OEA000101 LTE Air Interface ISSUE 1.05.pdf
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LTE Air Interface Training Manual
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Contents 1 The Air Interface ................................................................................................................... 1-1 1.1 Evolution of Cellular Networks............................................................................................................... 1-2 1.1.1 First Generation Mobile Systems ................................................................................................... 1-2 1.1.2 Second Generation Mobile Systems ............................................................................................... 1-2 1.1.3 Third Generation Mobile Systems .................................................................................................. 1-4 1.1.4 Fourth Generation Mobile Systems ................................................................................................ 1-5 1.2 3GPP Releases ........................................................................................................................................ 1-6 1.2.1 Pre-Release 99 ............................................................................................................................... 1-6 1.2.2 Release 99 ..................................................................................................................................... 1-7 1.2.3 Release 4 ....................................................................................................................................... 1-7 1.2.4 Release 5 ....................................................................................................................................... 1-7 1.2.5 Release 6 ....................................................................................................................................... 1-7 1.2.6 Release 7 ....................................................................................................................................... 1-8 1.2.7 Release 8 ....................................................................................................................................... 1-9 1.2.8 Release 9 and Beyond .................................................................................................................. 1-10 1.3 Radio Interface Techniques ................................................................................................................... 1-10 1.3.1 Frequency Division Multiple Access ............................................................................................ 1-10 1.3.2 Time Division Multiple Access .....................................................................................................1-11 1.3.3 Code Division Multiple Access .....................................................................................................1-11 1.3.4 Orthogonal Frequency Division Multiple Access .......................................................................... 1-12 1.4 Transmission Modes ............................................................................................................................. 1-12 1.4.1 Frequency Division Duplex.......................................................................................................... 1-13 1.4.2 Time Division Duplex .................................................................................................................. 1-13 1.5 Spectrum Usage.................................................................................................................................... 1-14 1.5.1 Frequency Bands ......................................................................................................................... 1-14 1.5.2 Existing Mobile Deployment ....................................................................................................... 1-16 1.5.3 LTE Release 8 Bands ................................................................................................................... 1-17 1.6 Channel Coding in LTE ........................................................................................................................ 1-20 1.6.1 Transport Block CRC................................................................................................................... 1-20 1.6.2 Code Block Segmentation and CRC Attachment........................................................................... 1-21 1.6.3 Channel Coding ........................................................................................................................... 1-23 1.6.4 Rate Matching ............................................................................................................................. 1-28
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1.6.5 Code Block Concatenation ........................................................................................................... 1-29 1.7 Principles of OFDM ............................................................................................................................. 1-30 1.7.2 Frequency Division Multiplexing ................................................................................................. 1-30 1.7.1 OFDM Subcarriers....................................................................................................................... 1-31 1.7.2 Fast Fourier Transforms ............................................................................................................... 1-31 1.7.3 LTE FFT Sizes ............................................................................................................................. 1-32 1.7.4 OFDM Symbol Mapping ............................................................................................................. 1-32 1.7.5 Time Domain Interference ........................................................................................................... 1-33 1.7.6 OFDM Advantages and Disadvantages......................................................................................... 1-35
2 LTE Physical Layer................................................................................................................ 2-1 2.1 The Uu Interface ..................................................................................................................................... 2-2 2.2 LTE Radio Interface Protocols ................................................................................................................ 2-2 2.2.1 Control and User Plane Protocols ................................................................................................... 2-3 2.2.2 Non Access Stratum....................................................................................................................... 2-3 2.2.3 RRC .............................................................................................................................................. 2-6 2.2.4 PDCP ............................................................................................................................................ 2-7 2.2.5 RLC .............................................................................................................................................. 2-7 2.2.6 MAC ............................................................................................................................................. 2-8 2.2.7 Physical ......................................................................................................................................... 2-9 2.3 LTE Channel Structure............................................................................................................................ 2-9 2.3.1 Logical Channels ........................................................................................................................... 2-9 2.3.2 Transport Channels .......................................................................................................................2-11 2.3.3 Physical Channels ........................................................................................................................ 2-12 2.3.4 Radio Channels............................................................................................................................ 2-13 2.3.5 Channel Mapping ........................................................................................................................ 2-13 2.4 LTE Frame Structure ............................................................................................................................ 2-14 2.4.1 Type 1 Radio Frames, Slots and Subframes .................................................................................. 2-14 2.4.2 Type 2 Radio Frames, Slots and Subframes .................................................................................. 2-16 2.5 OFDM Signal Generation ..................................................................................................................... 2-17 2.5.1 Codewords, Layers and Antenna Ports ......................................................................................... 2-18 2.5.2 Scrambling .................................................................................................................................. 2-19 2.5.3 Modulation Mapper ..................................................................................................................... 2-20 2.5.4 Layer Mapper .............................................................................................................................. 2-21 2.5.5 Precoding .................................................................................................................................... 2-22 2.5.6 Resource Element Mapper ........................................................................................................... 2-25 2.5.7 OFDM Signal Generation ............................................................................................................ 2-25 2.6 Downlink OFDMA............................................................................................................................... 2-25 2.6.1 General OFDMA Structure .......................................................................................................... 2-25 2.6.2 Physical Resource Blocks and Resource Elements ........................................................................ 2-26 2.7 LTE Physical Signals ............................................................................................................................ 2-27 2.8 Downlink Reference Signals ................................................................................................................. 2-30
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2.8.1 Cell Specific Reference Signals.................................................................................................... 2-30 2.8.2 MBSFN Reference Signals .......................................................................................................... 2-32 2.8.3 UE Specific Reference Signals ..................................................................................................... 2-33 2.9 Downlink LTE Physical Channels ......................................................................................................... 2-33 2.9.1 PBCH (Physical Broadcast Channel) ............................................................................................ 2-33 2.9.2 PCFICH (Physical Control Format Indicator Channel).................................................................. 2-34 2.9.3 PDCCH (Physical Downlink Control Channel)............................................................................. 2-36 2.9.4 PHICH (Physical Hybrid ARQ Indicator Channel) ....................................................................... 2-39 2.9.5 PDSCH (Physical Downlink Shared Channel) .............................................................................. 2-40 2.10 Downlink Control Signaling ............................................................................................................... 2-41 2.10.1 DCI Format 0............................................................................................................................. 2-41 2.10.2 DCI Format 1............................................................................................................................. 2-42 2.10.3 DCI Format 1A .......................................................................................................................... 2-42 2.10.4 DCI Format 1B .......................................................................................................................... 2-43 2.10.5 DCI Format 1C .......................................................................................................................... 2-43 2.10.6 DCI Format 1D .......................................................................................................................... 2-44 2.10.7 DCI Format 2............................................................................................................................. 2-44 2.10.8 DCI Format 2A .......................................................................................................................... 2-45 2.10.9 DCI Format 3............................................................................................................................. 2-45 2.10.10 DCI Format 3A ........................................................................................................................ 2-45 2.11 LTE Cell Search Procedure ................................................................................................................. 2-46 2.11.1 Cell Search ................................................................................................................................ 2-46 2.11.2 PSS Correlation ......................................................................................................................... 2-47 2.11.3 SSS Correlation ......................................................................................................................... 2-48 2.11.4 Master Information Block .......................................................................................................... 2-49 2.11.5 System Information Messages .................................................................................................... 2-49 2.11.6 PLMN Selection ........................................................................................................................ 2-54 2.11.7 Cell Selection............................................................................................................................. 2-56 2.12 Uplink Transmission Technique .......................................................................................................... 2-57 2.12.1 SC-FDMA Signal Generation ..................................................................................................... 2-57 2.13 OFDMA Verses SC-FDMA................................................................................................................. 2-60 2.14 Uplink LTE Physical Channels............................................................................................................ 2-60 2.14.1 PRACH (Physical Random Access Channel) .............................................................................. 2-61 2.14.2 PUSCH (Physical Uplink Shared Channel)................................................................................. 2-65 2.14.3 PUCCH (Physical Uplink Control Channel) ............................................................................... 2-67 2.15 Timing Relationships .......................................................................................................................... 2-68 2.16 Uplink Reference Signals.................................................................................................................... 2-69 2.16.1 Demodulation Reference Signal ................................................................................................. 2-70 2.16.2 Sounding Reference Signal ........................................................................................................ 2-71 2.17 Uplink Control Signaling .................................................................................................................... 2-74 2.17.1 PUCCH Format 1 ...................................................................................................................... 2-74 2.17.2 PUCCH Format 1a and 1b.......................................................................................................... 2-75
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2.18 LTE Random Access Procedure .......................................................................................................... 2-77 2.18.1 RRC Connection ........................................................................................................................ 2-77 2.18.2 PRACH Preambles .................................................................................................................... 2-78 2.18.3 Random Access Procedure Initialization ..................................................................................... 2-79 2.18.4 Random Access Response Window ............................................................................................ 2-81 2.18.5 Random Access Response .......................................................................................................... 2-81 2.18.6 Uplink Transmission .................................................................................................................. 2-82 2.19 Uplink Power Control ......................................................................................................................... 2-83 2.19.1 PUSCH Power Control .............................................................................................................. 2-83 2.19.2 PUCCH Power Control .............................................................................................................. 2-84 2.19.3 PRACH Power Control .............................................................................................................. 2-85 2.20 Paging Procedures .............................................................................................................................. 2-85 2.20.1 Discontinuous Reception for Paging........................................................................................... 2-85 2.20.2 Paging Frame............................................................................................................................. 2-86 2.21 HARQ Operation ................................................................................................................................ 2-87 2.21.1 Retransmission Types................................................................................................................. 2-87 2.21.2 HARQ Methods ......................................................................................................................... 2-87 2.21.3 HARQ in LTE............................................................................................................................ 2-89 2.21.4 HARQ In the Downlink ............................................................................................................. 2-90 2.21.5 HARQ In the Uplink .................................................................................................................. 2-90 2.21.6 ACK NACK Timing .................................................................................................................. 2-91 2.22 Diversity Options................................................................................................................................ 2-93 2.22.1 SU-MIMO and MU-MIMO ....................................................................................................... 2-93 2.22.2 MIMO and Transmission Options............................................................................................... 2-93 2.22.3 MIMO Modes ............................................................................................................................ 2-94 2.22.4 Spatial Multiplexing in LTE ....................................................................................................... 2-95 2.22.5 Feedback Reporting ................................................................................................................... 2-97
3 Dynamic Resource Allocation ............................................................................................. 3-1 3.1 Scheduling Principles and Signaling ....................................................................................................... 3-2 3.1.1 QoS in Packet Switched Networks ................................................................................................. 3-3 3.1.2 Key Factors Influencing Scheduling ............................................................................................... 3-4 3.1.3 Scheduling Methods ...................................................................................................................... 3-4 3.1.4 Downlink Scheduling..................................................................................................................... 3-5 3.1.5 PDSCH Resource Allocation .......................................................................................................... 3-6 3.1.6 Modulation and Coding Scheme..................................................................................................... 3-7 3.1.7 Uplink Scheduling ......................................................................................................................... 3-9 3.2 Scheduler Interaction .............................................................................................................................. 3-9 3.2.1 Radio Bearers ................................................................................................................................ 3-9 3.2.2 Scheduler Interaction with Layer 2 and Layer 1 .............................................................................. 3-9 3.3 Dynamic and Semi-persistent Scheduling.............................................................................................. 3-10 3.3.1 Dynamic Scheduling .....................................................................................................................3-11
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3.3.2 Downlink Semi-persistent Scheduling ...........................................................................................3-11 3.3.3 Uplink Semi-persistent Scheduling............................................................................................... 3-12
4 Intra LTE Mobility ................................................................................................................ 4-1 4.1 Intra-LTE Mobility ................................................................................................................................. 4-2 4.1.1 Idle State - Cell Reselection ........................................................................................................... 4-2 4.1.2 Active State Mobility ..................................................................................................................... 4-4 4.1.3 Handover Procedure ...................................................................................................................... 4-5 4.2 Reporting Options .................................................................................................................................. 4-6 4.2.1 Measurement Configuration Parameter........................................................................................... 4-6 4.2.2 Report Configuration Parameter ..................................................................................................... 4-7 4.3 Mobility Measurements .......................................................................................................................... 4-8 4.3.1 Measurement Gaps ........................................................................................................................ 4-8 4.3.2 Gap Configuration ......................................................................................................................... 4-9 4.3.3 UE Measurements.......................................................................................................................... 4-9
5 Glossary .................................................................................................................................. 5-1
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Figures
Figures Figure 1-1 Evolution of Cellular Networks ................................................................................................... 1-2 Figure 1-2 Second Generation Mobile Systems ............................................................................................ 1-3 Figure 1-3 Third Generation Mobile Systems ............................................................................................... 1-5 Figure 1-4 Forth Generation Mobile System ................................................................................................. 1-6 Figure 1-5 3GPP Releases ............................................................................................................................ 1-6 Figure 1-6 HSDPA....................................................................................................................................... 1-7 Figure 1-7 HSUPA....................................................................................................................................... 1-8 Figure 1-8 HSPA+ (Release 7) ..................................................................................................................... 1-9 Figure 1-9 Release 8 HSPA+ and LTE.......................................................................................................... 1-9 Figure 1-10 Release 9 and Beyond ............................................................................................................. 1-10 Figure 1-11 Radio Interface Techniques ..................................................................................................... 1-10 Figure 1-12 Frequency Division Multiple Access ........................................................................................1-11 Figure 1-13 Time Division Multiple Access ................................................................................................1-11 Figure 1-14 Code Division Multiple Access ............................................................................................... 1-12 Figure 1-15 Orthogonal Frequency Division Multiple Access ..................................................................... 1-12 Figure 1-16 Frequency Division Duplex..................................................................................................... 1-13 Figure 1-17 Time Division Duplex ............................................................................................................. 1-13 Figure 1-18 GSM Deployments ................................................................................................................. 1-16 Figure 1-19 Key UMTS Deployment Bands ............................................................................................... 1-17 Figure 1-20 EARFCN Calculation ............................................................................................................. 1-19 Figure 1-21 Example Downlink EARFCN Calculation ............................................................................... 1-19 Figure 1-22 Summary of LTE Transport Channel Processing ...................................................................... 1-20 Figure 1-23 Cyclic Redundancy Check Concept ......................................................................................... 1-21 Figure 1-24 CRC Parity Bits ...................................................................................................................... 1-21 Figure 1-25 Code Block Segmentation and CRC Attachment...................................................................... 1-22 Figure 1-26 Example Calculation for Segmentation and Filler Bits. ............................................................ 1-22
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Figure 1-27 Repetition Coding of the HI .................................................................................................... 1-24 Figure 1-28 Basic ½ Rate Convolutional Coder .......................................................................................... 1-25 Figure 1-29 Convolutional Coding Trellis .................................................................................................. 1-25 Figure 1-30 Example of Viterbi Decoding .................................................................................................. 1-26 Figure 1-31 Initializing Tail Biting Convolutional Coding .......................................................................... 1-27 Figure 1-32 LTE 1/3 Rate Tail Biting Convolutional Coding....................................................................... 1-27 Figure 1-33 LTE Turbo Coding .................................................................................................................. 1-28 Figure 1-34 LTE Rate Matching................................................................................................................. 1-28 Figure 1-35 Code Block Concatenation ...................................................................................................... 1-29 Figure 1-36 Use of OFDM in LTE ............................................................................................................. 1-30 Figure 1-37 FDM Carriers ......................................................................................................................... 1-30 Figure 1-38 OFDM Subcarriers.................................................................................................................. 1-31 Figure 1-39 Inverse Fast Fourier Transform ............................................................................................... 1-31 Figure 1-40 Fast Fourier Transform............................................................................................................ 1-32 Figure 1-41 OFDM Symbol Mapping ........................................................................................................ 1-33 Figure 1-42 OFDM PAPR (Peak to Average Power Ratio) .......................................................................... 1-33 Figure 1-43 Delay Spread .......................................................................................................................... 1-34 Figure 1-44 Inter Symbol Interference........................................................................................................ 1-34 Figure 1-45 Cyclic Prefix........................................................................................................................... 1-35 Figure 2-1 The LTE Air Interface ................................................................................................................. 2-2 Figure 2-2 LTE Control Plane and User Plane .............................................................................................. 2-3 Figure 2-3 E-UTRA Protocols ...................................................................................................................... 2-3 Figure 2-4 NAS Signaling............................................................................................................................ 2-4 Figure 2-5 Main RRC Functions .................................................................................................................. 2-6 Figure 2-6 PDCP Functions ......................................................................................................................... 2-7 Figure 2-7 RLC Modes and Functions.......................................................................................................... 2-8 Figure 2-8 Medium Access Control Functions .............................................................................................. 2-8 Figure 2-9 Physical Layer Functions ............................................................................................................ 2-9 Figure 2-10 LTE Channels ........................................................................................................................... 2-9 Figure 2-11 Location of Channels .............................................................................................................. 2-10 Figure 2-12 BCCH and PCCH Logical Channels........................................................................................ 2-10 Figure 2-13 CCCH and DCCH Signaling ....................................................................................................2-11 Figure 2-14 Dedicated Traffic Channel........................................................................................................2-11
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Figure 2-15 LTE Release 8 Transport Channels .......................................................................................... 2-12 Figure 2-16 Radio Channel ........................................................................................................................ 2-13 Figure 2-17 Downlink Channel Mapping ................................................................................................... 2-13 Figure 2-18 Uplink Channel Mapping ........................................................................................................ 2-14 Figure 2-19 LTE Frame Structure............................................................................................................... 2-15 Figure 2-20 Normal and Extended Cyclic Prefix ........................................................................................ 2-15 Figure 2-21 Normal CP Configuration ....................................................................................................... 2-16 Figure 2-22 Type 2 TDD Radio Frame ....................................................................................................... 2-17 Figure 2-23 Downlink Physical Layer Processing....................................................................................... 2-18 Figure 2-24 Codeword, Layer and Antenna Port Mapping .......................................................................... 2-18 Figure 2-25 Scrambling in LTE .................................................................................................................. 2-19 Figure 2-26 LTE Scrambling Code Generation ........................................................................................... 2-19 Figure 2-27 BPSK, QPSK and 16QAM Modulation Mapper ...................................................................... 2-20 Figure 2-28 64QAM Modulation Mapper ................................................................................................... 2-20 Figure 2-29 LTE Precoding Options ........................................................................................................... 2-23 Figure 2-30 Example of the Downlink Signal Generation Equation ............................................................ 2-25 Figure 2-31 OFDMA in LTE ...................................................................................................................... 2-26 Figure 2-32 Physical Resource Block and Resource Element ...................................................................... 2-27 Figure 2-33 Downlink Cell ID ................................................................................................................... 2-28 Figure 2-34 PSS and SSS Location for FDD .............................................................................................. 2-28 Figure 2-35 PSS and SSS Location for TDD .............................................................................................. 2-29 Figure 2-36 SSS Scrambling ...................................................................................................................... 2-30 Figure 2-37 Reference Signals - One Antenna Port ..................................................................................... 2-31 Figure 2-38 Reference Signal Physical Cell ID Offset ................................................................................ 2-31 Figure 2-39 Reference Signals - Two Antenna Ports (Normal CP)............................................................... 2-31 Figure 2-40 Reference Signals - Four Antenna Ports (Normal CP) .............................................................. 2-32 Figure 2-41 MBSFN Reference Signals ..................................................................................................... 2-33 Figure 2-42 UE Specific Reference Signals ................................................................................................ 2-33 Figure 2-43 Broadcast Signaling ................................................................................................................ 2-34 Figure 2-44 MIB to PBCH Mapping (FDD and Normal CP)....................................................................... 2-34 Figure 2-45 CFI to PCFICH Mapping ........................................................................................................ 2-35 Figure 2-46 FDD Downlink Control Region............................................................................................... 2-36 Figure 2-47 REG to CCE and PDCCH Mapping ........................................................................................ 2-37
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Figure 2-48 PDCCH to Control Region Mapping ....................................................................................... 2-37 Figure 2-49 CCE Allocation Levels............................................................................................................ 2-38 Figure 2-50 Common and UE-Specific Search Spaces ................................................................................ 2-38 Figure 2-51 PHICH Mapping..................................................................................................................... 2-39 Figure 2-52 Extended PHICH Example...................................................................................................... 2-40 Figure 2-53 Generic PDSCH Mapping ....................................................................................................... 2-40 Figure 2-54 Initial Procedures .................................................................................................................... 2-46 Figure 2-55 PSS and SSS for Cell Search (FDD Mode) .............................................................................. 2-46 Figure 2-56 Physical Cell Identities............................................................................................................ 2-47 Figure 2-57 PSS Correlation ...................................................................................................................... 2-47 Figure 2-58 SSS Correlation Example ........................................................................................................ 2-48 Figure 2-59 PBCH and the Master Information Block ................................................................................ 2-49 Figure 2-60 System Information Block Type 1 ........................................................................................... 2-50 Figure 2-61 Example of SI Mapping .......................................................................................................... 2-51 Figure 2-62 System Information Block Type 2 ........................................................................................... 2-52 Figure 2-63 System Information Block Type 3 ........................................................................................... 2-52 Figure 2-64 System Information Block Type 4 ........................................................................................... 2-52 Figure 2-65 System Information Block Type 5 ........................................................................................... 2-53 Figure 2-66 System Information Block Type 6 ........................................................................................... 2-53 Figure 2-67 System Information Block Type 7 ........................................................................................... 2-53 Figure 2-68 System Information Block Type 8 ........................................................................................... 2-54 Figure 2-69 System Information Block Type 9 ........................................................................................... 2-54 Figure 2-70 PLMN Selection ..................................................................................................................... 2-54 Figure 2-71 LTE Cell Selection .................................................................................................................. 2-56 Figure 2-72 SC-FDMA Subcarrier Mapping Concept ................................................................................. 2-58 Figure 2-73 SC-FDMA Signal Generation.................................................................................................. 2-59 Figure 2-74 SC-FDMA and the eNB .......................................................................................................... 2-59 Figure 2-75 Example of the Uplink Signal Generation Equation ................................................................. 2-60 Figure 2-76 Release 8 Uplink Physical Channels ........................................................................................ 2-61 Figure 2-77 PRACH Preamble ................................................................................................................... 2-61 Figure 2-78 PRACH Guard Period ............................................................................................................. 2-62 Figure 2-79 PRACH FDD Formats ............................................................................................................ 2-63 Figure 2-80 PRACH Configuration ............................................................................................................ 2-63
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Figure 2-81 PRACH Configuration and Preamble Sequences Per Cell ........................................................ 2-65 Figure 2-82 PUSCH Mapping .................................................................................................................... 2-66 Figure 2-83 Multiplexing Control Signaling ............................................................................................... 2-66 Figure 2-84 Mapping to Physical Resource Blocks for PUCCH .................................................................. 2-67 Figure 2-85 FDD Timing ........................................................................................................................... 2-68 Figure 2-86 Example of TDD Configuration 2 ........................................................................................... 2-69 Figure 2-87 Uplink Reference Signals ........................................................................................................ 2-69 Figure 2-88 DRS Sequence Group Selection .............................................................................................. 2-70 Figure 2-89 Uplink Demodulation Reference Signal (Normal CP) .............................................................. 2-71 Figure 2-90 Uplink Demodulation Reference Signal (Extended CP) ........................................................... 2-71 Figure 2-91 Requirement for SRS .............................................................................................................. 2-72 Figure 2-92 Example of SRS Frequency Hopping ...................................................................................... 2-72 Figure 2-93 Example SRS Allocation ......................................................................................................... 2-73 Figure 2-94 PUCCH Format 1a and 1b (Normal CP) .................................................................................. 2-75 Figure 2-95 PUCCH Format 2 (Normal CP)............................................................................................... 2-76 Figure 2-96 PUCCH Format 2 (Extended CP) ............................................................................................ 2-76 Figure 2-97 PUCCH Format 2a and 2b ACK/NACK Coding ...................................................................... 2-77 Figure 2-98 Overall Random Access Procedure .......................................................................................... 2-77 Figure 2-99 Random Access RRC Signaling Procedure .............................................................................. 2-78 Figure 2-100 PRACH Probing ................................................................................................................... 2-78 Figure 2-101 Allocating Preambles to Group A and Group B ...................................................................... 2-80 Figure 2-102 Random Access Response Window ....................................................................................... 2-81 Figure 2-103 MAC Random Access Response ........................................................................................... 2-81 Figure 2-104 Random Access - Assigned UL-SCH ..................................................................................... 2-82 Figure 2-105 MAC Contention Resolution ................................................................................................. 2-83 Figure 2-106 Uplink Power Control ........................................................................................................... 2-83 Figure 2-107 Paging Issues ........................................................................................................................ 2-85 Figure 2-108 System with DRX Reception of Paging ................................................................................. 2-86 Figure 2-109 ARQ Verses HARQ............................................................................................................... 2-87 Figure 2-110 Basic Concept of SAW .......................................................................................................... 2-88 Figure 2-111 HARQ Parallel Processes ...................................................................................................... 2-88 Figure 2-112 HARQ Methods .................................................................................................................... 2-88 Figure 2-113 Example of Redundancy Versions and Soft Bits ..................................................................... 2-89
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Figures
Figure 2-114 FDD HARQ Processes .......................................................................................................... 2-90 Figure 2-115 Downlink FDD HARQ Timing.............................................................................................. 2-92 Figure 2-116 Uplink FDD HARQ Timing .................................................................................................. 2-92 Figure 2-117 SU-MIMO and MU-MIMO ................................................................................................... 2-93 Figure 2-118 Spatial Multiplexing MIMO .................................................................................................. 2-94 Figure 2-119 Spatial Multiplexing Interference Issues ................................................................................ 2-94 Figure 2-120 MIMO Space Time Coding ................................................................................................... 2-95 Figure 2-121 AMS Concept ....................................................................................................................... 2-95 Figure 2-122 PDSCH Processing ............................................................................................................... 2-96 Figure 2-123 Feedback Reporting .............................................................................................................. 2-97 Figure 2-124 4-bit CQI Table ..................................................................................................................... 2-97 Figure 3-1 IP Scheduling ............................................................................................................................. 3-2 Figure 3-2 Basic Scheduling in a Cell .......................................................................................................... 3-2 Figure 3-3 Packet Classifier and Packet Scheduler ....................................................................................... 3-3 Figure 3-4 Key Factors Influencing Scheduling ............................................................................................ 3-4 Figure 3-5 Possible Scheduling Method ....................................................................................................... 3-4 Figure 3-6 Type 0 Resource Allocation......................................................................................................... 3-6 Figure 3-7 Type 1 Resource Allocation......................................................................................................... 3-7 Figure 3-8 Type 2 Resource Allocation......................................................................................................... 3-7 Figure 3-9 Using the TBS Size..................................................................................................................... 3-8 Figure 3-10 Scheduler Interaction .............................................................................................................. 3-10 Figure 3-11 Dynamic Scheduling ................................................................................................................3-11 Figure 3-12 Semi Persistent Scheduling ..................................................................................................... 3-12 Figure 4-1 Intra-LTE Mobility ..................................................................................................................... 4-2 Figure 4-2 Intra-Frequency and Inter-frequency ........................................................................................... 4-2 Figure 4-3 Sintrasearch Parameter ..................................................................................................................... 4-3 Figure 4-4 Impact to Treselection ................................................................................................................. 4-4 Figure 4-5 Ranking Equation ....................................................................................................................... 4-4 Figure 4-6 Intra-LTE Mobility ..................................................................................................................... 4-5 Figure 4-7 LTE Handover Procedure ............................................................................................................ 4-5 Figure 4-8 Measurement Configuration Parameters ...................................................................................... 4-6 Figure 4-9 Report Configuration Parameters ................................................................................................ 4-7 Figure 4-10 Periodic and Event Reporting .................................................................................................... 4-8
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Figure 4-11 Non Gap Assisted...................................................................................................................... 4-8 Figure 4-12 Gap Assisted ............................................................................................................................. 4-9 Figure 4-13 Gap Configuration .................................................................................................................... 4-9
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Tables
Tables Table 1-1 2.5G and 2.75G GSM/GPRS Systems ........................................................................................... 1-3 Table 1-2 IMT Advanced Features................................................................................................................ 1-5 Table 1-3 GSM Frequency Bands ............................................................................................................... 1-14 Table 1-4 UMTS FDD Frequency Bands .................................................................................................... 1-15 Table 1-5 UMTS TDD Frequency Bands .................................................................................................... 1-15 Table 1-6 LTE Release 8 Frequency Bands ................................................................................................. 1-18 Table 1-7 Transport Channel Coding Options ............................................................................................. 1-23 Table 1-8 Control Information Coding Options........................................................................................... 1-23 Table 1-9 CFI Encoding ............................................................................................................................. 1-24 Table 1-10 Convolutional Coding Example ................................................................................................ 1-25 Table 1-11 Standard Convolutional Coding Verses Tail Biting Convolutional Coding .................................. 1-27 Table 1-12 LTE Sub-block Interleaver ........................................................................................................ 1-29 Table 1-13 LTE Channel and FFT Sizes...................................................................................................... 1-32 Table 2-1 NAS EMM and ESM Procedures .................................................................................................. 2-4 Table 2-2 Downlink CP Parameters ............................................................................................................ 2-16 Table 2-3 Type 2 Radio Frame Switching Points......................................................................................... 2-17 Table 2-4 Layer Mapper Configuration....................................................................................................... 2-21 Table 2-5 Codeword to Layer Mapping for Spatial Multiplexing................................................................. 2-21 Table 2-6 Codeword to Layer Mapping for Transmit Diversity ................................................................... 2-22 Table 2-7 Codebook for Transmission for Two Antenna Ports ..................................................................... 2-24 Table 2-8 Downlink PRB Parameters ......................................................................................................... 2-27 Table 2-9 Example of SSS Indices.............................................................................................................. 2-29 Table 2-10 CFI Mapping ............................................................................................................................ 2-35 Table 2-11 CFI Codewords......................................................................................................................... 2-36 Table 2-12 DCI Formats............................................................................................................................. 2-41 Table 2-13 DCI Ambiguous Sizes of Information Bits ................................................................................ 2-42
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Tables
Table 2-14 Precoding Information Field for 4 Antenna Ports (Open Loop) .................................................. 2-45 Table 2-15 Cell Selection Parameters ......................................................................................................... 2-56 Table 2-16 SC-FDMA verses OFDMA ....................................................................................................... 2-60 Table 2-17 Random Access Preamble Parameters ....................................................................................... 2-62 Table 2-18 PRACH Configuration Index .................................................................................................... 2-64 Table 2-19 “K” Values for TDD Configurations.......................................................................................... 2-68 Table 2-20 PUCCH Formats ...................................................................................................................... 2-74 Table 2-21 Parameters for Random Access ................................................................................................. 2-79 Table 2-22 FDD Subframe Patterns ............................................................................................................ 2-86 Table 2-23 TDD Subframe Patterns ............................................................................................................ 2-87 Table 2-24 TDD HARQ Processes ............................................................................................................. 2-90 Table 2-25 UL HARQ Operation ................................................................................................................ 2-91 Table 2-26 Codebook Precoding................................................................................................................. 2-96 Table 3-1 Modulation and TBS index table for PDSCH ................................................................................ 3-7
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Objectives On completion of this section the participants will be able to: 1.1 Describe the evolution of cellular networks. 1.2 Summarize the evolution of 3GPP releases, from release 99 to release 8. 1.3 Describe radio interface techniques. 1.4 Explain the difference between FDD and TDD mode. 1.5 Describe flexible spectrum usage. 1.6 Explain the concepts of channel coding and FEC (Forward Error Correction). 1.7 Describe the principles for OFDM.
