ELP 4003 LTE Air Interface

LTE Air Interface ESB 4003 R2D

Student text

Disclaimer This book is a training document and contains simplifications. Therefore, it must not be considered as a specification of the system. The contents of this document are subject to revision without notice due to ongoing progress in methodology, design and manufacturing. ENKI Adam Girycki assumes no legal responsibility for any error or damage resulting from the usage of this document.

c April 28, 2014 by ENKI Adam Girycki. Copyright ⃝ This document was produced in Poland by ENKI Adam Girycki. It is used for training purpose only and may not be copied or reproduced in any manner without the express written consent of ENKI. This document number ESB 4003 R2D supports course number ELP 4003 R2D. 2

Contents 1 OFDMA principles 1.1

1.2

1.3

1.4

1.5

1.6

5

Two way communication . . . . . . . . . . . . . . . . . . . . . . . . . 1.1.1 FDD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1.2 TDD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Access network evolution overview . . . . . . . . . . . . . . . . . . . . 1.2.1 1G FDMA . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.2 2G TDMA . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.3 3G WCDMA . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.4 4G OFDMA . . . . . . . . . . . . . . . . . . . . . . . . . . . Complex numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.1 Rectangular notation . . . . . . . . . . . . . . . . . . . . . . . 1.3.2 Polar notation . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.3 Relation between rectangular and polar notation . . . . . . . . 1.3.4 Euler’s formula . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.5 Exponential notation . . . . . . . . . . . . . . . . . . . . . . . Fourier analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4.1 Fourier Transform (FT) . . . . . . . . . . . . . . . . . . . . . 1.4.2 Discrete Fourier Transform (DFT) and Fast Fourier Transform (FFT) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Orthogonal Frequency Division Multiplexing (OFDM) concept . . . . . 1.5.1 OFDM transmitter . . . . . . . . . . . . . . . . . . . . . . . . 1.5.2 OFDM receiver . . . . . . . . . . . . . . . . . . . . . . . . . . Modulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2 EPS architecture 2.1 2.2

2.3

2.4

2.5 2.6 2.7

5 6 6 6 6 7 9 9 12 12 13 14 14 14 15 15 15 20 22 22 22

27

LTE requirements . . . . . . . . . . . . . . EPS architectural principles . . . . . . . . 2.2.1 Evolved Packet Core (EPC) . . . . 2.2.2 Evolved UTRAN (E-UTRAN) . . . Strata . . . . . . . . . . . . . . . . . . . . 2.3.1 Non-Access Stratum (NAS) . . . . 2.3.2 Access Stratum (AS) . . . . . . . EPS Bearer and QoS . . . . . . . . . . . . 2.4.1 EPS Bearer . . . . . . . . . . . . . 2.4.2 Quality of Service (QoS) . . . . . . Integration with 2G and 3G . . . . . . . . Interfaces overview . . . . . . . . . . . . . Evolved Packet Core (EPC) functions . . . 2.7.1 Mobility Management Entity node 3

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CONTENTS 2.7.2

2.8

2.7.3 Long 2.8.1 2.8.2

Packet Data Network Gateway (P-GW) and Serving Gateway (S-GW) nodes . . . . . . . . . . . . . . . . . . . . . . . . . . Mobility Management Entity (MME) and S-GW pooling concept Term Evolution (LTE) functions . . . . . . . . . . . . . . . . . . LTE general principles . . . . . . . . . . . . . . . . . . . . . . Evolved Node B (eNB) functionality . . . . . . . . . . . . . .

3 LTE signalling 3.1 3.2 3.3

3.4

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User plane . . . . . . . . . . . . . . . . . . . . . . . Control plane . . . . . . . . . . . . . . . . . . . . . Protocols . . . . . . . . . . . . . . . . . . . . . . . 3.3.1 Radio Resource Control (RRC) . . . . . . . 3.3.2 Packet Data Convergence Protocol (PDCP) 3.3.3 Radio Link Control (RLC) . . . . . . . . . . 3.3.4 Medium Access Control (MAC) . . . . . . . Radio interface structure . . . . . . . . . . . . . . .

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4 LTE radio interface introduction 4.1

4.2

4.3 4.4 4.5

4.6

5.6 5.7

5.8

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Channel structure . . . . . . . . . . . . . 4.1.1 Logical channels . . . . . . . . . 4.1.2 Transport channels . . . . . . . . 4.1.3 Physical channels . . . . . . . . Time domain structure . . . . . . . . . . 4.2.1 Frequency Division Duplex (FDD) 4.2.2 Time Division Duplex (TDD) . . Frequency domain structure . . . . . . . Scheduling Block . . . . . . . . . . . . . Virtual Resource Block . . . . . . . . . . 4.5.1 VRB of localized type . . . . . . 4.5.2 VRB of distributed type . . . . . System spectral efficiency . . . . . . . .

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5 LTE downlink physical channels 5.1 5.2 5.3 5.4 5.5

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57 57 58 59 61 61 62 63 65 65 67 67 67

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Cell search . . . . . . . . . . . . P-SS . . . . . . . . . . . . . . . . S-SS . . . . . . . . . . . . . . . . RS . . . . . . . . . . . . . . . . . PBCH . . . . . . . . . . . . . . . 5.5.1 MIB . . . . . . . . . . . . 5.5.2 SIB . . . . . . . . . . . . PCFICH . . . . . . . . . . . . . . PDCCH . . . . . . . . . . . . . . 5.7.1 PDCCH usage . . . . . . 5.7.2 PDCCH mapping . . . . . 5.7.3 PDCCH format . . . . . . 5.7.4 PDCCH processing . . . . 5.7.5 PDCCH blind decoding . PDSCH . . . . . . . . . . . . . . 5.8.1 CRC attachment . . . . . 5.8.2 Code block segmentation 4

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CONTENTS 5.8.3 Channel coding . . . . . . . . . . . . . . 5.8.4 Rate matching . . . . . . . . . . . . . . 5.8.5 Code block concatenation . . . . . . . . 5.8.6 Scrambling . . . . . . . . . . . . . . . . 5.8.7 Modulation mapper . . . . . . . . . . . 5.8.8 Layer mapper . . . . . . . . . . . . . . . 5.8.9 Precoding . . . . . . . . . . . . . . . . 5.8.10 Resource element mapping . . . . . . . 5.9 PHICH . . . . . . . . . . . . . . . . . . . . . . 5.10 PMCH . . . . . . . . . . . . . . . . . . . . . . . 5.11 Downlink physical channels modulation summary

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Timing advance . . . . . . . . . . . . . . . . . . . . . . 7.1.1 Uplink-downlink frame timing . . . . . . . . . . 7.1.2 Timing advance range . . . . . . . . . . . . . . 7.1.3 Random access . . . . . . . . . . . . . . . . . . 7.1.4 Other cases . . . . . . . . . . . . . . . . . . . . 7.1.5 Maintenance of uplink time alignment . . . . . Random Access (RA) . . . . . . . . . . . . . . . . . . . Resource allocation . . . . . . . . . . . . . . . . . . . . 7.3.1 Resource allocation type 0 . . . . . . . . . . . . 7.3.2 Resource allocation type 1 . . . . . . . . . . . . 7.3.3 Resource allocation type 2 . . . . . . . . . . . . MIMO . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4.1 Spatial multiplexing . . . . . . . . . . . . . . . 7.4.2 Transmit diversity . . . . . . . . . . . . . . . . 7.4.3 Transmission modes . . . . . . . . . . . . . . . 7.4.4 MIMO antennas . . . . . . . . . . . . . . . . . UE reporting . . . . . . . . . . . . . . . . . . . . . . . 7.5.1 CQI definition . . . . . . . . . . . . . . . . . . 7.5.2 Aperiodic CQI/PMI/RI reporting using PUSCH 7.5.3 Periodic CQI/PMI/RI reporting using PUCCH . Modulation order and transport block size determination 7.6.1 Modulation determination . . . . . . . . . . . . 7.6.2 Transport block size determination . . . . . . . UL power control . . . . . . . . . . . . . . . . . . . . . 7.7.1 PUSCH power control . . . . . . . . . . . . . . 7.7.2 PUSCH power control example . . . . . . . . .

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6 LTE uplink physical channels 6.1 6.2

6.3

6.4

PUSCH . . . . . . . . Uplink reference signals 6.2.1 RS . . . . . . 6.2.2 SRS . . . . . . PUCCH . . . . . . . . 6.3.1 PUCCH format 6.3.2 PUCCH format 6.3.3 PUCCH format PRACH . . . . . . . .

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7 Physical layer procedures 7.1

7.2 7.3

7.4

7.5

7.6

7.7

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113

5

113 113 113 113 115 116 116 117 119 120 120 120 121 123 123 124 126 127 127 129 130 131 131 134 134 137

CONTENTS

8 LTE mobility 8.1

8.2

141

Idle mode mobility . . . . . . . . 8.1.1 PLMN selection . . . . . 8.1.2 Cell selection . . . . . . . 8.1.3 Cell reselection . . . . . . Connected mode mobility . . . . . 8.2.1 X2 handover . . . . . . . 8.2.2 Event triggered reporting 8.2.3 A3 event . . . . . . . . .

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141 142 144 146 148 149 151 154

A System information

157

List of Figures

165

List of Tables

167

Acronyms

175

6

1 OFDMA principles

1.1

Two way communication

In order to provide two way communication, so called duplex, two directions of transmission must exist and they must be separated from each other to avoid collisions. Transmission from the User Equipment (UE) to the Base Station (BS) is referred to as Uplink (UL), while the transmission from the BS to the UE is referred to as Downlink (DL), see Figure 1.1.

Figure 1.1: Two way communication. The UL and DL transmissions can be separated in frequency or time domain, as presented in Figure 1.2.

Figure 1.2: Frequency Division Duplex (FDD) and Time Division Duplex (TDD). 7

1 OFDMA principles

1.1.1

FDD

The FDD system uses different frequency bands for UL and DL, separated by the duplex distance, see Figure 1.2. In case of FDD, the UL is usually placed on the lower frequency band because the transmission of lower frequency radio wave requires less energy comparing to the higher frequency band, on which the DL is placed. In FDD solution the transmission and reception may take place continuously or discontinuously. An example of the FDD system is Global System for Mobile communication (GSM).

1.1.2

TDD

The TDD system uses the same frequency band for both UL and DL, which is time shared as presented in Figure 1.2. TDD requires only one frequency to realise two way communications, which may be an advantage when the availability of radio resources is a limiting factor. On the other hand, to avoid any collisions, TDD system requires a time structure (synchronisation) to separate the UL and DL transmission, which is always discontinuous. An examples of the TDD system is cordless telephony system.

1.2

Access network evolution overview

Apart from duplex transmission separation, a harmonised access of multiple UEs to the shared radio resources must exist, see Figure 1.3. In uplink direction a number UEs transmit to the base station. Thus the multiple access technology is required, which allows the base station to separate transmissions from different UEs. In downlink direction a single base station has to keep a connection with multiple users. For that reason a multiple access method is applied, which allows multiplexing of signals at the base station and demultiplexing the signal at the receiving side.

Figure 1.3: Multiple access. Each generation of cellular telecommunications system provided different, more effective, radio access technology, which are discuss in the next sections. The briefly summary of cellular technologies evolution is presented in Figure 1.4. 8

1.2 Access network evolution overview

Figure 1.4: Cellular technologies evolution.

1.2.1

1G FDMA

1st Generation (1G), the first generation of wireless telecommunications technology, was introduced in the 1980s and it was an analogue system. The offered service was the voice. Examples of 1G system: • Nordic Mobile Telephony (NMT) introduced in 1981 and developed in Nordic countries, Switzerland, Netherlands, Eastern Europe and Russia • Advanced Mobile Phone Systems (AMPS) introduced in 1983 and developed in North America and Australia In 1G system, Frequency Division Multiple Access (FDMA) method of radio resources usage was applied. The available radio resources were divided in frequency domain. For each connection a separate bandwidth was allocated and the user transmission on the allocated channel was continuous, which is illustrated in Figure 1.5. The allocated one way channel had bandwidth of 25 kHz in NMT and 30 kHz in case AMPS, resulting in a total of 50 kHz in NMT and 60 kHz in AMPS for each duplex channel.

1.2.2

2G TDMA

2G, the second generation of wireless telecommunications technology, was introduced in 1990s. It was a digital system. The offered services were voice, Short Message Service (SMS), Circuit Switched (CS) data transfer with the rate of 9.6 kbit/s. Examples of 2G systems: 9

1 OFDMA principles

Figure 1.5: Frequency Division Multiple Access (FDMA). • GSM introduced in 1991 and used across more than 212 countries and territories. • Digital Advanced Mobile Phone Systems (D-AMPS) introduced 1991 and used in North America. • CDMAOne introduced in 1995 and used in the Americas and parts of Asia. GSM, which is the dominant 2G system, employs the Time Division Multiple Access (TDMA) method of radio resources usage combined with FDMA. Available radio resources are first divided into Radio Frequency (RF) channels of 200 kHz bandwidth (FDMA concept) and next each RF channel is divided in time domain into timeslots (TDMA concept). A certain number of timeslots create so called TDMA frame. The number of timeslots in the TDMA frame is system specific. In GSM system eight timeslots make up the TDMA frame. A user has a cyclic access to the common radio resources during the allocated timeslot. Thus the transmission is discontinuous. The Figure 1.6 presents the TDMA system with 4 timeslots in the TDMA frame.

Figure 1.6: Time Division Multiple Access (TDMA). General Packet Radio Service (GPRS), which is an add-on to the CS GSM also called 2.5G, offers Packet Switched (PS) data transfer with the rate of approximately 50 kbps. 10

1.2 Access network evolution overview Enhanced GPRS (EGPRS), also called 2.75G, offers higher date rate of PS data transfer with the maximum rate of approximately 500 kbps, thanks to higher order modulation.

1.2.3

3G WCDMA

3G, the third generation of wireless telecommunication technology, was introduced in 2000s. Examples of 3G system: • CDMA2000 introduced in 2000 in South Korea and used in Asia, America and Africa. • Universal Mobile Telecommunications System (UMTS) introduced in 2001 in Japan and used in Europe, Asia and Africa. 3G systems employ Code Division Multiple Access (CDMA) method of radio resources usage. CDMA allows for simultaneous transmission of multiple users in the same frequency band, which is presented in Figure 1.7. Separation of different connections is achieved by means of different codes. The codes must be orthogonal (independent of each other).

Figure 1.7: Code Division Multiple Access (CDMA). In CDMA2000 the initial frequency band width was 1.25 MHz, which was next tripled to 3x1.25 MHz. In UMTS, the Wideband Code Division Multiple Access (WCDMA) method is applied, which utilizes wide frequency band of 5 MHz. Wide frequency channel allows for lowering the power density, thus signal may be even weaker than thermal noise level. High Speed Packet Access (HSPA) provides downlink throughput of approximately 14 Mbps, while Evolved HSPA (also called HSPA+) provides throughput of 84 Mbps. 11

1 OFDMA principles

1.2.4

4G OFDMA

4G, the fourth generation of mobile telecommunications technology, must support 1 Gbit/s downlink bit rate. Currently there is no system, that is able to support mobile communications with the required bit rate. However there are two technologies, which are on a way to achieve this goal in the nearest future: • LTE offers approximately 100 Mbit/s bit rate. The world’s first publicly available LTE-service was opened in the two Scandinavian capitals Stockholm and Oslo on the 14 December 2009. • Worldwide Interoperability for Microwave Access (WiMAX) offers approximately 40 Mbit/s bit rate. WiMAX access was used to assist with communications in Aceh, Indonesia, after the tsunami in December 2004. Both LTE and WiMAX employ Orthogonal Frequency Division Multiple Access (OFDMA) method of radio resources usage. Theoretical foundation of OFDMA had been already laid in 1960’, but due to high costs and lack of appropriate technologies for a long time it remained purely theoretical. This situation has changed with advent of cheap, small and fast microchips capable of processing the FFT and Inverse Fast Fourier Transform (IFFT) algorithms. Nowadays, OFDM is widely used in wireless networking (Wireless Local Area Network (WLAN)), digital television (Digital Video Broadcasting – Terrestrial (DVB-T)), audio broadcasting (Digital Audio Broadcasting (DAB)) and broadband wireless communications (WiMAX, LTE). OFDMA is a special type of the FDMA. OFDMA allows for transfer messages simultaneously, using multiple narrow ranges of frequencies, called subcarriers, see Figure 1.8.

Figure 1.8: Orthogonal Frequency Division Multiple Access (OFDMA). To avoid Inter Carrier Interference (ICI), in ordinary FDMA system, all such subcarriers are separated in frequency domain with guard bands, therefore some spectrum is wasted. OFDM provides much better spectrum efficiency, as it does not need gaps between subcarrier bands. Moreover, the subcarrier bands are overlapping, which allows to additionally save some spectrum. ICI is mitigated here by taking advantage of the fact that under the following conditions the subcarriers are orthogonal with one another: 12

1.2 Access network evolution overview • The careful choice of subcarrier spacing. The subcarrier spacing ∆f should be exactly equal to the reciprocity of the OFDMA symbol duration Tsymbol , see Figure 1.9, which provides that the subcarriers are mathematically orthogonal and thus independent. • Keeping the synchronisation in the frequency domain, providing there are no frequency shifts, e.g. due to Doppler effects.

Figure 1.9: OFDM subcarriers. Multiplexing and demultiplexing of OFDMA symbols into subcarriers can be performed using Inverse Inverse Discrete Fourier Transform (IDFT) and DFT. These mathematical procedures, that transform signal from frequency to time domain and opposite, can be implemented with IFFT and FFT algorithms. The presented above FDMA, TDMA and CDMA multiple access methods are single carrier modulation. OFDMA is a multi carrier modulation. In other words, it means that a large number of closely spaced orthogonal subcarriers are used to carry data. Each subcarrier is modulated with a conventional modulation scheme (such as quadrature amplitude modulation or phase shift keying) at a low symbol rate, maintaining total data rates similar to conventional single carrier modulation schemes in the same bandwidth. Advantages of OFDMA: • OFDMA effectively diminishes also the problem of multipath selective fading. Due to multipath radio waves propagation in typical urban environment, signal at the receiver can be constructively or destructively interfered by the same signal delayed over different path. This effect can dramatically change depending on frequency used as a signal carrier – some of the frequencies will suffer from deep fading, while neighbouring ones may not be affected at all. As 13

1 OFDMA principles OFDMA uses very small subcarrier widths, the fading within every subcarrier can be considered as relatively flat. • Another problem mitigated by OFDMA is Inter Symbol Interference (ISI). One of the causes of this effect is signal reflection from distant object (typically mountain). The delayed signal, which propagates over much longer path, interferes with the direct signal because it carries another (older) symbol than the direct signal and therefore the receiver is unable to detect the correct symbol. The ISI effect is diminished when the symbol duration is longer, thus only very far objects will lead to ISI. But the signal reflected from very far object is usually week enough and does not lead to interference. In OFDMA, the symbol duration can be lengthened, because a few symbols can be transmitted simultaneously on different subcarriers. As already mentioned, longer symbol makes the radio path less vulnerable to ISI. Additionally, to avoid overlapping, the adjacent symbols are always separated in time by short guard period. In the guard period, from technical reasons, it is not effective to stop transmission at all, thus, so called, cyclic prefix is inserted here, which is simply a copy of the signal tail end. • OFDMA can achieve a higher Multiple Input Multiple Output (MIMO) spectral efficiency due to providing flatter frequency channels than a CDMA rake receiver can. • No cell size breathing as more users connect. Recognised disadvantages of OFDMA: • Higher sensitivity to frequency offsets and phase noise. • Asynchronous data communication services such as web access are characterised by short communication bursts at high data rate. Few users in a base station cell are transferring data simultaneously at low constant data rate. • The complex OFDMA electronics, including the FFT algorithm and forward error correction, is constantly active independent of the data rate, which is inefficient from power consumption point of view, while OFDMA combined with data packet scheduling may allow that the FFT algorithm hibernates during certain time intervals. • The OFDMA diversity gain, and resistance to frequency-selective fading, may partly be lost if very few sub-carriers are assigned to each user, and if the same carrier is used in every OFDMA symbol. Adaptive sub-carrier assignment based on fast feedback information about the channel, or sub-carrier frequency hopping, is therefore desirable. • Dealing with co-channel interference from nearby cells is more complex in OFDMA than in CDMA. It would require dynamic channel allocation with advanced coordination among adjacent base stations. • The fast channel feedback information and adaptive sub-carrier assignment is more complex than CDMA fast power control. 14

1.3 Complex numbers

1.3

Complex numbers

Complex numbers are used in OFDMA signal processing. A complex number is a number comprising a real (Re) and imaginary (Im) part.

1.3.1

Rectangular notation

The complex number can be written in the form of rectangular notation (also called Cartesian notation) a + ib, where a and b are real numbers, and i is the standard imaginary unit with the property i2 = −1. Figure 1.10 shows geometric representation of a complex number z = a+ib in the complex plane. The complex plane can be thought of as a Cartesian plane, with the real part of a complex number represented by a displacement along the x-axis, and the imaginary part by a displacement along the y-axis.

Figure 1.10: Geometric representation of a complex number in the rectangular notation in a complex Cartesian plane. Each complex number z has a conjugate z ∗ , which has the same real part but opposite imaginary part, see 1.11: z = a + ib

(1.1)

∗

(1.2)

z = a − ib

1.3.2

Polar notation

Figure 1.12 presents another notation, so called polar notation, of a complex number. In the polar plane the complex number is represented by its modulus (absolute value) r and argument (angle) φ.

1.3.3

Relation between rectangular and polar notation

Relation between rectangular and polar notation of a complex number is the following: a = r cos φ

(1.3)

b = r sin φ

(1.4) 15

1 OFDMA principles

Figure 1.11: Conjugate z ∗ of a complex number z.

Figure 1.12: Geometric representation of a complex number in the polar notation. Thus, the complex number z = a + ib may be expressed as follows: z = a + ib = r cos φ + ir sin φ = r(cos φ + i sin φ)

1.3.4

(1.5)

Euler’s formula

Leonhard Euler, Swiss mathematician and physicist, discovered a mathematical relationship between the trigonometric functions (sin and cos) and the complex exponential function (see also Figure 1.13): cos φ + i sin φ = eiφ

(1.6)

Euler’s formula was called by Richard Feynman ”one of the most remarkable, almost astounding, formulas in all of mathematics”.

1.3.5

Exponential notation

Using the Euler’s formula the complex number z may be written as follows, which is called the exponential notation of a complex number: z = r(cos φ + i sin φ) = reiφ

(1.7) 16

1.4 Fourier analysis

Figure 1.13: Euler’s formula. In the exponential notation certain calculations, particularly multiplication and division of complex numbers, are easier than in rectangular notation. On the other hand, addition and subtraction are easier with the use of rectangular notation. The exponential notation of a complex number is in widespread use in engineering and science. Using the Euler’s formula the conjugate z ∗ may be written as: z ∗ = e−iφ

1.4 1.4.1

(1.8)

Fourier analysis Fourier Transform (FT)

Fourier Transform (FT) is an operation that transforms time domain function into frequency domain function. Therefore FT is often called the frequency domain representation of the original time domain function, see Figure 1.14.

1.4.2

Discrete Fourier Transform (DFT) and Fast Fourier Transform (FFT)

Discrete Fourier Transform (DFT) is a specific kind of FT. The input to the DFT is a finite sequence of real or complex numbers making the DFT ideal for processing information stored in computers. In particular, the DFT is widely employed in signal processing and related fields to analyse the frequencies contained in a sampled signal, to solve partial differential equations, and to perform other operations such as convolutions or multiplying large integers. DFT transforms the sequence of N complex numbers a0 , a1 , ..., aN −1 (usually in time domain) into a sequence of A0 , A1 , ..., AN −1 complex numbers (usually in frequency 17

1 OFDMA principles

Figure 1.14: Fourier Transform (FT) principles.

18

1.4 Fourier analysis domain) according to the following formula: Ak =

N −1 ∑

k = 0, ..., N − 1

an wkn

(1.9)

n=0

w=e

− 2π i N

(1.10)

The inverse transform to the DFT, which transforms the sequence of complex numbers Ak back to the sequence of complex values an , is called Inverse Discrete Fourier Transform (IDFT) and is given by the following formula: N −1 1 ∑ Ak w−kn an = N

n = 0, ..., N − 1

(1.11)

k=0

In practice, the DFT can be computed efficiently using a Fast Fourier Transform (FFT) algorithm and IDFT using Inverse Fast Fourier Transform (IFFT) algorithm.

DFT example We are going to apply the DFT to the following sequence of N = 8 numbers in the time domain: a = [2, 1, 0, 1, 2, 1, 0, 1]

(1.12)

We will show that the DFT of the above sequence is the following sequence of numbers in the frequency domain:

A = [8, 0, 4, 0, 0, 0, 4, 0]

(1.13)

Figure 1.15 shows the graphical presentation of the example, where the sequence of real numbers an is transformed into the sequence of complex numbers Ak . The complex numbers Ak are expressed by their modulus r and argument φ (see section 1.3). The modulus r represents the amplitude of the cosinusoidal signal of a given frequency f and the argument φ corresponds to the phase shift of the cosinusoidal signal. Because, in this example, the phase shift of the cosinusoidal signals is zero (which means that the imaginary parts of complex numbers Ak are equal zero) therefore Ak are actually real numbers. For the sequence of 8 numbers, the DFT formula may be expressed by the following matrix form:              

A0 A1 A2 A3 A4 A5 A6 A7





            =            

1 1 1 1 1 1 1 1

1 1 w w2 w2 w4 w3 w6 w4 w8 w5 w10 w6 w12 w7 w14

1 w3 w6 w9 w12 w15 w18 w21

1 w4 w8 w12 w16 w20 w24 w28 19

1 w5 w10 w15 w20 w25 w30 w35

1 w6 w12 w18 w24 w30 w36 w42

1 w7 w14 w21 w28 w35 w42 w49

              ·            

a0 a1 a2 a3 a4 a5 a6 a7

             

(1.14)

1 OFDMA principles

Figure 1.15: Example of the Discrete Fourier Transform (DFT).

w = e−

2π i 8

= e− 4 i = cos π

(π ) 4

− i sin

(π ) 4

i 1 =√ −√ 2 2

(1.15)

When raising the coefficient w to any integral power, one of eight values is obtained, which are illustrated in Figure 1.16. Let us denote these eight complex values by arrows according to Figure 1.16. Now, the matrix form of DFT can be noted in the following way:

             

A0 A1 A2 A3 A4 A5 A6 A7





            =            

→ → → → → → → →

→ ↘ ↓ ↙ ← ↖ ↑ ↗

→ ↓ ← ↑ → ↓ ← ↑

→ ↙ ↑ ↘ ← ↗ ↓ ↖

→ ← → ← → ← → ←

→ ↖ ↓ ↗ ← ↘ ↑ ↙

→ ↑ ← ↓ → ↑ ← ↓

→ ↗ ↑ ↖ ← ↙ ↓ ↘

              ·            

2 1 0 1 2 1 0 1

             

(1.16)

We may calculate Ak numbers from the above matrix notation. As an example A0 , A1 and A2 are calculated below: A0 = 1 · 2 + 1 · 1 + 1 · 0 + 1 · 1 + 1 · 2 + 1 · 1 + 1 · 0 + 1 · 1 = 8 20

(1.17)

1.4 Fourier analysis Im

Re

Figure 1.16: The coefficient wn in the DFT for N = 8.

(

) ( ) 1 i 1 i A1 = 1 · 2 + √ − √ · 1(−i) · 0 + − √ − √ · 1+ 2 2 2 2 ( ) ( ) 1 i 1 i + (−1) · 2 + − √ + √ ·1+i·0+ √ + √ ·1=0 2 2 2 2

(1.18)

A2 = 1 · 2 − i · 1 − 1 · 0 + i · 1 + 1 · 2 − i · 1 − 1 · 0 + i · 1 = 4

(1.19)

You may calculate the remaining Ak values to confirm that the DFT transforms the sequence a = [2, 1, 0, 1, 2, 1, 0, 1] into the sequence A = [8, 0, 4, 0, 0, 0, 4, 0]. It is important to observe that the duration of our signal sample in the time domain was 8 s, while the shift between transformed signals in frequency domain is equal 1 1 8 s = 8 Hz.

Inverse Discrete Fourier Transform (IDFT) example We are going show that the IDFT transforms the sequence of N = 8 numbers in the frequency domain: A = [8, 0, 4, 0, 0, 0, 4, 0]

(1.20)

back into the following sequence of numbers in the time domain: a = [2, 1, 0, 1, 2, 1, 0, 1]

(1.21)

Values an may be calculated from formula 1.11, as presented below, and values w−n are shown in Figure 1.17: a0 =

N −1 1 ∑ 1 Ak = (8 + 0 + 4 + 0 + 0 + 0 + 4 + 0) = 2 N 8 k=0

21

(1.22)

1 OFDMA principles

a1 =

N −1 ) 1 ∑ 1 ( −0·1 8w + 4w−2·1 + 4w−6·1 = Ak w−k = N 8 k=0

(1.23)

1 = (8 + 4i − 4i) = 1 8

a2 =

N −1 ) 1 ∑ 1 ( −0·2 Ak w−2k = 8w + 4w−2·2 + 4w−6·2 = N 8 k=0

(1.24)

1 = (8 − 4 − 4) = 0 8

Figure 1.17: The coefficient w−n in the IDFT for N = 8. When comparing with Figure 1.16 notice that w−n is a conjugate of wn . Figure 1.18 shows the graphical presentation of the IDFT example.

1.5

OFDM concept

The OFDM concept, which uses DFT, is shown in Figure 1.19. In the picture, the information to be transmitted is represented by different Ak values. The process of converting bits into Ak values is called modulation. Each of the Ak values is sent on another subcarrier. In the picture there are N = 10 subcarriers. Ak values, which are sent on different subcarriers, are represented by different heights of the bars. With the use of IDFT the Ak values are transformed to signal in time domain, which is physically transmitted during symbol time Tsymbol . The time domain signal is denoted by an values and represented by circles. Because there are 10 bars in the frequency domain before DFT, therefore there are also 10 circles of the time domain signal after IDFT. As already mentioned, the 10 time domain samples are to be transmitted during Tsymbol , therefore the time between samples is equal T Ts = symbol 10 . 22

1.5 OFDM concept

Figure 1.18: Graphical presentation of the IDFT example.

