lte for 4g mobile broadband air interface technologies and performance

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LTE for 4G Mobile Broadband
Do you need to get up-to-speed quickly on Long-Term Evolution (LTE)?
Understand the new technologies of the LTE standard and how they contribute to
improvements in system performance with this practical and valuable guide, written by an
expert on LTE who was intimately involved in the drafting of the standard. In addition to
a strong grounding in the technical details, you’ll also get fascinating insights into why
particular technologies were chosen in the development process.
Core topics covered include:
•
Network architecture and protocols;
•
OFDMA downlink access;
•
Low-PAPR SC-FDMA uplink access;
•
Transmit diversity and MIMO spatial multiplexing;
•
Channel structure and bandwidths;
•
Cell search, reference signals and random access;
•
Turbo coding with contention-free interleaver;
•
Scheduling, link adaptation, hybrid ARQ and power control;
•
Uplink and downlink physical control signaling;
•
Inter-cell interference mitigation techniques;
•

Single-frequency network (SFN) broadcast;
•
MIMO spatial channel model;
•
Evaluation methodology and system performance.
With extensive references, a useful discussion of technologies that were not included in the
standard, and end-of-chapter summaries that draw out and emphasize all the key points,
this book is an essential resource for practitioners in the mobile cellular communications
industry and for graduate students studying advanced wireless communications.
Farooq Khan is Technology Director at the Samsung Telecom R&D Center, Dallas, Texas,
where he manages the design, performance evaluation, and standardization of next-
generation wireless communications systems. Previously, he was a Member of Technical
Staff at Bell Laboratories, where he conducted research on the evolution of cdma2000 and
UMTS systems towards high-speed packet access (HSPA). He also worked at Ericsson
Research in Sweden, contributing to the design and performance evaluation of EDGE and
WCDMA technologies. He has authored more than 30 research papers and holds over 50
US patents, all in the area of wireless communications.

LTE for 4G Mobile Broadband
Air Interface Technologies and Performance
FAROOQ KHAN
Telecom R&D Center
Samsung Telecommunications, America
CAMBRIDGE UNIVERSITY PRESS
Cambridge, New York, Melbourne, Madrid, Cape Town, Singapore, São Paulo
Cambridge University Press
The Edinburgh Building, Cambridge CB2 8RU, UK
First published in print format
ISBN-13 978-0-521-88221-7
ISBN-13 978-0-511-51666-5

© Cambridge University Press 2009
2009
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To my wonderful wife, Mobeena; our three precious children, Nemul, Haris
and Alisha; my father and to the memory of my late mother.

Contents
Preface page xiii
1 Introduction 1
1.1 Beyond 3G systems 2
1.2 Long-Term Evolution (LTE) 3

1.3 Evolution to 4G 4
References 4
2 Network architecture and protocols 5
2.1 Network architecture 5
2.2 QoS and bearer service architecture 8
2.3 Layer 2 structure 9
2.4 Protocol states and states transitions 13
2.5 Seamless mobility support 14
2.6 Multicast broadcast system architecture 15
2.7 Summary 19
References 19
3 Downlink access 20
3.1 OFDM 20
3.2 Downlink capacity comparison 28
3.3 Effect of frequency selectivity on OFDM performance 31
3.4 Single-carrier with FDE 36
3.5 Frequency diversity 51
3.6 OFDM/OQAM with pulse-shaping 62
3.7 Summary 67
References 68
4 Single-carrier FDMA 70
4.1 Single-carrier FDMA 71
4.2 Uplink capacity comparison 75
4.3 Capacity results 80
4.4 Hybrid uplink access 83
4.5 FFT precoding complexity 85
viii Contents
4.6 Summary 86
References 87
5 Reducing uplink signal peakiness 88