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1.1 Evolution of Cellular Networks Cellular mobile networks have been evolving for many years. The initial networks are referred to as “First Generation”. These have now been replaced with “Second Generation” and “Third Generation” networks. It is only now that 4G or “Fourth Generation” systems are being deployed. Figure 1-1 Evolution of Cellular Networks
1.1.1 First Generation Mobile Systems The 1G (First Generation) mobile systems were not digital, i.e. they utilized analogue modulation techniques. The main systems included:
AMPS (Advanced Mobile Telephone System) - This first appeared in 1976 in the United States. It was mainly implemented in the Americas, Russia and Asia. Various issues including weak security features made the system prone to hacking and handset cloning.
TACS (Total Access Communications System) - This was the European version of AMPS with slight modifications, as well as operating in different frequency bands. It was mainly used in the United Kingdom, as well as parts of Asia.
ETACS (Extended Total Access Communication System) - This provided an improved version of TACS. It enabled a greater number of channels and therefore facilitated more users.
These analogue systems were all proprietary based FM (Frequency Modulation) systems and therefore they all lacked security, any meaningful data service and international roaming capability.
1.1.2 Second Generation Mobile Systems 2G (Second Generation) systems utilize digital multiple access technology, such as TDMA (Time Division Multiple Access) and CDMA (Code Division Multiple Access). Figure 1-2 illustrates some of the different 2G mobile systems, these include:
1-2
GSM (Global System for Mobile communications) - this is the most successful of all 2G technologies. It was initially developed by ETSI (European Telecommunications Standards Institute) for Europe and designed to operate in the 900MHz and 1800MHz frequency bands. It now has world-wide support and is available for deployment on many other frequency bands, such as 850MHz and 1900MHz. A mobile described as tri-band or quad-band indicates support for multiple frequency bands on the same device. GSM is TDMA, such that it employs 8 timeslots on a 200kHz radio carrier.
cdmaOne - this is a CDMA (Code Division Multiple Access) system based on IS-95 (Interim Standard 95). It uses a spread spectrum technique and utilizes a mixture of codes and timing to identify cells and channels. The system bandwidth is 1.25MHz.
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D-AMPS (Digital - Advanced Mobile Phone System) - this is based on IS-136 (Interim Standard 136) and is effectively an enhancement to AMPS which provides a TDMA access technique. It has been primarily used on the North American continent, as well as in New Zealand and parts of Asia-Pacific.
Figure 1-2 Second Generation Mobile Systems
In addition to being digital, as well as improving capacity and security, these 2G digital systems also offer enhanced services such as SMS (Short Message Service) and circuit switched data.
2.5G Systems Most 2G systems are being evolved. For example, GSM was extended with GPRS (General Packet Radio System) to support efficient packet data services, as well as increasing the data rates. As this feature does not meet 3G requirements, GRPS is often referred to as 2.5G. A comparison between 2G and 2.5G systems is illustrated in Table 1-1.
2.75G Systems GSM/GPRS systems also added EDGE (Enhanced Data Rates for Global Evolution). This nearly quadruples the throughput of GPRS. The theoretical data rate of 473.6kbit/s enables service providers to efficiently offer multimedia services. Like GPRS, since it does not comply with all the features of a 3G system, EDGE is usually categorized as 2.75G.
Table 1-1 2.5G and 2.75G GSM/GPRS Systems System
Service
Theoretical Data Rate
Typical Data Rate
2G GSM
Circuit Switched Data Service
9.6kbit/s or 14.4kbit/s
9.6kbit/s or 14.4kbit/s
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2.5G GPRS
Packet Switched Data
171.2kbit/s
4kbit/s to 50kbit/s
2.75G EDGE
Packet Switched Data
473.6kbit/s
120kbit/s
1.1.3 Third Generation Mobile Systems 3G (Third Generation) systems are defined by IMT2000 (International Mobile Telecommunications - 2000). IMT2000 defines that a 3G system should provide higher transmission rates, for example: 2Mbit/s for stationary or nomadic use and 348kbit/s in a moving vehicle. The main 3G technologies are illustrated in Figure 1-3.These include:
1-4
WCDMA (Wideband CDMA) - This was developed by the 3GPP (Third Generation Partnership Project). There are numerous variations on this standard, including TD-CDMA and TD-SCDMA. WCDMA is the main evolutionary path from GSM/GPRS networks. It is a FDD (Frequency Division Duplex) based system and occupies a 5MHz carrier. Current deployments are mainly at 2.1GHz, however deployments at lower frequencies are also being seen, e.g. UMTS1900, UMTS850, UMTS900 etc. WCDMA supports voice and multimedia services with an initial theoretical rate of 2Mbit/s, with most service providers initially offering 384kbit/s per user. However, this technology is continuing to evolve and later 3GPP releases have increased the rates to in excess of 40Mbit/s.
TD-CDMA (Time Division CDMA) - This is typically referred to as UMTS TDD (Time Division Duplex) and is part of the UMTS specifications, however it has only limited support. The system utilizes a combination of CDMA and TDMA to enable efficient allocation of resources.
TD-SCDMA (Time Division Synchronous CDMA) - This was jointly developed by Siemens and the CATT (China Academy of Telecommunications Technology). TD-SCDMA has links to the UMTS specifications and is often identified as UMTS-TDD LCR (Low Chip Rate). Like TD-CDMA, it is also best suited to low mobility scenarios in micro or pico cells.
CDMA2000 - This is a multi-carrier technology standard which uses CDMA. CDMA2000 is actually a set of standards including CDMA2000 EV-DO (Evolution-Data Optimized) which has various “revisions”. It is worth noting that CDMA2000 is backward compatible with cdmaOne.
WiMAX (Worldwide Interoperability for Microwave Access) - This is another wireless technology which satisfies IMT2000 3G requirements. The air interface is part of the IEEE (Institute of Electrical and Electronics Engineers) 802.16 standard which originally defined PTP (Point-To-Point) and PTM (Point-To-Multipoint) systems. This was later enhanced to provide mobility and greater flexibility. The success of WiMAX is mainly down to the “WiMAX Forum”, which is an organization formed to promote conformity and interoperability between vendors.
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Figure 1-3 Third Generation Mobile Systems
1.1.4 Fourth Generation Mobile Systems 4G (Fourth Generation) cellular wireless systems need to meet the requirements set by the ITU (International Telecommunication Union) as part of IMT Advanced (International Mobile Telecommunications Advanced). These features are illustrated in Table 1-2 and enable IMT Advanced to address evolving user needs. Table 1-2 IMT Advanced Features Key IMT Advanced Features A high degree of commonality of functionality worldwide while retaining the flexibility to support a wide range of services and applications in a cost efficient manner. Compatibility of services within IMT and with fixed networks. Capability of interworking with other radio access systems. High quality mobile services. User equipment suitable for worldwide use. User-friendly applications, services and equipment. Worldwide roaming capability. Enhanced peak data rates to support advanced services and applications (100Mbit/s for high and 1Gbit/s for low mobility were identified as targets). The main three 4G systems include:
LTE Advanced - LTE (Long Term Evolution) is part of 3GPP, however it does not meet all IMT Advanced features, as such it is sometimes referred to as 3.99G. In contrast, LTE Advanced is part of a later 3GPP Release and has been designed specifically to meet 4G requirements.
WiMAX 802.16m - The IEEE and the WiMAX Forum have identified 802.16m as their offering for a 4G system.
UMB (Ultra Mobile Broadband) - This is identified as EV-DO Rev C. It is part of 3GPP2 however most vendors and service providers have decided to promote LTE instead.
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Figure 1-4 Forth Generation Mobile System
1.2 3GPP Releases The development of GSM, GPRS, EDGE, UMTS, HSPA and LTE is in stages known as 3GPP releases. Hardware vendors and software developers use these releases as part of their development roadmap. Figure 1-5 illustrates the main 3GPP Releases that enhance the radio interface. Figure 1-5 3GPP Releases
3GPP Releases enhance various aspects, not just the radio interface. For example, Release 5 started the introduction of the IMS (IP Multimedia Subsystem) in the core network.
1.2.1 Pre-Release 99 Pre-Release 99 saw the introduction of GSM, as well as the addition of GPRS. The main GSM Phases and 3GPP Releases include:
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GSM Phase 1.
GSM Phase 2.
GSM Phase 2+ (Release 96).
GSM Phase 2+ (Release 97).
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GSM Phase 2+ (Release 98).
1.2.2 Release 99 3GPP Release 99 saw the introduction of UMTS, as well as the EDGE enhancement to GPRS. UMTS contains all features needed to meet the IMT-2000 requirements as defined by the ITU. It is able to support both CS (Circuit Switched) voice and video services, as well PS (Packet Switched) data services over common and dedicated bearers. Initial data rates for UMTS were 64kbit/s, 128kbit/s and 384kbit/s. Note that the theoretical maximum was 2Mbit/s.
1.2.3 Release 4 Release 4 included enhancements to the core network. The concept of “All IP Networks” was included and service providers were able to deploy Soft Switch based networks, i.e. the MSC (Mobile Switching Centre) was replaced by MSC Servers and MGW (Media Gateways).
1.2.4 Release 5 Release 5 is the first major addition to the UMTS air interface. It adds HSDPA (High Speed Downlink Packet Access) which improves capacity and spectral efficiency. Figure 1-6 illustrates some of the main features which include:
Adaptive Modulation - In addition to the original UMTS modulation scheme, QPSK (Quadrature Phase Shift Keying), HSDPA also includes support for 16 QAM (Quadrature Amplitude Modulation).
Flexible Coding - Based on fast feedback from the mobile in the form of a CQI (Channel Quality Indicator) the UMTS base station, i.e. the Node B, is able to modify the effective coding rate and thus increase system efficiency.
Fast Scheduling - HSDPA includes a 2ms TTI (Time Transmission Interval), which enables the Node B scheduler to quickly and efficiently allocate resources to mobiles.
HARQ (Hybrid Automatic Repeat Request) - In the event a packet does not get through to the UE (User Equipment) successfully, the system employs HARQ (Hybrid Automatic Repeat Request). This improves the retransmission timing, thus requiring less reliance on the RNC (Radio Network Controller).
Figure 1-6 HSDPA
1.2.5 Release 6 Release 6 adds various features, with HSUPA (High Speed Uplink Packet Data) being of most interest to RAN development. Even though the term HSUPA is widespread, this 3GPP enhancement also goes under the term “Enhanced Uplink”. It is also worth noting that
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HSDPA and HSUPA work in tandem and thus the term HSPA (High Speed Packet Access) is used. HSUPA, like HSDPA adds functionality to improve packet data. Figure 1-7 illustrates the three main enhancements which include:
Flexible Coding - HSUPA has the ability to dynamically change the coding and therefore improve the efficiency of the system.
Fast Power Scheduling - A key fact of HSUPA is that it provides a method to schedule the power from different mobiles. This scheduling can use either a 2ms or 10ms TTI.
HARQ - Like HSDPA, HSUPA also utilizes HARQ. The main difference is the timing relationship for the retransmission.
Figure 1-7 HSUPA
1.2.6 Release 7 The main RAN based feature of Release 7 is HSPA+. This, like HSDPA and HSUPA, provides various enhancements to improve packet switched data delivery. Figure 1-8 illustrates the main features which include:
1-8
64 QAM - This is added to the DL (Downlink) and enables HSPA+ to operate at a theoretical rate of 21.6Mbit/s.
16 QAM - This is added to the UL (Uplink) and enables the uplink to theoretically achieve 11.76Mbit/s.
MIMO (Multiple Input Multiple Output) Operation - this is added to HSPA+ Release 7 and offers various benefits including the ability to offer a theoretical 28.8Mbits/s in the downlink.
Power Enhancements -Various enhancements such as CPC (Continuous Packet Connectivity) have been included. Thus enabling DTX (Discontinuous Transmission), DRX (Discontinuous Reception) and HS-SCCH (High Speed - Shared Control Channel) Less Operation. Collectively these improve the mobile’s battery consumption.
Less Overhead - The downlink includes an enhancement to the MAC (Medium Access Control) layer which effectively means that fewer headers are required. This in turn improves the system efficiency.
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Figure 1-8 HSPA+ (Release 7)
1.2.7 Release 8 There are many additions to the RAN functionality in Release 8, such as enhancements to HSPA+. However the main aspect is the inclusion of LTE (Long Term Evolution). Figure 1-9 illustrates some of the main features for Release 8 HSPA+ and LTE. Release 8 HSPA+ enables various key enhancements, these include:
64 QAM and MIMO - Release 8 enables the combination of 64 QAM and MIMO, thus quoting a theoretical rate of 42Mbit/s, i.e. 2 x 21.6Mbit/s.
Dual Cell Operation - DC-HSDPA (Dual Cell - HSDPA) is a Release 8 feature which is further enhanced in Release 9 and Release 10. It enables a mobile to effectively utilize two 5MHz UMTS carriers. Assuming both are using 64 QAM (21.6Mbit/s), the theoretical maximum is 42Mbps. Note that in Release 8 a mobile is not able to combine MIMO and DC-HSDPA.
Less Uplink Overhead - In a similar way to Release 7 in the downlink, the Release 8 uplink has been enhanced to reduce overhead.
Figure 1-9 Release 8 HSPA+ and LTE
LTE provides a new radio access technique, as well as enhancements in the E-UTRAN (Evolved - Universal Terrestrial Radio Access Network). These enhancements are further discussed as part of this course. Issue 01 (2010-05-01)
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1.2.8 Release 9 and Beyond Even though LTE is a Release 8 system, it is further enhanced in Release 9. There are a huge number of features in Release 9. One of the most important is the support of additional frequency bands. Figure 1-10 Release 9 and Beyond
Release 10 includes the standardization of LTE Advanced, i.e. the 3GPP’s 4G offering. As such it includes modification to the LTE system to facilitate 4G services.
1.3 Radio Interface Techniques In wireless cellular systems, mobiles have to share a common medium for transmission. There are various categories of assignment, the main four include: FDMA (Frequency Division Multiple Access), TDMA (Time Division Multiple Access), CDMA (Code Division Multiple Access) and OFDMA (Orthogonal Frequency Division Multiple Access). Figure 1-11 Radio Interface Techniques
1.3.1 Frequency Division Multiple Access In order to accommodate various devices on the same wireless network, FDMA divides the available spectrum into sub-bands or channels. The concept of FDMA is illustrated in Figure 1-12. Using this technique a dedicated channel can be allocated to a user, whilst other users occupy other channels, i.e. frequencies. In a cellular system mobiles typically occupy multiple channels; one for the downlink and one for the uplink. This does however make FDMA less efficient since most data applications are downlink intensive.
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Figure 1-12 Frequency Division Multiple Access
FDMA channels also suffer since they cannot be close together due to the energy from one transmission affecting the adjacent/neighboring channels. To combat this, additional guard bands between channels are required, which also reduces the system’s spectral efficiency.
1.3.2 Time Division Multiple Access In TDMA systems the channel bandwidth is shared in the time domain. Figure 1-13 illustrates the concept of TDMA. It shows how each device is allocated a time on the channel, known as a “timeslot”. These are then grouped into a TDMA frame. The number of timeslots in a TDMA frame is dependent on the system, for example GSM utilizes 8 timeslots. Figure 1-13 Time Division Multiple Access
Devices must be allocated a timeslot; therefore it is usual to have one or more timeslots reserved for common control and system access. TDMA systems are normally digital and therefore offer additional features such as ciphering and integrity. In addition, they can employ enhanced error detection and correction schemes including FEC (Forward Error Correction). This enables the system to be more resilient to noise and interference and therefore they have a greater spectral efficiency when compared to FDMA systems.
1.3.3 Code Division Multiple Access The concept of CDMA is slightly different to that of FDMA and TDMA. Instead of sharing resources in the time or frequency domain, the devices are able to use the system at the same time and using the same frequency/bandwidth. This is possible due to the fact that each transmission is separated using a unique code. There are two main types of CDMA, FHSS (Frequency Hopping Spread Spectrum) and DSSS (Direct Sequence Spread Spectrum), with all the current cellular systems utilizing DSSS.
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Figure 1-14 illustrates the basic concept of CDMA. The narrowband signals are spread with a wideband code and then transmitted. The receivers are designed to extract the encoded signal (with the correct code) and reject everything else as noise. Figure 1-14 Code Division Multiple Access
UMTS, cdmaOne and CDMA2000 all use CDMA. However the implementation of the codes and the bandwidths used is different. For example UMTS utilizes a 5MHz channel bandwidth, whereas cdmaOne uses only 1.25MHz.
1.3.4 Orthogonal Frequency Division Multiple Access OFDMA (Orthogonal Frequency Division Multiple Access) is the latest addition to cellular systems. It provides a multiple access technique based on OFDM (Orthogonal Frequency Division Multiplexing). Figure 1-15 illustrates the basic view of OFDMA. It can be seen that the bandwidth is broken down to smaller units known as “subcarriers”. These are grouped together and allocated as a resource to a device. It can also be seen that a device can be allocated different resources in both the time and frequency domain. Additional detail on OFDM and OFDMA is provided in Section 1.7 and 2.6 . Figure 1-15 Orthogonal Frequency Division Multiple Access
1.4 Transmission Modes Cellular systems can be designed to operate in two main transmission modes, namely FDD (Frequency Division Duplex) and TDD (Time Division Duplex).
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1.4.1 Frequency Division Duplex The concept of FDD is illustrated in Figure 1-16. A separate uplink and downlink channel are utilized, enabling a device to transmit and receive data at the same time (assuming the device incorporates a duplexer). The spacing between the uplink and downlink channel is referred to as the duplex spacing. Figure 1-16 Frequency Division Duplex
Normally the uplink channel (mobile transmit) operates on the lower frequency. This is done because higher frequencies suffer greater attenuation than lower frequencies and therefore it enables the mobile to utilize lower transmit levels. Some systems also offer half-duplex FDD mode, where two frequencies are utilized, however the mobile can only transmit or receive, i.e. not transmit and receive at the same time. This allows for reduced mobile complexity since no duplex filter is required.
1.4.2 Time Division Duplex TDD mode enables full duplex operation using a single frequency band and time division multiplexing the uplink and downlink signals. One advantage of TDD is its ability to provide asymmetrical uplink and downlink allocation. Depending on the system, other advantages include dynamic allocation, increased spectral efficiency, as well as the improved use of beamforming techniques - this is due to having the same uplink and downlink frequency characteristics. Figure 1-17 Time Division Duplex
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1.5 Spectrum Usage One of the main factors in any cellular system is the frequency of deployment. Most 2G, 3G and 4G systems offer multiple options. For example, GSM can be deployed at various bands including: 900MHz, 1800MHz, 1900MHz, 850MHz etc.
1.5.1 Frequency Bands Each cellular system defines its own set of frequency bands it is able to operate on. In order to identify possible LTE bands it is worth noting the bands used by other technologies such as GSM, UMTS etc.
GSM Bands Table 1-3 illustrates the main frequency bands defined for GSM. However, this does not guarantee that the spectrum is available since there may be regulatory issues, as well as limitations in some handsets and base stations. The initial GSM band was referred to as P-GSM (Primary GSM). This was mainly defined to replace the TACS system which was also in the 900MHz band. Other 900MHz bands which were added include E-GSM (Extended GSM) and R-GSM (Railways GSM) bands, providing more channels and support of a railway based variant. Finally, other bands away from the 900MHz band are also available; however the support for 450MHz and 480MHz is limited. The terms DCS (Digital Cellular Service) and PCS (Personal Communications Service) are typically used in Europe and North America respectively to identify the higher frequency deployment options. It was expected that these frequencies would offer a better re-use in built up areas and therefore provide additional capacity. Table 1-3 GSM Frequency Bands Operating Band
Frequency Band
Uplink Frequency (MHz)
Downlink Frequency (MHz)
GSM 400
450
450.4 - 457.6
460.4 - 467.6
GSM 400
480
478.8 - 486.0
488.8 - 496.0
GSM 850
850
824.0 - 849.0
869.0 - 894.0
GSM 900 (P-GSM)
900
890.0 - 915.0
935.0 - 960.0
GSM 900 (E-GSM)
900
880.0 - 915.0
925.0 - 960.0
GSM-R (R-GSM)
900
876.0 - 880.0
921.0 - 925.0
DCS 1800
1800
1710.0 - 1785.0
1805.0 - 1880.0
PCS 1900
1900
1850.0 - 1910.0
1930.0 - 1990.0
UMTS Bands UMTS, like GSM, has a number of frequency bands defined. These are identified by an “Operating Band” number which is illustrated in Table 1-4, along with the associated Uplink and downlink frequency ranges.
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Table 1-4 UMTS FDD Frequency Bands Operating Band
Frequency Band
Uplink Frequency (MHz)
Downlink Frequency (MHz)
I
2100
1920 - 1980
2110 - 2170
II
1900
1850 - 1910
1930 - 1990
III
1800
1710 - 1785
1805 - 1880
IV
1700
1710 - 1755
2110 - 2155
V
850
824 - 849
869 - 894
VI
800
830 - 840
875 - 885
VII
2600
2500 - 2570
2620 - 2690
VIII
900
880 - 915
925 - 960
IX
1700
1749.9 - 1784.9
1844.9 - 1879.9
X
1700
1710 - 1770
2110 - 2170
XI
1500
1427.9 - 1452.9
1475.9 - 1500.9
XII
700
698 - 716
728 - 746
XIII
700
777 - 787
746 - 756
XIV
700
788 - 798
758 - 768
In addition to the previous UMTS FDD bands, various UMTS TDD bands are also defined. Table 1-5 illustrates the main TDD bands, however the majority of these have never been implemented. Table 1-5 UMTS TDD Frequency Bands Frequency Band 1900 - 1920 2010 - 2025 1850 - 1910 1930 - 1990 1910 - 1930 2570 - 2620
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1.5.2 Existing Mobile Deployment The list of current mobile service providers is constantly increasing. The latest list of GSM/UMTS and LTE operators is maintained by the GSMA (GSM Association).
GSM Deployments Figure 1-18 summarizes the main GSM deployment bands. It can be seen that GSM 900 and GSM 1800 are used in most parts of the world, i.e. Europe, Middle East, Africa and most of Asia/Pacific. In contrast, GSM 850 and GSM 1900 are mainly used in North America and Canada, as well as many other locations. Finally, the lower frequency bands, i.e. GSM 400/450 has limited support. Figure 1-18 GSM Deployments
Main UMTS Deployments The main UMTS deployment bands are illustrated in Figure 1-19, these include:
Band I (WCDMA 2100) - This is mainly used in Europe, Africa, Asia, Australia, New Zealand and Brazil.
Band II (WCDMA 1900) - This is used in North and South America.
