbps 21.6 Mbps 28.8 Mbps10

Rbit (Net) 1.8 Mbps 3.6 Mbps 7.2 Mbps 10.8 Mbps 16.2 Mbps 1.8 Mbps

Table 7.10: Example of peak bit rate calculation for several devices categories While doing the same calculation for a UE which supports MIMO operation, the final result can be multiplied by 2. Because in the MIMO scheme, where 2 transport blocks are multiplexed on the same TTI, the peak bit rates are doubled.

7.10. SERVING HS-DSCH CELL CHANGE

7.10

239

Serving HS-DSCH Cell Change

Figure 7.16: Inter-Node B serving HS-DSCH cell change (TS 25.308)

According to 3GPP TS 25.308, “A serving HS-DSCH cell change facilitates the transfer of the role of ‘serving HS-DSCH radio link’ from one radio link belonging to the source HS-DSCH cell to a radio link belonging to the target HS-DSCH cell”. As discussed in chapter 5, mobile in CELL DCH mode performs soft-handover or hardhandover in order to maintain the connectivity with UTRAN. However, for HS-PDSCH allocation for a given UE belongs to only one of the radio links assigned to the UE, the serving HS-DSCH radio link. The cell associated with the serving HS-DSCH radio link is defined as the serving HS-DSCH cell. While moving, UE can perform serving HS-DSCH cell change. Quite often, people call it HSDPA Serving Cell Change (SCC). This mechanism is almost similar to a hard handover with a small difference that during transition UE may perform soft handover on A-DCH channels with source and target cells. Hence for HS-DSCH, UE does not perform Soft handover but for the associatedDCH (A-DCH) it does. The source and the target cells can be controlled by the same Node B or two different Node Bs. Thus, we need to discuss two different mobility scenarios: In 3GPP 25.308, several ways to classify the Serving HS-DSCH Cell change procedures are defined. We will discuss the classification which is based on the serving HS-DSCH Node B. The signalling scenarios related to these procedures are discussed in chapter 9.

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Intra-Node B serving HS-DSCH cell change: In this scenario, the source and the target cells are two adjacent sectors of the same site (Node B). Therefore, the unacknowledged data which is buffered at Node B can be transmitted to the user using new radio link. There is no need to flush the data. Intra-Node B SCC has less interruption in service. Inter-Node B serving HS-DSCH cell change: In contrast to the earlier case, in this case, the source and the target cells are controlled by two different Node Bs. Therefore, when the user moves into the new cell, the unacknowledged buffer data at old Node B must be flushed and the new Node B must get the same from RNC. As expected, this causes delay and increases the service interruption time. For UE it is irrelevant whether the serving HS-DSCH cell change procedure is of a intraNode B or inter-Node B nature. The cell change decisions are always made by UTRAN. Hence SCC procedure of HSDPA is known as network-controlled serving HS-DSCH cell change. A network controlled HS-DSCH cell change is performed as an RRC layer signalling procedure and is based on the existing handover procedures in CELL DCH state. The detailed signalling between UE and RNC related to both Inter-Node B and IntraNode B Serving Cell Change is described in the in chapter 9 along with other intersting signalling scenarios related to UMTS and HSPA.

7.11. SUMMARY: HSDPA OPERATION IN SHORT

7.11

241

Summary: HSDPA Operation in Short

The whole communication between UE and Node B can be explained using the 3 physical channels designed for HSDPA operation. This procedure is illustrated in figure 7.17. In short, the various steps are as following:

Figure 7.17: HSDPA operation explained using the physical channels. Channel

Direction

HS-PDSCH

↓

Function Carries DL user data

SF

HS-SCCH

↓

Carries scheduling info

128

QPSK

HSDPCCH

↑

Used to send UL feedback

256

BPSK

16

Modulation QPSK & 16QAM

Table 7.11: Summary of HSDPA channels

Ch. Coding 1/3 Turbo coding 1/3 Convolutional coding ACK = ‘1111111111’ NACK = ‘0000000000’ CQI = (20,5) Block coding

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Copyright Notices In order to create some figures, tables and text-sections, the following reference material has been used. Information has been interpreted and presented in a simplified manner. The original references are provided here. Main reference material for this book has been technical specifications (TSs) and technical reports (TRs) of 3rd Generation Partnership Project (3GPP).

Table 7.3 on page 215 Table 7A of 3GPP TS 25.214 v 9.1.0. Table 7.4 on page 216 Table 7G of 3GPP TS 25.214 v 9.1.0. Figure 7.10 on page 228 Figure 2A of 3GPP TS 25.211 v 9.1.0. Figure 7.11 on page 229 Figure 26A of 3GPP TS 25.211 v 9.1.0. Figure 7.12 on page 232 Figure 26B of 3GPP TS 25.211 v 9.1.0. Table 7.6 on page 233 Table 26 of 3GPP TS 25.211 v 9.1.0. Figure 7.14 on page 234 Figure 12B of 3GPP TS 25.211 v 9.1.0. Table 7.7 on page 235 Table 16C of 3GPP TS 25.211 v 9.1.0. Text in section 7.7 on page 235 Section 7 of 3GPP TS 25.211 v 9.1.0. TM c ⃝2009. 3GPP TSs and TRs are the property of ARIB, ATIS, ETSI, CCSA, TTA, and TTC who jointly own the copyright for them. They are subject to further modifications and are therefore provided to you “as is” for information purposes only. Further use is strictly prohibited. Text in section 7.5.1 on page 221 Section 4.2.3.3 of 3GPP TS 25.321 v 7.7.0. Text in section 7.5.2 on page 224 Section 4.2.4.3 of 3GPP TS 25.321 v 7.7.0. Figure 7.6 on page 222 Figure 4.2.3.3.1 of 3GPP TS 25.321 v 7.7.0. Figure 7.8 on page 224 Figure 4.2.4.3.1 of 3GPP TS 25.321 v 7.7.0. TM c ⃝2008. 3GPP TSs and TRs are the property of ARIB, ATIS, ETSI, CCSA, TTA, and TTC who jointly own the copyright for them. They are subject to further modifications and are therefore provided to you “as is” for information purposes only. Further use is strictly prohibited.

