_____________________________________ COMPARISON BETWEEN WiMAX AND 3GPP LTE ______________________________________

_____________________________________ COMPARISON BETWEEN WiMAX AND 3GPP LTE ______________________________________
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_____________________________________
COMPARISON BETWEEN WiMAX AND
3GPP LTE
Syed Hamid Ali Shah
Mudasar Iqbal
Tassadaq Hussain
This thesis is presented as part of Degree of
Master of Science in Electrical Engineering
Blekinge Institute of Technology
August 2009
______________________________________
Blekinge Institute of Technology
School of Computing
Examiner: Dr. Doru Constantinescu
Supervisor: Dr. Doru Constantinescu
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ABSTRACT
Mobile communication technology evolved rapidly over the last few years due to increasing
demands such as accessing Internet services on mobile phones with a better quality of the
offered services. In order to fulfil this, wireless telecommunication industry worked hard and
defined a new air interface for mobile communications which enhances the overall system
performance by increasing the capacity of the system along with improving spectral
efficiencies while reducing latencies.
For this, two technologies, called Worldwide Interoperability for Microwave Access
(WiMAX) and Third Generation Partnership Project Long Term Evolution (3GPP LTE),
emerged with an aim of providing voice, data, video and multimedia services on mobile
phones at high speeds and cheap rates.
In this thesis, we have conducted a detailed comparative study between WiMAX and 3GPP
LTE by focusing on their first two layers, i.e. Physical and MAC layer. The comparison
specifically includes system architecture, radio aspects of the air interface (such as frequency
band, radio access modes, multiple access technologies, multiple antenna technologies and
modulation), protocol aspects of the air interface (in terms of protocol architecture,
modulation and frame structure), mobility and Quality of Service (QoS). We have also given a
brief comparative summary of both technologies in our thesis.
In the thesis, we investigated the LTE uplink and performed link level simulations of Single
Carrier Frequency Domain Equalization (SC-FDE) and Single Carrier Frequency Division
Multiple Access (SC-FDMA) in comparison with Orthogonal Frequency Division
Multiplexing (OFDM). The comparison has been in terms of Signal-to-Noise Ratio (SNR)
and Symbol Error Rate (SER). In order to verify the theoretical results, we simulated the Peak
to Average Power Ratio (PAPR) of SC-FDMA system in comparison with OFDMA. We also
simulated the capacity of Multiple Input Multiple Output (MIMO) systems in comparison
with Single Input Single Output (SISO) systems.
The simulation was performed on a PC running MATLAB 7.40 (R2007a). The operating
system used in the simulation was Microsoft Windows Vista.
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ACKNOWLEDGEMENTS
First of all, we are grateful to ALLAH ALMIGHTY, the most merciful, the most beneficent,
who gave us strength, guidance and abilities to complete this thesis in a successful manner.
We are thankful to our parents and our teachers that guided us throughout our career path
especially in building up our base in education and enhance our knowledge. We are indebted
to our advisor, Dr. Doru Constantinescu for his kind supervision. His co-operation and
support really helped us completing our project.
We are also thankful to our siblings for their support and guidance during our thesis work.
Finally, we would like to thank our friends and roommates for their moral support. I, Syed
Hamid Ali Shah, would like to say a special thank to Muhammad Saad Khan for his moral
support and strong motivation during my thesis.
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DEDICATION
I, Syed Hamid Ali Shah dedicate my thesis work to my parents, siblings and my beloved
nephew Syed Ajmal Ali Shah.
I, Tassadaq Hussain would like to dedicate my thesis to my family, especially my nephews
and nieces.
I, Mudasar Iqbal dedicate my thesis project and degree to my parents.
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Table of Contents
__________________________________________________
List of Figures
ix
List of Tables
xii
List of Acronyms
xiv
1. Introduction
1
1.1
Objective
2
1.2
Thesis outline
2
2.
Introduction to WiMAX
3
2.1
Overview of WiMAX
3
2.2
IEEE 802.16 Standards
3
2.2.1 IEEE 802.16-2001
3
2.2.2 IEEE 802.16a-2003
3
2.2.3 IEEE 802.16c
4
2.2.4 IEEE 802.16d-2004
4
2.2.5 IEEE 802.16e-2005
4
2.3
Fixed Vs Mobile WiMAX
4
2.4
IEEE 802.16 Protocol Layers
6
2.5
Physical Layer of IEEE 802.16
7
2.5.1 WirelessMAN OFDM PHY
7
2.5.2 Overview of OFDM
8
2.5.3 Time Domain OFDM
8
2.5.4 Frequency Domain OFDM
9
2.5.5 Parameters of OFDM
9
2.6
2.5.5.1
OFDM PHY for Fixed WiMAX
9
2.5.5.2
OFDMA PHY for Mobile WiMAX
10
2.5.6 Advantages and Disadvantages of OFDM
11
2.5.7 Features of WirelessMAN OFDM PHY
11
MAC Layer of IEEE 802.16
12
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2.7
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2.6.1 MAC Frame Format
13
2.6.2 Aggregation
15
2.6.3 Fragmentation
15
2.6.4 Transmission and Connection setup
16
2.6.5 Automatic Repeat Request
17
2.6.7 Features of MAC Layer
17
Multi Antenna Technologies
18
2.7.1 Smart Antenna System
19
2.7.1.1
Switch Beam Antenna
19
2.7.1.2
Adaptive Array Antenna
19
2.7.2 Diversity Techniques
20
2.7.3 MIMO
20
2.7.3.1
Open loop MIMO System
20
2.7.3.2
Closed loop MIMO System
20
2.8
Network Architecture of WiMAX
21
3.
Long Term Evolution
22
3.1
Overview of 3GPP Long Term Evolution
22
3.2
LTE Performance Targets
22
3.3
LTE Physical Layer
23
3.3.1 General Frame Structure
23
3.3.2 LTE Physical Layer for downlink Transmission
25
3.3.2.1
Modulation Parameters
25
3.3.2.2
Downlink Physical Resource
26
3.2.2.3
LTE Physical Channels for Downlink
27
3.2.2.4
LTE Downlink Physical Signals
28
3.2.2.5
LTE Downlink Transport Channel
30
3.2.2.6
Mapping of Downlink Transport Channels to Downlink
31
Physical Channels
3.2.2.7
OFDMA Basics
31
3.2.2.8
Downlink Physical Layer Processing
34
3.3.3 Uplink Physical Layer
3.3.3.1
36
Modulation Parameters
vi
36
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3.3.3.2
Uplink Physical Resource
36
3.3.3.3
LTE Uplink Physical Channels
38
3.3.3.4
Uplink Physical Signals
39
3.3.3.5
LTE Uplink Transport Channels
41
3.3.3.6
Mapping of Uplink Transport Channels to Uplink
41
Physical Channels
3.3.3.7
Single Carrier FDMA Basics
41
3.3.3.8
Uplink Physical layer Processing
45
3.3.4 Multi Antenna Techniques in LTE
3.3.4.1
LTE MIMO
47
3.3.4.2
Downlink MIMO
47
3.3.4.2.1
Spatial Multiplexing
47
3.3.4.2.2
Transmit Diversity
48
3.3.4.3
3.4
47
Uplink MIMO
48
LTE MAC Layer
49
3.4.1 Logical Channels
50
3.4.2 Mapping of Logical Channels to Transport Channels
51
3.4.3 Data Flow in MAC
51
4.
Comparison between WiMAX and LTE
54
4.1
Introduction
54
4.2
System Architecture
54
4.2.1 WiMAX Architecture
54
4.2.1.1
Network Reference Model
4.2.2 LTE Architecture
4.3
55
56
4.2.2.1
Core Network
57
4.2.2.2
Access Network
58
Radio Aspects of Air Interface
60
4.3.1 Frequency Bands
61
4.3.2 Radio Access Modes
62
4.3.3 Data Rates
62
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4.3.4 Multiple Access Technology
62
4.3.4.1
OFDMA
62
4.3.4.2
SC-FDMA
62
4.3.5 Modulation Parameters
63
4.3.6 Multiple Antenna Techniques
64
Protocol Aspects of Air Interface
64
4.4.1 Protocol Architecture
64
4.4.2 Modulation
66
4.4.3 Frame Structure
66
4.5
Quality of Service
68
4.6
Mobility
69
4.7
Comparative Summary
69
5.
Simulation
72
5.1
Introduction
72
5.2
Link Level Simulation of SC-FDE
72
5.2.1 SER for SC-FDE and OFDM using MMSE as Equalization Scheme
74
5.2.2 SER for SC-FDE and OFDM using Zero Forcing
76
5.2.3 Comparison of SC-FDE and OFDM with/without CP
78
5.3
Link Level Simulation of SC-FDMA
80
5.4
Peak-to-Average Power Ratio
82
5.4.1 PAPR-SC-FDMA Calculation using QPSK
84
5.4.2 PAPR-SC-FDMA Calculation using 16-QAM
85
5.4.3 PAPR Calculation for OFDM
85
5.5
Capacity of MIMO System
87
6.
Conclusions and Future Work
89
6.1
6.2
Conclusions
Future Work
89
89
4.4
References
90
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List of Figures
__________________________________________________
Figure 1.1 Evolution Path of Mobile Technologies towards 4G
2
Figure 2.1 Protocol Stack of IEEE 802.16
6
Figure 2.2 Comparison between Conventional FDM and OFDM
8
Figure 2.3 Cyclic Prefix in Time Domain
9
Figure 2.4 WiMAX OFDM Symbol in Frequency Domain
9
Figure 2.5 Architecture of WiMAX
13
Figure 2.6(a) Generic MAC Frame Having Management Payload
13
Figure 2.6(b) Generic MAC Frame Having Transport Payload
14
Figure 2.7 Generic MAC Header (GMH)
14
Figure 2.8 Multiple MSDUs Packed into MPDU
15
Figure 2.9 Single MSDU Packed into Multiple MAC Packets Data Units (MPDUs)
16
Figure 2.10 ARQ MAC Frame Format
17
Figure 2.11 Switched Beam Antenna
19
Figure 2.12 Adaptive Array System
19
Figure 2.13 General MIMO System
20
Figure 2.14 WiMAX Multiple Antenna Implementation Organization Chart
21
Figure 2.15 WiMAX Network Architecture
21
Figure 3.1 Generic Frame Structure for Downlink and Uplink LTE
24
Figure 3.2 Downlink and Uplink Subframe Assignment for FDD
24
Figure 3.3(a) Downlink Subframe Assignment for TDD
24
Figure 3.3(b) Uplink Subframe Assignment for TDD
25
Figure 3.4 LTE Downlink Physical Resource
27
Figure 3.5 Cell Specific Reference Signals
29
Figure 3.6 Mapping of Downlink Transport Channels to Physical Channels
31
Figure 3.7 FFT Operation Applied to Various Inputs in Time Domain
32
Figure 3.8 Transmitter-Receiver Block diagram of OFDMA
33
Figure 3.9 Structures of OFDMA Resource Blocks
34
Figure 3.10 LTE Physical Layer Processing in Downlink
34
Figure 3.11 Downlink Scrambling
35
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Figure 3.12 Downlink Modulation
35
Figure 3.13(a) Uplink Slot Structure in Case of Normal CP
37
Figure 3.13(b) Uplink Slot Structure in Case of Extended CP
37
Figure 3.14 Resource Grid for LTE Uplink
38
Figure 3.15 Random Access Preamble Format
39
Figure 3.16 Format of Random Access Preamble
40
Figure 3.17 Random Access Preamble Functionality
40
Figure 3.18 Mapping of Uplink Transport and Physical Channels
41
Figure 3.19 SC-FDMA Transmitter
42
Figure 3.20(a) Localized FDMA
42
Figure 3.20(b) Distributed FDMA
43
Figure 3.21 SC-FDMA Receiver
43
Figure 3.22 LTE Resource Grid for SC-FDMA
44
Figure 3.23 LTE Uplink Transport Channels Processing
46
Figure 3.24 CRC Insertion per Transport Block
46
Figure 3.25 Spatial Multiplexing
48
Figure 3.26 LTE Protocol Stack
49
Figure 3.27 Downlink Mapping of Logical and Transport Channels
51
Figure 3.28 Uplink Mapping of Logical and Transport Channels
51
Figure 3.29 MAC PDU Format
52
Figure 3.30 MAC Header Format
53
Figure 4.1 Network Reference Model for WiMAX
55
Figure 4.2 Evolved Packet System (EPS) Network Elements
57
Figure 4.3 Architecture of LTE Access Network (E-UTRAN)
59
Figure 4.4 3D Visualization of OFDMA
63
Figure 4.5 Protocol Architecture of WiMAX
65
Figure 4.6 Protocol Architecture of LTE
66
Figure 4.7(a) Generic Frame Structure for LTE (FDD)
67
Figure 4.7(b) Alternative Frame Structure for LTE (TDD)
67
Figure 4.8 WiMAX TDD Frame Structure
68
Figure 5.1 Block Diagram of SC-FDE Link level Simulator
73
Figure 5.2 Block Diagram of OFDM Link Level Simulator
74
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Figure 5.3 Comparison of SC-FDE and OFDM using MMSE in Pedestrian A,
75
Vehicular A and AWGN Channels
Figure 5.4 Comparison of SC-FDE and OFDM using Zero Forcing in
77
Pedestrian A, Vehicular A and AWGN Channels
Figure 5.5 Comparison of SC-FDE and OFDM with or without CP in Vehicular A
79
Channel
Figure 5.6 System Model of SC-FDMA
81
Figure 5.7 Comparison of SER with Various Subcarrier Mapping Schemes
81
Figure 5.8 SER Performance of SC-FDMA System Using Various Subcarrier
82
Mapping Schemes
Figure 5.9 Simulation Model of PAPR Calculations for SC-FDMA System
83
Figure 5.10 Comparison of CCDF of PAPR for DFDMA, IFDMA and
84
LFDMA using QPSK
Figure 5.11 Comparison of CCDF of PAPR for IFDMA, DFDMA and
85
LFDMA using 16-QAM
Figure 5.12 Simulation Model of PAPR Calculations for OFDMA
85
Figure 5.13 Simulation Model of PAPR Calculations for OFDMA System
86
Figure 5.14 Comparison of MIMO and SISO system in terms of Capacity
87
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List of Tables
Table 2.1
Fixed vs. Mobile WiMAX
5
Table 2.2
Physical Layer Interfaces of IEEE 802.16
7
Table 2.3
OFDM Parameters used in Fixed and Mobile WiMAX
10
Table 2.4
Modulation and Coding Schemes Supported by WiMAX
12
Table 2.5: WiMAX MAC Layer Features
18
Table 3.1
Performance Targets for Long Term Evolution
22
Table 3.2
Modulation Parameters for Downlink
26
Table 3.3
Number of Physical Resource Blocks (PRB) for Various Transmission
26
Bandwidths
Table 3.4
Modulation Schemes for Downlink Physical Signals
30
Table 3.5
SC-FDMA Parameters for LTE
45
Table 4.1
Description of Reference Points
56
Table 4.2
Reported Frequency Bands used for WiMAX
60
Table 4.3(a) LTE FDD Frequency Bands
61
Table 4.3(b) LTE TDD Frequency Bands
61
Table 4.4
Peak Data Rates of LTE and WiMAX
62
Table 4.5
Modulation Parameters for LTE and WiMAX
63
Table 4.6
MIMO aspects for WiMAX and LTE
64
Table 4.7
Comparative Summary of WiMAX and LTE
69
Table 5.1
Simulation Parameters and Assumptions
73
Table 5.2
Comparison between SC-FDE and OFDM in Various Channels
75
Using MMSE Equalization
Table 5.3
Comparison between SC-FDE and OFDM in Vehicular A Channels
76
Using MMSE Equalization
Table 5.4
Comparison between SC-FDE and OFDM in Various Channels Using
77
Zero Forcing
Table 5.5
Performance of SC-FDE and OFDM in Vehicular A Channel Using
Zero Forcing
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Table 5.6
Comparison of SC-FDE and OFDM With or Without CP
80
Table 5.7