Band IV (WCDMA 1700) - This is typically referred to as the AWS (Advanced Wireless Services) band. Certain service providers in North America and Canada have access to this band.
Band V (WCDMA 850) - This is found mainly in North and South America, as well as Australia, New Zealand, Canada, Israel, Poland and Asia.
Band VIII (WCDMA 900) - This is now being found in Europe, Asia, Australia, New Zealand and Venezuela. This list and usage of bands is not exclusive. As such other countries, as well as other cellular systems may exist.
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Figure 1-19 Key UMTS Deployment Bands
1.5.3 LTE Release 8 Bands The LTE Radio interface, namely the E-UTRA (Evolved - Universal Terrestrial Radio Access), is able to operate in many different radio bands. Table 1-6 illustrates the Release 8 frequency bands as well as other parameters which are used to identify centre frequencies. FDD requires two centre frequencies, one for the downlink and one for the uplink. These carrier frequencies are each given an EARFCN (E-UTRA Absolute Radio Frequency Channel Number) which ranges from 0 to 65535. In contrast, TDD only has one EARFCN. The parameters required to calculate the EARFCN(s) include:
FDL_low - This is the lower frequency of the downlink band.
FDL_high - This is the higher frequency of the downlink band.
NOffs-DL - This is a parameter used as part of the downlink EARFCN calculation.
NDL - This is the actual downlink EARFCN number.
FUL_low - This is the lower frequency of the uplink band.
FUL_high - This is the higher frequency of the uplink band.
NOffs-UL - This is a parameter used as part of the uplink EARFCN calculation.
NUL - This is the actual uplink EARFCN number.
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Table 1-6 LTE Release 8 Frequency Bands Band
Duplex
FDL_low
FDL_high
(MHz)
(MHz)
NOffs-DL
NDL
FUL_low
FUL_high
(MHz)
(MHz)
NOffs-UL
NUL
1
FDD
2110
2170
0
0-599
1920
1980
18000
18000-18599
2
FDD
1930
1990
600
600-1199
1850
1910
18600
18600-19199
3
FDD
1805
1880
1200
1200-1949
1710
1785
19200
19200-19949
4
FDD
2110
2155
1950
1950-2399
1710
1755
19950
19950-20399
5
FDD
869
894
2400
2400-2649
824
849
20400
20400-20649
6
FDD
875
885
2650
2650-2749
830
840
20650
20650-20749
7
FDD
2620
2690
2750
2750-3449
2500
2570
20750
20750-21449
8
FDD
925
960
3450
3450-3799
880
915
21450
21450-21799
9
FDD
1844.9
1879.9
3800
3800-4149
1749.9
1784.9
21800
21800-22149
10
FDD
2110
2170
4150
4150-4749
1710
1770
22150
22150-22749
11
FDD
1475.9
1500.9
4750
4750-4999
1427.9
1452.9
22750
22750-22999
12
FDD
728
746
5000
5000-5179
698
716
23000
23000-23179
13
FDD
746
756
5180
5180-5279
777
787
23180
23180-23279
14
FDD
758
768
5280
5280-5379
788
798
23280
23280-23379
17
FDD
734
746
5730
5730-5849
704
716
23730
23730-23849
33
TDD
1900
1920
36000
36000-36199
1900
1920
36000
36000-36199
34
TDD
2010
2025
36200
36200-36349
2010
2025
36200
36200-36349
35
TDD
1850
1910
36350
36350-36949
1850
1910
36350
36350-36949
36
TDD
1930
1990
36950
36950-37549
1930
1990
36950
36950-37549
37
TDD
1910
1930
37550
37550-37749
1910
1930
37550
37550-37749
38
TDD
2570
2620
37750
37750-38249
2570
2620
37750
37750-38249
39
TDD
1880
1920
38250
38250-38649
1880
1920
38250
38250-38649
40
TDD
2300
2400
38650
38650-39649
2300
2400
38650
38650-39649
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Carrier Frequency EARFCN Calculation The EARFCN is calculated using a combination of the equations in Figure 1-20 and the values in Table 1-6. The channel raster for LTE is 100kHz for all bands, i.e. the carrier centre frequency must be an integer multiple of 100kHz. This is represented in the equation by the “0.1” value. Figure 1-20 EARFCN Calculation
The channel numbers that designate carrier frequencies close to the edges of the operating band are not used. This implies that the first 7, 15, 25, 50, 75 and 100 channel numbers at the lower operating band edge and the last 6, 14, 24, 49, 74 and 99 channel numbers at the upper operating band edge are not used for channel bandwidths of 1.4, 3, 5, 10, 15 and 20 MHz respectively.
Example It is possible to utilize the previous equations to calculate the frequency for a given EARFCN. In addition, it is possible to calculate the EARFCN for a given frequency. Figure 1-21 illustrates an example with a defined uplink and downlink frequency. The calculation shown in the figure translates a downlink frequency of 2127.4MHz to an EARFCN equal to 174. Figure 1-21 Example Downlink EARFCN Calculation
100kHz Raster Uplink
Downlink
1937.4MHz
2127.4MHz
Frequency
FDL = FDL_low + 0.1(NDL - NOffs-DL) NDL =
NDL =
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+ NOffs-DL
(2127.4 - 2110) + 0 = 174 0.1
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1.6 Channel Coding in LTE The term “channel coding” can be used to describe the overall coding for the LTE channel. It can also be used to describe one of the individual stages. LTE channel coding is typically focused on a TB (Transport Block). This is a block of information which is provided by the upper layer, i.e. MAC (Medium Access Control). Figure 1-22 summarizes the typical processes performed by the PHY (Physical Layer), these include:
CRC (Cyclic Redundancy Check) attachment for the Transport Block.
Code block segmentation and CRC attachment.
Channel Coding.
Rate Matching.
Code Block Concatenation.
Figure 1-22 Summary of LTE Transport Channel Processing
The coding stages in Figure 1-22 are indicative of the LTE DL-SCH (Downlink Shared Channel) and the PCH (Paging Channel). Other channels, such as the UL-SCH (Uplink Shared Channel), BCH (Broadcast Channel) etc. are different but they can still utilize similar processes, e.g. they all have a “channel coding” stage.
1.6.1 Transport Block CRC The error detection method across the air interface is based on the addition of a CRC (Cyclic Redundancy Check). Figure 1-23 illustrates the basic concept of attaching a CRC to the Transport Block. The purpose of the CRC is to detect errors which may have occurred when the data was being sent. In LTE the CRC is based on complex parity checking.
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Figure 1-23 Cyclic Redundancy Check Concept
The LTE transport block is used to calculate the CRC parity bits. The size of the CRC is set to 24bits, 16bits or 8bits. This is typically indicated by higher layer signaling, i.e. RRC (Radio Resource Control). Figure 1-24 illustrates the CRC parity bits, where A is the size of the transport block and L is the number of parity bits. In addition, the lowest order information bit a0 is mapped to the most significant bit of the transport block. Figure 1-24 CRC Parity Bits
The parity bits are generated by one of the following cyclic generator polynomials: gCRC24A(D) = D24 + D23 + D18 + D17 + D14 +D11 + D10 + D7 + D6 + D5 + D4 + D3 + D + 1 gCRC16(D) = D16 + D12 + D5 + 1 gCRC8(D) = D8 + D7 + D4 + D3 + D + 1
Parity Checking The encoding is performed in a systematic form, which means that in GF(2) (Galois Field (2)), the polynomial: a0DA+23 + a1DA+22 +…+ aA-1D24 + p0D23 + … + p1D22+ p22D1 + p23 yields a remainder equal to 0 when divided by the corresponding 24bit CRC generator polynomial. Note that the 16bit and 8bit CRC generators each have a different polynomial which also yields a remainder equal to 0.
1.6.2 Code Block Segmentation and CRC Attachment The next stage in the processing of the transport block is code block segmentation and CRC attachment. Figure 1-25 illustrates the concept of code block segmentation. This process ensures that the size of each block is compatible with later stages of processing, i.e. the turbo interleaver. In addition, each code bock (segment) has a CRC included for the turbo coding.
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Figure 1-25 Code Block Segmentation and CRC Attachment
The input bit sequence to the code block segmentation is denoted by b0 , b1 ,….bB−1. Segmentation is performed if B is larger than the maximum code block size Z (6144bits). Finally, an additional CRC sequence of 24bits is attached to each code block. Note that if B < 40, filler bits are added to the beginning of the code block.
The code block CRC is different to the one used by the transport blocks. The polynomial is: gCRC24B(D) = D24 + D23 + D6 + D5 + D + 1 The verification polynomial is the same one used for the gCRC24A transport block which also yields a remainder equal to 0.
Example Figure 1-26 illustrates an example for segmentation when B=8000. In this instance the initial segment size is 4200bits (including the 24bit transport block CRC) which gets a 24bit code block CRC. The remaining 3800bits also get a 24bit code block CRC, however an additional 16bits of filler is required to ensure that the segments meet a valid turbo coding code block size. Figure 1-26 Example Calculation for Segmentation and Filler Bits.
In this example the total number of bits sent is 8064bits, thus an extra 64bits are sent (24bits +24bits +16bits).
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1.6.3 Channel Coding Channel coding in LTE facilitates FEC (Forward Error Correction) across the air interface. There are four main types:
Repetition Coding
Block Coding.
Tail Biting Convolutional Coding.
Turbo Coding.
The actual method used is linked to the type of LTE transport channel (Table 1-7) or the control information type (Table 1-8). Additional information on LTE channel types and control information is discussed in Section 2.1 . Table 1-7 Transport Channel Coding Options Transport Channel
Coding Method
Rate
Turbo Coding
1/3
Tail Biting Convolutional Coding
1/3
DL-SCH UL-SCH PCH MCH BCH
Table 1-8 Control Information Coding Options Control Information
Coding Method
Rate
DCI
Tail Biting Convolutional Coding
1/3
CFI
Block Code
1/16
HI
Repetition Code
1/3
UCI
Block Code
Variable
Tail Biting Convolutional Coding
1/3
Repetition Coding Repetition coding is used for coding the HI (HARQ Indicator) bit. The HI bit set to “1” is termed an ACK (Acknowledgement) and the HI bit set to “0” is a NACK (Negative Acknowledgement). The process of repetition coding is applied to increase the channel robustness. As such, for one initial bit, three bits are generated. These three bits are then map to an orthogonal sequence. The use of the HI bit, as well as the orthogonal sequences, is discussed in Section 2.21 .
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Figure 1-27 Repetition Coding of the HI
Block Coding The main utilization of block coding in LTE is for the CFI (Control Format Indicator). This parameter is used to convey vital information about the size of the downlink control region. Table 1-9 illustrates how the CFI values are encoded into a 32bit CFI codeword. Table 1-9 CFI Encoding CFI
CFI Codeword
< b0, b1, …, b31 >
1
2
3
4 (Reserved)
The utilization of the CFI and the mapping to the Physical Channels is discussed in Section 2.9.2 .
Concept of Convolutional Coding Prior to detailing the operation of tail biting convolutional coding and turbo coding in LTE it is worth examining the basics of a CC (Convolutional Coder) and the decoding process. Figure 1-28 illustrates a basic convolutional ½ rate coder, i.e. for 1bit input, 2bits are generated. It also has a constraint value of 3, meaning that three consecutive bits are used to calculate the output. For standard convolutional coders, before any information is sent, the registers are set to zero. This ensures that the initial information sent in the channel is at a known state at the receiver. For each subsequent input bit the previous input bit is used to load the registers S1 and S2 in turn.
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Figure 1-28 Basic ½ Rate Convolutional Coder
It can be seen in this simple coder that the output is dependent on the input and the state of the registers at any given time. What is also important is to understand how the output will change for any given input. For example, if the first input bit is “0” (bit A) and S1 and S2 are both at “0”, both outputs will be “0”. As the next bit arrives (bit B) it affects the output, such that G0 and G1 are both set to “1”. Table 1-10 illustrates bit B (in bold) clocking through the shift registers, as well as the output for the given sequence. Table 1-10 Convolutional Coding Example Input
S1
S2
G0
G1
0
0
0
0
0
1
0
0
1
1
1
1
0
0
1
0
1
1
0
1
Using the example coder from Figure 1-28 there are two possible outputs from each state. Figure 1-29 illustrates these, as well as the relationship for an input of 0 or 1. Figure 1-29 Convolutional Coding Trellis
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Concept of Viterbi Decoding The Viterbi algorithm is one of the main methods for decoding standard convolutional coded signals and it provides a valuable insight to how similar encoded signals can be decoded. The Viterbi method is based on a concept of maximum-likelihood decoding. Figure 1-30 illustrates the concept of trellis decoding by mapping the encoded data and current state to one of two outputs. The values shown on the input lines indicate the number of error(s) when comparing the received signal with the encoding trellis in Figure 1-29. For example, when the first bit “0” is encoded the output is “00”. If this is received without error then from the initiating state (00) there are only two possibilities:
“0” was sent - This is the example shown, therefore there are “0” errors indicated on the initial input=0 line.
“1” was sent - This is not the example shown, however the “2” on the input=1 line illustrates 2 errors, i.e. if the original input sequence was a 1, i.e. coded as “11” two errors must have happened on the air interface.
Figure 1-30 Example of Viterbi Decoding
In order for the Viterbi decoding trellis to work all possible states are considered for the sequence of bits. If errors did occur, it is the “maximum-likelihood” path which is chosen, i.e. the one with the least amount of errors.
Tail Biting Convolutional Coding As previously mentioned, LTE utilizes tail biting convolutional coding for the downlink BCH (Broadcast Channel) and DCI (Downlink Control Information), as well as possibly for the UCI (Uplink Control Information). Table 1-11 illustrates the main difference between the tail biting convolutional coding and standard convolutional coding.
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Table 1-11 Standard Convolutional Coding Verses Tail Biting Convolutional Coding Standard Convolutional Coding
Tail Biting Convolutional Coding
Initializes the shift register with zeros.
Initializes the shift register with the last bits of the stream, i.e. zeros are not added for initialization.
Padded with zeros.
The shift register finishes, such that the last bits of input are the same as what was used to initialize the shift registers.
The initial value of the shift registers are set to the values corresponding to the last 6 information bits in the input stream as illustrated in Figure 1-31. This ensures that the initial and final states of the shift registers are the same for the decoding process. Figure 1-31 Initializing Tail Biting Convolutional Coding
Last 6bits used to initialize coder.
Input Bits
Tail Biting Convolutional Coding
The actual LTE tail biting convolutional coder is shown in Figure 1-32. There are six shift registers and hence 6bits are required to initialize the coder. The input bit stream is identified by ck, dk(0), dk(1) and dk(2) correspond to the first, second and third parity streams, respectively. Figure 1-32 LTE 1/3 Rate Tail Biting Convolutional Coding
Turbo Coding Turbo coding defines a high-performance FEC mechanism. The term “Turbo coding” can be used to describe many different types of encoders. For example, in LTE the turbo encoder is known as a PCCC (Parallel Concatenated Convolutional Code) and it has two 8 state constituent encoders and one contention-free QPP (Quadratic Permutation Polynomial) turbo code internal interleaver. As previously mentioned, the coding rate of the LTE turbo encoder is 1/3, i.e. for each input bit, three bits are produced. The structure of a turbo encoder is illustrated in Figure 1-33.
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Figure 1-33 LTE Turbo Coding
Systematic Bits The dotted lines are part of the trellis termination.
1st Constituent Encoder
D
Turbo Code Internal Interleaver
D
D
Parity Bits
D
Parity Bits
2nd Constituent Encoder
D
D
The LTE turbo encoder employs two recursive convolutional encoders connected in parallel, with the QPP turbo interleaver preceding the second encoder. The outputs of the constituent encoders are punctured and repeated to achieve the correct output. It can be seen that the turbo coder encodes the input block twice, i.e. with and without interleaving, to generate two distinct sets of parity bits.
1.6.4 Rate Matching The rate matching for turbo coded transport channels is defined per coded block and consists of interleaving the three information bit streams dk(0), dk(1) and dk(2), followed by the collection of bits and the generation of a circular buffer as illustrated in Figure 1-34. Figure 1-34 LTE Rate Matching
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The sub-block interleaver is a row-column interleaver with 32 columns. Table 1-12 illustrates the column permutations. Table 1-12 LTE Sub-block Interleaver Number of Columns
Inter-column Permutation Pattern
32
< 0, 16, 8, 24, 4, 20, 12, 28, 2, 18, 10, 26, 6, 22, 14, 30, 1, 17, 9, 25, 5, 21, 13, 29, 3, 19, 11, 27, 7, 23, 15, 31 >
The sub-block interlearver works by writing each stream of bits row-by-row into a matrix with 32 columns. In so doing, the number of rows is based on the stream size. In addition, padding is added to the front of each stream so that the matrix is complete. The output of the sub-block interleaver consists of the columns read out in the permutation order, i.e. 0, 16, 8 etc. The bit collection block provides a circular buffer which can be read during “bit selection and pruning”. The circular buffer is formed by concatenating the rearranged systematic bits with the two rearranged/interlaced parity bit streams. Finally, the bit selection and pruning block performs a very important function. It provides a rate matching output, ek, of the correct length and utilizing the correct RV (Redundancy Version). The redundancy version is identified by the parameter rvidx and can have the values 0, 1, 2 or 3. As such, this value impacts the HARQ (Hybrid ARQ) operation, enabling the system to select and prune different sets of bits.
1.6.5 Code Block Concatenation Code block concatenation effectively concatenates the previously segmented code blocks. Figure 1-35 Code Block Concatenation
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1.7 Principles of OFDM The LTE air interface utilizes two different multiple access techniques both based on OFDM (Orthogonal Frequency Division Multiplexing):
OFDMA (Orthogonal Frequency Division Multiple Access) used on the downlink.
SC-FDMA (Single Carrier - Frequency Division Multiple Access) used on the uplink.
Figure 1-36 Use of OFDM in LTE
The concept of OFDM is not new and is currently being used on various systems such as Wi-Fi and WiMAX. In addition, it was even considered for UMTS back in 1998. One of the main reasons why it was not chosen at the time was the handset’s limited processing power and poor battery capabilities. LTE was able to choose OFDM based access due to the fact mobile handset processing capabilities and battery performance have both improved. In addition, there is continual pressure to produce more spectrally efficient systems.
1.7.2 Frequency Division Multiplexing OFDM is based on FDM (Frequency Division Multiplexing) and is a method whereby multiple frequencies are used to simultaneously transmit information. Figure 1-37 illustrates an example of FDM with four subcarriers. These can be used to carry different information and to ensure that each subcarrier does not interfere with the adjacent subcarrier, a guard band is utilized. In addition, each subcarrier has slightly different radio characteristics and this may be used to provide diversity. Figure 1-37 FDM Carriers
Guard Band
Subcarrier
Frequency Channel Bandwidth
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FDM systems are not that spectrally efficient (when compared to other systems) since multiple subcarrier guard bands are required.
1.7.1 OFDM Subcarriers OFDM follows the same concept as FDM but it drastically increases spectral efficiency by reducing the spacing between the subcarriers. Figure 1-38 illustrates how the subcarriers can overlap due to their orthogonality with the other subcarriers, i.e. the subcarriers are mathematically perpendicular to each other. As such, when a subcarrier is at its maximum the two adjacent subcarriers are passing through zero. In addition, OFDM systems still employ guard bands. These are located at the upper and lower parts of the channel and reduce adjacent channel interference. Figure 1-38 OFDM Subcarriers
The centre subcarrier, known as the DC (Direct Current) subcarrier, is not typically used in OFDM system due to its lack of orthogonality.
1.7.2 Fast Fourier Transforms OFDM subcarriers are generated and decoded using mathematical functions called FFT (Fast Fourier Transform) and IFFT (Inverse Fast Fourier Transform). The IFFT is used in the transmitter to generate the waveform. Figure 1-39 illustrates how the coded data is first mapped to parallel streams before being modulated and processed by the IFFT. Figure 1-39 Inverse Fast Fourier Transform
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At the receiver side, this signal is passed to the FFT which analyses the complex/combined waveform into the original streams. Figure 1-40 illustrates the FFT process. Figure 1-40 Fast Fourier Transform
1.7.3 LTE FFT Sizes Fast Fourier Transforms and Inverse Fast Fourier Transforms both have a defining size. For example, an FFT size of 512 indicates that there are 512 subcarriers. In reality, not all 512 subcarriers can be utilized due to the channel guard bands and the fact that a DC (Direct Current) subcarrier is also required. Table 1-13 illustrates the LTE channel bandwidth options, as well as the FFT size and associated sampling rate. Using the sampling rate and the FFT size the subcarrier spacing can be calculated, e.g. 7.68MHz/15kHz = 512. Table 1-13 LTE Channel and FFT Sizes Channel Bandwidth
FFT Size
Subcarrier Bandwidth
Sampling Rate
1.4MHz
128
1.92MHz
3MHz
256
3.84MHz
5MHz
512
7.68MHz 15kHz
10MHz
1024
15.36MHz
15MHz
1536
23.04MHz
20MHz
2048
30.72MHz
The subcarrier spacing of 15kHz is also used in the calculation to identify the OFDM symbol duration.
1.7.4 OFDM Symbol Mapping The mapping of OFDM symbols to subcarriers is dependent on the system design. Figure 1-41 illustrates an example of OFDM mapping. The first 12 modulated OFDM symbols are mapped to 12 subcarriers, i.e. they are transmitted at the same time but using different 1-32
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subcarriers. The next 12 subcarriers are mapped to the next OFDM symbol period. In addition, a CP (Cyclic Prefix) is added between the symbols. Figure 1-41 OFDM Symbol Mapping
LTE allocates resources in groups of 12 subcarriers. This is known as a PRB (Physical Resource Block).
In the previous example 12 different modulated OFDM symbols are transmitted simultaneously. Figure 1-42 illustrates how the combined energy from this will result in either constructive peaks (when the symbols are the same) or destructive nulls (when the symbols are different). This means that OFDM systems have a high PAPR (Peak to Average Power Ratio). Figure 1-42 OFDM PAPR (Peak to Average Power Ratio)
1.7.5 Time Domain Interference The OFDM signal provides some protection in the frequency domain due to the orthogonality of the subcarriers. The main issue is with delay spread, i.e. multipath interference. Figure 1-43 illustrates two of the main multipath effects, namely delay and attenuation. The delayed signal can manifest itself as ISI (Inter Symbol Interference), whereby one symbol impacts the next. This is illustrated in Figure 1-44.
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Figure 1-43 Delay Spread
ISI (Inter Symbol Interference) is typically reduced with “equalizers”. However, for the equalizer to be effective a known bit pattern or “training sequence” is required. However, this reduces the system capacity, as well as impacts processing on a device. Instead, OFDM systems employ a CP (Cyclic Prefix). Figure 1-44 Inter Symbol Interference
Cyclic Prefix A CP (Cyclic Prefix) is utilized in most OFDM systems to combat multipath delays. It effectively provides a guard period for each OFDM symbol. Figure 1-45 illustrates the Cyclic Prefix and its location in the OFDM Symbol. Notice that the Cyclic Prefix is effectively a copy taken from the back of the original symbol which is then placed in front of the symbol to make the OFDM symbol (Ts). The size of the Cyclic Prefix relates to the maximum delay spread the system can tolerate. As such, systems designed for macro coverage, i.e. large cells, should have a large CP. This does however impact the system capacity since the number of symbols per second is reduced.
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Figure 1-45 Cyclic Prefix
LTE has two defined Cyclic Prefix sizes, normal and extended. The extended Cyclic Prefix is designed for larger cells.
1.7.6 OFDM Advantages and Disadvantages OFDM Advantages OFDM systems typically have a number of advantages:
OFDM is almost completely resistant to multi-path interference due to very long symbol duration. higher spectral efficiency for wideband channels.
flexible spectrum utilization.
relatively simple implementation using FFT and IFFT.
OFDM Disadvantages OFDM also has some disadvantages:
frequency errors and phase noise can cause issues. Doppler shift impacts subcarrier orthogonality.
some OFDM systems can suffer from high PAPR.
required accurate frequency and time synchronization.
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2 LTE Physical Layer
2
LTE Physical Layer
Objectives On completion of this section the participants will be able to: 2.1 Detail the channel structure of the radio interface. 2.2 Detail the radio interface protocols. 2.3 Describe the physical signals in the UL and DL. 2.4 Detail the time-domain structure in the radio interface in the UL and DL for both FDD and TDD mode. 2.5 Have a good understanding of the OFDM principle, signal generation and processing. 2.6 Detail the DL transmission technique. 2.7 Detail the DL synchronization signals. 2.8 Detail the reference symbols in the DL. 2.9 Detail the DL physical Channels. 2.10 Detail the DL control signaling and formats. 2.11 Explain the cell search procedure. 2.12 Detail the UL transmission technique. 2.12 Have a good understanding of the SC-FDMA principle, signal generation and processing. 2.13 Explain the pros and cons with OFDM and SC-FDMA. 2.14 Detail the UL Physical Channels. 2.15 Explain the timing relationships between the UL and DL. 2.16 Detail the reference signals. 2.17 Detail the UL control signaling and formats. 2.18 Detail the random access procedure. 2.19 Describe the Power Control in the UL. 2.20 Detail the paging procedures. 2.21 Explain HARQ. Issue 01 (2010-05-01)
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2.22 Describe the concepts of layers, channel rank, spatial multiplexing, open and closed loop spatial multiplexing, TX diversity, beamforming, SU-MIMO and MU-MIMO.
2.1 The Uu Interface The LTE air interface is identified as the E-UTRA (Evolved - Universal Terrestrial Radio Access) and can support varying bandwidth options ranging from 1.4MHz to 20MHz. The interface is identified as “Uu”, with the capital “U” indicating the “User to Network” interface and the lower case “u” indicating Universal. The UE (User Equipment) will utilize a channel bandwidth based on the configuration of the eNB (Evolved Node B). However, the eNB may implement multiple channels to improve capacity or as part of a frequency reuse mechanism. Figure 2-1 The LTE Air Interface
2.2 LTE Radio Interface Protocols The E-UTRA interface provides connectivity between the User Equipment and the eNB. It can be logically split into a control plane and a user plane. There are effectively two control planes, the first is provided by RRC (Radio Resource Control) and carries signaling between the User Equipment and the eNB. The second carries NAS (Non Access Stratum) signaling messages to the MME (Mobility Management Entity), which are carried by RRC. Figure 2-2 illustrates the RRC and NAS control planes, as well as the user plane which focuses on the delivery of IP datagrams to and from the EPC (Evolved Packet Core), namely the S-GW (Serving Gateway) and PDN-GW (Packet Data Network - Gateway).
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Figure 2-2 LTE Control Plane and User Plane
2.2.1 Control and User Plane Protocols The control and user plane lower layer protocols are the same. As such, they both utilize the services of PDCP (Packet Data Convergence Protocol), RLC (Radio Link Control) and MAC (Medium Access Control), as well as the PHY (Physical Layer). Figure 2-3 illustrates the radio interface protocol stacks. It can be seen that the NAS signaling uses the services of RRC, which is then mapped into PDCP. On the user plane IP datagrams are also mapped into PDCP. Figure 2-3 E-UTRA Protocols
2.2.2 Non Access Stratum The term Non Access Stratum, or NAS, identifies the layer(s) above the AS (Access Stratum). The access stratum defines the procedures and protocols associated with the RAN (Radio Access Network), i.e. the E-UTRAN. There are two main aspects to NAS, namely higher layer signaling and user data.