5.1 Measures of signal peakiness 89
5.2 PAPR of QAM modulations 91
5.3 Peakiness of SC-FDMA signal 95
5.4 Low PAPR modulations 96
5.5 Spectrum shaping 98
5.6 Coverage gain due to low signal peakiness 105
5.7 Summary 108
References 109
6 Transmit diversity 110
6.1 Transmit diversity schemes 111
6.2 Downlink transmission chain 140
6.3 Codeword to layer mapping 141
6.4 Transmit diversity precoding 142
6.5 Summary 145
References 146
7 MIMO spatial multiplexing 147
7.1 MIMO capacity 147
7.2 Codewords and layer mapping 149
7.3 Downlink MIMO transmission chain 153
7.4 MIMO precoding 156
7.5 CDD-based precoding 161
7.6 Open-loop spatial multiplexing 168
References 171
8 Channel structure and bandwidths 172
8.1 Channel bandwidths 172
8.2 UE radio access capabilities 173
8.3 Frame and slot structure 174
8.4 Frame structure type 2 177
8.5 Downlink distributed transmission 178
8.6 Uplink hopping 182

8.7 Summary 184
References 186
9 Cell search and reference signals 187
9.1 PN sequence 187
9.2 Zadoff−Chu (ZC) sequences 189
9.3 Downlink frame structure 190
9.4 Synchronization signals 191
Contents ix
9.5 Broadcast channel 195
9.6 Downlink reference signals 199
9.7 Uplink reference signals 209
9.8 Reference signals overhead 222
9.9 Summary 224
References 225
10 Random access 226
10.1 Random access preamble formats 226
10.2 RA sequence length and resource mapping 228
10.3 Random access configurations 231
10.4 RA preamble cyclic shifts 233
10.5 Signal peakiness of RA sequences 243
10.6 Random access MAC procedures and formats 244
10.7 Summary 248
References 250
11 Channel coding 251
11.1 LDPC codes 251
11.2 Channel coding schemes in LTE 253
11.3 Cyclic redundancy check 255
11.4 Codeblock segmentation 261
11.5 Turbo coding 264
11.6 Tail-biting convolutional code 271

11.7 Circular-buffer rate matching for turbo code 273
11.8 Circular-buffer rate matching for convolutional code 281
11.9 Codeblock concatenation 285
11.10 Channel interleaver 285
11.11 Summary 287
References 289
12 Scheduling, link adaptation and hybrid ARQ 291
12.1 Channel-sensitive scheduling 292
12.2 Frequency-selective multi-user scheduling 298
12.3 Proportional fair scheduling 301
12.4 Link adaptation 303
12.5 Hybrid ARQ 308
12.6 Hybrid ARQ in the LTE system 325
12.7 Summary 328
References 328
13 Power control 329
13.1 Uplink power control 329
13.2 Downlink power control 334
x Contents
13.3 Summary 341
References 341
14 Uplink control signaling 342
14.1 Data control multiplexing 343
14.2 Control signaling contents 345
14.3 Periodic reporting 349
14.4 Aperiodic reporting 359
14.5 Summary 365
References 367
15 Downlink control signaling 368
15.1 Data control multiplexing 369

15.2 Resource element groups 370
15.3 Control format indicator channel 372
15.4 Downlink resource allocation 374
15.5 Downlink control information 378
15.6 Hybrid ARQ indicator 396
15.7 Summary 407
References 408
16 Inter-cell interference control 409
16.1 Inter-cell interference 409
16.2 Inter-cell interference mitigation 411
16.3 Cell-edge performance 412
16.4 Cell-center performance 416
16.5 Fractional frequency reuse 419
16.6 Fractional loading 421
16.7 ICI co-ordination in the LTE system 424
16.8 Summary 425
References 425
17 Single frequency network broadcast 426
17.1 Multicast broadcast system 427
17.2 Single frequency network 427
17.3 Multiplexing of MBSFN and unicast 431
17.4 MBSFN and unicast superposition 438
17.5 Summary 445
References 447
18 Spatial channel model 448
18.1 Multi-path fading 448
18.2 SCM channel scenarios 450
18.3 Path-loss models 451
Contents xi
18.4 SCM user parameters 453

