elp 4003 lte air interface

LTE Air InterfaceESB 4003 R2Ae

Student text

Disclaimer

This book is a training document and contains simplifications. Therefore, it mustnot be considered as a specification of the system.

The contents of this document are subject to revision without notice due to ongoingprogress in methodology, design and manufacturing.

ENKI Adam Girycki assumes no legal responsibility for any error or damage resultingfrom the usage of this document.

Copyright c September 7, 2013 by ENKI Adam Girycki.This document was produced in Poland by ENKI Adam Girycki. It is used fortraining purpose only and may not be copied or reproduced in any manner withoutthe express written consent of ENKI.

This document number ESB 4003 R2Ae supports course number ELP 4003 R2Ae.

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Contents

1 OFDMA principles 71.1 Two way communication . . . . . . . . . . . . . . . . . . . . . . . . 7

1.1.1 FDD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

1.1.2 TDD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

1.2 Access network evolution overview . . . . . . . . . . . . . . . . . . . 81.2.1 1G FDMA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

1.2.2 2G TDMA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

1.2.3 3G WCDMA . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

1.2.4 4G OFDMA . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

1.3 Complex numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151.3.1 Rectangular notation . . . . . . . . . . . . . . . . . . . . . . . 15

1.3.2 Polar notation . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

1.3.3 Relation between rectangular and polar notation . . . . . . . . . 15

1.3.4 Eulers formula . . . . . . . . . . . . . . . . . . . . . . . . . . 16

1.3.5 Exponential notation . . . . . . . . . . . . . . . . . . . . . . . 16

1.4 Fourier analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171.4.1 Fourier Transform (FT) . . . . . . . . . . . . . . . . . . . . . . 17

1.4.2 Discrete Fourier Transform (DFT) and Fast Fourier Transform(FFT) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

1.5 Orthogonal Frequency Division Multiplexing (OFDM) concept . . . 221.5.1 OFDM transmitter . . . . . . . . . . . . . . . . . . . . . . . . 24

1.5.2 OFDM receiver . . . . . . . . . . . . . . . . . . . . . . . . . . 25

1.6 Modulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

2 EPS architecture 292.1 LTE requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . 292.2 EPS architectural principles . . . . . . . . . . . . . . . . . . . . . . . 31

2.2.1 Evolved Packet Core (EPC) . . . . . . . . . . . . . . . . . . . . 31

2.2.2 Evolved UTRAN (E-UTRAN) . . . . . . . . . . . . . . . . . . . 32

2.3 Strata . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 332.3.1 Non-Access Stratum (NAS) . . . . . . . . . . . . . . . . . . . . 33

2.3.2 Access Stratum (AS) . . . . . . . . . . . . . . . . . . . . . . . 34

2.4 EPS Bearer and QoS . . . . . . . . . . . . . . . . . . . . . . . . . . . 352.4.1 EPS Bearer . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

2.4.2 Quality of Service (QoS) . . . . . . . . . . . . . . . . . . . . . 36

2.5 Integration with 2G and 3G . . . . . . . . . . . . . . . . . . . . . . . 372.6 Interfaces overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . 392.7 Evolved Packet Core (EPC) functions . . . . . . . . . . . . . . . . . 44

2.7.1 Mobility Management Entity node . . . . . . . . . . . . . . . . 44

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CONTENTS

2.7.2 Packet Data Network Gateway (P-GW) and Serving Gateway(S-GW) nodes . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

2.7.3 Mobility Management Entity (MME) and S-GW pooling concept 452.8 Long Term Evolution (LTE) functions . . . . . . . . . . . . . . . . . 47

2.8.1 LTE general principles . . . . . . . . . . . . . . . . . . . . . . . 472.8.2 Evolved Node B (eNB) functionality . . . . . . . . . . . . . . . 47

3 LTE signalling 513.1 User plane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 513.2 Control plane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 523.3 Protocols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

3.3.1 Radio Resource Control (RRC) . . . . . . . . . . . . . . . . . . 523.3.2 Packet Data Convergence Protocol (PDCP) . . . . . . . . . . . 543.3.3 Radio Link Control (RLC) . . . . . . . . . . . . . . . . . . . . . 553.3.4 Medium Access Control (MAC) . . . . . . . . . . . . . . . . . . 55

3.4 Radio interface structure . . . . . . . . . . . . . . . . . . . . . . . . 57

4 LTE radio interface introduction 594.1 Channel structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

4.1.1 Logical channels . . . . . . . . . . . . . . . . . . . . . . . . . . 594.1.2 Transport channels . . . . . . . . . . . . . . . . . . . . . . . . 604.1.3 Physical channels . . . . . . . . . . . . . . . . . . . . . . . . . 61

4.2 Time domain structure . . . . . . . . . . . . . . . . . . . . . . . . . 634.2.1 Frequency Division Duplex (FDD) . . . . . . . . . . . . . . . . 634.2.2 Time Division Duplex (TDD) . . . . . . . . . . . . . . . . . . . 63

4.3 Frequency domain structure . . . . . . . . . . . . . . . . . . . . . . . 654.4 Scheduling Block . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 674.5 Virtual Resource Block . . . . . . . . . . . . . . . . . . . . . . . . . 68

4.5.1 VRB of localized type . . . . . . . . . . . . . . . . . . . . . . . 684.5.2 VRB of distributed type . . . . . . . . . . . . . . . . . . . . . . 68

4.6 System spectral efficiency . . . . . . . . . . . . . . . . . . . . . . . . 69

5 LTE downlink physical channels 735.1 Cell search . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 735.2 P-SS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 745.3 S-SS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 755.4 RS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 775.5 PBCH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

5.5.1 MIB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 805.5.2 SIB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81

5.6 PCFICH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 815.7 PDCCH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82

5.7.1 PDCCH usage . . . . . . . . . . . . . . . . . . . . . . . . . . . 825.7.2 PDCCH mapping . . . . . . . . . . . . . . . . . . . . . . . . . 835.7.3 PDCCH format . . . . . . . . . . . . . . . . . . . . . . . . . . 835.7.4 PDCCH processing . . . . . . . . . . . . . . . . . . . . . . . . 835.7.5 PDCCH blind decoding . . . . . . . . . . . . . . . . . . . . . . 85

5.8 PDSCH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 865.8.1 CRC attachment . . . . . . . . . . . . . . . . . . . . . . . . . 905.8.2 Code block segmentation . . . . . . . . . . . . . . . . . . . . . 90

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CONTENTS

5.8.3 Channel coding . . . . . . . . . . . . . . . . . . . . . . . . . . 90

5.8.4 Rate matching . . . . . . . . . . . . . . . . . . . . . . . . . . . 91

5.8.5 Code block concatenation . . . . . . . . . . . . . . . . . . . . . 94

5.8.6 Scrambling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94

5.8.7 Modulation mapper . . . . . . . . . . . . . . . . . . . . . . . . 94

5.8.8 Layer mapper . . . . . . . . . . . . . . . . . . . . . . . . . . . 94

5.8.9 Precoding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97

5.8.10 Resource element mapping . . . . . . . . . . . . . . . . . . . . 101

5.9 PHICH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1015.10 PMCH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1025.11 Downlink physical channels modulation summary . . . . . . . . . . . 102