7.11. SUMMARY: HSDPA OPERATION IN SHORT

Table 7.1 on page 206 Table 5.1a of 3GPP TS 25.306 v 9.1.0. Table 7.8 on page 237 Table 5.1a of 3GPP TS 25.306 v 9.1.0. Table 7.9 on page 237 Table 5.1a of 3GPP TS 25.306 v 9.1.0. Text in section 7.6.2 on page 229 Section 4.6 of 3GPP TS 25.212 v 9.3.0. c ⃝2010. 3GPPTM TSs and TRs are the property of ARIB, ATIS, ETSI, CCSA, TTA, and TTC who jointly own the copyright for them. They are subject to further modifications and are therefore provided to you “as is” for information purposes only. Further use is strictly prohibited.

243

BIBLIOGRAPHY [1] 3GPP TS 25.301 ver. 7.0.0 ;‘Radio Interface Protocol Architecture’ [2] 3GPP TS 25.308 ver. 7.0.0 ;‘High Speed Downlink Packet Access (HSDPA); Overall Description;’ [3] 3GPP TS 25.306 ver. 9.0.0 ;‘UE Radio Access capabilities’ [4] 3GPP TS 25.211 ver. 6.0.0 ;‘Physical channels and mapping of transport channels onto physical channels (FDD)’ [5] 3GPP TS 25.212 ver. 6.0.0 ;‘Multiplexing and Channel Coding (FDD)’ [6] 3GPP TS 25.213 ver. 6.0.0 ;‘Spreading and Modulation (FDD)’ [7] 3GPP TS 25.214 ver. 6.0.0 ;‘Physical Layer Procedures (FDD)’ [8] 3GPP TS 25.321 ver. 7.0.0 ;‘MAC protocol specification’ [9] 3GPP TS 25.331 ver. 7.0.0 ;‘Radio Resource Control (RRC) protocol specification’ [10] 3GPP TS 25.401 Ver. 7.0.0 ;‘UTRAN overall description’ [11] 3GPP TS 25.413 Ver. 7.0.0 ;‘UTRAN Iu Interface: RANAP Signalling’ [12] 3GPP TS 25.433 Ver. 7.0.0 ;‘UTRAN Iub Interface: NBAP Signalling’ [13] 33GPP TR 25.931 ver. 8.0.0 ;‘UTRAN functions, examples on signalling procedures’ [14] H.Holma and A. Toskala, ‘WCDMA for UMTS’ , 5th Edition, John Wiley & Sons. [15] H.Holma and A. Toskala, ‘HSDPA/HSUPA for UMTS’ , 1st Edition, John Wiley & Sons. [16] Chris Johnson, ‘Radio Access Networks For UMTS ; Principles And Practice’ , John Wiley & Sons.

244

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8 HIGH SPEED UPLINK PACKET ACCESS

Source:

3GPP TS 25.319; Enhanced uplink; Overall description

After learning the important facts about High Speed Downlink Packet Access (HSDPA) in the previous chapter, the next logical step is to investigate the improvements in uplink. These new set of improvements are known as “High Speed Uplink Packet Access (HSUPA) or Enhanced Uplink1 ”. The first release of HSDPA standard was available in 3GPP Rel-5. HSUPA was standardized in 3GPP Rel-6. Once again, the design targets are very similar to HSDPA. But in Uplink, there are some additional requirements which need to be met. These requirements are discussed in 3GPP 25.319. Some of those points are mentioned below.

8.1

Requirements

• The uplink coverage for R99 DCH channel is generally very limited. Therefore, the end user experience in wide area cells is not very good. 1

Enhanced Uplink (EUL) is the official name chosen by 3GPP but due to popularity of HSDPA, the term HSUPA is also very widely used.

245

246

CHAPTER 8. HIGH SPEED UPLINK PACKET ACCESS The Enhanced Uplink feature is targeted at providing a significant improvements in terms of user experience (throughput and delay) and/or capacity. The coverage is one of the aspects which affect the user experience. For an operator, it is desirable to have good coverage to provide consistency of performance across the whole cell area. Therefore, it is expected that HSUPA should serve a wider cell area. Hence, coverage will be one of the main design criterion while development. In contrast to this, in HSDPA, the focus was on DL throughput. • HSUPA should be designed to serve urban, sub-urban and rural deployment scenarios. • HSUPA should support full mobility. Undoubtedly, the system is best optimized for stationary users but it should perform well for fast-moving users as well. • HSUPA should be designed with least complexity so that the user equipment’s (UE) & network elements’ cost is not very high. In R99 specification, there were a lot of features that are practically not used anywhere. In HSUPA development, such features should be avoided so that the time-to-market can be reduced. • It is required that HSUPA should provide improved QoS compared to R99 UL dedicated channels. The main focus should be on the services of streaming, interactive and background traffic classes. • There is always a trade-off between performance improvement & complexity of upgrades. HSDPA introduced a lot of changes in hardware and protocol architecture. It is desirable that changes caused by HSUPA should be as little as possible. According to 3GPP TS 25.319: “New techniques or group of techniques shall therefore provide significant incremental gain for an acceptable complexity”. • The improvements should be designed in such a away that HSUPA can be introduced to a network which has UEs of different radio capabilities, i.e., R6 UEs and the UEs from R99, R4 and R5. • A terminal supporting the Enhanced Uplink feature must support HSDPA. Therefore, the term HSPA can be used to describe the combination of HSDPA and HSUPA. In our further discussions, we will use HSPA as a synonym for HSUPA.