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NAS Signaling In terms of NAS signaling, messages pass between the User Equipment and the MME. This is illustrated in Figure 2-4. Figure 2-4 NAS Signaling
Two categories of NAS signaling exist:
EMM (EPS Mobility Management).
ESM (EPS Session Management).
Table 2-1 illustrates the main EMM and ESM LTE procedures. Table 2-1 NAS EMM and ESM Procedures EMM Procedures
ESM Procedures
Attach
Default EPS Bearer Context Activation
Detach
Dedicated EPS Bearer Context Activation
Tracking Area Update
EPS Bearer Context Modification
Service Request
EPS Bearer Context Deactivation
Extended Service Request
UE Requested PDN Connectivity
GUTI Reallocation
UE Requested PDN Disconnect
Authentication
UE Requested Bearer Resource Allocation
Identification
UE Requested Bearer Resource Modification
Security Mode Control
ESM Information Request
EMM Status
ESM Status
EMM Information NAS Transport Paging
EMM Procedures The key EMM procedures include:
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Attach - this is used by the UE to attach to an EPC (Evolved Packet Core) for packet services in the EPS (Evolved Packet System). Note that it can be also used to attach to non-EPS services.
Detach - this is used by the UE to detach from EPS services. In addition, it can also be used for other procedures such as disconnecting from non-EPS services.
Tracking Area Updating - this procedure is always initiated by the UE and is used for the various purposes. The most common include normal and periodic tracking area updating.
Service Request - this is used by the UE to get connected and establish the radio and S1 bearers when uplink user data or signaling is to be sent.
Extended Service Request - this is used by the UE to initiate a Circuit Switched fallback call or respond to a mobile terminated Circuit Switched fallback request from the network.
GUTI Reallocation - This is used to allocate a GUTI (Globally Unique Temporary Identifier) and optionally to provide a new TAI (Tracking Area Identity) list to a particular UE.
Authentication - this is used for AKA (Authentication and Key Agreement) between the user and the network.
Identification - this is used by the network to request a particular UE to provide specific identification parameters, e.g. the IMSI (International Mobile Subscriber Identity) or the IMEI (International Mobile Equipment Identity).
Security mode control - this is used to take an EPS security context into use, and initialize and start NAS signaling security between the UE and the MME with the corresponding NAS keys and security algorithms.
EMM Status - this is sent by the UE or by the network at any time to report certain error conditions.
EMM Information - this allows the network to provide information to the UE.
Transport of NAS messages - this is to carry SMS (Short Message Service) messages in an encapsulated form between the MME and the UE.
Paging - this is used by the network to request the establishment of a NAS signaling connection to the UE. Is also includes the Circuit Switched Service Notification
EMM Procedures The key ESM procedures include:
Default EPS Bearer Context Activation - this is used to establish a default EPS bearer context between the UE and the EPC.
Dedicated EPS Bearer Context Activation - this is to establish an EPS bearer context with specific QoS (Quality of Service) and TFT (Traffic Flow Template) between the UE and the EPC. The dedicated EPS bearer context activation procedure is initiated by the network, but may be requested by the UE by means of the UE requested bearer resource allocation procedure.
EPS Bearer Context Modification - this is used to modify an EPS bearer context with a specific QoS and TFT.
EPS Bearer Context Deactivation - this is used to deactivate an EPS bearer context or disconnect from a PDN by deactivating all EPS bearer contexts to the PDN.
UE Requested PDN Connectivity - this is used by the UE to request the setup of a default EPS bearer to a PDN.
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UE Requested PDN Disconnect - this is used by the UE to request disconnection from one PDN. The UE can initiate this procedure to disconnect from any PDN as long as it is connected to at least one other PDN.
UE Requested Bearer Resource Allocation - this is used by the UE to request an allocation of bearer resources for a traffic flow aggregate.
UE Requested Bearer Resource Modification - this is used by the UE to request a modification or release of bearer resources for a traffic flow aggregate or modification of a traffic flow aggregate by replacing a packet filter.
ESM Information Request - this is used by the network to retrieve ESM information, i.e. protocol configuration options, APN (access Point Name), or both from the UE during the attach procedure.
ESM Status - this is used to report at any time certain error conditions detected upon receipt of ESM protocol data.
NAS User The NAS user plane is based on IP (Internet Protocol). As such, IP datagrams are passed to the lower layers, i.e. PDCP, for processing.
2.2.3 RRC The main air interface control protocol is RRC (Radio Resource Control). For RRC messages to be transferred between the UE and the eNB it uses the services of PDCP, RLC, MAC and PHY. Figure 2-5 identifies the main RRC functions. In summary, RRC handles all the signaling between the UE and the E-UTRAN, with signaling between the UE and Core Network, i.e. NAS (Non Access Stratum) signaling, being carried by dedicated RRC messages. When carrying NAS signaling, RRC does not alter the information but instead, provides the delivery mechanism. RRC provides the main configuration and parameters to the lower layers. As such, the PHY layer will get information from RRC on how to configure certain aspects of the Physical Layer.
Figure 2-5 Main RRC Functions
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2.2.4 PDCP LTE implements PDCP in both the user plane and control plane. This is unlike UMTS, where PDCP was only found in the user plane. The main reason for the difference is that PDCP in LTE takes on the role of security, i.e. encryption and integrity. In addition, Figure 2-6 illustrates some of the other functions performed by PDCP. Figure 2-6 PDCP Functions
In the control plane, PDCP facilitates encryption and integrity checking of signaling messages, i.e. RRC and NAS. The user plane is slightly different since only encryption is performed. In addition, the user plane IP datagrams can also be subjected to IP header compression techniques in order to improve the system’s performance and efficiency. Finally, PDCP also facilitates sequencing and duplication detection.
2.2.5 RLC The RLC (Radio Link Control) protocol exists in the UE and the eNB. As its name suggests it provides “radio link” control, if required. In essence, RLC supports three delivery services to the higher layers:
TM (Transparent Mode) - This is utilized for some of the air interface channels, e.g. broadcast and paging. It provides a connectionless service for signaling.
UM (Unacknowledged Mode) - This is like Transparent Mode, in that it is a connectionless service; however it has the additional features of sequencing, segmentation and concatenation.
AM (Acknowledged Mode) - This offers an ARQ (Automatic Repeat Request) service. As such, retransmissions can be used.
These modes, as well as the other RLC features are illustrated in Figure 2-7. In addition to ARQ, RLC offers segmentation, re-assembly and concatenation of information.
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Figure 2-7 RLC Modes and Functions
2.2.6 MAC MAC (Medium Access Control) provides the interface between the E-UTRA protocols and the E-UTRA Physical Layer. In doing this it provides the following services:
Mapping - MAC maps the information received on the LTE Logical Channels into the LTE transport channels. These channels and their mapping are discussed further in Section 2.3 .
Multiplexing - The information provided to MAC will come from a RB (Radio Bearer) or multiple Radio Bearers. The MAC layer is able to multiplex different bearers into the same TB (Transport Block), thus increasing efficiency.
HARQ (Hybrid Automatic Repeat Request) - MAC utilizes HARQ to provide error correction services across the air. HARQ is a feature which requires the MAC and Physical Layers to work closely together. This is discussed further in Section 2.21 .
Radio Resource Allocation - QoS (Quality of Service) based scheduling of traffic and signaling to users is provided by MAC. There are various scheduling options, these are described further in Section 3 .
In order to support these features the MAC and Physical layers need to pass various indications on the radio link quality, as well as the feedback from HARQ operation. Figure 2-8 Medium Access Control Functions
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2.2.7 Physical The PHY (Physical Layer) in LTE provides a new and flexible channel. It does however utilize features and mechanisms defined in earlier systems, i.e. UMTS. Figure 2-9 illustrates the main functions provided by the Physical Layer. Figure 2-9 Physical Layer Functions
2.3 LTE Channel Structure The concept of “channels” is not new. Both GSM and UMTS defined various channel categories, however LTE terminology is closer to UMTS. Broadly there are four categories of channel. Figure 2-10 LTE Channels
2.3.1 Logical Channels In order to describe Logical Channels it is best to identify where Logical Channels are located in relation to the LTE protocols and the other channel types. Figure 2-11 shows Logical Channels located between the RLC and the MAC layers.
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Figure 2-11 Location of Channels
Logical channels are classified as either Control Logical Channels, which carry control data such as RRC signaling, or traffic Logical Channels which carry user plane data.
Control Logical Channels The various forms of these Control Logical Channels include:
BCCH (Broadcast Control Channel) - This is a downlink channel used to send SI (System Information) messages from the eNB. These are defined by RRC.
PCCH (Paging Control Channel) - This downlink channel is used by the eNB to send paging information.
Figure 2-12 BCCH and PCCH Logical Channels
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CCCH (Common Control Channel) - This is used to establish a RRC (Radio Resource Control) connection, also known as a SRB (Signaling Radio Bearer). The SRB is discussed further in Section 2.18 . The SRB is also used for re-establishment procedures. SRB 0 maps to the CCCH.
DCCH (Dedicated Control Channel) - This provides a bidirectional channel for signaling. Logically there are two DCCH activated: −
SRB 1 - This is used for RRC messages, as well as RRC messages carrying high priority NAS signaling.
−
SRB 2 - This is used for RRC carrying low priority NAS signaling. Prior to its establishment low priority signaling is sent on SRB1.
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Figure 2-13 CCCH and DCCH Signaling
Traffic Logical Channels Release 8 LTE has one type of Logical Channel carrying traffic, namely the DTCH (Dedicated Traffic Channel). This is used to carry DRB (Dedicated Radio Bearer) information, i.e. IP datagrams. Figure 2-14 Dedicated Traffic Channel
The DTCH is a bidirectional channel that can operate in either RLC AM or UM mode. This is configured by RRC and is based on the QoS (Quality of Service) of the E-RAB (EPS Radio Access Bearer).
2.3.2 Transport Channels Historically, Transport Channels were split between common and dedicated channels. However, LTE has moved away from dedicated channels in favor of the common/shared channels and the associated efficiencies provided. The main Release 8 Transport Channels include:
BCH (Broadcast Channel) - This is a fixed format channel which occurs once per frame and carries the MIB (Master Information Block). Note that the majority of System Information messages are carries on the DL-SCH (Downlink - Shared Channel).
PCH (Paging Channel) - This channel is used to carry the PCCH, i.e. paging messages. It also utilizes DRX (Discontinuous Reception) to improve UE battery life.
DL-SCH (Downlink - Shared Channel) - This is the main downlink channel for data and signaling. It supports dynamic scheduling, as well as dynamic link adaptation. In addition, it supports HARQ (Hybrid Automatic Repeat Request) operation to improve performance. As previously mentioned it also facilitates the sending of System Information messages.
RACH (Random Access Channel) - This channel carries limited information and is used in conjunction with Physical Channels and preambles to provide contention resolution procedures.
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UL-SCH (Uplink Shared Channel) - Similar to the DL-SCH, this channel supports dynamic scheduling (eNB controlled) and dynamic link adaptation by varying the modulation and coding. In addition, it too supports HARQ (Hybrid Automatic Repeat Request) operation to improve performance.
Figure 2-15 LTE Release 8 Transport Channels
2.3.3 Physical Channels The Physical Layer facilitates transportation of MAC Transport Channels, as well as providing scheduling, formatting and control indicators. Sections 2.9 and 2.14 examines the Physical Channels in greater detail.
Downlink Physical Channels There are a number of downlink Physical Channels in LTE. These include:
PBCH (Physical Broadcast Channel) - This channel carries the BCH.
PCFICH (Physical Control Format Indicator Channel) - This is used to indicate the number of OFDM symbols used for the PDCCH.
PDCCH (Physical Downlink Control Channel) - This channel is used for resource allocation.
PHICH (Physical Hybrid ARQ Indicator Channel) - This channel is part of the HARQ process.
PDSCH (Physical Downlink Shared Channel) - This channel carries the DL-SCH.
Uplink Physical Channels There are a number of Uplink Physical Channels in LTE. These include:
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PRACH (Physical Random Access Channel) - This channel carries the Random Access Preamble. The location of the PRACH is defined by higher layer signaling, i.e. RRC signaling.
PUCCH (Physical Uplink Control Channel) - This channel carries uplink control and feedback. It can also carry scheduling requests to the eNB.
PUSCH (Physical Uplink Shared Channel) - This is the main uplink channel and is used to carry the UL-SCH (Uplink Shared Channel) Transport Channel. It carries both signaling and user data, in addition to uplink control. It is worth noting that the UE is not allowed to transmit the PUCCH and PUSCH at the same time.
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2.3.4 Radio Channels The term “Radio Channel” is typically used to describe the overall channel, i.e. the downlink and uplink carrier for FDD or the single carrier for TDD. Figure 2-16 Radio Channel
2.3.5 Channel Mapping There are various options for multiplexing multiple bearers together, such that Logical Channels may be mapped to one or more Transport Channels. These in turn are mapped into Physical Channels. Figure 2-17 and Figure 2-18 illustrate the mapping options. Figure 2-17 Downlink Channel Mapping
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Figure 2-18 Uplink Channel Mapping
In order to facilitate the multiplexing from Logical Channels to Transport Channels, the MAC Layer typically adds a LCID (Logical Channel Identifier).
2.4 LTE Frame Structure In LTE, devices are allocated blocks of subcarriers for a period of time. These are referred to as a PRB (Physical Resource Block). The resource blocks are contained within the LTE generic frame structure. Two types are defined, namely type 1 and type 2 radio frames.
2.4.1 Type 1 Radio Frames, Slots and Subframes The type 1 radio frame structure is used for FDD and is 10ms in duration. It consists of 20 slots, each lasting 0.5ms. Two adjacent slots form one subframe of length 1ms. For FDD operation 10 subframes are available for downlink transmission and 10 subframes are available for uplink transmission, with each transmission separated in the frequency domain. Figure 2-19 illustrates the FDD frame structure, as well as highlighting the slots and subframe concept. In addition, it illustrates how the slots are numbered 0 to 19.
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Figure 2-19 LTE Frame Structure
LTE Time Unit The LTE time unit is identified as Ts and is calculated as 1/(15000x2048), which equates to approximately 32.552083ns. Various aspects of LTE utilize this parameter, or multiple of it, to identify timing and configuration.
Cyclic Prefix Options Section 1.7.5 introduced the concept of a CP (Cyclic Prefix) in OFDM systems. In LTE, it was chosen to have two different cyclic prefix sizes, namely “Normal” and “Extended”. In order to facilitate these, two different slot formats are available. Figure 2-20 illustrates the 7 and 6 ODFM symbol options. Obviously, to facilitate a larger cyclic prefix one of the symbols is sacrificed, thus the symbol rate is reduced. Figure 2-20 Normal and Extended Cyclic Prefix
The use of the extended cyclic prefix is intended for scenarios when the range of the cell needs to be extended, e.g. for planning purposes.
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Table 2-2 illustrates the sizes of the cyclic prefix for different configurations. It can be seen that the CP size can vary during a slot, such that the first CP is larger than the rest when the normal CP size is chosen. Table 2-2 Downlink CP Parameters Configuration
CP Length (Ts)
Time
Delay Spread
160 for symbol 0
~ 5.208µs
~ 1.562km
144 for symbol 1, 2, …6
~ 4.688µs
~ 1.406km
Normal Cyclic Prefix
∆f = 15kHz
Extended Cyclic Prefix
∆f = 15kHz
512 for symbol 0, 1, …5
~16.67µs
~ 5km
∆f = 7.5kHz
1024 for symbol 0, 1, 2
~ 33.33 µs
~ 10km
The 7.5kHz option is part of MBSFN (MBMS over Single Frequency Network) which is still in the Release 8 PHY specifications, however the MBMS feature which utilizes this has been delayed until Release 9. In addition, this option (7.5kHz) is only available in the downlink.
The symbol (Ts) consists of a guard period, i.e. the cyclic prefix, and the Tb data duration which is 2048 LTE time units for both the normal and extended 15kHz option. Figure 2-21 illustrates an example of the normal cyclic prefix configuration for a slot. Figure 2-21 Normal CP Configuration
2.4.2 Type 2 Radio Frames, Slots and Subframes The type 2 radio frame structure is used for TDD. One key addition to the TDD frame structure is the concept of “special subframes”. This includes a DwPTS (Downlink Pilot Time Slot), GP (Guard Period) and UpPTS (Uplink Pilot Time Slot). These have configurable individual lengths and a combined total length of 1ms. For TDD operation the 10 subframes are shared between the uplink and the downlink. A 5ms and 10ms switch-point periodicity is supported however subframes 0 and 5 must be allocated to the downlink as these contain the PSS (Primary Synchronization Signal) and SSS (Secondary Synchronization Signal), as well as the broadcast information in subframe 0. The PSS and SSS are discussed in Section 2.7
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Figure 2-22 Type 2 TDD Radio Frame
There are various frame configuration options supported for TDD. Table 2-3 illustrates the different options. Configuration options 0, 1, 2 and 6 have a 5ms switching point and therefore require 2 special subframes, whereas the rest are based on a 10ms switching point. In the table, the letter “D” is reserved for downlink transmissions, “U” denotes subframes reserved for uplink transmissions and “S” denotes a special subframe with the three fields DwPTS, GP and UpPTS. Table 2-3 Type 2 Radio Frame Switching Points Configuration
Switching Point Periodicity
Subframe Number 0
1
2
3
4
5
6
7
8
9
0
5ms
D
S
U
U
U
D
S
U
U
U
1
5ms
D
S
U
U
D
D
S
U
U
D
2
5ms
D
S
U
D
D
D
S
U
D
D
3
10ms
D
S
U
U
U
D
D
D
D
D
4
10ms
D
S
U
U
D
D
D
D
D
D
5
10ms
D
S
U
D
D
D
D
D
D
D
6
5ms
D
S
U
U
U
D
S
U
U
D
The DwPTS and UpPTS in a special frame may carry information. For example the DwPTS can include scheduling information and the UpPTS can be configured to facilitate random access bursts.
2.5 OFDM Signal Generation There are various Physical Layer stages involved in the generation of the downlink and uplink signals. Figure 2-23 illustrates the possible stages for a PDSCH.
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Figure 2-23 Downlink Physical Layer Processing
2.5.1 Codewords, Layers and Antenna Ports Prior to identifying the various stages it is worth clarifying the concept of codewords, layers and antenna ports. The use of layers and multiple antenna ports is related to diversity and MIMO (Multiple Input Multiple Output). In addition, the term “rank” is typically applied to the number of layers. In LTE, when discussing the Physical Layer processing, a “codeword” corresponds to a TB (Transport Block). One or two codewords can be used and these are mapped onto layers. The number of layers can vary from one up to a maximum which is equal to the number of antenna ports. When there is one codeword, i.e. one transport block, a single layer is used. In contrast, two codewords, i.e. two transport blocks, can be used with two or more layers. Figure 2-24 illustrates the mapping options. Figure 2-24 Codeword, Layer and Antenna Port Mapping
It is important to note that the number of modulation symbols on each layer needs to be the same. As such, when operating with three layers, the second codeword is twice as large as the first. This can be achieved due to the supported TB sizes and the other Physical Layer stages.
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2.5.2 Scrambling The initial stage of the Physical Layer processing is “scrambling”. This stage is applied to the signal in order to provide interference rejection properties. Scrambling effectively randomizes interfering signals using a pseudo-random scrambling process. Figure 2-25 illustrates the concept of scrambling, showing a Physical Resource Block on each of the cells using the same frequency. The scrambling feature statistically improves the interference by scrambling the information with a scrambling code based on the physical cell ID and RNTI. Figure 2-25 Scrambling in LTE
Figure 2-26 illustrates the generation of the scrambling code which is applied to most of the Physical Channels. It is worth noting that scrambling is not used on the downlink PHICH and on certain parts of the uplink. Figure 2-26 LTE Scrambling Code Generation
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2.5.3 Modulation Mapper The modulation mapper converts the scrambled bits to complex-valued modulation symbols (BPSK, QPSK, 16QAM or 64QAM). Figure 2-27 BPSK, QPSK and 16QAM Modulation Mapper
Figure 2-28 64QAM Modulation Mapper
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2.5.4 Layer Mapper The layer mapper effectively maps the complex-valued modulation symbols onto one or several transmission layers, thus splitting the data into a number of layers. Depending on the transmission mode, various options are available. Table 2-4 Layer Mapper Configuration Mapper Configuration
Layers ( )
Antenna Ports ( P )
Single Antenna
1
P 1
Transmit Diversity
P
P 1 (2 or 4)
Spatial Multiplexing
1 P
P 1 (2 or 4)
The complex-valued modulation symbols for each of the codewords to be transmitted are mapped onto one or several layers. Complex-valued modulation symbols (q) d ( q ) (0),..., d ( q ) ( M symb 1) for codeword q are mapped onto the layers
T
layer layer x(i) x (0) (i ) ... x ( 1) (i) , i 0,1,..., M symb 1 where is the number of layers and M symb
is the number of modulation symbols per layer.
Single Antenna For transmission on a single antenna port, a single layer is used, 1 , and the mapping is layer (0) defined by x ( 0 ) (i ) d ( 0) (i ) with M symb M symb .
Spatial Multiplexing For spatial multiplexing, the layer mapping is illustrated in Table 2-5. The number of layers is less than or equal to the number of antenna ports P used for transmission of the physical channel. The case of a single codeword mapped to two layers is only applicable when the number of antenna ports is 4. Table 2-5 Codeword to Layer Mapping for Spatial Multiplexing layer Codeword to Layer Mapping i 0,1,..., M symb 1
Number of Layers
Number of Codewords
1
1
x (0 ) (i ) d ( 0) (i )
layer ( 0) M symb M symb
2
2
x ( 0 ) (i ) d ( 0 ) (i )
layer (0) (1) M symb M symb M symb
x (1) (i ) d (1) (i )
2
1
x (0) (i) d (0) (2i) (1)
x (i) d
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(0 )
layer (0) M symb M symb 2
(2i 1)
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3
2
layer ( 0) (1) M symb M symb M symb 2
x ( 0 ) (i ) d ( 0 ) (i ) x (1) (i ) d (1) ( 2i ) x ( 2) (i ) d (1) ( 2i 1)
4
2
layer ( 0) (1) M symb M symb 2 M symb 2
x (0) (i) d (0) (2i) x (1) (i) d (0) (2i 1) x ( 2) (i ) d (1) ( 2i ) x (3) (i ) d (1) ( 2i 1)
Transmit Diversity For transmit diversity there is only one codeword and the number of layers is equal to the number of antenna ports used for transmission of the physical channel. Table 2-6 Codeword to Layer Mapping for Transmit Diversity layer Number Number Codeword to Layer Mapping i 0,1,..., M symb 1 of of Code Layers words
2
1
x (0 ) (i ) d (0 ) (2i )
layer ( 0) M symb M symb 2
x (1) (i ) d (0 ) (2i 1)
4
1
x ( 0) (i ) d ( 0) ( 4i ) x (1) (i ) d ( 0) (4i 1) x ( 2 ) (i ) d (0 ) ( 4i 2) x (3) (i) d ( 0) ( 4i 3)
(0 ) (0) M symb 4 if M symb mod 4 0 layer M symb ( 0) ( 0) M symb 2 4 if M symb mod 4 0 ( 0) If M symb mod 4 0 two null symbols are
( 0) appended to d ( 0) ( M symb 1)
2.5.5 Precoding The next stage is precoding the complex-valued modulation symbols on each layer for transmission. Figure 2-29 illustrates the different precoding options:
Single Antenna Port.
Transmit Diversity.
Spatial Multiplexing - This includes two options, i.e. with CDD (Cyclic Delay Diversity) and without. CDD (Cyclic Delay Diversity) is a method whereby a delayed version of the same OFDM symbol is transmitted from multiple antennas. It provides a method for transforming spatial diversity into frequency diversity thus avoiding Inter Symbol Interference.
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Figure 2-29 LTE Precoding Options
Precoding Concept
T
The precoder takes as input a block of vectors x(i) x (0) (i ) ... x ( 1) (i) , layer i 0,1,..., M symb 1 from the layer mapping and generates a block of vectors
T
ap y(i) ... y ( p) (i) ... , i 0,1,..., M symb 1 to be mapped onto resources on each of the antenna
ports, where y ( p ) (i ) represents the signal for antenna port .
Precoding for Single Antenna Port For transmission on a single antenna port, precoding is defined by y ( p ) (i ) x ( 0 ) (i ) where p 0,4,5 is the number of the single antenna port used for transmission of the physical ap ap layer channel and i 0,1,..., M symb 1 , M symb M symb .
Precoding for Transmit Diversity The precoding operation for transmit diversity is defined for two and four antenna ports. For
T
transmission on two antenna ports, p 0,1 , the output y(i) y (0) (i) y (1) (i) , ap i 0,1,..., M symb 1 of the precoding operation is defined by:
y (0 ) (2i ) j 0 Re x (0 ) (i ) 1 0 (1) j Re x (1) (i ) y (2i ) 1 0 1 0 y (0) (2i 1) 0 j Im x (0) (i ) 2 0 1 (1) (1) y (2i 1) 1 0 j 0 Im x (i ) layer ap layer for i 0,1,..., M symb 1 with M symb 2M symb .
It is worth noting that any two columns of the coding matrix are orthogonal. In addition, the precoding has facilitated space-frequency transmit diversity, i.e. coding in frequency domain. ap layer The precoding for four antenna ports is similar, however typically M symb . 4M symb
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Precoding for Spatial Multiplexing Spatial multiplexing supports two or four antenna ports and the set of antenna ports used is p 0,1 or p 0,1,2,3 , respectively. Without Cyclic Delay Diversity), precoding for spatial multiplexing is defined by: y (0) (i ) x (0) (i ) W (i ) y ( P 1) (i ) x ( 1) (i ) ap ap layer where the precoding matrix W (i ) is of size P and i 0,1,..., M symb 1 , M symb M symb .
Note that the values of W (i ) are selected among the precoder elements in the codebook configured in the eNodeB and the UE. For large-delay CDD, precoding for spatial multiplexing is defined by y (0) (i ) x (0) (i ) W (i ) D (i )U y ( P 1) (i ) x ( 1) (i ) ap ap layer where the precoding matrix W (i ) is of size P and i 0,1,..., M symb 1 , M symb M symb .
Compared to none CDD precoding, D(i) provides the CDD (Cyclic Delay Diversity) diagonal matrix, whereas U uses a square matrix.
Spatial Multiplexing Codebook for Precoding The size of the codebook varies for two and four antenna transmissions. The two antenna ports, p 0,1 , the precoding matrix W (i ) is selected from Table 6.3.4.2.3-1 or a subset thereof. For the closed-loop spatial multiplexing transmission mode, the codebook index 0 is not used when the number layers is 2 . Table 2-7 Codebook for Transmission for Two Antenna Ports Codebook Index
Number of layers
1
2
0
1 1 2 1
1
1 1 2 1
1 1 1 2 1 1
2
1 1 2 j
1 1 1 2 j j
3
1 1 2 j
-
1 1 0 2 0 1
Note that for transmission on four antenna ports there are 16 codebook indexes to choose from.