18.5 SCM channel coefficients 463
18.6 SCM extension 464
18.7 Summary 466
References 466
19 LTE performance verification 468
19.1 Traffic models 468
19.2 System simulations scenarios and parameters 475
19.3 Link to system performance mapping 479
19.4 System performance 482
19.5 Summary 485
References 487
Index 488

Preface
The Global system for mobile communications (GSM) is the dominant wireless cellular
standard with over 3.5 billion subscribers worldwide covering more than 85% of the global
mobile market. Furthermore, the number of worldwide subscribers using high-speed packet
access (HSPA) networkstopped 70million in2008. HSPAis a 3G evolution of GSM supporting
high-speed data transmissions using WCDMA technology. Global uptake of HSPAtechnology
among consumers and businesses is accelerating, indicating continued traffic growth for high-
speed mobile networks worldwide. In order to meet the continued traffic growth demands,
an extensive effort has been underway in the 3G Partnership Project (3GPP) to develop a
new standard for the evolution of GSM/HSPA technology towards a packet-optimized system
referred to as Long-Term Evolution (LTE).
The goal of the LTE standard is to create specifications for a new radio-access technology
geared to higher data rates, low latency and greater spectral efficiency. The spectral efficiency
target for the LTE system is three to four times higher than the current HSPA system. These
aggressive spectral efficiency targets require pushing the technology envelope by employing
advanced air-interface techniques such as low-PAPR orthogonal uplink multiple access
based on SC-FDMA(single-carrier frequency division multiple access) MIMO multiple-input

multiple-output multi-antenna technologies, inter-cell interference mitigation techniques, low-
latency channel structure and single-frequency network (SFN) broadcast. The researchers
and engineers working on the standard come up with new innovative technology proposals
and ideas for system performance improvement. Due to the highly aggressive standard
development schedule, these researchers and engineers are generally unable to publish
their proposals in conferences or journals, etc. In the standards development phase, the
proposals go through extensive scrutiny with multiple sources evaluating and simulating
the proposed technologies from system performance improvement and implementation
complexity perspectives. Therefore, only the highest-quality proposals and ideas finally make
into the standard.
The book provides detailed coverage of the air-interface technologies and protocols that
withstood the scrutiny of the highly sophisticated technology evaluation process typically
used in the 3GPP physical layer working group. We describe why certain technology choices
were made in the standard development process and how each of the technology components
selected contributes to the overall system performance improvement. As such, the book serves
as a valuable reference for system designers and researchers not directly involved in the
standard development phase.
I am indebted to many colleagues at Samsung, in particular to Zhouyue (Jerry) Pi,
Jianzhong (Charlie) Zhang, Jiann-An Tsai, Juho Lee, Jin-Kyu Han and Joonyoung Cho. These
colleagues and other valued friends, too numerous to be mentioned, have deeply influenced my
xiv Preface
understanding of wireless communications and standards. Without the unprecedented support
of Phil Meyler, Sarah Matthews, Dawn Preston and their colleagues at Cambridge University
Press, this monograph would never have reached the readers. Finally my sincere gratitude
goes to the numerous researchers and engineers who contributed to the development of the
LTE standard in 3GPP, without whom this book would not have materialized.
1 Introduction
The cellular wireless communications industry witnessed tremendous growth in the past
decade with over four billion wireless subscribers worldwide. The first generation (1G)
analog cellular systems supported voice communication with limited roaming. The second