From the end-user point of view, HSUPA is an enhancement to Rel-99 UTRAN which allows him to achieve higher Uplink peak bit rates in a wider service area compared to classical R99 solution for Uplink data transmission. This is an important upgrade because the UL bit rates of R99 DCH are very low when the UE is at cell-edge.

8.2. COMPARISON WITH HSDPA

8.2

247

Comparison with HSDPA

As the names indicate, HSDPA and HSUPA sound very similar. Therefore, we commonly assume that HSUPA is nothing but ‘HSDPA for Uplink’. This is not exactly true. To investigate this issue further, let’s discuss the commonalities and differences between the two technologies.

8.2.1

Commonalities with HSDPA

Node B based scheduling: The transport channels used to carry the user data in R99 UMTS are RACH (↑), FACH (↓) & DCH (↕). All of these channels are scheduled by RNC’s packet scheduler. This concept was explained in HSDPA module. HSDPA transport channel HS-DSCH and HSUPA transport channel E-DCH are both scheduled by Node B based packet scheduler. Fast L1 H-ARQ: In HSUPA, the data transmitted via E-DCH transport channel required immediate acknowledgements from Node B. This concept was introduced in HSDPA where the role of transmitter is played by Node B and UE sends the acknowledgment. Multicode Operation: The peak UE bit rates in HSDPA are achieved by sending data to a user on multiple SF16 codes. Similarly, in HSUPA, UE can send uplink data on either 1, 2 or 4 channelization codes. Link Adaptation: Based on the UE radio conditions, data volume and many other conditions, UE resource allocation can be modified. This concept is common in HSDPA and HSUPA. • Rel-5 HSDPA devices support QPSK & 16QAM modulation2 . Therefore, link adaptation happens by adaptive modulation and coding (AMC). • Rel-6 HSUPA devices support only BPSK modulation. Therefore, the link adaptation happens mainly by adaptive coding (AC) only. Shorter TTI: The Rel-99 transport channel DCH supports the TTI length of 10, 20, 40 or 80 ms. HSDPA utilizes a significantly shorter TTI of 2 ms. In HSUPA: • 10 ms TTI is a mandatory for every network and the UE. This is to ensure that UE will be able to use HSUPA when it finds itself in a poor coverage area. • 2 ms TTI is optional. 2 ms will allow the user to achieve higher peak bit rates and lower latency, but 2ms TTI can be used only if the UE is in good radio conditions. 2

Except special category 11 & 12 UEs

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8.2.2

Differences from HSDPA

Power Control: In HSDPA, all 3 slots in a subframe are transmitted at a constant power. In other words, there is no fast inner loop PC for HSDPA. But in HSUPA, the power control is very crucial for minimizing the near-far effect. In HSDPA, there is a central transmitter in Node B whereas in HSUPA, the transmitters are distributed across the whole cell coverage area. Therefore, management of interference requires more signalling in HSUPA. Soft Handover: HSDPA performs a (hard) Serving Cell Change whereas, in HSUPA the E-DCH channel can be in soft-handover with up to 3 Cells. Variable Spreading factor: In HSDPA, the SF = 16, which is fixed. But in HSUPA, UE gets a resource grant from Node B and decides which SF to use. It is allowed to use eight possible spreading factors (SF 256, 128, ..., 2). This is shown in table 8.4. Overall Procedure: In HSDPA, the scheduler resides in Node B and data is also buffered at Node B & RNC side. Therefore, Node B can very well decide how much bitrate will satisfy the need of the user. On the contrary, in uplink, the scheduler needs to know the status of the UE buffer. There has to be some periodic reporting of the buffer status. • In HSUPA, we have some kind of Request → Grant → Data Transmission → Acknowledgement mechanism. Whereas, • In HSDPA, we have Notification to UE → Data Transmission → Acknowledgement type of mechanism.

8.3

HSUPA User Plane Protocols

A detailed description of PDCP, RLC and MAC-d protocols is available in chapter 6. HSUPA related MAC protocols are MAC-e and MAC-es, which are explained later in this chapter. In this section, we will examine the HSUPA transmission only in an overview manner. 1. On UE Side • PDCP layer compresses the headers belonging to higher layers e.g., TCP/IP or RTP/UDP/IP. • RLC layer performs segmentation on the big data block received from PDCP layer. The size of RLC PDU is explicitly signalled to the user via RRC signalling3 . RLC also performs ciphering. If the Acknowledged Mode (AM) of RLC is configured, then RLC layer keeps track of L2 retransmissions. 3

in RB setup or RB Reconfiguration message.

8.3. HSUPA USER PLANE PROTOCOLS

249

Figure 8.1: HSUPA User Plane Protocols • MAC-d layer in UE generates the MAC-d flows. MAC-d layer is transparent in HSUPA. Therefore, the MAC-d PDU is exactly the same as RLC PDU. • MAC-es layers combines MAC-d PDUs of the same logical channel and same size into one MAC-es PDU. MAC-es layer adds a Transmission Sequence Number which will help the RNC to re-order the correctly received MAC-es PDUs. • MAC-e layer in UE, combines several MAC-es PDUs and form a MAC-e PDU. • Physical layer in UE carries on L1 processing and transmits the data on WCDMA air interface. 2. On Node B Side • Node B’s Physical layer receives the data coming from UE’s physical layer. • MAC-e layer in Node B checks for L1 H-ARQ and decides whether an Ack or Nack has to be sent. • MAC-e layer demultiplexes the MAC-e PDU and extracts the MAC-es PDUs which are sent towards RNC. 3. On RNC Side • By looking into the ‘transmission sequence number (TSN)’, MACes layer of RNC re-orders the correctly received MAC-es PDUs. It demultiplexes the MAC-es PDUs to extract the MAC-d pdus. In HSUPA, UE can be in soft handover with more than one cell. Therefore, MAC-es layer also performs Macro Diversity Combining (MDC) to achieve the link diversity. • Finally, the correctly received MAC-d PDUs are forwarded to the MAC-d layer. • RLC layer in RNC checks whether a L2 Ack or Nack has to be sent to the UE. On RNC side, the RLC layer performs reassembly of several RLC blocks and constructs a big data block to be delivered to the PDCP layer.