23

1 OFDMA principles So far we had to do with digital operations (modulation and IDFT are digital operations). Next, the 10 time domain circles an are used to generate an analogue signal, which is physically transmitted from an antenna.

Figure 1.19: OFDM concept. The receiver performs an opposite operation. It samples the time domain signal every Ts and collects 10 time domain samples an , which are next transformed, with use of the DFT, to frequency domain values Ak . The frequency domain values Ak carry information about bits which were transmitted. The bits are retrieved by demodulation of values Ak . After time Tsymbol the next symbol may be transmitted. Figure 1.19 illustrates transmission of 3 symbols. Please observe that there could be a break between consecutive symbols transmission. This break is used to transmit cyclic prefix.

1.5.1

OFDM transmitter

In OFDM the carrier signal is a sum of orthogonal subcarriers. In each subcarrier processing Quadrature Amplitude Modulation (QAM) or Phase Shift Keying (PSK) can be used. A simplified scheme of an OFDM transmitter has been shown in Figure 1.20. s[i] is input bit stream. First, bits are separated into N parallel streams. Streams are assigned for Quadrature Amplitude Modulation (QAM) or Phase Shift Keying (PSK) modulation. Depending on the modulation, subcarriers may have different transmission bit rate. Next, IFFT is computed for the sequence of complex data symbols A0 , ..., AN −1 , which results in a sequence of complex time symbols a0 , ..., aN −1 of the signal. For each symbol, after imaginary and real part separation, both parts are converted to analogue in Digital-to-Analogue converter (D/A). Next, analogue signals are quadrature modulated (multiplied by cosine and sine functions) and summed up giving the output modulated signal s(t). 24

1.6 Modulation

Figure 1.20: OFDM transmitter.

1.5.2

OFDM receiver

Figure 1.21 presents the simplified OFDM receiver model. Receiver is detecting the signal rx(t). Besides the wanted signal also signal with 2f frequency is created. Therefore low pass filter is used to filter it out. Next, the signal is sampled and converted to digital by the Analogue-to-Digital converter (A/D). The series of complex time symbols is then corrected for frequency drifts and global phase offsets (not shown in the diagram). In the next step FFT is carried out and frequency symbol detection takes place, which results in N parallel bit streams, joined finally into one initial bit stream s(i).

Figure 1.21: OFDM receiver.

1.6

Modulation

In telecommunications, modulation is the process of conveying a message signal, for example digital information bit stream, inside another signal that can be physically 25

1 OFDMA principles transmitted. Modulation of a sine waveform is used to transform a baseband message signal to a passband signal, for example a RF signal. Electrical signals can only be transferred over a limited passband frequency spectrum, with specific (non-zero) lower and upper cut-off frequencies. Modulating a sine wave carrier makes it possible to keep the frequency content of the transferred signal as close as possible to the centre frequency (typically the carrier frequency) of the passband. For the purpose of LTE it is a good idea to think about the modulation as a technique, which changes a digital signal of bits into another digital signal of complex numbers. The complex numbers represent amplitude and phase shift of OFDM subcarriers. The modulation techniques used in LTE are based on phase and amplitude modulation of the carrier frequency, see also Figure 1.22: • Binary Phase Shift Keying (BPSK) allows for transmission of one information bit during one modulation symbol. • Quadrature Phase Shift Keying (QPSK) allows for transmission of two information bits during one modulation symbol. • 16 Quadrature Amplitude Modulation (16QAM) allows for transmission of 4 information bits during one modulation symbol. • 64 Quadrature Amplitude Modulation (64QAM) is the fastest modulation used in LTE and allows for transmission of 6 information bits during one modulation symbol. Only QPSK, 16QAM and 64QAM are used in LTE for user data bit. QPSK is only used for some control information bits, which require robust modulation.

26

1.6 Modulation

Figure 1.22: LTE modulations.

27

1 OFDMA principles

28

2 EPS architecture 2.1

LTE requirements

Operators around the world have been rapidly deploying 3rd Generation (3G) network technologies, including UMTS, HSPA, and CDMA2000 1xEV-DO, to support increasing subscriber demand for mobile broadband services. LTE is a step toward the 4th Generation (4G). LTE requirements are specified by TS 25.913: • Capability-related requirements. ◦ Peak data rate. Evolved UMTS Terrestrial Radio Access Network (E-UTRAN) should support significantly increased instantaneous peak data rates. The supported peak data rate should scale according to size of the spectrum allocation. Note that the peak data rates may depend on the numbers of transmit and receive antennas at the UE. The targets for DL and UL peak data rates are specified in terms of a reference UE configuration comprising: 1. DL capability – 2 receive antennas at UE. 2. UL capability – 1 transmit antenna at UE. For this baseline configuration, the system should support an instantaneous downlink peak data rate of 100 Mbps within a 20 MHz downlink spectrum allocation (5 bps/Hz) and an instantaneous uplink peak data rate of 50 Mbps (2.5 bps/Hz) within a 20 MHz uplink spectrum allocation. The peak data rates should then scale linearly with the size of the spectrum allocation. In case of spectrum shared between downlink and uplink transmission, E-UTRAN does not need to support the above instantaneous peak data rates simultaneously. ◦ Control Plane (CP) latency. Transition time (excluding downlink paging delay and Non-Access Stratum (NAS) signalling delay) of less than 100 ms from a camped-state (Idle Mode) to an active state, in such a way that the User Plane (UP) is established. ◦ User Plane (UP) latency. 29

2 EPS architecture E-UTRAN UP latency reduced to less than 5 ms in unload condition for small Internet Protocol (IP) packets. ◦ CP capacity. The system should be able to support a large number of users per cell with quasi instantaneous access to radio resources in the active state. It is expected that at least 200 users per cell should be supported in the active state for spectrum allocations up to 5 MHz, and at least 400 users for higher spectrum allocation. A much higher number of users is expected to be supported in the camped state. • System performance requirements. ◦ DL user throughput. Target for user throughput per MHz at the 5% point of the C.D.F., 2 to 3 times Release 6 HSDPA. Target for averaged user throughput per MHz, 3 to 4 times Release 6 HSDPA. Both targets should be achieved assuming Release 6 reference performance is based on a single Tx antenna at the Node B with enhanced performance type 1 receiver in UE whilst the E-UTRA may use a maximum of 2 Tx antennas at the Node B and 2 Rx antennas at the UE. The supported user throughput should scale with the spectrum bandwidth. ◦ UL user throughput. Target for user throughput per MHz at the 5% point of the C.D.F., 2 to 3 times Release 6 Enhanced Uplink (deployed with a single Tx antenna at the UE and 2 Rx antennas at the Node B). Target for averaged user throughput per MHz, 2 to 3 times Release 6 Enhanced Uplink (deployed with a single Tx antenna at the UE and 2 Rx antennas at the Node B). Both should be achievable by the E-UTRAN using a maximum of a single Tx antenna at the UE and 2 Rx antennas at the Node B. Greater user throughput should be achievable using multiple Tx antennas at the UE. The user throughput should scale with the spectrum bandwidth provided that the maximum transmit power is also scaled. ◦ Spectrum efficiency. Downlink. In a loaded network, target for spectrum efficiency (bits/sec/Hz/site), 3 to 4 times Release 6 HSDPA. This should be achieved assuming Release 6 reference performance is based on a single Tx antenna at the Node B with enhanced performance type 1 receiver in UE whilst 30

2.2 EPS architectural principles the E-UTRA may use a maximum of 2 Tx antennas at the Node B and 2 Rx antennas at the UE. Uplink In a loaded network, target for spectrum efficiency (bits/sec/Hz/site), 2 to 3 times Release 6 Enhanced Uplink (deployed with a single Tx antenna at the UE and 2 Rx antennas at the Node B). This should be achievable by the E- UTRA using a maximum of a single Tx antenna at the UE and 2Rx antennas at the Node B. ◦ Mobility. The E-UTRAN shall support mobility across the cellular network and should be optimised for low mobile speed from 0 to 15 km/h. Higher mobile speed between 15 and 120 km/h should be supported with high performance. Mobility across the cellular network shall be maintained at speeds from 120 km/h to 350 km/h (or even up to 500 km/h depending on the frequency band). ◦ Coverage. E-UTRAN should support the maximum cell range of 100 km.

2.2

EPS architectural principles

The LTE radio network is called E-UTRAN. System Architecture Evolution (SAE) is the core network architecture of the LTE wireless communication standard. SAE is the evolution of the GPRS Core Network. The main component of the SAE architecture is the Evolved Packet Core (EPC). The Long Term Evolution/System Architecture Evolution (LTE/SAE) system, which consists of E-UTRAN and EPC, is called Evolved Packet System (EPS), see Figure 2.1. LTE/SAE is specified from 3GPP Technical Specification (3GPP TS) Release 8.

2.2.1

Evolved Packet Core (EPC)

The EPC provides access to external data networks (e.g., Internet, corporate networks) and operator services (e.g., Multimedia Messaging Services (MMS)1 , Multimedia Broadcast and Multicast Services (MBMS)2 ). It also performs functions related to security (authentication, key agreement), subscriber information, charging and inter-access mobility (GSM EDGE Radio Access Network (GERAN)/Universal Terrestrial Radio Access Network (UTRAN)/E-UTRAN/Interworking Wireless 1 Multimedia Messaging Services (MMS) is a standard way to send messages that include multimedia content to and from mobile phones. It extends the core SMS capability which only allows exchange of text messages up to 160 characters in length. The most popular use of MMS is to send photographs from camera-equipped handsets, although it is also popular as a method of delivering news and entertainment content including videos, pictures, text pages and ringtones. 2 Multimedia Broadcast and Multicast Services (MBMS) is a broadcasting service, which may be offered via existing GSM and UMTS cellular networks. The main application is mobile TV. The infrastructure offers an option to use an uplink channel for interaction between the service and the user.

31

2 EPS architecture

Figure 2.1: EPS architecture. Local Area Network (IWLAN)/Code Division Multiple Access 2000 (CDMA2000) etc.). The EPC also tracks the mobility of inactive terminals (i.e., terminals in power saving state). The number of user plane nodes3 in the core network has been reduced from two in Release 6 (Serving GPRS Support Node (SGSN) and Gateway GPRS Support Node (GGSN)) to only one in EPS called Packet Data Network/Serving Gateway (P/S-GW). The P/S-GW can be divided into a S-GW and P-GW but often resides in the same physical node referred to as P/S-GW or System Architecture Evolution Gateway (SAE-GW). In typical implementations the P/S-GW is realised by software upgrade of GGSN. The control plane node is called MME and it may be realised by software upgrade of SGSN.

2.2.2

Evolved UTRAN (E-UTRAN)

E-UTRAN performs all radio related functions for active terminals (i.e. terminals sending data). The number of user plane nodes in E-UTRAN has been reduced to one only and the node is called Evolved Node B (eNB) The interface between the 3

User plane is a communication strata responsible for user data transmission, in contrast to control plane, which is responsible for signalling transmission. The strata concept is explained in the next section.

32

2.3 Strata EPC and the E-UTRAN is called S1 and the interface between the eNBs is called X2.

2.3

Strata

To keep the questions of mobility and connection management independent of the air interface technology, the concept of communication strata has been employed in UMTS and it is also used in LTE/SAE. The stack of protocols has been divided into: • NAS – containing Core Network (CN) protocols between the CN and UE, which do not terminate in the E-UTRAN, but in the CN itself. E-UTRAN is completely transparent for these protocols, and hence they can be independent of the radio technology used. • Access Stratum (AS) – containing radio access protocols between the UE and the E-UTRAN. These protocols are different in GSM, UMTS and LTE, since the radio access technology is different here (OFDMA instead of TDMA or WCDMA).

2.3.1

Non-Access Stratum (NAS)

The concept of Non-Access Stratum (NAS) is almost the same as in UMTS, however it is implemented in much different way. The UMTS uses the same mobility and connection management protocols as the earlier generation networks (GSM, GPRS) and they are the following: • Connection Management (CM) and Mobility Management (MM) for the CS part of the network, • Session Management (SM) and GPRS Mobility Management (GMM) for the PS part. The fact that LTE/SAE is totally packet oriented eliminates the protocols connected with the CS network part and modifies the NAS operation in PS part (i.e. the entire network). Consequently, the NAS in EPS: • Introduces the new EPS Mobility Management (EMM) layer, • Inherits the SM layer after UMTS. From the changes presented above, one can deduct that lower layer EMM had to be redefined for EPS to meet the requirements of the new concept of UE mobility for the PS transmission only. The SM remains the same due to the fact of common way of handling the session management in LTE/SAE, UMTS and GSM (GPRS) systems. The examples of functions performed by NAS: • Mobility management for idle UEs, • UE authentication, 33

2 EPS architecture • EPS bearer management, • Configuration and control of security, • Paging initiation for idle UEs. The NAS messages are transported by the Radio Resource Control (RRC) layer – the signalling layer of the AS. There are two ways to transport the NAS messages by RRC, either by concatenating the NAS messages with other Radio Resource Control (RRC) messages, or by including the NAS messages in dedicated RRC messages without concatenation. The NAS messages are protected using the ciphering and integrity protection services provided by the Packet Data Convergence Protocol (PDCP) layer. However, NAS is also protected by its own security functions terminated in the UE and MME, respectively. On the network side, the NAS layers are in 3rd Generation Partnership Project (3GPP) agreed to be terminated by the MME. The NAS state model is based on a two-dimensional model which consists of EMM states describing the mobility management states that result from the mobility management procedures e.g. attach and Tracking Area Update (TAU) procedures, and of EPS Connection Management (ECM) states describing the signalling connectivity between the UE and the EPC. The ECM and EMM states are independent of each other and when the UE is in EMM-CONNECTED state this does not imply that the user plane (radio and S1 bearers) is established.

2.3.2

Access Stratum (AS)

The services, access signalling, mobility and subscriber management specific to CN are completely outside the AS, and are transferred transparently through the Radio Access Network (RAN). AS protocols are specific to the RAN being used by the mobile system. This RAN may be implemented as the GSM Base Station System (BSS), GERAN, UTRAN or E-UTRAN. AS provides radio access bearers for both connection-oriented, packet-switched services and connectionless (store-andforward) services. In LTE/SAE there is no CS network part thus the AS differs significantly from the one in older technologies. The AS provides the connectivity between the nodes in the E-UTRAN. There are three interfaces that are involved in the AS concept: • Radio interface – connectivity between the UE and the E-UTRAN node – the eNB. • S1 – connectivity between eNB and the core network nodes: ◦ S1-MME – eNB and MME, responsible for control plane. ◦ S1-U – eNB and S-GW, responsible for user plane. • X2 – connectivity between eNBs in E-UTRAN. 34

2.4 EPS Bearer and QoS

2.4

EPS Bearer and QoS

The EPS defines bearers for services and strictly binds them with QoS level provided. This strict mapping leads to definition of certain QoS level for certain applications using the bearers in the network. Consequently, the bearers will always obtain appropriate QoS classes, according to the requirements of the service provided by the application the UE utilises.

2.4.1

EPS Bearer

Similarly to UMTS, EPS implements a bearer concept for supporting end-user data services. The EPS bearer (similar to a Packet Data Protocol (PDP) context of previous 3GPP releases) is defined between the UE and the P-GW node in the EPC (which provides the end-users IP point of presence towards external networks), see Figure 2.2.

Figure 2.2: EPS bearer concept. End-to-end services (e.g. IP services) are multiplexed on different EPS Bearers. There is a many-to-one relation between end-to-end services and EPS Bearers. An UL Traffic Flow Template (TFT) in the UE binds an Service Data Flow (SDF) to an EPS Bearer in the uplink direction. Multiple SDFs can be multiplexed onto the same EPS Bearer by including multiple uplink packet filters in the UL TFT. A DL TFT in the P-GW binds an SDF to an EPS Bearer in the downlink direction. Multiple SDFs can be multiplexed onto the same EPS Bearer by including multiple downlink packet filters in the DL TFT. The EPS Bearer is further sub-divided into a E-UTRAN Radio Access Bearer (E-RAB) and S5/S8 Bearer. An E-RAB transports the packets of an EPS Bearer between the UE and the EPC. When an E-RAB exists, there is a one-to-one mapping between this E-RAB and an EPS Bearer. An S5/S8 Bearer transports the packets of an EPS Bearer between a S-GW and a P-GW. A Radio Bearer transports the packets of an EPS Bearer between a UE and an eNB. When a Radio Bearer exists, there is a one-to-one mapping between this 35

2 EPS architecture Radio Bearer and the EPS Bearer/E-RAB. An S1 Bearer transports the packets of an E-RAB between an eNB and a S-GW. A UE stores a mapping between an uplink packet filter and a Radio Bearer to create the binding between an SDF and a Data Radio Bearer in the uplink. A P-GW stores a mapping between a downlink packet filter and an S5/S8a Bearer to create the binding between an SDF and an S5/S8a Bearer in the downlink. An eNB stores a one-to-one mapping between a Radio Bearer and an S1 Bearer to create the binding between a Radio Bearer and an S1 Bearer in both the uplink and downlink. A S-GW stores a one-to-one mapping between an S1 Bearer and an S5/S8a Bearer to create the binding between an S1 Bearer and an S5/S8a Bearer in both the uplink and downlink.

2.4.2

Quality of Service (QoS)

QoS concept QoS has been defined by the International Telecommunication Union (ITU) as: the collective effect of service performance, which determines the degree of satisfaction of a user of a service. Thus, QoS is connected with the way the user perceives the service. The user is not interested in how a service is provided but only whether or not he or she is satisfied with that service. So, from a user’s perspective the QoS level is a very subjective thing and if the network does not provide the desired level of satisfaction, the user may simply stop using the service and possibly change to some other operator offering a similar service with the desired QoS level.

QoS classes In UMTS four different QoS classes (referred also to as traffic classes) have been defined. These QoS classes are: • Conversational class, • Streaming class, • Interactive class, and • Background class. The main distinction between these QoS classes follows from how delay-sensitive the traffic is: Conversational class is meant for traffic, which is very delay-sensitive, while Background class is the most delay-insensitive traffic class.

QoS Class Identifier (QCI) In case of LTE, 3GPP in Release 8 introduces another concept: QoS Class Identifier (QCI). QCI is a scalar that is used as a reference to node specific parameters that 36

2.5 Integration with 2G and 3G control packet forwarding treatment. They should be pre-configured by the operator owning the node. QCI values indicate the QoS characteristics for edge-to-edge packet forwarding between UE and Policy and Charging Enforcement Function (PCEF). Each QCI is associated with the following standardized performance characteristics: • Resource Type (Guaranteed Bit Rate (GBR) or Non-GBR), • Priority, • Packet Delay Budget, • Packet Error Loss Rate. To control the edge-to-edge packet forwarding QCI is signaled to different network nodes while the above standardized characteristics are not. It is up to the operator to map QCI values to the corresponding performance characteristics. The characteristics of QCI from 1 to 9 are standardized though and should be considered as guidelines when pre-configuring the node specific parameters. The goal of this operation is to ensure that applications mapped to a particular QCI receive the same minimum level of QoS regardless access network they use (e.g. when UE is roaming or if the network operator uses equipment from different vendors). Table 2.1 presents standardized QCI values mapped to the corresponding performance characteristics, as specified in 3GPP TS 23.203.

Mapping between QCI and QoS classes In order to provide backward compatibility, the mapping between QCI and QoS classes parameters was specified in Time Slot (TS) 23.401. It is presented in Table 2.2.

2.5

Integration with 2G and 3G

When an E-UTRAN system is deployed in a network already supporting GERAN and/or UTRAN it is possible to use a common core network for all accesses. In practice this means that the P-GW will provide GGSN functionality towards the existing GPRS CN. Therefore an E-UTRAN/UTRAN/GERAN capable terminal will not need to change the GGSN (i.e., the IP point of presence towards external networks) when it changes Radio Access Technology (RAT)) and switches between GERAN, UTRAN or E-UTRAN. Figure 2.3 shows how the EPS inter-works with existing 2nd Generation (2G)/3G networks. The figure presents the UTRAN when utilizing the GPRS one tunnel approach standardized in 3GPP Release 7. This feature makes it possible to bypass the SGSN in the user plane. Figure 2.4 shows a standardization view on how GERAN, UTRAN and E-UTRAN are integrated into the SAE. It should however be noted that the SGSN and MME shares a lot of common functionality. It is also required that the CN protocols, SM and MM, used in 2G/3G are compatible with the respective protocols used in EPS meaning that the SGSN and MME share a common evolution in the 3GPP standard. In a typical implementation/deployment view, it is likely that the 2G/3G SGSN and the MME are merged into one node, as illustrated in Figure 2.4. This 37

2 EPS architecture

QCI

Resource type

Packet

Packet Priority

error

delay

loss

budget

rate

2 4

100 ms 150 ms

10−2 10−3

3 5

50 ms 300 ms

10−3 10−6

5 6

1 6

100 ms 300 ms

10−6 10−6

7

7

100 ms

10−3

8

300 ms

10−6

9

300 ms

10−6

1 2 GBR

3 4

8

non-GBR

9

Example service

Conversational voice Conversational video (live streaming) Real-time gaming Non-conversational video (buffered streaming) IMS signalling Video buffered streaming,TCP based services (e.g. www, e-mail, chat, ftp, p2p file sharing, progressive video, etc.) Voice, video live streaming, interactive gaming ”Premium bearer” for video buffered streaming, TCP based services (e.g. www, e-mail, chat, ftp, p2p file sharing, progressive video, etc) for premium subscribers ”Default bearer” for video, TCP based services (etc. for non-privilaged subscribers

Table 2.1: QoS Class Identifier (QCI) defined for LTE/SAE.

QCI

Traffic class

1 2 3 4 5 6 7 8 9

Conversational Conversational Conversational Streaming Interactive Interactive Interactive Interactive Background

Traffic Handling Priority N/A N/A N/A N/A 1 1 2 3 N/A

Signalling indication

Source statistics descriptor

N/A N/A N/A N/A Yes No No No N/A

Speech Unknown Unknown Unknown N/A N/A N/A N/A N/A

Table 2.2: Mapping between standardized QCIs and pre-Relese-8 QoS parameter values.

38

2.6 Interfaces overview

Figure 2.3: E-UTRAN, UTRAN and GERAN architecture. GPRS one tunnel approach. will make it possible to support intra SGSN/MME and inter P/S-GW/GGSN node mobility between the different accesses.

2.6

Interfaces overview

This section contains a brief overview of the LTE/SAE interfaces.

Gi Gi is the interface to external packet data networks (e.g. Internet) and contains the end-user’s IP Point of Presence (PoP). All user-plane and control-plane functions that use the Gi interface are handled above the end-user’s IP layer, whereas all terminal mobility within 3GPP is handled below the Gi interface.

S1 S1 is the interface between eNB and MME and between eNB and S-GW. In the user plane this interface will be based on GTP User data tunnelling (GTP-U) (similar to Iu and Gn interface in UMTS). In the control plane the interface is more similar 39

2 EPS architecture

Figure 2.4: Typical implementation of LTE/SAE. Combined SGSN/MME one tunnel approach. to RAN Application Part (RANAP), with some simplifications and changes due to the different functional split and mobility within EPS. It has been agreed to split the S1 interface into a S1-CP (control plane) and S1UP (user plane) part. The signalling transport on S1-CP will be based on Stream Control Transmission Protocol (SCTP). The signalling protocol for S1 is called S1 Application Protocol (S1AP). S1AP protocol has the following functions: • EPS Bearer management function. This overall functionality is responsible for setting up, modifying and releasing EPS bearers, which are triggered by the MME The release of EPS bearers may be triggered by the eNB as well. • Initial context transfer function. This functionality is used to establish an S1 UE context in the eNB, to setup the default IP connectivity, to setup one or more SAE bearer(s) if requested by the MME, and to transfer NAS signalling related information to the eNB if needed. • Mobility functions for UEs in LTE ACTIVE in order to enable: ◦ a change of eNB within LTE/SAE (inter MME/S-GW handovers) via the 40

2.6 Interfaces overview S1 interface (with EPC involvement), ◦ a change of RAN nodes between different RAT (inter-3GPP-RAT handovers) via the S1 interface (with EPC involvement). • Paging. This functionality provides the EPC the capability to page the UE. • S1 interface management functions: ◦ Reset functionality to ensure a well defined initialisation on the S1 interface. ◦ Error Indication functionality to allow a proper error reporting/handling in cases where no failure messages are defined. ◦ Overload function to indicate the load situation in the control plane of the S1 interface. • NAS signaling transport function between the UE and the MME is used to: ◦ transfer NAS signalling related information and to establish the S1 UE context in the eNB, ◦ transfer NAS signalling related information when the S1 UE context in the eNB is already established. • S1 UE context release function. This functionality is responsible to manage the release of UE specific context in the eNB and the MME. S1 is a many-to-many interface.

X2 X2 is the interface between eNBs. The interface is mainly used to support active mode UE mobility (Packet Forwarding). This interface may also be used for multicell Radio Resource Management (RRM) functions. The X2-CP interface consists of a signalling protocol called X2 Application Protocol (X2AP) on top of SCTP. The X2-UP interface is based on GTP-U. The X2-UP interface is used to support loss-less mobility (packet forwarding). The X2-AP protocol provides the following functions: • Mobility Management (MM). This function allows the eNB to move the responsibility of a certain UE to another eNB. Forwarding of user plane data is a part of the mobility management. • Load management. This function allows eNBs to indicate overload and traffic load to each other. • Reporting of general error situations. 41

2 EPS architecture This function allows reporting of general error situations, for which function specific error messages have not been defined. The X2 interface is a many-to-many interface.

S3 S3 is a control interface between the MME and 2G/3G SGSNs. The interface is based on Gn/GTP Control plane (GTP-C) (SGSN-SGSN), possibly with some new functionality to support signalling free idle mode mobility between E-UTRAN and UTRAN/GERAN. S3 will not support packet forwarding; instead this will be supported on the S4 interface. S3 is a many-to-many interface. The S3 interface is similar to the S10 interface between MMEs which will be used for intra-LTE mobility between two MME pool areas.

S4 S4 is the interface between the P-GW and 2G/3G SGSNs. The interface is based on Gn/GPRS Tunnelling Protocol (GTP) (SGSN-GGSN). The user plane interface is based on GTP-U (same as S1-UP and Iu-UP) and the control plane is based on GTP-C (similar to S11). S4 is a many-to-many interface. The S4 interface is backwards compatible with the Gn interface.

S6 S6a enables transfer of subscription and authentication data for authenticating/authorizing user access to the evolved system (Authentication, authorisation and accounting (AAA) interface) between MME and Home Subscriber Server (HSS). S6d is between the SGSN and the HSS. S6 is based on Diameter.

S5/S8 S5/S8 is the interface between the S-GW and P-GW. In principle S5 and S8 is the same interface, the difference being that S8 is used when roaming between different operators while S5 is network internal. The S5/S8 interface will exist in two variants one based on Gn/GTP (SGSN-GGSN) and the other will use the Internet Engineering Task Force (IETF) specified Proxy Mobile IP (PMIP) for mobility control with additional mechanism to handle QoS. The usage of PMIP or GTP on S5/S8 will not be visible over the S1 interface or in the terminal. In the non roaming case the S-GW and P-GW functions can be performed in one physical node. It has been agreed in 3GPP that the usage of PMIP or GTP on S5 and S8 should not impact RAN behaviour or impact the terminals. 42

2.6 Interfaces overview In the roaming case S8 is providing user and control plane between the S-GW in the Visited PLMN (VPLMN) and the P-GW in the Home PLMN (HPLMN). S8 is the inter Public Land Mobile Network (PLMN) variant of S5. S5/S8 is a many-to-many interface.

S9 S9 provides transfer of QoS policy and charging control information between the Home Policy and Charging Rules Function (PCRF) and the Visited PCRF in order to support local breakout function.

S10 S10 is a control interface between the MMEs which will be very similar to the S3 interface between the SGSN and MME. The interface is based on Gn/GTP-C (SGSN-SGSN) with additional functionality. S10 is a many-to-many interface.

S11 S11 is the interface between the MME and S-GW. The interface is based on Gn/GTP-C (interface between SGSN and GGSN) with some additional functions for paging coordination, mobility compared to the legacy Gn/GTP-C (SGSN-GGSN) interface. S11 is a many-to-many interface.

S12 S12 is the interface between UTRAN and S-GW for user plane tunnelling when direct tunnel is established. It is based on the Iu-u/Gn-u reference point using the GTP-U protocol as defined between SGSN and UTRAN or respectively between SGSN and GGSN. Usage of S12 is an operator configuration option.

S13 S13 enables UE identity check procedure between MME and Equipment Identify Register (EIR).

SGi SGi is the interface between the P-GW and the packet data network. Packet data network may be an operator external public or private packet data network or an intra operator packet data network, e.g. for provision of IP Multimedia Subsystem (IMS) services. This interface corresponds to Gi for 3GPP accesses.

Rx Rx is the interface between the application server and the PCRF 43

2 EPS architecture

Gx Gx provides transfer of QoS policy and charging rules from PCRF to PCEF in the P-GW.

2.7

Evolved Packet Core (EPC) functions

EPC is the core network of the SAE system and is built up with P/S-GW nodes, together with MME nodes.

2.7.1

Mobility Management Entity node

The EPS architecture defines MME node, which contains core network control functionality. Although the functionality is not entirely the same, the MME conceptually constitutes a control plane SGSN node. The CP terminal protocols terminate at the MME, which also manages the mobility contexts of the UEs. The same MME remains in control of a UE as long as the UE moves within an MME pool area. The MME handles the mobility and session management functions listed below: • UE attach/detach handling. This allows UE to register and de-register to the network. • Security. The MME implements functions for Authentication and Authorization to verify users’ identities, grant access to the network and track users’ activities, respectively. In addition, the MME performs ciphering and integrity protection of NAS message signalling. • EPS Bearer handling. The MME manages the setting up, modification and tearing down of EPS Bearers. It is assumed that a UE in E-UTRAN will always have one default EPS Bearer established at the time of attachment to the network. • MM for idle mode UEs. The MME manages mobility of idle mode UEs. Idle mode UEs are tracked with the granularity of Tracking Areas (TAs).