generation (2G) digital systems promised higher capacity and better voice quality than
did their analog counterparts. Moreover, roaming became more prevalent thanks to fewer
standards and common spectrum allocations across countries particularly in Europe. The two
widely deployed second-generation (2G) cellular systems are GSM (global system for mobile
communications) and CDMA (code division multiple access). As for the 1G analog systems,
2G systems were primarily designed to support voice communication. In later releases of
these standards, capabilities were introduced to support data transmission. However, the data
rates were generally lower than that supported by dial-up connections. The ITU-R initiative
on IMT-2000 (international mobile telecommunications 2000) paved the way for evolution to
3G. A set of requirements such as a peak data rate of 2 Mb/s and support for vehicular mobility
were published under IMT-2000 initiative. Both the GSM and CDMA camps formed their
own separate 3G partnership projects (3GPP and 3GPP2, respectively) to develop IMT-2000
compliant standards based on the CDMAtechnology. The 3G standard in 3GPP is referred to as
wideband CDMA (WCDMA) because it uses a larger 5 MHz bandwidth relative to 1.25 MHz
bandwidth used in 3GPP2’s cdma2000 system. The 3GPP2 also developed a 5 MHz version
supporting three 1.25 MHz subcarriers referred to as cdma2000-3x. In order to differentiate
from the 5 MHz cdma2000-3x standard, the 1.25 MHz system is referred to as cdma2000-1x
or simply 3G-1x.
The first release of the 3G standards did not fulfill its promise of high-speed data
transmissions as the data rates supported in practice were much lower than that claimed
in the standards. A serious effort was then made to enhance the 3G systems for efficient data
support. The 3GPP2 first introduced the HRPD (high rate packet data) [1] system that used
various advanced techniques optimized for data traffic such as channel sensitive scheduling,
fast link adaptation and hybrid ARQ, etc. The HRPD system required a separate 1.25 MHz
carrier and supported no voice service. This was the reason that HRPD was initially referred
to as cdma2000-1xEVDO (evolution data only) system. The 3GPP followed a similar path
and introduced HSPA (high speed packet access) [2] enhancement to the WCDMA system.
The HSPA standard reused many of the same data-optimized techniques as the HRPD system.
A difference relative to HRPD, however, is that both voice and data can be carried on the same
5 MHz carrier in HSPA. The voice and data traffic are code multiplexed in the downlink. In

parallel to HRPD, 3GPP2 also developed a joint voice data standard that was referred to as
cdma2000-1xEVDV (evolution data voice) [3]. Like HSPA, the cdma2000-1xEVDV system
supported both voice and data on the same carrier but it was never commercialized. In the
2 Introduction
later release of HRPD, VoIP (Voice over Internet Protocol) capabilities were introduced to
provide both voice and data service on the same carrier. The two 3G standards namely HSPA
and HRPD were finally able to fulfill the 3G promise and have been widely deployed in major
cellular markets to provide wireless data access.
1.1 Beyond 3G systems
While HSPA and HRPD systems were being developed and deployed, IEEE 802 LMSC
(LAN/MAN Standard Committee) introduced the IEEE 802.16e standard [4] for mobile
broadband wireless access. This standard was introduced as an enhancement to an earlier
IEEE 802.16 standard for fixed broadband wireless access. The 802.16e standard employed a
different access technology named OFDMA (orthogonal frequency division multiple access)
and claimed better data rates and spectral efficiency than that provided by HSPA and HRPD.
Although the IEEE 802.16 family of standards is officially called WirelessMAN in IEEE, it
has been dubbed WiMAX (worldwide interoperability for microwave access) by an industry
group named the WiMAX Forum. The mission of the WiMAX Forum is to promote and certify
the compatibility and interoperability of broadband wireless access products. The WiMAX
system supporting mobility as in IEEE 802.16e standard is referred to as Mobile WiMAX. In
addition to the radio technology advantage, Mobile WiMAX also employed a simpler network
architecture based on IP protocols.
The introduction of Mobile WiMAX led both 3GPP and 3GPP2 to develop their own
version of beyond 3G systems based on the OFDMA technology and network architecture
similar to that in Mobile WiMAX. The beyond 3G system in 3GPP is called evolved universal
terrestrial radio access (evolved UTRA) [5] and is also widely referred to as LTE (Long-Term
Evolution) while 3GPP2’s version is called UMB (ultra mobile broadband) [6] as depicted
in Figure 1.1. It should be noted that all three beyond 3G systems namely Mobile WiMAX,
GSM WCDMA/HSPA LTE LTE-advancedLTE-advanced
CDMA