Simulation Parameters of SC-FDMA
80
Table 5.8
Parameters Used in the Simulation of PAPR Calculation for SCFDMA
84
Table 5.9
Parameters Used in the Simulation of PAPR-Calculation for OFDMA
86
Table 5.10 Comparison between MIMO and SISO System with SNR=5 dB
88
Table 5.11 Comparison between MIMO and SISO System with SNR=14 dB
88
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List of Acronyms
3GPP
16-QAM
64-QAM
AAS
AP
ARQ
AS
ASN
ASN GW
ATM
AuC
AWGN
BCCH
BCH
BPSK
BSN
BWA
CCCH
CCDF
CDD
CDMA
CI
CID
CIR
CN
CPS
CQI
CRC
CS
CSN
DAC
DC
DCCH
DFDMA
DHCP
DRX
DSL
DTCH
EC
EKS
3rd Generation Partnership Project
16-Quadrature Amplitude Modulation
64-Quadrature Amplitude Modulation
Adaptive Antenna System
Access Point
Automatic Repeat reQuest
Access Stratum
Access Service Network
Access Service Network Gateway
Asynchronous Transmission Mode
Authentication Centre
Additive White Gaussian Noise
Broadcast Control Channel
Broadcast Channel
Binary Phase Shift Keying
Block Sequence Number
Broadband Wireless Access
Common Control Channel
Complementary Cumulative Distribution Function
Cyclic Delay Diversity
Code Division Multiple Access
Cyclic Redundancy Indicator
Connection Identifier
Channel Impulse Response
Core Network
Common Part Sublayer
Channel Quality Indicator
Cyclic Redundancy Check
Convergence Sublayer
Connectivity Service Network
Digital to Analog Convertor
Direct Current
Dedicated Control Channel
Distributed Frequency Division Multiple Access
Dynamic Host Control Protocol
Discontinuous Reception
Direct Subscriber Line
Dedicated Traffic Channel
Encryption Control
Encryption Key Sequence
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EPC
ErtPS
E-UTRA
E-UTRAN
FBSS
FDD
FDM
FEC
FFT
FSN
FTP
GMH
GT
HARQ
HCS
HHO
HLR
HSDPA
HSS
HT
ICI
IDFT
IFFT
IP
IRC
ISI
ISP
ITU
LFDMA
LLC
LOS
LTE
MAC
MAN
MBMS
MBSFN
MCCH
MCH
MCM
MDHO
MIMO
MIP-HA
MMSE
MME
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Evolved Packet Core
Extended Real Time Polling Service
Evolved UMTS Terrestrial Radio Access
Evolved Universal Terrestrial Radio Access Network
Fast Base Station Switching
Frequency Division Duplexing
Frequency Division Multiplexing
Forward Error Correction
Fast Fourier Transform
Fragment Sequence Number
File Transfer Protocol
Generic MAC Header
Guard Time
Hybrid Automatic Repeat reQuest
Header Check Sequence
Hard Handover
Home Location Register
High Speed Downlink Packet Access
Home Subscriber Station
Header Type
Inter Carrier Interference
Inverse Discrete Fourier Transform
Inverse Fourier Transform
Internet Protocol
Interference Rejection Combining
Inter Symbol Interference
Internet Service Provider
International Telecommunication Union
Localized Frequency Division Multiple Access
Logical Link Control
Line Of Sight
Long Term Evolution
Medium Access Control
Metropolitan Area Network
Multimedia Broadcast Multimedia Service
Mobile Broadcast Single Frequency Network
Multicast Control Channel
Multicast Channel
Multicarrier Modulation
Macro Diversity Handover
Multiple Input Multiple Output
Mobile IP Home Agent
Minimum Mean Square Error
Mobility Management Entity
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MPDU
MRT
MS
MSDU
MTCH
MU-MIMO
MTCH
NAS
NLOS
NSP
nrtPS
N-WEST
NWG
OFDM
OFDMA
OSI
OSS
PAPR
PBCH
PBFICH
PCCH
PCEF
PCMCIA
PDCCH
PDSCH
PDCP
PDU
P-GW
PHICH
PHY
PMCH
PMP
PRACH
PRN
P-SCH
PSTN
PTP
PUSCH
QoS
QPP
QPSK
RB
RE
RLC
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MAC Packet Data Unit
Maximum Ratio Transmission
Mobile Station
MAC Single Data Unit
Multicast Traffic Channel
Multi User-Multiple Input Multiple Output
Multicast Traffic Channel
Non Access Stratum
Non Line Of Sight
Network Service Provider
Non Real Time Polling Service
National Wireless Electronics Systems Testbed
Network Working Group
Orthogonal Frequency Division Multiplexing
Orthogonal Frequency Division Multiple Access
Open System Interface
Operation Supports System
Peak-To-Average Power Ratio
Physical Broadcast Channel
Physical Control Format Indicator Channel
Paging Control Channel
Policy Control Enforcement Function
Personal Computer Memory Cards International Association
Physical Downlink Control Channel
Physical Downlink Shared Channel
Packet Data Convergence Protocol
Packet Data Unit
Packet Data Network Gateway
Physical HARQ Indicator Channel
Physical Layer
Physical Multicast Channel
Point-to-Multipoint
Physical Random Access Channel
Pseudo Random Numerical
Primary Synchronous Channel
Public Switch Telephone Network
Point-to-Point
Physical Uplink Shared Channel
Quality of Service
Quadratic Polynomial Permutation
Quadrature Phase Shift Keying
Resource Block
Resource Element
Radio Link control
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RP
RRC
RRM
rtPS
SAS
SAE
SAP
SC-FDE
SER
S-GW
SM
SNR
SOFDMA
SS
S-SCH
STBC
TDM
TDMA
TTI
UE
UL-SCH
UMTS
VoIP
WAN
WCDMA
WiMAX
WMAN
ZF
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Reference Point
Radio Resource Control
Radio Resource Management
Real Time Polling Service
Smart Antenna System
System Architecture Evolution
Service Access Point
Single Carrier with Frequency Domain Equalization
Symbol Error Rate
Serving Gateway
Spatial Multiplexing
Signal-to-Noise Ratio
Scalable Orthogonal Frequency Division Multiple Access
Subscriber Station
Secondary Synchronous Channel
Space Time Block coding
Time Division Multiplexing
Time Division Multiple Access
Transmission Time Interval
User Equipment
Uplink Shared Channel
Universal Mobile Telecommunication System
Voice over Internet Protocol
Wide Area Network
Wideband Code Division Multiple Access
WiMAX
Wireless Metropolitan Area Network
Zero Forcing
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Chapter 1: Introduction
__________________________________________________
Worldwide Interoperability for Microwave Access (WiMAX) technology, also known as the
IEEE 802.16 standard, is based on WMAN (Wireless Metropolitan Area Network). It
provides data rates up to 75 Mbps over the distance of 50 km. WiMAX uses frequency bands
of 10-66 GHz, covering long geographical areas using licensed or unlicensed spectrum.
WiMAX uses OFDMA (Orthogonal Frequency Division Multiple Access) as multiplexing
technique in uplink and downlink directions. The mode of operation used for communication
between multiple subscriber stations and base station is Point-to-Multipoint (PMP), whereas
the mode of operation used between two base stations is Point-to-Point (PTP).
Other versions of WiMAX include IEEE 802.16-2004 and IEEE 802.16-2005. IEEE 802.162004 is known as fixed WiMAX, has no mobility and is used for fixed and nomadic access.
Since fixed WiMAX has no mobility it does not support handovers. IEEE 802.16-2005 is
known as mobile WiMAX, which is an extension of fixed WiMAX, introducing many new
features to support enhanced Quality of Service (QoS) to provide high mobility. The mobile
WiMAX supports data rate of up to 75 Mbps.
The Long Term Evolution (LTE) is an evolution of the third generation technology based on
Wideband Code Division Multiple Access (WCDMA). LTE uses OFDM for downlink, i.e.
from base station to the terminal. There are three physical channels such as Physical
Downlink Shared Channel (PDSCH), Physical Multicast Channel (PMCH), Physical
Broadcast Channel (PBCH) in the downlink used for data transmission, broadcast
transmission and system information within a cell. The modulation schemes used are
Quadrature Phase Shift Keying (QPSK), 16-Quadrature Amplitude Modulation (16-QAM)
and 64-QAM.
LTE uses a precoded version of Orthogonal Frequency Division Multiplexing (OFDM) using
a single carrier for uplink called Single Carrier Frequency Division Multiplexing (SCFDMA). SC-FDMA is used to minimize Peak-to-Average Power Ratio (PAPR) caused by
OFDM. PAPR is the ratio of peak signal power to the average signal power. There are two
physical channels, Physical Random Access Channel (PRACH) and Physical Uplink
Synchronization Channel (PUSCH), used in the LTE uplink. For initial access PRACH is
used whereas when the User Equipment (UE) is not synchronized the data is send on PUSCH.
The modulations schemes used for LTE uplink are QPSK, 16-QAM, 64-QAM.
The Figure 1.1 shows the wireless technology evolution of WiMAX and LTE.
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Technology Evolution of 3G
3G EV-DO
WCDMA
Wi-Fi
OFDM
3.5G
EV-DO Rev A
HSDPA
3G Evolution,
LTE EV-DO Rev
B, CFDMA, MCOFDMA
802.16e-2005
OFDMA
802.16e-2005
MIMO-BF
OFDMA
4G
(IMT
Advanced)
OFDMA
Based
Evolution of Broadband Wireless Technology
Figure 1.1: Evolution Path of Mobile Technologies towards 4G [1]
1.1 Objective
The objective of this thesis is to conduct a brief comparison between WiMAX and 3GPP
LTE. The comparison is performed by discussing the physical and MAC layers of WiMAX
and LTE including their multiplexing schemes. Link level simulations of the LTE uplink
correspond to the main part of our thesis. Link level simulation of OFDM by using
equalization schemes as Minimum Mean Square Error (MMSE) and Zero Forcing (ZF) in
ITU Pedestrian A, ITU vehicular A and AWGN channels in comparison with SC-FDE and
SCFDMA is also included in our thesis. The comparison is taken in terms of Symbol Error
Rates (SER) and Signal-to-Noise Ratio (SNR). In addition, the Peak-to-Average Power Ratio
(PAPR) is calculated for both the SC-FDMA and the OFDMA systems.
1.2 Thesis Outline
Chapter 2 gives a technical overview of the WiMAX technology including its different
standards and air interfaces. This chapter also discusses Physical and MAC layers of
WiMAX.
Chapter 3 gives a brief description of 3GPP LTE including its architectures, air interfaces,
uplink, downlink, multiple antenna techniques and layers (Physical and MAC layer).
Chapter 4 underlines the main differences between WiMAX and LTE. The comparison is
conducted in terms of system architecture, radio and protocol aspects of air interfaces,
mobility and QoS.
Chapter 5 includes our simulation results. It also includes the link level simulation of LTE
uplink in comparison with an OFDM system. In addition to this, the capacity of MIMO
system is in comparison with a SISO system also discussed.
Chapter 6 concludes the thesis and provides some suggestions for future work.
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Chapter 2: Introduction to WiMAX
__________________________________________________
2.1 Overview of WiMAX
WiMAX, also known as IEEE 802.16, provides wireless data services by using the 10-66
GHz frequency bands and provides data rates up to 70 Mbps over distance of 50 km. WiMAX
covers large geographical areas using licensed or unlicensed spectrum in order to provide
wireless Internet services to users with high data rates. It is based on WMAN which is not
only an alternative to wired T1 and Digital Subscriber Lines (DSL) but it also provides
wireless broadband services within a building from an Internet Service Provider (ISP) and can
be used to connect many Wi-Fi networks across different campuses or cities.
WiMAX works like any other cellular technology and uses a base station to establish the
wireless connection to the subscriber such as Universal Mobile Telecommunication Systems
(UMTS). The communication between two or more WiMAX base stations could be Point to
Point/ Line of Sight (LOS) whereas between the base station and the subscriber can be Point
to Multi Point/ Non Line of Sight (NLOS).
2.2 IEEE 802.16 Standards
Telecommunication equipment manufacturers started introducing products for Broadband
Wireless Access (BWA) at the end of the 90’s. But they were still looking for interoperable
standard. The National Wireless Electronics Systems Testbed (N-WEST) called a meeting in
1998, about the need of an interoperable standard which resulted in the IEEE 802 standard. A
lot of efforts were made in this regard which resulted later in the formation of IEEE 802.16
standard. Initially, the main focus of this group was to develop the radio interface for BWA
which used the radio spectrum from the 10-66 GHz range. It also supports the LOS based
Point to Multipoint (PMP) broadband wireless system.
2.2.1 IEEE 802.16-2001
The standard was developed in December 2001. It uses the spectrum range of 10-66 GHz to
provide fixed broadband wireless connectivity and single carrier modulation techniques such
as 16-QAM, 64-QAM and QPSK in physical layer and Time division Multiplexed (TDM)
techniques in MAC layer. The standard includes Differential QoS techniques for the
improvement of LOS based conditions. The standard uses Time Division Duplex (TDD) and
Frequency Division Duplex (FDD) as duplexing techniques.
2.2.2 IEEE 802.16a-2003
The standard amended the basic IEEE 802.16 by using a frequency range of 2-11 GHz which
includes both licensed and license free frequency bands. Due to inclusion of the low
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frequencies, below 11 GHz, NLOS communication is possible. The NLOS operations
introduced the multipath propagation effects which have been overcome through the
adaptation of multicarrier modulation techniques in the physical layer. OFDM was chosen as
modulation technique. The standard improved also security issues by making the features of
privacy layer mandatory.
2.2.3 IEEE 802.16c
The standard developed the profile details of 10-66 GHz frequency band and corrected the
inconsistencies involved in the previous standard.
2.2.4 IEEE 802.16d-2004
Is the amendment of IEEE 802.16a. It was initially considered as the revision of IEEE 802.16
standard and was named IEEE 802.16 REVd. But in September 2004, due to the credibility of
the amendments, it was named IEEE.802.16d. The standard was designed for fixed, nomadic
and portable users so as to provide fixed BWA. It supports both TDD and FDD transmission
modes. The most important feature of this standard is the provision of support for advance
antenna systems and adaptive modulation and coding techniques.