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2.5.6 Resource Element Mapper Following on from the precoding stage the resource element mapper maps the complex-valued symbols to the allocated resources. For each of the antenna ports used for transmission of the Physical Channel, the block of complex-valued symbols is mapped in sequence to resource elements (see Section 2.6.2 .) which meet all of the following criteria:
They are in the PRB (Physical Resource Blocks) assigned for transmission.
They are not used for transmission of PBCH, synchronization signals or reference signals.
They are not in an OFDM symbol used for PDCCH.
Additional information on the physical resources is provided in Section 2.6.2 .
2.5.7 OFDM Signal Generation The final Physical Layer processing stage is the actual OFDM signal generation, i.e. the generation of time-domain signals for each antenna. This is a purely mathematical process with various equations and parameters being used. Figure 2-30 illustrates the downlink equation; however the detail is not discussed as part of this course. Figure 2-30 Example of the Downlink Signal Generation Equation
sl p t
1
ak p ,l e j 2kf tNCP,lTs DL RB
N DLN RB / 2 RB sc
k NRB Nsc / 2
j 2kf t NCP,lTs p ak ,l e k 1
2.6 Downlink OFDMA 2.6.1 General OFDMA Structure The E-UTRA downlink is based on OFDMA. As such, it enables multiple devices to receive information at the same time but on different parts of the radio channel. In most OFDMA systems this is referred to as a “Subchannel”, i.e. a collection of subcarriers. However, in E-UTRA, the term subchannel is replaced with the term PRB (Physical Resource Block). Figure 2-31 illustrates the concept of OFDMA, whereby different users are allocated one or more resource blocks in the time and frequency domain, thus enabling efficient scheduling of the available resources.
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Figure 2-31 OFDMA in LTE
It is also worth noting that a device is typically allocated 1ms of time, i.e. a subframe, and not an individual PRB.
2.6.2 Physical Resource Blocks and Resource Elements A PRB (Physical Resource Block) consists of 12 consecutive subcarriers and lasts for one slot, i.e. 0.5ms. Figure 2-32 illustrates the size of a PRB. The NRBDL parameter is used to define the number of RB (Resource Blocks) used in the DL (Downlink). This is dependent on the channel bandwidth. In contrast, NRBUL is used to identify the number of resource blocks in the uplink. Each RB (Resource Block) consists of NSCRB subcarriers, which for standard operation is set to 12. In addition, another configuration is available when using MBSFN and a 7.5kHz subcarrier spacing. The PRB is used to identify an allocation. It typically includes 6 or 7 symbols, depending on whether an extended or normal cyclic prefix is configured. The term RE (Resource Element) is used to describe one subcarrier lasting one symbol. This can then be assigned to carry modulated information, reference information or nothing.
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Figure 2-32 Physical Resource Block and Resource Element
The different configurations for the downlink E-UTRA PRB are illustrated in Table 2-8. Table 2-8 Downlink PRB Parameters Configuration Normal Cyclic Prefix
NSCRB ∆f = 15kHz
NSymbDL 7
12 Extended Cyclic Prefix
∆f = 15kHz ∆f = 7.5kHz
6 24
3
The uplink PRB configuration is similar; however the 7.5kHz option is not available.
2.7 LTE Physical Signals In order to acquire the system, the eNB must broadcast various downlink signals. In addition, since the downlink is scalable from 1.4MHz to 20MHz and the device may not be aware of the eNB configuration, the method of finding the system needs to be consistent. Consequently, synchronization and cell identity information must appear on the downlink in a fixed place irrespective of the radio spectrum configuration. Figure 2-33 illustrates the structure of the NIDcell (Physical Cell Identity).
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Figure 2-33 Downlink Cell ID
In LTE there are two synchronization sequences, known as the PSS (Primary Synchronization Signal) and the SSS (Secondary Synchronization Signal). The location of these is dependent on the transmission mode, i.e. FDD or TDD, as well as the use of the normal or extended cyclic prefix. Figure 2-34 PSS and SSS Location for FDD
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Figure 2-35 PSS and SSS Location for TDD
Primary Synchronization Signal The PSS provides downlink synchronization information for the device. The signal sent is one of three ZC (Zadoff-Chu) sequences. This provides a pseudo noise pattern which devices can correlate, whilst at the same time enabling adjacent cells/sectors on the eNB to utilize different synchronization signals. The NID(2) (0,1 or 2) is mapped to a Zadoff-Chu root index which is used in the sequence generation process.
Secondary Synchronization Signal The SSS is generated from the interleaved concatenation of two length-31 binary sequences which are cyclic shifted based on the value of NID(1). Table 2-9 illustrates the indices generated from NID(1). It is worth noting that additional algorithms are used, as well as a different combination for subframe 0 and subframe 10. Table 2-9 Example of SSS Indices N ID
1
m0
m1
N ID
1
m0
m1
N ID
1
m0
m1
N ID
1
m0
m1
N ID
1
m0
m1
0
0
1
34
4
6
68
9
12
102
15
19
136
22
27
1
1
2
35
5
7
69
10
13
103
16
20
137
23
28
2
2
3
36
6
8
70
11
14
104
17
21
138
24
29
3
3
4
37
7
9
71
12
15
105
18
22
139
25
30
2
9
.
.
.
.
.
.
.
.
.
167
33
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5
67
8
11
101
14
18
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21
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The secondary synchronization sequence is an interleaving of two length-31 sequences s0(m0) and s1(m1) scrambled with sequences c0 and c1, which are based on NID(2), as well as scrambled with a z sequence. Figure 2-36 illustrates the concept mapping the sequences to the 62 subcarriers in subframes 0 and 5. Figure 2-36 SSS Scrambling
The concatenated sequence is scrambled with a scrambling sequence given by the primary synchronization signal.
2.8 Downlink Reference Signals Unlike other systems, the LTE air interface does not employ a frame preamble. Instead it utilizes various RS (Reference Signals) to facilitate coherent demodulation, channel estimation, channel quality measurements and timing synchronization etc. Fundamentally there are three types of downlink reference signals:
Cell Specific (non-MBSFN).
MBSFN (MBMS over Single Frequency Network).
UE Specific.
2.8.1 Cell Specific Reference Signals In LTE, the cell specific reference signals are arranged in a two dimensional lattice of time and frequency. This has been done so that they are equidistant and therefore provides a minimum mean squared error estimate for the channel. In addition, the spacing in time between the Reference Symbols is an important factor for channel estimation and relates to the maximum Doppler spread supported, i.e. speed. In LTE, this works out at 2 Reference symbols per slot. The spacing in the frequency domain is also an important factor, as this relates to the expected coherent bandwidth and delay spread of the channel. In LTE there is a 6 subcarrier separation of reference signals, however these are staggered in time such that they appear every 3 subcarriers.
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One Antenna Port Configuration The location of the RSs is dependent on the number of antennas and use of a Normal CP or Extended CP. Figure 2-37 illustrates the two options. Figure 2-37 Reference Signals - One Antenna Port
This is used for a single TX (Transmit) antenna. The reference signals are transmitted during the first and fifth OFDM symbols of each slot when the normal CP is used and during the first and fourth OFDM symbols when the extended CP is used.
Cell ID Offset It is worth noting that the position of the reference signals is dependent on the value of the Physical Cell ID. As such, the system performs a calculation (Physical Cell ID mod 6) to determine the correct offset. Figure 2-38 illustrates two cells, each producing a different offset. Figure 2-38 Reference Signal Physical Cell ID Offset
Two Antenna Port Configuration LTE is designed to operate with multiple transmit antennas for MIMO, or Transmit Diversity. The concept of reference signals is used to define different patterns for multiple antenna ports. Figure 2-39 illustrates the concept for two antennas. The RS pattern corresponding to a given antenna port enables the device to derive channel estimation. Figure 2-39 Reference Signals - Two Antenna Ports (Normal CP)
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Whilst Reference Symbols are transmitted on one antenna, the other antennas resource element is null. In addition, like the single antenna port configuration the location of the reference signals is offset based on the Physical Cell ID.
Four Antenna Port Configuration LTE supports up to four cell-specific antenna ports (0 to 3). As such, the device is required to derive up to four separate channel estimates. Figure 2-40 illustrates the configuration for four antenna ports. Figure 2-40 Reference Signals - Four Antenna Ports (Normal CP)
Antenna port “2” and antenna port “3” both have a reduced number of reference symbols. This is to reduce the reference signal overhead. It does also have a negative impact on the system since the lack of reference signals will mean that in high mobility, i.e. fast channel variations, the channel estimation will not be as accurate. This however can be offset by the fact that spatial multiplexing MIMO with 4 antennas will mostly be performed in low mobility scenarios. In addition, like the single antenna port configuration the location of the reference signals is offset based on the Physical Cell ID.
2.8.2 MBSFN Reference Signals The LTE system also defines a set of reference signal for MBSFN. This is referred to as “antenna port 4”. Figure 2-41 illustrates the two MBSFN reference signal configurations, one for 15kHz and one for 7.5kHz.
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Figure 2-41 MBSFN Reference Signals
2.8.3 UE Specific Reference Signals UE specific reference signals are supported for single antenna port transmission on the PDSCH and are transmitted on antenna port 5. It is typically used for beamforming when non-codebook based precoding is applied. Figure 2-42 UE Specific Reference Signals
Since the device has no information on the beamforming attributes applied by the eNB it needs to estimate these as part of the channel estimation process.
2.9 Downlink LTE Physical Channels In Release 8 there are five downlink Physical Channels.
2.9.1 PBCH (Physical Broadcast Channel) Along with synchronization information the eNB also schedules a MIB (Master Information Block) over the logical BCCH (Broadcast Control Channel). This is mapped into the transport BCH (Broadcast Channel) and ultimately into the PBCH (Physical Broadcast Channel).
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Figure 2-43 Broadcast Signaling
The coded BCH TB (Transport Block) is mapped into four subframes within a 40ms interval. This 40ms timing is blindly detected by the UE and the information within the subframe is assumed to be self decodable. This means that it is not dependent on information in subsequent transmissions of Transport Blocks on the PBCH. The PBCH is located in 4 symbols of slot 1 only (symbols 0, 1, 2 and 3). Figure 2-44 MIB to PBCH Mapping (FDD and Normal CP)
CRC Channel Coding Rate Matching Scrambling Modulation Layer Mapping Precoding Mapping to REs
System Bandwidth
MIB
PBCH
10ms Frame
Only the MIB is carried in the PBCH, other SIB (System Information Blocks) are sent using the PDSCH.
Section 2.11.4 discusses the LTE SI (System Information) messages and scheduling options.
2.9.2 PCFICH (Physical Control Format Indicator Channel) The PCFICH (Physical Control Format Indicator Channel) is used to inform the UE about the number of OFDM symbols used for the PDCCH in a subframe. This channel consists of 32bits which are cell-specific and scrambled prior to modulation and mapping.
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Table 2-10 CFI Mapping CFI Value
Number of OFDM Symbols Assigned to PDCCH DL
DL
N RB 10
N RB 10
1
1
2
2
2
3
3
3
4
The control area within a PRB is grouped into multiple REG (Resource Element Group), with one REG containing four Resource Elements. It is worth noting that the REG does not use Resource Elements assigned to Reference Signals. Figure 2-45 CFI to PCFICH Mapping
CFI k Channel Coding (Block1/16) Scrambling Modulation Layer Mapping Precoding Mapping to REs
OFDM Symbols allocated to PDCCH
PCFICH NRBDL
Cell
DL k = (NRB sc /2)∙(NID mod 2NRB)
Reserved RSs
k=k DL k = k + NRB )/2 ∙ NRB sc /2 DL
RB
k = k + 2NRB)/2 ∙ Nsc /2 RB
k = k + 3NDL RB)/2 ∙ Nsc /2
The PCFICH requires four REGs, i.e. 16 Resource Elements, which are distributed over the channel bandwidth. The location of these varies depending on the system bandwidth (NSCRB) and the NIDcell. Figure 2-45 illustrates the processes involved in mapping the CFI (Control Format Indicator) to the correct REGs. In addition, the calculations required are also illustrated. Table 2-11 illustrates the CFI codewords which are mapped to the PCFICH. These can change every subframe, i.e. 1ms.
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Table 2-11 CFI Codewords CFI
CFI Codeword < b0, b1, …, b31 >
1
2
3
4 (Reserved)
Since there are 2bits, i.e. four combinations, coded to 32bits the result is 1/16 Block Coding.
2.9.3 PDCCH (Physical Downlink Control Channel) The PDCCH control area size is defined by the PCFICH, i.e. 1, 2 or 3 OFDM symbols. The PDCCH carries scheduling assignments and other control information. Figure 2-46 illustrates the downlink control region. In addition, it shows how the size of the region can vary per subframe. Figure 2-46 FDD Downlink Control Region
In TDD the control regions are only available on the downlink subframes and the DwPTS.
The PDCCH is transmitted on an aggregation of one or several consecutive CCE (Control Channel Element), where a CCE corresponds to nine REGs. The number of REGs not assigned to PCFICH or PHICH (Physical Hybrid ARQ Indicator Channel) is NREG. The CCEs available in the system are numbered from 0 and NCCE -1, where NCCE = NREG / 9. The PDCCH supports multiple formats, these include:
2-36
PDCCH Format 0 - This consist of one CCE.
PDCCH Format 1 - This consist of two CCE.
PDCCH Format 2 - This consist of four CCE.
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PDCCH Format 3 - This consist of eight CCE.
Figure 2-47 illustrates the PDCCH mapping process. Figure 2-47 REG to CCE and PDCCH Mapping
PDCCH Mapping Figure 2-48 illustrates the concept of mapping the PDCCH to REGs. It assumes that the PCFICH indicated 2 symbols, as well as two antennas and one PHICH. The numbers in the control region relate to the grouping of REs into a REG. Figure 2-48 PDCCH to Control Region Mapping
Each control channel carries downlink or uplink scheduling information for one MAC identity, namely a C-RNTI (Cell - Radio Network Temporary Identifier). This is implicitly encoded in the CRC.
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There are various rules governing when a PDCCH can start in a subframe. Effectively there is a tree based method to the aggregation of CCE, these include:
1 - CCE - these start on any CCE position (0, 1, 2, 3, 4, ...).
2 - CCE - these start every second location (0, 2, 4, 6, ...).
4 - CCE - these start on every fourth (0, 4, 8, ...).
8 - CCE - these start on every eighth position (0, 8, ...).
Figure 2-49 illustrates how CCEs could be mapped. Figure 2-49 CCE Allocation Levels
Search Spaces The set of PDCCH candidates to monitor are defined in terms of search spaces. The diagram illustrates the concept of search spaces and the relationship to the CCEs. Figure 2-50 Common and UE-Specific Search Spaces
Candidate Aggregation Set for Common Control
Candidate Aggregation Set for UE-specific Control
8 - CCE 4 - CCE 2 - CCE CCE
1 - CCE 0 1 2 3 4 5 6 7 8 9 Common Search Space
UE-specific Search Space
There are two types of search spaces, namely common and UE specific. The common search space corresponds to CCEs 0-15 at two levels:
4-CCE - CCEs 0-3, 4-7, 8-11, 12-15.
8-CCE - CCEs 0-7, 8-15.
These are monitored by all UEs in the cell and can be used for any PDCCH signaling. In addition, a UE must monitor one UE specific search space at each of the aggregation levels 1, 2, 4 and 8. This may overlap with the common control search space. The location of the UE-specific search space is based on the C-RNTI (Cell - Radio Network Temporary Identity).
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The number of available CCEs in a cell is dependent on a number of attributes including:
Bandwidth.
Number of antenna ports.
PHICH configuration.
PCFICH value (1, 2 or 3).
2.9.4 PHICH (Physical Hybrid ARQ Indicator Channel) The PHICH carries HARQ (Hybrid ARQ) ACK/NAKs and is transmitted in PHICH groups. A PHICH group consists of up to eight ACK/NACK processes and requires three REGs for transmission. Each PHICH within the same PHICH group is separated through different orthogonal sequences. There are two PHICH frame formats:
Frame structure type 1 - the number of PHICH groups remains constant.
Frame structure type 2 (TDD) - the number of PHICH groups may vary between downlink subframes; this is achieved through different configuration formats.
The amount of PHICH resources (Ng) is signaled on the PBCH, as part of the MIB. Figure 2-51 illustrates how the number of PHICH groups is calculated using this parameter. Figure 2-51 PHICH Mapping
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Normal and Extended PHICH It is worth noting that the different REGs belonging to a PHICH group may be transmitted on different symbols. Figure 2-52 Extended PHICH Example
2.9.5 PDSCH (Physical Downlink Shared Channel) The PDSCH is used to send various Transport Channels, such as the PCH and DL-SCH. Figure 2-53 illustrates PDSCH mapping for one subframe. In this example the PDSCH symbols are mapped, avoiding the control region and symbols reserved for reference signals. Figure 2-53 Generic PDSCH Mapping
Subframe
PDSCH Symbols PDSCH Symbol Mapping Reserved for Control
2-40
x
R
x
R
R
x
R
x
x
R
x
R
R
x
R
x
x
R
x
R
R
x
R
x
x
R
x
R
R
x
R
x
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2.10 Downlink Control Signaling The LTE system uses a set of DCI (Downlink Control Information) messages to convey control and scheduling information to devices. The set of Downlink Control Information messages is defined LTE Release 8. Note that future releases could include additional formats. Table 2-12 illustrates the DCI Formats. Table 2-12 DCI Formats DCI Format
Usage
0
Scheduling of PUSCH
1
Scheduling of one PDSCH codeword
1A
Compact scheduling of one PDSCH codeword and random access procedure initiated by a PDCCH order
1B
Compact scheduling of one PDSCH codeword with precoding information (Rank-1 transmission)
1C
Very compact scheduling of one PDSCH codeword
1D
Compact scheduling of one PDSCH codeword with precoding and power offset information (multi-user MIMO)
2
Scheduling PDSCH to UEs configured in closed-loop spatial multiplexing MIMO
2A
Scheduling PDSCH to UEs configured in open-loop spatial multiplexing MIMO
3
Transmission of TPC (Transmit Power Control) commands for PUCCH and PUSCH with 2-bit power adjustments
3A
Transmission of TPC (Transmit Power Control) commands for PUCCH and PUSCH with 1-bit power adjustments DCI formats 0, 1A, 3, and 3A have the same payload size.
The size of the DCI format depends on its function, as well as the system bandwidth. There are various rules associated with the formatting of the DCI messages. As such, padding is typically added to ensure the rules are met.
2.10.1 DCI Format 0 This is used when scheduling the PUSCH. The following information is sent:
Flag for format0/format1A differentiation - 1 bit, where value 0 indicates format 0 and value 1 indicates format 1A.
Hopping flag.
Resource block assignment and hopping resource allocation.
Modulation and coding scheme and redundancy version.
New data indicator.
TPC command for scheduled PUSCH.
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Cyclic shift for DM RS.
UL index - This field is present only for TDD operation with uplink-downlink configuration 0.
DAI (Downlink Assignment Index) - This field is present only for TDD operation with uplink-downlink configurations 1-6.
CQI Request.
2.10.2 DCI Format 1 This is used when scheduling one PDSCH codeword. The following information is sent:
Resource allocation header (resource allocation type 0 / type 1).
Resource block assignment.
Modulation and coding scheme.
HARQ process number.
New data indicator.
Redundancy version.
TPC command for PUCCH.
Downlink Assignment Index - This field is present in TDD.
It is important that the size of a DCI format 1 message does not match other DCI messages. If the number of information bits in DCI format 1 is equal to that for format 0/1A, one zero is added. In addition, if the number of information bits in DCI format 1 belongs to one of the sizes in Table 2-13, one or more zeros can be added. Table 2-13 DCI Ambiguous Sizes of Information Bits Ambiguous Sizes of Information Bits 12, 14, 16 ,20, 24, 26, 32, 40, 44, 56
2.10.3 DCI Format 1A This is used for compact scheduling of one PDSCH codeword and random access procedure initiated by a PDCCH order. When used for the random access procedure initiated by a PDCCH order the CRC is scrambled with C-RNTI and the following information is sent:
2-42
Flag for format0/format1A differentiation - 1 bit, where value 0 indicates format 0 and value 1 indicates format 1A.
Localized/Distributed VRB assignment flag - This is 1 bit and set to 0.
Resource block assignment - all bits are set to 1.
Preamble Index.
PRACH Mask Index.
All the remaining bits are set to zero.
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Otherwise, when used for compact scheduling of one PDSCH codeword the following information is sent:
Flag for format0/format1A differentiation - 1 bit, where value 0 indicates format 0 and value 1 indicates format 1A.
Localized/distributed VRB (Virtual Resource Block) assignment flag.
Resource block assignment (localized VRB /distributed VRB).
Modulation and coding scheme.
HARQ process number.
New data indicator. Redundancy version.
TPC command for PUCCH.
Downlink Assignment Index - This is present in TDD and is applicable to TDD configurations 1-6.
Like format 0, various rules apply to the size of the message, such that zeros may need to be inserted. In addition, depending on the channel usage, i.e. the CRC is scrambled with random access, paging or system information RNTIs, certain fields may be reserved.
2.10.4 DCI Format 1B This is used for compact scheduling of one PDSCH codeword with precoding information (Rank-1 transmission). The message contains the following information:
Localized/Distributed VRB assignment flag.
Resource block assignment - different for localized and distributed VRB.
Modulation and coding scheme.
HARQ process number.
New data indicator.
Redundancy version.
TPC command for PUCCH.
Downlink Assignment Index - This is present in TDD and is applicable to TDD configurations 1-6.
TPMI information for precoding - The TPMI (Transmitted Precoding Matrix Indicator) information indicates which codebook index is used corresponding to the single-layer transmission.
PMI (Precoding Matrix Indicator) confirmation for precoding - This indicates whether precoding is based on the indicated TPMI or on the latest PMI report sent on the PUSCH.
If the number of information bits in format 1B belongs to one of the sizes in Table 2-13, one zero bit is added.
2.10.5 DCI Format 1C This is used for very compact scheduling of one PDSCH codeword. The messages include:
Gap value - This indicates if N gap,1 or N gap,2 is to be utilized.
Resource block assignment.
Transport block size index.
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2.10.6 DCI Format 1D This is used for compact scheduling of one PDSCH codeword with precoding and power offset information. The following information is sent:
Localized/Distributed VRB assignment flag.
Resource block assignment.
Modulation and coding scheme.
HARQ process number - the size of this varies depending on FDD or TDD mode.
New data indicator.
Redundancy version.
TPC command for PUCCH.
Downlink Assignment Index - This is present in TDD and is applicable to TDD configurations 1-6.
TPMI information for precoding.
Downlink power offset - This is required for multi-user MIMO scheduling in the downlink.
If the number of information bits in format 1D belongs to one of the sizes in Table 2-13, one zero bit is added.
2.10.7 DCI Format 2 This is used for scheduling PDSCH to UEs configured in closed-loop SM (Spatial Multiplexing). The concept of MIMO and SM is discussed in Section 2.22 . The following information is sent as part of DCI format 2:
Resource allocation header - This indicates resource allocation type 0 or type 1.
Resource block assignment - This is for type 0 or 1 information.
TPC command for PUCCH.
Downlink Assignment Index - This is present in TDD and is applicable to TDD configurations 1-6.
HARQ process number - the size of this varies depending on FDD or TDD mode.
Transport block to codeword swap flag - This determines the transport block to codeword mapping. However, if one of the transport blocks is disabled the mapping is different.
For the first Transport Block:
2-44
−
Modulation and coding scheme.
−
New data indicator.
−
Redundancy version.
For the second Transport Block: −
Modulation and coding scheme.
−
New data indicator.
−
Redundancy version.
Precoding information - This is either 3bits or 6bits depending on the number of antenna ports.
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2.10.8 DCI Format 2A This is for scheduling PDSCH to UEs configured in open-loop spatial multiplexing MIMO, i.e. without PMI feedback. The format of DCI format 2A is the same as format 2, except that the precoding information parameter is not used for 2 antenna ports (0 bits) and carries transmission rank information (2bits) if 4 antenna ports are used. Table 2-14 Precoding Information Field for 4 Antenna Ports (Open Loop) One codeword: Codeword 0 enabled, Codeword 1 disabled
Two codewords: Codeword 0 enabled, Codeword 1 enabled
Bit field mapped to index
Message
Bit field mapped to index
Message
0
4 layers: Transmit diversity
0
2 layers: precoder cycling with large delay CDD
1
2 layers: precoder cycling with large delay CDD
1
3 layers: precoder cycling with large delay CDD
2
Reserved
2
4 layers: precoder cycling with large delay CDD
3
Reserved
3
Reserved
2.10.9 DCI Format 3 DCI format 3 is for the transmission of TPC (Transmit Power Control) commands for PUCCH and PUSCH with 2-bit power adjustments. The following information is transmitted:
TPC command number 1, TPC command number 2,…, TPC command number N, where:
L N format 0 , 2
The parameter Lformat 0 is equal to the payload size of format 0 before CRC attachment. A power control parameter, namely tpc-Index, is provided by higher layers. This is utilized by the mobile to determine the index to the TPC command for a given UE. Power control is discussed in Section 2.19 .
2.10.10 DCI Format 3A Transmission of TPC (Transmit Power Control) commands for PUCCH and PUSCH with 1-bit power adjustments. The following information is transmitted by means of the DCI format 3A:
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2.11 LTE Cell Search Procedure The LTE device needs to perform an LTE Attach procedure, i.e. transition from the LTE Detached to LTE Active State, to connect to the EPC (Evolved Packet Core) and ultimately services. Figure 2-54 Initial Procedures
In order to access a cell the device must find and synchronize to the cell. It is then able to decode the System Information messages and perform PLMN (Public Land Mobile Network) and Cell Selection. Once this has been completed, the device is in a position to access the cell and establish a RRC connection, i.e. a SRB (Signaling Radio Bearer).
2.11.1 Cell Search The downlink in LTE is based on scalable OFDMA with channels ranging from 1.4MHz to 20MHz (Note that not all bandwidths are available at the different frequency bands). Initially the UE is unaware of the downlink configuration of the cell, unless it has stored information from when it was previously attached. Assuming no information, the synchronization process must be quick and concise. Figure 2-55 illustrates the location of the PSS and SSS. Figure 2-55 PSS and SSS for Cell Search (FDD Mode)
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In order for the UE to identify the cell and synchronize with the downlink transmission, the eNB sends synchronization signals over the centre 72 sub-carriers. For FDD mode (using a normal CP) this is in the first and sixth subframes of each downlink frame. These synchronization signals comprise of the PSS (Primary Synchronization Signal) and SSS (Secondary Synchronization Signal). Together they enable the UE to become downlink synchronized and identify the Physical Cell Identity. There are 504 unique physical cell identities, divided into 168 cell identity groups each containing three cell identities (sectors). Figure 2-56 Physical Cell Identities
The Physical Cell ID is able to be reused based on the cell and frequency reuse mechanism employed.