Cdma2000/
HRPD
UMB
802.16e/
WiMAX
802.12.16m802.16m
2G 3G/IMT-2000 Beyond 3G and 4G/IMT-advanced
3GPP
3GPP2
IEEE
802
LMSC
Figure 1.1. Cellular systems evolution.
1.2 Long-Term Evolution (LTE) 3
Table 1.1. LTE system attributes.
Bandwidth 1.25–20 MHz
Duplexing FDD, TDD, half-duplex FDD
Mobility 350 km/h
Multiple access
Downlink OFDMA
Uplink SC-FDMA
MIMO
Downlink 2 × 2, 4 ×2, 4 ×4
Uplink 1 × 2, 1 × 4
Peak data rate in 20 MHz
Downlink 173 and 326 Mb/s for 2 × 2 and 4 × 4
MIMO, respectively
Uplink 86 Mb/s with 1×2 antenna configuration
Modulation QPSK, 16-QAM and 64-QAM
Channel coding Turbo code

Other techniques Channel sensitive scheduling, link
adaptation, power control, ICIC and
hybrid ARQ
LTE and UMB meet IMT-2000 requirements and hence they are also part of IMT-2000 family
of standards.
1.2 Long-Term Evolution (LTE)
The goal of LTE is to provide a high-data-rate, low-latency and packet-optimized radio-
access technology supporting flexible bandwidth deployments [7]. In parallel, new network
architecture is designed with the goal to support packet-switched traffic with seamless
mobility, quality of service and minimal latency [8].
The air-interface related attributes of the LTE system are summarized in Table 1.1. The
system supports flexible bandwidths thanks to OFDMA and SC-FDMA access schemes. In
addition to FDD (frequency division duplexing) and TDD (time division duplexing), half-
duplex FDD is allowed to support low cost UEs. Unlike FDD, in half-duplex FDD operation
a UE is not required to transmit and receive at the same time. This avoids the need for a costly
duplexer in the UE. The system is primarily optimized for low speeds up to 15 km/h. However,
the system specifications allow mobility support in excess of 350 km/h with some performance
degradation. The uplink access is based on single carrier frequency division multiple access
(SC-FDMA) that promises increased uplink coverage due to low peak-to-average power ratio
(PAPR) relative to OFDMA.
The system supports downlink peak data rates of 326 Mb/s with 4 × 4 MIMO (multiple
input multiple output) within 20 MHz bandwidth. Since uplink MIMO is not employed in
the first release of the LTE standard, the uplink peak data rates are limited to 86 Mb/s within
20 MHz bandwidth. In addition to peak data rate improvements, the LTE system provides two
to four times higher cell spectral efficiency relative to the Release 6 HSPA system. Similar
improvements are observed in cell-edge throughput while maintaining same-site locations
as deployed for HSPA. In terms of latency, the LTE radio-interface and network provides
capabilities for less than 10 ms latency for the transmission of a packet from the network to
the UE.
4 Introduction

1.3 Evolution to 4G
The radio-interface attributes for Mobile WiMAX and UMB are very similar to those of
LTE given in Table 1.1. All three systems support flexible bandwidths, FDD/TDD duplexing,
OFDMA in the downlink and MIMO schemes. There are a few differences such as uplink
in LTE is based on SC-FDMA compared to OFDMA in Mobile WiMAX and UMB. The
performance of the three systems is therefore expected to be similar with small differences.
Similar to the IMT-2000 initiative, ITU-R Working Party 5D has stated requirements for
IMT-advanced systems. Among others, these requirements include average downlink data
rates of 100 Mbit/s in the wide area network, and up to 1 Gbit/s for local access or low-
mobility scenarios. Also, at the World Radiocommunication Conference 2007 (WRC-2007),
a maximum of a 428 MHz new spectrum is identified for IMT systems that also include a
136 MHz spectrum allocated on a global basis.
Both 3GPP and IEEE 802 LMSC are actively developing their own standards for submission
to IMT-advanced. The goal for both LTE-advanced [9] and IEEE 802.16 m [10] standards
is to further enhance system spectral efficiency and data rates while supporting backward
compatibility with their respective earlier releases. As part of the LTE-advanced and IEEE
802.16 standards developments, several enhancements including support for a larger than
20 MHz bandwidth and higher-order MIMO are being discussed to meet the IMT-advanced
requirements.
References
[1] 3GPP2 TSG C.S0024-0 v2.0, cdma2000 High Rate Packet Data Air Interface Specification.
[2] 3GPP TSG RAN TR 25.848 v4.0.0, Physical Layer Aspects of UTRA High Speed Downlink
Packet Access.
[3] 3GPP2 TSG C.S0002-C v1.0, Physical Layer Standard for cdma2000 Spread Spectrum Systems,
Release C.
[4] IEEE Std 802.16e-2005,Air Interface for Fixed and Mobile Broadband WirelessAccess Systems.
[5] 3GPP TSG RAN TR 25.912 v7.2.0, Feasibility Study for Evolved Universal Terrestrial Radio
Access (UTRA) and Universal Terrestrial Radio Access Network (UTRAN).
[6] 3GPP2 TSG C.S0084-001-0 v2.0, Physical Layer for Ultra Mobile Broadband (UMB) Air
Interface Specification.