2.7.2

P-GW and S-GW nodes

The EPS architecture defines the Packet Data Network/Serving Gateway (P/S-GW) node. The P/S-GW is the anchor point for the user plane for a terminal moving between eNBs. The S-GW is only changed when the UE move to a new S-GW pool area while the P-GW is normally kept as long as the UE is attached to the network. The P/S-GW functionality is very similar to the existing GGSN node. The main additions are adding support for packet buffering during E-UTRAN paging and additional support for Non-3GPP interworking (e.g. CDMA2000, WLAN). The 44

2.7 Evolved Packet Core (EPC) functions P-GW provides an interface to the outside world (e.g. the Internet). The P/S-GW can mainly be seen as a user plane node, however it also performs some QoS related signalling (it terminates the interface for policy control). The P/S-GW is involved in the following control plane functions: • EPS Bearer Handling. The P/S-GW triggers the setup of EPS Bearers upon request from the policy control functions. • Mobility Anchor – IP PoP. The P-GW acts as a mobility anchor point which hides UE mobility from the fixed network. When a UE attaches to the network it is assigned an IP address from a P-GW, which then also assumes the role of mobility anchor to the UE. While the control of a UE may be transferred to another MME or S-GW as a consequence of a Handover (HO), the UE’s IP PoP will remain at the P-GW. Thus, the mobility of UEss is transparent to the fixed network. Further, the P/S-GW handles the following user plane functions: • QoS Policy Control and Enforcement. To simplify bearer requests from an application point of view, increase operator’s control over its network resources and limit the potential for abuse by users, EPS QoS is network controlled. The policy control and enforcement functions associate users’ traffic flows with appropriate QoS classes and execute rate policing to prohibit users or flows from exceeding the QoS limits specified in users’ subscription agreements. DL traffic is policed in the P/S-GW whereas UL traffic is policed in the eNB. • Charging. The charging function is responsible for charging the user for its traffic according to the rate that applies for a particular service, subscription etc. • Lawful Intercept. This function enables communications to be electronically intercepted, or eavesdropped, by law enforcement agencies, should it be authorized by judicial or regulatory mandates.

2.7.3

MME and S-GW pooling concept

It is possible to pool a number of MME and S-GW nodes together in order to eliminate the risk that one node failure will cause parts of the network to be out of service. This is possible since there is a many-to-many relation interface between eNBs and EPC nodes where each eNB is associated with a set of MMEs and S-GWs called an MME and S-GW pool. The resulting network is non-hierarchical. Independent pooling of MME and S-GW are supported, it is however not possible to change a S-GW without involving the MME. An operator may pool MMEs and S-GWs into one or several pools depending on organisation, regulatory requirements, transport providers etc. This is illustrated in Figure 2.5. The flexibility of the pooling concept makes it possible to enable partial 45

2 EPS architecture sharing of networks; i.e., to use only a part of the operator’s network as a shared network.

Figure 2.5: Inter-pool mobility. The individual pooled MMEs and S-GW do not have to be located on the same physical site, but can be distributed in the network. All pools of a particular operator are assumed to be interconnected by means of an interface similar to the S3/S4/S10/S11 interface. When a UE attaches to the network, it is assigned to one of the MMEs that belong to the MME pool associated with the eNB through which the UE is attaching, the MME then selects an S-GW in the S-GW pool. No change of MME or S-GW is required while the UE moves around among eNBs belonging to the same MME or S-GW pool. If the UE moves out of the pools coverage it is reassigned to an MME or S-GW in the pool associated with the new eNB. The P-GW, which performs charging, policy enforcement and UE’s IP PoP is not changed when the S-GW is relocated. The main purpose of the S-GW is to act as a local mobility anchor and to buffer packets during E-UTRAN paging. In some equipment vendors views (for example Ericsson) S-GWs are rare and in most cases the S-GW and P-GW functions are performed by the same physical node. MME relocation may be more motivated since there may be limits on how many eNBs the MME is connected to. 46

2.8 LTE functions Partially overlapping pools will also be supported. Overlapping pools may have some benefits since it makes it possible to avoid some of the negative effects of hard pool borders, however it comes with extra complexity.

2.8

LTE functions

LTE is a synonym for the new system’s radio access network, which officially is referred to in 3GPP specifications as E-UTRAN. This radio network is functionally an evolution of the 3G UTRAN, although the radio transmission technology has been changed completely.

2.8.1

LTE general principles

The radio interface in LTE is developed according to the requirements of spectrum flexibility, spectrum efficiency, cost effectiveness etc. Robustness against time dispersion has influenced the choice of transmission technique in both UL and DL. Spectrum flexibility incorporates the possibility to use both paired and unpaired spectrum, i.e. LTE should support both FDD and TDD based duplex arrangements, respectively. Also, the support for operation in six different bandwidths, 1.4, 3, 5, 10, 15 and 20 MHz, plays an important role of the spectrum flexibility part in the standardisation of the radio interface. Actually, the LTE radio interface implementation supports operation in any bandwidth between 1.4 and 20 MHz in steps of one resource block, which corresponds to 12 subcarriers or 180 kHz. High spectrum efficiency is achieved by the use of higher order modulation schemes, like 16QAM and 64QAM and advanced antenna solutions, including transmit and receive diversity, beamforming and spatial multiplexing (MIMO). Furthermore, the ISI is reduced by the choice of OFDM in the DL and Single Carrier Frequency Division Multiple Access (SC-FDMA) in UL. Both of these methods results in a long symbol time and thus a reduced ISI, which increases the performance in highly time-dispersive radio environments. The UL and DL has a similar time-domain structure.

2.8.2

eNB functionality

E-UTRAN consists solely of the eNB, which is responsible for all radio interface functionality. eNB is the RAN node in the EPS architecture that is responsible for radio transmission to and reception from UEs in one or more cells. The eNB is connected to EPC nodes by means of an S1 interface. The eNB is also connected to its neighbour eNBs by means of the X2 interface. Some significant changes have been made to the eNB functional allocation compared to UTRAN. Most Release 6 Radio Network Controller (RNC) functionality has been moved to the E-UTRAN eNB. Below follows a description of the functionality provided by eNB. • Cell control and MME pool support. 47

2 EPS architecture eNB owns and controls the radio resources of its own cells. Cell resources are requested by and granted to MMEs in an ordered fashion. This arrangement supports the MME pooling concept. S-GW pooling is managed by the MMEs and is not really seen in the eNB. • Mobility control. The eNB is responsible for controlling the mobility for terminals in active state. This is done by ordering the UE to perform measurement and then performing handover when necessary. • Control Plane (CP) and User Plane (UP) security. The ciphering of user plane data over the radio interface is terminated in the eNB. Also the ciphering and integrity protection of RRC signalling is terminated in the eNB. • Shared channel handling. Since the eNB owns the cell resources, the eNB also handles the shared and random access channels used for signalling and initial access. • Segmentation/concatenation. Radio Link Control (RLC) Service Data Units (SDUs) received from the PDCP layer consist of whole IP packets and may be larger than the transport block size provided by the physical layer. Thus, the RLC layer must support segmentation and concatenation to adapt the payload to the transport block size. • Hybrid Automatic Repeat reQuest (HARQ). Medium Access Control (MAC) HARQ layer with fast feedback provides a means for quickly correcting most errors from the radio channel. To achieve low delay and efficient use of radio resources, the HARQ operates with a native error rate which is sufficient only for services with moderate error rate requirements such as for instance Voice over IP (VoIP). Lower error rates are achieved by letting an outer Automatic Repeat reQuest (ARQ) layer in the eNB handle the HARQ errors. • Scheduling. Scheduling with support for QoS provides for efficient scheduling of UP and CP data. • Multiplexing and mapping. The eNB performs mapping of logical channels onto transport channels. • Physical layer functionality. The eNB handles the physical layer processing such as scrambling, Transmit (TX) diversity, beamforming and OFDM modulation. The eNB also handles layer one functions like link adaptation and power control. • Measurements and reporting. eNB provides functions for configuring and making measurements on the radio environment and eNB-internal variables and conditions. The collected data is 48

2.8 LTE functions used internally for RRM but can be reported for the purpose of multi-cell RRM.

49

2 EPS architecture

50

3 LTE signalling 3.1

User plane

The protocols performing the user plane functions in the radio interface are as follows: • Packet Data Convergence Protocol (PDCP), which maps the EPS bearer onto the E-UTRAN radio bearer and performs Robust Header Compression (ROHC). • Radio Link Control (RLC), which maps the E-UTRAN radio bearer to a logical channel and performs segmentation, in-sequence delivery and retransmissions. • Medium Access Control (MAC), which maps the logical channel to a transport channel and is responsible for HARQ and scheduling. • The physical layer, which maps the transport channel onto a physical channel and performs channel coding, modulation etc. The LTE radio interface protocol architecture for User Plane is shown in Figure 3.1.

Figure 3.1: User plane for LTE.

51

3 LTE signalling

3.2

Control plane

The protocols performing the control plane functions in the radio interface are as follows: • RRC protocol, which is used to transfer the NAS information over the radio interface. • PDCP. • RLC. • MAC. • The physical layer. The Figure 3.2 presents the LTE radio interface protocol architecture for the control plane.

Figure 3.2: Control plane for LTE.

3.3 3.3.1

Protocols Radio Resource Control (RRC)

The following control plane functions are agreed in 3GPP to be performed by the Radio Resource Control (RRC) layer: • Broadcast of System Information (SI) related to the NAS, • Broadcast of SI related to the AS, • Paging, • Establishment, maintenance and release of an RRC connection between the UE and E-UTRAN including: ◦ Allocation of temporary identifiers between UE and E-UTRAN, 52

3.3 Protocols ◦ Configuration of radio resources for RRC connection including Signalling Radio Bearer (SRB), • Establishment, maintenance and release of point to point radio bearers, • Mobility functions including: ◦ UE measurement reporting and control of the reporting for inter-cell and Inter Radio Access Technology (Inter-RAT) mobility, ◦ Inter-cell handover, ◦ UE cell selection and reselection and control of cell selection and reselection, ◦ UE context transfer between eNBs, • Notification for MBMS services, • Establishment, configuration, maintenance and release of radio bearers for MBMS services, • QoS management functions. (Note: These functions are spread across multiple layers), • UE measurement reporting and control of the reporting, • MBMS control, • NAS direct message transfer to/from NAS from/to UE. On the network side, the RRC layer is terminated by the eNB.

RRC specification aspects The RRC specification includes a hierarchy of procedures, where the highest level is called ”High-level procedures” covering e.g. Broadcast Control Channel (BCCH) acquisition, paging, RRC connection establishment, reestablishment, re-configuration and release. The content of high level procedure messages may then trigger Elementary Procedures that execute e.g. measurement, radio resource or security configuration. Mobility is also described as an elementary procedure. A single high-level procedure may in some cases trigger multiple elementary procedures.

Relation between NAS and AS The relation between NAS and AS states is characterised by the following principles, which is also illustrated in Figure 3.3. • EMM-Deregistered & ECM-Idle ⇒ RRC IDLE: ◦ Mobility: PLMN selection, ◦ UE position: not known by the network. • EMM-Registered & ECM-Idle ⇒ RRC IDLE: ◦ Mobility: cell reselection, ◦ UE position: known by MME at tracking area level. 53

3 LTE signalling

Figure 3.3: Relation between NAS and AS. • EMM-Registered & ECM-Connected with radio bearers established ⇒ RRC CONNECTED: ◦ Mobility: handover, ◦ UE position: known by the network at cell level.

3.3.2

Packet Data Convergence Protocol (PDCP)

Packet Data Convergence Protocol (PDCP) provides its services to the NAS/RRC at the UE or the relay at the eNB. The PDCP supports the following functions: • Header compression and decompression of IP data flows using the ROHC protocol, at the transmitting and receiving entity, respectively. • Transfer of data (user plane or control plane). This function is used for conveyance of data between users of PDCP services. • Maintenance of PDCP sequence numbers for radio bearers mapped on RLC acknowledged mode. • In-sequence delivery of upper layer Packet Data Units (PDUs) at handover. • Duplicate elimination of lower layer SDUs at handover for radio bearers mapped on RLC acknowledged mode. • Ciphering and deciphering of user plane data and control plane data • Integrity protection of control plane data. • Timer based discard. PDCP uses the services provided by the RRLC sublayer. 54

3.3 Protocols

3.3.3

Radio Link Control (RLC)

The Radio Link Control (RLC) protocol supports an Unacknowledged Mode (UM) and an Acknowledged Mode (AM). Whether UM or AM is used is configured per radio bearer. For example, UM could be used for VoIP while AM is used to carry Transmission Control Protocol (TCP)-based traffic. An RLC transparent mode exists as well, but it shall be only used to send RRC messages when no RLC UM or AM entity is set up, yet. The RLC layer supports segmentation and concatenation of RLC SDUs. Depending on the scheduler decision, a certain amount of data is selected from the RLC SDU buffer and segmented and/or concatenated depending on the size of the SDUs. This selected data block becomes the RLC PDU to which a sequence number is assigned. This means that one transport block contains only a single RLC PDU per radio bearer except if an RLC retransmission is required. In this case an RLC PDU containing new data might be multiplexed at the MAC layer with an RLC PDU retransmission. In order to allow the RLC SDU reassembly at the receiver, the RLC header carries the required segmentation, re-segmentation and concatenation information. The RLC sequence number will also be used at the receiver for insequence delivery to the RLC SDU reassembly entity. In AM, RLC is responsible for correcting residual HARQ errors by operating another ARQ protocol since it would be expensive in terms of transmit power to reach the required residual error rates of 10−5 or less in the MAC HARQ protocol. The ARQ retransmission units are RLC PDUs or RLC PDU segments. If an RLC retransmission is required and the radio quality has changed significantly compared to the original RLC transmission, the RLC protocol is able to perform a re-segmentation. In this case RLC segments a PDU into smaller PDU segments. The number of RLC re-segmentations of an RLC PDU is unlimited. RLC performs reordering of received RLC PDUs and PDU segments in order to ensure that RLC SDUs are delivered in sequence to higher layers. Retransmissions are initiated either by status reports sent by the RLC receiver or by local triggers from MAC layer in case of reaching the maximum number of HARQ transmissions. Status Reports are triggered either by polls sent from the RLC sender or by detecting missing PDUs after the PDUs have passed the reordering entity. Similar to UTRAN, the LTE RLC supports a status prohibit timer and a poll timer. Finally, RLC provides means for protocol error detection and recovery (e.g. reset) and duplicate detection.

3.3.4

Medium Access Control (MAC)

The Medium Access Control (MAC) layer for the LTE access can be compared to the Release 6 MAC-hs/MAC-e and covers mainly similar functionality: HARQ, priority handling (scheduling), transport format selection and Discontinuous Reception (DRX) control (not part of MAC in Release 6). The HARQ protocol is very similar to the solution adopted for High Speed Downlink Packet Access (HSDPA), i.e., the protocol uses multiple stop-and-wait hybrid 55

3 LTE signalling ARQ processes. The motivation for this type of protocol is to allow continuous transmission, which cannot be achieved with a single stop-and-wait scheme, while at the same time having some of the simplicity of a stop-and-wait protocol. The functionality and performance is similar to that of a window based selective repeat protocol but only single-bit HARQ feedback is required. The protocol is modelled as a number of parallel HARQ processes, where each process uses a simple stop-and-wait protocol. By using NHARQ parallel HARQ processes, where NHARQ > Round trip time/Subframe length, a continuous transmission is achieved. The maximum UE processing time before sending a HARQ feedback has been specified such that 8 HARQ processes are needed for continuous transmission in FDD with a typical eNB implementation. In 3GPP, the current working assumption is to use a synchronous HARQ for the uplink and an asynchronous HARQ for the downlink. That is, for the uplink, the subframe when the retransmission occurs is known at the receiver, while for the downlink the scheduler has the freedom to choose the subframe for the retransmission dynamically. For both up- and downlink a synchronous, single-bit HARQ feedback Acknowledge (ACK)/Negative Acknowledge (NACK) is sent providing feedback about the success of the previous transmission. The HARQ protocol is adaptive in both uplink and downlink, meaning that the scheduler can decide to use a different resource for a retransmission compared to that one used for the previous (re)transmission. The redundancy version of a (re)transmission needs to be known by the receiver. Thus, the redundancy version and an indication whether the transmission contains a first transmission or a retransmission is indicated on the Physical Downlink Control Channel (PDCCH). In case the data is a retransmission of previously stored data, the received data is soft combined with the data stored in the soft buffer. In case the received data is not a retransmission or a retransmission of data that has not been stored, the soft buffer is cleared and only the latest received data is placed in the buffer. The Figure 3.4 presents the principle of HARQ operation for MAC layer.

Figure 3.4: HARQ principle - four multiple HARQ processes. The MAC layer does not support in-order delivery to RLC. HARQ retransmissions will lead to that MAC PDUs are received in a different order than they were sent. Due to the lack of MAC sequence numbers it is up to the RLC receivers to restore the original sequence and to provide in-order delivery to higher layers. 56

3.4 Radio interface structure The MAC layer supports the ARQ in the RLC layer with certain triggers if residual HARQ errors are detected, e.g., if the maximum number of HARQ transmissions has been reached. Finally, MAC also allows flows from a single user to be multiplexed. Correspondingly, the MAC header carries multiplexing information used to de-multiplex RLC PDUs to different flows.

3.4

Radio interface structure

The radio interface is structured in a layered model, similar to WCDMA, with a layer 2 bearer (here called EPS Bearer Service), which corresponds to a PDPcontext in Release 6, carrying layer 3 data and the end-to-end service. The EPS bearer is carried by the E-UTRAN Radio Bearer Service in the radio interface. The E-UTRAN radio bearer is carried by the radio channels. The radio channel structure is divided into logical, transport and physical channels. The logical channels are carried by transport channels, which in turn are carried by the physical channels as illustrated in Figure 3.5.

57

3 LTE signalling

Figure 3.5: LTE radio interface structure for DL.

58

4 LTE radio interface introduction 4.1

Channel structure

The physical layer provides transport channels to the L2. These transport channels differ in their characteristics how data is transmitted and are mapped to different logical channels provided by the MAC layer. Logical channels describe which type of data is conveyed.

4.1.1

Logical channels

The logical channels can be divided into control channels and traffic channels. The control channels are used for transfer of control plane information and the traffic channels are used for the transfer of user plane information. The following logical channels are supported for LTE: • Control channels: ◦ Broadcast Control Channel (BCCH). A downlink channel for broadcasting system control information. ◦ Paging Control Channel (PCCH). A downlink channel that transfers paging information. This channel is used when the network does not know the location cell of the UE. ◦ Common Control Channel (CCCH). This channel is used by the UEs having no RRC connection with the network. CCCH would be used by the UEs when accessing a new cell or after cell reselection. ◦ Multicast Control Channel (MCCH). A point-to-multipoint downlink channel used for transmitting MBMS scheduling and control information from the network to the UE, for one or several Multicast Traffic Channels (MTCHs). After establishing an RRC connection this channel is only used by UEs that receive MBMS. ◦ Dedicated Control Channel (DCCH). A point-to-point bidirectional channel that transmits dedicated control information between a UE and the network. Used by UEs having an RRC connection. 59

4 LTE radio interface introduction • Traffic channels: ◦ Dedicated Traffic Channel (DTCH). A DTCH is a point-to-point channel, dedicated to one UE, for the transfer of user information. A DTCH can exist in both uplink and downlink. ◦ Multicast Traffic Channel (MTCH). A point-to-multipoint downlink channel for transmitting traffic data from the network to the UE using MBMS.

4.1.2

Transport channels

An effort has been made to keep a low number of transport channels in order to avoid unnecessary switches between different channel types, which are found to be time consuming in UMTS. In fact there is currently only one transport channel in downlink and one in uplink carrying user data, i.e., channel switching is not needed. For LTE, the following transport channels are provided by the physical layer: • Downlink: ◦ Broadcast Channel (BCH). A low fixed bit rate channel broadcast in the entire coverage area of the cell. Beamforming is not applied. ◦ Downlink Shared Channel (DL-SCH). A channel with possibility to use HARQ and link adaptation by varying the modulation, coding and transmit power. The channel is possible to broadcast in the entire cell and beamforming may be applied. UE power saving (DRX) is supported to reduce the UE power consumption. MBMS transmission is also supported. ◦ Paging Channel (PCH). A channel that is broadcast in the entire cell. DRX is supported to enable power saving. ◦ Multicast Channel (MCH). A separate transport channel for multicast MBMS. This channel is broadcast in the entire coverage area of the cell. Combining of MBMS transmissions from multiple cells Multicast Broadcast Single Frequency Network (MBSFN) is supported. • Uplink: ◦ Uplink Shared Channel (UL-SCH). A channel with possibility to use HARQ and link adaptation by varying the transmit power, modulation and coding. Beamforming may be applied. ◦ Random Access Channel (RACH). A channel used to obtain timing synchronization (asynchronous random access) and to transmit information needed to obtain scheduling grants (synchronous random access). The transmission is typically contention 60

4.1 Channel structure based. For UEs having an RRC connection there is some limited support for contention free access.

4.1.3

Physical channels

The physical layer offers services to the MAC layer in the form of transport channels. User data to be transmitted is delivered to the physical layer from the MAC layer in the form of transport blocks. The MAC layer at the transmitter side also provides the physical layer with control information necessary for transmission and/or reception of the user data. The physical layer defines physical channels and physical signals. • A physical channel corresponds to a set of physical resources used for transmission of data and/or control information from the MAC layer. • A physical signal, which also corresponds to a set of physical resources, is used to support physical-layer functionality but does not carry any information from the MAC layer. From a specification perspective, the interface between 3GPP TS 36.211 and 36.212 is defined in terms of physical channels, while physical signals are generated inside 36.211. Figure 4.1 illustrates the logical channels and their mapping to transport channels and physical channels. • Physical channels: ◦ Physical Downlink Shared Channel (PDSCH). Transmission of the DL-SCH transport channel. ◦ Physical Uplink Shared Channel (PUSCH). Transmission of the UL-SCH transport channel. ◦ Physical Control Format Indicator Channel (PCFICH). Indicates the PDCCH format. ◦ Physical Downlink Control Channel (PDCCH). DL Layer 1 (L1)/Layer 2 (L2) control signalling. ◦ Physical Uplink Control Channel (PUCCH). UL L1/L2 control signalling. ◦ Physical Hybrid ARQ Indicator Channel (PHICH). Carries DL HARQ ACK/NACK. ◦ Physical Broadcast Channel (PBCH). DL transmission of the BCH transport channel. ◦ Physical Multicast Channel (PMCH). DL transmission of the MCH transport channel. ◦ Physical Random Access Channel (PRACH). UL transmission of the random access preamble as given by the RACH transport channel. • Physical signals: 61

4 LTE radio interface introduction

Figure 4.1: LTE channels mapping.

62

4.2 Time domain structure ◦ Reference Signals (RS). Support measurements (for example for cell selection process) and coherent demodulation. Transmitted in both downlink and uplink. ◦ Primary Synchronisation Signals (P-SS) and Secondary Synchronisation Signals (S-SS). They are transmitted in the downlink and used in the cell search procedure. They transmit a parameter, which is used to identify a cell on the air interface. P-SS transmits a cell parameter physicalLayerSubCellId = {0, 1, 2}. S-SS transmits a cell parameter physicalLayerCellIdGroup = {0, 1, ....167}. The two parameters together indicated the CellID according the below formula: CellID = 3 · physicalLayerCellIdGroup + physicalLayerSubCellId (4.1) ◦ Sounding Reference Signal (SRS). Supports UL channel quality measurements for scheduling purpose. Transmitted in UL in wide frequency band to let the eNB discover and allocate the best subcarriers for UL PUSCH.

4.2 4.2.1

Time domain structure FDD

For the LTE FDD mode of operation, the time domain structure is divided into 10 ms long radio frames. Each radio frame consists of ten equally sized subframes of 1 ms length, which is illustrated in Figure 4.2. Each subframe, in turn, consists of two equally sized slots of 0.5 ms length. The subframe is the typical scheduling unit of LTE, while slots are relevant in case of frequency hopping. Figure 4.2 is valid for both the downlink and uplink transmission direction. As a result of OFDMA and applied subcarrier spacing of 15 kHz, the length of the 1 OFDMA symbol is 15 kHz = 66.67 µs. To the beginning of each OFDM symbol, a cyclic prefix is appended, which is a guard time to combat ISI due to multipath propagation. Cyclic prefix is a copy of the ending part of the OFDM symbol and when it is appended to the beginning of the OFDM symbol then the frequency domain content of the transmitted signal is unchanged, see Figure 4.3 and Figure 1.18. With cyclic prefix the transmission of time domain signal takes longer, but when receiver makes FFT of the received time domain signal then it obtains exactly the same frequency representation of the signal as it would get without cyclic prefix. 500 µs One slot could theoretically fit 7.5 symbols ( 66.67 µs = 7.5), therefore a slot contains maximum 7 symbols and the remaining time of half of a symbol duration is used as the cyclic prefixes for all 7 symbols, according to Table 4.1. In large cells, with higher delay spread of the radio channel, the cyclic prefix must be extended and only 6 symbols may be placed in a slot.

63

4 LTE radio interface introduction

Figure 4.2: LTE FDD time domain structure.

Figure 4.3: Cyclic prefix concept.

4.2.2

TDD

In case of TDD, some of the subframes, in 10 ms long frame, are reserved for downlink transmission, some subframes are reserved for uplink transmission and one or two subframes have special structure, because they are used as switch points between downlink and uplink. Seven uplink-downlink configurations are supported, see Table 4.2. All subframes, which are not special subframes, are defined as two slots of length 0.5 ms in each subframe. The special subframes consist of the three fields, see Figure 4.4:

Figure 4.4: LTE TDD frame structure for UL-DL configuration 2. • Downlink Pilot Time Slot (DwPTS), 64

4.3 Frequency domain structure

Prefix type

Number of symbols in a slot

∆f

7

15 kHz

6 3

15 kHz 7.5 kHz

Normal prefix Extended prefix

Propagation path difference

Cyclic prefix length 5.2 µs for first symbol 4.7 µs for other symbols 16.7 µs 33.3 µs

1.4 km 5.0 km 10 km

Table 4.1: Cyclic prefix types. UL-DL configuration

UL-DL switch point periodicity

1

Subframe number 2 3 4 5 6 7

0

8

9

0

5 ms

↓

S

↑

↑

↑

↓

S

↑

↑

↑

1

5 ms

↓

S

↑

↑

↓

↓

S

↑

↑

↓

2

5 ms

↓

S

↑

↓

↓

↓

S

↑

↓

↓

3

10 ms

↓

S

↑

↑

↑

↓

↓

↓

↓

↓

4

10 ms

↓

S

↑

↑

↓

↓

↓

↓

↓

↓

5

10 ms

↓

S

↑

↓

↓

↓

↓

↓

↓

↓

6

5 ms

↓

S

↑

↑

↑

↓

S

↑

↑

↓

Table 4.2: Uplink-downlink configuration for LTE TDD. ↓ denotes a subframe reserved for downlink transmission. ↑ denotes a subframe reserved for uplink transmission. S denotes a special subframe. • Guard Period (GP), • and Uplink Pilot Time Slot (UpPTS). DwPTS, GP and UpPTS have configurable individual lengths (see Table 4.3 and Figure 4.5) and a total length of 1 ms.

4.3

Frequency domain structure

LTE downlink transmission is based on the OFDMA with the subcarrier bandwidth of 15 kHz. The LTE downlink physical resource can thus be seen as a time-frequency grid, which consists of Resource Elements (REs), as illustrated in Figure 4.6. The RE corresponds to one symbol duration in the time domain and subcarrier width (15 kHz) in frequency domain. Since the idea of OFDMA is to divide the available channel bandwidth into many narrow subcarriers and to allocate to a user several simultaneous subcarriers, therefore a concept of a Resource Block (RB) is created. The RB corresponds to 12 consecutive subcarriers (12·15 kHz = 180 kHz) used during one slot (0.5 ms), therefore the RB consists of 12·7 = 84 RE. To provide user with higher bit rate, a user may get simultaneously several RBs on one E-UTRAN carrier. The number of RBs for the different LTE channel bandwidths is listed in Table 4.4. 65

4 LTE radio interface introduction

Special subframe config. 0 1 2 3 4 5 6 7 8

Normal CP in downlink UpPTS Normal Extended DwPTS CP in CP in uplink uplink

Normal CP in downlink UpPTS Normal Extended DwPTS CP in CP in downlink downlink

6592·Ts 19760·Ts 21952·Ts 24144·Ts 26336·Ts 6592·Ts 19760·Ts 21952·Ts 24144·Ts

7680·Ts 20480·Ts 23040·Ts 25600·Ts 7680·Ts 20480·Ts 23040·Ts – –

2192·Ts

4384·Ts

2560·Ts

5120·Ts

2192·Ts

2560·Ts

4384·Ts

5120·Ts

– –

– –

Table 4.3: Special subframe configuration.

Figure 4.5: Special subframe configuration.

66

4.4 Scheduling Block

Figure 4.6: LTE downlink physical resource. For example for channel bandwidth 5 MHz there are 25 RBs, which can be allocated to users. The remaining frequency band is unused and needed as band guard, see also Figure 4.7. Channel bandwidth [MHz] Transmission bandwidth configuration [RB]

1.4

3

5

10

15

20

6

15

25

50

75

100

Table 4.4: Number of RBs for different channel bandwidths in FDD and TDD.

4.4

Scheduling Block

Data is allocated to the UEs in form of Scheduling Block (SB). One SB consists of two RBs in the same subframe. In DL, one UE can be allocated integer multiples of one SB in the frequency domain. These SBs do not have to be adjacent to each other. In the time domain, the scheduling decision can be modified every Transmission Time Interval (TTI) of 1 ms. The scheduling decision is done in the eNB. The scheduling algorithm has to take into account the radio link quality situation of different users, the overall interference situation, QoS requirements, service priorities, etc. Figure 4.8 shows an example of downlink data allocation to different users.

4.5

Virtual Resource Block

Resource blocks are used to describe the mapping of certain physical channels to resource elements. Both physical and virtual resource blocks are defined by 3GPP TS 36.211 . 67

4 LTE radio interface introduction

Figure 4.7: Definition of channel bandwidth and transmission bandwidth configuration for one E-UTRAN carrier.

Figure 4.8: An example of DL resource allocation.

68

4.6 System spectral efficiency Physical Resource Block (PRB) is what we have been discussing so far, RB with the following properties: 180 kHz over 0.5 ms. A Virtual Resource Block (VRB) is of the same size as a PRB. Two types of VRBs are defined: • VRB of localised type, • VRB of distributed type.