250

CHAPTER 8. HIGH SPEED UPLINK PACKET ACCESS • PDCP layer in RNC performs header decompression and restores the original header of higher layer application. Summary of HSUPA Operation (according to TS 25.401): “The E-DCH MAC (MAC-e/MAC-es) entity in the UE transfers MAC-e PDUs to the peer MAC-e entity in the Node B and MAC-es PDUs to the peer MAC-es entity in the RNC using the services of the Physical Layer”.

8.4

HSUPA Configuration Options

At first sight, one can guess that E-DCH transport channel is mainly designed for carrying uplink user data (Logical channel DTCH). Similarly HS-DSCH transport channel is mainly designed to transmit downlink user data. But if we carefully examine options available for mapping the Signalling Radio Bearers (SRB) on transport channels, operators have two choices. This section investigates both options. The general channel structure of UMTS was discussed in chapter 4.

Figure 8.2: HSUPA Control Plane Protocols - SRB on DCH Option 1: SRB on DCH: In this configuration, HS-DSCH channel and E-DCH channel is used to carry the DTCH logical channel whereas the logical channel DCCH or RRC signalling4 is still sent via UL & DL DCH channels. Obviously this option is not the best option because it includes a lot of DCH overhead channels. The control plane protocol stack for this option is exactly the same as Rel-99 control plane protocol stack as shown in figure 8.2. SRB on DCH option does not reduce the control plane latency. 4

also known as Signalling Radio Bearer SRB or L3 Signalling

8.5. E-DCH UE CATEGORIES AND BIT RATES

251

Figure 8.3: HSUPA Control Plane Protocols - SRB on HSPA Option 2: SRB on HSPA: Alternatively, HS-DSCH and E-DCH channels can be configured to send both User data DTCH and DCCH. This option is commonly known as SRB on HSPA. The control plane protocol stack for this configuration is illustrated in figure 8.3. This option significantly reduces the amount of DCH overhead channels and control plane latency. SRB on HSPA is a pre-requisite for some other smart features, for example, FractionalDPCH (F-DPCH).

8.5

E-DCH UE Categories and Bit Rates Source:

3GPP TS 25.306 ; UE Radio Access capabilities

In HSDPA, we have learnt about a variety of UE categories. Up to Release 9, there are 28 HS-DSCH UE categories defined for HSDPA operation. Similarly, there exist some standard UE categories for HSUPA operation too. In order to follow the HSUPA development in chronological order, the E-DCH UE categories are illustrated in three different tables. • Rel. 6: Category 1, 2, 3, 4, 5 & 6 are introduced. • Rel. 7: Category 7 UE has been added to the list of UE categories. Main enhancement is 4-PAM modulation on E-DPDCH channel (which is quite often referred to as 16-QAM).

252

CHAPTER 8. HIGH SPEED UPLINK PACKET ACCESS • Rel. 8: No new Category defined in Rel-8. • Rel. 9: Category 8 & 9 Categories have been added which support Dual Cell-HSUPA (or DC-HSUPA) operation.

E-DCH Category

Cat. Cat. Cat. Cat. Cat. Cat.

1 2 3 4 5 6

Max no. of E-DCH Codes 1 2 2 2 2 4

Min. SF

Supported TTI

SF4 SF4 SF4 SF2 SF2 SF2

10 10 10 10 10 10

ms ms ms ms ms ms

only & 2 ms only & 2 ms only & 2 ms

Max. TB Size [bits] in 10 ms TTI 7110 14484 14484 20000 20000 20000

Max. TB Size [bits] in 2 ms TTI 2798 5772 11484

Modul.

BPSK BPSK BPSK BPSK BPSK BPSK

Table 8.1: E-DCH UE Categories introduced in 3GPP Rel. 6 (25.306) E-DCH Category

Cat. 7

Max no. of E-DCH Codes 4

Min. SF

Supported TTI

SF2

10 ms & 2 ms

Max. TB Size [bits] in 10 ms TTI 20000

Max. TB Size [bits] in 2 ms TTI 22996

Modul.

4-PAM

Table 8.2: Additional E-DCH UE Categories in 3GPP Rel. 7 (25.306) E-DCH Category

Cat. 8 Cat. 9

Max no. of E-DCH Codes 4 4

Min. SF

Supported TTI

SF2 SF2

10 ms & 2 ms 10 ms & 2 ms

Max. TB Size [bits] in 10 ms TTI 20000 20000

Max. TB Size [bits] in 2 ms TTI 11484 22996

Modul.

BPSK 4-PAM

Table 8.3: Additional E-DCH UE Categories in 3GPP Rel. 9 (25.306) After observing the tables 8.1, 8.2 & 8.3, we can make some remarks about the various UEs of different categories. 1. Multi-code Support: Some UEs do not support multi-code operation on E-DPDCH (for example Cat. 1 UE), some support upto 2 codes (for example Cat. 2, 3, 4, & 5) while some UEs support upto 4 code transmission (for example UE cat. 6, 7, 8 & 9).