6 Physical layer procedures 1036.1 Timing advance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103

6.1.1 Uplink-downlink frame timing . . . . . . . . . . . . . . . . . . . 103

6.1.2 Timing advance range . . . . . . . . . . . . . . . . . . . . . . . 103

6.1.3 Random access . . . . . . . . . . . . . . . . . . . . . . . . . . 103

6.1.4 Other cases . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105

6.1.5 Maintenance of uplink time alignment . . . . . . . . . . . . . . 106

6.2 Random access . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1066.3 Resource allocation . . . . . . . . . . . . . . . . . . . . . . . . . . . 107

6.3.1 Resource allocation type 0 . . . . . . . . . . . . . . . . . . . . 108



6.4 MIMO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1096.4.1 Spatial multiplexing . . . . . . . . . . . . . . . . . . . . . . . . 110

6.4.2 Transmit diversity . . . . . . . . . . . . . . . . . . . . . . . . . 112

6.4.3 Transmission modes . . . . . . . . . . . . . . . . . . . . . . . . 112

6.4.4 MIMO antennas . . . . . . . . . . . . . . . . . . . . . . . . . . 114

6.5 UE reporting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1156.5.1 CQI definition . . . . . . . . . . . . . . . . . . . . . . . . . . . 116

6.5.2 Aperiodic CQI/PMI/RI reporting using PUSCH . . . . . . . . . 117

6.5.3 Periodic CQI/PMI/RI reporting using PUCCH . . . . . . . . . . 119

6.6 Modulation order and transport block size determination . . . . . . 1196.6.1 Modulation determination . . . . . . . . . . . . . . . . . . . . . 120

6.6.2 Transport block size determination . . . . . . . . . . . . . . . . 120

6.7 UL power control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1246.7.1 PUSCH power control . . . . . . . . . . . . . . . . . . . . . . . 124

6.7.2 PUSCH power control example . . . . . . . . . . . . . . . . . . 127

7 LTE mobility 1317.1 Idle mode mobility . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131

7.1.1 PLMN selection . . . . . . . . . . . . . . . . . . . . . . . . . . 132

7.1.2 Cell selection . . . . . . . . . . . . . . . . . . . . . . . . . . . 134

7.1.3 Cell reselection . . . . . . . . . . . . . . . . . . . . . . . . . . 136

7.2 Connected mode mobility . . . . . . . . . . . . . . . . . . . . . . . . 1387.2.1 X2 handover . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139

7.2.2 Event triggered reporting . . . . . . . . . . . . . . . . . . . . . 141

7.2.3 A3 event . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144

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CONTENTS

A System information 147

List of Figures 157

List of Tables 160

Acronyms 167

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1 OFDMA principles

1.1 Two way communication

In order to provide two way communication, so called duplex, two directions oftransmission must exist and they must be separated from each other to avoid col-lisions. Transmission from the User Equipment (UE) to the Base Station (BS) isreferred to as Uplink (UL), while the transmission from the BS to the UE is referredto 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, aspresented in Figure 1.2.

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

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1 OFDMA principles

1.1.1 FDD

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

1.1.2 TDD

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

1.2 Access network evolution overview

Apart from duplex transmission separation, a harmonised access of multiple UEs tothe shared radio resources must exist, see Figure 1.3. In uplink direction a numberUEs transmit to the base station. Thus the multiple access technology is required,which allows the base station to separate transmissions from different UEs. Indownlink 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 ofsignals 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 ef-fective, radio access technology, which are discuss in the next sections. The brieflysummary of cellular technologies evolution is presented in Figure 1.4.

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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 wasthe voice.

Examples of 1G system:

Nordic Mobile Telephony (NMT) introduced in 1981 and developed in Nordiccountries, Switzerland, Netherlands, Eastern Europe and Russia

Advanced Mobile Phone Systems (AMPS) introduced in 1983 and developedin North America and Australia

In 1G system, Frequency Division Multiple Access (FDMA) method of radio re-sources usage was applied. The available radio resources were divided in frequencydomain. For each connection a separate bandwidth was allocated and the user trans-mission 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 incase AMPS, resulting in a total of 50 kHz in NMT and 60 kHz in AMPS for eachduplex channel.

1.2.2 2G TDMA

2G, the second generation of wireless telecommunications technology, was introducedin 1990s. It was a digital system. The offered services were voice, Short MessageService (SMS), Circuit Switched (CS) data transfer with the rate of 9.6 kbit/s.

Examples of 2G systems:

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1 OFDMA principles

Figure 1.5: Frequency Division Multiple Access (FDMA).

GSM introduced in 1991 and used across more than 212 countries and territo-ries.

Digital Advanced Mobile Phone Systems (D-AMPS) introduced 1991 and usedin 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 radioresources 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. Thenumber of timeslots in the TDMA frame is system specific. In GSM system eighttimeslots make up the TDMA frame. A user has a cyclic access to the common radioresources during the allocated timeslot. Thus the transmission is discontinuous. TheFigure 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 called2.5G, offers Packet Switched (PS) data transfer with the rate of approximately 50kbps.

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Enhanced GPRS (EGPRS), also called 2.75G, offers higher date rate of PS datatransfer with the maximum rate of approximately 500 kbps, thanks to higher ordermodulation.

1.2.3 3G WCDMA

3G, the third generation of wireless telecommunication technology, was introducedin 2000s.

Examples of 3G system:

CDMA2000 introduced in 2000 in South Korea and used in Asia, America andAfrica.

Universal Mobile Telecommunications System (UMTS) introduced in 2001 inJapan and used in Europe, Asia and Africa.

3G systems employ Code Division Multiple Access (CDMA) method of radio re-sources usage. CDMA allows for simultaneous transmission of multiple users inthe same frequency band, which is presented in Figure 1.7. Separation of differentconnections 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 nexttripled to 3x1.25 MHz.

In UMTS, the Wideband Code Division Multiple Access (WCDMA) method is ap-plied, which utilizes wide frequency band of 5 MHz. Wide frequency channel allowsfor lowering the power density, thus signal may be even weaker than thermal noiselevel.

High Speed Packet Access (HSPA) provides downlink throughput of approximately14 Mbps, while Evolved HSPA (also called HSPA+) provides throughput of 84Mbps.

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1 OFDMA principles

1.2.4 4G OFDMA

4G, the fourth generation of mobile telecommunications technology, must support 1Gbit/s downlink bit rate. Currently there is no system, that is able to support mobilecommunications 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 worlds first publicly avail-able LTE-service was opened in the two Scandinavian capitals Stockholm andOslo on the 14 December 2009.

Worldwide Interoperability for Microwave Access (WiMAX) offers approxi-mately 40 Mbit/s bit rate. WiMAX access was used to assist with communi-cations 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 OFDMAhad been already laid in 1960, but due to high costs and lack of appropriate tech-nologies for a long time it remained purely theoretical. This situation has changedwith advent of cheap, small and fast microchips capable of processing the FFT andInverse Fast Fourier Transform (IFFT) algorithms. Nowadays, OFDM is widelyused in wireless networking (Wireless Local Area Network (WLAN)), digital televi-sion (Digital Video Broadcasting Terrestrial (DVB-T)), audio broadcasting (Digi-tal Audio Broadcasting (DAB)) and broadband wireless communications (WiMAX,LTE).