2.2.5 IEEE 802.16e-2005
Is the amendment of IEEE 802.16d-2004 and provides support for mobility of subscribers,
who can move at vehicular speeds and provides services such as high speed handoffs due to
its technological advances. It enhances the overall system performance due to support of
Adaptive Antenna Systems (AAS) and MIMO. It facilitates mobile, fixed and portable users.
The standard updated the security feature included privacy sub-layer.
2.3 Fixed vs. Mobile WiMAX
IEEE 802.16-2004 is known as fixed WiMAX. The standard was originally developed as a
wireless extension of the wired infrastructure. It uses OFDM to mitigate the effects of
multipath and improves the propagation of signals in NLOS. Fixed WiMAX has no mobility
and this is also the reason why it does not support handovers. The IEEE 802.16-2005, also
known as mobile WiMAX, uses Scalable Orthogonal Frequency Division Multiplexing
Access (SOFDMA), which divides the carrier up to 2048 subcarriers. This division of the
carrier signal makes it possible to improve the signal penetration into the buildings and should
enable cheaper products for the end subscriber such as PC and USB cards.
The basic difference between fixed and mobile variants of WiMAX is their mobility. Mobile
WiMAX supports users moving at speeds of 120 km/h and enables the handoff mechanism
when a user moves from one Base Station (BS) to another. A comparison between Fixed and
Mobile WiMAX is shown in Table 2.1 [2].
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Standard
IEEE 802.16-2004
IEEE 802.16-2005
Release
June 2004
December 2005
Spectrum
2 to 11 GHz
Fixed:2 to 11 GHz
Mobile: 2 to 6 GHZ
Modulation Techniques
16-QAM,
QPSK
Propagation Schemes
NLOS
NLOS
Single Carrier
Single carrier
256-OFDM
2048-OFDM
Scalable OFDMA with 128,
256,512,1024
and
2048
subcarriers
Duplex Method
TDD/FDD
TDD/FDD
Data Rate
Maximum 70 Mbps for (20 Maximum 15 Mbps
MHz Channel)
(5MHz Channel)
Applications
Voice over IP (VoIP)
Mobile VoIP
Supported Services
Fixed, Nomadic and Portable
Mobile, Fixed and Portable
Service Providers
Digital
(DSL)
Wired ISP
Wired and wireless ISP
Wireless ISP
Modem Service Providers
PCMCIA card for Laptops
PCMCIA card
PHY Layer
Targeted Groups
64-QAM
and 16-QAM,
QPSK
64-QAM
Subscriber
User Equipment
Smart Phones
Mobility
NO
Yes
Coverage
Up to 50 km maximum
2-5 km approximately
Table 2.1: Fixed vs. Mobile WiMAX [2]
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2.4 IEEE 802.16 Protocol Layers
The IEEE 802.16 uses the first two layers of the Open System Interconnection (OSI) model.
The PHY layer uses OFDM and Orthogonal Frequency Division Multiple Access (OFDMA)
as transmission techniques whereas data link layer is divided into MAC and Logical Link
Control (LLC) sub-layers. The MAC layer is further divided into three sub-layers called
Security Sublayer, MAC Common Part Sublayer (MAC CPS) and Convergence Sublayer
(CS). The protocol stack of WiMAX is shown in Figure 2.1, and consists of the first two
layers (PHY and Data link) of OSI reference model. The upper layers include network,
transport, session, presentation and application layers of OSI model.
Upper Layers
Logical Link Control
Data
Link
Layer
Convergence sub layer (CS)
MAC Common Part Sublayer
(CPS)
MAC Layer
Security Sub Layer
Physical Layer
Figure 2.1 Protocol Stack of IEEE 802.16 [3]
PHY layer of WiMAX not only establishes the connection between communicating devices
but is also responsible for defining the modulation/demodulation type for transmission of the
incoming bit sequence. It uses OFDM and OFDMA as transmission schemes, which uses the
frequency band between 2-11 GHz. The frequency band below 11 GHz makes possible NLOS
wireless communication and the use of OFDM reduces multipath effects and Inter Symbol
Interference (ISI). PHY layer uses FDD and TDD as duplexing techniques.
MAC provides the interface between PHY layer and the transport. From a transmission
prospective, MAC layer takes the packets from the upper layers and organizes them in
Protocol Data Units (PDU’s) for transmission over the air. The CS of the MAC layer can
interface with the protocols of upper layers. Consequently, WiMAX supports both IP and
Ethernet protocol. The MAC CPS is the core part of the MAC layer and is responsible for
connection maintenance, bandwidth allocation, PDU framing, duplexing and channelization.
The security sublayer connects the MAC CPS and the PHY layer and provides the necessary
methods for encryption and decryption of data. Security sublayer is also used for
authentication and the secure exchange of keys.
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2.5 Physical Layer of IEEE 802.16
WiMAX supports five types of physical interfaces due to the use of various types of
modulation techniques. In this section, we will first define each type of PHY layer interface
and then will give a detailed description of the OFDM techniques used at the PHY layer.
 WirelessMAN-SC: The WirelessMAN-SC PHY uses single carrier modulation
technique for LOS transmission within 10-66 GHz frequency band.
 WirelessMAN-SCa: The WirelessMAN-SCa PHY uses single carrier modulation
techniques for the NLOS transmission in the frequency band of 2-11 GHz.
 WirelessMAN-OFDM: It is based on OFDM and is providing the NLOS transmission
in the frequency band of 2-11 GHz.
 WirelessMAN-OFDMA: The WirelessMAN-OFDMA PHY uses the licensed
frequency band of 2-11 GHz and supports the NLOS operation by using the 2048
subcarrier OFDM scheme.
 WirelessHUMAN: Is based on license free frequency band below 11 GHz. It can use
any of the air interfaces that use the 2-11 GHz frequency band. It uses TDD as
duplexing technique [4].
The description of physical layer interfaces is described in Table 2.2 [4].
PHY Interface
Duplexing
Modulation
Frequency
Bands
Propagation
Modes
WirelessMAN-SC
FDD and TDD
Single carrier
10-66 GHz
LOS
WirelessMAN-SCa
FDD and TDD
Single carrier
2-11 GHz
NLOS
WirelessMANOFDM
FDD and TDD
OFDM
2-11 GHz
NLOS
WirelessMANOFDMA
FDD and TDD
2048 subcarrier
OFDM Scheme.
2-11 GHz
NLOS
WirelessHUMAN
TDD
SC, OFDM,
OFDMA
License free NLOS
frequency
band below
11 GHz
Table 2.2: Physical Layer Interfaces of IEEE 802.16 [4]
2.5.1 WirelessMAN OFDM PHY
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It uses OFDM which enables high speed data services and multimedia communication in
NLOS environment. It can reduce multipath effects in NLOS and provides efficient data rates
for transmission.
2.5.2 Overview of OFDM
OFDM is based on a multicarrier modulation technique which, in turn, is based on the concept
of dividing incoming data streams of high bit rates into several data streams of lower bit rates.
OFDM modulates each stream onto separate carrier frequencies, known as subcarriers.
Multicarrier Modulation (MCM) techniques use guard band to in order eliminate or reduce the
ISI. The idea of OFDM is slightly different from that of MCM. In OFDM, subcarriers are
placed in such a manner that they are orthogonal to each other. Consequently, the Inter Carrier
Interference (ICI) is reduced and the available bandwidth is used more efficiently.
Figure 2.2: Comparison between Conventional FDM and OFDM [5]
The use of OFDM saves bandwidth as compared to the Frequency Division Multiplexing
(FDM) as shown in Figure 2.2. The orthogonal overlapping nature of OFDM subcarriers not
only reduces the ISI but also saves the bandwidth of system which is different from FDM
where ISI is reduced by the introduction of guard bands. The addition of guard band is the
wastage of power and bandwidth.
2.5.3 Time Domain OFDM
The Cyclic Prefix (CP) could be added at the beginning of the OFDM symbol before
transmission. The addition of CP maintains orthogonality and reduces the delay spread
introduced by multipath. The time occupied by CP is called Guard Time (TG) and is used in
computations of various data rates. The time occupied by data is called Td. In WiMAX the
ratio of TG/Td is known as Guard Interval (G). The choice of G depends upon the conditions
of radio channel. The values of G are 1/4, 1/8, 1/16, 1/32. The time domain description of CP
is shown in Figure 2.3.
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Data
TG
Td
Symbol Time (TS)
Figure 2.3: Cyclic Prefix in Time Domain
2.5.4 Frequency Domain OFDM
Useful data is not carried by all the subcarriers of an OFDM symbol. Four types of subcarriers
are used in WiMAX OFDM:
 Data Subcarriers: Carries useful data for transmission.
 Pilot Subcarriers: Used for synchronization and channel estimation.
 Null Subcarrier: Having no data for transmission, known as frequency guard bands.
 Direct Current Subcarrier: DC subcarrier is called Null subcarrier as it corresponds to
the zero frequency if the Fast Fourier Transform (FFT) signal is not modulated. The
FFT signal is obtained by taking the transformation of discrete signal into discrete
frequency domain. Normally, the DC subcarrier has a frequency equal to the RF centre
of frequency of the transmitting station.
The OFDM symbol of WiMAX in frequency domain is shown in Figure 2.4.
Pilot Subcarrier
Left Guard
Subcarrier
Data Subcarrier
Left Guard
Subcarrier
DC
_______________________________________________________________
Figure 2.4: WiMAX OFDM Symbol in Frequency Domain
2.5.5 Parameters of OFDM
As mentioned previously, WiMAX has five different implementations of the physical layer.
Here we will discuss the parameters of PHY for fixed and mobile WiMAX, based on OFDM
and OFDMA PHY layers respectively. In addition to different air interfaces, mobile WiMAX
also uses variable FFT size.
2.5.5.1 OFDM PHY for Fixed WiMAX
Fixed WiMAX is based on IEEE 802.16-2004 and uses the OFDM PHY layer. It uses 256
point FFT, where the size of FFT is fixed. From 256 points (subcarriers), 192 subcarriers
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carry data, 8 are used for estimation and synchronization, while the remaining 56 subcarriers
are used as a guard band. Due to the fixed size of FFT, subcarrier spacing increases as the
bandwidth increases which in turn decreases the symbol time. The reduction in symbol time
increases the delay spread which is undesirable. Consequently, in order to reduce the delay
spread, a large fraction of time needs to be allocated as guard time. For 3.5 MHz channel
bandwidth, the maximum delay spread is 16us.
2.5.5.2 OFDMA PHY for Mobile WiMAX
Mobile WiMAX uses a scalable size of FFT that varies between 128 to 2048 points. In mobile
WiMAX, when the bandwidth increases, the size of FFT increases such that the subcarrier
spacing is 10.94 kHz. The spacing of 10.94 kHz keeps the balance between Doppler spread
and delay spread requirements for both fixed and mobile WiMAX environments. Doppler
spread occurs in the signal by movement of communicating devices (mobile phones) or other
objects in the environment. The effect of Doppler spreading creates ICI by destroying the
orthogonality of the subcarriers. In addition, the subcarrier spacing of 10.94 kHz supports
delay spread values up to 20us and vehicular speed up to 125 km/h when operating in 3.5
GHz spectrum band. A scalable version of FFT also reduces cost due to support of various
transmission bandwidths (3.5 MHz, 5 MHz, 10 MHz and 20 MHz) without any change in
equipment. The OFDM parameters for OFDM PHY and OFDMA PHY layers are shown in
Table 2.3.
OFDM Parameter
OFDM PHY for Fixed
WiMAX
OFDMA PHY for
Mobile WiMAX
FFT Size
256
512
1024
2048
Number of Data
Subcarrier
192
360
720
1440
Number of Pilot
Subcarrier
8
60
120
240
Number of Null subcarrier
56
92
184
368
Cyclic Prefix (Guard
Time)
1/32
1/8
1/4
1/4
Channel Bandwidth
(MHz)
3.5
5
10
20
Subcarrier spacing (KHz)
15.625
10.94
OFDM symbol duration
(µs)
72
102.9
Useful symbol time (µs)
64
91.4
Table 2.3: OFDM Parameters used in Fixed and Mobile WiMAX [2]
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2.5.6 Advantages and disadvantages of OFDM
OFDM has many advantages when compared with a single carrier modulation scheme.
Advantages of OFDM:
 OFDM is simple to implement due to the use of FFT.
 OFDM is spectral efficient due to overlapping spectra and orthogonality.
 It is robust in NLOS transmissions.
 OFDM reduces the effects of ISI through the use of a cyclic prefix in a transmitted
symbol.
 In OFDM each subcarrier is modulated by different modulation techniques such as
BPSK, QAM and QPSK.
 It is robust against narrow band interference.
 It is useful for coherent demodulation because pilot based channel estimations are easy
to implement in OFDM systems.
Disadvantages of OFDM:
Here are some drawbacks of OFDM.
 OFDM has Peak to Average Power Ratio (PAPR) that causes nonlinearities and
clipping distortions.
 It is sensitive to phase noise which is acute at higher frequencies.
 It is sensitive to timing and frequency offset [6].
2.5.7 Features of WirelessMAN OFDM PHY
Flexible Channel Bandwidth
WiMAX IEEE 802.16-2004 standard allows flexible channel bandwidth to provide
compatibility with wireless technologies. It uses the channel bandwidth from 1.25 MHz to 20
MHz.
Adaptive Modulation and coding
WiMAX uses adaptive modulation techniques and allows the technique to be changed on
burst by burst basis per link, depending on channel conditions [7]. On basis of channel
quality, the base station scheduler assigns the modulation scheme that maximizes the
throughput within available Signal to Noise Ratio (SNR). The downlink and uplink of
WiMAX supports various modulation schemes including 16-QAM, QPSK and 64-QAM. The
use of 64-QAM is optional in the uplink direction. Table 2.4 [2] shows various types of
modulation and coding schemes used in the downlink and uplink of WiMAX.
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Downlink
Modulation
Uplink
BPSK, QPSK, 16-QAM, 64- BPSK, QPSK, 16-QAM, 64QAM.
QAM (optional)
Mandatory: Convolutional Mandatory: Convolutional
codes at rate: 1/2.2/3,3/4,5/6 codes at rate: 1/2.2/3,3/4,5/6
Optional:
Convolutional Optional:
Convolutional
Turbo
codes
at
rate: Turbo
codes
at
rate:
1/2.2/3,3/4,5/6
1/2.2/3,3/4,5/6
Coding
Repetition codes at rate: Repetition codes at rate:
1/2.1/3,1/6, LDPC, RS- 1/2.1/3,1/6, LDPC
Codes for OFDM PHY
Table 2.4: Modulation and Coding Schemes Supported by WiMAX [2]
Error Correction Mechanism
The WirelessMAN OFDM PHY provides robust error correction by using the Forward Error
Correction (FEC) control mechanism. It uses a two stages FEC. In the first stage, FEC uses
Reed Solomon Encoder that corrects burst errors at byte level and improves the OFDM link in
multipath propagations. In the second stage, FEC uses convolutional coder that corrects
independent bit errors. Convolutional coding reduces the overall number of bits needed to be
sent on the channel due to puncturing functionality [2]. Puncturing is the process of removing
certain bits before transmission and replacing the deleted bits with fixed values upon
reception.