8.6. STARTING OF HSUPA OPERATION

253

2. Min. SF: SF2 was introduced in 3GPP REL-6 for E-DPDCH channel. But all the UEs cannot use SF2. This aspect of their radio access capabilitis is shown in the column ‘Min. SF’ in the UE categories tables. 3. TTI Support: Both 2 ms and 10 ms TTI have their own advantages and disadvantages. 10 ms TTI is a mandatory feature which is supported by all UEs but 2 ms TTI operation is possible only for UE cat. 2, 4, 6, 7, 8 & 9. 4. Modulation: Cat. 1 to 6 and cat. 8 can transmit data using BPSK modulation (one bit per symbol) only whereas the UE category 7 & 9 can also use 4PAM; modulation (2 bits per symbol). 5. DC-HSUPA: Only Rel. 9 categories UEs, i.e. Category 8 & 9 UEs, can support DC-HSUPA operation.

8.6

Starting of HSUPA Operation

As discussed in the chapter 5 about the Radio Resource Management, we saw that RNC’s PS is responsible for deciding the ’transport channel type selection’. This procedure can yield 4 possible outcomes: 1. RACH & FACH 2. DCH & DCH 3. DCH & HS-DSCH 4. E-DCH & HS-DSCH As shown in figure 8.4, every HSUPA device starts the signalling procedure as if it were a simple R99 UE. After performing GPRS ATTACH, the serving SGSN, UE acquires a P-TMSI and knows about the Routing area ID of the cell. As a result of GPRS attach, there is a MM context stored in UE and SGSN. Later, UE establishes a PDP context and tries to acquire an IP address and negotiate the QoS. Later on, when UE feels the need of UL resources, it sends an UL capacity request to the RNC. RNC performs the channel type selection and decides one of the options listed above. Up to this point in signalling, a 3G R99 UE and HSUPA UE behave almost the same. In case, RNC chooses to use E-DCH in UL, the use of HS-DSCH becomes mandatary. If HS-DSCH resources are also available, RNC sends the information regarding the cell specific HSDPA and HSUPA details to user in a L3 RRC message Radio Bearer Reconfiguration.

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Figure 8.4: Signalling to initiate an HSUPA session

8.7

HSUPA Protocol Architecture Source: 3GPP TS 25.321; Medium Access Control (MAC) Protocol Specification

Earlier in section 8.3, the user plane protocol architecture of HSUPA & data-flow was described but the details about MAC-e and MAC-es were not discussed. In the same section, figure 8.1 described the overall functionality of HSUPA using the user plane protocol stack. In the following section, we will investigate the MAC layer of HSUPA in depth. On UTRAN side, for each UE that uses E-DCH, one MAC-e entity per Node-B and one MAC-es entity in the SRNC are configured. Whereas on UE side, both MAC-e & MAC-es are configured in the user equipment.

MAC-e/es entity - UE Side Figure 8.5 is copied from ‘Figure 4.2.3.4.1a: UE side MAC architecture / MAC-e/es details (FDD)’ of 3GPP TS 25.321. The MAC-es/e handles the E-DCH specific functions. The split between MAC-e and MAC-es in the UE is not detailed. In the model below, the

8.7. HSUPA PROTOCOL ARCHITECTURE

255

Figure 8.5: UE side MAC-e/es details (25.321) MAC-e/es comprises the following entities: 1. H-ARQ: The HARQ entity is responsible for handling the MAC functions relating to the HARQ protocol. It is responsible for storing MAC-e payloads and re-transmitting them. The detailed configuration of the hybrid ARQ protocol is provided by RRC over the MAC-Control SAP. The HARQ entity provides the ETFC, the retransmission sequence number (RSN), and the power offset to be used by L1. Redundancy version (RV) of the HARQ transmission is derived by L1 from RSN, CFN and in case of 2 ms TTI from the sub-frame number. RRC signalling can also configure the HARQ entity to use RV=0 for every transmission. 2. Multiplexing and TSN setting: Figure 8.6 illustrates multiplexing of multiple MACd PDUs into MAC-es PDU. After this, MAC-e layer further multiplexes several MAC-es PDUs into MAC-e PDUs, as shown by figure 8.7. PDU sizes directly affect the user bit rate. Therefore, these decisions are done by E-TFC selection function. UE also sets the TSN while concatenating multiple MAC-d PDUs into MAC-es PDUs. 3. E-TFC selection: This entity is responsible for E-TFC selection according to the scheduling information, Relative Grants and Absolute Grants, received from UTRAN

256

CHAPTER 8. HIGH SPEED UPLINK PACKET ACCESS via L1 and Serving Grant value signalled through RRC, and for arbitration among the different flows mapped on the E-DCH. The detailed configuration of the ETFC entity is provided by RRC over the MAC-Control SAP. The E-TFC selection function controls the multiplexing function.

Figure 8.6: MAC-es PDU

Figure 8.7: MAC-e PDU As shown in figure 8.1, MAC-es sits on top of MAC-e and receives PDUs directly from MAC-d. Figure 8.6 illustrates that MAC-es SDUs (i.e. MAC-d PDUs) of the same size, coming from a particular logical channel are multiplexed together into a single MAC-es

8.7. HSUPA PROTOCOL ARCHITECTURE

257

payload. There is one and only one MAC-es PDU per logical channel per TTI (since only one MAC-d PDU size is allowed per logical channel per TTI). To this payload is prepended the MAC-es header. The number of PDUs, as well as the one DDI value identifying the logical channel, the MAC-d flow and the MAC-es SDU size are included as part of the MAC-e header. In case sufficient space is left in the E-DCH transport block or if Scheduling Information needs to be transmitted, an SI will be included at the end of the MAC-e PDU. Multiple MAC-es PDUs from multiple logical channels, but only one MAC-e PDU can be transmitted in a TTI. In the example shown in figure 8.7, the field DDI0 is referring to the specific DDI value that indicates that there is an SI included in the MAC-e PDU. This header will not be associated with a new MAC-es payload.