2.11.2 PSS Correlation The device cross correlates 3 possible PSSs with the received signal. Figure 2-57 illustrates the cross correlation results. In this example PSS1 is found. Figure 2-57 PSS Correlation
At this stage the cell identity within the group is known. In addition, the location of the SSS is also known because it occupies the previous OFDM symbol (FDD mode). However, at this stage the frame synchronization is not known since subframe 0 and 5 both utilize the same PSS sequence.
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2.11.3 SSS Correlation As previously discussed in Section 2.7 the sequence used for the second synchronization signal is an interleaved concatenation of two length-31 binary sequences. The concatenated sequence is scrambled with a scrambling sequence given by the primary synchronization signal. The combination of two length-31 sequences defining the secondary synchronization signal differs between subframe 0 and subframe 5 according to: (m )
s 0 (n)c0 n in subframe 0 d (2n) 0( m ) s1 1 (n)c0 n in subframe 5 s ( m1 ) (n)c1 n z1( m0 ) n in subframe 0 d (2n 1) 1( m ) (m ) s 0 0 (n)c1 n z1 1 n in subframe 5
where 0 n 30 . (1) The indices m 0 and m1 are derived from the Physical Layer cell identity group N ID and are shown in Table 2-9.
The references to the m-sequences include:
The two sequences s0( m0 ) (n) and s1( m1 ) ( n) are defined as two different cyclic shifts of the m-sequence ~s (n) .
The two scrambling sequences c0 (n) and c1 (n) depend on the primary synchronization signal and are defined by two different cyclic shifts of the m-sequence c~(n) .
The scrambling sequences z1( m0 ) (n) and z1( m1 ) ( n ) are defined by a cyclic shift of the m-sequence ~z (n) .
Figure 2-58 illustrates the correlation of the SSS. Note that the device is monitoring/processing a number of different SSS possibilities, i.e. more than the two shown. Figure 2-58 SSS Correlation Example
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2.11.4 Master Information Block Once the device has decoded the PSS and SSS it is able to:
Decode cell specific Reference Signals (since their location is based on the Physical Cell ID).
Perform channel estimation procedures.
Decode the PBCH which carries the MIB (Master Information Block).
The MIB repeats every 40ms and uses a 40ms TTI (Time Transmission Interval), i.e. the message is interleaved over 4 frames. The MIB transmission is aligned to the SFN (System Frame Number) such that it starts when SFN mod 4 = 0. Figure 2-59 PBCH and the Master Information Block
The MIB is always transmitted in subframe 0. The MIB carries three very important bits of information. It indicates the downlink bandwidth, i.e. 6, 15, 25, 50, 75 or 100 Resource Blocks. This enables the device to know where it should be looking (subcarriers) for the downlink control information. In addition, the PHICH configuration parameter is included. This indicates that Ng is equal to 1/6, 1/2, 1 or 2 and whether “Normal” or “Extended” PHICH mode is being used. These are used by the device to determine the number of PHICH groups configured on the cell and their location. Finally, the SFN is also included. In addition, the PBCH is layer mapped and precoded. As such, the PBCH can employ transmit diversity over multiple antennas ports. Based on the MIB the UE is able to decode the PCFICH. This identifies the number of OFDM symbols assigned to the downlink control region in the subframe.
2.11.5 System Information Messages Limited system information is sent on the MIB. As such additional SIB (System Information Block) messages are required. SIBs, other than SIB 1 (System Information Block Type1), are carried in System Information messages which are then transmitted on the DL-SCH (Downlink - Shared Channel) based on various system parameters. SIB 1 is slightly different in that it has predefined rules on how it may be sent.
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System Information Block Type 1 System Information Block Type 1 contains key information about the cell and network. In addition, it defines the scheduling window for the other System Information messages. SIB1 is transmitted on subframe 5 when SFN mod 8=0. It is also repeated in subframe 5 when SFN mod 2=0. This is illustrated in Figure 2-60. Figure 2-60 System Information Block Type 1
PLMN Identity List Tracking Area Code E-CGI (Evolved Cell Global Identity) Cell Barred Indication Intra Frequency Reselection CSG Indication CSG Identity Qrxlevminoffset P-Max Frequency Band Indicator Scheduling Info List SIB Window Length (1, 2, 5, 10, 15, 20, 40ms) System Info Value Tag
NRB
Frame 0
1
2
3
4
5
6
7
8
9
Repetitions are scheduled in subframe #5 of all other radio frames for which SFN mod 2 = 0 The main information in SIB1 includes:
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PLMN Identity List - This is a list of PLMN identities. The first listed PLMN-Identity is the primary PLMN.
Tracking Area Code - This is a TAC (Tracking Area Code) that is common for all the PLMNs listed.
E-CGI - This is a 28bit cell identifier.
Cell Barred Indication.
Intra Frequency Reselection - This is used to control cell reselection to intra-frequency cells when the highest ranked cell is barred, or treated as barred by the UE.
CSG Indication - if set to “TRUE”, the UE CSG (Closed Subscriber Group) identity needs to match.
CSG Identity - This is the identity of the Closed Subscriber Group within the primary PLMN the cell belongs to. Qrxlevminoffset - This affects the minimum required Rx level in the cell.
P-Max - This is part of the cell selection process.
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Frequency Band Indicator.
SI Periodicity Mapping Information - This denotes a value in radio frames: rf8, rf16, rf32, rf64, rf128, rf256, rf512 and is used to calculate the occurrence of messages.
SIB Window Length - This is a common SI scheduling window for all SIB and indicates 1, 2, 5, 10, 15, 20 or 40ms.
System Info Value Tag - Common for all SIBs other than MIB, SIB1, SIB10 and SIB11.
Acquisition of an SI Message When acquiring an SI message, the UE performs various calculations to determine the start of the SI-window for the concerned SI message:
For the concerned SI message, determine the number n which corresponds to the order of entry in the list of SI messages configured by “schedulingInfoList” in SystemInformationBlockType1.
Determine the integer value x = (n – 1)*w, where w is the si-WindowLength.
The SI-window starts at the subframe #a, where a = x mod 10, in the radio frame for which SFN mod T = FLOOR(x/10), where T is the si-Periodicity of the concerned SI message.
In order to identify the scheduling of SI messages the UE looks for the SI-RNTI (System Information - Radio Network Temporary Identifier) on the PDCCH. Figure 2-61 Example of SI Mapping
E-UTRAN should configure an SI-window of 1 ms only if all SIs are scheduled before subframe #5 in radio frames for which SFN mod 2 = 0.
System Information Block Type 2 System Information Block Type 2 contains radio resource configuration information that is common for all UEs. This includes detailed information on the access channels and paging channels.
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Figure 2-62 System Information Block Type 2
System Information Block Type 3 System Information Block Type 3 contains cell reselection information common for intra-frequency, inter-frequency and/or inter-RAT cell reselection (i.e. applicable for more than one type of cell reselection but not necessarily all), as well as intra-frequency cell reselection information other than that which is neighbor cell related. Figure 2-63 System Information Block Type 3
System Information Block Type 4 System Information Block Type 4 contains neighboring cell related information relevant only for intra-frequency cell reselection. It includes cells with specific reselection parameters and blacklisted cells. Figure 2-64 System Information Block Type 4
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System Information Block Type 5 System Information Block Type 5 contains information relevant only for inter-frequency cell reselection i.e. information about other E-UTRA frequencies and inter-frequency neighboring cells relevant for cell reselection. It includes cell reselection parameters common for a frequency as well as cell specific reselection parameters. Figure 2-65 System Information Block Type 5
System Information Block Type 6 System Information Block Type 6 contains information relevant only for inter-RAT cell reselection i.e. information about UTRA frequencies and UTRA neighboring cells relevant for cell reselection. It includes cell reselection parameters common for a frequency as well as cell specific reselection parameters. Figure 2-66 System Information Block Type 6
Carrier Frequency List UTRA UTRA Reselection Information
System Information Block Type 7 The System Information Block Type 7 contains information relevant only for inter-RAT cell reselection i.e. information about GERAN frequencies relevant for cell reselection. It includes cell reselection parameters for each frequency. Figure 2-67 System Information Block Type 7
Carrier Frequency List GERAN GERAN Reselection Information
System Information Block Type 8 The System Information Block Type 8 contains information relevant only for inter-RAT cell reselection i.e. information about CDMA2000 frequencies and CDMA2000 neighboring cells relevant for cell reselection. It includes cell reselection parameters common for a frequency as well as cell specific reselection parameters.
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Figure 2-68 System Information Block Type 8
System Information Block Type 9 The System Information Block Type 9 contains a HeNB (Home eNB) name. Figure 2-69 System Information Block Type 9
System Information Block Type 10 and 11 SIB 10 and SIB 11 are used to send ETWS (Earthquake and Tsunami Warning System) primary notification and ETWS secondary notification respectfully.
2.11.6 PLMN Selection The transition from LTE Detached to LTE Active can be used to describe the processes through which the UE must progress in order to establish a point of attachment within the Evolved Packet Core and ultimately connect to services. The initial processes including scanning for downlink and uplink channels and synchronization are passive in that the information required to achieve this is broadcast from the eNB within the relevant E-UTRAN. Before the UE can access the network it must first select a suitable PLMN (Public Land Mobile Network) and then a suitable cell. Services may be available to the user through a choice of several serving networks in a given location, possibly using different types of Radio Access Network. Figure 2-70 PLMN Selection
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E-UTRA PLMN Selection In the UE, the Access Stratum reports available PLMNs to the NAS on request from the NAS or autonomously. During PLMN selection, based on the list of PLMN identities in priority order, the particular PLMN may be selected either automatically or manually. Each PLMN in the list of PLMN identities is identified by a 'PLMN identity'. In the system information on the broadcast channel, the UE can receive one or multiple 'PLMN identity' in a given cell. The UE scans all RF channels in the E-UTRA bands according to its capabilities to find available PLMNs. On each carrier, the UE searches for the strongest cell and read its system information, in order to find out which PLMN(s) the cell belongs to. If the UE can read one or several PLMN identities in the strongest cell, each found PLMN is reported to the NAS as a high quality PLMN (but without the RSRP value), provided that the following high quality criterion is fulfilled:
For an E-UTRAN cell, the measured RSRP value is greater than or equal to -110 dBm.
Found PLMNs that do not satisfy the high quality criterion, but for which the UE has been able to read the PLMN identities are reported to the NAS together with the RSRP value. The quality measure reported by the UE to NAS is the same for each PLMN found in one cell. Note that the UE may optimize the PLMN search by using stored information e.g. carrier frequencies and optionally also information on cell parameters from previously received measurement control information elements.
NAS PLMN Selection The UE utilizes all the information stored in the USIM (Universal Subscriber Identity Module) related to the PLMN selection; e.g. "HPLMN (Home PLMN) Selector with Access Technology", "Operator controlled PLMN Selector with Access Technology", "User Controlled PLMN Selector with Access Technology", "Forbidden PLMNs", "Equivalent HPLMN". Note that these are the same for UMTS PLMN selection. The PLMN/access technology combinations are listed in priority order. If no particular access technology is indicated in an entry, the UE assumes that all access technologies supported by the UE apply. In addition, the UE stores a list of EHPLMN (Equivalent HPLMN). This list is replaced or deleted as part of various EMM Procedures. The stored list consists of a list of equivalent PLMNs as downloaded by the network plus the PLMN code of the registered PLMN that downloaded the list. All PLMNs in the stored list, in all access technologies supported by the PLMN, are regarded as equivalent to each other for PLMN selection, cell selection/re-selection and handover. The UE selects and attempts registration on other PLMN/access technology combinations, if available and allowable, in the following order:
Either the HPLMN (if the EHPLMN list is not present or is empty) or the highest priority EHPLMN that is available (if the EHPLMN list is present).
Each PLMN/access technology combination in the "User Controlled PLMN Selector with Access Technology" data file in the SIM (in priority order).
Each PLMN/access technology combination in the "Operator Controlled PLMN Selector with Access Technology" data file in the SIM (in priority order).
Other PLMN/access technology combinations with received high quality signal in random order.
Other PLMN/access technology combinations in order of decreasing signal quality.
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Once the UE NAS has selected a PLMN, the cell selection procedure can be performed in order to select a suitable cell of that PLMN to camp on.
2.11.7 Cell Selection For LTE there are two cell selection procedures supported:
Initial Cell Selection - This is when the UE has no prior knowledge of the cell.
Stored Information Cell Selection - This is when the UE has stored information which is used to optimize the selection process, i.e. it stored information before it was switched off.
Once a UE has synchronized with the cell and decoded the necessary System Information messages, it must camp on it; or one of the surrounding cells. This is achieved through the cell selection process. The UE is aiming to find the cell which will provide the best quality radio link between it and the network. Figure 2-71 illustrates the S (Cell Selection) calculation. Figure 2-71 LTE Cell Selection
Table 2-15 identifies the parameters used as part of the Cell Selection process. Table 2-15 Cell Selection Parameters
2-56
Parameter
Description
Srxlev
Cell Selection RX level value (dB).
Qrxlevmeas
Measured cell RX level value (RSRP), where RSRP is defined as the linear average over the power contributions of the resource elements that carry cell specific reference signals within the considered measurement frequency bandwidth.
Qrxlevmin
Minimum required RX level in the cell (dBm).
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Qrxlevminoffset
Offset to the signaled Qrxlevmin taken into account in the Srxlev evaluation as a result of a periodic search for a higher priority PLMN while camped normally in a visited PLMN.
Pcompensation
max (PEMAX - PUMAX, 0), where PEMAX is the maximum allowed power configured by higher layers.
PUMAX
RF output power of the UE (dBm) according to the UE power class (this may vary depending on allowed tolerances).
In terms of the radio channel, the UE measures the RSRP (Reference Signal Received Power). The LTE downlink contains cell specific RS (Reference Signals) which are used for channel equalization and determining the RSRP (Reference Signal Received Power). The device calculates the Qrxlevmeas for each cell. It then gathers the related Qrxlevmin and other parameters from the SI messages (each cell may provide different parameters). Once it has gathered all the information it is able to calculate Srxlev for each cell. All cells that return a value of Srxlev greater than zero are considered candidates for selection. The cell with the most positive value is selected and becomes the camped on cell.
Cell Random Access Once a UE has selected a cell it performs a random access procedure on the PRACH/RACH. Section 2.18 details this procedure.
2.12 Uplink Transmission Technique The uplink in LTE, as previously mentioned, is based on SC-FDMA (Single Carrier Frequency Division Multiple Access). This was chosen for its low PAPR (Peak to Average Power Ratio) and flexibility which reduced complexity in the handset and improved power performance and battery life. SC-FDMA tries to combine the best characteristics of single carrier systems like low peak-to-average power ratio, with the advantages of multi carrier OFDM and as such, is well suited to the LTE uplink requirements.
2.12.1 SC-FDMA Signal Generation The basic transmitter and receiver architecture is very similar (nearly identical) to OFDM, and it offers the same degree of multipath protection. Importantly, because the underlying waveform is essentially single carrier, the PAPR is lower. It is quite difficult to visually represent SC-FDMA in the time and frequency domain. This section aims to illustrate the concept. Figure 2-72 illustrates the basic structure of the SC-FDMA process.
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Figure 2-72 SC-FDMA Subcarrier Mapping Concept
In Figure 2-72 the SC-FDMA signal generation process starts by creating a time domain waveform of the data symbols to be transmitted. This is then converted into the frequency domain, using a DFT (Discrete Fourier Transform). DFT length and sampling rate are chosen so that the signal is fully represented, as well as being spaced 15kHz apart. Each bin (subcarrier) will have its own fixed amplitude and phase for the duration of the SC-FDMA symbol. Next the signal is shifted to the desired place in the channel bandwidth using the zero insertion concept, i.e. subcarrier mapping. Finally, the signal is converted to a single carrier waveform using an IDFT (Inverse Discrete Fourier Transform) and other functions. Finally a cyclic prefix can be added. Note that additional functions such as S-P (Serial to Parallel) and P-S (Parallel to Serial) converters are also required as part of a detailed functional description. Figure 2-73 illustrates the concept of the DFT, such that a group of N symbols map to N subcarriers. However depending on the combination of N symbols into the DFT the output will vary. As such, the actual amplitude and phase of the N subcarriers is like a “code word”. For example the first combination represents the first set of symbols. Since the second set of symbols is different the amplitude and phase of the N subcarriers would then be different.
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Figure 2-73 SC-FDMA Signal Generation
The process at the eNB receiver takes the N subcarriers and reverses the process. This is achieved using an IDFT (Inverse Discrete Fourier Transform) which effectively reproduces the original N symbols. Figure 2-74 illustrates the basic view of how the subcarriers received at the eNB are converted back into the original signals. Note that the SC-FDMA symbols have a constant amplitude and phase and like ODFMA, a CP (Cyclic Prefix) is still required. Figure 2-74 SC-FDMA and the eNB
Time
Second N Symbols IDFT
Power Cyclic Prefix
First N Symbols IDFT
N Subcarriers
SC-FDMA Signal Generation Equation The previous diagrams go some way to visualizing the concept of SC-FDMA. However the true time-continuous signal sl t in SC-FDMA symbol l in an uplink slot is defined by the equation in Figure 2-75.
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Figure 2-75 Example of the Uplink Signal Generation Equation
sl t
UL RB N sc / 2 1 N RB
UL RB k N RB N sc / 2
ak ( ) ,l e
j 2 k 1 2 f t N CP ,lTs
RB for 0 t N CP ,l N Ts where k ( ) k N UL RB N sc 2 , N 2048 , f 15 kHz and ak , l is the content of resource element k, l .
The SC-FDMA symbols in a slot are transmitted in increasing order of l , starting with l 0 , where SC-FDMA symbol l 0 starts at time
l 1 l 0
( N CP ,l N )Ts within the slot.
2.13 OFDMA Verses SC-FDMA The main reason SC-FDMA was specified for the uplink was because of its PA (Power Amplifier) characteristics. Typically, the SC-FDMA signal will operate with a 2-3dB lower PAPR (Peak-to-Average Power Ratio). This makes the system more efficient, thus increasing the battery life for mobile users. SC-FDMA is also better when it comes to larger cell coverage. It must be noted that OFDMA is better in a number of areas, such as Inter-symbol orthogonality and the ability to provide a more flexible frequency domain scheduling mechanism. This increases the system performance. In addition, OFDMA is more suitable for uplink MIMO operation and associated high date rate services. Table 2-16 highlights three main features and indicates which technology is best suited.
Table 2-16 SC-FDMA verses OFDMA Feature
SC-FDMA
OFDMA
Low PAPR
Y
X
Performance
X
Y
Uplink MIMO
X
Y
2.14 Uplink LTE Physical Channels There are a number of Uplink Physical Channels in LTE. These include:
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PRACH (Physical Random Access Channel) - This channel carries the Random Access Preamble. The location of the PRACH is defined by higher layer signaling.
PUCCH (Physical Uplink Control Channel) - This channel carries UCI (Uplink Control Information) such as ACK/NAKs in response to downlink transmission, as well as CQI (Channel Quality Indicator) reports. It also carries scheduling request indicators and MIMO codeword feedback.
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PUSCH (Physical Uplink Shared Channel) - This is the main uplink channel and is used to carry the UL-SCH (Uplink Shared Channel) Transport Channel. It carries both signaling and user data, in addition to UCI.
Figure 2-76 Release 8 Uplink Physical Channels
2.14.1 PRACH (Physical Random Access Channel) The random access procedure is used in various scenarios, including initial access, handover, or re-establishment. Like other 3GPP systems the random access procedure provides a method for contention and non-contention based access. The PRACH (Physical Random Access Channel) includes RA (Random Access) preambles generated from ZC (Zadoff-Chu) sequences. Figure 2-77 illustrates the basic structure of the PRACH preamble. This is effectively an OFDM symbol. Figure 2-77 PRACH Preamble CP
Sequence
TCP
TSEQ
Guard Period
Preamble
The Guard Period is required since the eNB does not know when the preambles will arrive. Figure 2-78 illustrates an example with two UEs. The first is next to the eNB therefore there is very little delay. In contrast UE “B” is some distance from the eNB, as such the initial access preamble is delayed, i.e. there is a round trip delay. The eNB must allocate a large enough window such that the preambles from UE at the edge of the cell don’t arrive outside of this window.
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Figure 2-78 PRACH Guard Period
PRACH Frame Formats As well as the position of the PRACH, four PRACH frame formats for FDD are also defined. These contain a CP (Cyclic Prefix) and Zadoff Chu sequence. The formats are designed to enable efficient operation in different scenarios. For example, the varying length of CP can be employed to counter either small or large delay spread effects due to the mobile’s position. Table 2-17 illustrates the different PRACH formats. Table 2-17 Random Access Preamble Parameters Preamble Format
Allocated Subframes
TSEQ (Ts)
TCP (Ts)
TCP (µs)
TGT (Ts)
TGT (µs)
Max. Delay Spread (µs)
Max Cell Radius (km)
0
1
24576
3168
103.125
2976
96.875
5.208
14.531
1
2
24576
21024
684.375
15840
515.625
16.666
77.344
2
2
49152
6240
203.125
6048
196.875
5.208
29.531
3
3
49152
21024
684.375
21984
715.625
16.666
102.65
4 (TDD)
Special Frame
4096
448
14.583
576
18.75
16.666
4.375
Format 4 is only available for frame structure type 2 and special subframe configurations with UpPTS lengths 4384⋅Ts and 5120⋅Ts only.
For FDD format 0, 1 2 or 3 can be configured. Figure 2-79 visualizes the different formats. It is worth noting that they can occupy more than a subframe and in addition the guard period is not specified.
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Figure 2-79 PRACH FDD Formats
The actual PRACH channel utilizes 6 PRBs, i.e. it occupies 1.4MHz of uplink channel capacity. For FDD the subcarrier spacing is 1.25kHz and there are 839 subcarriers, whilst TDD utilizes a 7.5kHz subcarrier spacing and 139 carriers. As such for FDD the duration is 1/T = 1/1.25kHz = 0.8ms. Figure 2-80 PRACH Configuration
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The exact position of the PRACH is defined in the SI (System Information) messages by using the PRACH Configuration Index. This is based on a table and can vary from 0 to 63. Table 2-18 illustrates the first part of the table. Table 2-18 PRACH Configuration Index PRACH Configuration Index
Preamble Format
System Frame Number
Subframe Number
0
0
Even
1
1
0
Even
4
2
0
Even
7
3
0
Any
1
4
0
Any
4
5
0
Any
7
6
0
Any
1, 6
7
0
Any
2 ,7
8
0
Any
3, 8
9
0
Any
1, 4, 7
10
0
Any
2, 5, 8
11
0
Any
3, 6, 9
12
0
Any
0, 2, 4, 6, 8
13
0
Any
1, 3, 5, 7, 9
14
0
Any
0, 1, 2, 3, 4, 5, 6, 7, 8, 9
15
0
Even
9
.
.
.
.
.
.
.
.
63
3
Even
9
PRACH Sequence Generation The network configures the set of preamble sequences the UE is allowed to use. There are 64 preamble sequences per cell.
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Figure 2-81 PRACH Configuration and Preamble Sequences Per Cell
The random access preamble is generated from Zadoff-Chu sequences. These have key properties:
Constant Amplitude - This improves the PARP and increases the amplifier efficiency.
Autocorrelation - This enables the eNB to provide accurate timing.
Cross Correlation - This enables different base sequence cyclic shifts to be used. Additional mechanisms are required when the cyclic shift is greater than the time expected for round trip propagation and signal delay spread.
The set of 64 preamble sequences in a cell is found by including first, in the order of increasing cyclic shift, all the available cyclic shifts of a root Zadoff-Chu sequence with the logical index RACH_ROOT_SEQUENCE, where RACH_ROOT_SEQUENCE is broadcasted as part of the System Information. Additional preamble sequences, in case 64 preambles cannot be generated from a single root Zadoff-Chu sequence, are obtained from the root sequences with the consecutive logical indexes until all the 64 sequences are found. The relation between a logical root sequence index and physical root sequence index “u” is defined by various tables and calculations in the 3GPP 36.211 specification - Physical Channels and Modulation. The u th root Zadoff-Chu sequence is defined by: xu n e
j
un( n 1) N ZC
, 0 n N ZC 1
where the length N ZC of the Zadoff-Chu sequence, e.g. 829 for Format 0. Various rules apply to identify the chosen set. In addition, the parameter “Highspeed-flag” is provided by higher layers and determines if “unrestricted set” or “restricted set” is used. The restricted set adds additional rules on the cyclic shifts that can be used as preambles, i.e. taking Doppler spread into account.
2.14.2 PUSCH (Physical Uplink Shared Channel) Uplink resource scheduling is performed by the eNB. Note that Section 3 provides more information on resource allocation and scheduling. The eNB utilizes information, e.g. QoS parameters, buffer status, UE capabilities, CQI (Channel Quality Indicator) measurements, to identify the best scheduling of resources. Like the downlink, the uplink allocation is multiples of Resource Blocks, each consisting of 12 subcarriers. The Physical Uplink Shared Channel is the main delivery mechanism for higher layer Transport Channels. Figure 2-82 illustrates an example of the mapping of PUSCH symbols to
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the Resource Elements. Like the downlink, the uplink also has resource elements reserved for Reference Signals and control. Figure 2-82 PUSCH Mapping
Additional Resource Elements are typically required to carry extra control signaling, e.g. CQI (Channel Quality Information), ACK/NACK, etc.
Multiplexing of Control Signaling and UL-SCH Data There are various types of control signaling which may need to be sent in the same subframe as the allocated PUSCH. A device is not allowed to transmit the PUCCH and PUSCH in the same subframe; therefore the control information needs to be multiplexed with the UL-SCH Transport Channel before the DFT process. Figure 2-83 Multiplexing Control Signaling
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Figure 2-83 illustrates an example of control signaling to the PUSCH. In this example, three additional types of signaling are added:
ACK/NACK - These are part of the HARQ process and are located next to the RS. This ensures that they benefit from the best possible channel estimation. The information is punctured to make way for the ACK/NACK information.
CQI/PMI - The CQI (Channel Quality Information) and PMI (Precoding Matrix Indicator) can also be multiplexed onto the PUSCH. These are rate matched with the UL-SCH. The mapping of these is sequential on one subcarrier before continuing on the next.
RI - RI (Rank Indication) - These are placed next to the ACK/NACK.
Various rules on the mapping and coding of control information exist. In addition, it is also possible to send control information on the PUSCH without data, i.e. not the UL-SCH.
2.14.3 PUCCH (Physical Uplink Control Channel) The PUCCH carries UCI (Uplink Control Information); examples include: ACK/NAKs in response to downlink transmission, CQI (Channel Quality Indicator) reports, SR (Scheduling Requests) and MIMO feedback such as PMI (Precoding Matrix Indicator) and RI (Rank Indication). The PUCCH is transmitted on a reserved frequency region. This is configured by the higher layer. Figure 2-84 illustrates an example of this mapping. It is worth noting that the number of control regions is variable. Figure 2-84 Mapping to Physical Resource Blocks for PUCCH
The PUCCH resource blocks are located at both edges of the uplink bandwidth. It uses inter-slot hopping to improve frequency diversity. Note that a UE only uses the PUCCH when it does not have any data to transmit on the PUSCH, i.e. no allocated resources. There are various types of PUCCH formats associated with uplink control. Section 2.17 discusses these in detail.