OFDMA is a special type of the FDMA. OFDMA allows for transfer messagessimultaneously, using multiple narrow ranges of frequencies, called subcarriers, seeFigure 1.8.

Figure 1.8: Orthogonal Frequency Division Multiple Access (OFDMA).

To avoid Inter Carrier Interference (ICI), in ordinary FDMA system, all such subcar-riers are separated in frequency domain with guard bands, therefore some spectrumis wasted. OFDM provides much better spectrum efficiency, as it does not need gapsbetween subcarrier bands. Moreover, the subcarrier bands are overlapping, whichallows to additionally save some spectrum. ICI is mitigated here by taking advan-tage of the fact that under the following conditions the subcarriers are orthogonalwith one another:

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The careful choice of subcarrier spacing. The subcarrier spacing f should beexactly equal to the reciprocity of the OFDMA symbol duration Tsymbol, seeFigure 1.9, which provides that the subcarriers are mathematically orthogonaland thus independent.

Keeping the synchronisation in the frequency domain, providing there are nofrequency shifts, e.g. due to Doppler effects.

Figure 1.9: OFDM subcarriers.

Multiplexing and demultiplexing of OFDMA symbols into subcarriers can be per-formed using Inverse Inverse Discrete Fourier Transform (IDFT) and DFT. Thesemathematical procedures, that transform signal from frequency to time domain andopposite, can be implemented with IFFT and FFT algorithms.

The presented above FDMA, TDMA and CDMA multiple access methods are singlecarrier modulation. OFDMA is a multi carrier modulation. In other words, itmeans that a large number of closely spaced orthogonal subcarriers are used tocarry data. Each subcarrier is modulated with a conventional modulation scheme(such as quadrature amplitude modulation or phase shift keying) at a low symbolrate, maintaining total data rates similar to conventional single carrier modulationschemes in the same bandwidth.

Advantages of OFDMA:

OFDMA effectively diminishes also the problem of multipath selective fad-ing. Due to multipath radio waves propagation in typical urban environment,signal at the receiver can be constructively or destructively interfered by thesame signal delayed over different path. This effect can dramatically changedepending on frequency used as a signal carrier some of the frequencies willsuffer from deep fading, while neighbouring ones may not be affected at all. As

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1 OFDMA principles

OFDMA uses very small subcarrier widths, the fading within every subcarriercan 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 (typi-cally mountain). The delayed signal, which propagates over much longer path,interferes with the direct signal because it carries another (older) symbol thanthe direct signal and therefore the receiver is unable to detect the correct sym-bol. The ISI effect is diminished when the symbol duration is longer, thus onlyvery far objects will lead to ISI. But the signal reflected from very far objectis usually week enough and does not lead to interference. In OFDMA, thesymbol duration can be lengthened, because a few symbols can be transmittedsimultaneously on different subcarriers. As already mentioned, longer symbolmakes the radio path less vulnerable to ISI. Additionally, to avoid overlapping,the adjacent symbols are always separated in time by short guard period. Inthe guard period, from technical reasons, it is not effective to stop transmissionat all, thus, so called, cyclic prefix is inserted here, which is simply a copy ofthe signal tail end.

OFDMA can achieve a higher Multiple Input Multiple Output (MIMO) spec-tral efficiency due to providing flatter frequency channels than a CDMA rakereceiver 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 charac-

terised by short communication bursts at high data rate. Few users in a basestation cell are transferring data simultaneously at low constant data rate.

The complex OFDMA electronics, including the FFT algorithm and forwarderror correction, is constantly active independent of the data rate, which isinefficient from power consumption point of view, while OFDMA combinedwith data packet scheduling may allow that the FFT algorithm hibernatesduring certain time intervals.

The OFDMA diversity gain, and resistance to frequency-selective fading, maypartly be lost if very few sub-carriers are assigned to each user, and if the samecarrier is used in every OFDMA symbol. Adaptive sub-carrier assignmentbased on fast feedback information about the channel, or sub-carrier frequencyhopping, is therefore desirable.

Dealing with co-channel interference from nearby cells is more complex inOFDMA than in CDMA. It would require dynamic channel allocation withadvanced coordination among adjacent base stations.

The fast channel feedback information and adaptive sub-carrier assignment ismore complex than CDMA fast power control.

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1.3 Complex numbers

1.3 Complex numbers

Complex numbers are used in OFDMA signal processing. A complex number is anumber 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 calledCartesian notation) a + ib, where a and b are real numbers, and i is the standardimaginary unit with the property i2 = 1. Figure 1.10 shows geometric representa-tion of a complex number z = a+ib in the complex plane. The complex plane can bethought of as a Cartesian plane, with the real part of a complex number representedby a displacement along the x-axis, and the imaginary part by a displacement alongthe y-axis.

Figure 1.10: Geometric representation of a complex number in the rectangularnotation in a complex Cartesian plane.

Each complex number z has a conjugate z, which has the same real part butopposite imaginary part, see 1.11:

z = a+ ib (1.1)

z = a ib (1.2)

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) .

1.3.3 Relation between rectangular and polar notation

Relation between rectangular and polar notation of a complex number is the follow-ing:

a = r cos (1.3)

b = r sin (1.4)

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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+ ir sin = r(cos+ i sin) (1.5)

1.3.4 Eulers formula

Leonhard Euler, Swiss mathematician and physicist, discovered a mathematical re-lationship between the trigonometric functions (sin and cos) and the complex expo-nential function (see also Figure 1.13):

cos+ i sin = ei (1.6)

Eulers formula was called by Richard Feynman one of the most remarkable, almostastounding, formulas in all of mathematics.

1.3.5 Exponential notation

Using the Eulers formula the complex number z may be written as follows, whichis called the exponential notation of a complex number:

z = r(cos+ i sin) = rei (1.7)

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1.4 Fourier analysis

Figure 1.13: Eulers formula.

In the exponential notation certain calculations, particularly multiplication and di-vision of complex numbers, are easier than in rectangular notation. On the otherhand, addition and subtraction are easier with the use of rectangular notation. Theexponential notation of a complex number is in widespread use in engineering andscience.

Using the Eulers formula the conjugate z may be written as:

z = ei (1.8)


1.4.1 Fourier Transform (FT)

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

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

Discrete Fourier Transform (DFT) is a specific kind of FT. The input to the DFTis a finite sequence of real or complex numbers making the DFT ideal for processinginformation stored in computers. In particular, the DFT is widely employed insignal processing and related fields to analyse the frequencies contained in a sampledsignal, to solve partial differential equations, and to perform other operations suchas convolutions or multiplying large integers.

DFT transforms the sequence of N complex numbers a0, a1, ..., aN1 (usually in timedomain) into a sequence of A0, A1, ..., AN1 complex numbers (usually in frequency

17

1 OFDMA principles

Figure 1.14: Fourier Transform (FT) principles.