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MAC-es entity - UTRAN Side

Figure 8.8: UTRAN side MAC-es details(25.321) For each UE, there is one MAC-es entity in the SRNC. The MAC-es sublayer handles E-DCH specific functionality,which is not covered in the MAC-e entity in Node B. The MAC-es comprises the following entities: 1. Reordering Queue Distribution: The reordering queue distribution function routes the MAC-es PDUs to the correct reordering buffer based on the SRNC configuration.

8.7. HSUPA PROTOCOL ARCHITECTURE

259

2. Reordering: This function reorders received MAC-es PDUs according to the received TSN and Node B tagging i.e. (CFN, subframe number). MAC-es PDUs with consecutive TSNs are delivered to the disassembly function upon reception. Mechanisms for reordering MAC-es PDUs are left to the implementation. The number of reordering entities is controlled by the SRNC. There is one Reordering Queue per logical channel. 3. Macro diversity selection: The function is performed in the MAC-es, in case of soft handover with multiple Node Bs (The soft combining for all the cells of a Node B takes place in the Node B). This means that the reordering function receives MAC-es PDUs from each Node B in the E-DCH active set. The exact implementation is not specified. However, the model below is based on one Reordering Queue Distribution entity receiving all the MAC-d flow from all the Node Bs, and one MAC-es entity per UE. 4. Disassembly: The disassembly function is responsible for disassembly of MAC-es PDUs. When a MAC-es PDU is disassembled the MAC-es header is removed, the MAC-d PDU’s are extracted and delivered to MAC-d.

MAC-e entity - UTRAN Side There is one MAC-e entity in the Node B for each UE and one E-DCH scheduler function in the Node B. The MAC-e and E-DCH scheduler handle HSUPA specific functions in the Node B. In HSUPA, the MAC-e and E-DCH scheduler comprises the following entities:

1. E-DCH Scheduling: This function manages E-DCH cell resources between UEs. Based on scheduling requests, Scheduling Grants are determined and transmitted. The general principles of the E-DCH scheduling are described by 3GPP but the actual implementation is not specified (i.e. depends on RRM strategy). 2. E-DCH Control: The E-DCH control entity is responsible for reception of scheduling requests and transmission of Scheduling Grants. 3. De-multiplexing: This function provides de-multiplexing of MAC-e PDUs. MAC-es PDUs are forwarded to the associated MAC-d flow. 4. HARQ: One HARQ entity is capable of supporting multiple instances (HARQ processes) of stop and wait HARQ protocols. Each process is responsible for generating ACKs or NACKs indicating delivery status of E-DCH transmissions. The HARQ entity handles all tasks that are required for the HARQ protocol.

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Figure 8.9: UTRAN side MAC-e details(25.321)

8.8

Channels and Physical Layer

In the previous section, we learnt about the L2 MAC sub-layer functionality for HSUPA operation. Now we will take a closer look at L1 Physical layer and learn about the HSUPA physical channels. All the channels related to HSUPA operation have a name which start with ‘E-’. One by one, we will discuss the following physical channels: 1. E-DPDCH 2. E-DPCCH 3. E-RGCH 4. E-HICH 5. E-AGCH

8.8. CHANNELS AND PHYSICAL LAYER

8.8.1

261

E-DPDCH

Figure 8.10: Subframe Structure of E-DPDCH and E-DPCCH Channels

The E-DPDCH is the principal channel which is used to carry the E-DCH transport channel. There may be zero, one, 2 or 4 E-DPDCH on each radio link. The E-DPCCH is a physical channel used to transmit control information associated with the E-DCH. There is at most one E-DPCCH on each radio link. Figure 8.10 shows the E-DPDCH and E-DPCCH (sub)frame structure. Each radio frame is divided in 5 subframes, each of length 2 ms; the first subframe starts at the start of each radio frame and the 5th subframe ends at the end of each radio frame. Just like Rel. 99 DPDCH channel, REL-6 E-DPDCH channel can also have variable spreading factor. E-DPDCH support 8 different SF as shown in table 8.4 by row number 1 to 8. Various slot formats actually represent a combination of ‘SF and Modulation’. An E-DPDCH may use BPSK (all UE categories) or 4PAM modulation symbols (Category 7 and 9 only). Table 8.1, 8.2 & 8.3 show various UE categories and their physical layer capabilities. In the basic form of HSUPA (3GPP release 6), there are 6 UE categories defined. As an example, we try to calculate the peak L1 bitrate of category 6.

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Slot format #i 0 1 2 3 4 5 6 7 8 9

(BPSK) (BPSK) (BPSK) (BPSK) (BPSK) (BPSK) (BPSK) (BPSK) (4PAM) (4PAM)

Channel Bit Rate (kbps) 15 30 60 120 240 480 960 1920 1920 3840

Channel Symbol Rate (ksps) 15 30 60 120 240 480 960 1920 960 1920

SF

Bits/ E-DPDCH subframe 30 60 120 240 480 960 1920 3840 3840 7680

256 128 64 32 16 8 4 2 4 2

Table 8.4: E-DPDCH slot formats (from TS 25.211)

E-DPDCH Code # 1 :

SF4

=

960

ksps

+

E-DPDCH Code # 2 :

SF4

=

960

ksps

+

E-DPDCH Code # 3 :

SF2

=

1920

ksps

E-DPDCH Code # 4 :

SF2

=

1920

ksps

Sum of all 4 E-DPDCH Codes

=

5760

ksps

+ = or

Cat. 6 UE supports only BPSK Modulation ⇒ 5760 kbps

Hence, by sending Uplink data on 4 channelization codes ( 2 × SF2 + 2 × SF4 ), UE is able to achieve a L1 bit rate of 5.76 Mbps. The knowledge of channel coding rate is needed to find out the L2 user bit rate. Same calculation can be done for UE of category 7, which supports both BPSK and 4PAM modulation. 4PAM modulation uses 2 bits to generate one modulation symbol. For 4-PAM case, 5760 ksps = 11200 kbps or 11.2 Mbps.