4.5.1

VRB of localized type

When using localized VRBs then there is a direct mapping of VRB to the PRBs: nPRB = nVRB . It means that a SB consisting of two VRBs corresponds to two PRBs located at the same place in the frequency domain, see Figure 4.9. VRBs are DL − 1, where N DL = N DL . numbered from 0 to NVRB RB VRB

4.5.2

VRB of distributed type

When using distributed VRBs then the first PRB that belongs to the SB is transmitted on different subcarriers than the second PRB belonging to the same SB, see Figure 4.9. RB ≤ 49 , only one gap value The parameter Ngap is given by Table 4.5. For 6 ≤ NDL RB Ngap,1 is defined and Ngap = Ngap,1 . For 50 ≤ NDL ≤ 110, two gap values Ngap,1 and Ngap,2 are defined. Whether Ngap = Ngap,1 or Ngap = Ngap,2 is signalled as part of the downlink scheduling assignment. DL − 1, where VRBs of distributed type are numbered from 0 to NVRB DL = N DL DL NVRB VRB,gap1 = 2 · min(Ngap , NRB − Ngap ) for Ngap = Ngap,1 and N DL

DL = N DL RB NVRB VRB,gap2 = ⌊ 2Ngap ⌋ · 2Ngap for Ngap = Ngap,2 . DL = 100 then N DL = 2 · 48 = 96 for N DL For example, if NRB gap = Ngap,1 . The NVRB is VRB used in the interleaving process as presented in Figure 4.9. The interleaving decides about the PRB number used for the first slot. The PRB, which transmit the second slot of the SB, is shifted by Ngap compared to the first slot. RB NDL

6

7-8

9-10

11

12-19

20-26

27-44

45-49

50-63

64-79

80-110

RBG Ngap,1 Ngap,2

1 3 -

1 4 -

1 5 -

2 4 -

2 8 -

2 12 -

3 18 -

3 27 -

3 27 9

4 32 16

4 48 16

Table 4.5: RB gap values.

4.6

System spectral efficiency

Table 4.6 compares air interface characteristics of GSM, UMTS, WiMAX and LTE systems. System spectral efficiency shows how many bits per second the system can transmit for each Hz of channel band width allocated in a cell. UMTS high 69

4 LTE radio interface introduction

Figure 4.9: Localized and distributed VRB. The picture illustrates Ngap,1 = 48 RB = 100. for NDL

70

4.6 System spectral efficiency system spectral efficiency is achieved thanks to high symbol rate of 3840 ksym/s used for wide channel band width. In LTE higher system spectral efficiency is achieved even though the symbol rate is low, this is because the channel band width is narrow. User bit rate depends not only on the system spectral efficiency, but also on the frequency band width allocated for the user as well as Carrier (C) to Noise (N) and Interferer (I) conditions C/(N+I). Low C/(N+I) may unable usage of high modulation techniques like 64QAM. User bit rate may also be increased thanks to multiple antennas used for transmission and for reception, so called MIMO concept, which is discussed in further in the book.

71

4 LTE radio interface introduction

System

GSM UMTS WiMAX WiMAX LTE LTE

Radio access technique TDMA WCDMA OFDMA OFDMA OFDMA OFDMA

kHz 3.7 0.3 91.4 91.4 66.7 66.7

Symbol duration µs

(Sub)carrier bandwidth

200 5000 10.94 10.94 15 15

GMSK QPSK QPSK 64QAM QPSK 64QAM

Modulation Symbol Technique rate ksymb s

271 3840 10 10 14 14

1 2 2 6 2 6

bit symb

Scheme

271 7680 20 60 28 84

kbit s

(Sub)carrier bit rate

1.36 1.54 1.83 5.48 1.87 5.60

bit/s Hz

Spectral efficiency

freq cell 1 3 1 1 1 1 1 1 1 1 1 1

Frequency reuse

System spectral efficiency

bit/s Hz· cell

0.45 1.54 1.83 5.48 1.87 5.60

Table 4.6: GSM, UMTS, WiMAX and LTE comparison. The table presents gross bit rate, spectral efficiency and system spectral efficiency, which include not only user date bit rate but also system signalling. The table does not consider MIMO which can further increase spectral efficiency.

72

5 LTE downlink physical channels In the OFDMA technique, which is used in the LTE DL, each RE contains one complex number. The complex numbers, which are sent duration the same symbol, are input the IFFT to build the time domain signal. The time domain signal is next converted to analogue and transmitted by an antenna. The complex numbers compose different physical channels, in order to support the system with not only user data transmission (carried out on PDSCH), but also with all kinds of signalling necessary to support this transmission. This chapter presents the process of the complex numbers generation, which compose different physical channels.

5.1

Cell search

The cell search in a process of finding an LTE. The cell search is based on the Primary Synchronisation Signals (P-SS) and Secondary Synchronisation Signals (S-SS) as well as the SI transmitted on the PBCH and PDSCH. The first step of cell search in LTE is based on specific P-SS and S-SS. LTE uses a hierarchical cell search scheme similar to WCDMA. Thus, the P-SS and the S-SS are defined. The synchronization signals are transmitted twice per 10 ms on predefined slots, see Figure 5.1 for FDD and Figure 5.2 for TDD.

Figure 5.1: Primary/secondary synchronization signal and PBCH structure for FDD (normal cyclic prefix). 73

5 LTE downlink physical channels

Figure 5.2: Primary/secondary synchronization signal and PBCH structure for TDD (normal cyclic prefix).

5.2

P-SS

The P-SS is a sequence of 62 symbols transmitted on the 62 central subcarriers. The sequence is generated from a frequency-domain Zadoff-Chu1 sequence. The ZadoffChu sequence has an ideal periodic auto-correlation property (i.e. the periodic autocorrelation is zero for all time shifts other than zero). Thanks to this property and also thanks to location of the P-SS on the central subcarriers an UE may synchronize to the subcarrier structure (frequency domain synchronization). There are three different Zadoff-Chu root sequences defined in the 3GPP standard (3GPP TS 36.211) using root indices 25, 29, 34, corresponding to cell parameter physicalLayerId = {0, 1, 2}, see Figure 5.3. Identifying the sequence transmitted in a cell the UE can detect physicalLayerId parameter. Different physicalLayerId should be allocated to neighbouring cells located on the same site. The different Zadoff-Chu root sequences are not orthogonal, but exhibit low cross-correlation2 . The P-SS is transmitted without being scrambled. Since the P-SS occurs twice per frame it does not uniquely determine the frame timing, but has an ambiguity of 5 ms. 1

A Zadoff-Chu sequence is a complex-valued mathematical sequence which have the property that cyclicly shifted versions of the sequence comprising the signal do not cross-correlate with each other when the signal is recovered at the receiver. A generated Zadoff-Chu sequence that has not been shifted is known as a ”root sequence”. Zadoff-Chu sequences are used in the 3GPP LTE air interface in the definition of Primary Synchronization Signal (P-SS), random access preamble (sent on PRACH), HARQ ACK/NACK responses (sent on PUCCH) and Sounding Reference Signals (SRS). 2 Correlation is a dependence between two variables. Intuitively, correlation between two variables means, that if we know the value of one of them, then we are able, at least in some cases, to predict the value of the other variable with better accuracy than without this information. Crosscorrelation is a measure of similarity of two waveforms as a function of a time-lag applied to one of them. This is also known as a sliding dot product or inner-product.

74

5.3 S-SS

Figure 5.3: Zadoff-Chu sequence transmitted on 31 lower frequency band subcarriers for physicalLayerId = 0, which corresponds to root index u = 25. The mapping of P-SS as well as other physical channels and physical signals is illustrated in Figure 5.4

5.3

S-SS

The sequence d(n) used for the S-SS is an interleaved concatenation of two length-31 binary sequences s0 (n) and s1 (n), hence the total length is 62. The two sequences s0 (n) and s1 (n) are defined as two different cyclic shifts of a source sequence s(n), see Table 5.1.

Table 5.1: S-SS sequence generation.

The sequence s(n) is used to generate, by its cyclic shift, two sequences s0 and s1 . The Table 5.1 shows cyclic shifts of 1 and 4 respectively, which sent in subframe 0 encode physicalLayerCellIdGroup = 60. The sequences s0 (n) and s1 (n) are next 75

5 LTE downlink physical channels

Figure 5.4: Mapping of Physical Channels on DL for FDD mode. Time on horizontal axis and frequency on vertical axis.

76

5.4 RS concatenated with interleaving building 62-long sequence. The 3GPP standard specifies 168 different pairs of shifts, therefore one of 168 different 62-long concatenated binary sequences may be transmitted on the S-SS, which encode the parameter physicalLayerCellIdGroup = {0, 1, ....167}. The concatenated sequence is next scrambled with a scrambling sequence given by the P-SS.and, similar to the P-SS, transmitted on 62 central subcarriers. The combination of two length-31 sequences defining the S-SS differs between subframe 0 and subframe 5 and is used to resolve the ambiguity of 5 ms mentioned above. Parameters physicalLayerId and physicalLayerCellIdGroup compose the physical layer cell identity according the below formula: CellID = 3 · physicalLayerCellIdGroup + physicalLayerId

(5.1)

The above formula makes available 504 different physical layer cell identities. Both P-SS and S-SS must be transmitted on the same antenna port. Placing P-SS and SSS close to each other enables coherent detection of S-SS using the channel estimate obtained from P-SS. A drawback of this placement is that the duration between P-SS and S-SS depends on the length of the CP and its length must therefore be blindly estimated.

5.4

RS

Reference Signals (RS) are transmitted in both downlink and uplink. The downlink reference signals consist of so-called reference symbols, which are known symbols inserted within in the OFDM time/frequency grid. This section discusses the downlink RS, which enable: • Coherent demodulation of other symbols into bits in UE. Without these reference symbols it would be very difficult for the UE to demodulate symbols into bits in so dense modulations like 16-QAM and 64-QAM where different between different modulation constellations may be small. If an NodeB is uses 2 or 4 antennas for transmission then different RS are transmitted by each antenna. • Channel quality measurements for scheduling. Because the downlink RS are sent in whole frequency band of the carrier therefore measurements done by UE and provided to the NodeB may be used by the NodeB to allocate the optimal downlink subcarriers for downlink transmission. • Measurements for mobility. RS are transmitted with constant output power, therefore measurements on the m are good signal strength measure of a cell and are used in cell reselection and handover process. Specific predefined resource elements carry the cell specific reference signal, which consists of so called reference symbols. The reference symbols are transmitted every 6-th subcarrier across the whole band of the carrier. In case of normal cyclic prefix, the reference symbols are transmitted on symbols 0 and 4 in each slot (for one or two antenna ports) and also on symbol 1 for four antenna ports in a cell, see Figure 5.5. In case of extended cyclic prefix the reference symbols are transmitted on symbols 77

5 LTE downlink physical channels

Figure 5.5: Downlink reference signal structure in a cell supporting non-MBSFN transmission with normal cyclic prefix and CellID = 0.

78

5.5 PBCH 0 and 3 in each slot (for one or two antenna ports) and also on symbol 1 for four antenna ports in a cell. In case of one or two transmit antennas, each antenna has 4 reference symbols in a RB. In case of four transmit antennas in a cell, antenna ports 0 and 1 have four reference symbols in a RB, while antenna ports 2 and 3 have two reference symbols in a RB. The reference symbols, which are sent on a particular symbol every 6-th subcarrier across the carrier frequency band, compose a pseudo random sequence of QPSK modulation symbols. The sequence is generated with use of Gold codes3 and different pseudo random sequence is used for different symbols within a frame, but are repeated every 10 ms frame. The pseudo random sequences are different for each physical layer CellID. Not only the random sequence, but also the frequency domain location of the reference symbols depends on the CellID. The cell-specific frequency shift of the reference symbols is given by: νshif t = Cell ID mod 6

(5.2)

Figure 5.6 illustrates the above formula.

Figure 5.6: Cell specific RS frequency shift. Downlink RS are transmitted in all downlink subframes in a cell supporting nonMBSFN transmission. In case the subframe is used for transmission with MBSFN, only the first two OFDM symbols in a subframe can be used for transmission of cellspecific reference symbols. Downlink reference signals are defined for ∆f = 15 kHz only.

5.5

PBCH

As additional help during cell search a set of parameters, called System Information (SI), is broadcast to all UEs in the whole cell area by the logical channel BCCH. The SI is divided into two parts. The static part is called Master Information Block (MIB) and is carried out by transport channels BCH. The dynamic part contains different System Information Blocks (SIBs) and is carried out by DL-SCH as presented in Figure 5.7. 3

A Gold code, also known as Gold sequence, is a type of binary sequence, used in telecommunication (CDMA, LTE) and satellite navigation (GPS). Gold codes are named after Robert Gold. Gold codes have bounded small cross-correlations within a set, which is useful when multiple devices are broadcasting in the same range.

79

5 LTE downlink physical channels

Figure 5.7: System information.

5.5.1

MIB

The MIB contains a limited number of the most essential and most frequently transmitted parameters that are needed to acquire other information from the cell, and is transmitted on PBCH. The MIB contains 24 bits of information plus 16 bits of Cyclic Redundancy Check (CRC) and transmits the following parameters: • DL carrier bandwidth. • PHICH configuration. • System frame number. The MIB uses a fixed schedule with a periodicity of 40 ms and repetitions made within 40 ms. The first transmission of the MIB is scheduled in subframe number 0 of radio frames for which the SFN mod 4 = 0, and repetitions are scheduled in subframe number 0 of all other radio frames. The PBCH is mapped onto the first four OFDM symbols of the second slot in the first subframe of every frame. In the frequency domain PBCH uses the 72 centre subcarriers, which corresponds to six resource blocks. Over one radio frame this corresponds to 4 symbols · 72 subcarriers = 288 RE. • In case of normal cyclic prefix, 48 REs (8 reference symbols per RB and 6 RBs) are occupied by RS and thus 288 − 48 = 240 REs are used for PBCH per frame. This corresponds to 480 coded bits per frame, since QPSK is used. • In case of extended cyclic prefix, 72 resource elements (12 reference symbols per resource block and 6 resource block) are occupied by RS and thus 288 − 72 = 216 resource elements are used for PBCH per frame. This corresponds to 432 coded bits per frame, since QPSK is used. 80

5.6 PCFICH The BCH transport block is encoded with a convolutional encoder. The BCH TTI is 40 ms and thus, in case of normal cyclic prefix, a BCH transport block of 4 · 480 = 1920 bits is delivered to L1 every 40 ms. In case of extended cyclic the block size is of 4 · 432 = 1728 bits. The block of bits is scrambled with a cell-specific sequence prior to modulation.

5.5.2

SIB

The remaining parameters are divided thematically into blocks, so called SIBs: SIB1

contains information on e.g. access related information and scheduling information on how the other SIBs are scheduled.

SIB2

contains radio resource configuration information that is common for all UEs.

SIB3

transmits cell reselection parameters.

SIB4

contains info for intra frequency LTE neighbouring cell relevant for cell reselection.

SIB5

contains info for inter frequency LTE neighbouring cell relevant for cell reselection.

SIB6

contains info for UTRAN neighbouring cells relevant for cell reselection.

SIB7

contains info for GERAN neighbouring cells relevant for cell reselection.

SIB8

contains info for CDMA2000 neighbouring cells relevant for cell reselection.

SIB9

contains home eNB name.

SIB10

contains an Earthquake and Tsunami Warning System (ETWS) primary notification.

SIB11

contains and ETWS secondary notifications.

SIB12

contains a CMAS notification.

SIB13

contains the information required to acquire the MBMS control information associated with one or more MBSFN areas.

Each SIB is transmitted periodically. SIB1 uses a fixed schedule with a periodicity of 80 ms. SIBs other than SIB1 are carried in System Information (SI) messages. Mapping of SIBs to SI messages is flexibly configurable by schedulingInfoList included in SIB1. SIBs are transmitted on DL-SCH, which in turn is transmitted by the physical channel PDSCH. The scheduling of the SIB is indicated by sending a single System Information RNTI (SI-RNTI) on PDCCH. Parameters transmitted in SIBs are listed in Appendix A on page 157.

5.6

PCFICH

The Physical Control Format Indicator Channel (PCFICH) carries information about the number of OFDM symbols used for transmission of PDCCHs in a subframe. The 81

5 LTE downlink physical channels set of OFDM symbols possible to use for PDCCH in a subframe is given in Table 5.2. Subframe

DL > 10 NRB

DL ≤ 10 NRB

Subframe 1 and 6 in TDD MBSFN subframes on a carrier supporting both PMCH and PDSCH for 1 or 2 cell specific antenna ports MBSFN subframes on a carrier supporting both PMCH and PDSCH for 4 cell specific antenna ports MBSFN subframes on a carrier not supporting PDSCH All other cases

1, 2 1, 2

2 2

2

2

0 1, 2, 3

0 2, 3, 4

DL is the downlink Table 5.2: Number of OFDM symbols used for PDCCH. The NRB bandwidth configuration, expressed in number of RB, see Table 4.4.

Reception of the PCFICH is essential to correct operation of the system. If the PCFICH is incorrectly decoded the terminal will neither know where to find the control channels nor where the data region starts, and will therefore lose any DL-SCH data transmission intended for the terminal as well as uplink scheduling grants Two bits of information are coded into a 32-bit long sequence using a rate-1/16 simplex code. The coded bits are scrambled with a cell-specific sequence, modulated with QPSK modulation and mapped to 16 resource elements grouped into 4 groups of 4 elements each. The four groups are well-separated in frequency to obtain good diversity. Furthermore, to avoid inter-cell PCFICH collisions, the location of the four groups in the frequency domain depends on the CellID. The PCFICH is transmitted on the same set of antenna ports as the PBCH.

5.7 5.7.1

PDCCH PDCCH usage

The Physical Downlink Control Channel (PDCCH) carries scheduling assignments and other control information: • Downlink scheduling assignments indicating downlink transmission of PDSCH. • Uplink scheduling grants informing the UE about grants of PUSCH. The uplink scheduling grants include: ◦ PUSCH resource indication, ◦ Transport format (coding and modulation to apply by the UE), ◦ HARQ related information. • Power control commands of groups of terminals, which complements the power control commands included in scheduling decisions. 82

5.7 PDCCH

5.7.2

PDCCH mapping

Multiple PDCCHs can be transmitted in a subframe. PDCCHs are mapped on the first (up to four) OFDM symbols within a subframe, see Figure 5.4. The actual number of symbols used for the PDCCHs may vary per subframe and is indicated by PCFICH, see Table 5.2. Thus, each subframe can be said to be divided into a control region (including PCFICH, PHICH and PDCCH), followed by a data region (PDSCH). This maximises the spectral efficiency as the control signalling overhead can be adjusted to match the instantaneous traffic situation. Location of the PDCCH at the beginning of the subframe allows the terminal to decode the downlink scheduling assignment prior to the downlink transmission. The downlink transmission, takes place on the PDSCH, which is mapped on upper symbols numbers in the subframe. This minimises the delay in the DL-SCH decoding and thus the overall downlink transmission delay. Mobile terminals that are not scheduled may turn off their receiver circuitry for a large part of the subframe, with reduces terminal power consumption.

5.7.3

PDCCH format

A PDCCH carries messages listed in section 5.7.2. Because multiple mobile terminals can be scheduled simultaneously, on both downlink and uplink, there must be a possibility to transmit multiple scheduling messages within each subframe. The different scheduling messages have different payload sizes. For example, supporting spatial multiplexing with non-contiguous allocation of resource blocks in the frequency domain require a larger scheduling message than an uplink grant supporting frequency contiguous allocations only. Note, that PDCCHs, which are sent to different terminals located in different radio conditions, may require different codec rate. Matching of the codec rate to different radio conditions is supported and carried out by the Link Adaptation (LA) algorithm. Thus, the size of the PDCCH is variable and the PDCCH is transmitted on an aggregation of 1, 2, 4 or 8 consecutive Control Channel Elements (CCEs), where a CCE corresponds to 9 Resource Element Groups (REGs) and each REG consists of 4 RE, see Figure 5.8 and Table 5.3 (TS 36.211). PDCCH format

Number of CCEs

Number of REG

Number of PDCCH bits

0 1 2 3

1 2 4 8

9 18 36 72

72 144 288 576

Table 5.3: Supported PDCCH formats.

5.7.4

PDCCH processing

The PDCCH processing consists of the following steps, which are also illustrated in Figure 5.9: 83

5 LTE downlink physical channels

Figure 5.8: Control Channel Element (CCE).

Figure 5.9: Physical layer PDCCH processing.

84

5.7 PDCCH • CRC attachment. An CRC is attached to each PDCCH payload, where the MAC ID (Radio Network Temporary Identity (RNTI)) is included in the CRC calculation. Upon reception of a PDCCH, the terminal checks the CRC using its own RNTI. If the CRC checks, the message is declared to be correctly received and intended for the terminal. Thus, the identity of the terminal, which is supposed to receive the PDCCH message, is implicitly encoded in the CRC and not explicitly transmitted. • Channel coding and rate matching. PDCCHs, which are sent to different terminals located in different radio conditions, may require different codec rate. Matching of the codec rate to different radio conditions is supported and carried out by the Link Adaptation (LA) algorithm. The number of bits after coding and rate matching depends on the PDCCH format and is presented in Table 5.3. • Multiplexing of CCEs. The bits of coded PDCCHs are multiplexed in such a way, that bits of the first PDCCH are put first and they are followed by bits of the second PDCCH and so on. • Scrambling. The block of multiplexed bits is scrambled by the cell specific scrabbling sequence. • Modulation. The block of scrambled bits is modulated with QPSK modulation resulting in a block of complex-valued modulation symbols. • Layer mapping and precoding. The block of modulation symbols is mapped to layers to support the following TX schemes: ◦ Transmission on a single antenna port. ◦ Transmit diversity with 2 or 4 layers. In transmit diversity there is always one codeword and the number of layers is equal to the number of antenna ports. For details on precoding for transmit diversity see Section 5.8.9. The PDCCHs are transmitted on the same set of antenna ports as the PBCH.

5.7.5

PDCCH blind decoding

Each PDCCH may be of different format, see Table 5.3, and the number of CCEs building the PDCCH is a-priori unknown to the UE. Therefore, the UE needs to blindly detect the format of the PDCCH. Because the PDCCH must start at CCE, which is a multiple of its size, therefore the number of blind decoding is reduced. For example, a PDCCH of size 4 CCEs can only start at CCE 0, 4, 8, etc. If the control region consists of only 8 CCEs the number of PDCCH candidates for blind decoding 85

5 LTE downlink physical channels is 15, see Figure 5.10. The UE tries to apply the MAC ID to each one PDCCH candidates. First the UE assume that the PDCCH consists of 1 CCE, thereafter 2, 4 and 8 CCEs. The UE knows that the PDCCH is intended for it if the CRC is OK. If the number of CCEs is three times bigger then the number of channel PDCCH candidates triples as well. Figure 5.11 shows an example of mapping of PDCCHs into the Control Channel Element (CCE), when the control region consists of 24 CCEs. In order to reduce the number of decoding attempts the common search space and UE specific search spaces are also defined by the 3GPP: • Common search space is used to send PDCCHs for all users or a group of users (e.g. indications about paging). All UEs monitor the common search space on PDCCH. • The UE specific search space contains PDCCHs intended for one UE only (e.g. scheduling grants for transmitting UL data.) The UE uses its RNTI to find its specific search space.

5.8

PDSCH

The Physical Downlink Shared Channel (PDSCH) processing consists of two parts: 1. DL-SCH processing. Figure 5.12 shows the processing structure for each transport block for the DLSCH, PCH and MCH transport channels as described in TS 36.212. Data and control streams from/to MAC layer are encoded/decoded to offer transport and control services over the radio transmission link. Channel coding scheme is a combination of: error detection, error correcting, rate matching, interleaving and transport channel or control information mapping onto/splitting from physical channels. • CRC attachment, • Code block segmentation, • Channel coding, • Rate matching, • Code block concatenation. 2. Physical layer PDSCH processing. Physical layer PDSCH processing is described in 3GPP 36.211 clause 6.3. The processing consists of the following steps, which are also presented in Figure 5.13: • Scrambling. Scrambling of coded bits in each of the code words to be transmitted on a physical channel. 86

5.8 PDSCH

Figure 5.10: PDCH blind decoding example.

Figure 5.11: PDCH blind decoding.

87

5 LTE downlink physical channels

Figure 5.12: Transport channel processing for DL-SCH, PCH and MCH. • Modulation mapper. Mapping of scrambled bits to generate complex-valued modulation symbols. • Layer mapper. Mapping of the complex-valued modulation symbols onto one or several transmission layers. Layer mapper together with precoding are enables for MIMO. • Precoding. Precoding of the complex-valued modulation symbols on each layer for transmission on the antenna ports. • Resource element mapper on antenna ports. Mapping of complex-valued modulation symbols for each antenna port to resource elements. • OFDM signal generation. Generation of complex-valued time-domain OFDM signal for each antenna port. 88

5.8 PDSCH

Figure 5.13: Physical layer PDSCH processing.

89

5 LTE downlink physical channels

5.8.1

CRC attachment

Error detection is provided on transport blocks through a Cyclic Redundancy Check (CRC). CRC is used to detect if there are any uncorrected errors left after error correction. The entire transport block a0 , a1 , ..., aA−1 , where A is the size of the input sequence, is used to calculate 24 parity bits of the CRC, which is attached to the transport block bits a0 , a1 , ..., aA−1 as presented in Figure 5.14. Transmitter

Original data 10010111010011011... CRC generator Original data 10010111010011011...

Checksum 24 bits 11001110110101...

Radio frequency transmission path Received data 0 10010101010011011... CRC generator

Received checksum 11001110110101... If checksums do not match there is an error

Regenerated checksum 00001110011101...

Receiver

Figure 5.14: CRC concept.

5.8.2

Code block segmentation

The input bit sequence to the code block segmentation (see Figure 5.12)is denoted by b0 , b1 , ..., bB−1 , where B > 0. If B is larger than the maximum code block size Z, segmentation of the input bit sequence is performed and an additional CRC sequence of L = 24 bits is attached to each code block. The number of code blocks after segmentation is denoted by C and the code blocks are numbered accordingly r = 0, ..., C − 1. The maximum code block size Z = 6144 bits.

5.8.3

Channel coding

The correction of bit errors, which may happen during air interface propagation, is carried out by channel coding. Each code block is coded separately. The channel coding consists of encoding on the transmitting side and decoding on the receiving side. The encoding carried out by adding redundant bits (coding bits) to the user 90

5.8 PDSCH date bits on the transmitting side. The receiver performs decoding of the signal by removals of the additional encoding bits and correcting possible bit errors. The following channel coding schemes can be applied to TrCHs: • Convolutional coding with rate 1/3, see Figure 5.15. • Turbo coding. The scheme of turbo encoder is a Parallel Concatenated Convolutional Code (PCCC) with two 8-state constituent encoders and one turbo code internal interleaver. The coding rate of turbo encoder is 1/3. The structure of turbo encoder is illustrated in Figure 5.16. Usage of coding scheme and coding rate for the different types of TrCH is shown in Table 5.4. TrCh

Coding scheme

Coding rate

UL-SCH DL-SCH PCH MCH

Turbo coding

1/3

BCH

Tail biting convolutional coding

1/3

Table 5.4: Usage of channel coding scheme and coding rate for control information.

D

D

D

D

D

D

Figure 5.15: Rate 1/3 tail biting convolutional encoder. Due to reflections from different objects, like for example buildings, the radio waves propagate over several paths before they reach the receiver. The multipath propagation results in constructive or destructive interference of radio waves, which propagate over different paths. The destructive interference causes signal attenuation. The signal attenuation leads to bursty errors (consecutive erroneous bits) that appear repeatedly when receiver is moving. The decoder fails to recover bursty errors, but it successfully recovers single errors spread over the whole coding block. Therefore turbo coding has an internal Quadrature Permutation Polynomial (QPP) interleaver, as shown in Figure 5.16, which spreads bursty errors over the whole coding block and hence making the decoding more effective. The interleaver concept is presented in Figure 5.17.

5.8.4

Rate matching

Rate matching algorithm repeats or punctures the bits of a mother codeword to generate a requested number of bits according to a desired code rate that may 91

5 LTE downlink physical channels

1st constituent encoder D

Turbo code internal interleaver

D

D

2nd constituent encoder D

D

D

Figure 5.16: Structure of rate 1/3 turbo encoder (dotted lines apply for trellis termination only). The initial value of the shift registers of the 8-state constituent encoders is all zeros when starting to encode the input bits.

Transmitter 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 ... 39 Interleaver 0 13 6 19 12 25 18 31 24 37 30 3 36 9 2 15 8 21 14 27 20 33 26 39 ... 7 Amplitude Time

Radio frequency transmission path

0 13 6 19 12 25 18 31 24 37 30 3 36 9 2 15 8 21 14 27 20 33 26 39 ... 7 Consecutive errors

Deinterleaver

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 ... 39

Distributted errors Receiver

To decoder

Figure 5.17: Interleaver.

92

5.8 PDSCH be different from the mother code rate of the turbo coder. The rate matching algorithm also facilitates enhanced HARQ operation by minimising repetition of coded bits (when possible) for subsequent retransmissions of a packet in order to increase coding gains via Incremental Redundancy (IR). The rate matching for turbo coded transport channel is presented in Figure 5.18. For an input block size of K bits, the output of a turbo encoder consists of three lengthK streams, corresponding to the systematic bit d(0) and two parity bit streams d(1) and d(2) , referred to as P1 and P2 respectively. In the Circular Buffer Rate Matching (CBRM) method for rate-1/3 turbo codes, which is used in LTE, each of the three output streams of the turbo coder is rearranged with its own sub-block interleaver. Then, a single output buffer is formed by placing the rearranged systematic bits in the beginning followed by bit-by-bit interlacing of the two rearranged parity streams. Interlacing allows equal levels of protection for each constituent code.

Figure 5.18: Operations of circular buffer rate matching for turbo code. For a desired code rate, the number of coded bits Ndata to be selected for transmission is passed to the rate matching algorithm. The bit selection step of the CBRM simply reads out the first Ndata bits from the start of the buffer. In general, the bits to be selected for transmission can be read out starting from any point in the buffer. If the end of the buffer is reached, then the reading continues by wrapping around to the beginning of the buffer (hence the term circular buffer). Thus, puncturing and repetition is achieved using a unified method. IR based HARQ operation is a key performance enabler in LTE. Thus, an LTE RM algorithm is expected to provide different subsets, denoted by Redundancy Version (RV), of the codeword for different transmissions of a packet (i.e., minimise repetition 93

5 LTE downlink physical channels of coded bits when possible). In CBRM, different RVs can be specified by simply defining different starting points (to start reading out) in the CB. For the first transmission (RV = 0), it is conventionally assumed the bits are read out from the beginning of the circular buffer, which means that all systematic bits are always selected and puncturing, if needed, is applied to parity bits only.