2.6 MAC Layer of IEEE 802.16
MAC layer provides the interface between the physical layer and upper layers. It takes MAC
Service Data Units (MSDU) from the upper transport layers and organizes them in form of
MAC Packet Data Units (MPDU) for transmission over the air. MAC layer supports variable
length frames for transmission. In IEEE 802.16 MAC layer is divided in to three sub-layers:
 Service Specific Convergence Sublayer (SSCS)
 Common Part Sublayer (CPS)
 Security Sublayer (SS)
The CS accommodates upper layer protocols. The IEEE 802.16 MAC layer supports
Asynchronous Transmission Mode (ATM) and Ethernet (IEEE.802.3) which specifies two
types of traffic supported by CS; IP and ATM. CS takes the MSDU’s from upper layers and
do key processing such as payload compression. After payload compression the MSDU’s are
sent to CPS through Service Access Point (SAP). CS can accept data frames from the CPS.
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The CPS of IEEE 802.16 takes MSDU’s from the CS and organizes them in form of MPDU
by performing fragmentation and segmentation. CPS is the core part of the MAC layer and it
provides functions related to bandwidth allocations, connection initialization and
maintenance, QoS, duplexing and framing. CPS provides the connection identifier to identify
the serving MPDU, when MAC layer is connected to Subscriber Stations (SSs). The main
goal of the SS is to ensure privacy services to the subscribers across the wireless network and
give protection from theft of services to the operators. It provides encryption, authentication
and secure key exchange functions on MPDUs and sends them to the PHY layer for further
processing.
The data, control and management plane of WiMAX are shown in Figure 2.5.
CS SAP
Service Specific Convergence Sublayer
MAC SAP
Service Specific CS
Management Entity
MAC Common Part Sublayer (MAC CPS)
MAC CPS
Management Entity
Privacy Sublayer
Security
Management
PHY SAP
Physical layer
Management Entity
Physical Layer (PHY)
Data and Control Plane
Management Plane
Figure 2.5: Architecture of WiMAX [8]
2.6.1 MAC Frame Format
WiMAX supports two types of generic MAC frame formats. The first contains the
management information while the second has transport information.
Generic MAC
Header (GMH)
Subheader
MAC Management
Information
Forward
Error
Correction
(FEC)
Figure 2.6(a): Generic MAC Frame Having Management Payload
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Generic MAC
Header (GMH)
Subheader
MAC Transport Information
Forward
Error
Correction
(FEC)
Figure 2.6(b): Generic MAC Frame Having Transport Payload
EC
Type
1bit
1bit
6 bits
CI
1bit 1bit
EKS
Reserved
HT
Reserved
Figures 2.6(a) and 2.6(b) show the generic MPDU frame format. A generic MPDU contains
the GMH, subheader which is optional, payload information and error correction mechanisms,
in the form either CRC or FEC. The header of GMH is shown in Figure 2.7.
2bits 1bit
LEN
CID
11 bits
16 bits
HCS
8 bits
6 Bytes
Figure 2.7: Generic MAC Header (GMH)
From Figure 2.7, GMH consists of Header Type (HT), Encryption Control (EC), Type,
Reserved bits, Cyclic Redundancy Indicator (CI), Encryption Key Sequence (EKS), Length of
Number of bytes of MPDU (LEN), Connection Identifier (CI) and Header Check Sequence
(HCS). Each field of GMH has its specific function which is described below.
The GMH contains information about MPDU details. The 1 bit HT indicates the type of
header. The MAC layer supports two types of MPDUs, Generic MPDU and the Bandwidth
Request PDU. For Generic MPDU the “HT” contains the “0” value. The 1 bit “EC” indicates
the encryption of the payload. The “0” value in EC indicates the payload is not encrypted
while “1” indicates the payload is encrypted. The “Type” field indicates the type of payload
contents used. The payload content can be fragmentation, Automatic Repeat Request (ARQ),
mesh and Aggregation. The “CI” field indicates the status of CRC, whether it is present or
not. Value “0” indicates the absence of CRC while 1 indicates its presence. The “EKS” field
indicates the key used to encrypt the frame payload. The “LEN” field indicates the number of
bytes of MPDU. The “LEN” field is 11 bits allowing thus a maximum frame length of 2047
bytes. The “CID” indicates the connection where the MPDU has to be sent. The “HCS”
performs error check for the GMH. The second field of “Generic MPDU” is Subheader (SH)
which is optional. The “SH” defines the bits for aggregation, ARQ, fragmentation and mesh
feature of the MAC. The “Payload” field of MPDU contains fragments of MSDUs, single
MSDU, aggregates of fragments of MSDUs and aggregates of MSDUs, which depend on
aggregation or fragmentation rules for MAC.
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2.6.2 Aggregation
The CPS of MAC layer is capable of packing one or more MSDUs in a single MPDU due to
the variable size of MSDU. The size of the payload is determined by on-air timing slots and
feedback from SS. Figure 2.8 shows two complete MSDUs, where one partial MSDU is
packed to form the payload of MPDU. The concatenations of this type of MSDUs safe the
resources of the MPDU from wastage. The aggregation used in payload is indicated by the
“Type” field of GMH of MPDU. To indicate aggregation, the “type” bit is set and the
subheaders are used accordingly. Figure 2.8 shows multiple “SH” fields each followed by
fragmented MSDU and MSDU. The “SH” field is 1 byte long having three sub fields. The
“Fragmented Control (FC)” field indicates whether the MSDU is fragmented or not. The “00”
indicates the packet is not fragments while “01”, “10”, and “11” indicates the packet is
fragmented. The “Fragment Sequence Number (FSN)” indicates the sequence number of
fragmented MSDU. The “length field” indicates the start of next subheader in the payload.
Fragmented part
MAC Service
Data Units
GMH
SH
MSDU 1
FC (2 bits)
SH
MSDU 2
FSN (3 bits)
SH
Fragmented
MSDU
FEC
Length (3 bits)
Figure 2.8 Multiple MSDUs Packed into MPDU
2.6.3 Fragmentation
The CPS can fragment the single MSDU into multiple MPDUs. In this case, the payload of
MPDU is small to accommodate the complete MAC service data unit. Hence single MSDU is
fragmented and packed into multiple MPDUs for transmission.
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Fragmented
Part 1
Fragmented
Part 2
MAC SDU
GMH(6
Bytes )
SH
Fragmented
MSDU1
FEC
FC (2
bits)
GMH(6
Bytes )
FSN (3
bits)
SH
Fragmented
MSDU2
FEC
Length (3
bits)
Figure 2.9: Single MSDU Packed into Multiple MAC Packets Data Units (MPDUs)
Figure 2.9 shows the fragmentation of a single MSDU into multiple MPDUs. The “FC” field
indicates the fragment number in case of Fragmentation. The “10” in “FC” indicates the first
fragment, “01” indicates the last fragment and the “11” indicates the fragments in between
first and last. The “FSN” has the sequence number of the fragmented MSDU.
2.6.4 Transmission and Connection setup
The connection setup between SSs and the BS is established in three phases.
Phase 1: SS sends connection request
SS sends the ranging request packet to the BS which enables the timing, initial ranging and
power parameters of the BS. The request for service flow parameters is sent next to the
ranging packet request and turn on the variable size MSDUs. The service flow parameters
include bandwidth, frequency and peak services.
Phase 2: BS confirmation
When the ranging request packet is received, the BS transmits the ranging response to SS with
initial ranging, timing and power adjustment parameters. The service flow parameters are
agreed on this stage and CID is given to the subscriber station.
Phase 3: Transmission of MPDUs
The MSDUs provided by the MAC convergence layer are organized in MPDUs. The MSDUs
are either fragmented or packed into one or several MPDUs depending on the need. At the
start of transmission there is no feedback received from the receiver. When feedback is
received, the next MPDU is ready to transmit but it depends upon the type of feedback
response. If the response is positive the next MPDU is transmitted over the air while in case of
negative feedback the packets are retransmitted.
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2.6.5 Automatic Repeat Request (ARQ)
The mechanism of sending feedback use ARQ to check whether the packet is received
correctly or not. In WiMAX, ARQ is optional and used when needed. The header format of
ARQ is shown in Figure 2.10.
GMH (6 Bytes)
Subheader
Bytes)
ARQ Payload
FEC
3 Bytes
FSN (2bits)
BSN (11 bits)
Length (11 bits)
Figure 2.10: ARQ MAC Frame Format
To indicate ARQ, the “Type” field of the GMH has a specific value and the subheader is
extended. The ARQ MAC frame uses 11 bits Block Sequence Number (BSN) instead of using
FSN to store the sequence number of the block.
2.6.7 Features of MAC Layer
MAC layer is designed to support large amounts of traffic including voice and video services
by providing peak data rates over the channel. MAC layer is developed to sustain the PMP
frame with centralized BS. TDM is used as multiplexing technique in the downlink while the
uplink is shared between subscriber stations with TDMA.
The key features of MAC layer are summarized in Table 2.5 [9].
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Table 2.5: WiMAX MAC Layer Features [9]
2.7 Multi Antenna Technologies
WiMAX supports multi antenna technologies in order to provide data rates and spectral
efficiencies, which distinguish it from wireless technologies such as High Speed Downlink
Packet Access (HSDPA) and 1x EV-DO. WiMAX has two standards IEEE 802.16-2004 and
IEEE 802.16-2005, based on OFDM and OFDMA, respectively. Multiple antenna
technologies are easy to implement in WIMAX due to the simplicity of OFDM and OFDMA
based physical layers in the sense of orthogonality between subcarriers and support of flexible
bandwidths. These implementations increase the range, capacity, diversity, data rates and
efficiency of the system as compared to a single antenna system.
Multiple antenna technologies are normally divided into three types:
 Smart Antenna System (SAS)
 Diversity Techniques
 Multiple Input Multiple Output (MIMO)
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2.7.1 Smart Antenna System
SAS is known as Adaptive Antenna System (AAS). SAS constructs the channel model and
attains channel knowledge by using signal processing techniques in order to steer the beam
towards the desired subscriber while transmitting null steering towards the interferer [10]. The
null steering cancels out undesired portion of the signal and reduces the gain of radiation
pattern obtained from adaptive array antenna in the direction of interference source. This is
achieved by using beamforming and null steering towards desired user and interferer
respectively. The process of combining the radiated signal and focusing it in the desired
direction is called Beamforming [10]. SAS is divided as follows.
2.7.1.1 Switch Beam Antennas
Switch beam antenna forms several fixed beams to cover the coverage area. It selects the
beam pattern which has strong power towards the direction of intended user. As the mobile
moves, the beam switching algorithm determines when a particular beam should be selected
to enhance the quality of the mobile user. Switched beam antennas continuously scan the
output of each beam and select the beam having strongest output power. Figure 2.11 shows
the Switch beam antenna.
Figure 2.11: Switched Beam Antenna [11]
2.7.1.2 Adaptive Array Antenna
Adaptive array antenna has an infinite number of beam patterns that can be adjusted according
to real time scenarios. The adaptive array utilizes advanced signal processing techniques to
distinguish between the interferer, multipath and the desired subscriber. It continuously
monitors the changes between interfering desired signal locations, and maximizes the link
budget (estimation and determination of all gains and losses of transmitted signal upon arrival
at the receiver) due to its ability to track the interferer with null and users with main lobes.
Figure 2.13 shows the adaptive array antenna.
Figure 2.12: Adaptive Array System [11]
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2.7.2 Diversity Techniques
Diversity techniques enhance the performance of the wireless system by reducing the fading a
signal faces during its transmission. Time diversity, frequency diversity and space diversity
are common types of diversity.
2.7.3 Multiple Input Multiple Output (MIMO)
MIMO refers to a system having minimum two antennas at the base station as well as at the
mobile station. MIMO system enhances the performance of WiMAX including spatial
multiplexing, diversity and interference reduction. WiMAX supports two forms of MIMO
systems, Open loop MIMO and Closed loop MIMO systems. A general MIMO system is
shown in Figure 2.13.
.
.
.
.
.
.
Figure 2.13: General MIMO System
2.7.3.1 Open loop MIMO System
Open loop MIMO techniques are subdivided into Matrix A and Matrix B. Open loop MIMO
does not utilize the information of the channel. Matrix A refers to the Space Time Block
Coding (STBC) whereas Matrix B refers to the spatial multiplexing in WiMAX. Open loop
techniques increase the range and capacity of WiMAX.
2.7.3.2 Closed loop MIMO System
The transmitter collects information about the propagation channel in the closed loop MIMO
to further enhance coverage and capacity of WiMAX. Closed loop MIMO utilizes the
beamforming or Maximum Ratio Transmission (MRT).
The Multiple antenna organization chart for WiMAX is shown in Figure 2.14.
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Multiple Input Multiple Output
Open loop MIMO
Matrix A
(STBC)
Closed loop MIMO
Matrix B
(SM)
Beamforming
Figure 2.14: WiMAX Multiple Antenna Implementation Organization Chart
2.8 Network Architecture of WiMAX
IEEE 802.16e specifies the air interface but it does not define the end-to-end network
architecture for WiMAX. The Network Working Group (NWG) has developed a reference
network architecture used for the deployment of WiMAX. Interoperability between various
WiMAX equipments and operators can be ensured by this framework. The network
architecture is based on IP services and can be divided logically into three parts: Mobile
Station (MS), Connectivity Service Network (CSN) and Access Service Network (ASN).
Reference network architecture is shown in Figure 2.15 [12].
Internet
AAA
ASP
MS
BS
MIP-HA
MS
MS
BS
Access
Network
ASNGW
IP Network
BS
Access Service
Network (ASN)
IP
Network
Connectivity
Service
Network
(CSN)
OSS/BSS
PSTN
AAA: Authentication, Authorization, Accounting
ASN-GW: Access Service Network Gateway
MIP-HA: Mobile IP Home Agent
OSS: Operational Support System
MS: Mobile Station
BS: Base Station
Gateway
Figure 2.15: WiMAX Network Architecture [12].
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Chapter 3: Long Term Evolution
__________________________________________________
3.1 Overview of 3GPP Long Term Evolution
The 3rd Generation Partnership Project (3GPP) started working on 3G cellular system
evolution in November, 2004. The 3GPP is the collaboration agreement for promotion of
mobile standards in order to cope future needs (high data rates, spectral efficiencies, etc.). The
3GPP LTE (Long Term Evolution) was developed to provide higher data rates, lower
latencies, wider spectrum and packet optimized radio technology.
Like other cellular technologies LTE uses OFDM as multiplexing technique. LTE uses
OFDMA as downlink and Single Carrier FDMA (SC FDMA) as uplink transmission
technique. The use of SC FDMA in LTE reduces the Peak to Average Power Ratio (PAPR)
which is the main drawback of OFDM.