18


domain) according to the following formula:

Ak =N1n=0

anwkn k = 0, ..., N 1 (1.9)

w = e2piNi (1.10)

The inverse transform to the DFT, which transforms the sequence of Ak complexnumbers back to the sequence of an complex values, is called Inverse Discrete FourierTransform (IDFT) and is given by the following formula:

an =1

N

N1k=0

Akwkn n = 0, ..., N 1 (1.11)

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

DFT example

We are going to apply the DFT to the following sequence of N = 8 numbers in thetime 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 ofnumbers 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 sequenceof real numbers an is transformed into the sequence of complex numbers Ak. Thecomplex numbers Ak are expressed by their modulus r and argument (see section1.3). The modulus r represents the amplitude of the cosinusoidal signal of a givenfrequency f and the argument corresponds to the phase shift of the cosinusoidalsignal. 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 DFTformula may be expressed by the following matrix form:

A0A1A2A3A4A5A6A7

=

1 1 1 1 1 1 1 1

1 w w2 w3 w4 w5 w6 w7

1 w2 w4 w6 w8 w10 w12 w14

1 w3 w6 w9 w12 w15 w18 w21

1 w4 w8 w12 w16 w20 w24 w28

1 w5 w10 w15 w20 w25 w30 w35

1 w6 w12 w18 w24 w30 w36 w42

1 w7 w14 w21 w28 w35 w42 w49

a0a1a2a3a4a5a6a7

(1.14)

19

1 OFDMA principles

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

w = e2pi8i = e

pi4i = cos

(pi4

) i sin

(pi4

)=

12 i

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 byarrows according to Figure 1.16. Now, the matrix form of DFT can be noted in thefollowing way:

A0A1A2A3A4A5A6A7

=

2

1

0

1

2

1

0

1

(1.16)

We may calculate Ak numbers from the above matrix notation. As an exampleA0, A1 and A2 are calculated below:

A0 = 1 2 + 1 1 + 1 0 + 1 1 + 1 2 + 1 1 + 1 0 + 1 1 = 8 (1.17)

20


Re

Im

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

A1 = 1 2 +(

12 i

2

) 1(i) 0 +

( 1

2 i

2

) 1+

+ (1) 2 +( 1

2+

i2

) 1 + i 0 +

(12

+i2

) 1 = 0

(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 transformsthe sequence a = [2, 1, 0, 1, 2, 1, 0, 1] into the sequence A = [8, 0, 4, 0, 0, 0, 4, 0]. Itis important to observe that the duration of our signal sample in the time domainwas 8 s, while the shift between transformed signals in frequency domain is equal1

8 s =18Hz.

Inverse Discrete Fourier Transform (IDFT) example

We are going show that the IDFT transforms the sequence of N = 8 numbers in thefrequency 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 wn

are shown in Figure 1.17:

a0 =1

N

N1k=0

Ak =1

8(8 + 0 + 4 + 0 + 0 + 0 + 4 + 0) = 2 (1.22)

21

1 OFDMA principles

a1 =1

N

N1k=0

Akwk =

1

8

(8w01 + 4w21 + 4w61

)=

=1

8(8 + 4i 4i) = 1

(1.23)

a2 =1

N

N1k=0

Akw2k =

1

8

(8w02 + 4w22 + 4w62

)=

=1

8(8 4 4) = 0

(1.24)

Figure 1.17: The coefficient wn in the IDFT for N = 8. When comparing withFigure 1.16 notice that wn 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, theinformation to be transmitted is represented by different Ak values. The process ofconverting bits into Ak values is called modulation. Each of the Ak values is senton another subcarrier. In the picture there are N = 10 subcarriers. Ak values,which are sent on different subcarriers, are represented by different heights of thebars. 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 signalis denoted by an values and represented by circles. Because there are 10 bars inthe frequency domain before DFT, therefore there are also 10 circles of the timedomain signal after IDFT. As already mentioned, the 10 time domain samplesare to be transmitted during Tsymbol, therefore the time between samples is equal

Ts =Tsymbol

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 opera-tions). 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 signalevery Ts and collects 10 time domain samples an, which are next transformed, withuse of the DFT, to frequency domain values Ak. The frequency domain values Akcarry information about bits which were transmitted. The bits are retrieved bydemodulation of values Ak.

After time Tsymbol the next symbol may be transmitted. Figure 1.19 illustratestransmission of 3 symbols. Please observe that there could be a break betweenconsecutive 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 subcarrierprocessing Quadrature Amplitude Modulation (QAM) or Phase Shift Keying (PSK)can be used. A simplified scheme of an OFDM transmitter has been shown in Figure1.20.

s[i] is input bit stream. First, bits are separated into N parallel streams. Streamsare assigned for Quadrature Amplitude Modulation (QAM) or Phase Shift Keying(PSK) modulation. Depending on the modulation, subcarriers may have differenttransmission bit rate.

Next, IFFT is computed for the sequence of complex data symbols A0, ..., AN1,which results in a sequence of complex time symbols a0, ..., aN1 of the signal. Foreach symbol, after imaginary and real part separation, both parts are convertedto analogue in Digital-to-Analogue converter (D/A). Next, analogue signals arequadrature modulated (multiplied by cosine and sine functions) and summed upgiving 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 thesignal 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 andconverted to digital by the Analogue-to-Digital converter (A/D). The series of com-plex time symbols is then corrected for frequency drifts and global phase offsets (notshown in the diagram). In the next step FFT is carried out and frequency symboldetection takes place, which results in N parallel bit streams, joined finally into oneinitial bit stream s(i).

Figure 1.21: OFDM receiver.

1.6 Modulation

In telecommunications, modulation is the process of conveying a message signal, forexample 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 messagesignal to a passband signal, for example a RF signal. Electrical signals can onlybe transferred over a limited passband frequency spectrum, with specific (non-zero)lower and upper cut-off frequencies. Modulating a sine wave carrier makes it possibleto keep the frequency content of the transferred signal as close as possible to thecentre 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 tech-nique, which changes a digital signal of bits into another digital signal of complexnumbers. The complex numbers represent amplitude and phase shift of OFDMsubcarriers.

The modulation techniques used in LTE are based on phase and amplitude modu-lation of the carrier frequency, see also Figure 1.22:

Binary Phase Shift Keying (BPSK) allows for transmission of one informationbit during one modulation symbol.

Quadrature Phase Shift Keying (QPSK) allows for transmission of two infor-mation bits during one modulation symbol.

16 Quadrature Amplitude Modulation (16QAM) allows for transmission of 4information bits during one modulation symbol.

64 Quadrature Amplitude Modulation (64QAM) is the fastest modulation usedin LTE and allows for transmission of 6 information bits during one modulationsymbol.

Only QPSK, 16QAM and 64QAM are used in LTE for user data bit. QPSK is onlyused 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) net-work technologies, including UMTS, HSPA, and CDMA2000 1xEV-DO, to supportincreasing subscriber demand for mobile broadband services. LTE is a step towardthe 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) shouldsupport significantly increased instantaneous peak data rates. The sup-ported peak data rate should scale according to size of the spectrumallocation.