5.8.5

Code block concatenation

If the transport block was segmented into code blocks, see section 5.8.2, then the code blocks are concatenated. The number of code blocks is denoted by C and the code blocks are numbered accordingly r = 0, ..., C − 1. The bits input to the code block concatenation are denoted by er0 , er1 , ..., er(Er −1) where Er is the number of rate matched bits for the r-th code block, compare with Figure 5.12.

5.8.6

Scrambling (q)

Each codeword q = 0, 1, the block of bits b(q) (0), b(q) (1), ..., b(q) (M bit − 1), where (q) M bit is the number of bits in code word transmitted on the physical channel in one subframe, is scrambled prior to modulation, resulting in a block of scrambled bits ˜b(q) (0), ˜b(q) (1), ..., ˜b(q) (M (q) − 1) according to: bit ( ) ˜b(q) (i) = b(q) (i) + c(q) (i) mod 2

(5.3)

The scrambling sequence c(q) (i) is different for each code word, CellID as well as RNTI associated with the PDSCH transmission. Up to two code words can be transmitted in one subframe.

5.8.7

Modulation mapper

(q) For each codeword, the block of scrambled bits ˜b(q) (0), ˜b(q) (1), ..., ˜b(q) (M bit − 1) is modulated using one of the three modulation schemes QPSK, 16QAM or 64QAM, presented in section 1.6, resulting in a block of complex-valued modulation symbols (q) d(q) (0), d(q) (1), ..., b(q) (M symb − 1).

5.8.8

Layer mapper

The complex-valued modulation symbols, for each of the codewords to be transmitted, are mapped onto one or several layers. A layer is an isolated (from other layers) stream of modulation symbols that will be sent to the UE. Up to four layers may be transmitted to the UE parallely increasing the downlink throughput. The actual number of layers used for the transmission depends on the downlink radio conditions and is decided by the eNB on the bases of the UE report, mainly Rank Indicator (RI), see section 7.5. Each layer has the same number of symbols, but modulation and coding may differ between the codewords. 94

5.8 PDSCH

Single antenna port For transmission on a single antenna port, a single layer is used, υ = 1, and the mapping is defined by: x(0) (i) = d(0) (i)

(5.4)

(0)

layer with M symb = M symb .

Spatial multiplexing For spatial multiplexing, the layer mapping is done according to Table 5.5, which is also illustrated in Figure 5.19. The number of layers υ = 1 is less than or equal to the number of antenna ports P used for transmission of the physical channel. The case of a single code word mapped to two layers is only applicable when the number of antenna ports is 4.

Figure 5.19: Codeword-to-layer mapping for spatial multiplexing and transmit diversity. The picture also presents the precoding for transmit diversity. The size of the codeword(s) correspond to the maximum throughput possible to achieve for particular layer mapping. It can be observed that in spatial multiplexing maximum throughput increases with the the number of layers. In transmit diversity, regardless of the number of antennas, the maximum throughput is not increased.

Trasmit diversity For transmit diversity, the layer mapping is done according to Table 5.6, which is also illustrated in Figure 5.19. There is only one codeword and the number of layers υ = 1 is equal to the number of antenna ports P used for transmission of the physical channel. 95

5 LTE downlink physical channels

Number of layers

Number of codewords

1

1

2

2

2

1

3

2

4

2

Codeword-to-layer mapping layer i = 0, 1, ..., M symb −1 x(0) (i) = d(0) (i) x(0) (i) = d(0) (i) x(1) (i) = d(1) (i) x(0) (i) = d(0) (2i) x(1) (i) = d(0) (2i + 1) x(0) (i) = d(0) (i) x(1) (i) = d(1) (2i) x(2) (i) = d(1) (2i + 1) 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)

(0)

layer M symb = M symb (0)

(1)

layer M symb = M symb = M symb (0)

layer M symb = M symb /2

(0)

(1)

layer M symb = M symb = M symb /2

(0)

(1)

layer M symb = M symb /2 = M symb /2

Table 5.5: Codeword-to-layer mapping for spatial multiplexing.

Number of layers

Number of codewords

2

1

4

1

Codeword-to-layer mapping layer i = 0, 1, ..., M symb −1 x(0) (i) = d(0) (2i) x(1) (i) = d(0) (2i + 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)

layer M symb = M symb /2

layer M symb = M symb /4∗ (0)

Table 5.6: Codeword-to-layer mapping for transmit diversity. (0) ∗ In case when M (0) (0) symb mod 4 ̸= 0 then two null symbols are appended to d (M symb − 1).

96

5.8 PDSCH

5.8.9

Precoding

The precoder maps layers onto resources on each of the antenna ports. There are several variants of precoding: • Precoding for transmission on a single antenna port, • Precoding for spatial multiplexing, ◦ Precoding without Cyclic Delay Diversity (CDD), ◦ Precoding for large delay CDD, • Precoding for diversity.

Single antenna port Precoding for transmission on a single antenna port is defined by: y (p) (i) = x(0) (i)

(5.5)

where p ∈ {0, 4, 5, 7, 8} is the number of the single antenna port used for transmission ap ap layer of the physical channel and i = 0, 1, M symb − 1, M symb = M symb .

Spatial multiplexing Precoding for spatial multiplexing using antenna ports with cell-specific reference signals is only used in combination with layer mapping 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. • Precoding without CDD is defined by: 

  (0)  y (0) (i) x (i)     .. ..   = W (i)   . . (P −1) (υ−1) y (i) x (i)

(5.6)

ap where the precoding matrix W (i) is of size P × υ and i = 0, 1, ..., M symb − 1, ap layer M symb = M symb .

For spatial multiplexing, the values of W (i) are selected among the precoder elements in the codebook configured in the eNB and the UE. The eNB can further confine the precoder selection in the UE to a subset of the elements in the codebook using codebook subset restrictions. For 2 antenna ports, a codebook index from Table 5.7 must be selected. Different code book is specified for 4 antenna transmission. Figure 5.20 presents the precoding matrix W (i) selection for a relatively simple case of spatial multiplexing with one layer and two antenna ports. This technique is also called beamforming. The possible precoding matrixes, which may be applied in this case, are presented in Table 5.7 in the column υ = 1. 97

5 LTE downlink physical channels

Codebook index 0 1 2 3

Number of layers υ 1 2 ] [ ] [ 1 1 0 √1 √1 2 1 2 0 1 [ ] [ ] 1 1 1 1 √1 2 2 −1 1 −1 [ ] [ ] 1 1 1 1 1 √ 2 2 j j −j [ ] 1 √1 – 2 −j

Table 5.7: Codebook for transmission on antenna ports {0, 1}.

Figure 5.20: Spatial multiplexing with one layer and two antenna ports.

98

5.8 PDSCH [ Let us assume that currently W =

√1 2

] 1 1

is used. Signals transmitted from

antennas can be calculated as follows: [

y (0) y (1)

]

y (0) = y (1) =

1 =√ 2

√1 (1 2 √1 (1 2

[

] 1 1

· x(0) ) = · x(0) ) =

x(0)

(5.7)

√1 x(0) 2 √1 x(0) 2

(5.8) [

] 1 It can be seen from the above formulas that, for W = , both antennas 1 transmit exactly the same signal. It means, that the transmitted signals have the same phase. This is advantageous for the UE located in front of the transmitted antennas, where it has equal distances to both antennas. Signals transmitted from both antennas will change their phases during propagation, but, because they cover the same distance, they will reach the UE with the same phase leading to constructive interference and producing a gain in the UE antenna. √1 2

The UE may change its location in the cell and move to an area, where it is closer to the antenna TX0 (denoted by red colour in the figure) than to the antenna TX1 (denoted by blue colour in the figure). If the difference in the paths is equal λ/4, then the blue wave reaches the UE later, with a phase delay of 90 degrees, compared to the red wave. To compensate the phase shift, the blue antenna should start its transmission earlier, which is achieved by shifting its phase by −90 degrees. The phase shift takes place in the precoder by multiplying the transmuted symbol x(0) by −j. Multiplication by −j, which π in the exponential notation is equal to e− 2 j , results in −90 degree (− π2 ) phase shift. (For exponential notation of complex numbers see section [ 1.3.5). ] The 1 required phase shift is achieved by precoding matrix W = √12 , which −j does not apply any phase shift (coefficient 1) to antenna port 0 and applies a phase shift of −90 degrees (coefficient −j) to the antenna port 1. Figure 7.13 shows that, when the user moves further to the side of the cell, the difference between paths increases and may reach λ/2. λ/2 is a distance between TX antennas, if polarisation diversity is not used, see Figure 5.20. If the phase of the signals is not modified by the precoded, the two signals reach UE with opposite phase leading to destructive interference and signal cancelation. Because the blue signal has half of the wave length longer path to cover, it reaches the UE with a phase shift of 180 degrees compared to the red signal. 180 degree (π) phase shift is realised by multiplying a signal by ejπ = −1. the best precoding matrix for this UE location [ Therefore, ] 1 is W = √12 , which does not apply any phase shift (coefficient 1) to −1 99

5 LTE downlink physical channels antenna port 0 and a phase shift of 180 degrees (coefficient −1) to the antenna port 1. It can be seen from Table 5.7, that, for spatial multiplexing with one layer, no phase shift is applied to the signal transmitted from antenna port 0 (in all matrixes the coefficient corresponding to the antenna port 0 is equal 1). The phase shift is applied to the signal transmitted from antenna port 1 and the possible phase shifts are: 0 degree (coefficient 1), 180 degrees (coefficient −1), 90 degrees (coefficient j) and −90 degrees (coefficient −j). • Precoding with large delay CDD is defined by:    (0) x (i) y (0) (i)     .. ..   = W (i)D(i)U   . . (υ−1) (P −1) x (i) y (i) 

(5.9)

ap where the precoding matrix W (i) is of size P × υ and i = 0, 1, ..., M symb − 1, ap layer M symb = M symb . The diagonal size-υ × υ matrix D(i) supports cyclic delay diversity and is specified in TS 36.211 6.3.4.2.2. The size-υ × υ matrix U is also specified in TS 36.211 6.3.4.2.2.

Transmit diversity Precoding for transmit diversity is only used in combination with layer mapping for transmit diversity described above. The precoding operation for transmit diversity is defined for two and four antenna ports. For transmission on two antenna ports, p ∈ {0, 1}, the output [ ]T ap y(i) = y (0) (i) y (1) (i) , i = 0, 1, M symb − 1 of the precoding operation is defined by: 

  y (0) (2i) 1 0 j  (1)   1 y (2i) 0 −1 0     (0) = √  y (2i + 1) 0 2 0 1 (1) y (2i + 1) 1 0 −j

 ( ) 0 Re x(0) (i) )  ( j  Re x(1) (i)  )  ( j  Im x(0) (i)  ( ) 0 Im x(1) (i)

(5.10)

ap ap layer for i = 0, 1, ..., M symb − 1 with M symb = 2M symb .

It can be seen from the equation 5.10 that in the transmit diversity two different modulation symbols x(0) (i) and x(1) (i), that come from different layers, are transmitted simultaneously by different antennas and are transmitted twice, which is also illustrated in Figure 5.19. First antenna port 0 transmits modulation symbol x0 (i) and at the same time antenna port 1 transmits modulation symbol x1 (i). Next antenna port 0 transmits modulation symbol x(1) (i) and at the same time antenna port 1 transmits modulation x(0) (i), but with changed phase. This technique results in sending each modulation symbol twice in different directions and therefore increases probability of successful reception. For transmission on four antenna ports p ∈ {0, 1, 2, 3} the output y(i) is described in TS 36.211 6.3.4.3 and not presented in this book. 100

5.9 PHICH

5.8.10

Resource element mapping

For each of the antenna ports used for transmission of the physical channel, the block ap of complex-valued symbols y (p) (0), ..., y (p) (M symb −1) is mapped in sequence starting (p) with y (0) to resource elements (k, l) which meet all of the following criteria: • they are in the physical resource blocks corresponding to the virtual resource blocks assigned for transmission, and • they are not used for transmission of PBCH, synchronisation signals, cellspecific reference signals, MBSFN reference signals or UE-specific reference signals, and • they are not in an OFDM symbol used for PDCCH. The mapping to resource elements (k, l) on antenna port p not reserved for other purposes shall be in increasing order of first the index k over the assigned physical resource blocks and then the index l, starting with the first slot in a subframe.

5.9

PHICH

The Physical Hybrid ARQ Indicator Channel (PHICH) is used for transmission of hybrid-ARQ acknowledgements in response to UL-SCH transmission. There is one PHICH present for each terminal expecting an acknowledgement in the subframe. Each PHICH carries one bit, which is repeated three times, modulated, spread with a spreading factor of four and mapped to three REGs. Multiple PHICHs form a PHICH group and the PHICHs within a PHICH group are code-multiplexed using different orthogonal sequences and share the same set of resource elements, see Figure 5.21. There is in total eight 3GPP defined orthogonal sequences available when using normal CP. Four orthogonal sequences are available in case of extended CP. The use of use of code division multiplexing is motivated by power control of the PHICH, because with code division multiplexing the power difference between subcarriers is not as large as with pure frequency division multiplexing. The capacity of PHICH depends on the configured number of PHICH groups. Each PHICH group is assigned a unique frequency resource. Typically, the PHICH is transmitted in the first OFDM symbol only. However, in some propagation environments, this would unnecessarily restrict the PHICH coverage. To alleviate this, it is possible to configure a PHICH duration of three OFDM symbols10. In this case the control region is three OFDM symbols long in all subframes. The PHICH configuration is part of the system information (MIB on the BCH); one bit indicates whether the duration is one or three OFDM symbols and two bits indicate the amount of resources set aside for PHICHs. 101

5 LTE downlink physical channels

Figure 5.21: PHICH.

5.10

PMCH

The Physical Multicast Channel carries DL transmission of the MCH transport channel.

5.11

Downlink physical channels modulation summary

Table 5.8 shows modulations used for different physical channels and physical signals, which were discussed in this chapter. Physical channel or physical signal P-SS S-SS RS PBCH PCFICH PDCCH PDSCH PHICH

Modulation Zadoff-Chu sequence Interleaved concatenation of two length-31 binary sequences Gold sequence (pseudo random) of QPSK symbols QPSK QPSK QPSK QPSK, 16-QAM, 64-QAM BPSK

Table 5.8: DL physical channels modulation.

102

6 LTE uplink physical channels During the LTE development phase different alternatives for the optimum uplink transmission scheme were investigated. While OFDMA is seen optimum to fulfil the LTE requirements in DL, OFDMA properties are less favourable for the UL. This is mainly due to worse Peak-to-Average Power Ratio (PAPR) properties of an OFDMA signal, resulting in worse UL coverage. Thus, the LTE UL transmission scheme for FDD and TDD mode is based on Single Carrier Frequency Division Multiple Access (SC-FDMA) with cyclic prefix. SC-FDMA signals have better PAPR properties compared to an OFDMA signal, see Figure 6.1. This was one of the main reasons for selecting SC-FDMA as LTE UL access scheme. The PAPR characteristics are important for cost-effective design of UE power amplifiers. Still, SC-FDMA signal processing has some similarities with OFDMA signal processing, so parametrisation of downlink and uplink can be harmonised.

Figure 6.1: SC-FDMA versus OFDMA spectral power distribution. There are different possibilities of SC-FDMA signal generation. Discrete Fourier Transform spread-OFDM (DFT-s-OFDM) has been selected for LTE. The principles of the DFT-s-OFDM are illustrated in Figure 6.2. A size-M DFT is first applied to a block of M modulation symbols (i.e. complex numbers). QPSK, 16QAM or 64QAM may be used as uplink modulation schemes, the latter being optional for the UE. The DFT transforms the M modulation symbols into another M modulation symbols in the frequency domain. The result is mapped onto the M available UL subcarriers, that is inputs of the size-N IDFT. Unused inputs of the IDFT are set to zero. In UL, only localised transmission on consecutive M subcarriers is allowed. An size-N IDFT, where N > M , is then performed as in OFDM (see Figure 1.20), followed by addition of the cyclic prefix and parallel to serial conversion. M is the number of transmitted subcarriers and changes during UL transmission. For example, if the currently transmitted bandwidth by the UE is equal to 6 RBs 103

6 LTE uplink physical channels

Figure 6.2: Block diagram of the UL DFT-s-OFDM transmitter. then M = 6 · 12 = 72. N is the size of the IDFT build in the UE microprocessor and in LTE is equal to N = 211 = 2048, the same as in DL. If the DFT size M would equal the IDFT size N , the cascaded DF T and IDF T blocks of Figure 6.2 would completely cancel out each other. However, if M is smaller than N and the remaining inputs to the IDF T are set to zero, the output of the IDF T will be a signal with ’single-carrier’ properties, i.e. a signal with low power variations, and with a bandwidth that depends on M .

6.1

PUSCH

The UL SC-FDMA subcarrier spacing equals ∆f = 15 kHz and RBs, consisting of 12 subcarriers in the frequency domain, are defined also for the UL. However, in contrast to the DL, no unused DC subcarrier is defined for the UL as this would destroy the ’single-carrier’ property of the UL transmission (single-carrier characteristics require the transmission of consecutive subcarriers). Similar to the DL, the SC-FDMA used for the UL, also allows for a very high degree of flexibility in terms of transmission bandwidth by allowing for, in essence, any number of UL subcarriers. However, from a DFT implementation point of view, the DFT size M should preferably be constrained to a power of 2 (M = 2n ). On the other hand, such constraint is in direct conflict with a desired flexibility of UL bandwidth allocation to different terminals. From a flexibility point of view, all possible values of M should rather be allowed. For LTE, a middle way has been adopted where the DFT size is limited to products of the integers two, three and five (M = 2α · 3β · 5γ , where α, β, γ = 0, 1, 2, ...). Thus, as an example, DFT of 104

6.1 PUSCH sizes 84 is not allowed, because 84 = 2 · 2 · 3 · 7. Observe, that M = 84 = 12 · 7 correspond to 7 RBs, therefore 7 RBs allocation is not allowed. As a consequence, the number of UL RBs allocation is also limited to products of the integers two, three and five.

Figure 6.3: UL resource allocation. Also in terms of the more detailed time-domain structure the LTE UL is very similar to the DL. Each 1 ms UL subframe consists of two slots of length Tslot = 0.5 ms, see Figure 6.4. Each slot consists of seven or six DFT-s-OFDM blocks including the cyclic prefix. Also similar to the downlink, two cyclic prefix lengths, a normal cyclic prefix (for seven DFT-s-OFDM blocks symbol) and an extended cyclic prefix (for six DFT-s-OFDM blocks symbol) are defined for the UL.

Figure 6.4: UL subframe structure for normal cyclic prefix. In Figure 6.3, UEs gets radio resources on the same subcarriers in the two slots. As an alternative, inter slot frequency hopping may be applied for the LTE uplink. In this case different frequencies are used for transmission in the two slots of a subframe as presented in Figure 6.5. There are two potential benefits with UL frequency hopping if the hopping pattern are different in neighbouring cells. • Frequency diversity. • Interference averaging. 105

6 LTE uplink physical channels

Figure 6.5: UL frequency hopping.

6.2

Uplink reference signals

There are two types of UL reference signals in LTE: • Reference Signals (RS) for channel estimation to support coherent uplink transmission. • Sounding Reference Signal (SRS) to support UL frequency dependent scheduling.

6.2.1

RS

As illustrated in Figure 6.6, the uplink RSs used for channel estimation are transmitted within the fourth DFT-s-OFDM block of each uplink slot1 and with an instantaneous bandwidth equal to the bandwidth of the data transmission.

Figure 6.6: UL RS. The UL RS use cyclic extensions of Zadoff-Chu sequences at allocations of three RBs (36 subcarriers) or more. The exceptions are the allocation of 1 or 2 RBs (12 or 24 subcarriers), which instead use QPSK-based sequences. This is because there are too few Zadoff-Chu sequences available at such short sequence lengths. 1

This assumes the normal cyclic prefix, i.e. seven DFT-s-OFDM blocks per slot.

106

6.3 PUCCH

6.2.2

SRS

Channel dependent scheduling, in both the time and frequency domain, is a key LTE technology. The RS allow for UL channel estimation on the subcarriers, which are currently used by the UE’s PUSCH. The intention with the SRS is for the network to estimate the channel quality of the uplink frequencies, which are currently not used by UE’s PUSCH transmission. The sounding reference signals can also be used to estimate the timing of UE transmissions and to derive timing control commands for UL time alignment. SRS are transmitted independently of the UE’s PUSCH transmission, i.e. a UE may transmit the SRS also in subframes, where it does not have any data transmission. Furthermore, the bandwidth of SRS can be, and typically is, different from that of the UE’s PUSCH. The SRSs are transmitted within the last DFT-s-OFDM block of a subframe as shown in Figure 6.7. The SRS resources are shared by a number of UEs by their multiplexing in the time, frequency and orthogonal codes domain: • SRS in the time domain. Different UEs may by configured to transmit SRS in different subframes by providing the UE with SRS periodicity and SRS subframe offset. The periodicity of the SRS transmission is selected from the set {2, 5, 10, 20, 40, 80, 160, 320} ms or subframes. • SRS in the frequency domain. UE may be configured to transmit SRS in the whole or a fraction of the UL carrier bandwidth. For example, if srs-BandwidthConfig = 2 in a cell with 5 MHz UL bandwidth then some UEs in the cell may be configured to transmit SRS in the bandwidth of 24 RBs, some in the bandwidth 4 RBs and some in the bandwidth of 4 RBs. • SRS orthogonal codes. Similar to the RS, the SRS is a Zadoff-Chu sequence. With cyclic shifts up to 8 shift can be configured, which implies that up to 8 UEs can transmit SRS in the same time and in the same bandwidth but with different orthogonal (independent) sequences. Example of the SRS allocation is illustrated in Figure 6.7.

6.3

PUCCH

PUCCH supports uplink L1/L2 control signalling, which carriers one (or more) of the following singling information: • HARQ acknowledgements related to reception of DL-SCH transport. HARQ acknowledgements are sent by PUCCH format 1A, 1B or PUSCH. • Scheduling requests, used by the terminal to request UL-SCH resources in case it does not have a valid scheduling grant. Scheduling request are transmitted on the PUCCH format 1. 107

6 LTE uplink physical channels

Figure 6.7: UL SRS. • Channel Quality Indicator (CQI) indicating the downlink channel quality perceived by the terminal. CQI is used by the network for DL modulation and coding scheme selection. The CQI reports are transmitted periodically on PUCCH format 2 or aperiodically on PUSCH. UE reporting is discuss in section 7.5. As illustrated in Figure 6.8, these resources are located at the edges of the total available system bandwidth. Each such resource consists of 12 subcarriers (one resource block) within each slot of an uplink subframe. To provide frequency diversity, these frequency resources are frequency hopping on the slot boundary, that is one L1/L2 control resource consists of 12 subcarriers at the upper part of the spectrum within the first slot of a subframe and an equally sized resource at the lower part of the spectrum during the second slot of the subframe or vice versa, and it is referred to as a resource block pair. If more resources are needed for the uplink L1/L2 control signalling, for example, in case of very large overall transmission bandwidth supporting a large number of users, additional resources blocks can be assigned next to the previously assigned resource blocks.

Figure 6.8: PUCCH resources. 108

6.3 PUCCH

6.3.1

PUCCH format 1A/1B

PUCCH format 1A and 1B are used for transmission of HARQ acknowledgements. • Format 1A supports one bit acknowledgement to one code word. • Format 1B supports two bits acknowledgement to two code words sent to the UE during one subframe, which is the case of spacial multiplexing. One (format 1A) or two (format 1B) acknowledgement bits are modulated using BPSK or QPSK, respectively, resulting in one complex number (modulation symbol). A length-12 Constant Amplitude Zero AutoCorrelation (CAZAC) sequence is applied to each symbol in order to spread the symbol over 12 symbols sent on different subcarriers of an RB. Different cyclic shift of the length-12 CAZAC sequence are applied by different users, therefore 12 UEs feedbacks can be transmitted over the same subcarriers in the same time. Then scrambling is applied to all the symbols, see Figure 6.9. Different scrambling codes are used in the two different slots within one subframe. The 12 complex numbers are further multiplied by an orthogonal cover sequence. Orthogonal cover sequences are applied to both the four information symbols in a slot as well as to the three reference signal symbols. Thus, with three reference symbols per slot, up to three orthogonal cover sequences can be used. This implies three different UEs acknowledgements can be transmitted at the same cyclic shift of the length-12 CAZAC sequence resulting in up to 3 · 12 = 36 UEs with PUCCH format 1A/1B sharing one resource block pair. The same PUCCH structure is used in the two slots of a subframe. To further randomise the inter-cell interference between PUCCH resource blocks, cyclic shift hopping (per OFDM symbol) and orthogonal cover hopping (per slot) are used.

Figure 6.9: PUCCH format 1. HARQ acknowledgements are transmitted at a fixed time after the reception of a DL-SCH transport block (4 subframes in case of FDD). Furthermore, the PUCCH resource to use is derived from the index of the first control channel element in the 109

6 LTE uplink physical channels PDCCH used for scheduling the downlink transmission (or from RRC signalling in case of persistent scheduling).

6.3.2

PUCCH format 1

PUCCH format 1 is used for transmitting scheduling requests. The overall structure is similar to that used for HARQ acknowledgements. Each active terminal is assigned a dedicated resource for scheduling request through RRC signalling, providing the possibility to request an uplink grant every x subframe. If the UE do not want more scheduling, then it will not transmit anything on the dedicated resources.

6.3.3

PUCCH format 2

PUCCH format 2 is used for CQI reports. The CQI reports are coded to 20 bits and scrambled. The scrambling sequence depends on the CellID, slot number and Cell RNTI (C-RNTI). The scrambled bits are then modulated using QPSK, resulting in 10 complex valued symbols, see Figure 6.10. Each of the QPSK symbols (assuming normal cyclic prefix) is multiplied by a cyclically shifted length-12 CAZAC sequence and transmitted in one DFT-s-OFDM symbol. As the same underlying principle of cyclically shifted CAZAC sequences is used for PUCCH format 2 as for format 1A/1B, CQI from different terminals can be transmitted on the same time-frequency resource by assigning different cyclic shifts. In theory, it is possible to use 12 different cyclic shifts, hence twelve different UE’s CQI can be transmitted in the same resource block pair.

Figure 6.10: PUCCH format 2. It is also possibility for one UE to send CQI reports together with ACK/NACK. In that case format 2A or 2B is used. However, it is also possible to mix different 110

6.4 PRACH formats, i.e. different UEs transmit different feedback (e.g. CQI and ACK/NACK) in the same resource block. This is then signalled by higher layers.

6.4

PRACH

In the LTE, the UE uses the Random Access (RA) process to gain an access to a cell for the following reasons: • Initial access to the network from the RRC IDLE state. • Regaining access to the network after a radio link failure. • As part of the handover process to gain timing synchronisation with a new cell. • Before uplink data transfers when the UE is in RRC CONNECTED, but not UL time synchronised with the cell. When UE is RRC CONNECTED and UL synchronised then it uses scheduling request on PUCCH to request for UL transmission. In both RRC IDLE and RRC CONNECTED the UE is time synchronise to the DL BCCH, however, due to the propagation (round trip) delay, there is a timing uncertainty in the uplink. Therefore, the RA process is used by the UE to obtain time synchronisation. The PRACH shall reserve a sufficient time window to accommodate various arrival times. During this time the UE transmits RA preamble. Five formats of RA preamble exist (0, 1, 2, 3 and 4) (TS 36.211), see Table 6.1, which is illustrated in Figure 6.11. Format 4 is used in TDD only. Preamble format

TCP

TSEQ

0 1 2 3 4∗

3168 · Ts 21024 · Ts 6240 · Ts 21024 · Ts 448 · Ts

24576 · Ts 24576 · Ts 2 · 24576 · Ts 2 · 24576 · Ts 4096 · Ts

Table 6.1: Random access preamble parameters. Ts ≃ 32.55 ns, see equation 7.1. ∗ TDD mode and special subframe configurations with UpPTS lengths 4384 · T and s 5120 · Ts only. The RA preamble has different subcarrier spacing than other UL channels. Duration of the RA preamble symbol is 0.8 ms, therefore RA subcarrier spacing is 1/800 ms = 1250 Hz. The RA preamble consists of 840 such subcarriers leading to the total effective bandwidth of 840 · 1250 Hz = 1.05 MHz. The bandwidth reserved for a RA opportunity is 1.08 MHz (6 RBs), so it is slightly bigger leaving small spectral guard bands on each side of the RA preamble. This is necessary since RA and regular UL data are separated in frequency domain, but are not completely orthogonal. The parameter prach-ConfigIndex specifies the preamble format and subframes where where PRACH is allowed (TS 36.211). Location of the PRACH in the frequency do111

6 LTE uplink physical channels

Figure 6.11: Preamble formats.

112

6.4 PRACH main is defined by the parameter prach-FreqOffset, which indicated the RB allocated for the PRACH opportunity, which is illustrated in Figure 6.12.

Figure 6.12: Time-frequency structure of non-synchronised RA for FDD. Example for prach-ConfigIndex = 6 and prach-FreqOffset = 1.

113

6 LTE uplink physical channels

114

7 Physical layer procedures 7.1

Timing advance

7.1.1

Uplink-downlink frame timing

From the eNodeB perspective, the uplink and downlink frames have defined time shift equal to NTA offset Ts , where NTA offset = 0 for frame structure 1 used in FDD and NTA offset = 614 for frame structure 2 used in TDD, as presented in the upper part of Figure 7.2. Ts is the sampling time, which is the time unit used in LTE and specified in 3GPP 36.211 as follows: Ts =

1s 15000

2048

≃ 32.55 ns,

(7.1)

1s where 15000 is the symbol duration and 2048 is the FFT size. Thus, in case of the frame structure 2 the uplink frame starts 614Ts ≃ 20.0µs earlier than downlink frame.