LTE uses wider spectrum, up to 20 MHz, to provide compatibility with existing cellular
technologies such as UMTS and HSPA+, and increases the capacity of the system. LTE uses
flexible spectrum which makes it possible to be deployed in any bandwidth combinations.
This makes LTE suitable for various sizes of spectrum resources.
LTE uses both FDD and TDD as duplexing techniques to accommodate all types of spectrum
resources.
3.2 LTE Performance Targets
The LTE performance targets are shown in Table 3.1.
Downlink
Requirements
Peak data transmission rate
> 100 Mbps
Peak Spectral Efficiency
> 5 b/s/Hz
Comment
LTE Bandwidth = 20 MHz
Duplexing Mode = FDD
Spatial Multiplexing = 2x2
Spectral Efficiency of cell > 0.04 – 0.06 bps/Hz/user
Edge
Assumed 10 Users/Cell
Average Cell
Efficiency
Spatial Multiplexing = 2x2
Receiver = IRC
(Interference Rejection
Combining)
Broadcost Specral
Efficiency
Spectral > 1.6 – 2.1 bps/Hz/cell
1 bps/Hz
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Uplink
Requirements
Peak Data Transmission
Rate
> 50 Mbps
Peak Spectral Efficiency
> 2.5 bps/Hz
Comments
LTE Bandwidth = 20 MHz
Duplexing Mode = FDD
Transmission = Single
Antenna
Spectral Efficiency of Cell > 0.02 – 0.03 Bps/Hz/user Single Antenna
Edge
transmission
System
Receiver =IRC
Average Spectral
Efficiency
> 0.66 – 1.0 bps/Hz/cell
Assumed 10 Users/Cell
Operating Bandwidth
1.4 MHz to 20 MHz
Initially starts at 1.25 MHz
User Plane Latency
< 10 ms
Connection set up Latency
< 100 ms
From Idle mode to Active
Table 3.1: Performance Targets for Long Term Evolution
3.3 LTE Physical Layer
The physical layer of LTE conveys data and control information between E-UTRAN NodeB
(eNodeB) and user equipment (UE) in an efficient way. It employs advanced technologies
such as OFDM and MIMO for data transmission. In addition, LTE uses OFDMA and SCFDMA for downlink and uplink data transmissions. The use of SC-FDMA in the uplink
reduces PAPR. A detail description of LTE physical layer is provided below.
3.3.1 Generic Frame Structure
The generic frame of LTE has a length of 10ms and is subdivided into ten sub-frames of 1ms
length. Each sub-frame is further divided into two slots of 0.5ms having six or seven OFDM
symbols depending upon the length of CP. Each slot uses 7 OFDM symbols in case of normal
CP whereas 6 OFDM symbols in case of extended CP. Sub-frames can be assigned for either
uplink or downlink. The generic frame structure of LTE downlink and uplink is shown in
Figure 3.1.
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0
1
2
3
4
5
6
Cyclic Prefix
0
1
2
………
1 slot = 0.5ms
14
15
16
17
18
19
7 OFDM symbols
0
1 subframe=1ms
1
2
3
4
5
6
Radio Frame of 10ms
Figure 3.1: Generic Frame Structure for Downlink and Uplink of LTE
In case of FDD, all subframes are used either for downlink or for uplink data transmissions.
For TDD, subframe 1 and 6 are used for downlink transmission whereas the rest of the frames
are used either for uplink or downlink. Subframes 1 and 6 contain synchronization signals for
downlink. Figure 3.2 shows downlink and uplink subframe assignments for FDD.
1 Frame of 10msec
Figure 3.2: Downlink and Uplink Subframe Assignment for FDD
Uplink transmission
Downlink Transmission
Figure 3.3(a): Downlink Subframe Assignment for TDD
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Subframe 1 and 6 assigned for downlink transmission
Downlink transmission
Uplink transmission
Figure 3.3(b): Uplink Subframe Assignment for TDD
Figure 3.3(a) and Figure 3.3(b) show the uplink subframe assignments for FDD and TDD.
3.3.2 LTE Physical Layer for downlink Transmission
3.3.2.1 Modulation Parameters
The transmission scheme used in downlink is OFDM using a cyclic prefix. The basic
subcarrier spacing is 15 kHz with OFDM symbol duration of 66.67us. The downlink uses a
subcarrier spacing of 7.5 kHz with OFDM symbol duration of 133us in case of Mobile
Broadcast Single Frequency Network (MBSFN). MBSFN refers to a mobile network using a
single band on which broadcasted and dedicated signals are sharing single frequency [13].
Two types of cyclic prefixes are used, depending on the delay dispersion characteristics of the
radio cell (channel delay spread). The normal CP is used in urban or high frequency areas
whereas extended CP is used in rural and low frequency areas.
The modulation parameters for various transmission bandwidth configurations for LTE are
shown in Table 3.2.
Parameters
Transmission
Bandwidth (MHz)
Values
1.25
2.5
5
Subcarrier Spacing
Sampling
Frequency
10
15
20
23.04
MHz
30.72
MHz
15 kHz
1.92 MHz 3.84
(1/2x3.84
MHz
MHz)
7.68 MHz 15.36
(2x3.84
MHz
MHz)
(4x3.84
MHz)
FFT Size
128
256
512
1024
1536
2048
No. of occupied
subcarrier
76
151
301
601
901
1201
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Parameters
Values
Number of OFDM
symbols/slot
CP
(4.69/9) x
lengths
Normal 6,
(us/sam
(5.21/10) x
ple)
1
Extende
d
7 for Normal CP and 6 for Extended CP
(4.69/18
)x6
(4.69/36)x (4.69/72)x (4.69/108
6
6
)x6
(4.69/144
)x6
(5.21/10
)x1
(5.21/40)
x1
(5.21/80)
x1
(5.21/120
)x1
(5.21/160
)x1
(16.67/
64)
(16.67/
128)
(16.67/
256)
(16.67/
512)
(16.67/
1024)
(16.67/32)
Table 3.2: Modulation Parameters for Downlink [13]
3.2.2.2 Downlink Physical Resource
The downlink physical resource consists of Physical Resource Blocks (PRBs) where a PRB
consists of 12 consecutive subcarriers for one slot (1 slot = 0.5msec). The bandwidth of PRB
is 180 kHz. A resource element corresponds to one subcarrier for the duration of one OFDM
symbol. Thus depending on the cyclic prefix length, a PRB comprises 84 OFDM symbols in
case of normal CP and 72 OFDM symbol in case of extended CP. The number of resource
blocks depends upon the transmission bandwidth of LTE i.e. 1.25 MHz to 20 MHz. Table 3.3
shows the number of PRBs for various transmission bandwidths.
Transmission
Bandwidth
(MHZ)
1.25
2.5
5
Subcarrier BW
(kHz)
15
PRB BW
(kHz)
180
Number of
available PRB
6
12
25
10
15
20
50
75
100
Table 3.3: Number of Physical Resource Blocks (PRB) for Various Transmission
Bandwidths [14]
The Downlink physical resource in time frequency grid is shown in Figure 3.4 [15].
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Figure 3.4: LTE Downlink Physical Resource [15]
Figure 3.4 shows that, a PRB is comprised of 12 consecutive subcarriers with a subcarrier
spacing of 15 kHz and 7 OFDM symbols for the duration of 0.5ms in case of normal cyclic
prefix. Thus a PRB of 84 resource elements (12 x7 = 84) corresponds to one slot in the time
domain whereas a PRB of 180 kHz (15 kHz x 12 = 180 kHz) corresponds to the frequency
domain.
3.2.2.3 LTE Physical Channels for Downlink
Physical channels convey information from upper layers of the LTE stack. Physical channels
are mapped onto transport channels. The transport channels act as an interface or Service
Access Points (SAPs) between the MAC and physical layer. Every physical channel has
defined the algorithms for bit scrambling, modulation, layer mapping, Cyclic Delay Diversity
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(CDD) precoding and resource elements. LTE supports various types of physical channels in
the downlink.
Physical Broadcast Channel (PBCH)
It carries paging and control signaling information. The coded broadcast channel transport
block is mapped on four subframes within 40ms interval, blindly detected (no explicit
signaling) [16]. The subframes are assumed to be self decodable. QPSK is used as modulation
technique in this channel [14].
Physical Control Format Indicator Channel (PBFICH)
PBFICH contains the number of OFDM symbols used for Physical Downlink Control
Channel (PDCCH) and it informs the UE about this. PBFICH is transmitted in every
subframe.
Physical Downlink Control Channel (PDCCH)
PDCCH is used to carry out the control signaling information to UE. PDCCH is used by the
eNodeB. It carries ACK/NACK response to the uplink channel, resource allocation
information for UE and scheduling grant for UL [16]. Multiple PDCCH can be transmitted in
one subframe. PDCCH is mapped onto resource elements in up to the first three OFDM
symbols in the first slot of a subframe. It uses QPSK as a modulation technique.
Physical Hybrid ARQ Indicator Channel (PHICH)
It carries the ACK/NAK responses of Hybrid ARQ. It uses QPSK as modulation technique.
Physical Downlink Shared Channel (PDSCH)
It is utilized for transportation of data and multimedia services. Due to requirement of high
data rates, it uses modulation techniques such as QPSK, 16 and 64-QAM. Spatial
multiplexing is implemented in PDSCH.
Physical Multicast Channel (PMCH)
It carries multicast data. It uses QPSK, 16-QAM and 64-QAM as modulation techniques.
3.2.2.4 LTE Downlink Physical Signals
Physical signals use assigned resource elements in the physical resource. They do not convey
information to (or from) upper layers of the LTE stack.
There are two types of physical signals used in LTE:
Reference Signals
Reference signals are generated as a combination of Pseudo Random Numerical (PRN)
sequence and an orthogonal sequence. They are used to determine the Channel Impulse
Response (CIR). Reference signals consist of known reference symbols that are inserted in
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the first and third OFDM symbol of every slot. There are 510 unique reference signals.
Reference signals are of three types:
 Cell specific reference signals.
 User equipment specific reference signals.
 Mobile Broadcast Single Frequency Network (MBSFN) reference signals.
Cell specific reference signals are associated with non MBSFN transmission. They use 1, 2 or
4 antenna ports for the transmission.
MBSFN reference signals are associated with MBSFN transmission. They are transmitted on
antenna port.
UE reference signals support single antenna port transmissions of PDSCH in the frame
structure of type 2.
Figure 3.5 [17] shows the cell specific reference signals.
Figure 3.5: Cell Specific Reference Signals [17]
Figure 3.5 shows that reference signals are transmitted on first and fourth OFDM symbol of
every slot, which depend on the antenna port and type of the frame structure.
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Synchronization Signals
Synchronization signals are used for cell identification and slot synchronization. For this
purpose they use Primary Synchronization Channel (P-SCH) and Secondary Synchronization
Channel (S-SCH). Synchronization signals are transmitted on 72 subcarriers centered around
the DC subcarrier during every 0 and 10 frame slot [14]. The modulation schemes used in
physical signals are shown in Table 3.4.
Physical Signals
Modulation Scheme
Reference Signals
Orthogonal Sequence of binary PN sequence
Primary Synchronization Channel (P-SCH)
Cycle of 3 Zadoff-Chu sequence
Secondary Synchronization Channel (S-SCH)
Two 31 bit BPSK M sequences
Table 3.4: Modulation Schemes for Downlink Physical Signals
3.2.2.5 LTE Downlink Transport Channel
Transport channels act as an interface between MAC and the physical layer [18]. They
transfer the information to MAC and upper layers. The description of downlink transport
channel is described below.
Broadcast Channel (BCH)
Broadcast channel is used to broadcast the system parameters (such as random access related
parameters) to enable the devices accessing the system.
 Fixed transport format
 Broadcast the information in the entire cell coverage area.
Downlink Shared Channel (DL-SCH)
It carries user data information for point to point connection in the downlink. DL-SCH is
characterized as:
 Dynamic link adaptations supported by varying the coding, modulation and transmit
power.
 Suitable to use with beamforming.
 Hybrid ARQ.
 Can be broadcasted in the entire cell coverage area.
 Support for semi static and dynamic resource allocation.
 MBMS transmission.
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Paging Channels
Paging channels are used to carry paging information to move the device from RRC_IDLE
state to RRC_CONNECTED state. In RRC_CONNECTED state a mobile has established
RRC connection with SGSN (Serving GPRS Support Node) and Radio Access Network
(RAN). Paging channels are characterized as follows:
 Requirement for broadcast over whole cell coverage area.
 Mapped to physical resources which can be allocated dynamically for traffic channels.
Multicast Channel (MCH)
Multicast channel is used to transfer multicast data to the UE in the downlink.
 Requirement for broadcast over whole cell coverage area.
 Provides support for MBSFN.
 Semi static resource allocation.
3.2.2.6 Mapping of Downlink Transport channels to Downlink Physical Channels
Figure 3.6 shows the mapping of downlink transport channel to physical channel. The PCH
and DL-SCH are mapped on PDSCH. BCH is mapped on PBCH and MCH is mapped on his
related downlink PMCH physical channel.
PCH
------
BCH
PDSCH
------
PBCH
- - - - - - DL-SCH
--- -
PMCH
- - - - - - - MCH
- - - - - PDCCH
------
- - - - - PHICH
Figure 3.6: Mapping of Downlink Transport Channels to Physical Channels [16]
Transport channels provide the structure for transferring data to or from upper layers, the
mechanism for configuring the physical layer, peer to peer signaling for upper layers and
status indicators (Channel-Quality Indicator (CQI), packet errors) to upper layers.
3.2.2.7 OFDMA Basics
OFDMA is an extension of OFDM and is used in the downlink of LTE. OFDMA distributes
subcarriers to different users at the same time so that multiple users can receive data
simultaneously while in OFDM, a single user can receive data on all subcarriers at any given
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time. Subcarriers are allocated in contiguous groups with a subcarrier spacing of 15 kHz in
order to reduce the overhead of indicating which subcarriers have been allocated to each user
[19].
OFDMA is based on Discrete Fourier Transform (DFT) and Inverse Discrete Fourier
Transform (IDFT) to switch between time and frequency domain. The time domain
representation of various inputs applied to FFT are shown in Figure 3.7 [20].
Figure 3.7: FFT Operation Applied to Various Inputs in Time Domain [20]
FFT converts the time domain signal to frequency domain. For a sinusoidal wave, FFT
operation results in a peak at the corresponding frequency and zeros elsewhere, while in case
of square wave FFT the operation results in having multiple peaks on various frequencies.
The bigger peak of square wave corresponds to the fundamental frequency (f = 1 / T) while
rests are the odd harmonics of it.
OFDMA Transmitter and Receiver
OFDMA transmitter uses narrow and orthogonal subcarriers such that at the sampling instant
of one subcarrier, the remaining subcarriers have zero value. In LTE, OFDMA uses fixed 15
kHz frequency spacing between the subcarriers regardless of the transmission bandwidth. In
the OFDMA transmitter, first high data rate bit stream is passed through the modulator. The
modulator uses various coding schemes such as QAM. The modulated bits are converted from
serial to parallel which becomes the input of IFFT block. The inputs to the IFFT block are the
subcarriers converted into the time domain signal. CP is added in the signal by copying the
part of the symbol at the end and inserted in the beginning. The advantage of adding cyclic
prefix is to avoid the ISI. The length of CP should be larger than the channel delay spread or
channel impulse response in order to avoid the ISI at the receiver.