Note that the peak data rates may depend on the numbers of transmitand receive antennas at the UE. The targets for DL and UL peak datarates 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 instanta-neous downlink peak data rate of 100 Mbps within a 20 MHz downlinkspectrum allocation (5 bps/Hz) and an instantaneous uplink peak datarate of 50 Mbps (2.5 bps/Hz) within a 20 MHz uplink spectrum alloca-tion. The peak data rates should then scale linearly with the size of thespectrum allocation.

In case of spectrum shared between downlink and uplink transmission,E-UTRAN does not need to support the above instantaneous peak datarates simultaneously.

Control Plane (CP) latency.Transition time (excluding downlink paging delay and Non-Access Stra-tum (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 forsmall Internet Protocol (IP) packets.

CP capacity.The system should be able to support a large number of users per cellwith quasi instantaneous access to radio resources in the active state. Itis expected that at least 200 users per cell should be supported in theactive state for spectrum allocations up to 5 MHz, and at least 400 usersfor higher spectrum allocation.

A much higher number of users is expected to be supported in the campedstate.

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 6HSDPA Both targets should be achieved assuming Release 6 referenceperformance is based on a single Tx antenna at the Node B withenhanced performance type 1 receiver in UE whilst the E-UTRA mayuse a maximum of 2 Tx antennas at the Node B and 2 Rx antennasat the UE.

The supported user throughput should scale with the spectrum band-width.

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 Txantenna at the UE and 2 Rx antennas at the Node B).

Target for averaged user throughput per MHz, 2 to 3 times Release 6Enhanced Uplink (deployed with a single Tx antenna at the UE and2 Rx antennas at the Node B).

Both should be achievable by the E-UTRAN using a maximum ofa single Tx antenna at the UE and 2 Rx antennas at the Node B.Greater user throughput should be achievable using multiple Tx an-tennas at the UE.

The user throughput should scale with the spectrum bandwidth pro-vided 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 assumingRelease 6 reference performance is based on a single Tx antenna atthe 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 Band 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 Txantenna at the UE and 2 Rx antennas at the Node B). This should beachievable by the E- UTRA using a maximum of a single Tx antennaat the UE and 2Rx antennas at the Node B.

Mobility.The E-UTRAN shall support mobility across the cellular network andshould be optimised for low mobile speed from 0 to 15 km/h. Highermobile speed between 15 and 120 km/h should be supported with highperformance. Mobility across the cellular network shall be maintained atspeeds from 120 km/h to 350 km/h (or even up to 500 km/h dependingon 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. SAEis the evolution of the GPRS Core Network. The main component of the SAEarchitecture is the Evolved Packet Core (EPC).

The Long Term Evolution/System Architecture Evolution (LTE/SAE) system, whichconsists of E-UTRAN and EPC, is called Evolved Packet System (EPS), see Figure2.1. LTE/SAE is specified from 3GPP Technical Specification (3GPP TS) Release8.

2.2.1 Evolved Packet Core (EPC)

The EPC provides access to external data networks (e.g., Internet, corporate net-works) and operator services (e.g., Multimedia Messaging Services (MMS)1, Multi-media Broadcast and Multicast Services (MBMS)2). It also performs functions re-lated to security (authentication, key agreement), subscriber information, chargingand inter-access mobility (GSM EDGE Radio Access Network (GERAN)/Univer-sal Terrestrial Radio Access Network (UTRAN)/E-UTRAN/Interworking Wireless

1Multimedia Messaging Services (MMS) is a standard way to send messages that include mul-timedia content to and from mobile phones. It extends the core SMS capability which only allowsexchange of text messages up to 160 characters in length. The most popular use of MMS is to sendphotographs from camera-equipped handsets, although it is also popular as a method of deliveringnews and entertainment content including videos, pictures, text pages and ringtones.

2Multimedia Broadcast and Multicast Services (MBMS) is a broadcasting service, which maybe offered via existing GSM and UMTS cellular networks. The main application is mobile TV. Theinfrastructure offers an option to use an uplink channel for interaction between the service and theuser.

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 inpower saving state).

The number of user plane nodes3 in the core network has been reduced from twoin Release 6 (Serving GPRS Support Node (SGSN) and Gateway GPRS SupportNode (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 residesin the same physical node referred to as P/S-GW or System Architecture EvolutionGateway (SAE-GW). In typical implementations the P/S-GW is realised by softwareupgrade of GGSN.

The control plane node is called MME and it may be realised by software upgradeof SGSN.

2.2.2 Evolved UTRAN (E-UTRAN)

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

3User plane is a communication strata responsible for user data transmission, in contrast tocontrol plane, which is responsible for signalling transmission. The strata concept is explained inthe next section.

32

2.3 Strata

EPC and the E-UTRAN is called S1 and the interface between the eNBs is calledX2.

2.3 Strata

To keep the questions of mobility and connection management independent of theair interface technology, the concept of communication strata has been employed inUMTS and it is also used in LTE/SAE. The stack of protocols has been dividedinto:

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 iscompletely transparent for these protocols, and hence they can be independentof the radio technology used.

Access Stratum (AS) containing radio access protocols between the UE andthe E-UTRAN. These protocols are different in GSM, UMTS and LTE, sincethe radio access technology is different here (OFDMA instead of TDMA orWCDMA).

2.3.1 Non-Access Stratum (NAS)

The concept of Non-Access Stratum (NAS) is almost the same as in UMTS, howeverit is implemented in much different way.

The UMTS uses the same mobility and connection management protocols as theearlier generation networks (GSM, GPRS) and they are the following:

Connection Management (CM) and Mobility Management (MM) for the CSpart of the network,

Session Management (SM) and GPRS Mobility Management (GMM) for thePS part.

The fact that LTE/SAE is totally packet oriented eliminates the protocols connectedwith the CS network part and modifies the NAS operation in PS part (i.e. the entirenetwork).

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 tobe redefined for EPS to meet the requirements of the new concept of UE mobilityfor the PS transmission only. The SM remains the same due to the fact of commonway 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 messagesby RRC, either by concatenating the NAS messages with other Radio ResourceControl (RRC) messages, or by including the NAS messages in dedicated RRCmessages without concatenation.

The NAS messages are protected using the ciphering and integrity protection servicesprovided by the Packet Data Convergence Protocol (PDCP) layer. However, NASis 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 EMMstates describing the mobility management states that result from the mobility man-agement procedures e.g. attach and Tracking Area Update (TAU) procedures, andof EPS Connection Management (ECM) states describing the signalling connectivitybetween the UE and the EPC.

The ECM and EMM states are independent of each other and when the UE is inEMM-CONNECTED state this does not imply that the user plane (radio and S1bearers) is established.

2.3.2 Access Stratum (AS)

The services, access signalling, mobility and subscriber management specific to CNare completely outside the AS, and are transferred transparently through the Ra-dio Access Network (RAN). AS protocols are specific to the RAN being used bythe mobile system. This RAN may be implemented as the GSM Base Station Sys-tem (BSS), GERAN, UTRAN or E-UTRAN. AS provides radio access bearers forboth connection-oriented, packet-switched services and connectionless (store-and-forward) services. In LTE/SAE there is no CS network part thus the AS differssignificantly from the one in older technologies.

The AS provides the connectivity between the nodes in the E-UTRAN. There arethree interfaces that are involved in the AS concept:

Radio interface connectivity between the UE and the E-UTRAN node theeNB.