7.1.2

Timing advance range

In order to keep the alignment of downlink and uplink frames at the eNB as specified by the NTA offset , the UE must advance its uplink transmission compared to the signal received on downlink. The time advance compensates the radio waves propagation delay from the eNB to the UE and back to the eNB. Therefore, from the UE perspective, transmission of the uplink radio frame number shall start (NTA + NTA offset )Ts earlier than the start of the corresponding downlink radio frame at the UE, where 0 ≤ NTA ≤ 20512. The maximum timing advance 20512Ts ≃ 667.7µs corresponds to the cell range of 100 km. Figure 7.1 and Figure 7.2 present the time advance compensation for FDD and TDD respectively. Initially NTA is received by the UE from the eNB in the timing advance command during random access and next is continuously adjusted by timing advance commands sent in the MAC control element.

7.1.3

Random access

Initial time alignment is performed by the random access process. Random access response carries 11-bit timing advance command TA = 0, 1, 2, ..., 1282 and indicates 115

7 Physical layer procedures

Figure 7.1: Uplink-downlink timing relation from UE perspective for FDD.

Figure 7.2: Uplink-downlink time relation from UE perspective for TDD.

116

7.1 Timing advance NTA value (3GPP TS 36.213), which is presented in Figure 7.3: NTA = 16TA ,

(7.2)

which means that the maximum timing advance value sent on the random access channel is 16 · 1282 · Ts ≃ 66.77 ms and corresponds to the distance of 100 km.

Figure 7.3: Random access timing advance. The granularity of the timing advance is 16Ts ≃ 0.52 µs and during this time radio waves cover the distance of 156 m. This distance is the sum of downlink and uplink path, therefore one step of timing advance corresponds to the distance change between the UE and the eNB of 78 m.

7.1.4

Other cases

The actual timing advance is continuously adjusted by timing advance command sent as MAC control element. The timing advance command MAC control element is identified by MAC PDU subheader with LCID = 11101, as specified in 3GPP TS 36.321. The timing advance command field is 6 bits TA = 0, 1, 2, ..., 63 and indicates adjustment of the current NTA value (NTA,old ) to the new NTA value (NTA,new ) expressed in multiples of 16 Ts , as specified by 3GPP TS 36.213: NTA,new = NTA,old + 16(TA − 31).

(7.3)

Adjustment of NTA value by a positive or a negative amount indicates advancing or delaying the uplink transmission timing by a given amount respectively, as presented in Figure 7.4. The maximum timing advance adjustment is equal to 16(63 − 31)Ts ≃ 16.7 µs and corresponds to the distance change of 2.5 km. For a timing advance command received on subframe n, the corresponding adjustment of the timing shall apply from the beginning of subframe n + 6. When the UEs uplink transmissions in subframe n and subframe n + 1 are overlapped due to the timing adjustment, the UE shall transmit complete subframe n and not transmit the overlapped part of subframe n + 1. 117

7 Physical layer procedures

Figure 7.4: Adjustment of timing advance by MAC control element.

7.1.5

Maintenance of uplink time alignment

The UE has a configurable timer timeAlignmentTimer, which is used to control how long the UE is considered uplink time aligned. The timer is sent in the System Information Block Type 2. T imeAlignmentT imer = {sf500, sf750, sf1280, sf1920, sf2560, sf5120, sf10240, infinity}.

(7.4)

Value in number of sub-frames. Value sf500 corresponds to 500 sub-frames, sf750 corresponds to 750 sub-frames and so on (3GPP TS 36.331). The UE starts the timeAlignmentTimer after random access response message is received and restarts the timer after each received timing advance command MAC control element. When timeAlignmentTimer expires the UE shall: • flush all HARQ buffers, • notify RRC to release PUCCH/SRS, • clear any configured downlink assignments and uplink grants. To get time alignment a new the UE must initiate the random access process, see Figure 7.5.

7.2

Random Access (RA)

From the physical layer perspective, the L1 RA procedure encompasses the transmission of RA preamble and RA response. The remaining messages are scheduled for transmission by the higher layer on the shared data channel and are not considered part of the L1 random access procedure. A RA channel occupies 6 RBs in a subframe or set of consecutive subframes reserved for RA preamble transmissions. The eNB is not prohibited from scheduling data in the resource blocks reserved for PRACH transmission. 118

7.3 Resource allocation

Figure 7.5: UE time synchronisation. Since the initial access attempt cannot be scheduled by the network, the RA procedure is by definition contention based. Collisions may occur and an appropriate contention-resolution scheme needs to be implemented. The process of the RA is presented in Figure 7.6. UE sends the RA preamble with initial power, which calculated based on the parameter preambleInitialReceivedTargetPower and waits for the response in the response window configured by the parameter ra-ResponseWindowSize. If the UE does not receive the RA response then retransmits the RA increasing the power by powerRampingStep. The RA response is recognised by its Random Access Radio Network Temporary Identity (RA-RNTI) and contains the allocation of the PDSCH, which includes the RA preamble identity, timing and UL scheduling grant. The UE uses the granted PUSCH resources to send message 3, which contains RRC CONNECTION REQUEST command to the eNB.

7.3

Resource allocation

The resource allocation is sent to the UE on PDCCH. The downlink assignment includes: • PDSCH resource indication, • Transport format (applied coding and modulation), • Transport block size, • HARQ related information, • MIMO related information (if applicable), • PUCCH power order commands. Resource indications can be of three different types: 0, 1, and 2 as described in 3GPP TS 36.213, see also Figure 7.7. Type 0 and 1 use a bitmap to support noncontiguous allocation in the frequency domain. Type 0 does not allow to address a single RB, therefore, type 0 is complemented by type 1. Type 2 allows to address only continuous RBs. 119

7 Physical layer procedures

Figure 7.6: RA process.

Figure 7.7: DL resource allocation.

120

7.3 Resource allocation The Downlink Control Information (DCI) on the PDCCH has several supported formats. The UE interprets the resource allocation field depending on the PDCCH DCI format detected. A resource allocation field in each PDCCH includes two parts: • resource allocation header, • resource block assignment. PDCCH DCI formats 1, 2, 2A and 2B with type 0 and PDCCH DCI formats 1, 2, 2A and 2B with type 1 resource allocation have the same format and are distinguished from each other via the single bit resource allocation header field which exists depending on the downlink system bandwidth, where type 0 is indicated by 0 value and type 1 is indicated otherwise. PDCCH with DCI format 1A, 1B, 1C and 1D have a type 2 resource allocation while PDCCH with DCI format 1, 2, 2A and 2B have type 0 or type 1 resource allocation. PDCCH DCI formats with a type 2 resource allocation do not have a resource allocation header field. A UE shall discard PDSCH resource allocation in the corresponding PDCCH if consistent control information is not detected.

7.3.1

Resource allocation type 0

In resource allocation of type 0 the RBs are grouped into P consecutive RBs called Resource Block Group (RBG). The reason for grouping RBs into RBG is to reduce the size of the bitmap used for resource allocation. P is than the size of the RBG and is a function of the system bandwidth, as specified in Table 7.1. System bandwidth RB NDL

RBG size (P)

≤ 10 11-26 27-63 64-110

1 2 3 4

Table 7.1: Type 0 resource allocation RBG size vs. downlink system bandwidth.

The total number of RBGs for the downlink system bandwidth is given by ⌈

NRBG

RB NDL = P

⌉ (7.5)

One RBG may be of size lower than P. Assignment information includes a bitmap indicating the RBGs that are allocated to the scheduled UE. The bitmap is of size NRBG bits with one bitmap bit per RBG such that each RBG is addressable. The RBG is allocated to the UE if the corresponding bit value in the bitmap is 1, the RBG is not allocated to the UE otherwise. The resource allocation type 0 allows to allocate all RBs if needed, but it allows to allocate a single RB. 121

7 Physical layer procedures

7.3.2

Resource allocation type 1

In resource allocations of type 1, the RBGs of size P are additionally divided into P subsets. A UE may get assignment on RBs belonging to one subset only. The resource allocation consists of a field, which indicates the selected subset, and a bitmap, which indicates allocated RBs within the set of RBs belonging to the selected subset. The resource allocation type 1 does not allow to allocate all RBs to the UE, but it allows to allocate one RB, if needed.

7.3.3

Resource allocation type 2

In resource allocations of type 2, the resource block assignment information indicates to a scheduled UE a set of contiguously allocated RBs. Resource allocation type 2 allows to allocate all RBs to UE or one RB only if needed. However it does not allow for full allocation flexibility, because only continuous RBs may be allocated.

7.4

MIMO

Multiple Input Multiple Output (MIMO) refers to the use of multiple antennas at transmitter and receiver side. The concept of multiple transmitting and receiving antennas is extensively used in LTE. MIMO systems form an essential part of LTE in order to achieve the ambitious requirements for throughput and spectral efficiency. For the LTE downlink, a 2x2 configuration for MIMO is assumed as baseline configuration, i.e. two transmit antennas at the base station and two receive antennas at the terminal side. Configurations with four transmit or receive antennas are also foreseen and reflected in specifications. The 3GPP TS 36.211 defines two downlink modes of MIMO, which were described in sections 5.8.8 and 5.8.9: • Spatial multiplexing ◦ without Cyclic Delay Diversity (CDD) – also called closed loop special multiplexing, which requires UE feedback concerning received phase shifts from transmitting antennas, ◦ with CDD – also called open loop special multiplexing, which does not require UE feedback. • Transmit diversity. Depending on the MIMO mode that is used different gains can be achieved, see Figure 7.8: • With use of spatial multiplexing different data streams may be transmitted from antennas resulting in data rate multiplication. • With use of spatial multiplexing modulation symbols from the same layer may be transmitted from several antennas simultaneously. By adjusting the phase 122

7.4 MIMO of the modulation symbols transmitted from different antennas a constructive interference may be achieved in desired direction. This technique is called beamforming and it results in signal strength gain (beamforming gain). • With use of transmit diversity the same data stream may be transmitted twice and in different directions resulting in reduced signal fading.

Figure 7.8: Multi antenna possibilities. In the following sections, a general description of spatial multiplexing and transmit diversity is provided.

7.4.1

Spatial multiplexing

Spatial multiplexing allows for transmission of different data streams simultaneously on the same resource block(s) by exploiting the spatial dimension of the radio channel. These data streams can belong to (see Figure 7.9): • one single user – Single User MIMO (SU-MIMO), which increases the data rate of one user and it may be applied for DL, because UE may have multiple receiving antennas. • different users – Multi User MIMO (MU-MIMO), which allows for increase of the overall capacity and it may be applied in the UL, because UE has only one transmit antenna.

Figure 7.9: SU-MIMO and MU-MIMO. 123

7 Physical layer procedures Figure 7.10 shows the principles of spatial multiplexing. In this figure, antennas TX0 and TX1 transmit different modulation symbols x(0) and x(1) respectively1 . The signal received by antenna RX0 is a sum of signals transmitted by antenna TX0 and TX1. The signal transmitted by antenna TX0 is attenuated by pathloss h00 between antenna TX0 and RX0, while the signal transmitted by antenna TX1 is attenuated by pathless h10 between antenna TX1 and RX0. In the same way, also signal received by antenna RX1 comes from antenna TX0 and TX1, but the transmitted signals are attenuated by path losses h01 and h11 respectively, see equations in Figure 7.10. The path losses are measured by UE thanks to reference signals. It is important that when one of the antennas transmits its reference signals then all others antennas are silent. This lets the UE antennas measure the pathloss to this particular antenna. Because all the path losses are known, as well as the measured signals z(0) and z(1) , the UE may solve the set of equations in Figure 7.10 for x(0) and x(1) . This is how the UE is able to detect signals transmitted simultaneously from two antennas.

Figure 7.10: Spatial multiplexing principles. Let us consider a system with more than two TX antennas. A signal from each TX antenna can be considered as an unknown. Therefore the number of unknowns is equal to the number of TX antennas. On the other hand, for each RX antenna one equation may be created, so the number of equations is equal to the number of receiving antennas. To be able to solve a set of equations the number of equations must be equal to or more than the number of unknowns. Therefore having 4 TX antennas (unknowns) we must also have 4 receiving antennas (equations). Having one or two receiving antennas would not let the UE to detect all 4 transmitted signals. Each antenna may transmit different layer. In spatial multiplexing, the number of layers used for the transmission is equal to the bit rate multiplication. In order to achieve bit rate multiplication of 4, four layers must be transmitted simultaneously, which requires four TX and four RX antennas. rmax = min{number of TX antennas, number of RX antennas}

(7.6)

Spatial multiplexing is only possible if the radio channel allows for it. Depending on the radio channel properties, it may be impossible to transmit 4 independent layers 1

This is one of the spatial multiplexing transmission type. The complex numbers may also be weighted and added so, that each antenna actually transmits a combination of the symbols x(0) and x(1) . This process is called precoding, see section 5.8.9.

124

7.4 MIMO between the transmitter and receiver. In this case the number of layers used for the transmission may be less then rmax . In the DL, the UE estimates the spatial properties of the radio channel by measuring the DL reference symbols from different antenna ports. This estimation is reported to the eNB, so that the eNB can use an appropriate number of layers an make optimal antenna mapping. The report consists of CQI, Precoding Matrix Indicator (PMI) and RI (for details see section 7.5): • CQI indicates the channel quality and is used whether or not spatial multiplexing is used. • RI indicates the number of useful layers and it must be equal to or less then the maximum number of layers. RI ≤ rmax

(7.7)

The maximum number of layers depends on the number of TX and RX antennas. • PMI indicates the precoder matrix that the UE considers as the best (gives the highest estimated Signal to Interference and Noise Ratio (SINR)).

7.4.2

Transmit diversity

Instead of increasing data rate or capacity, MIMO can be used to exploit diversity and increase the robustness of data transmission. Transmit diversity schemes are already known from WCDMA release 99 and are also a part of LTE. Each transmit antenna transmits essentially the same stream of data, so the receiver gets replicas of the same signal, see Figure 7.11. This increases the signal to noise ratio at the receiver side and thus the robustness of data transmission especially in fading scenarios. Typically an additional antenna-specific coding is applied to the signals before transmission to increase the diversity effect. Often, space-time coding is used.

Figure 7.11: Transmit diversity

7.4.3

Transmission modes

Switching between two MIMO transmission schemes of transmit diversity and spatial multiplexing is possible depending on channel conditions as presented in Figure 125

7 Physical layer procedures 7.12.

Figure 7.12: Transmission mode 3: spatial multiplexing with large delay CDD or transmit diversity. In order to support different transmission schemes as well as switching between different transmission schemes, eight transmission modes have been defined by the 3GPP TS 36.213, which are presented in Table 7.2. A transmission mode can use one or more transmission schemes. Typically, the transmission mode is set up at session establishment and does not changed during the session, while the transmission scheme is dynamically decided every TTI.

7.4.4

MIMO antennas

The antennas used for MIMO should be uncorrelated. A suitable way of achieving uncorrelated antenna elements is to use polarisation diversity. A cross-polarisation antenna (XPol) is a common solution for 2x2 MIMO. Two cross-polarised antennas (XXPol) are used for 4x4 MIMO, see Figure 7.13.

Figure 7.13: MIMO antenna solutions. 126

7.4 MIMO

Transmission mode

DCI format

1

1A

2

1 1A

3

1 1A 2A

4

1A 2

5

1A

6

1D 1A 1B

Search space

Transmission scheme of PDSCH

Common and UE specific UE specific Common and UE specific UE specific Common and UE specific UE specific

Single-antenna port, port 0

Common and UE specific UE specific Common and UE specific UE specific Common and UE specific UE specific

7

1A

Common and UE specific

8

1 1A

UE specific Common and UE specific

2B

UE specific

Single-antenna port, port 0 Transmit diversity Transmit diversity Transmit diversity Large delay CDD or Transmit diversity Transmit diversity Closed-loop spatial multiplexing or Transmit diversity Transmit diversity Multi-user MIMO Transmit diversity Closed-loop spatial multiplexing with a single transmission layer If the number of PBCH antenna ports is one, Single-antenna port, port 0; otherwise Transmit diversity Single-antenna port, port 5 If the number of PBCH antenna ports is one, Single-antenna port, port 0; otherwise Transmit diversity Dual layer transmission, port 7 and 8 or Single-antenna port, port 7 or 8

Table 7.2: PDSCH transmission scheme.

127

7 Physical layer procedures

7.5

UE reporting

The UE reporting is used to support optimal radio resource allocation for the downlink transmission towards UE. It means, the UE reporting is used by eNB to select: • Transport Format (TF) and • frequency subbands. The UE report may include indicators presented in Figure 7.14: • Channel Quality Indicator (CQI), which is a measure of DL quality and it is used by eNodeB to choose the optimal modulation and coding rate for downlink transmission. • Rank Indicator (RI), which is the optimal number of layers for the DL transmission for spatial multiplexing. For transmit diversity RI is equal to one. • Precoding Matrix Indicator (PMI), which is used for precoding matrix selection when operating with MIMO.

Figure 7.14: UE reporting. The time and frequency resources that can be used by the UE to report CQI, PMI, and RI are controlled by the eNB. The UE reporting is periodic or aperiodic. • Periodic CQI/PMI, or RI reports are send by UE on PUCCH or PUSCH if it collides in time domain with PUCCH. • Aperiodic CQI/PMI, and RI reports are transmitted by UE on PUSCH if the conditions specified hereafter are met. For aperiodic CQI reporting, RI reporting is transmitted only if configured CQI/PMI/RI feedback type supports RI reporting. Regarding the reported frequency band the CQI reporting is of two kinds: • Frequency non-selective. One CQI value is reported by the UE for the whole frequency band. 128

7.5 UE reporting • Frequency selective. UE provides several CQI values, one for each sub band of the carrier. Frequency selective reporting is used for channel dependent scheduling and it is always aperiodic and transmitted on PUSCH only, see also Table 7.3. Scheduling mode Periodic CQI Aperiodic CQI reporting channels reporting channels Frequency non-selective Frequency selective

PUCCH PUCCH

PUSCH

Table 7.3: Physical Channels for Aperiodic or Periodic CQI reporting. The reporting described in this section is not used for the best cell selection or handover, as different event triggered reporting of Reference Signal Received Power (RSRP) and Reference Signal Received Quality (RSRQ) is specified to support UE mobility.

7.5.1

CQI definition

In Table 7.4 a list of 4-bit CQIs corresponding to the 16 possible combinations of modulation scheme and code rate is shown, which is specified by 3GPP TS 36.213. As can be seen in this table, CQI = 1 refers to the most robust transmission parameters i.e. QPSK as modulation scheme and the lowest code rate of 78 user information symbols for 1024 transmitted symbols, which is selected for the worst channel quality. With increasing channel quality, higher order modulation schemes and higher code rates can be selected. The highest order of modulation and highest code rate, which can be selected are 64QAM and code rate of 948/1024 = 0.93 respectively and correspond to a CQI value of 15. Depending on the SINR a the biggest CQI value is selected, which ensures that the Block Error Rate (BLER) is less than 0.1.

7.5.2

Aperiodic CQI/PMI/RI reporting using PUSCH

A UE performs aperiodic CQI, PMI and RI reporting using the PUSCH upon receiving a DCI format 0 or a Random Access Response Grant, if the respective CQI request field is set to 1 and is not reserved. The possible reporting modes on the PUSCH are presented in Table 7.5. For each of the transmission modes, see Table 7.2, only some of the the reporting modes are supported as specified in Table 7.6. The aperiodic CQI reporting mode, which UE should use is given by the parameter cqi-ReportModeAperiodic. The UE may be configured to report one CQI value for the whole carrier band (so called wideband CQI) or to divide the whole carrier band into several sub bands and provide one CQI value for each band (subband reporting). The subband reporting may be of two different kinds: 129

7 Physical layer procedures

CQI index

Modulation

Code rate x 1024 out of range 78 120 193 308 449 602

Efficiency (information bits (per symbol)

0 1 2 3 4 5 6

QPSK QPSK QPSK QPSK QPSK QPSK

7 8 9

16QAM 16QAM 16QAM

378 490 616

1.4766 1.9141 2.4063

10 11 12 13 14 15

64QAM 64QAM 64QAM 64QAM 64QAM 64QAM

466 567 666 772 873 948

2.7305 3.3223 3.9023 4.5234 5.1152 5.5547

0.1523 0.2344 0.3770 0.6016 0.8770 1.1758

Table 7.4: 4-bit CQI Table.

PMI Feedback Type No PMI Single PMI Multiple PMI PUSCH CQI Feedback Type

Wideband (wideband CQI) UE Selected (subband CQI) Higher Layer configured (subband CQI)

Mode 1-2 Mode 2-0 Mode 3-0

Mode 2-2 Mode 3-1

Table 7.5: CQI and PMI Feedback Types for PUSCH reporting modes.

130

7.5 UE reporting Transmission mode

Reporting mode

1 2 3 4 5 6 7 8

2-0, 3-0 2-0, 3-0 2-0, 3-0 1-2, 2-2, 3-1 3-1 1-2, 2-2, 3-1 2-0, 3-0 1-1, 2-2, 3-1 if the UE is configured with PMI/RI reporting 2-0, 3-0 if the UE is configured without PMI/RI reporting

Table 7.6: PUSCH reporting modes for different transmission modes. • Higher layer-configured subband feedback, • UE selected subband feedback. In both cases a wideband average is computed and used as a reference. In addition, M subbands (M could be fixed or configured) of size k (see Table 7.7) are selected and encoded differentially using two bits relative to the wide-band average. In the case UE selected subband feedback the UE selects M subbands to report. The UE internal procedure to select subbands is not specified but the selected subbands should correspond to the highest CQI values. The subbands selected by UE are N )⌉ bits. signalled using L = ⌈log2 ( M System Bandwidth DL NRB

Higher layer-configured Subband size k [RB]

6-7 8-10 11-26 27-63 64-110

N/A 4 4 6 8

UE-selected Subband size k M [RB] N/A 2 2 3 4

N/A 1 3 5 6

Table 7.7: Subband size (k) vs. System Bandwidth.

7.5.3

Periodic CQI/PMI/RI reporting using PUCCH

A UE is semi-statically configured by higher layers to periodically feed back different CQI, PMI, and RI on the PUCCH using the reporting modes given in Table 7.8 and described below. 131

7 Physical layer procedures PMI Feedback Type No PMI Single PMI PUCCH CQI Feedback Type

Wideband (wideband CQI) UE Selected (subband CQI)

Mode 1-0

Mode 1-1

Mode 2-0

Mode 2-1

Table 7.8: CQI and PMI Feedback Types for PUCCH reporting modes. For each of the transmission modes defined in Table 7.2, the reporting modes specified in Table 7.9 are supported on PUCCH. The periodic CQI reporting mode is given by the parameter cqi-FormatIndicatorPeriodic, which is configured by higherlayer signalling. The periodicity of the QCI/PMI reporting is defined by the parameter cqi-PUCCHResourceIndex (TS 36.331) and can be set between 2 ms to 160 ms for FDD (TS 36.213). The periodicity of RI reporting is set by the parameter ri-ConfigIndex and can be set between 1 to 32 ms. Transmission mode

Reporting mode

1 2 3 4 5 6 7 8

1-0, 2-0 1-0, 2-0 1-0, 2-0 1-1, 2-1 1-1, 2-1 1-1, 2-1 1-0, 2-0 1-1, 2-1 if the UE is configured with PMI/RI reporting 1-0, 2-0 if the UE is configured without PMI/RI reporting

Table 7.9: PUCCH reporting modes for different transmission modes.

7.6

Modulation order and transport block size determination

The DL Modulation and Coding Scheme (MCS) is selected by eNB. The eNB decision may be based on CQI feedback and buffer content. The eNB algorithm used for modulation and transport block size determination is often referred to as LA. Rapid interference variations make it difficult to predict the link quality accurately, and select MCS based on such knowledge. Therefore, decision which MCS to used 132

7.6 Modulation order and transport block size determination is based on averaged link quality and next adjusted depending if the objective is to provide high throughput or low delay: • If the objective is to provide low delay (few retransmissions), a margin to the interference variations can be included. This however leads to limited throughput, as often an unnecessary robust MCS is used. • To reach high throughput, a low margin (even negative) is used. This will instead lead to a larger number of retransmissions, and hence a larger delay. The risk of throughput loss or large delays in case of negative margins is reduced by the use of incremental redundancy for retransmissions.

7.6.1

Modulation determination

The eNB decision, which modulation and coding is used for the PDSCH, is communicated to the UE by the 5-bit“modulation and coding scheme” field IMCS in the DCI presented in Table 7.10.

7.6.2

Transport block size determination

The Transport Blok Size (TBS), that is the number user bits in the transport block, is determined depending on the value of IMCS in the following way: • For 0 ≤ IMCS ≤ 28 the UE determines the TBS index ITBS using Table 7.10. ◦ For transport blocks not mapped to two-layer spatial multiplexing, the TBS is given by the (ITBS , NPRB ) entry of Table 7.11. ◦ For transport block mapped to two-layer spatial multiplexing: for 1 ≤ NPRB ≤ 55, the TBS is given by the (ITBS , 2NPRB ) entry of Table 7.11. It means that the transport block is twice as much as in case of one-layer spatial multiplexing, transmit diversity or no MIMO. for 56 ≤ NPRB ≤ 110 there is different way of deriving the TBS. It results in the TBS a little less then twice as much as in case of one-layer spatial multiplexing, transmit diversity or no MIMO. • For 29 ≤ IMCS ≤ 31, the TBS is assumed to be as determined from DCI transported in the latest PDCCH for the same transport block using 0 ≤ IMCS ≤ 28. For example, if the IMCS = 28 and NPRB = 100 then from Table 7.10 the ITBS = 26 and from Table 7.11 the TBS = 75376 bits. It means that the transport block will be sent over 100 RBs and will contains 75376 user data bits. Taking into account that the transport block transmission time is 1 ms, the momentary MAC layer user bit throughput will be 75376 ≃ 75 Mbps and it is the maximum possible throughput 1 ms per one layer in LTE. In two layer spatial multiplexing the throughput will be twice bigger, that is 300 Mbps.

133

7 Physical layer procedures

MSC index IMCS

Modulation order Qm

TBS index ITBS

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

2 2 2 2 2 2 2 2 2 2 4 4 4 4 4 4 4 6 6 6 6 6 6 6 6 6 6 6 6 2 4 6

0 1 2 3 4 5 6 7 8 9 9 10 11 12 13 14 15 15 16 17 18 19 20 21 22 23 24 25 26 reserved

Table 7.10: Modulation and TBS index table for PDSCH.

134

7.6 Modulation order and transport block size determination

ITBS 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

1

2

3

4

5

NPRB 6

16 24 32 40 56 72 328 104 120 136 144 176 208 224 256 280 328 336 376 408 440 488 520 552 584 616 712

32 56 72 104 120 144 176 224 256 296 328 376 440 488 552 600 632 696 776 840 904 1000 1064 1128 1192 1256 1480

56 88 144 176 208 224 256 328 392 456 504 584 680 744 840 904 968 1064 1160 1288 1384 1480 1608 1736 1800 1864 2216

88 144 176 208 256 328 392 472 536 616 680 776 904 1000 1128 1224 1288 1416 1544 1736 1864 1992 2152 2280 2408 2536 2984

120 176 208 256 328 424 504 584 680 776 872 1000 1128 1256 1416 1544 1608 1800 1992 2152 2344 2472 2664 2856 2984 3112 3752

152 208 256 328 408 504 600 712 808 936 1032 1192 1352 1544 1736 1800 1928 2152 2344 2600 2792 2984 3240 3496 3624 3752 4392

7

98

99

100

176 224 296 392 488 600 712 840 968 1096 1224 1384 1608 1800 1992 2152 2280 2536 2792 2984 3240 3496 3752 4008 4264 4392 5160

2728 3624 4392 5736 6968 8760 10296 11832 13536 15264 16992 19848 22152 25456 28336 30576 31704 35160 39232 42368 45352 48936 52752 57336 59256 61664 73712

2728 3624 4392 5736 6968 8760 10296 12216 14112 15840 17568 19848 22920 25456 28336 30576 31704 35160 39232 42368 46888 48936 52752 57336 61664 63776 73712

2792 3624 4584 5736 7224 8760 10296 12216 14112 15840 17568 19848 22920 25456 28336 30576 32856 36696 39232 43816 46888 51024 55056 57336 61664 63776 75376

Table 7.11: Transport block size table.

135

7 Physical layer procedures

7.7

UL power control

The 3GPP TS 36.213 specifies algorithms of power control on PUCCH and PUSCH. Both algorithms are similar. The standard specifies open loop and closed loop power control algorithms for PUCCH and PUSCH: • In open loop power control the UE calculates the output power based on downlink measurements and controlling parameters sent by eNB, see Figure 7.15.

Figure 7.15: Open loop power control. • In closed loop power control additional correction of the open loop power control algorithm is provided to the UE. The correction provided by eNB indicates if the UE should increase or decrease its transmit power compared to the open loop algorithm, see Figure 7.16.

Figure 7.16: Closed loop power control.