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2.15 Timing Relationships FDD Timing In LTE there are various rules associated with timing between the downlink and uplink transmissions. The timing for FDD is illustrated in Figure 2-85. If a UE detects a PDCCH with DCI format 0 and/or a PHICH transmission in subframe n intended for the UE, it will adjust the corresponding PUSCH transmission in subframe n+4 according to the PDCCH and PHICH information. This offset is identified as K, such that K=4 for FDD. Figure 2-85 FDD Timing
PDCCH
Subframe 3
Subframe 4
Subframe 5
Subframe 6
Subframe 7
Subframe 8 Downlink
Subframe 6
Subframe 7
Subframe 8
FDD: K=4
Subframe 3
Subframe 4
Subframe 5
Uplink 4 Subframe Delay
PUSCH
TDD Timing For TDD the timing relationship is more complex. As such, it now depends on the UL/DL TDD configurations, namely 0 to 6. Table 2-19 illustrates the different K values for TDD. Table 2-19 “K” Values for TDD Configurations TDD UL/DL Configuration
K value for DL Subframe Number 0
1
0
4*
6*
1
4
5
6
4*
6*
4
7
8
6
4
9
4 4 4
4
4
4
4
5
4
6
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3
6
2 3
2
4
7
7
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5
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The UE, upon detection of a PDCCH with DCI format 0 and/or a PHICH transmission in subframe n intended for the UE, adjusts the corresponding PUSCH transmission in subframe n+k, with k given in Table 2-19. Figure 2-86 illustrates an example of frame configuration 2. In this configuration, K=4 in subframes 3 and 8. This relates to transmission being scheduled for subframes 7 and 2 respectively. Figure 2-86 Example of TDD Configuration 2
2.16 Uplink Reference Signals In addition to the higher layer control and data being sent on the uplink, lower layer Reference Signals are also required. Like other Reference Signals these require good auto correlation and cross correlation properties. In addition, there needs to be a sufficient number of sequences to minimize interference. There are two variants of uplink Reference Signal supported:
DRS (Demodulation Reference Signal) - This is associated with transmission of PUSCH or PUCCH.
SRS (Sounding Reference Signal) - This is not associated with transmission of PUSCH or PUCCH.
Figure 2-87 Uplink Reference Signals
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Base Sequences Reference Signals are generated using “Base Sequences”, with the same set of base sequences used for demodulation and sounding Reference Signals. These sequences need to support different bandwidth options whilst at the same time having auto correlation and cross correlation properties. In addition, they need to have acceptable cubic metric values.
2.16.1 Demodulation Reference Signal The DRS (Demodulation Reference Signal) is used for channel estimation to help the demodulation of the control and data channels in the eNB. There are two different demodulation Reference Signals; these are used for the PUSCH and PUCCH respectively. There are various RS sequences defined, as well as different lengths. As a result, the DRS is defined using four parameters:
Sequence length - This is part of the uplink allocation.
Sequence Groups (0-29) - This is cell specific.
Sequence - Each group contains one sequence for each length up to 5PRB, and two sequences for each length from 6PRB.
12 Cyclic Shift options.
Sequence Group Selection In any given slot, the reference sequences used within a cell are from the same group. However the group assignment may change. There are two group assignment methods. Figure 2-88 DRS Sequence Group Selection
When using a fixed group, i.e. not group hopping, the same group is used for all slots. However, the group number to use is dependent on the channel type. As such, the PUCCH group number is based on the cell identity and the PUSCH group number is influenced by a higher layer parameter. If using group hopping, the group number changes with slots based on an equation. There are 17 different hopping patterns and 30 different sequence-shift patterns. As such, the PUCCH and PUSCH have the same hopping pattern but may have different sequence-shift patterns.
PUSCH DRS The DRS varies in its location depending on a number of attributes, such as the use of a normal or extended cyclic prefix. Figure 2-89 illustrates the DRS location for the PUSCH and a normal CP. In this case the DRS is located on the 4th symbol in each slot and uses the same transmission bandwidth allocated to the UEs in the uplink. Reference Signals for different UEs are derived by different cyclic shifts from the same base sequence.
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Figure 2-89 Uplink Demodulation Reference Signal (Normal CP)
In contrast, if the system is utilizing an extended CP then the DRS is located in a different OFDM symbol. Figure 2-90 Uplink Demodulation Reference Signal (Extended CP)
2.16.2 Sounding Reference Signal The SRS (Sounding Reference Signal) provides the eNB with uplink channel quality information which can be used for scheduling. The UE sends a Sounding Reference Signal in different parts of the allocated bandwidth where no uplink data transmission is available. Figure 2-91 illustrates an example whereby a UE has been allocated resources in the uplink. The eNB is able to use the DRS to provide channel estimation in this sub-band. However the eNB does not know how the UE will perform in the other bands. As such, if the eNB was to allocate resources in these other bands, the conditions may not be “favorable” and additional errors could be introduced. Effectively there are two modes for transmitting SRS, either wideband mode or frequency hopping mode. In wideband mode, the SRS occupies the bandwidth required. This could however lead to poor channel quality estimates. In contrast, frequency hopping mode sends multiple SRS signals using a narrowband transmission. This will, over time, cover the same bandwidth.
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Figure 2-91 Requirement for SRS
Subframe
Subframe No Channel Information
Assigned Resources
No Channel Information
The configuration of the sounding signal, e.g. bandwidth, duration and periodicity, are given by higher layers. The SRS is transmitted in the last symbol of the subframe. Figure 2-92 illustrates an example, whereby the eNB has configured the mobile to send SRS over a desired portion of the band. Figure 2-92 Example of SRS Frequency Hopping
Since the SRS can be sent when the UE has no current PUSCH or PUCCH assignment, mechanisms must exist to stop the UE interfering with other users’ PUSCHs. This is done by making sure all UEs know when the SRS are transmitted, such that the last symbol of the subframe where SRS is transmitted is not used by any mobiles for their PUSCH.
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SRS Transmission There are various Sounding Reference Symbol parameters defined. Most are UE semi-statically configurable by higher layers:
Transmission comb.
Starting physical resource block assignment.
Duration of SRS transmission: single or indefinite (until disabled).
SRS configuration index ISRS for SRS periodicity and SRS subframe offset Toffset .
SRS bandwidth BSRS .
Frequency hopping bandwidth, b hop .
cs Cyclic shift n SRS .
In addition, “cell specific” parameters, SRS transmission bandwidths ( C SRS ) and subframe transmission are configured by higher layers. Figure 2-93 illustrates an example of multiplexing the SRS from different users. Notice that multiple UEs can send the SRS at the same time, using different resources as well as a different cyclic shift. Figure 2-93 Example SRS Allocation
Note that the SRS may need to interact with ACK/NACK, CQI or SR information. If interacting with ACK/NACK the SRS may be dropped or the ACK/NACK punctured. In contrast, when interacting with the CQI and SR information, the SRS is dropped.
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2.17 Uplink Control Signaling The PUCCH supports multiple formats; these are illustrated in Table 2-20. Table 2-20 PUCCH Formats PUCCH Format
Description
Modulation Type
Bits per subframe
1
Scheduling Request
N/A
N/A
1a
ACK/NACK
BPSK
1
QPSK
2
QPSK
20
QPSK+BPSK
21
QPSK+QPSK
22
ACK/NACK+SR 1b
ACK/NACK ACK/NACK+SR
2
CQI/PMI or RI (CQI/PMI or RI)+ACK/NACK (Extended CP only)
2a
(CQI/PMI or RI)+ACK/NACK (normal CP only)
2b
(CQI/PMI or RI)+ACK/NACK (normal CP only)
Demodulation Reference Signal on the PUCCH The PUCCH formats include DRS (Demodulation Reference Signal). The location of these is dependent on the format type and the use of normal or extended CP. In summary these are:
Format 1, 1a and 1b (Normal CP) - DRS is symbols 2, 3 and 4.
Format 1, 1a and 1b (Extended CP) - DRS is symbols 2 and 3.
Format 2, 2a and 2b (Normal CP) - DRS is symbols 1 and 5.
Format 2 (Extended CP) - DRS is symbol 3.
If a UE has a scheduling request or CQI to send, higher layer signaling configures the resource.
2.17.1 PUCCH Format 1 For PUCCH format 1, information is carried by the presence/absence of transmission of the PUCCH from the UE. The UE is assigned a resource index which indicates a resource every nth frame that can be used to transmit a SR (Scheduling Request). The size of PUCCH format 1 is 0bits. However, the eNB knows when to expect a scheduling request from a UE. As such, if the eNB detects energy on the PUCCH it can assume it came from the “scheduled” UE.
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Note that various rules apply to the sending of scheduling requests, especially if the UE is multiplexing it with CQI and/or ACK/NAK on PUCCH. In this case:
CQI: Drop CQI when SR is transmitted.
ACK/NAK: Support multiplexing of SR and ACK/NAK.
2.17.2 PUCCH Format 1a and 1b The PUCCH Format 1a and 1b carry 1 or 2 HARQ bits. Figure 2-94 illustrates the process for one of the slots. The BPSK/QPSK symbol are applied to a cyclically shifted length-12 sequence ru(,v) (n) . Finally, an orthogonal cover code (Walsh Code) is applied. The example shows a Normal CP option with 3 DRS included. A length 3 code is applied to these, enabling the eNB to perform channel estimations for devices sharing the same resource. Figure 2-94 PUCCH Format 1a and 1b (Normal CP)
1 or 2 bit ACK/NACK To Next Slot
BPSK/QPSK Cyclically shifted length-12 sequence IFFT
IFFT
IFFT
IFFT
Length 4 Sequence W0
W1
UL RS
W2
UL RS
W3
UL RS
Slot
For an extended CP, there are six symbols and only two UL RS (Reference Signals).
Interference Issues There should be no intra cell interference in a RB since the system is using the same base reference sequence with different cyclic shifts and orthogonal codes. However there may be inter cell interference. This is improved with the use of different cyclic shifts and orthogonal codes, as well as applying different hopping patterns (since these are cell specific too).
PUCCH Format 2 Format 2 is used when CQI/PMI is transmitted without ACK/NACK or when CQI/PMI and ACK/NACK are jointly coded for the case of the extended cyclic prefix. Format 2 is characterized as follows:
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It is bit scrambled by a UE specific scrambling sequence.
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The initialization of the scrambling sequence generator is the same as that of the PUSCH.
It contains CS (Cyclic Shift) based sequences.
CS hopping is performed on a symbol basis.
Figure 2-95 PUCCH Format 2 (Normal CP)
Figure 2-96 PUCCH Format 2 (Extended CP)
PUCCH Format 2a and 2b (ACK/NACK and CQI) These formats are only supported when using the normal CP. They are characterized as follows:
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They are bit scrambled by a UE specific scrambling sequence.
The initialization of the scrambling sequence generator is the same as that of the PUSCH.
BPSK (2a) or QPSK (2b) modulation for the 2nd RS symbol in each slot is used. This carries ACK/NACK.
Format 2a: QPSK CQI + BPSK ACK/NACK
Format 2b: QPSK CQI + QPSK ACK/NACK
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Figure 2-97 PUCCH Format 2a and 2b ACK/NACK Coding
2.18 LTE Random Access Procedure Prior to registering on the network the UE must first establish a SRB (Signaling Radio Bearer) to the eNB that it has “camped on” during the cell selection process. Figure 2-98 illustrates the overall processes required, typically termed the RACH (Random Access Channel) process. Figure 2-98 Overall Random Access Procedure
2.18.1 RRC Connection The SRB is also termed the “RRC Connection”, i.e. the UE has moved into the RRC-Connected State. In order to achieve this signaling between the eNB and the UE is required. Figure 2-99 illustrates the main signaling messages to establish a SRB. Note: some of these are messages or indicators at the PHY or MAC layer. The sequence starts with the probing of the network on the PRACH. Once the UE has successfully probed for uplink resources and has been allocated these on the UL-SCH, the RRC Connection is established through a three way signaling handshake on the UL-SCH and the DL-SCH respectively.
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Figure 2-99 Random Access RRC Signaling Procedure
2.18.2 PRACH Preambles Figure 2-100 illustrates the probing process. The UE send an initial probe based on the PRACH configuration parameter, discussed in Section 2.14.1 and open loop power control. This is discussed in Section 2.19.3 . Figure 2-100 PRACH Probing
In this example the initial probe is below the noise/interference level and thus is not heard. The UE increases its power based on a step size until a response is heard on the PDCCH.
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2.18.3 Random Access Procedure Initialization The Random Access procedure is initiated by the MAC sublayer or by a PDCCH Order. The UE is required to gather various parameters before it can initiate the random access procedure. Table 2-21 lists the main parameters. Table 2-21 Parameters for Random Access Parameter
Description
PRACH-ConfigInfo
This contains: prach-ConfigIndex, highSpeedFlag, zeroCorrelationZoneConfig and prach-FreqOffset.
ra-ResponseWindowSize
Random access response window size in subframes (sf2, sf3, sf4, sf5, sf6, sf7, sf8 or sf10).
powerRampingStep
Power ramping factor (dB0, dB2,dB4 or dB6).
preambleTransMax
Maximum number of preamble transmission (n3, n4, n5, n6, n7, n8, n10, n20, n50, n100 or n200).
preambleInitialReceivedTargetPower
Initial preamble power (-120, -118, -116, -114, -112, -110, -108, -106, -104, -102, -100, -98, -96, -94, -92 or -90 dBm).
DELTA_PREAMBLE
Preamble format based offset.
maxHARQ-Msg3Tx
Maximum number of Msg3 HARQ transmissions (1 to 8).
mac-ContentionResolutionTimer
Contention Resolution Timer (sf8, sf16, sf24, sf32, sf40, sf48, sf56 or sf64).
numberOfRA-Preambles
Number of preambles used (n4, n8, n12, n16 ,n20, n24, n28, n32, n36, n40, n44, n48, n52, n56, n60 or n64).
sizeOfRA-PreamblesGroupA
Number of preambles assigned to group A (n4, n8, n12, n16 ,n20, n24, n28, n32, n36, n40, n44, n48, n52, n56 or n60).
messagePowerOffsetGroupB
Part of the power equation to identify which group to use (minusinfinity, dB0, dB5, dB8, dB10, dB12, dB15, or dB18).
messageSizeGroupA
Part of the size equation to identify which group to use (b56, b144, b208, b256}.
ra-PreambleIndex
The preamble to use as part of dedicated configuration (0 to 63).
ra-PRACH-MaskIndex
The resource to use as part of dedicated configuration (0 to 15).
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Allocation of Preamble Groups The LTE random access procedure can group the access preambles into one of two groups. In so doing, it enables the UE to indicate power or payload size requirements to the eNB for the initial UL-SCH allocation. Figure 2-101 illustrates how the random access preambles are grouped into group A and group B. Two key parameters are required to make the groups: numberOfRA-Preambles and sizeOfRA-PreamblesGroupA. The preambles in random access preamble group A are the preambles 0 to sizeOfRA-PreamblesGroupA - 1 and, if it exists, the preambles in random access preamble group B are the preambles sizeOfRA-PreamblesGroupA to numberOfRA-Preambles - 1 from the set of 64 preambles. Figure 2-101 Allocating Preambles to Group A and Group B
If sizeOfRA-PreamblesGroupA is equal to numberOfRA-Preambles then there is no Random Access Preambles group B.
Group Utilization For the first Msg3 (Higher Layer Message) the selection of group B is based on message size and pathloss attributes:
Data size plus MAC and control is greater than messageSizeGroupA.
Pathloss is less than (PCMAX – preambleInitialReceivedTargetPower – deltaPreambleMsg3 – messagePowerOffsetGroupB).
For retransmissions the UE uses the same group as was used for the initial preamble transmission attempt.
PDCCH Access Order If a UE receives a PDCCH transmission consistent with a PDCCH order masked with its C-RNTI, it initiates a Random Access procedure.
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2.18.4 Random Access Response Window Once the UE has transmitted the randomly selected preamble from the appropriate group, it monitors the PDCCH for Random Access Response(s) identified by the RA-RNTI (Random Access - RNTI) in the RA Response window. This starts at the subframe that contains the end of the preamble transmission plus three subframes and has length ra-ResponseWindowSize subframes. Figure 2-102 Random Access Response Window
The RA-RNTI is calculated using the formula: 1 + t_id+10*f_id, where t_id is the index of the first subframe of the specified PRACH (Physical Random Access Channel) resource and f_id is the index of the specified PRACH resource within that subframe.
2.18.5 Random Access Response On receiving the preamble, the eNB sends a Random Access Response on the DL-SCH. This is addressed to the RA-RNTI on the PDCCH (Physical Downlink Control Channel). It includes the RAPID (Random Access Preamble Identifier), TA (Timing Alignment) information, initial UL (Uplink) grant and assignment of a Temporary C-RNTI. Figure 2-103 MAC Random Access Response
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The UL grant contains 20bits of information, including:
Hopping flag - 1bit.
Fixed size resource block assignment - 10bits.
Truncated modulation and coding scheme - 4bits.
TPC command for scheduled PUSCH - 3bits.
UL delay - 1bit.
CQI request - 1bit.
The UE utilizes these parameters to access the resource.
2.18.6 Uplink Transmission If the UE decodes a PDCCH with the correct RA-RNTI identified, it decodes the DL-SCH transport block to check if the RAPID is included. If so, it transmits an UL-SCH transport block in the first subframe n+k1, where k1 ≥ 6. Figure 2-104 Random Access - Assigned UL-SCH
The UE would postpone the PUSCH transmission to the next available UL subframe if the UL Delay field is set to 1.
If no random access response is received in the RA response window, the UE is able to transmit a new preamble sequence. This should happen no later than 4 subframes after the end of the RA response window. Figure 2-105 illustrates the MAC contention resolution process. This is achieved by the UE sending its identity to the eNB in the first UL-SCH message. Granted, this resource could be contention based, i.e. another UE sent the same access preambles in the same subframe. Consequently, each would include their own higher layer identity. The eNB then adds the UE identity in the MAC header. Other UEs with different identifiers realize that a collision has taken place and then re-access the system, i.e. they send a new preamble.
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Figure 2-105 MAC Contention Resolution
2.19 Uplink Power Control The E-UTRA, like most cellular systems, requires power control to be implemented. This reduces interference and enables it to be managed/optimized by the eNB. Uplink power control determines the average power over a SC-FDMA symbol in which the Physical Channel is transmitted. Figure 2-106 Uplink Power Control
2.19.1 PUSCH Power Control The setting of the UE Transmit power PPUSCH (dBm) for the Physical Uplink Shared Channel transmission in subframe i is defined by:
PPUSCH (i ) min{PCMAX , 10 log10 ( M PUSCH ( i )) PO_PUSCH ( j ) ( j ) PL TF (i ) f ( i )} Where:
PCMAX - This is the configured UE transmitter power. It relates to either the maximum allowed by the eNB or the UE power class.
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M PUSCH (i ) - This is related to the bandwidth of the PUSCH resource assignment expressed in number of resource blocks.
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PO_PUSCH ( j ) - This parameter is the sum of various cell and UE specific parameters. It is
also based on retransmission and scheduling options.
- This is a 3bit cell specific parameter provided by higher layers (0, 0.4, 0.5, 0.6,….1).
PL (Pathloss) - This is the downlink pathloss estimate calculated in the UE. Note pathloss is calculated based on the reference signal power and other higher layer filter configurations.
ΔTF - This is a UE specific parameter which relates to the MCS (Modulation and Coding Scheme) and TF (Transport Format), i.e. TBS (Transport Blok Size).
F - This enables UE specific power control, i.e. TPC (Transmit Power Control). Different options can be configured, e.g. accumulation or current absolute power.
Power headroom The LTE System also defines UE PH (Power Headroom) as:
PH (i) PCMAX 10 log10 ( M PUSCH (i )) PO_PUSCH ( j ) ( j ) PL TF (i ) f (i ) db
A PHR (Power Headroom Report) is typically sent by the UE when the “prohibitPHR-Timer” expires, or when the power headroom reporting functionality is configured or re-configured.
2.19.2 PUCCH Power Control The UE power calculation whilst on the PUCCH (Physical Uplink Control Channel) is defined as:
PPUCCH i minPCMAX , P0_PUCCH PL h nCQI , n HARQ F_PUCCH F g i dBm
Where:
PCMAX - This is the configured UE transmitter power. It relates to either the maximum allowed by the eNB or the UE power class.
PO_PUCCH - This is a parameter is the sum of cell specific and UE specific parameters.
PL (Pathloss) - This is the downlink pathloss estimate calculated in the UE. Note pathloss is calculated based on the reference signal power and other higher layer filter configurations.
hn - This is a PUCCH format dependent value, where nCQI relates to the number of
CQI bits and n HARQ is the number of HARQ bits.
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F_PUCCH ( F ) - This is provided by higher layers and provides a frame format dB offset.
g (i ) - This is the current PUCCH power control and enables UE specific power control, i.e. TPC (Transmit Power Control).
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2.19.3 PRACH Power Control The UE power calculation whilst on the PRACH (Physical Random Access Channel), i.e. for preambles, is determined as: PPRACH = min{ PCMAX , PREAMBLE_RECEIVED_TARGET_POWER + PL} dBm Where:
PCMAX - This is the configured UE transmitter power. It relates to either the maximum allowed by the eNB or the UE power class.
PREAMBLE_RECEIVED_TARGET_POWER - This is set to the preambleInitialReceivedTargetPower + DELTA_PREAMBLE + (PREAMBLE_TRANSMISSION_COUNTER – 1) * powerRampingStep.
PL (Pathloss) - This is the downlink pathloss estimate calculated in the UE. Note pathloss is calculated based on the Reference Signal Power and other higher layer filter configurations.
2.20 Paging Procedures 2.20.1 Discontinuous Reception for Paging A UE in the Idle State is required to listen for paging messages. However, if left unmanaged the UE would potentially have to look at every subframe for a possible paging message. Figure 2-107 illustrates the issue this would cause, i.e. a reduction in battery performance. Figure 2-107 Paging Issues
To combat this, LTE supports DRX (Discontinuous Reception) of paging messages. Figure 2-108 illustrates the concept, whereby a UE looks at pre-determined times.
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Figure 2-108 System with DRX Reception of Paging
The eNB may have to buffer the paging message until a UE’s paging occasion occurs. The UE is given various parameters which enable it to identify a time when it should listen. This is termed a PO (Paging Occasion) and relates to a subframe. In addition, the DRX parameters also define a PF (Paging Frame), i.e. Radio Frame, which may contain one or multiple Paging Occasion(s). The system information messages provide the necessary DRX parameters to enable a UE to calculate listening times. Alternatively they can be sent to a specific UE as part of higher layer signaling.
2.20.2 Paging Frame The PF is given by the following equation: SFN mod T= (T div N)*(UE_ID mod N). This indicates the frames in which the PO (Paging Occasion) could occur. In addition, to derive the PO, a subframe pattern table and calculation is used to derive the i_s (Index). The calculation is defined as: i_s = floor(UE_ID/N) mod Ns. The following Parameters are used for the calculation of the PF and i_s:
T - This is a range of DRX values: 32, 64, 128, 256 radio frames. Note that shorter UE specific values override T.
N - This is calculated as: min(T,nB).
nB -This is defined as: 4T, 2T, T, T/2, T/4, T/8, T/16, T/32.
Ns - This is calculated as: max(1,nB/T). UE_ID - This is calculated as: IMSI mod 1024.
The i_s and Ns parameters are used to identify the PO pattern from the pattern tables. Table 2-22 illustrates the subframe patterns for FDD. Table 2-22 FDD Subframe Patterns
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Ns
PO when i_s=0
PO when i_s=1
PO when i_s=2
PO when i_s=3
1
9
N/A
N/A
N/A
2
4
9
N/A
N/A
4
0
4
5
9
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Table 2-23 illustrates the subframe patterns for TDD. Table 2-23 TDD Subframe Patterns Ns
PO when i_s=0
PO when i_s=1
PO when i_s=2
PO when i_s=3
0
0
N/A
N/A
N/A
2
0
5
N/A
N/A
4
0
1
5
6
2.21 HARQ Operation 2.21.1 Retransmission Types There are two types of retransmissions, namely ARQ (Automatic Repeat Request) and HARQ (Hybrid Automatic Repeat Request). The ARQ is performed by RLC (Radio Link Control), whereas the HARQ is part of the MAC (Medium Access Control) and Physical Layer. Figure 2-109 illustrates some of the features/issues of ARQ, as well the benefits of HARQ. Figure 2-109 ARQ Verses HARQ
2.21.2 HARQ Methods HARQ provides a Physical Layer retransmission function that significantly improves performance and adds robustness. The retransmission protocol selected in LTE is SAW (Stop And Wait) due to the simplicity of this form of ARQ. In SAW, the transmitter persists on the transmission of the current transport block until it has been successfully received, before initiating the transmission of the next one. Figure 2-110 illustrates the basic concept of SAW. It also highlights a possible issue associated with sending more packets between each transmission.
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Figure 2-110 Basic Concept of SAW
The mechanism for sending more packets between each transmission is relatively simple; have a number of HARQ processes that can run in parallel. Figure 2-111 illustrates the concept of the HARQ processes. In LTE there are various rules and options for how many HARQ processes are configured, i.e. it depends on downlink, uplink, FDD or TDD. This example illustrates the downlink FDD frame where 8 HARQ processes are used. It also highlights one of these processes, namely process “3”, being sent by the eNB and initially acknowledged by the UE. Whilst the eNB is awaiting the ACK (Acknowledgement) for this, the additional processes can be utilized to ensure the UE can receive a stream of packets. Figure 2-111 HARQ Parallel Processes
If the mobile identified an error in the transmission it is able to send a NACK (Negative Acknowledgement) to the eNB. The eNB is then able to quickly re-schedule the data. There are two main concepts of HARQ, namely CC (Chase Combining) and IR (Incremental Redundancy). Figure 2-112 HARQ Methods
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Chase Combining Chase Combining ensures that each retransmission is simply a replica of the data first transmitted. The decoder at the receiver combines these multiple copies (of the same information). This type of combining provides time diversity and soft combining gain at a low complexity cost and imposes the least demanding UE memory requirements of all H-ARQ methods.