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The receiver does the inverse procedure by first removing the CP extension followed by serial
to parallel conversion. The subcarriers are then passed to FFT block which converts them into
a frequency domain signal. The frequency domain signal is equalized and demodulated.
The Transmitter-Receiver block diagram of OFDMA is shown in Figure 3.8 [21].
Figure 3.8: Transmitter-Receiver Block Diagram for OFDMA [21]
OFDMA arranges the subcarrier on the basis of resource blocks instead of individual
subcarriers. A resource block is comprised of 12 consecutive subcarriers with 15 kHz
frequency spacing in the frequency domain for a duration of 0.5ms in time domain. The size
of RB is 180 kHz in the frequency domain while having 84 OFDM symbols (12 x 7 = 84) in
the time domain as in the case of normal CP. One OFDM symbol corresponds to a Resource
Element (RE). The OFDMA resource blocks in LTE are shown in Figure 3.9 [21].
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Figure 3.9: Structure of OFDMA Resource Blocks [21]
3.2.2.8 Downlink Physical Layer Processing
Physical layer interfaces to MAC layer by mean of transport channels. The LTE physical
layer receives data in the form of transport blocks of a certain size. The downlink transport
channel processing consists of the steps depicted in Figure 3.10.
CRC Insertion
Channel coding
Hybrid ARQ Processing
Channel Interleaving
Scrambling
Modulation
Layer mapping and Pre-coding
Antenna Mapping
Resource Management
Figure 3.10: LTE Physical Layer Processing in Downlink [22]
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CRC Insertion: A 24 bits CRC is inserted in the beginning of the transport blocks. CRC
detects residual errors at the receiver by decoding the transport blocks.
Channel Coding: It uses turbo coding based on Quadratic Polynomial Permutation (QPP)
inner interleaving with trellis termination [23].
Hybrid ARQ (HARQ) processing: The functionality of downlink hybrid ARQ is to extract the
bits from the blocks of code bits delivered by the channel encoder and to transmit the exact
set of bits within a given Transmission Time Interval (TTI). The number of extracted bits
depends on the modulation scheme, assigned resource size and spatial multiplexing order.
If the number of coded bits from the channel encoder is larger than the number of bits to be
transmitted, the hybrid ARQ will extract the subsets of code bits with an effective rate Reff >
1/3.
If the number of encoded bits from the channel is smaller than the number of bits that have to
be transmitted, the hybrid ARQ will repeat the subset of bits or total bits with an effective rate
of Reff < 1/3.
Hybrid AQR transmits the various code bits set in case of a retransmission.
Scrambling: “Scrambling of coded data ensures that the receiver side decoding can utilize the
processing gain provided by the channel code” [24]. In LTE, scrambling is applied on the bits
delivered from the HARQ by multiplying with the scrambling sequence. The downlink
scrambling is shown in Figure 3.11.
M bits Scrambling Sequence
M bits from HARQ
M Bits
Figure 3.11: Downlink Scrambling
Scrambling is applied to DL-SCH, PCH, and BCH while MCH uses cell common scrambling.
Modulation: The LTE downlink supports 16-QAM, 64-QAM and QPSK as modulation
schemes. Modulation is performed on the scrambled bits and results in the M/L modulation
symbols where L = 2, 4, 6 for QPSK, 16-QAM and 64-QAM respectively. BCH uses QPSK
as modulation scheme. The block diagram for downlink modulation is shown in Figure 3.12.
M Bits as result
of scrambling
Data Modulator
Modulation
Symbols (M/L)
Figure 3.12 Downlink Modulation
Antenna Mapping: Antenna mapping jointly processes the modulation symbols,
corresponding to two transport blocks in general and maps the output to various antenna ports.
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In LTE, antenna mapping can be configured to support spatial multiplexing, transmit diversity
and multi-antenna schemes.
Resource Block Mapping: It maps the symbols which are the outputs of the antenna port to
the resource elements of the resource blocks. The resource blocks are assigned by the MAC
scheduler for the transport block(s) transmission to terminal.
3.3.3 Uplink Physical Layer
3.3.3.1 Modulation Parameters
Uplink uses a frequency spacing of 15 kHz between subcarriers. The subcarriers are grouped
in the form of RBs comprised of 12 consecutive SCFDMA subcarriers for the duration of one
slot (0.5ms) using normal or extended cyclic prefixes. A slot uses 7 and 6 SC-FDMA symbols
in the case of normal and extended CPs respectively. The duration of normal and extended
CPs are as
Normal CP: TCP = 160×Ts = 5.2 us (SC-FDMA symbol #0), TCP = 144×Ts = 4.7 us (SCFDMA symbol #1 to #6).
Extended CP: TCP-e = 512×Ts = 16.67 us (OFDM symbol #0 to OFDM symbol #5)
Due to the fixed size of RBs in LTE, uplink supports a number of resource blocks ranging
from NRB-min = 6 to NRB-max = 110 in frequency domain, where Ts = 1/2048x∆f and ∆f is
subcarrier spacing.
𝑁𝑁𝑅𝑅𝑅𝑅 (𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁 𝑜𝑜𝑜𝑜 𝑅𝑅𝑅𝑅) =
𝑁𝑁𝑅𝑅𝑅𝑅−𝑚𝑚𝑚𝑚𝑚𝑚 =
𝑁𝑁𝑅𝑅𝑅𝑅−𝑚𝑚𝑚𝑚𝑚𝑚 =
𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇 𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵ℎ 𝑜𝑜𝑜𝑜 𝐿𝐿𝐿𝐿𝐿𝐿
𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵ℎ 𝑜𝑜𝑜𝑜 𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅 𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵
𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀 𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇 𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵ℎ 𝑜𝑜𝑜𝑜 𝐿𝐿𝐿𝐿𝐿𝐿
1.25 𝑀𝑀𝑀𝑀𝑀𝑀
=
=6
𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵ℎ 𝑜𝑜𝑜𝑜 𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅 𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵
180 𝑘𝑘𝐻𝐻𝐻𝐻
𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀 𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇 𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵ℎ 𝑜𝑜𝑜𝑜 𝐿𝐿𝐿𝐿𝐿𝐿
20 𝑀𝑀𝑀𝑀𝑀𝑀
=
= 110
𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵ℎ 𝑜𝑜𝑜𝑜 𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅 𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵
180 𝑘𝑘𝐻𝐻𝐻𝐻
Data is mapped onto the QPSK, 16-QAM and 64-QAM in the LTE uplink. The modulated
symbols are fed into a serial-to-parallel convertor instead of modulating the QPSK/QAM
symbols directly in the LTE downlink OFDM. The FFT block takes the parallel modulated
symbols as an input and transforms them into discrete frequency domain sequences. The
discrete Fourier terms are mapped to the subcarriers and converted back into the time domain
by using IFFT. The CP is added and the signal is sent for transmission.
The use of SC-FDMA in the uplink minimizes the PAPR as compared to OFDM and is
bandwidth efficient.
3.3.3.2 Uplink Physical Resource
The uplink physical resources can be shown in form of time-frequency resource grids. Uplink
supports two frame structures similar to the downlink LTE. We consider a generic frame
structure of LTE and discuss the resources according to it. The generic frame structure is
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comprised of 10 subframes with a duration of 10msec. The subframes are further divided into
slots of 0.5msec per slot. Every slot consists of 7 or 6 SC-FDMA symbols depending on the
type of cyclic prefix. A slot is comprised of 7 SC-FDMA symbols in case of normal CP and 6
for extended CP.
The slot structure for normal CP and extended CP for LTE uplink is shown in Figure 3.13 (a)
and 3.13 (b).
1st CP = 5.21us
Symbol
#0
CP = 4.7 us (from symbol #1 to symbol #6)
Symbol
#1
Symbol
#2
Symbol
#3
Symbol
#4
Symbol
#5
Symbol
#6
One Slot (1msec)
Figure 3.13(a): Uplink Slot Structure in Case of Normal CP
Extended CP = 16.67 us
Symbol
#0
Symbol
#1
Symbol
#2
Symbol
#3
Symbol #4
Symbol #5
One Slot (1msec)
Figure 3.13(b): Uplink Slot Structure in Case of Extended CP
The uplink resources are grouped in RBs where every RB consists of 12 consecutive
subcarriers for the duration of one slot in the LTE frame structure. Hence a RB consists of
(12x7 = 84 SCFDMA symbols) or (12x6 = 72 SCFDMA symbols) for the normal and
extended CP, respectively. The frequency spacing is 15 kHz between the subcarriers.
The transmitted signal in every uplink slot is comprised of NRBUL x NSCRB subcarriers and
NSymbUL SC-FDMA symbols as shown in Figure 3.14 [25]. The NRBUL depends on the
transmission bandwidth due to its fixed size. The NRBUL ranges from 6 to 110 resource blocks
while the transmission bandwidth ranges from 1.25 MHz to 20 MHz. The elements in the
resource grid are called resource elements (REs). We can access specific RE by time-
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frequency coordinates (k, l) where k is subcarrier number and ‘l’ is the SC-FDMA symbol.
The resource grid for LTE uplink is shown in Figure 3.14 [25].
Figure 3.14: Resource Grid for LTE Uplink [25]
3.3.3.3 LTE Uplink Physical Channels:
LTE Uplink supports three types of physical channels:
 Physical Random Access Channel (PRACH).
 Physical Uplink Shared Channel (PUSCH).
 Physical Uplink Control Channel (PUCCH).
Physical Random Access Channel (PRACH)
The PRACH carries the random access preamble. The random access preamble consists of CP
length and sequence length. There are four types of preamble formats. The random access
preambles are generated from Zadoff-Chu sequences [26] with zero correlation zone
generated from one or several root Zadoff-Chu sequences. Zadoff-Chu sequence is a complex
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mathematical sequence and it generates signals of constant amplitude. The use of Zadoff-Chu
sequences reduces the PAPR and BER of LTE uplink. The format of Random Access
Preamble is shown in Figure 3.15.
Cyclic
Prefix
Sequence
TCP
TSEQ
Figure 3.15 Random Access Preamble Format
Physical Uplink Shared Channel (PUSCH)
PUSCH carries user data for transmission (generates time domain SC-FDMA signals for
every antenna port). Transmission time is 1msec which is similar to downlink transmission.
PUSCH uses QPSK, 16-QAM and 64-QAM modulations.
Physical Uplink Control Channel (PUCCH)
PUCCH carries the uplink control information. It is not simultaneously transmitted with
PUSCH for the UE. PUCCH will be mapped to the uplink control channel resource which is
defined by a code and two resource blocks, consecutive in time, with hopping at slot boundary
[26].
The signaling for uplink can differ depending on presence or absence of time synchronization.
In case of time synchronization PUCCH performs the following duties:
 Carries Channel Quality Indicators (CQI) reports.
 Scheduling Request.
 Carries HARQ ACK/NACK responses in reaction of downlink transmission.
 It uses QPSK and BPSK modulation.
The CQI tells the scheduler about the channel conditions seen by UE. The HARQ consists of
a single ACK/NACK bit per HAQR process.
3.3.3.4 Uplink Physical Signals
Uplink physical signals are used by the physical layer but they do not carry data from upper
layers of LTE. There are two types of uplink physical signals:
 Reference Signals.
 Random Access Preamble.
Reference Signals
In case of normal CP, uplink reference signals are transmitted in the fourth block in every slot
in order to facilitate coherent demodulation [27]. There are two types of reference signals:
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 Demodulation reference signals: Transmitted in the fourth SC-FDMA symbol in every
slot and facilitate coherent demodulation. They are based on Zadoff-Chu sequences.
 Sounding reference signals: Based on Zadoff-Chu sequences and facilitate frequency
selective scheduling.
Random Access Preamble
Random access procedure includes upper layers and physical layer of the LTE stack. As the
transmission of random access preamble starts, the UE initiates the cell search procedure. If it
is successful, a random access sequence is received from the eNodeB. Zadoff-Chu sequences
are used to derive the random access preambles. Random access preambles are grouped in 72
contiguous subcarriers for transmission.
The random access preamble shown in Figure 3.16 consists of CP, preamble and a guard
period.
CP
TCP = 0.1 ms
Preamble
Guard
Time (GT)
TPRE = 0.8 ms
TGT = 0.1 ms
TRA = 1 ms
Figure 3.16: Format of Random Access Preamble [14]
Random access preamble is 1ms in duration and comprised of TCP of 0.1 ms, preamble of 0.8
ms and TGT of 0.1 ms. In GT no transmission takes place. Upper layers provide preamble
sequences, initial transmission power, available random access channels and maximum
number of retries to the PHY. The basic functionality of random access preamble is shown in
Figure 3.17 [17].
Figure 3.17: Random Access Preamble Functionality [17]
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3.3.3.5 LTE Uplink Transport Channels
Uplink transport channels act as an interface between physical and upper layers of LTE stack.
The description of uplink transport channels are as follows:
Uplink Shared Channel (UL-SCH)
 Support for beamforming (optional).
 Hybrid ARQ.
 Dynamic link adaptations are supported by varying coding and modulation.
 Semi-static and dynamic resource allocation.
Random Access Channel (RACH)
 Carries minimal control information.
 Transmission may be lost due to collision.
3.3.3.6 Mapping of Uplink Transport Channels to Uplink Physical Channels
Uplink transport channels are mapped to their respective uplink physical channel as shown in
Figure 3.18. UL-SCH is mapped on PUSCH and RACH is mapped on PRACH.
UL-SCH
PUCH
-------
RACH
- - - - - - - - PRACH
- - - - - - - - - - - - - - Uplink Transport Channels
----
PUCCH - -- -Uplink physical Channels
Figure 3.18: Mapping of Uplink Transport and Physical Channels
3.3.3.7 Single Carrier FDMA Basics
Single Carrier-FDMA (SC-FDMA) is an extension of OFDMA and is used in the uplink of
LTE. Unlike OFDMA, SC-FDMA reduces the PAPR by adding additional blocks of DFT and
IDFT at transmitter and receiver. The transmitter and receiver structure of SC-FDMA is as
follows.
SC-FDMA Transmitter
The SC-FDMA transmitter consists of function blocks similar to OFDMA. The block
diagram of SC-FDMA is shown in Figure 3.19. The input data stream is first modulated to
single carrier symbols by using QPSK, 16-QAM or 64-QAM. The resultant modulated
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symbols become the inputs of the functional blocks of SC-FDMA. The description of every
functional block is described below.
Serial
to
Parallel
Subcarrier
MApping
N-Point DFT
M-Point IDFT
Parallel
to
Serial
ADD
Cyclic
Prefix
Digital
to
Analog
Convers
ion
(DAC)
Figure 3.19: SC-FDMA Transmitter Structure
Serial to Parallel Convertor (S-to-P): The modulated symbols are converted into parallel
symbols and organized into blocks.