7.7.1

PUSCH power control

The UE calculates its output, which will transmit in a subframe i, on the bases of the below formula. The formula is common for open loop and closed loop power control. The difference is, that in closed loop power control the eNB provides the 136

7.7 UL power control UE with a Transmit Power Control (TPC) command that includes δPUSCH . The δPUSCH is used to calculate the closed loop power adjustment f (i): PPUSCH (i) = min{PCMAX , (7.8) 10 log10 (MPUSCH (i)) + P0 PUSCH (j) + α(j) · P L + ∆TF (i) + f (i)} [dBm] where, • PCMAX is the maximum allowed power by the terminal and depends on the UE power class, • MPUSCH (i) is the bandwidth of the PUSCH resource assignment expressed in number of resource blocks valid for subframe i. • P0 PUSCH (j) is the parameter composed of the sum of a cell specific nominal component P0 NOMINAL PUSCH (j) sent in the SIB2 for j = 0 and 1 and a UE specific component P0 UE PUSCH (j) sent in dedicated signalling for layers for j = 0 and 1. ◦ For PUSCH (re)transmissions corresponding to a semi-persistent grant then j = 0. ◦ For PUSCH (re)transmissions corresponding to a dynamic scheduled grant then j = 1. ◦ For PUSCH (re)transmissions corresponding to the random access response grant then j = 2 and: P0 UE PUSCH (2) = 0 P0 NOMINAL PUSCH (2) = P0 PRE + ∆P REAM BLE

M sg3 ,

(7.9)

where the parameter PREAMBLE INITIAL RECEIVED TARGET POWER (P0 PRE ) and ∆P REAM BLE M sg3 are signalled from higher layers. • α is the pathloss compensation factor. For j = 0 or 1, α ∈ {0, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1} is a 3-bit cell specific parameter sent in SIB2. For j = 2, α = 0. • P L is the downlink pathloss estimate calculated in the UE in dB and P L = ref erenceSignalP ower − higher layer filtered RSRP, where ref erenceSignalP ower is transmitted in SIB2. • ∆FT (i) is the Transport Format dependent compensation offset. The value of the offset depends on the UE specific parameter deltaM CS-Enabled provided by higher layers. • f (i) is the current closed loop PUSCH power control adjustment. There are two methods of the power control adjustments: accumulated and absolute. User specific parameter Accumulation-enabled informs the UE which one to use. In both cases the power control adjustment f (i) depends on the UE specific correction value δPUSCH , also referred to as TPC command. The TPC command is transmitted on PDCCH and is used to calculate the power control 137

7 Physical layer procedures adjustment differently for accumulated and absolute method: f (i) = f (i − 1) + δPUSCH (i − KP U SCH ) f (i) = δPUSCH (i − KP U SCH )

for accumulated for absolute

(7.10)

◦ KP U SCH is equal 4 for FDD and for TDD is equal 4, 6 or 7 depending on the UL/DL TDD configuration. ◦ δPUSCH is the TPC command given in dB. δPUSCH values are signalled on the PDCCH with DCI format 0 or jointly coded with other TPC commands in PDCCH with DCI format 3/3A whose CRC parity bits are scrambled with TPC-PUSCH-RNTI. The δPUSCH values signalled on the PDCCH with DCI format 0 are given in Table 7.12. The δPUSCH values signalled on the PDCCH with 3/3A are one of SET1 given in Table 7.12 or SET2 given in Table 7.13 as determined by the parameter TCP-Index provided by higher layers. TCP Command Field in DCI format 0/3

Accumulated δPUSCH [dB]

Absolute δPUSCH [dB] only DCI format 0

0 1 2 3

-1 0 1 3

-4 -1 1 4

Table 7.12: Mapping of TPC Command Field in DCI format 0/3 to absolute and accumulated δPUSCH values. TCP Command Field in DCI format 3A

Accumulated δPUSCH

0 1

-1 1

Table 7.13: Mapping of TPC Command Field in DCI format 3A to accumulated δPUSCH values. In the accumulated mode: • If UE has reached maximum power, positive TPC commands are not accumulated. • If UE has reached minimum power, negative TPC commands shall not accumulated. • UE reset accumulation: ◦ when P0 UE PUSCH value is changed by higher layers. ◦ when UE receives random access response. In the absolute mode: • f (i) = f (i−1) for a subframe where no PDCCH with DCI format 0 is decoded. 138

7.7 UL power control For both accumulated and absolute method the first value is set as follows: f (0) = 0 f (0) = ∆Prampup + δmsg2

if P0 UE PUSCH value is changed other cases

(7.11)

• δmsg2 is the TPC command indicated in the random access response, • ∆rampup is provided by higher layers and corresponds to the total power rampup from the first to the last preamble. Figures 7.17 and 7.18 present accumulated and absolute closed loop power control adjustment.

Figure 7.17: Accumulated method of the closed loop power control adjustment.

Figure 7.18: Absolute method of the closed loop power control adjustment.

7.7.2

PUSCH power control example

Figure 7.19 illustrates the open loop power control for the path loss compensation parameter α = 1 (full path loss compensation). For α = 1 then drop of the RSRP by 10 dB, results in increase of the UE transmit power by 10 dB. This UE power increase fully compensate the path loss increase and leads to constant P SDRX at 139

7 Physical layer procedures the eNB in accordance to the setting of the parameter P0 PUSCH = −109 dBm. But for far enough distance UE reaches its maximum transmit power and it cannot anymore compensate the path loss increase. In this example it happens for the RSRP equal to −117 dBm. Therefore, when the UE moves further from the eNB and RSRP drops below −117 dBm, the P SDRX at the eNB drops below the design threshold P0 PUSCH = −109 dBm resulting in throughput reduction. It should also be noted, that, as long as the P SDRX at the eNB is equal to the design threshold P0 PUSCH = −109, the UE UL throughput is constant regardless of the UE location in a cell.

Figure 7.19: Transmitted power and signal at eNB as a function of the RSRP for the following parameters setting: PCMAX = 23 dBm, MPUSCH = 1, P0 PUSCH = −109 dBm, α = 1, ref erenceSignalP ower = 15 dBm.

Figure 7.20 shows dependence of P SDRX and the TBS on the number of allocated RBs to the UE. For low number of allocated RBs the UE is able to keep required target P SDRX in accordance with the parameter P0 PUSCH = −109 dBm. To do so, the UE must transmit more power when more RBs are allocated to it. Therefore the transmitted power of the UE grows linearly with the number of allocated RB. Accordance to the power control algorithm, UE transmits the same power for each RB. Because the target P SDRX is achieved the same coding and modulation is used and the TBS (that is also throughput) grows linearly with the number of allocated RBs. At some number of RBs the maximum power of the UE is achieved and the UE cannot further increase its power. Therefore the output power of the UE is distributed evenly between the transmitted RBs leading to the power per RB below the target P SDRX . To handle lower signal-to-interference ratio at the eNB, the eNB’s link 140

7.7 UL power control adaptation algorithm decides about more robust coding or modulation to be used by UE for the UL transmission. TBS still grows due to more RBs allocated, but due to more robust coding and modulation the grow is less then linear.

Figure 7.20: The target P SDRX and the TBS.

P0 PUSCH In this example the parameter P0 PUSCH = −109 dBm. This section show the process of the parameters calculation. The throughput depends on the bandwidth, which is used for the signal transmission, and SINR. Stronger the signal above noise and interference level, bigger the throughput. This theoretical relation is know as Snannon theorem: T hroughput = B log2 (1 + SINR),

(7.12)

where B is the bandwidth used for the transition. Let us assume that the wanted UL SINR, which provides satisfactory throughput, is SINRUL = −2 dB and the expected UL noise rise due to UEs transmitting in neighbouring cells is Imarg,UL = 12 dB. With these assumptions, and also assuming a typical noise figure of the eNB, the minimum value of P0 PUSCH can be calculated as presented in Table 7.14. 141

7 Physical layer procedures No.

Element

1 2 3 4 5

Value

6

Boltzmann constant, k Temperature, T Thermal noise power density, kT Bandwidth, BRB eNB noise figure, Nf ) ( ·BRB Thermal noise, NRB,UL = 10 log kT1 mW + Nf

7 8 9

Interference margin, Imarg,UL SINRUL SeNB = NRB,UL + Imarg,UL + SINRUL

10

P0 PUSCH ≥ SeNB

1.3806·10−23 J/K 290 K 4.00 · 10−21 J 180000 Hz 5 dB -119.4 dBm 12 dB -2 dB -109.4 dBm -109 dBm

Table 7.14: P0 PUSCH calculation.

ref erenceSignalP ower In this example the parameter and the ref erenceSignalP ower = 15 dBm. This sections shows the process of the parameter calculation. No.

Element

Value

1 2 3

eNB transmit power, PeNB Number of RBs in the carrier bandwidth, NRB Antenna feeder loss, Lf

40 W 50 RB 3 dB

4

RB ) ref erenceSignalP ower = 10 log PeNB1/(12N − Lf mW

Table 7.15: ref erenceSignalP ower calculation.

142

15 dBm

8 LTE mobility This chapter describes UE mobility in LTE with a focus on the algorithms, that are used to choose the best cell to serve the UE. In the RRC IDLE the cell selection algorithm S and cell reselection algorithm R are used by the UE to choose a cell. Also the PLMN selection algorithm is presented in this chapter. In RRC CONNECTED the eNB chooses a cell, but its decision is supported by the UE measurements. The UE measurement reports are triggered by events. For example, the UE may send a measurement report when it finds a neighbouring cell that is better than serving. This event may be used by the eNB to trigger a handover to the reported better neighbouring cell. This chapter also presents a flow graph of the handover process.

8.1

Idle mode mobility

In idle mode (RRC IDLE) the UE has no connection to the radio network, i.e. no RRC connection is established. The purpose of keeping UE in idle mode is to minimise the resource usage both for the UE and for the network. Yet the UE should still be able to access the system and be reached by the system with acceptable delays. In idle mode the UE: • Monitors system information, that system and cell specific parameters transmitted to all UEs in a cell. • Selects the PLMN. • Selects a suitable cell of the selected PLMN to camp on by using the cell selection algorithm. • After the cell selection the UE attaches and registers to the CN supported by the PLMN. This process is called location registration. • Performs cell reselection based on radio measurements. Cell reselection makes sure that the UE is always camping on the cell that gives the highest probability for successful establishment of a connection. The cell reselection process may imply a change of the RAT i.e. (GSM/ GPRS/WCDMA/CDMA2000 LTE). • Monitors paging. 143

8 LTE mobility • May initiate a connection by sending random access. Figure 8.1 illustrates relation between PLMN selection, cell selection and reselection and location registration according to the TS 36.304.

Figure 8.1: Overall idle mode process.

8.1.1

PLMN selection

The PLMN selection process aims at finding an operator, where the UE can find a suitable cell and access available services. The PLMN selection process is described in TS 22.011. The following concepts are use in the PLMN selection process: • PLMN selector lists. • Equivalent HPLMN (EHPLMN). • Forbidden TAs or LAs lists. The above concepts are explained in the next sections and next the actual PLMN selection algorithm is described.

PLMN selector lists There are two PLMN priority lists stored on the Universal Subscriber Identity Module (USIM): • Operator Controlled PLMN Selector list. • User Controlled PLMN Selector list. Both PLMN selector lists may contain a list of preferred PLMNs in priority order. It shall be possible to have an associated access technology identifier e.g., E-UTRAN, 144

8.1 Idle mode mobility UTRAN, or GERAN associated with each entry in the PLMN selector lists. A PLMN in a selector list may have multiple occurrences with different access technology identifiers. The UE ignores those PLMN + access technology entries in the PLMN selector lists where the associated access technology is not supported by the UE.

EHPLMN It shall be possible to handle cases where one network operator accepts access from access networks with different network IDs. It shall also be possible to indicate to the UE that a group of PLMNs are equivalent to the registered PLMN regarding PLMN selection, cell selection/reselection and handover. It shall be possible for the home network operator to identify alternative network IDs as the Home PLMN (HPLMN). It shall be possible for the home network operator to store in the USIM an indication to the UE that a group of PLMNs are treated as the HPLMN regarding PLMN selection. Any PLMN to be declared as an equivalent to the HPLMN shall be present within the EHPLMN list and is called an EHPLMN. The EHPLMN list replaces the HPLMN derived from the International Mobile Subscriber Identity (IMSI). When the EHPLMN list is present, any PLMN in this list shall be treated as the HPLMN in all the network and cell selection procedures.

Forbidden TAs and LAs for roaming When a registration attempt by the UE is rejected by a network, the UE stores the tracking area identity or the location area identity in the list of “forbidden TAs or LAs for roaming” respectively. The lists of forbidden TAs and LAs are maintained in the UE to avoid unnecessary registration attempts.

PLMN selection algorithm Depending on the user setting the UE follows one of the following procedures for network selection: • Automatic network selection mode. The default behaviour for a UE is to select the last registered PLMN. As an alternative option to this, if the UE is in automatic network selection mode and it finds coverage of the HPLMN or any EHPLMN, the UE may register on the HPLMN (if the EHPLMN list is not present) or the highest priority EHPLMN of the available EHPLMNs (if the EHPLMN list is present) and not return to the last registered PLMN. If the EHPLMN list is present and not empty, it shall be used. The operator shall be able to control by USIM configuration whether an UE that supports this option shall follow this alternative behaviour. The UE selects and attempts registration on other PLMNs, if available and allowable, if the location area is not in the list of “forbidden LAs for roaming” and the tracking area is not in the list of “forbidden TAs for roaming”, in the following order, which is also illustrated in Figure 8.2: 145

8 LTE mobility 1. An EHPLMN, if the EHPLMN list is present, or the HPLMN (derived from the IMSI), if the EHPLMN list is not present for preferred access technologies, in the order specified. In the case that there are multiple EHPLMNs present then the highest priority EHPLMN shall be selected. 2. Each entry in the User Controlled PLMN Selector list with access technology data field in the SIM/USIM (in priority order). 3. Each entry in the Operator Controlled PLMN Selector list with access technology data field in the SIM/USIM (in priority order). 4. Other PLMN/access technology combinations with sufficient received signal quality in random order. A PLMN is considered to have sufficient received signal quality if: ◦ for LTE cell: RSRP ≥ −110 dBm (TS 36.304), ◦ for WCDMA FDD cell: RSCP ≥ −95 dBm (TS 25.304), ◦ for WCDMA TDD cell: RSCP ≥ −84 dBm (TS 25.304), ◦ for GSM cell: rxlev > −85 dBm (TS 43.022). 5. All other PLMN/access technology combinations in order of decreasing signal quality. • Manual network selection mode. 1. A registered PLMN is selected if available. 2. A list of available PLMNs is presented to a user and the user selects one of the PLMNs manually. If the registration cannot be achieved on the selected PLMN, the UE shall indicate “No Service”. The user may then select and attempt to register on another or the same PLMN. Once the UE has selected a PLMN, the cell selection procedure shall be performed in order to select a suitable cell of that PLMN to camp on.

8.1.2

Cell selection

After a UE has switched on and a PLMN has been selected, the cell selection process takes place. This process allows the UE to select a suitable cell where to camp on in order to access available services. In this process the UE can use stored information (stored information cell selection) or not (initial cell selection).

Description To select a cell the UE uses one of the following two cell selection procedures: 1. Initial cell selection. This procedure requires no prior knowledge of which RF channels are Evolved Universal Terrestrial Radio Access (E-UTRA) carriers. The UE shall scan all RF channels in the E-UTRA bands according to its capabilities to find a suitable cell. On each carrier frequency, the UE need only search for the strongest cell. Once a suitable cell is found this cell shall be selected. 146

8.1 Idle mode mobility

Figure 8.2: Automatic PLMN selection process.

147

8 LTE mobility 2. Stored information cell selection. This procedure requires stored information of carrier frequencies and optionally also information on cell parameters, from previously received measurement control information elements or from previously detected cells. Once the UE has found a suitable cell the UE shall select it. If no suitable cell is found the initial cell selection procedure shall be started. NOTE: Priorities between different frequencies or RATs provided to the UE by system information or dedicated signalling are not used in the cell selection process.

Cell selection criterion The cell selection criterion S is fulfilled when: S>0

(8.1)

where S = Qmeas,s − (q-RxLevMin + q-RxLevMinOffset) − Pcompensation

(8.2)

where S–

cell selection level value [dB].

Qmeas,s –

measured cell RX level value RSRP [dBm].

q-RxLevMin –

minimum required RX level [dBm] in the cell sent in SIB1 (Table A.2).

q-RxLevMinOffset – offset [dB] to the signalled q-RxLevMin taken into account in the Srxlev evaluation as a result of a periodic search for a higher priority PLMN while camped normally in a VPLMN. Sent in SIB1 (Table A.2). Pcompensation =

max(p-Max − PPowerClass , 0) [dB].

p-Max –

maximum TX power level [dBm] an UE may use when transmitting on the uplink in the cell. The parameter is sent in SIB1 (Table A.2).

PPowerClass –

maximum RF output power of the UE according to the UE power class [dBm].

8.1.3

Cell reselection

Following rules are used by the UE to limit needed measurements: • If the cell parameter S-IntraSearch is sent in the SIB3 of the serving cell and Ss > S-IntraSearch, UE may choose to not perform intra-frequency measurements. • If Ss ≤ S-IntraSearch, or S-IntraSearch is not sent in the serving cell UE shall perform intra-frequency measurements. The Ss is the S value of the serving cell as specified by formula 8.2. 148

8.1 Idle mode mobility

Cell reselection criteria The reselection criterion discussed in this section applies for: • Intra-frequency cell re-election. • Equal priority inter-frequency cell reselection. All cells that fulfill the cell selection criterion S (formula 8.1) are ranked according to the R criteria specified as follows: Rs = Qmesa,s + q-Hyst Rn = Qmesa,n − Qoffset

(8.3)

Rs –

ranking criteria for serving cell [dBm].

Rn –

ranking criteria for neighbouring cell [dBm].

Qmeas,s –

averaged measured RSRP value for serving cell [dBm].

Qmeas,n –

averaged measured RSRP value for neighbouring cell [dBm].

q-Hyst –

cell reselection hysteresis parameter [dB] broadcast in the SIB3 of the serving cell (Table A.4). { q-OffsetCells,n q-OffsetFreq + q-OffsetCells,n

Qoffset =

for intra LTE frequency neighbour for inter LTE frequency neighbour (8.4)

q-OffsetCells,n –

neighbour relation specific offset [dB] sent in SIB4 for intra LTE frequency neighbouring cells (Table A.5) and in SIB5 for inter LTE frequency neighbouring cells (Table A.6).

q-OffsetFreq –

frequency specific offset [dB] for equal priority E-UTRAN frequencies sent in SIB5 (Table A.6).

The UE reselects the new cell, if the cell reselection criteria are fulfilled during the time interval t-ReselectionEUTRA, which is illustrated in Figure 8.3.

Mobility states of UE Besides normal mobility state a high mobility state and a medium mobility state are applicable. Reduced value of q-Hyst and t-ReselectionEUTRA are applied for UE in high or medium mobility state, which result in earlier reselections compared to the normal mobility state: • High mobility state. UE enters high mobility state if number of cell reselections during time period t-Evaluation exceeds n-CellChangeHigh. Hysteresis and reselection time for high mobility state: q-Hyst + q-HystSF(sf-High) t-ReselectionEUTRA · t-ReselectionEUTRA-SF(sf-High) 149

(8.5)

8 LTE mobility

Figure 8.3: Cell reselection criterion. • Medium mobility state. UE enters medium mobility stets if number of cell reselections during time period t-Evaluation exceeds n-CellChangeMedium and not exceeds n-CellChangeHigh. Hysteresis and reselection time for medium mobility state: q-Hyst + q-HystSF(sf-Medium) t-ReselectionEUTRA · t-ReselectionEUTRA-SF(sf-Medium)

(8.6)

q-HystSF and t-ReselectionEUTRA-SF are transmitted in the SIB3 of the serving cell (see Table A.4).

8.2

Connected mode mobility

In RRC CONNECTED, the eNB controls UE mobility, i.e. the eNB decides when the UE shall move to which cell (which may be on another frequency or RAT). For network controlled mobility in RRC CONNECTED, handover is the only procedure that is defined. The eNB triggers the handover procedure e.g. based on radio conditions and load. There are two cases of EPS handovers: • X2 handover. The HO procedure is performed without EPC involvement, i.e. preparation messages are directly exchanged between the eNBs. The release of the resources at the source side during the HO completion phase is triggered by the eNB. • S1 handover. 150

8.2 Connected mode mobility The HO procedure is performed with MME involvement. The MME and S-GW may be reallocated.

8.2.1

X2 handover

The Figure 8.4 depicts the basic handover scenario where neither MME nor S-GW changes (TS 36.300):

Figure 8.4: X2 handover. 1. To facilitate the handover decision the source eNB configures the UE to perform measurement reporting. 2. UE is triggered to send MEASUREMENT REPORT by the rules set by i.e. system information, specification etc. 3. Source eNB makes decision based on MEASUREMENT REPORT and RRM information to hand off UE. The network may also initiate handover blindly, i.e. without having received measurement reports from the UE. 151

8 LTE mobility 4. The source eNB issues a HANDOVER REQUEST message to the target eNB passing necessary information to prepare the HO at the target side (UE X2 signalling context reference at source eNB, UE S1 EPC signalling context reference, target cell ID, KeNB∗ , RRC context including the C-RNTI of the UE in the source eNB, AS-configuration, E-RAB context and physical layer ID of the source cell + MAC for possible Radio Link Failure (RLF) recovery). UE X2/UE S1 signalling references enable the target eNB to address the source eNB and the EPC. The E-RAB context includes necessary Radio Network Layer (RNL) and Transport Network Layer (TNL) addressing information, and QoS profiles of the E-RABs. 5. Admission Control may be performed by the target eNB depending on the received E-RAB QoS information. The target eNB configures the required resources according to the received E-RAB QoS information and reserves a C-RNTI and optionally a RACH preamble. 6. Target eNB prepares HO with L1/L2 and sends the HANDOVER REQUEST ACKNOWLEDGE to the source eNB. The HANDOVER REQUEST ACKNOWLEDGE message includes a transparent container to be sent to the UE as an RRC message to perform the handover. The container includes a new C-RNTI, target eNB security algorithm identifiers for the selected security algorithms, may include a dedicated RACH preamble, and possibly some other parameters i.e. access parameters, SIBs, etc. The HANDOVER REQUEST ACKNOWLEDGE message may also include RNL/TNL information for the forwarding tunnels, if necessary. NOTE: As soon as the source eNB receives the HANDOVER REQUEST ACKNOWLEDGE, or as soon as the transmission of the handover command is initiated in the downlink, data forwarding may be initiated. 7. The target eNB generates the RRC message to perform the handover, i.e RRCConnectionReconfiguration message including the mobilityControlInformation, to be sent by the source eNB towards the UE. The UE does not need to delay the handover execution for delivering the HARQ/ARQ responses to source eNB. 8. The source eNB sends the SN STATUS TRANSFER message to the target eNB to convey the uplink PDCP Sequence Number (SN) receiver status and the downlink PDCP SN transmitter status of E-RABs for which PDCP status preservation applies (i.e. for RLC AM). The uplink PDCP SN receiver status includes at least the PDCP SN of the first missing UL SDU and may include a bit map of the receive status of the out of sequence UL SDUs that the UE needs to retransmit in the target cell, if there are any such SDUs. The downlink PDCP SN transmitter status indicates the next PDCP SN that the target eNB shall assign to new SDUs, not having a PDCP SN yet. The source eNB may omit sending this message if none of the E-RABs of the UE shall be treated with PDCP status preservation. 9. After receiving the RRCConnectionReconfiguration message including the mobilityControlInformation, UE performs synchronisation to target eNB and accesses the target cell via RACH, following a contention-free procedure if a dedicated RACH preamble was indicated in the mobilityControlInformation, 152

8.2 Connected mode mobility or following a contention-based procedure if no dedicated preamble was indicated. UE derives target eNB specific keys and configures the selected security algorithms to be used in the target cell. 10. The target eNB responds with UL allocation and timing advance. 11. When the UE has successfully accessed the target cell, the UE sends the RRCConnectionReconfigurationComplete message (C-RNTI) to confirm the handover, along with an uplink Buffer Status Report, whenever possible, to the target eNB to indicate that the handover procedure is completed for the UE. The target eNB verifies the C-RNTI sent in the RRCConnectionReconfigurationComplete message. The target eNB can now begin sending data to the UE. 12. The target eNB sends a PATH SWITCH message to MME to inform that the UE has changed cell. Observe, that so far the handover process was carried out without interaction with MME and S-GW. 13. The MME sends an UPDATE USER PLANE REQUEST message to the S-GW. 14. The S-GW switches the downlink data path to the target side. The S-GW sends one or more “end marker” packets on the old path to the source eNB and then can release any UP/TNL resources towards the source eNB. 15. Serving Gateway sends an UPDATE USER PLANE RESPONSE message to MME. 16. The MME confirms the PATH SWITCH message with the PATH SWITCH ACKNOWLEDGE message. 17. By sending UE CONTEXT RELEASE, the target eNB informs success of HO to source eNB. 18. Upon reception of the UE CONTEXT RELEASE message, the source eNB can release radio and CP related resources associated to the UE context. Any ongoing data forwarding may continue.

8.2.2

Event triggered reporting

The UE reports measurement information in accordance with the measurement configuration as provided by the eNB. eNB provides the measurement configuration applicable for a UE in RRC CONNECTED by means of dedicated signalling, i.e. using the RRCConnectionReconfiguration message, which is step 1 in Figure 8.4. The UE can be requested to perform the following types of measurements (TS 36.331): • Intra frequency measurements: measurements at the downlink carrier frequency of the serving cell. • Inter frequency measurements: measurements at frequencies that differ from the downlink carrier frequency of the serving cell. • Inter RAT measurements of UTRAN frequencies. 153

8 LTE mobility • Inter RAT measurements of GERAN frequencies. • Inter RAT measurements of CDMA2000 High Rate Packet Data (HRPD) or CDMA2000 1x Radio Transmission Technology (1xRTT) frequencies. The measurement configuration includes the following parameters (TS 36.331): 1. Measurement objects: The objects on which the UE shall perform the measurements. • For intra frequency and inter frequency measurements a measurement object is a single E-UTRAN carrier frequency. Associated with this carrier frequency, E-UTRAN can configure a list of cell specific offsets and a list of “blacklisted” cells. Blacklisted cells are not considered in event evaluation or measurement reporting. • For inter RAT UTRAN measurements a measurement object is a set of cells on a single UTRAN carrier frequency. • For inter RAT GERAN measurements a measurement object is a set of GERAN carrier frequencies. • For inter RAT CDMA2000 measurements a measurement object is a set of cells on a single (HRPD or 1xRTT) carrier frequency. 2. Reporting configurations: A list of reporting configurations where each reporting configuration consists of the following: • Reporting criterion: The criterion that triggers the UE to send a measurement report. This can either be periodical or a single event description. • Reporting format: The quantities that the UE includes in the measurement report and associated information (e.g. number of cells to report). 3. Measurement identities: A list of measurement identities where each measurement identity links one measurement object with one reporting configuration. By configuring multiple measurement identities it is possible to link more than one measurement object to the same reporting configuration, as well as to link more than one reporting configuration to the same measurement object. The measurement identity is used as a reference number in the measurement report. 4. Quantity configurations: One quantity configuration is configured per RAT type. The quantity configuration defines the measurement quantities and associated filtering used for all event evaluation and related reporting of that measurement type. One filter can be configured per measurement quantity. 5. Measurement gaps: Periods that the UE may use to perform measurements, i.e. no (UL, DL) transmissions are scheduled. The reporting criterion, which is a part of reporting configuration, can either be periodical or a single event. The following events are specified for reporting: A1: Serving becomes better than threshold. A2: Serving becomes worse than threshold. A3: Neighbour becomes offset better than serving. 154

8.2 Connected mode mobility A4: Neighbour becomes better than threshold. A5: Serving becomes worse than threshold1 and neighbour becomes better than threshold2. B1: Inter RAT neighbour becomes better than threshold. B2: Serving becomes worse than threshold1 and inter RAT neighbour becomes better than threshold2. An example of measurement configuration is presented in Figure 8.5.

Figure 8.5: Measurement configuration.

155

8 LTE mobility

8.2.3

A3 event

To illustrate the event triggered reporting, this section describes details of event A3. Conditions to enter the event, reporting parameters and condition to leave the event are presented. Event A3 is the normal event, which is used to trigger intra LTE frequency handover and this is the reason it was selected as an example. When UE is configured to reports measurements upon event A3 takes place, then the UE will sent measurements if it finds cells, which are several dB (so called offset) stronger than the serving cell. Figure 8.6 illustrates the event together with parameters controlling UE reporting when the condition to enter the event is met.

Figure 8.6: Event A3: Neighbour becomes offset better than serving. Frequency specific offsets (Ofn and Ofs) as well as cell specific offsets (Ocn and Ocs) are assumed to be set to zero in this figure. Condition to enter event A3 (TS 36:331): M n + Ofn + Ocn − hysteresis > M s + Ofs + Ocs + a3-Offset

(8.7)

Condition to leave event A3: M n + Ofn + Ocn + hysteresis < M s + Ofs + Ocs + a3-Offset

(8.8)

where Mn –

the measurement result of the neighbouring cell, not taking into account any offsets. Expressed in dBm in case of RSRP, or in dB in case of RSRQ

Ofn –

the frequency specific offset of the frequency of the neighbour cell (i.e. offsetFreq as defined within measObjectEUTRA corresponding to the frequency of the neighbour cell) [dB]. 156

8.2 Connected mode mobility Ocn –

the cell specific offset of the neighbour cell (i.e. cellIndividualOffset as defined within measObjectEUTRA corresponding to the frequency of the neighbour cell), and set to zero if not configured for the neighbour cell [dB].

Ms –

the measurement result of the serving cell, not taking into account any offsets. Expressed in dBm in case of RSRP, or in dB in case of RSRQ

Ofs –

the frequency specific offset of the serving frequency (i.e. offsetFreq as defined within measObjectEUTRA corresponding to the serving frequency) [dB].

Ocs –

the cell specific offset of the serving cell (i.e. cellIndividualOffset as defined within measObjectEUTRA corresponding to the serving frequency), and is set to zero if not configured for the serving cell [dB].

hysteresis –

the hysteresis parameter for this event as defined within reportConfigEUTRA for this event [dB].

a3-Offset –

the offset parameter for this event as defined within reportConfigEUTRA for this event [dB].

s-Measure –

defines when the UE is required to perform measurements on neighbouring cells.

riggerQuantity – the quantities used to evaluate the triggering condition for the event (RSRP or RSRQ). timeToTrigger – time during which specific criteria for the event needs to be met in order to trigger a measurement report. ReportInterval – indicates the interval between periodical reports. reportQuantity – the quantities to be included in the measurement report. The value both means that both the RSRP and RSRQ quantities are to be included in the measurement report. reportAmount – number of measurement reports sent.

157

8 LTE mobility

158

A System information MIB dl-Bandwidth, phich-Config phich-Duration, phich-Resource, systemFrameNumber, spare,

ENUMERATED {n6, n15, n25, n50, n75, n100} ENUMERATED {normal, extended} ENUMERATED {oneSixth, half, one, two} BIT STRING (SIZE (8)) BIT STRING (SIZE (10))

Table A.1: Master Information Block (MIB).