8.8.2

E-DPCCH

The E-DPCCH is a physical channel carrying control information for the E-DPDCH. The E-DPCCH is sent with a power offset relative to the DPCCH. The power offset is signalled by RRC. E-DPCCH has a fixed spreading factor 256 which allows UE to send 15 kbps control signalling. In a 2 ms subframe, UE can send maximum 30 bits on E-DPCCH. Out of these 30 bits, only 10 carry useful information and the remaining 20 bits are used for the reliability or channel coding. The details can be found in 3GPP TS 25.212.

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263

For both 2 ms and 10 ms TTI, the information carried on the E-DPCCH consists of 10 bits in total. E-TFCI, 7 bits: An E-DCH Transport Format Combination Indicator (E-TFCI) identifies the transport block size on E-DCH (7 bits). SRNC signals which E-DCH Transport Block Size table should be used by the UE5 . RSN, 2 bits: The Retransmission Sequence Number (RSN) is used to convey the uplink HARQ transmission number. The combination of the RSN and the transmission timing allows the receiver to determine the exact transmission number. The length of the RSN field is 2 bits. 2 bits of RSN are interpreted as: • ‘00’ ⇒ Original Transmission • ‘01’ ⇒ First Re-transmission • ‘10’ ⇒ Second Re-transmission • ‘11’ ⇒ Third or higher retransmission Happy Bit, 1 bit: One bit of the E-DPCCH is used to indicate whether or not the UE is satisfied (‘happy’) with the current Serving Grant. This bit is always be present during uplink transmission of E-DPCCH. According to section 11.8.1.5 of 25.321, UE indicates that it is ‘unhappy’ if the following criteria are met: 1. UE is transmitting as much scheduled data as allowed by the current Serving Grant; 2. UE has enough power available to transmit at higher data rate; 3. Total buffer status would require more than Happy Bit Delay Condition ms to be transmitted with the current Serving Grant. ‘Happy Bit Delay Condition’ is an operator configurable parameter.

5

3GPP TS 25.321, annexure B shows all the tables for E-DCH FDD mode.

• If the UE is configured with E-TFCI table 0 and 2ms TTI, use Annex B.1 • If the UE is configured with E-TFCI table 1 and 2ms TTI, use Annex B.2 • If the UE is configured with E-TFCI table 2 and 2ms TTI, use Annex B.2a • If the UE is configured with E-TFCI table 3 and 2ms TTI, use Annex B.2b • If the UE is configured with E-TFCI table 0 and 10ms TTI, use Annex B.3 • If the UE is configured with E-TFCI table 1 and 10ms TTI, use Annex B.4

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Slot format #i 0 (BPSK )

Channel Bit Rate (kbps) 15

Channel Symbol Rate (ksps) 15

SF 256

Bits/ E-DPDCH subframe 30

Table 8.5: E-DPCCH slot formats (from TS 25.211)

8.8.3

E-AGCH

The E-DCH Absolute Grant Channel (E-AGCH) is a downlink physical channel with fixed spreading factor (SF=256). In other words, the E-AGCH has a bit rate 30 kbps. In a subframe of 2 ms, Node B can send 60 bits. E-AGCH transmission is: • Over one sub-frame if E-DCH TTI is set to 2ms. • Over one frame if E-DCH TTI is set to 10ms. The sequence of 60 bits are mapped to the corresponding E-AGCH sub-frame. If the EDCH TTI is equal to 10 ms, the same sequence of bits is transmitted in all the E-AGCH sub-frames of the E-AGCH radio frame. In other words, the same 2 ms sub-frame of E-AGCH is re-transmitted four times (sent total 5 times). E-AGCH channel is used to carry the uplink E-DCH Absolute Grant. Figure 8.11 illustrates the frame and sub-frame structure of the E-AGCH.

Figure 8.11: Subframe Structure of E-AGCH The absolute grant channel carries six bits which are concatenated with 16 bit CRC. The user identity E-RNTI is masked on the CRC. After channel coding, E-AGCH becomes 90 bits long. A Rate Matching procedure is used to select selected 60 bits and those bits are transmitted in E-AGCH sub-frame. The six data bits of E-AGCH channel are:

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265

Index 31 30 29 28 27 26 25 24 23 22 21 20 19 .. .

Absolute Grant Value (168/15)2 ∗ 6 (150/15)2 ∗ 6 (168/15)2 ∗ 4 (150/15)2 ∗ 4 (134/15)2 ∗ 4 (119/15)2 ∗ 4 (150/15)2 ∗ 2 (95/15)2 ∗ 4 (168/15)2 (150/15)2 (134/15)2 (119/15)2 (106/15)2 .. .