[7] 3GPP TSG RAN TR 25.913 v7.3.0, Requirements for Evolved Universal Terrestrial Radio
Access (UTRA) and Universal Terrestrial Radio Access Network (UTRAN).
[8] 3GPP TSG RAN TR 23.882 v1.15.1, 3GPP SystemArchitecture Evolution: Report on Technical
Options and Conclusions.
[9] 3GPP TSG RAN TR 36.913 v8.0.0, Requirements for Further Advancements for E-UTRA
(LTE-Advanced).
[10] IEEE 802.16m-07/002r4, TGm System Requirements Document (SRD).
2 Network architecture and protocols
The LTE network architecture is designed with the goal of supporting packet-switched traffic
with seamless mobility, quality of service (QoS) and minimal latency. A packet-switched
approach allows for the supporting of all services including voice through packet connections.
The result in a highly simplified flatter architecture with only two types of node namely evolved
Node-B (eNB) and mobility management entity/gateway (MME/GW). This is in contrast to
many more network nodes in the current hierarchical network architecture of the 3G system.
One major change is that the radio network controller (RNC) is eliminated from the data
path and its functions are now incorporated in eNB. Some of the benefits of a single node in
the access network are reduced latency and the distribution of the RNC processing load into
multiple eNBs. The elimination of the RNC in the access network was possible partly because
the LTE system does not support macro-diversity or soft-handoff.
In this chapter, we discuss network architecture designs for both unicast and broadcast
traffic, QoS architecture and mobility management in the access network. We also briefly
discuss layer 2 structure and different logical, transport and physical channels along with
their mapping.
2.1 Network architecture
All the network interfaces are based on IP protocols. The eNBs are interconnected by means of
an X2 interface and to the MME/GW entity by means of an S1 interface as shown in Figure 2.1.
The S1 interface supports a many-to-many relationship between MME/GW and eNBs [1].
The functional split between eNB and MME/GW is shown in Figure 2.2. Two logical
gateway entities namely the serving gateway (S-GW) and the packet data network gateway
(P-GW) are defined. The S-GW acts as a local mobility anchor forwarding and receiving

packets to and from the eNB serving the UE. The P-GW interfaces with external packet
data networks (PDNs) such as the Internet and the IMS. The P-GW also performs several IP
functions such as address allocation, policy enforcement, packet filtering and routing.
The MME is a signaling only entity and hence user IP packets do not go through MME. An
advantage of a separate network entity for signaling is that the network capacity for signaling
and traffic can grow independently. The main functions of MME are idle-mode UE reachability
including the control and execution of paging retransmission, tracking area list management,
roaming, authentication, authorization, P-GW/S-GW selection, bearer management including
dedicated bearer establishment, security negotiations and NAS signaling, etc.
Evolved Node-B implements Node-B functions as well as protocols traditionally
implemented in RNC. The main functions of eNB are header compression, ciphering and
reliable delivery of packets. On the control side, eNB incorporates functions such as admission
6 Network architecture and protocols
eNB
MME/GW MME/GW
X
2
X2
1S
eNB
eNB
1S
X
2
S
1
S
1
Figure 2.1. Network architecture.
eNB