159

A System information

SIB1 cellAccessRelatedInfo plmn-IdentityList, SEQUENCE (SIZE (1..6)) OF PLMN-IdentityInfo PLMN-IdentityInfo plmn-Identity mcc, SEQUENCE (SIZE (3)) OF MCC-MNC-Digit mnc, SEQUENCE (SIZE (2..3)) OF MCC-MNC-Digit cellReservedForOperatorUse, ENUMERATED {reserved, notReserved} trackingAreaCode, BIT STRING (SIZE (16)) cellIdentity, BIT STRING (SIZE (28) cellBarred, ENUMERATED barred, notBarred intraFreqReselection, ENUMERATED allowed, notAllowed csg-Indication, BOOLEAN csg-Identity, BIT STRING (SIZE (27)) cellSelectionInfo q-RxLevMin, INTEGER (-70..-22) q-RxLevMinOffset, INTEGER (1..8) p-Max, INTEGER (-30..33) freqBandIndicator, INTEGER (1..64) schedulingInfoList si-Periodicity, ENUMERATED {rf8, rf16, rf32, rf64, rf128, rf256, rf512} sib-MappingInfo, SEQUENCE (SIZE (0..maxSIB-1)) OF SIB-Type tdd-Config subframeAssignment, ENUMERATED {sa0, sa1, sa2, sa3, sa4, sa5, sa6} specialSubframePatterns, ENUMERATED {ssp0, ssp1, ssp2, ssp3, ssp4, ssp5, ssp6, ssp7, ssp8} si-WindowLength, ENUMERATED {ms1, ms2, ms5, ms10, ms15, ms20, ms40} systemInfoValueTag, INTEGER (0..31) nonCriticalExtension spare bits set to zero Table A.2: SIB1.

160

SIB2 ac-BarringInfo ac-BarringForEmergency, BOOLEAN ac-BarringForMO-Signalling, AC-BarringConfig ac-BarringForMO-Data, AC-BarringConfig AC-BarringConfig ac-BarringFactor, ENUMERATED {p00, p05, p10, p15, p20, p25, p30, p40, p50, p60, p70, p75, p80, p85, p90, p95} ac-BarringTime, ENUMERATED {s4, s8, s16, s32, s64, s128, s256, s512} ac-BarringForSpecialAC, BIT STRING (SIZE(5)) radioResourceConfigCommon rach-ConfigCommon preambleInfo numberOfRA-Preambles, ENUMERATED {n4, n8, n12, n16 ,n20, n24, n28, n32, n36, n40, n44, n48, n52, n56, n60} preamblesGroupAConfig powerRampingParameters powerRampingStep, ENUMERATED {dB0, dB2,dB4, dB6} preambleInitialReceivedTargetPower, ENUMERATED {dBm-120, dBm-118, dBm-116, dBm-114, dBm-112,dBm-110, dBm-108, dBm-106, dBm-104, dBm-102, dBm-100, dBm-98, dBm-96, dBm-94, dBm-92, dBm-90} ra-SupervisionInfo preambleTransMax, ENUMERATED {n3, n4, n5, n6, n7, n8, n10, n20, n50, n100, n200} ra-ResponseWindowSize, ENUMERATED {sf2, sf3, sf4, sf5, sf6, sf7, sf8, sf10} mac-ContentionResolutionTimer, ENUMERATED {sf8, sf16, sf24, sf32, sf40, sf48, sf56, sf64} maxHARQ-Msg3Tx, INTEGER (1..8) bcch-Config pcch-Config prach-Config pdsch-ConfigCommon referenceSignalPower, INTEGER (-60..50) p-b, INTEGER (0..3) pusch-ConfigCommon pusch-ConfigBasic n-SB, INTEGER (1..4) hoppingMode, ENUMERATED {interSubFrame, intraAndInterSubFrame} pusch-HoppingOffset, INTEGER (0..98) enable64QAM, BOOLEAN 161

A System information

ul-ReferenceSignalsPUSCH pucch-ConfigCommon soundingRS-UL-ConfigCommon uplinkPowerControlCommon p0-NominalPUSCH, INTEGER (-126..24) alpha, ENUMERATED {al0, al04, al05, al06, al07, al08, al09, al1} p0-NominalPUCCH, INTEGER (-127..-96) deltaFList-PUCCH deltaPreambleMsg3, INTEGER (-1..6) ul-CyclicPrefixLength, ENUMERATED {len1, len2} ue-TimersAndConstants t300, ENUMERATED {ms100, ms200, ms300, ms400, ms600, ms1000, ms1500, ms2000} t301, ENUMERATED {ms100, ms200, ms300, ms400, ms600, ms1000, ms1500, ms2000} t310, ENUMERATED {ms0, ms50, ms100, ms200, ms500, ms1000, ms2000} n310, ENUMERATED {n1, n2, n3, n4, n6, n8, n10, n20} t311, ENUMERATED {ms1000, ms3000, ms5000, ms10000, ms15000, ms20000, ms30000} n311, ENUMERATED {n1, n2, n3, n4, n5, n6, n8, n10} freqInfo ul-CarrierFreq, ARFCN-ValueEUTRA ul-Bandwidth, ENUMERATED {n6, n15, n25, n50, n75, n100} additionalSpectrumEmission, INTEGER (1..32) mbsfn-SubframeConfigList timeAlignmentTimerCommon, ENUMERATED {sf500, sf750, sf1280, sf1920, sf2560, sf5120, sf10240, infinity} Table A.3: SIB2.

162

SIB3 cellReselectionInfoCommon q-Hyst, ENUMERATED {B0, dB1, dB2, dB3, dB4, dB5, dB6, dB8, dB10, dB12, dB14, dB16, dB18, dB20, dB22, dB24} speedStateReselectionPars mobilityStateParameters t-Evaluation, ENUMERATED {s30, s60, s120, s180, s240, spare3, spare2, spare1} t-HystNormal, ENUMERATED {s30, s60, s120, s180, s240, spare3, spare2, spare1} n-CellChangeMedium, INTEGER (1..16) n-CellChangeHigh, INTEGER (1..16) q-HystSF sf-Medium, ENUMERATED {dB-6, dB-4, dB-2, dB0} sf-High, ENUMERATED {dB-6, dB-4, dB-2, dB0} cellReselectionServingFreqInfo s-NonIntraSearch, INTEGER (0..31) ThreshServingLow, INTEGER (0..31) cellReselectionPriority, INTEGER (0..7) intraFreqCellReselectionInfo q-RxLevMin, INTEGER (-70..-22) p-Max, INTEGER (-30..33) s-IntraSearch, INTEGER (0..31) allowedMeasBandwidth, ENUMERATED {mbw6, mbw15, mbw25, mbw50, mbw75, mbw100} presenceAntennaPort1, BOOLEAN neighCellConfig, BIT STRING (SIZE (2)) t-ReselectionEUTRA, INTEGER (0..7) t-ReselectionEUTRA-SF sf-Medium, ENUMERATED {oDot25, oDot5, oDot75, lDot0} sf-High, ENUMERATED {oDot25, oDot5, oDot75, lDot0} Table A.4: SIB3.

163

A System information

SIB4 intraFreqNeighCellList, intraFreqBlackCellList, csg-PhysCellIdRange,

SEQUENCE (SIZE (1..maxCellIntra)) OF IntraFreqNeighCellInfo SEQUENCE (SIZE (1..maxCellBlack)) OF PhysCellIdRange PhysCellIdRange

IntraFreqNeighCellInfo physCellId, INTEGER (0..503) q-OffsetCell, ENUMERATED {dB-24, dB-22, dB-20, dB-18, dB-16, dB-14, dB-12, dB-10, dB-8, dB-6, dB-5, dB-4, dB-3, dB-2, dB-1, dB0, dB1, dB2, dB3, dB4, dB5, dB6, dB8, dB10, dB12 dB14, dB16, dB18, dB20, dB22, dB24} PhysCellIdRange start, INTEGER (0..503) range, ENUMERATED {n4, n8, n12, n16, n24, n32, n48, n64, n84, n96, n128, n168, n252, n504, spare2, spare1} Table A.5: SIB4.

164

SIB5 interFreqCarrierFreqList,

SEQUENCE (SIZE (1..maxFreq)) OF InterFreqCarrierFreqInfo

InterFreqCarrierFreqInfo dl-CarrierFreq, INTEGER (0..maxEARFCN) q-RxLevMin, INTEGER (-70..-22) p-Max, INTEGER (-30..33) t-ReselectionEUTRA, INTEGER (0..7) t-ReselectionEUTRA-SF sf-Medium, ENUMERATED {oDot25, oDot5, oDot75, lDot0} sf-High, ENUMERATED {oDot25, oDot5, oDot75, lDot0} threshX-High, INTEGER (0..31) threshX-Low, INTEGER (0..31) allowedMeasBandwidth, ENUMERATED {mbw6, mbw15, mbw25, mbw50, mbw75, mbw100} presenceAntennaPort1, BOOLEAN cellReselectionPriority, INTEGER (0..7) neighCellConfig, BIT STRING (SIZE (2)) q-OffsetFreq, dB-24, dB-22, dB-20, dB-18, dB-16, dB-14, dB-12, dB-10, dB-8, dB-6, dB-5, dB-4, dB-3, dB-2, dB-1, dB0, dB1, dB2, dB3, dB4, dB5, dB6, dB8, dB10, dB12, dB14, dB16, dB18, dB20, dB22, dB24} interFreqNeighCellList, InterFreqNeighCellList interFreqBlackCellList, SEQUENCE (SIZE (1..maxCellBlack)) OF PhysCellIdRange InterFreqNeighCellInfo physCellId, INTEGER (0..503) q-OffsetCell, dB-24, dB-22, dB-20, dB-18, dB-16, dB-14, dB-12, dB-10, dB-8, dB-6, dB-5, dB-4, dB-3, dB-2, dB-1, dB0, dB1, dB2, dB3, dB4, dB5, dB6, dB8, dB10, dB12, dB14, dB16, dB18, dB20, dB22, dB24} Table A.6: SIB5.

165

A System information

166

List of Figures 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 1.10 1.11 1.12 1.13 1.14 1.15 1.16 1.17 1.18 1.19 1.20 1.21 1.22 2.1 2.2 2.3

Two way communication. . . . . . . . . . . . . . . . . . . . . . . . . Frequency Division Duplex (FDD) and Time Division Duplex (TDD). Multiple access. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cellular technologies evolution. . . . . . . . . . . . . . . . . . . . . . Frequency Division Multiple Access (FDMA). . . . . . . . . . . . . Time Division Multiple Access (TDMA). . . . . . . . . . . . . . . . Code Division Multiple Access (CDMA). . . . . . . . . . . . . . . . Orthogonal Frequency Division Multiple Access (OFDMA). . . . . . OFDM subcarriers. . . . . . . . . . . . . . . . . . . . . . . . . . . . Geometric representation of a complex number in the rectangular notation in a complex Cartesian plane. . . . . . . . . . . . . . . . . Conjugate z ∗ of a complex number z. . . . . . . . . . . . . . . . . . Geometric representation of a complex number in the polar notation. Euler’s formula. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fourier Transform (FT) principles. . . . . . . . . . . . . . . . . . . . Example of the Discrete Fourier Transform (DFT). . . . . . . . . . The coefficient wn in the DFT for N = 8. . . . . . . . . . . . . . . . The coefficient w−n in the IDFT for N = 8. When comparing with Figure 1.16 notice that w−n is a conjugate of wn . . . . . . . . . . . Graphical presentation of the IDFT example. . . . . . . . . . . . . . OFDM concept. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . OFDM transmitter. . . . . . . . . . . . . . . . . . . . . . . . . . . . OFDM receiver. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . LTE modulations. . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5 5 6 7 8 8 9 10 11 13 13 14 15 16 17 18 20 21 24 24 24 25 30 33

2.5

EPS architecture. . . . . . . . . . . . . . . . . . . . . . . . . . . . . EPS bearer concept. . . . . . . . . . . . . . . . . . . . . . . . . . . . E-UTRAN, UTRAN and GERAN architecture. GPRS one tunnel approach. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Typical implementation of LTE/SAE. Combined SGSN/MME one tunnel approach. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Inter-pool mobility. . . . . . . . . . . . . . . . . . . . . . . . . . . .

3.1 3.2 3.3 3.4 3.5

User plane for LTE. . . . . . . . . . . . . . . . . . Control plane for LTE. . . . . . . . . . . . . . . . Relation between NAS and AS. . . . . . . . . . . HARQ principle - four multiple HARQ processes. LTE radio interface structure for DL. . . . . . . .

. . . . .

49 50 52 54 56

4.1 4.2

LTE channels mapping. . . . . . . . . . . . . . . . . . . . . . . . . . LTE FDD time domain structure. . . . . . . . . . . . . . . . . . . .

60 62

2.4

167

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37 38 44

LIST OF FIGURES 4.3 4.4 4.5 4.6 4.7 4.8 4.9

5.1

Cyclic prefix concept. . . . . . . . . . . . . . . . . . . . . . . . . . . LTE TDD frame structure for UL-DL configuration 2. . . . . . . . Special subframe configuration. . . . . . . . . . . . . . . . . . . . . . LTE downlink physical resource. . . . . . . . . . . . . . . . . . . . Definition of channel bandwidth and transmission bandwidth configuration for one E-UTRAN carrier. . . . . . . . . . . . . . . . . . . . An example of DL resource allocation. . . . . . . . . . . . . . . . . Localized and distributed VRB. The picture illustrates Ngap,1 = 48 RB = 100. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . for NDL

62 62 64 65 66 66 68

5.20 5.21

Primary/secondary synchronization signal and PBCH structure for FDD (normal cyclic prefix). . . . . . . . . . . . . . . . . . . . . . . . 71 Primary/secondary synchronization signal and PBCH structure for TDD (normal cyclic prefix). . . . . . . . . . . . . . . . . . . . . . . 72 Zadoff-Chu sequence transmitted on 31 lower frequency band subcarriers for physicalLayerId = 0, which corresponds to root index u = 25. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 Mapping of Physical Channels on DL for FDD mode. Time on horizontal axis and frequency on vertical axis. . . . . . . . . . . . . . . 74 Downlink reference signal structure in a cell supporting non-MBSFN transmission with normal cyclic prefix and CellID = 0. . . . . . . . 76 Cell specific RS frequency shift. . . . . . . . . . . . . . . . . . . . . 77 System information. . . . . . . . . . . . . . . . . . . . . . . . . . . . 78 Control Channel Element (CCE). . . . . . . . . . . . . . . . . . . . 82 Physical layer PDCCH processing. . . . . . . . . . . . . . . . . . . . 82 PDCH blind decoding example. . . . . . . . . . . . . . . . . . . . . 85 PDCH blind decoding. . . . . . . . . . . . . . . . . . . . . . . . . . 85 Transport channel processing for DL-SCH, PCH and MCH. . . . . . 86 Physical layer PDSCH processing. . . . . . . . . . . . . . . . . . . . 87 CRC concept. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88 Rate 1/3 tail biting convolutional encoder. . . . . . . . . . . . . . . 89 Structure of rate 1/3 turbo encoder (dotted lines apply for trellis termination only). The initial value of the shift registers of the 8state constituent encoders is all zeros when starting to encode the input bits. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90 Interleaver. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90 Operations of circular buffer rate matching for turbo code. . . . . . 91 Codeword-to-layer mapping for spatial multiplexing and transmit diversity. The picture also presents the precoding for transmit diversity. The size of the codeword(s) correspond to the maximum throughput possible to achieve for particular layer mapping. It can be observed that in spatial multiplexing maximum throughput increases with the the number of layers. In transmit diversity, regardless of the number of antennas, the maximum throughput is not increased. 93 Spatial multiplexing with one layer and two antenna ports. . . . . . 96 PHICH. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100

6.1 6.2 6.3

SC-FDMA versus OFDMA spectral power distribution. . . . . . . 101 Block diagram of the UL DFT-s-OFDM transmitter. . . . . . . . . 102 UL resource allocation. . . . . . . . . . . . . . . . . . . . . . . . . . 103

5.2 5.3

5.4 5.5 5.6 5.7 5.8 5.9 5.10 5.11 5.12 5.13 5.14 5.15 5.16

5.17 5.18 5.19

168

LIST OF FIGURES 6.4 6.5 6.6 6.7 6.8 6.9 6.10 6.11 6.12

7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 7.9 7.10 7.11 7.12 7.13 7.14 7.15 7.16 7.17 7.18 7.19

7.20 8.1 8.2 8.3 8.4 8.5 8.6

UL subframe structure for normal cyclic prefix. . . . . . . . . . . . . UL frequency hopping. . . . . . . . . . . . . . . . . . . . . . . . . . UL RS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . UL SRS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . PUCCH resources. . . . . . . . . . . . . . . . . . . . . . . . . . . . . PUCCH format 1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . PUCCH format 2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . Preamble formats. . . . . . . . . . . . . . . . . . . . . . . . . . . . . Time-frequency structure of non-synchronised RA for FDD. Example for prach-ConfigIndex = 6 and prach-FreqOffset = 1. . . . . . . .

103 104 104 106 106 107 108 110

Uplink-downlink timing relation from UE perspective for FDD. . . . Uplink-downlink time relation from UE perspective for TDD. . . . . Random access timing advance. . . . . . . . . . . . . . . . . . . . . Adjustment of timing advance by MAC control element. . . . . . . . UE time synchronisation. . . . . . . . . . . . . . . . . . . . . . . . . RA process. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . DL resource allocation. . . . . . . . . . . . . . . . . . . . . . . . . . Multi antenna possibilities. . . . . . . . . . . . . . . . . . . . . . . SU-MIMO and MU-MIMO. . . . . . . . . . . . . . . . . . . . . . . . Spatial multiplexing principles. . . . . . . . . . . . . . . . . . . . . Transmit diversity . . . . . . . . . . . . . . . . . . . . . . . . . . . Transmission mode 3: spatial multiplexing with large delay CDD or transmit diversity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . MIMO antenna solutions. . . . . . . . . . . . . . . . . . . . . . . . UE reporting. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Open loop power control. . . . . . . . . . . . . . . . . . . . . . . . . Closed loop power control. . . . . . . . . . . . . . . . . . . . . . . . Accumulated method of the closed loop power control adjustment. . Absolute method of the closed loop power control adjustment. . . . Transmitted power and signal at eNB as a function of the RSRP for the following parameters setting: PCMAX = 23 dBm, MPUSCH = 1, P0 PUSCH = −109 dBm, α = 1, ref erenceSignalP ower = 15 dBm. . The target P SDRX and the TBS. . . . . . . . . . . . . . . . . . . .

114 114 115 116 117 118 118 121 121 122 123

Overall idle mode process. . . . . . . . . . . . . . . . . . . . . . . . Automatic PLMN selection process. . . . . . . . . . . . . . . . . . . Cell reselection criterion. . . . . . . . . . . . . . . . . . . . . . . . . X2 handover. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Measurement configuration. . . . . . . . . . . . . . . . . . . . . . . . Event A3: Neighbour becomes offset better than serving. Frequency specific offsets (Ofn and Ofs) as well as cell specific offsets (Ocn and Ocs) are assumed to be set to zero in this figure. . . . . . . . . . . .

142 145 148 149 153

169

111

124 124 126 134 134 137 137

138 139

154

LIST OF FIGURES

170

List of Tables 2.1 2.2

4.1 4.2

4.3 4.4 4.5 4.6

5.1 5.2

5.3 5.4 5.5 5.6

QoS Class Identifier (QCI) defined for LTE/SAE. . . . . . . . . . . Mapping between standardized QCIs and pre-Relese-8 QoS parameter values. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

35

Cyclic prefix types. . . . . . . . . . . . . . . . . . . . . . . . . . . . Uplink-downlink configuration for LTE TDD. ↓ denotes a subframe reserved for downlink transmission. ↑ denotes a subframe reserved for uplink transmission. S denotes a special subframe. . . . . . . . . Special subframe configuration. . . . . . . . . . . . . . . . . . . . . . Number of RBs for different channel bandwidths in FDD and TDD. RB gap values. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . GSM, UMTS, WiMAX and LTE comparison. The table presents gross bit rate, spectral efficiency and system spectral efficiency, which include not only user date bit rate but also system signalling. The table does not consider MIMO which can further increase spectral efficiency. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

63

S-SS sequence generation. . . . . . . . . . . . . . . . . . . . . . . . . DL is the downNumber of OFDM symbols used for PDCCH. The NRB link bandwidth configuration, expressed in number of RB, see Table 4.4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Supported PDCCH formats. . . . . . . . . . . . . . . . . . . . . . . Usage of channel coding scheme and coding rate for control information. Codeword-to-layer mapping for spatial multiplexing. . . . . . . . . . Codeword-to-layer mapping for transmit diversity. ∗ In case when M (0) symb mod 4 ̸= 0 then two null symbols are appended

36

63 64 65 67

70 73

80 81 89 94

(0)

5.7 5.8

to d(0) (M symb − 1). . . . . . . . . . . . . . . . . . . . . . . . . . . . 94 Codebook for transmission on antenna ports {0, 1}. . . . . . . . . . 96 DL physical channels modulation. . . . . . . . . . . . . . . . . . . . 100

6.1

Random access preamble parameters. . . . . . . . . . . . . . . . . . 109

7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 7.9

Type 0 resource allocation RBG size vs. downlink system bandwidth. 119 PDSCH transmission scheme. . . . . . . . . . . . . . . . . . . . . . . 125 Physical Channels for Aperiodic or Periodic CQI reporting. . . . . . 127 4-bit CQI Table. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128 CQI and PMI Feedback Types for PUSCH reporting modes. . . . . 128 PUSCH reporting modes for different transmission modes. . . . . . 129 Subband size (k) vs. System Bandwidth. . . . . . . . . . . . . . . . 129 CQI and PMI Feedback Types for PUCCH reporting modes. . . . . 130 PUCCH reporting modes for different transmission modes. . . . . . 130 171

LIST OF TABLES 7.10 Modulation and TBS index table for PDSCH. . . . . . . . . . . . . 7.11 Transport block size table. . . . . . . . . . . . . . . . . . . . . . . . 7.12 Mapping of TPC Command Field in DCI format 0/3 to absolute and accumulated δPUSCH values. . . . . . . . . . . . . . . . . . . . . . . 7.13 Mapping of TPC Command Field in DCI format 3A to accumulated δPUSCH values. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.14 P0 PUSCH calculation. . . . . . . . . . . . . . . . . . . . . . . . . . . 7.15 ref erenceSignalP ower calculation. . . . . . . . . . . . . . . . . . .

132 133

A.1 A.2 A.3 A.4 A.5 A.6

157 158 160 161 162 163

Master Information Block SIB1. . . . . . . . . . . . SIB2. . . . . . . . . . . . SIB3. . . . . . . . . . . . SIB4. . . . . . . . . . . . SIB5. . . . . . . . . . . .

(MIB). . . . . . . . . . . . . . . . . . . . . . . . . .

172

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136 136 140 140

Acronyms 1G

1st Generation

16QAM

16 Quadrature Amplitude Modulation

1xRTT

1x Radio Transmission Technology

2G

2nd Generation

3G

3rd Generation

3GPP

3rd Generation Partnership Project

3GPP TS

3GPP Technical Specification

4G

4th Generation

64QAM

64 Quadrature Amplitude Modulation

AAA

Authentication, authorisation and accounting

ACK

Acknowledge

A/D

Analogue-to-Digital converter

AM

Acknowledged Mode

AMPS

Advanced Mobile Phone Systems

ARQ

Automatic Repeat reQuest

AS

Access Stratum

AS

Application Server

BCCH

Broadcast Control Channel

BCH

Broadcast Channel

BLER

Block Error Rate

BPSK

Binary Phase Shift Keying

BS

Base Station

BSS

Base Station System

C

Carrier

CAZAC

Constant Amplitude Zero AutoCorrelation

CCCH

Common Control Channel 173

ACRONYMS CCE

Control Channel Element

DCI

Downlink Control Information

CDD

Cyclic Delay Diversity

CDMA

Code Division Multiple Access

CDMA2000

Code Division Multiple Access 2000

CM

Connection Management

CN

Core Network

CP

Control Plane

CQI

Channel Quality Indicator

CRC

Cyclic Redundancy Check

C-RNTI

Cell RNTI

CS

Circuit Switched

D/A

Digital-to-Analogue converter

DAB

Digital Audio Broadcasting

D-AMPS

Digital Advanced Mobile Phone Systems

DCCH

Dedicated Control Channel

DFT

Discrete Fourier Transform

DFT-s-OFDM Discrete Fourier Transform spread-OFDM DL

Downlink

DL-SCH

Downlink Shared Channel

DRX

Discontinuous Reception

DTCH

Dedicated Traffic Channel

DwPTS

Downlink Pilot Time Slot

DVB-T

Digital Video Broadcasting – Terrestrial

ECM

EPS Connection Management

EDGE

Enhanced Data rates for GSM Evolution

EGPRS

Enhanced GPRS

EHPLMN

Equivalent HPLMN

EIR

Equipment Identify Register

EMM

EPS Mobility Management

eNB

Evolved Node B

EPC

Evolved Packet Core

EPS

Evolved Packet System 174

E-RAB

E-UTRAN Radio Access Bearer

ETWS

Earthquake and Tsunami Warning System

E-UTRA

Evolved Universal Terrestrial Radio Access

E-UTRAN

Evolved UMTS Terrestrial Radio Access Network

FDD

Frequency Division Duplex

FDMA

Frequency Division Multiple Access

FFT

Fast Fourier Transform

FT

Fourier Transform

GBR

Guaranteed Bit Rate

GERAN

GSM EDGE Radio Access Network

GGSN

Gateway GPRS Support Node

GP

Guard Period

GPRS

General Packet Radio Service

GMM

GPRS Mobility Management

GSM

Global System for Mobile communication

GTP

GPRS Tunnelling Protocol

GTP-C

GTP Control plane

GTP-U

GTP User data tunnelling

HARQ

Hybrid Automatic Repeat reQuest

HO

Handover

HPLMN

Home PLMN

HRPD

High Rate Packet Data

HSDPA

High Speed Downlink Packet Access

HSPA

High Speed Packet Access

HSS

Home Subscriber Server

I

Interferer

ICI

Inter Carrier Interference

IDFT

Inverse Discrete Fourier Transform

IETF

Internet Engineering Task Force

IFFT

Inverse Fast Fourier Transform

IMS

IP Multimedia Subsystem

IMSI

International Mobile Subscriber Identity

IP

Internet Protocol 175

ACRONYMS Inter-RAT

Inter Radio Access Technology

ISI

Inter Symbol Interference

ITU

International Telecommunication Union

IWLAN

Interworking Wireless Local Area Network

L1

Layer 1

L2

Layer 2

LA

Link Adaptation

LTE

Long Term Evolution

LTE/SAE

Long Term Evolution/System Architecture Evolution

MAC

Medium Access Control

MBMS

Multimedia Broadcast and Multicast Services

MBSFN

Multicast Broadcast Single Frequency Network

MCCH

Multicast Control Channel

MCH

Multicast Channel

MCS

Modulation and Coding Scheme

MIB

Master Information Block

MIMO

Multiple Input Multiple Output

MM

Mobility Management

MME

Mobility Management Entity

MMS

Multimedia Messaging Services

MTCH

Multicast Traffic Channel

MU-MIMO

Multi User MIMO

N

Noise

NACK

Negative Acknowledge

NAS

Non-Access Stratum

NMT

Nordic Mobile Telephony

OFDM

Orthogonal Frequency Division Multiplexing

OFDMA

Orthogonal Frequency Division Multiple Access

PAPR

Peak-to-Average Power Ratio

PBCH

Physical Broadcast Channel

PCCH

Paging Control Channel

PCEF

Policy and Charging Enforcement Function

PCFICH

Physical Control Format Indicator Channel 176

PCH

Paging Channel

PCRF

Policy and Charging Rules Function

PDCCH

Physical Downlink Control Channel

PDCP

Packet Data Convergence Protocol

PDP

Packet Data Protocol

PDSCH

Physical Downlink Shared Channel

PDU

Packet Data Unit

P-GW

Packet Data Network Gateway

PHICH

Physical Hybrid ARQ Indicator Channel

PLMN

Public Land Mobile Network

PMCH

Physical Multicast Channel

PMI

Precoding Matrix Indicator

PMIP

Proxy Mobile IP

PoP

Point of Presence

PRACH

Physical Random Access Channel

PRB

Physical Resource Block

PS

Packet Switched

P/S-GW

Packet Data Network/Serving Gateway

PSK

Phase Shift Keying

P-SS

Primary Synchronisation Signals

PUCCH

Physical Uplink Control Channel

PUSCH

Physical Uplink Shared Channel

QAM

Quadrature Amplitude Modulation

QCI

QoS Class Identifier

QoS

Quality of Service

QPSK

Quadrature Phase Shift Keying

RA

Random Access

RACH

Random Access Channel

RAN

Radio Access Network

RANAP

RAN Application Part

RA-RNTI

Random Access Radio Network Temporary Identity

RAT

Radio Access Technology

RB

Resource Block 177

ACRONYMS RE

Resource Element

REG

Resource Element Group

RF

Radio Frequency

RI

Rank Indicator

RLC

Radio Link Control

RLF

Radio Link Failure

RNC

Radio Network Controller

RNL

Radio Network Layer

RNTI

Radio Network Temporary Identity

ROHC

Robust Header Compression

RRC

Radio Resource Control

RRM

Radio Resource Management

RS

Reference Signals

RSRP

Reference Signal Received Power

RSRQ

Reference Signal Received Quality

S1AP

S1 Application Protocol

SAE

System Architecture Evolution

SAE-GW

System Architecture Evolution Gateway

SB

Scheduling Block

SC-FDMA

Single Carrier Frequency Division Multiple Access

SCTP

Stream Control Transmission Protocol

SDF

Service Data Flow

SDU

Service Data Unit

SGSN

Serving GPRS Support Node

S-GW

Serving Gateway

SI

System Information

SIB

System Information Block

SINR

Signal to Interference and Noise Ratio

SI-RNTI

System Information RNTI

SM

Session Management

SMS

Short Message Service

SN

Sequence Number

SRB

Signalling Radio Bearer 178

SRS

Sounding Reference Signal

S-SS

Secondary Synchronisation Signals

SU-MIMO

Single User MIMO

TA

Tracking Area

TAU

Tracking Area Update

TBS

Transport Blok Size

TCP

Transmission Control Protocol

TDD

Time Division Duplex

TDMA

Time Division Multiple Access

TFT

Traffic Flow Template

TNL

Transport Network Layer

TPC

Transmit Power Control

TS

Time Slot

TTI

Transmission Time Interval

TX

Transmit

UE

User Equipment

UL

Uplink

UM

Unacknowledged Mode

UMTS

Universal Mobile Telecommunications System

UP

User Plane

UpPTS

Uplink Pilot Time Slot

UL-SCH

Uplink Shared Channel

USIM

Universal Subscriber Identity Module

UTRAN

Universal Terrestrial Radio Access Network

VoIP

Voice over IP

VPLMN

Visited PLMN

VRB

Virtual Resource Block

WCDMA

Wideband Code Division Multiple Access

WiMAX

Worldwide Interoperability for Microwave Access

WLAN

Wireless Local Area Network

X2AP

X2 Application Protocol

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ACRONYMS

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