N-Point DFT (Discrete Fourier Transform): Converts time domain single carrier blocks into N
discrete frequency tones.
Subcarrier Mapping: Controls the frequency allocation, and maps N-discrete frequency tones
to subcarriers for transmission. The mapping can be localized or distributed. In localized
mapping, N-discrete frequency tones are mapped on N consecutive subcarriers where as in
distributed mapping, N-discrete frequency tones are mapped on uniformly spaced subcarriers.
Figures 3.20(a) and 3.20(b) show the localized and distributed mapping respectively. LTE
uses localized mapping because it exploits frequency selective gain by channel dependent
scheduling [28].
Adding 0’s
.
.
DFT
IDFT
.
.
.
.
.
.
Adding 0’s
Figure 3.20(a): Localized FDMA
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Adding 0’s
.
.
.
.
IDFT
DFT
.
.
.
.
.
.
Adding 0’s
Figure 3.20(b): Distributed FDMA
M-Point IDFT: Converts the mapped subcarriers to time domain. For efficient computations
of IDFT M>N.
Parallel to Serial Convertor (P-to-S): The time domain subcarriers are converted back from
parallel to serial.
Add Cyclic Prefix: CP is added to avoid ISI. The length of CP is larger than the channel delay
spread in order to avoid ISI at the receiver.
Digital to Analog Convertor (DAC): Converts the digital signal to analog signal and up
convert (convert set of values to higher set of values) to RF for transmission over the channel.
SC-FDMA Receiver
The SC-FDMA receiver does the inverse of SC-FDMA transmitter. The block diagram of
receiver is shown in Figure 3.21.
Detect
Parallel
to
Serial
N-Point
IDFT
Subcarrier
DeMpping/
Equalizati
on
M-Point
DFT
Serial
to
parallel
Remove
Cyclic
Prefix
Analog
to
Digital
Convers
ion
Figure 3.21: SC-FDMA Receiver
The receiver converts the analog signal to digital and removes the cyclic extension. The
output of “Remove CP” block is converted serial to parallel and become the input to M point
DFT, which results into M-mapped subcarriers in the frequency domain. The M-mapped
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subcarriers are de-mapped which results in N-discrete frequency tones. The N-frequency
tones are converted back into time by using IDFT and passed to the PS convertor and converts
parallel time domain symbols to serial data stream. The serial data stream is passed through
detector which results in single carrier modulation symbols in the time domain. The single
carrier symbols are demodulated in order to get the input bit stream.
SC-FDMA Resources
SC-FDMA arranges subcarriers in RBs similar to the downlink OFDMA. A RB is comprised
of 12 consecutive subcarriers for the duration of one time slot of LTE frame (1slot = 0.5 ms).
Two types of CP are used in uplink, the normal and extended CP having 7 and 6 SC-FDMA
symbols respectively. Due to the fixed size of RB’s, uplink supports flexible transmission
bandwidths similar to downlink.
The SC-FDMA Resource Grid for LTE is shown in Figure 3.22 [29].
Figure 3.22: LTE Resource Grid for SC-FDMA [29]
Where NRB = number of Resource Blocks
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NscRB= Number of subcarriers in a resource block
NRB x NscRB = Total transmission bandwidth (LTE supports bandwidth ranges from 1.4 MHz
to 20 MHz).
Nsymb= Number of SC-FDMA symbols in one slot.
Nsymb x NscRB= Number of REs in one RB.
The SC-FDMA parameters used in LTE are described in Table 3.5
Transmission Bandwidth
1.4
3
5
10
15
20
FFT Size
128
256
512
1024
1536
2048
Sampling Rate:
(N/M x 3.84 MHz)
½
1/1
2/1
4/1
6/1
8/1
Number of
subcarriers
72
180
300
600
900
1200
Number of
Resource blocks
6
15
25
50
75
100
Bandwidth
Efficiency (%)
77.1
90
90
90
90
90
Table 3.5: SC-FDMA Parameters for LTE [29]
In Table 3.5, N>M in order to perform efficient computations of IDFT.
3.3.3.8 Uplink Physical layer Processing:
The LTE uplink transport channel for Uplink-Shared Channel (UL-SCH) is shown in Figure
3.23. The uplink transport channel processing is somewhat similar to downlink transport
however, uplink transport channel processing did not define transmit diversity and spatial
multiplexing for the LTE uplink. In addition, there is no explicit multi-antenna mapping
functions defined for the processing of the uplink transport channel. In contrast to downlink, a
single transport block of dynamic size is transmitted for every Transmission Time Interval
(TTI).
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Transport block of dynamic size from MAC layer
Transport Block
CRC Insertion
Channel Coding
Hybrid-ARQ
Scrambling
Data Modulation
To SC-FDMA (DFTS-OFDM) modulation including
Mapping of assigned frequency resource
Figure 3.23: LTE Uplink Transport Channel Processing [30]
CRC Insertion: A 24-bits CRC is calculated and appended at the end of every transport block.
CRC allows receiver to detect residual errors from the decoded transport block. The block
diagram for CRC insertion is shown in Figure 3.24.
Transport Block
CRC
Transport Block
CRC
Figure 3.24: CRC Insertions per Transport Block
Channel Coding: Uplink channel coding uses turbo codes, including QPP based inner interleaver similar to downlink.
HARQ functionality: The task of physical layer hybrid ARQ is to extract the exact set of bits
from the blocks of code bits delivered by the encoder. The extracted bits are transmitted
within a given TTI. The uplink HARQ functionality is similar to downlink however the uplink
and downlink have different HARQ protocols i.e. asynchronous vs. synchronous operation.
Scrambling: Uplink scrambling is applied to the code bits coming from the HARQ. The
purpose of uplink scrambling is to randomize the interference which ensures fully utilization
of processing gain provided by the channel code. The uplink scrambling is specific to mobile
terminal where every terminal uses a unique scrambling sequence.
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Data Modulation: Transforms the scrambled bits into complex modulation symbols. The ULSCH uses 16-QAM, QPSK and 64-QAM for modulation in LTE uplink. UL-SCH supports a
similar set of modulation techniques as the DL-SCH in the downlink.
The block of modulation symbols is applied to DFTS-OFDM for processing, which maps the
complex modulation symbols to discrete frequency tones followed by the mapping of discrete
tones to specific subcarriers. The subcarriers are converted back to time domain by IDFT and
the cyclic prefix is inserted. The time domain signal with CP is converted from digital to
analog and sent for transmission.
3.3.4 Multi- Antenna Techniques in LTE
LTE supports transmissions with 2 or 4 antennas in the downlink. LTE uses maximum two
codewords for fixed mapping between codewords to layers. A codeword is formed by a
sequence of bits.
LTE MIMO
MIMO is a technique used to increase data rates to fulfill the needs of next generation mobile
networks. It also fulfills the needs of high capacity and extended coverage. In order to achieve
high data rates, multiple antennas can be used such as 2x2 or 4x4 MIMO whereas to achieve
extended coverage, beam-forming is used.
Downlink MIMO:
In LTE downlink, 2x2 and 4x4 configurations are used. Different MIMO modes i.e. spatial
multiplexing and transmit diversity are dependent on the condition of the channels.
Spatial Multiplexing
Spatial multiplexing transmits different data streams on the same downlink block. These
streams of data belong to a single user also called Single User MIMO (SU-MIMO) or to
multiple users called Multi User MIMO (MU-MIMO). In SU-MIMO, data rate is increased
whereas in MU-MIMO, overall capacity is increased. This is only possible if the mobile
channel allows it. The basic spatial multiplexing principle can be seen in Figure 3.25.
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TX
RX
Nt Trabsmit antennas
at eN odeB
Nr receive antennas
at UE
Figure 3.25: Spatial Multiplexing [31]
Figure 3.25 shows that each transmitter ‘Tx’, transmitting different streams of data and each
antenna at the receiver ‘Rx’ is receiving data streams from all transmitters. The channel is
specified by the following matrix H.
Where Nt represents the number of antennas at transmitter and Nr represents the number of
antennas at receiver. The hij are the coefficients of the channel from Tx j to Rx i.
The matrix rank H limits the data that can be sent over the MIMO channel and is given by
min {Nt, Nr}. When the matrix H is singular , the quality of the transmission degrades
significantly. This is possible when both the antennas i.e. Tx and Rx are too close.
3.3.4.2.2 Transmit Diversity
The transmit diversity scheme is used when the conditions of the channel do not allow spatial
multiplexing. This means that in transmit diversity a single stream of data is transmitted,
whereas in spatial multiplexing multiple streams are transmitted.
3.3.4.3 Uplink MIMO
The baseline used in the UL-MIMO is MU-MIMO. The MU-MIMO reception at eNodeB is
supported by allocating the same time frequecy resources to multiple UEs, transmitting on
single antenna [32]. The closed loop transmit diversity is only supported by FDD, and is
optional for UE.
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3.4 LTE MAC Layer
The MAC layer is the part of logical link layer (Layer 2) of the radio protocol stack of LTE as
shown in Figure 3.26. MAC layer is connected to Radio Link Control (RLC) and physical
layers through logical and transport channels respectively. MAC layer sends/receives the
MAC PDUs to/from the physical layer through transport channels. The connection to RLC
layer is through logical channels by means of RLC Service Data Units (SDUs).
MAC layer performs HARQ transmissions/retransmissions, multiplexing/de-multiplexing of
logical channels and downlink/uplink scheduling.
Layer 3
Radio Resource
Control (RRC)
Radio Link Control (RLC)
Logical Channels
Layer 2
Medium Access Layer (MAC)
Transport Channels
L
Physical Layer
Physical Channels
Transceiver
Figure 3.26: LTE Protocol Stack
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3.4.1 Logical Channels
MAC layer transfers data to/from RLC layer through logical channels. Logical channels are
categorized into control logical channels and traffic logical channels. Control logical channels
carry control information whereas user plan information is carried out by traffic control
channels. The control and traffic channels used in LTE are as follows:
Control Logical Channels
Carry the control data such as Radio Resource Control (RRC) signaling.
Dedicated Control Channel (DCCH)
Transmits dedicated control information to/from the specific UE. It configures the UEs
individually such as handover massages. It is used when UE has a RRC connection with the
eNodeB [33].
Broadcast Control Channel (BCCH)
Broadcast the system control information to the mobile terminals within a cell. It is necessary
for every mobile terminal to have system control information prior to accessing it in order to
have knowledge about system configuration and how to behave within a cell.
Paging Control Channel (PCCH)
Transmit the paging control information when the location cell of UE is unknown to the
network.
Common Control Channel (CCCH)
Used for regular transmission of control information between UEs and eNodeB and it does
not care whether the UEs have RRC connection with eNodeB or not.
Multicast Control Channel (MCCH)
Used for transmission of MBMS control information from network to UE for one or several
multicast traffic channels. It is used by UEs that receive MBMS [16].
Traffic Logical Channels
Dedicated Traffic Channel (DTCH)
DTCH is used to transmit user information dedicated to one UE. Further it is used in uplink
and non MBFSN downlink transmissions.
Multicast Traffic Channel (MTCH)
MTCH is used to transmit user data in the downlink MBMS services.
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3.4.2 Mapping of Logical Channels to Transport Channels
The logical channels are mapped to specific transport channels in downlink and uplink
direction. Figure 3.27 shows the mapping of downlink logical channels to downlink transport
channels where PCCH is mapped on PCH transport channel, BCCH is mapped on BCH and
DL-SCH. The CCCH, DCCH, DTCH, MCCH and MTCH are mapped on DL-SCH while
MCCH and MTCH are mapped on MCH.
PCCH
LC- - -
BCCH
---
CCCH
DTCH
---
Transport Channels- - - -
---
--PCH
DCCH
- -- - -
--BCH
MTCH
---
MCCH
--
--
- - - - - - - - - - - ---- -- - - - - - - DL-SCH
MCH
Where, “LC” stands for Logical Channels.
Figure 3.27: Downlink Mapping of Logical and Transport Channels [16]
Figure 3.28 shows the mapping of uplink logical channels to uplink transport channels.
Logical Channels- - -
CCCH
----
Transport Channels - - - - - - - - - - - - - -
DTCH
-----
DCCH
UL-SCH
- - - - - - - RACH
------
-----
Figure 3.28: Uplink Mapping of Logical and Transport Channels [16]
3.4.3 Data Flow in MAC
MAC layer receives data as MAC SDUs from RLC layer. The MAC SDUs are combined
along with the attachment of MAC header and MAC control elements to form MAC PDUs.
The MAC header is further divided into subheaders where every subheader contains the
Logical Control Identification (LCID) and length field. The LCID indicates which type of
control elements are used in the MAC payload field or indicates the type of channel. The
length field indicates the length of MAC SDUs or MAC control elements.
MAC control elements perform control functionalities in the uplink and downlink direction.
In uplink related to UL-SCH, MAC payload contains control elements such as:
 Buffer Status Report: Contains information about data in the UE, waiting for
transmission and information about the priority of data in the buffer.
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 Power Headroom Report: Contains information about available power resources in the
uplink direction.
 Contention Resolution Procedure Information (C-RNTI and CCCH).
In the downlink, the control elements related to DL-SCH are as follow:
 Timing advance commands: Used to adjust the timing of uplink.
 Contention resolution information.
 Used Discontinuous Reception (DRX) commands to control the DRX operation.
The general MAC PDU structure is described in Figure 3.29.
Subheader - - - - - - Subheader
MAC Header
Header
Payload
MAC PDU
MAC CE 1
--------
MAC CE N
MAC SDU1
--------
MAC SDU N
Padding
MAC Payload
Figure 3.29: MAC PDU Format [34]
The “MAC CE” corresponds to a MAC control element. In case of PCCH and BCCH, the
MAC header does not contain the LCID field because there is no multiplexing used in PCCH
and BCCH. The format of MAC header in MAC PDU in case of UL-SCH and DL-SCH is
shown in Figure 3.30.
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DL-SCH: Types of PE
UL-SCH: Types of PE

CCCH

CCCH

UE contention
and resolution
identity

Power
Headroom
Report

Timing Advance

C-RNTI

DRX command

Buffer
Report
LCID
Length
Figure 3.30: MAC Header Format
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Chapter 4: Comparison between WiMAX and LTE
__________________________________________________
4.1 Introduction
In recent years, communication industry have been keen to develop and formulate new
standards in order to provide high speed broadband mobile access in a single air interface and
network architecture for reasonable cost for end-users and mobile operators [35]. WiMAX
and LTE are two leading standards as the results of above efforts.
WiMAX belongs to the IEEE family of standards and refers to IEEE 802.16 standard. It
enhances the WLAN (IEEE 802.11) by extending the wireless access to Wide Area Network
(WAN) and Metropolitan Area Network (MAN). It uses OFDMA as physical layer radio
access technology in the downlink and uplink. The initial versions of WiMAX, IEEE 802.162004 (fixed WiMAX) supports fixed and nomadic access, while IEEE 802.16-2005 (mobile
WiMAX) supports enhanced QoS and mobility up to 120 km/h. Mobile WiMAX uses IP
based services to provide downlink peak data rates up to 75 Mbps depending on the
modulation technique and antenna configuration used. WiMAX supports LOS and NLOS
propagations across 10 GHz to 66 GHz and 2 GHz to 11 GHz respectively.