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 applicationsusing the bearers in the network. Consequently, the bearers will always obtainappropriate QoS classes, according to the requirements of the service provided bythe application the UE utilises.

2.4.1 EPS Bearer

Similarly to UMTS, EPS implements a bearer concept for supporting end-user dataservices. The EPS bearer (similar to a Packet Data Protocol (PDP) context ofprevious 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), seeFigure 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. AnUL Traffic Flow Template (TFT) in the UE binds an Service Data Flow (SDF) toan EPS Bearer in the uplink direction. Multiple SDFs can be multiplexed onto thesame EPS Bearer by including multiple uplink packet filters in the UL TFT. ADL 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 multipledownlink 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 Bearerbetween the UE and the EPC. When an E-RAB exists, there is a one-to-one map-ping between this E-RAB and an EPS Bearer. An S5/S8 Bearer transports thepackets 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 aneNB. 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 ofan E-RAB between an eNB and a S-GW.

A UE stores a mapping between an uplink packet filter and a Radio Bearer tocreate the binding between an SDF and a Data Radio Bearer in the uplink. AP-GW stores a mapping between a downlink packet filter and an S5/S8a Bearer tocreate 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 tocreate the binding between a Radio Bearer and an S1 Bearer in both the uplink anddownlink.

A S-GW stores a one-to-one mapping between an S1 Bearer and an S5/S8a Bearerto create the binding between an S1 Bearer and an S5/S8a Bearer in both the uplinkand 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 ofsatisfaction of a user of a service.

Thus, QoS is connected with the way the user perceives the service. The user isnot interested in how a service is provided but only whether or not he or she issatisfied with that service. So, from a users perspective the QoS level is a very sub-jective thing and if the network does not provide the desired level of satisfaction, theuser may simply stop using the service and possibly change to some other operatoroffering a similar service with the desired QoS level.

QoS classes

In UMTS four different QoS classes (referred also to as traffic classes) have beendefined. These QoS classes are:

Conversational class, Streaming class, Interactive class, and Background class.

The main distinction between these QoS classes follows from how delay-sensitivethe 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 operatorowning the node.

QCI values indicate the QoS characteristics for edge-to-edge packet forwarding be-tween UE and Policy and Charging Enforcement Function (PCEF). Each QCI isassociated 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 networknodes while the above standardized characteristics are not. It is up to the operatorto map QCI values to the corresponding performance characteristics. The charac-teristics of QCI from 1 to 9 are standardized though and should be considered asguidelines when pre-configuring the node specific parameters. The goal of this op-eration is to ensure that applications mapped to a particular QCI receive the sameminimum level of QoS regardless access network they use (e.g. when UE is roamingor if the network operator uses equipment from different vendors).

Table 2.1 presents standardized QCI values mapped to the corresponding perfor-mance 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 QoSclasses parameters was specified in Time Slot (TS) 23.401. It is presented in Table2.2.

2.5 Integration with 2G and 3G

When an E-UTRAN system is deployed in a network already supporting GERANand/or UTRAN it is possible to use a common core network for all accesses. Inpractice this means that the P-GW will provide GGSN functionality towards theexisting GPRS CN. Therefore an E-UTRAN/UTRAN/GERAN capable terminalwill not need to change the GGSN (i.e., the IP point of presence towards externalnetworks) when it changes Radio Access Technology (RAT)) and switches betweenGERAN, UTRAN or E-UTRAN. Figure 2.3 shows how the EPS inter-works withexisting 2nd Generation (2G)/3G networks. The figure presents the UTRAN whenutilizing the GPRS one tunnel approach standardized in 3GPP Release 7. Thisfeature makes it possible to bypass the SGSN in the user plane.

Figure 2.4 shows a standardization view on how GERAN, UTRAN and E-UTRANare integrated into the SAE. It should however be noted that the SGSN and MMEshares 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 inEPS meaning that the SGSN and MME share a common evolution in the 3GPPstandard. In a typical implementation/deployment view, it is likely that the 2G/3GSGSN and the MME are merged into one node, as illustrated in Figure 2.4. This

37

2 EPS architecture

QCIResource

typePriority

Packet

delay

budget

Packet

error

loss

rate

Example service

1

GBR

2 100 ms 102 Conversational voice2 4 150 ms 103 Conversational video (live

streaming)

3 3 50 ms 103 Real-time gaming4 5 300 ms 106 Non-conversational video

(buffered streaming)

5

non-GBR

1 100 ms 106 IMS signalling6 6 300 ms 106 Video buffered

streaming,TCP basedservices (e.g. www, e-mail,chat, ftp, p2p file sharing,progressive video, etc.)

7 7 100 ms 103 Voice, video live streaming,interactive gaming

8 8 300 ms 106 Premium bearer for videobuffered streaming, TCPbased services (e.g. www,e-mail, chat, ftp, p2p filesharing, progressive video,etc) for premium subscribers

9 9 300 ms 106 Default bearer for video,TCP based services (etc. fornon-privilaged subscribers

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

QCI Traffic class

Traffic

Handling

Priority

Signalling

indication

Source statistics

descriptor

1 Conversational N/A N/A Speech

2 Conversational N/A N/A Unknown

3 Conversational N/A N/A Unknown

4 Streaming N/A N/A Unknown

5 Interactive 1 Yes N/A

6 Interactive 1 No N/A



9 Background N/A N/A N/A

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

38

2.6 Interfaces overview

Figure 2.3: E-UTRAN, UTRAN and GERAN architecture. GPRS one tunnelapproach.

will make it possible to support intra SGSN/MME and inter P/S-GW/GGSN nodemobility between the different accesses.


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 theend-users IP Point of Presence (PoP). All user-plane and control-plane functionsthat use the Gi interface are handled above the end-users IP layer, whereas allterminal 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 userplane this interface will be based on GTP User data tunnelling (GTP-U) (similarto 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 onetunnel approach.

to RAN Application Part (RANAP), with some simplifications and changes due tothe different functional split and mobility within EPS.

It has been agreed to split the S1 interface into a S1-CP (control plane) and S1-UP (user plane) part. The signalling transport on S1-CP will be based on StreamControl Transmission Protocol (SCTP). The signalling protocol for S1 is called S1Application Protocol (S1AP). S1AP protocol has the following functions:

EPS Bearer management function.This overall functionality is responsible for setting up, modifying and releasingEPS bearers, which are triggered by the MME The release of EPS bearers maybe triggered by the eNB as well.

Initial context transfer function.This functionality is used to establish an S1 UE context in the eNB, to setupthe default IP connectivity, to setup one or more SAE bearer(s) if requestedby the MME, and to transfer NAS signalling related information to the eNBif 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


S1 interface (with EPC involvement),

a change of RAN nodes between different RAT (inter-3GPP-RAT han-dovers) 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 inter-

face.

Error Indication functionality to allow a proper error reporting/handlingin cases where no failure messages are defined.

Overload function to indicate the load situation in the control plane ofthe S1 interface.

NAS signaling transport function between the UE and the MME is usedto:

transfer NAS signalling related information and to establish the S1 UEcontext in the eNB,

transfer NAS signalling related information when the S1 UE context inthe eNB is already established.