RB Control
Connection Mobility Cont.
eNB Measurement
Configuration & Provision
Dynamic Resource
Allocation (Scheduler)
PDCP
PHY
MME
S-GW
MAC
Inter Cell RRM
Radio Admission Control
RLC
RRC
Mobility
Anchoring
EPS Bearer Control
Idle State Mobility Handling
NAS Security
UE IP
address
allocation
Packet
Filtering
MME/GW
S1
PDN
(e.g. Internet)
P-GW

Figure 2.2. Functional split between eNB and MME/GW.
control and radio resource management. Some of the benefits of a single node in the access
network are reduced latency and the distribution of RNC processing load into multiple eNBs.
The user plane protocol stack is given in Figure 2.3. We note that packet data convergence
protocol (PDCP) and radio link control (RLC) layers traditionally terminated in RNC on
2.1 Network architecture 7
eNB
PDCP
PHY
MAC
RLC
Gateway
IP
UE
PDCP
PHY
MAC
RLC
IP
Figure 2.3. User plane protocol.
eNB
PDCP
PHY
MAC
RLC
MME
NAS
UE
PDCP
PHY

MAC
RLC
NAS
RRC
RRC
Figure 2.4. Control plane protocol stack.
the network side are now terminated in eNB. The functions performed by these layers are
described in Section 2.2.
Figure 2.4 shows the control plane protocol stack. We note that RRC functionality
traditionally implemented in RNC is now incorporated into eNB. The RLC and MAC layers
perform the same functions as they do for the user plane. The functions performed by the
RRC include system information broadcast, paging, radio bearer control, RRC connection
management, mobility functions and UE measurement reporting and control. The non-access
stratum (NAS) protocol terminated in the MME on the network side and at the UE on the
terminal side performs functions such as EPS (evolved packet system) bearer management,
authentication and security control, etc.
The S1 and X2 interface protocol stacks are shown in Figures 2.5 and 2.6 respectively.
We note that similar protocols are used on these two interfaces. The S1 user plane interface
(S1-U) is defined between the eNB and the S-GW. The S1-U interface uses GTP-U (GPRS
tunneling protocol – user data tunneling) [2] on UDP/IP transport and provides non-guaranteed
delivery of user plane PDUs between the eNB and the S-GW. The GTP-U is a relatively simple
IP based tunneling protocol that permits many tunnels between each set of end points. The
S1 control plane interface (S1-MME) is defined as being between the eNB and the MME.
Similar to the user plane, the transport network layer is built on IP transport and for the reliable
8 Network architecture and protocols
PHY
Data link layer
IP
UDP
GTP-U

User-plane PDUs
PHY
Data link layer
IP
SCTP
S1-AP
User plane (eNB-S-GW) Control plane (eNB-MME)
Figure 2.5. S1 interface user and control planes.
PHY
Data link layer
IP
UDP
GTP-U
User-plane PDUs
PHY
Data link layer
IP
SCTP
X2-AP
User plane (eNB-S-GW) Control plane (eNB-MME)
Figure 2.6. X2 interface user and control planes.
transport of signaling messages SCTP (stream control transmission protocol) is used on top of
IP. The SCTP protocol operates analogously to TCP ensuring reliable, in-sequence transport
of messages with congestion control [3]. The application layer signaling protocols are referred
to as S1 application protocol (S1-AP) and X2 application protocol (X2-AP) for S1 and X2
interface control planes respectively.
2.2 QoS and bearer service architecture
Applications such as VoIP, web browsing, video telephony and video streaming have special
QoS needs. Therefore, an important feature of any all-packet network is the provision of
a QoS mechanism to enable differentiation of packet flows based on QoS requirements. In

EPS, QoS flows called EPS bearers are established between the UE and the P-GW as shown
in Figure 2.7. A radio bearer transports the packets of an EPS bearer between a UE and an
eNB. Each IP flow (e.g. VoIP) is associated with a different EPS bearer and the network can