Incremental Redundancy The IR (Incremental Redundancy) method ensures that retransmissions include additional redundant information that is incrementally transmitted if the decoding fails on the first attempt. This causes the effective coding rate to increase based on the number of retransmissions sent. Incremental Redundancy can be further classified in Partial IR and Full IR. Partial IR includes the systematic bits in every coded word, which implies that every retransmission is self-decodable, whereas Full IR only includes parity bits, and therefore its retransmissions are not self-decodable. Figure 2-113 illustrates an example showing how rate matching and redundancy versions are used for retransmission. In addition, it highlights the concept of the “effective” code rate. Figure 2-113 Example of Redundancy Versions and Soft Bits
2.21.3 HARQ in LTE The HARQ within the MAC sublayer is designed to transmit and retransmit transport blocks. For FDD, there are 8 HARQ processes in the downlink. In contrast the uplink has 8 HARQ processes for non-subframe bundling operation, i.e. normal HARQ operation, and 4 HARQ processes in the uplink for subframe bundling operation. The concept of subframe bundling is discussed in Section 3 0as part of LTE scheduling options. Various HARQ scheduling parameters are required, such as NDI (New Data Indicator) and TB (Transport Block) size. In addition, the DL-DSCH HARQ information also includes the HARQ process ID. For UL-SCH transmission the HARQ info also includes RV (Redundancy Version). In case of spatial multiplexing, i.e. MIMO, on the DL-SCH the HARQ information comprises a set of NDI and TB size for each transport block.
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Figure 2-114 FDD HARQ Processes
The number of HARQ processes for TDD is related to the frame configuration and varies between 4 and 15. Table 2-24 illustrates the different TDD HARQ configurations. Table 2-24 TDD HARQ Processes TDD UL/DL Configuration
Maximum Number of HARQ Processes
0
4
1
7
2
10
3
9
4
12
5
15
2.21.4 HARQ In the Downlink The downlink HARQ is summarized by:
Asynchronous adaptive HARQ.
Uplink ACK/NAKs in response to downlink (re)transmissions are sent on PUCCH or PUSCH.
PDCCH signals the HARQ process number, indicating transmission or retransmission.
Retransmissions are always scheduled through PDCCH.
2.21.5 HARQ In the Uplink The uplink HARQ is summarized by:
2-90
Synchronous HARQ.
Maximum number of retransmissions configured per UE (as opposed to per Radio Bearer).
Downlink ACK/NAKs in response to uplink (re)transmissions are sent on PHICH.
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HARQ operation in uplink is governed by the following principles:
Regardless of the content of the HARQ feedback (ACK or NACK), when a PDCCH for the UE is correctly received, the UE follows what the PDCCH asks the UE to do i.e. perform a transmission or a retransmission (referred to as adaptive retransmission).
When no PDCCH addressed to the C-RNTI of the UE is detected, the HARQ feedback dictates how the UE performs retransmissions: −
NACK - the UE performs a non-adaptive retransmission i.e. a retransmission on the same uplink resource as previously used by the same process.
−
ACK - the UE does not perform any uplink (re)transmission and keeps the data in the HARQ buffer. A PDCCH is then required to perform a retransmission i.e. a non-adaptive retransmission cannot follow.
Measurement gaps (part of the measurements for mobility) are of higher priority than HARQ retransmissions: whenever an HARQ retransmission collides with a measurement gap, the HARQ retransmission does not take place.
The sequence of redundancy versions is 0, 2, 3, 1.
Table 2-25 illustrates the UE behavior in various situations. Table 2-25 UL HARQ Operation HARQ feedback seen by the UE
PDCCH seen by the UE
UE behaviour
ACK or NACK
New Transmission
New transmission according to PDCCH.
ACK or NACK
Retransmission
Retransmission according to PDCCH (adaptive retransmission).
ACK
None
No (re)transmission, keep data in HARQ buffer and a PDCCH is required to resume retransmissions.
NACK
None
Non-adaptive retransmission.
2.21.6 ACK NACK Timing FDD Mode In FDD mode, when data is sent on the PDSCH for a UE, the DCI scheduling messages provide the UE with the necessary information to decode the message. Based on the validation of a CRC the UE then sends an ACK or NACK to the eNB. Figure 2-115 illustrates the ACK/NACK in the transmission in subframe i+4, where subframe i is associated with the PDSCH.
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Figure 2-115 Downlink FDD HARQ Timing
Figure 2-116 illustrates an ACK/NACK received on the PHICH assigned to a UE in subframe i, where the associated PUSCH was in transmission subframe i-4. Figure 2-116 Uplink FDD HARQ Timing
TDD ACK Modes In TDD, an ACK/NACK received on the PHICH assigned to a UE in subframe i is associated with the PUSCH transmission in the subframe i-k, where k is dependent on the TDD configuration mode table. In addition, TDD has two ACK/NACK feedback modes defined:
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ACK/NACK bundling feedback mode - This is used when the associated HARQ ACK/NACK from multiple PDSCH subframes map into the same uplink subframe. It utilizes a logical “AND” operation across the downlink subframes.
ACK/NACK multiplexing feedback mode - This uses spatial ACK/NACK bundling across multiple codewords within a downlink subframe and is performed by a logical “AND” operation of all the corresponding individual ACK/NACKs.
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2.22 Diversity Options Cellular systems are continually improving the performance and spectral efficiency achieved on the radio interface. One method of providing this is through the inclusion of diversity techniques. This may be through schemes like SFBC (Space Frequency Block Coding) and FSTD (Frequency Shift Time Diversity), as well as various types of MIMO (Multiple Input Multiple Output).
2.22.1 SU-MIMO and MU-MIMO MIMO relates to the use of multiple antennas at both the transmitter (multiple input) and receiver (multiple output). The terminology and methods used in MIMO can differ from system to system, however most fall into one of two categories:
SU-MIMO (Single User - MIMO) - this utilizes MIMO technology to improve the performance towards a single user.
MU-MIMO (Multi User - MIMO) - this enables multiple users to be served through the use of spatial multiplexing techniques.
Figure 2-117 SU-MIMO and MU-MIMO
Increases capacity since a single user benefits from multiple data streams.
MU-MIMO
SU-MIMO
Increases sector capacity by allowing users to share streams.
2.22.2 MIMO and Transmission Options The LTE system supports various “modes” of transmission, some of which include TD (Transmit Diversity) techniques. Some techniques are “open-loop”, i.e. no feedback, which are mainly used for common downlink channels that are not able to benefit from channel selective scheduling.
Transmission Modes In the downlink, the method of transmission is sent when a mobile is semi-statically configured via higher layer signaling to receive PDSCH data. LTE includes the following Transmission Modes:
Mode 1 - Single-Antenna transmission, port 0, no MIMO.
Mode 2 - Transmit diversity.
Mode 3 - Transmit diversity or with Large Delays CDD (Cyclic Delay Diversity) is used.
Mode 4 - Transmit diversity or Closed-loop spatial multiplexing.
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Mode 5 - Transmit diversity or multi user MIMO (more than one UE is assigned to the same resource block).
Mode 6 - Transmit diversity or closed loop precoding for rank=1 (i.e. no spatial multiplexing, but precoding is used).
Mode 7 - Single-antenna port, port 5 (beamforming).
2.22.3 MIMO Modes LTE supports MIMO (Multiple Input Multiple Output), or multi-antenna transmission, with 2 or 4 transmit antennas. The maximum number of codewords is two, irrespective of the number of antennas with fixed mapping between code words to layers.
Spatial Multiplexing The most common MIMO category is referred to as SM (Spatial Multiplexing). This allocates multiple modulation symbol streams to a single UE using the same time/frequency. The differentiation of signals is achieved by the different Reference Signals which were sent as part of the PRB (Physical Resource Block). Figure 2-118 illustrates the concept of Spatial Multiplexing using a 2x2 MIMO system. Figure 2-118 Spatial Multiplexing MIMO
The main issue with Spatial Multiplexing in a cellular system is associated with high levels of interference, especially at the cell edge. Unfortunately, this can affect both spatial streams and, as such, twice as many errors could be introduced. Hence, SM is typically used close to the eNB, i.e. not at the cell edge. Figure 2-119 Spatial Multiplexing Interference Issues
If a UE was at the cell edge it could still benefit from MIMO. However it would rely on different implementations, such as using a single stream precoding. Figure 2-120 illustrates the basic concept of precoding using STC (Space Time Coding) as a visual example. Note that precoding is more involved.
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Figure 2-120 MIMO Single Stream
AMS (Adaptive MIMO Switching) To truly optimize the channel efficiency, some systems offer the ability to support AMS (Adaptive MIMO Switching). Figure 2-121 illustrates how a system could utilize a mixture of Spatial Multiplexing and other methods, such as Space Time Coding, to optimize the eNB performance. Figure 2-121 AMS Concept Spatial Multiplexing
AMS Point Other Methods
Low SNR
High SNR
Other Techniques In addition, the following techniques are supported in LTE:
Code-book-based pre-coding.
Rank adaptation with single rank feedback. Note: the eNB can override a rank report.
2.22.4 Spatial Multiplexing in LTE LTE allows up to two code words to be mapped onto different layers. The system uses precoding to enable spatial multiplexing. Figure 2-122 illustrates the processing undertaken by the PDSCH. This was previously introduced in Section 2.5 with the concept of rank transmission and layers.
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Figure 2-122 PDSCH Processing
In order for the signal to be spatially multiplexed onto the different antenna ports various mathematical processes are required. In addition, variances occur for 2 and 4 antenna configurations, as well as open and closed loop spatial multiplexing.
Codebook Based Precoding A key part of the system is the codebook based coding mechanism. It uses a:
7 element codebook for 2 antenna ports.
16 element codebook for 4 antenna ports.
Table 2-26 illustrates the mapping of codebook indexs onto layers for a 2 transmit antenna configuration. Note that the 3GPP 36.211 specification includes the detail of precoding and layer mapping equations for the different techniques and also for 4 antenna configurations. Table 2-26 Codebook Precoding Codebook Index
Number of Layers 1
2
0
1 1 2 1
1
1 1 2 1
1 1 1 2 1 1
2
1 1 2 j
1 1 1 2 j j
3
1 1 2 j
-
1 1 0 2 0 1
For the closed-loop spatial multiplexing transmission mode, the codebook index 0 is not used when the number of layers is equal to 2.
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2.22.5 Feedback Reporting In order to optimize the system’s performance, the UE can provide various feedback information about the radio channel environment. LTE has various feedback reporting options which depend on the MIMO and eNB configuration. The reporting may consist of the following elements. Figure 2-123 Feedback Reporting
CQI (Channel Quality Indicator) This provides an indication of the downlink channel quality and effectively identifies an optimum modulation and coding scheme for the eNB to use. There are various coding options for the CQI; Figure 2-124 illustrates the main CQI index. Figure 2-124 4-bit CQI Table CQI Index
Modulation
Code Rate x 1024
Efficiency
0
out of range
1
QPSK
78
0.1523
2
QPSK
120
0.2344
3
QPSK
193
0.3770
4
QPSK
308
0.6016
5
QPSK
449
0.8770
6
QPSK
602
1.1758
7
16QAM
378
1.4766
8
16QAM
490
1.9141
9
16QAM
616
2.4063
10
64QAM
466
2.7305
11
64QAM
567
3.3223
12
64QAM
666
3.9023
13
64QAM
772
4.5234
14
64QAM
873
5.1152
15
64QAM
948
5.5547
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The system defines multiple types of CQI, whereby the term “wideband CQI” relates to the entire system bandwidth. In contrast, “sub-band CQI” relates to a value per sub-band. This is defined and configured by the higher layers and relates to the number of resource blocks. It is also worth noting that a CQI per codeword is reported for MIMO spatial multiplexing. Depending on the scheduling mode, Periodic and Aperiodic CQI reporting can be used. In “Frequency Non-selective” and “Frequency selective” mode the PUCCH is used to carry periodic CQI reports. In contrast, for “Frequency selective” mode, the PUSCH is used to carry aperiodic CQI reports.
PMI (Precoding Matrix Indicator) This enables the mobile to select an optimal precoding matrix. The PMI value relates to a codebook table within the specifications. Like sub-band CQI, the eNB defines which resource blocks are related to a PMI report. The PMI reports are used in various mode, including: closed loop spatial multiplexing, multi-user MIMO and closed-loop rank 1 precoding.
RI (Rank Indication) This indicates the number of useful transmission layers when spatial multiplexing is used. Thus, in case of transmit diversity, rank is equal to 1 (RI=1).
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3
Dynamic Resource Allocation
Objectives On completion of this section the participants will be able to: 3.1 Describe UL and DL Scheduling principles and signaling 3.2 Explain how the scheduler interactions with other functions 3.3 Explain the concepts of dynamic and semi-persistent scheduling
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3.1 Scheduling Principles and Signaling LTE air interface scheduling is the responsibility of the eNB, however additional scheduling and QoS (Quality of Service) handling could take place in the EPC (Evolved Packet Core). Typically, the main goal of scheduling is to meet the different users’ expectations. Historically the radio interface is the “weak link” or “bottle neck” in the overall end-to-end service. This is typically due to limited physical resources, i.e. limited bandwidth or channels. The scheduling in previous systems, such as GSM and UMTS, was easier. This was due to the fact that voice was the main service and required a dedicated channel. As such, the number of channels (or elements) on the base station limited the number of simultaneous calls. Systems are now evolving, e.g. UMTS has evolved into HSPA and HSPA+, towards packet based services. LTE is the same, such that it is a pure packet based system. In so doing, all services utilize IP (Internet Protocol). Figure 3-1 IP Scheduling
Since LTE is 100% packet based it makes the system design easier. This is because the eNB does not have to “interwork” its scheduling algorithms with dedicated functions. Figure 3-2 illustrates the basic scheduling concept. In this example three users, each with a defined QoS, have data to send. Figure 3-2 Basic Scheduling in a Cell
This is a simple example but it does highlight some of the fundamental concepts:
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Multiple users can have different amounts of data in the eNB buffers, as well as in their uplink buffers.
UEs could be in different locations and hence features such as MIMO may or may not be available.
Users and specifically the services (QoS) supported could have different priorities, thus requiring the eNB to prioritize traffic. In the previous example, User B’s data was scheduled, however User A’s data was delayed until the next subframe. This could have been based on the service, e.g. a guaranteed service.
The eNB only has a finite amount of resources. This can vary based on a number of factors. One such factor is the location of users, whereby if they were all close to the eNB, the scheduler could allocate SM MIMO resources.
3.1.1 QoS in Packet Switched Networks Packet switched technologies are designed to provide enhance network utilization and converge multiple data types (multimedia). Unfortunately, services such as voice and multimedia have various issues associated with delay and jitter. To combat this, the LTE packet switches / bearer managers are QoS aware, in that they are able to classify packets, as well as enforce forwarding characteristics. The eNB (Evolved Node B), S-GW (Serving Gateway) and PDN-GW (Packet Data Network - Gateway) all get involved in the managing of QoS. Figure 3-3 illustrates the concept of packet classifiers and packet schedulers. Note that most of the packets have already been classified by the time they reach the eNB. Figure 3-3 Packet Classifier and Packet Scheduler
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3.1.2 Key Factors Influencing Scheduling Figure 3-4 illustrates a number of factors which influence the scheduling process. This is not a complete list and some of the factors may contain a lot of other aspects. For example, “eNB configuration” could relate to:
Frequency planning.
Cell size.
Power limitations.
MIMO feature support.
Etc.
Figure 3-4 Key Factors Influencing Scheduling
3.1.3 Scheduling Methods One of the other big influences in the performance of the eNB and the scheduler is the actual algorithm used and its associated efficiency for the type(s) of traffic being scheduled. Broadly speaking, there is a handful of basic scheduling methods, which are then customized into proprietary scheduling algorithms. Most schedulers use QoS classes of the services for radio resource allocation. Figure 3-5 Possible Scheduling Method
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Proportional Fair This is a very common scheduling method. It effectively allocates the same amount of resources to all the users. In so doing, each user will get the resources they require (throughput demand) or they will get an equal share. This is effectively the total amount of resources divided by the total number of users.
MAX C/I In order to achieve the “best” eNB throughput rates it makes sense to allocate resources to those users with the best signal, i.e. C/I (Carrier to Interference). In this way features such as MIMO SM and high order modulation schemes (64 QAM) can be used. In so doing this increases the system’s spectral efficiency. Unfortunately, this means that users closer to the eNB continually get resources allocated up to their maximum demanded rate. At the same time, users at the cell edge will be limited to their minimum guaranteed rate. This could be detrimental to the “marketing plan”, since users of LTE will expect higher data rates.
Biased (QoS Based) The biased scheduling method relates to the user’s services and their QoS class, such that users with high QoS service attributes are allocated the resources first. If multiple users shared the same QoS, e.g. they are both performing a VoIP session, then the system revertes back to another method (usually Proportional Fair).
3.1.4 Downlink Scheduling The signaling required for scheduling downlink resources is firstly dependent on the type of resources being scheduled. The LTE system defines various DCI (Downlink Control Information) messages which were introduced in Section 2.10 . These enable both downlink and uplink scheduling, as well as linking to different MIMO and diversity options. For the purpose of this section, DCI Format 2 for FDD is re-visited.
DCI Format 2 This is used for scheduling PDSCH to UEs configured in closed-loop SM (Spatial Multiplexing). The following information is sent as part of DCI format 2:
Resource allocation header - This indicates resource allocation type 0 or type 1. These are detailed in Section 3.1.5 .
Resource block assignment - This is for type 0 or 1 information.
TPC command for PUCCH - Previous discussed under power control.
HARQ process number.
Transport block to codeword swap flag - This determines the transport block to codeword mapping. However, if one of the transport blocks is disabled the mapping is different.
For the first Transport Block:
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Modulation and coding scheme.
−
New data indicator.
−
Redundancy version.
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−
Modulation and coding scheme.
−
New data indicator.
−
Redundancy version.
Precoding information - This is either 3bits or 6bits depending on the number of antenna ports.
3.1.5 PDSCH Resource Allocation The UE interprets the DCI resource allocation field depending on the PDCCH DCI format message. A resource allocation field in each PDCCH includes two parts. These are:
A resource allocation header field.
Information consisting of the actual resource block assignment.
There are three types of resource allocation.
Type 0 Resource Allocation The resource block assignment information includes a bitmap indicating the RBG (Resource Block Groups) that are allocated to the scheduled UE, where a RBG is a set of consecutive PRBs. Resource Block Group size (P) is a function of the system bandwidth, examples include: 5MHz P=2, 10MHz P=3 and for 15MHz and 20MHz P=4. Figure 3-6 Type 0 Resource Allocation
Type 1 Resource Allocation Type 1 - resource block assignment information of size NRBG indicates to a scheduled UE the PRBs from the set of PRBs from one of P RBG subsets. A RBG subset“p”, where 0 ≤ p < P , consists of every Pth RBG starting from RBG p . The resource block assignment information consists of three fields:
3-6
The first field is used to indicate the selected RBG subset among P RBG subsets.
The second field with one bit is used to indicate a shift of the resource allocation span within a subset. A bit value of 1 indicates a shift is triggered. Otherwise a shift is not triggered.
The third field includes a bitmap, where each bit of the bitmap addresses a single PRB in the selected RBG subset in such a way that MSB to LSB of the bitmap are mapped to the PRBs in the increasing frequency order.
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Figure 3-7 Type 1 Resource Allocation
Type 2 Resource Allocation Type 2 - resource block assignment information indicates to a scheduled UE a set of contiguously allocated localized VRB (Virtual Resource Block) or distributed VRB, which are then mapped onto physical resource blocks. The information field for the resource block assignment carried on the PDCCH contains a RIV (Resource Indication Value) from which a starting VRB and a length in terms of contiguously allocated virtual resource blocks, can be derived. Figure 3-8 Type 2 Resource Allocation
3.1.6 Modulation and Coding Scheme One of the key parameters in the DCI messages is the MCS Index Parameter. Table 3-1 illustrates the mapping of the MCS index to the modulation and TBS (Transport Block Set) Index. Table 3-1 Modulation and TBS index table for PDSCH MCS Index
TBS Index
MCS Index
I MCS
Modulation Order Qm
TBS Index
I MCS
Modulation Order Qm
I TBS
0
2
0
16
4
15
1
2
1
17
6
15
2
2
2
18
6
16
3
2
3
19
6
17
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4
2
4
20
6
18
5
2
5
21
6
19
6
2
6
22
6
20
7
2
7
23
6
21
8
2
8
24
6
22
9
2
9
25
6
23
10
4
9
26
6
24
11
4
10
27
6
25
12
4
11
28
6
26
13
4
12
29
2
Reserved
14
4
13
30
4
15
4
14
31
6
The modulation order parameter indicates whether the scheduled transmission is QPSK (2 bits), 16QAM (4bits) or 64QAM (6bits). The UE is able to use this information, in conjunction with the physical number of Resource Blocks, i.e. symbols, to receive all the bits. Figure 3-9 illustrates an example of a scheduled message. As previously mentioned the resource allocation, modulation order and precoding information enables the UE to determine the number and location of the physical bits. The TBS (Transport Block Set) parameter in the previous table enables the UE to identify the size of the transport block(s) using a mixture of a table and equation. Since the coding is all predefined, the UE is able to replicate the number of coded bits (pre puncturing) and therefore, using the RV (Redundancy Version) parameter, identify which bits the eNB would have punctured/rate matched. Using this it can now attempt to decoded the transport block and verify the CRC.
5MHz (25 Resource Blocks)
Figure 3-9 Using the TBS Size
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3.1.7 Uplink Scheduling The uplink scheduling process is similar. The eNB provides the relevant parameters in the DCI Format 0 message. In order to simplify uplink signal processing and the DFT (Discrete Fourier Transform) design the PUSCH can only be allocated in factors of 2, 3, and 5, i.e. 7 RBs are not allowed.
LTE TTI Bundling LTE also supports subframe bundling where a bundle of PUSCH transmissions consists of four consecutive uplink subframes in both FDD and TDD. The subframe bundling operation is configured by the parameter “ttiBundling” provided by higher layers.
3.2 Scheduler Interaction A good scheduler is one that is harmonized with all the flexibilities of the LTE air interface. In so doing, it can quickly adapt to various issues and changes in the radio environment. As previously mentioned, the scheduler needs to be QoS aware for different users and their services. This is achieved by the scheduler interacting with different functions and the process which manages those functions. In addition, it must have a mixture of pre-configured and dynamic variables it can use and possibly change.
3.2.1 Radio Bearers The scheduling of user information can be broadly broken down into three areas:
SRB (Signaling Radio Bearer) - each UE on the network will establish a SRB, i.e. RRC connection, when it moves to the LTE Active state. There are three types of SRB, namely SRB 0, SRB 1 and SRB 2. Each have different scheduling requirements.
Default EPS Bearer - The process of attaching to the network causes a default EPS bearer to be established. The QoS attributes for this are part of the user’s subscription. This is passed to the eNB as part of the Initial Context Setup procedure.
Dedicated EPS Bearers - In addition to the default bearer, one or more dedicated bearers can be established (each with their own QoS attributes). The process of E-RAB setup from the MME typically activates these.
3.2.2 Scheduler Interaction with Layer 2 and Layer 1 It is mainly the responsibility of RRM and RRC to configure the bearers between the eNB and the UE. As a result, configuration parameters can be sent for various layer 2 management functions looking after scheduling, link adaptation, RLC, HARQ etc. Figure 3-10 illustrates some of the interaction that may take place within the eNB.
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Figure 3-10 Scheduler Interaction
In order to correctly schedule resources, various layer 1 and layer 2 indications and configurations are required. The link adaptation function manages the current MCS (Modulation and Coding Scheme) based on feedback from both layer 1 in the eNB and the UE. In addition, SRS (Sounding Reference Signals) provide intelligence about the channel. Other reports from the eNB Layer 1, as well as UCI (Uplink Control Information) from the UE, can be collated to provide an up-to-date representation of the channel. The scheduler also needs to interact closely with HARQ, since layer 1 NACKs and subsequent retransmissions impact resources. Additional functionality which monitors the relationship between retransmissions, the choice of MSC and power control is also vital, enabling the system to adapt to the channel conditions.
3.3 Dynamic and Semi-persistent Scheduling LTE supports Dynamic and Semi-persistent scheduling, the latter being used to reduce the amount of control channel overhead/signaling. This enables the eNB scheduler to efficiently schedule resources for application/bearers which have a continual allocation requirement, e.g. VoIP. The semi-persistent allocation persists until the eNB scheduler changes it. When Semi-Persistent Scheduling is enabled by RRC, the following information is provided:
3-10
Semi-Persistent Scheduling C-RNTI.
semiPersistSchedIntervalUL - This is the uplink Semi-Persistent Scheduling interval.
implicitReleaseAfter - This is the number of empty transmissions before implicit release if Semi-Persistent Scheduling is enabled for the uplink.
semiPersistSchedIntervalDL - This is the downlink Semi-Persistent Scheduling interval .
numberOfConfSPS-Processes - This is the number of configured HARQ processes for Semi-Persistent Scheduling (downlink).
Whether twoIntervalsConfig is enabled or disabled for uplink (only for TDD).
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3.3.1 Dynamic Scheduling Figure 3-11illustrates the concept of dynamic scheduling, whereby an individual scheduling message allocates a resource. Note that in the uplink TTI bundling could also be used. Figure 3-11 Dynamic Scheduling
3.3.2 Downlink Semi-persistent Scheduling After a Semi-Persistent downlink assignment is configured, the UE considers that the assignment recurs in each subframe for which: (10 * SFN + subframe) = [(10 * SFNstart time + subframestart time) + N * semiPersistSchedIntervalDL] modulo 10240, for all N>0. Where SFNstart time and subframestart time are the SFN (System Frame Number) and subframe, respectively, at the time the configured downlink assignment were (re-)initialised. Figure 3-12 illlustrates the basic concept of uplink Semi-Persistent Scheduling.
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Figure 3-12 Semi Persistent Scheduling
3.3.3 Uplink Semi-persistent Scheduling When a Semi-Persistent Scheduling uplink grant is configured, the UE considers that the grant recurs in each subframe for which: (10 * SFN + subframe) = [(10 * SFNstart time + subframestart time) + N * semiPersistSchedIntervalUL + Subframe_Offset * (N modulo 2)] modulo 10240, for all N>0. Where SFNstart time and subframestart time are the SFN (System Frame Number) and subframe at the time the configured uplink grant were (re-)initialised. In additon, the Subframe_Offset is set to 0 unless twoIntervalsConfig is enabled. In this case the Subframe_Offset is set according to a table in the 3GPP 36.321 specification. Retransmissions for Semi-Persistent Scheduling can continue after clearing the configured uplink grant.
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Intra LTE Mobility
Objectives On completion of this section the participants will be able to: 4.1 Describe intra-LTE mobility in ECM-CONNECTED and ECM-IDLE mode. 4.2 Explain the concept of event triggered periodical reporting. 4.3 Describe the mobility measurements.
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4.1 Intra-LTE Mobility Intra-LTE mobility can be split into Idle State mobility and Active State mobility. Figure 4-1 Intra-LTE Mobility
A UE in the Idle State has previously registered on the network and is performing two main procedures, namely cell reselection and listening to paging messages.
4.1.1 Idle State - Cell Reselection The E-UTRA cell reselection process is similar (not identical) to the one used in UMTS. In addition, various parameters are used to define if intra and inter frequency measurements should be taken. Figure 4-2 illustrates the concept of intra-frequency and inter-frequency. Figure 4-2 Intra-Frequency and Inter-frequency
Intra-Frequency Measurements Criteria to perform intra-frequency measurements are as follows.
If Squal > Sintrasearch - the UE may choose not to perform intra-frequency measurements.
If Squal
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