S1 UE context release function.This functionality is responsible to manage the release of UE specific contextin 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 activemode UE mobility (Packet Forwarding). This interface may also be used for multi-cell Radio Resource Management (RRM) functions. The X2-CP interface consistsof 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 supportloss-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 toanother eNB. Forwarding of user plane data is a part of the mobility manage-ment.

Load management.This function allows eNBs to indicate overload and traffic load to each other.

Reporting of general error situations.

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2 EPS architecture

This function allows reporting of general error situations, for which functionspecific 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 isbased on Gn/GTP Control plane (GTP-C) (SGSN-SGSN), possibly with some newfunctionality to support signalling free idle mode mobility between E-UTRAN andUTRAN/GERAN. S3 will not support packet forwarding; instead this will be sup-ported on the S4 interface.


The S3 interface is similar to the S10 interface between MMEs which will be usedfor intra-LTE mobility between two MME pool areas.

S4

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


The S4 interface is backwards compatible with the Gn interface.

S6

S6a enables transfer of subscription and authentication data for authenticating/au-thorizing user access to the evolved system (Authentication, authorisation and ac-counting (AAA) interface) between MME and Home Subscriber Server (HSS). S6dis 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 thesame interface, the difference being that S8 is used when roaming between differentoperators while S5 is network internal. The S5/S8 interface will exist in two variantsone based on Gn/GTP (SGSN-GGSN) and the other will use the Internet Engineer-ing Task Force (IETF) specified Proxy Mobile IP (PMIP) for mobility control withadditional mechanism to handle QoS.

The usage of PMIP or GTP on S5/S8 will not be visible over the S1 interface orin the terminal. In the non roaming case the S-GW and P-GW functions can beperformed in one physical node.

It has been agreed in 3GPP that the usage of PMIP or GTP on S5 and S8 shouldnot impact RAN behaviour or impact the terminals.

42


In the roaming case S8 is providing user and control plane between the S-GW in theVisited PLMN (VPLMN) and the P-GW in the Home PLMN (HPLMN). S8 is theinter 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 theHome Policy and Charging Rules Function (PCRF) and the Visited PCRF in orderto support local breakout function.

S10

S10 is a control interface between the MMEs which will be very similar to theS3 interface between the SGSN and MME. The interface is based on Gn/GTP-C(SGSN-SGSN) with additional functionality.


S11

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


S12

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

S13

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

SGi

SGi is the interface between the P-GW and the packet data network. Packet datanetwork may be an operator external public or private packet data network or anintra 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 theP-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 func-tionality. Although the functionality is not entirely the same, the MME conceptuallyconstitutes a control plane SGSN node. The CP terminal protocols terminate at theMME, which also manages the mobility contexts of the UEs. The same MME re-mains 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 ver-ify users identities, grant access to the network and track users activities,respectively. In addition, the MME performs ciphering and integrity protec-tion of NAS message signalling.

EPS Bearer handling.The MME manages the setting up, modification and tearing down of EPSBearers. It is assumed that a UE in E-UTRAN will always have one defaultEPS 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 trackedwith 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 movingbetween eNBs. The S-GW is only changed when the UE move to a new S-GWpool area while the P-GW is normally kept as long as the UE is attached to thenetwork.

The P/S-GW functionality is very similar to the existing GGSN node. The mainadditions are adding support for packet buffering during E-UTRAN paging andadditional 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-GWcan mainly be seen as a user plane node, however it also performs some QoS relatedsignalling (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 policycontrol functions.

Mobility Anchor IP PoP.The P-GW acts as a mobility anchor point which hides UE mobility from thefixed network. When a UE attaches to the network it is assigned an IP addressfrom 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 aconsequence of a Handover (HO), the UEs 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 op-erators control over its network resources and limit the potential for abuseby users, EPS QoS is network controlled. The policy control and enforcementfunctions associate users traffic flows with appropriate QoS classes and executerate policing to prohibit users or flows from exceeding the QoS limits speci-fied in users subscription agreements. DL traffic is policed in the P/S-GWwhereas UL traffic is policed in the eNB.

Charging.The charging function is responsible for charging the user for its traffic accord-ing to the rate that applies for a particular service, subscription etc.

Lawful Intercept.This function enables communications to be electronically intercepted, or eaves-dropped, by law enforcement agencies, should it be authorized by judicial orregulatory 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 elimi-nate 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 andEPC nodes where each eNB is associated with a set of MMEs and S-GWs calledan MME and S-GW pool. The resulting network is non-hierarchical. Independentpooling of MME and S-GW are supported, it is however not possible to change aS-GW without involving the MME.

An operator may pool MMEs and S-GWs into one or several pools depending onorganisation, regulatory requirements, transport providers etc. This is illustrated inFigure 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 operators network as a sharednetwork.

Figure 2.5: Inter-pool mobility.

The individual pooled MMEs and S-GW do not have to be located on the samephysical site, but can be distributed in the network. All pools of a particular op-erator are assumed to be interconnected by means of an interface similar to theS3/S4/S10/S11 interface.

When a UE attaches to the network, it is assigned to one of the MMEs that belongto the MME pool associated with the eNB through which the UE is attaching, theMME then selects an S-GW in the S-GW pool. No change of MME or S-GW isrequired while the UE moves around among eNBs belonging to the same MME orS-GW pool. If the UE moves out of the pools coverage it is reassigned to an MMEor S-GW in the pool associated with the new eNB.

The P-GW, which performs charging, policy enforcement and UEs IP PoP is notchanged when the S-GW is relocated. The main purpose of the S-GW is to act asa local mobility anchor and to buffer packets during E-UTRAN paging. In someequipment vendors views (for example Ericsson) S-GWs are rare and in most casesthe S-GW and P-GW functions are performed by the same physical node. MMErelocation may be more motivated since there may be limits on how many eNBs theMME is connected to.

46

2.8 LTE functions

Partially overlapping pools will also be supported. Overlapping pools may havesome benefits since it makes it possible to avoid some of the negative effects of hardpool borders, however it comes with extra complexity.

2.8 LTE functions

LTE is a synonym for the new systems radio access network, which officially isreferred to in 3GPP specifications as E-UTRAN. This radio network is functionallyan evolution of the 3G UTRAN, although the radio transmission technology hasbeen changed completely.

2.8.1 LTE general principles

The radio interface in LTE is developed according to the requirements of spectrumflexibility, spectrum efficiency, cost effectiveness etc. Robustness against time disper-sion has influenced the choice of transmission technique in both UL and DL.

Spectrum flexibility incorporates the possibility to use both paired and unpairedspectrum, 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 partin the standardisation of the radio interface. Actually, the LTE radio interfaceimplementation supports operation in any bandwidth between 1.4 and 20 MHz insteps 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 andreceive diversity, beamforming and spatial multiplexing (MIMO).