11 10 9 8 7 6 5 4 3 2 1 0

(42/15)2 (38/15)2 (34/15)2 (30/15)2 (27/15)2 (24/15)2 (19/15)2 (15/15)2 (11/15)2 (7/15)2 ZERO GRANT INACTIVE

Table 8.6: Mapping of Absolute Grant Value (from 3GPP TS 25.321) Absolute Grant Value, 5 bits: This field indicates the maximum E-DCH traffic to pilot ratio (E-DPDCH/DPCCH) that the UE is allowed to use in the next transmission. The length of the Absolute Grant Value field is 5 bits. Absolute Grant =

Ptx,E-DPDCH Ptx,DPCCH

Absolute Grant Scope, 1 bit: This field indicates the applicability of the Absolute Grant. It can take two different values, “Per HARQ process” or “All HARQ processes”, allowing to indicate whether the HARQ process activation/de-activation

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CHAPTER 8. HIGH SPEED UPLINK PACKET ACCESS will affect one or all processes. The Absolute Grant Scope is encoded in 1 bit. When the E-DCH is configured with 10ms TTI, only the value “All HARQ processes” is valid. In case, Identity Type is ‘Secondary’, only the value “All HARQ processes” is valid.

8.8.4

E-RGCH

The E-DCH Relative Grant Channel (E-RGCH) is a downlink physical channel with fixed spreading factor (SF=128). Hence, this channel can carry information at 60 kbps. ERGCH carries dedicated uplink E-DCH relative grants. The word ‘Relative’ means, in comparison to the current grant used by UE. Figure 8.15 illustrates the structure of the E-RGCH. A relative grant can have one of the following three values. • UP • DOWN • HOLD E-RGCH channel can be transmitted either in 3, 12 or 15 consecutive slots and in each slot a sequence of 40 ternary values is transmitted (Up, Down or Hold). E-RGCH transmission on 3 slots: Used on an E-RGCH transmitted to UEs for which the cell transmitting the E-RGCH is in the E-DCH serving radio link set and for which the E-DCH TTI is 2 ms. E-RGCH transmission on 12 slots: Used on an E-RGCH transmitted to UEs for which the cell transmitting the E-RGCH is in the E-DCH serving radio link set and for which the E-DCH TTI is 10 ms. E-RGCH transmission on 15 slots: Used on an E-RGCH transmitted to UEs for which the cell transmitting the E-RGCH is not in the E-DCH serving radio link set. For non-serving E-DCH RLS, the duration of E-RGCH transmission is irrespective of the E-DCH TTI. The next section has been written with the help of 3GPP TS 25.321 as reference material. For the following discussion, it is assumed that UE’s E-DCH active set is more than one. Hence, UE is in soft handover for E-DCH with two or more cells. Serving Relative Grant: Transmitted in downlink on the E-RGCH from all cells in the serving E-DCH RLS, the serving relative grant allows the Node B scheduler to incrementally adjust the serving grant of UEs under its control. By definition, there can only be one serving relative grant command received at any time. This indication can take three different values, ‘UP’, ‘DOWN’ or ‘HOLD’.

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267

Non-serving Relative Grant: Transmitted in downlink on the E-RGCH from a nonserving E-DCH RL, the non-serving relative grant allows neighboring Node Bs to adjust the transmitted rate of UEs that are not under their control in order to avoid overload situations. By definition, there could be multiple non-serving relative grant commands received by MAC at any time. This indication can take two different values, ‘DOWN’ or ‘HOLD’.

Figure 8.12: Subframe Structure of E-RGCH & E-HICH The sequence bi,0 , bi,1 . . . , bi,39 is calculated as [bi,0 , bi,1 . . . , bi,39 ] = a ∗ [40 bit long Signature Sequence],

where:

  +1 if Relative Grant is ‘UP’, 0 if Relative Grant is ‘HOLD’, a=  −1 if Relative Grant is ‘DOWN’. The orthogonal signature sequences are defined by 3GPP TS 25.21. Figure 8.14 shows a table with all of these 40 signature sequences which are numbered from CSS,40,0 to CSS,40,39 . Each HSUPA user is assigned one signature sequence for E-HICH and another sequence for E-RGCH. Hence, every user requires at least two signature sequences. This is a nice trick which consumes only one channelization code for E-RGCH and E-HICH for up to 20 HSUPA users. The principle of creating 40 dedicated channels using only one channelization code is illustrated in figure 8.13. In figure 8.13, UE1 has been assigned a signature sequence # 0 for E-RGCH and # 1 for E-HICH. Similarly the other users are also assigned 2 signature sequences per UE. If there are more than 20 HSUPA users in a cell, then additional channelization codes must be allocated for additional E-RGCH and E-HICH channels.

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Figure 8.13: Signature Multiplexing Scheme for E-RGCH and E-HICH Which Grant to be Respected: AGCH or RGCH? According to 3GPP TS 25.321, UEs configured with an E-DCH transport channel shall maintain a Serving Grant and the list of active HARQ processes based on the absolute and relative grant commands decoded on the configured E-AGCH and E-RGCH(s). The UE will only act on a relative grant command if all of the following are true: • The current serving grant is not set to ZERO GRANT. • The UE has not received a new absolute grant within 1 HARQ Round Trip Time (40 ms for 10 ms TTI, 16 ms for 2 ms TTI). • The UE was expecting to receive an ack/nack on the E-HICH at the same time as the network sent the E-RGCH command (an ack/nack sent for a E-DPDCH transmission that just contained MAC-e Scheduling Information alone does not meet this criteria). Now the question is: “If Serving Relative Grant is UP, how much should the SG be increased? Similarly, if serving Relative Grant is down, how much should the SG be decreased?” According to TS 25.321, the answer to this question can be found by using two parameters: “3-index-step threshold” and “2-index-step threshold” that are configured by higher layers.

8.8. CHANNELS AND PHYSICAL LAYER

269

Figure 8.14: E-RGCH and E-HICH signature sequences (from TS 25.211) If the UE received a Serving Relative Grant ‘UP’: UE determine the Serving Grant as follows: • if SG < “3-index-step threshold”: Serving Grant (SG) = [MIN(SG + 3, 37)]. • if “3-index-step threshold”