Furthermore, the ISI is reduced by the choice of OFDM in the DL and Single CarrierFrequency Division Multiple Access (SC-FDMA) in UL. Both of these methodsresults in a long symbol time and thus a reduced ISI, which increases the performancein 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 interfacefunctionality.

eNB is the RAN node in the EPS architecture that is responsible for radio transmis-sion to and reception from UEs in one or more cells. The eNB is connected to EPCnodes by means of an S1 interface. The eNB is also connected to its neighbour eNBsby means of the X2 interface. Some significant changes have been made to the eNBfunctional allocation compared to UTRAN. Most Release 6 Radio Network Con-troller (RNC) functionality has been moved to the E-UTRAN eNB. Below followsa 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 arerequested by and granted to MMEs in an ordered fashion. This arrangementsupports the MME pooling concept. S-GW pooling is managed by the MMEsand 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 performinghandover when necessary.

Control Plane (CP) and User Plane (UP) security.The ciphering of user plane data over the radio interface is terminated inthe eNB. Also the ciphering and integrity protection of RRC signalling isterminated in the eNB.

Shared channel handling.Since the eNB owns the cell resources, the eNB also handles the shared andrandom access channels used for signalling and initial access.

Segmentation/concatenation.Radio Link Control (RLC) Service Data Units (SDUs) received from the PDCPlayer consist of whole IP packets and may be larger than the transport blocksize provided by the physical layer. Thus, the RLC layer must support seg-mentation 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 ameans for quickly correcting most errors from the radio channel. To achievelow delay and efficient use of radio resources, the HARQ operates with anative error rate which is sufficient only for services with moderate error raterequirements such as for instance Voice over IP (VoIP). Lower error rates areachieved by letting an outer Automatic Repeat reQuest (ARQ) layer in theeNB handle the HARQ errors.

Scheduling.Scheduling with support for QoS provides for efficient scheduling of UP andCP 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 handleslayer one functions like link adaptation and power control.

Measurements and reporting.eNB provides functions for configuring and making measurements on the radioenvironment 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-cellRRM.

49

2 EPS architecture

50

3 LTE signalling

3.1 User plane

The protocols performing the user plane functions in the radio interface are asfollows:

Packet Data Convergence Protocol (PDCP), which maps the EPS beareronto the E-UTRAN radio bearer and performs Robust Header Compression(ROHC).

Radio Link Control (RLC), which maps the E-UTRAN radio bearer to alogical channel and performs segmentation, in-sequence delivery and retrans-missions.

Medium Access Control (MAC), which maps the logical channel to atransport channel and is responsible for HARQ and scheduling.

The physical layer, which maps the transport channel onto a physical chan-nel 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 asfollows:

RRC protocol, which is used to transfer the NAS information over the radiointerface.

PDCP. RLC. MAC. The physical layer.

The Figure 3.2 presents the LTE radio interface protocol architecture for the controlplane.

Figure 3.2: Control plane for LTE.

3.3 Protocols

3.3.1 Radio Resource Control (RRC)

The following control plane functions are agreed in 3GPP to be performed by theRadio Resource Control (RRC) layer:

Broadcast of system snformation related to the non-access stratum, Broadcast of System information related to the access stratum, 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 [! ([!)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 andInter Radio Access Technology (Inter-RAT) mobility,

Inter-cell handover, UE cell selection and reselection and control of cell selection and reselec-

tion,

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 multiplelayers),

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 in 3GPP agreed to be terminated by theeNB.

RRC specification aspects

The RRC specification includes a hierarchy of procedures, where the highest level iscalled High-level procedures covering e.g. Broadcast Control Channel (BCCH) ac-quisition, paging, RRC connection establishment, reestablishment, re-configurationand release. The content of high level procedure messages may then trigger Elemen-tary Procedures that execute e.g. measurement, radio resource or security configu-ration. Mobility is also described as an elementary procedure. A single high-levelprocedure 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 atthe UE or the relay at the eNB. The PDCP supports the following functions:

Header compression and decompression of IP data flows using the ROHC pro-tocol, at the transmitting and receiving entity, respectively.

Transfer of data (user plane or control plane). This function is used for con-veyance of data between users of PDCP services.

Maintenance of PDCP sequence numbers for radio bearers mapped on RLCacknowledged 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 perradio bearer. For example, UM could be used for VoIP while AM is used to carryTransmission Control Protocol (TCP)-based traffic. An RLC transparent modeexists as well, but it shall be only used to send RRC messages when no RLC UM orAM entity is set up, yet.

The RLC layer supports segmentation and concatenation of RLC SDUs. Dependingon the scheduler decision, a certain amount of data is selected from the RLC SDUbuffer and segmented and/or concatenated depending on the size of the SDUs. Thisselected 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 radiobearer except if an RLC retransmission is required. In this case an RLC PDUcontaining new data might be multiplexed at the MAC layer with an RLC PDUretransmission. In order to allow the RLC SDU reassembly at the receiver, theRLC header carries the required segmentation, re-segmentation and concatenationinformation. The RLC sequence number will also be used at the receiver for in-sequence delivery to the RLC SDU reassembly entity.

In AM, RLC is responsible for correcting residual HARQ errors by operating anotherARQ protocol since it would be expensive in terms of transmit power to reach therequired residual error rates of 105 or less in the MAC HARQ protocol.

The ARQ retransmission units are RLC PDUs or RLC PDU segments. If an RLCretransmission is required and the radio quality has changed significantly com-pared to the original RLC transmission, the RLC protocol is able to perform are-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 toensure that RLC SDUs are delivered in sequence to higher layers.

Retransmissions are initiated either by status reports sent by the RLC receiver or bylocal triggers from MAC layer in case of reaching the maximum number of HARQtransmissions. Status Reports are triggered either by polls sent from the RLCsender or by detecting missing PDUs after the PDUs have passed the reorderingentity. Similar to UTRAN, the LTE RLC supports a status prohibit timer and apoll 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 comparedto the Release 6 MAC-hs/MAC-e and covers mainly similar functionality: HARQ,priority handling (scheduling), transport format selection and Discontinuous Recep-tion (DRX) control (not part of MAC in Release 6).

The HARQ protocol is very similar to the solution adopted for High Speed Down-link 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 continuoustransmission, which cannot be achieved with a single stop-and-wait scheme, whileat the same time having some of the simplicity of a stop-and-wait protocol. Thefunctionality and performance is similar to that of a window based selective repeatprotocol but only single-bit HARQ feedback is required.

The protocol is modelled as a number of parallel HARQ processes, where eachprocess uses a simple stop-and-wait protocol. By using NHARQ parallel HARQprocesses, where NHARQ > Round trip time/Subframe length, a continuous trans-mission is achieved. The maximum UE processing time before sending a HARQfeedback has been specified such that 8 HARQ processes are needed for continuoustransmission in FDD with a typical eNB implementation.

In 3GPP, the current working assumption is to use a synchronous HARQ for theuplink and an asynchronous HARQ for the downlink. That is, for the uplink, thesubframe when the retransmission occurs is known at the receiver, while for thedownlink the scheduler has the freedom to choose the subframe for the retransmis-sion dynamically. For both up- and downlink a synchronous, single-bit HARQ feed-back Acknowledge (ACK)/Negative Acknowledge (NACK) is sent providing feed-back about the success of the previous transmission. The HARQ protocol is adap-tive in both uplink and downlink, meaning that the scheduler can decide to use adifferent 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 afirst transmission or a retransmission is indicated on the Physical Downlink ControlChannel (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 casethe received data is not a retransmission o

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