LTE is the part of 3GPP and evolved from the evolution of UMTS/HSPA cellular technology
to meet current user demands of high data rates and spectral efficiencies. LTE specifications
are jointly based on E-UTRA and E-UTRAN. The version specification for LTE is released in
3GPP Release 8. LTE uses OFDMA radio access technology in downlink and SC-FDMA in
the uplink. The use of SC-FDMA in the uplink reduces PAPR as compared to OFDMA. The
downlink peak data rates range from 100 Mbps to 326.4 Mbps depending on the modulation
technique and antenna configuration used. LTE aims at providing data rates, IP backbone
services, flexible spectrum, lower power consumptions and simple network architecture with
open interfaces.
In this chapter we do a comparative study of WiMAX and LTE in context of system
architecture, air interfaces radio and protocol aspects (including multiple access techniques,
access modes and modulation. Further, we provide a comparative summary that concludes
this chapter.
4.2 System Architecture
In this section we will discuss system architecture in the context of WiMAX and LTE.
4.2.1 WiMAX Architecture
The WiMAX architecture is based on a network reference model to define end-to-end
WiMAX network.
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4.2.1.1 Network Reference Model (NRM)
The network reference model for WiMAX was developed by the WiMAX Network Working
Group (NWG). The model defines the entire WiMAX network. The NRM ensures
interoperability between various WiMAX enabled devices and operators. The network
architecture is based on IP services and it can be logically divided into three parts; Mobile
Station, Access Service Network and Connectivity Service Network. The network reference
model is described in Figure 4.1 [37].
Mobile Station (MS): Used to access the network.
Access Service Network (ASN): Comprised of ASN GWs (Gateways) and BSs to form
Radio Access Network (RAN) at the edge.
Base Station: Provides air interface to MS. In addition, BS is responsible for handoff
triggering, radio resource management, enforcement of QoS policy, Dynamic Host Control
Protocol (DHCP) proxy, session management, key management and multicast group
management.
Access Service Network Gateway: Acts as layer 2 traffic aggregation point within an ASN
[36]. In addition, ASN-GW performs AAA client functionality, establish and manage mobility
tunnel with BSs, foreign agent functionality for mobile IP and outing towards selected
Connectivity Service Network (CSN).
Figure 4.1 Network Reference Model for WiMAX [37]
Connectivity Service Network: Provides IP connectivity to internet, PSTN (Public Switched
Telephone Network), ASP and corporate networks. In addition, it provides core IP functions.
CSN is owned by the Network Service Provider (NSP), and is comprised of AAA servers,
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Mobile IP Home Agent (MIP-HA), Operation Supports Systems (OSS) and gateways. AAA
servers are used to authenticate devices, users and specific services. CSN has following
responsibilities:
 IP address Management.
 Mobility, roaming and location management between ASN’s.
 Roaming between NSPs by Inter-CSN tunneling.
The logical link that connects two functional groups is called Reference Point (RP). The NRM
shown in Figure 4.1 has 8 RPs ranges from R1 to R8. The description of RPs is given in Table
4.1.
Reference
Points
Description
R1
Connect Mobile Station (MS) and ASN
R2
Connect MSN and CSN
R3
Connect ASN and CSN
R4
Connect two ASNs
R5
Connect two CSN
R6
Connect BS and ASN- GW
R7
Represents the internal communication within
the gateway.
R8
Connect two Base Stations (BSs)
Table 4.1: Description of Reference Points
4.2.2 LTE Architecture
LTE supports packet data services unlike previous cellular systems that support circuit
switched data model. In addition, LTE provides seamless IP connectivity between Packet
Data Network (PDN) and UE. LTE architecture is comprised of Core Network (CN) and
Access Network (AN), where CN corresponds to the Evolved Packet Core (EPC) which
comes from System Architecture Evolution (SAE). The AN refers to E-UTRAN. The CN and
AN together correspond to Evolved Packet System (EPS). EPS connects the users to PDN by
IP address in order to access the internet and services like Voice over IP (VoIP). Typically,
the EPS bearer is associated with QoS. Multiple bearers can be established for a user to
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provide connectivity to different PDNs or QoS streams. The overall network architecture
including various EPS elements is shown in Figure 4.2 [38].
Figure 4.2: Evolved Packet System (EPS) Network Elements [38]
EPS elements are inter-linked with standard interfaces which allow the operators to source the
network elements from various vendors.
4.2.2.1 Core Network
Core network is known as EPC in SAE. The key responsibilities of CN include bearer
establishment and control of UE. EPC is made of various logical nodes.
 Mobility Management Entity (MME).
 Packet Data Network Gateway (P-GW).
 Serving Gateway (S-GW).
 Policy Control and Charging Rules Function (PCRF).
 Home Subscriber Server (HSS).
Mobility Management Entity
It is the control node used to process signaling information between CN and UE. The
protocols running between CN and UE are called Non Access Stratum (NAS) protocols. The
key functions of MME are:
 Bearer Management Functions: Handled by the session management layer in the
NAS protocol and used to establish, maintain and release bearers.
 Connection Management Functions: Handled by the mobility management or the
connection management layer in the NAS protocol. They are used to manage security
and connection establishment between UE and network.
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Packet Data Network Gateway
Used to allocate the IP address for UE as well as flow based charging and QoS enforcement.
It filters the downlink user IP packets into the bearers typically based on QoS. This is based
on traffic flow templates. In addition, P-GW acts as mobility anchor to work with Non-3GPP
technologies i.e. WiMAX and CDMA2000.
Serving Gateway
S-GW is responsible for transferring user IP packets. It stores local mobility information for
data bearers when UE runs between various eNodeBs. S-GW acts as mobility anchor to work
with 3GPP technologies (UMTS, GPRS etc). In addition, it collects information about legal
interception and charging, i.e. the volume of data sent to or received from the user is called
charging.
Policy Control and Charging Rules Functions
PCRF controls flow based charging functions which are part of Policy Control Enforcement
Function (PCEF) as well as it organize decision making control policy. PCEF is part of PGW. The key responsibility of PCRF is to provide QoS authorization i.e. bit rate and QoS
class identifier. QoS authorization decides the method of treating certain data flows in the
PCEF and ensures that the data flow is in accordance with the subscription of the user profile.
Home Subscriber Server (HSS):
HSS is also called Home Location Register (HLR). It contains the SAE subscription data of
users such as roaming restrictions and EPS subscribed QoS profiles. HSS contains the
information of PDN in the form of AP or PDN address. In addition, HSS contains dynamic
information i.e. identity of the MME to which an user is connected currently. The vectors for
security keys and authentication are generated as the result of AuC (Authentication centre)
integration.
4.2.2.2 Access Network
The Access Network (EUTRAN) is comprised of network of eNodeBs connected to each
other through interfaces called X2. The Architecture of E-UTRAN is flat due to the absence
of a centralized controller in the case of normal traffic (as opposed to broadcast). The eNodeB
is connected to EPC via S1 interface and to MME through S1-MME interface. The eNodeB
and S-GW are interlinked by means of S1-U interface. The S1-U interface carries user data
between serving GW and eNodeB. The protocols which run between eNodeB and UE are
known as Access Stratum (AS) Protocols. The Architecture of Access Network is shown in
Figure 4.3 [39].
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Figure 4.3: Architecture of LTE Access Network (E-UTRAN) [39]
The key responsibilities of E-UTRAN are as follows:
 Radio Resource Management (RRM): RRM includes radio bearers related functions
such as radio admission control, radio bearer control, scheduling, radio mobility
control and dynamic allocation of resources in downlink and uplink to UEs.
 Header Compression: Due to IP header compression, the radio interface can be
utilized efficiently in case of small IP packets.
 Security: The data sent to the radio interface is secured by encryption.
 Connectivity to the EPC: Connectivity to the EPC consists of bearer path towards SGW and signaling towards the MME.
The functions described above reside in the eNodeB for network prospective. In contrast to
previous generation technologies, LTE embed radio controller functionalities into eNodeB
which allows tight interaction between the protocol layers of AN. This distributed control
eliminates the need for a processing intensive radio controller which in turn reduces the cost
and avoids a “single point of failure”. In addition, due to absence of the radio controller
improve the efficiency of the network by reducing the latency. There is no soft handover in
LTE, which eliminates the need for a centralized data combining function.
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4.3 Radio Aspects of Air Interface
4.3.1 Frequency Bands
Frequency bands play an important role for providing broadband wireless services. WiMAX
uses license-exempt and licensed frequency bands for providing broadband wireless access.
Every frequency band has unique characteristics which have significant impact on overall
system performance.
Licensed Frequency bands: The licensed frequency bands used by WiMAX are: 2.3 GHz,
2.5 GHz, 3.3 GHz and 3.5 GHz.
License-Exempt Frequency Bands: WiMAX uses unlicensed frequency band of 5 GHz. The
fixed profile of WiMAX created in 2004 used 5.8 GHz unlicensed frequency band. In
addition, various frequency bands between 5 GHz and 6 GHz are under consideration for
unlicensed WiMAX.
Table 4.2 summarizes the frequency bands used for WiMAX globally.
Frequency Bands for WiMAX (in GHz)
Regions
License Bands
License-Exempt
Bands
USA
2.3 and 2.5
5.8
Europe
3.5 and 2.5
5.8
South East Asia
2.3, 2.5, 3.3 and 3.5
5.8
Middle East
3.5
5.8
Africa
3.5
5.8
South and Central America
2.5 and 3.5
5.8
Table 4.2: Reported Frequency Bands used for WiMAX
LTE can be deployed in paired and unpaired spectrum. For paired spectrum, uplink and
downlink use separate frequency bands for transmissions. In addition, we can deploy FDD
systems in paired spectrum. TDD systems are deployed in unpaired spectrum. In unpaired
spectrum, uplink and downlink use same frequency band for transmissions. The summary of
LTE FDD and LTE TDD frequency bands in various regions are shown in Table 4.2(a) and
Table 4.2(b) respectively.
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Operating Band
Regions
Downlink
Uplink
I
Europe, Asia
1920 - 1980 MHz
2110 -2170 MHz
II
America
1850 -1910 MHz
1930 -1990 MHz
III
Europe, Asia
1710-1785 MHz
1805-1880 MHz
IV
America
1710-1755 MHz
2110-2155 MHz
V
America
824 - 849 MHz
869-894 MHz
VI
Japan
830-840 MHz
875-885 MHz
VII
Europe, Asia
2500-2570 MHz
2620-2690 MHz
VIII
Europe, Asia
880 - 915 MHz
925 - 960 MHz
IX
Japan
1749.9-1784.9 MHz
1844.9-1879.9 MHz
X
America
1710-1770 MHz
2110-2170 MHz
XI
Japan
1427.9 - 1452.9 MHz
1475.9 - 1500.9 MHz
XII
America
698 – 716 MHz
728 – 746 MHz
XIII
America
777 - 787 MHz
746 - 756 MHz
XIV
America
788 – 798 MHz
758 – 768 MHz
Table 4.3(a): LTE FDD Frequency Bands [40]
Band
A
Regions
Downlink and Uplink (in MHz)
1900-1920 (UL and DL transmission)
Asia (not Japan), Europe
2010-2025 (UL and DL transmission)
B
1850-1910 (UL and DL transmission)
-
1930-1990 (UL and DL transmission)
C
-
1910-1930 (UL and DL transmission)
D
Europe
2570-2620 (UL and DL transmission)
E
Europe, Asia
2300-2400 ( UL and DL transmission)
Table 4.3(b): LTE TDD Frequency Bands [40]
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4.3.2 Radio Access Modes
LTE and WiMAX use FDD and TDD as radio access modes. In FDD, BS and mobile user
transmit and receive simultaneously due to allocation of separate frequency bands. While in
TDD, downlink and uplink transmit in different times due to sharing of same frequency. The
radio mode currently specified by WiMAX is TDD whereas LTE is specified for FDD. The
spectral holdings of operator’s will be a key decision factor for selecting the technology
(based on FDD or TDD).
4.3.3 Data Rates
The peak data rates of LTE and WiMAX depend upon multiple antenna configuration and
modulation scheme used. The peak data rates of LTE and WiMAX in DL and UL are
illustrated in Table 4.4.
Downlink (DL)
Uplink (UL)
WiMAX
75 Mbps
25 Mbps
LTE
100 Mbps
50 Mbps
Table 4.4 Peak Data Rates of LTE and WiMAX
4.3.4 Multiple Access Technology
The multiple access technologies used by WiMAX and LTE are quite similar having
modification in the uplink. The multiple access technology adopted in the downlink of LTE
and uplink/downlink of WiMAX is OFDMA, whereas uplink of LTE is based on SC-FDMA.
The benefit of SC-FDMA in the uplink is the reduction of the PAPR.
4.3.4.1 OFDMA
It is an extension OFDM and is used in downlink of LTE and uplink/downlink of WiMAX. In
OFDMA, subcarriers are allocated dynamically to users in different time slots. OFDMA has
various advantages as compared to OFDM where single user can transmit/receive in the entire
time frame. Due to this, OFDM suffers from PAPR. OFDMA reduces PAPR by distributing
the entire bandwidth to multiple mobile stations with low transmit power. In addition,
OFDMA accommodates multiple users with widely varying applications, QoS requirements
and data rates.
4.3.4.2 SC-FDMA
SC-FDMA is an extension of OFDMA and is used in the uplink of LTE. SC-FDMA
significantly reduces PAPR as compared to OFDMA by adding additional blocks of DFT and
IDFT at transmitter and receiver. However, due to existing similarities with OFDMA,
parameterization of LTE in the uplink and downlink can be harmonized. The 3D visualization
of OFDMA is shown in Figure 4.4.
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Figure 4.4: 3D Visualization of OFDMA
4.3.5 Modulation Parameters
WiMAX and LTE support flexible bandwidth ranges from 1.25 MHz to 20 MHz. Due to the
flexible bandwidth of the two technologies, they use various modulation parameters (FFT
size, subcarrier spacing etc). A detail comparison of modulation parameters used by LTE and
WiMAX is described in Table 4.5.
Parameter
Fixed WiMAX
Transmission B.W
(in MHz)
3.5
1.25
5
10
20
1.25
2.5
5
10
15
20
FFT Size
256
128
512
1024
2048
128
256
512
1024
1536
2048
Subcarrier Spacing
(in kHz)
15.625
10.94
15
Subframe Duration
(in ms)
5
2 to 20 but focus on 5
1
Cyclic Prefix
Number of
OFDM/SC-FDMA
Symbols
OFDM/SC-FDMA
Symbol duration (in
us)
Mobile WiMAX
LTE
1/32, 1/16, 1/8 (typically for mobile WiMAX)
and 1/4
69 OFDM
Symbols
72
48 OFDM Symbols
102.9
5us for Normal CP and 16.67us for
extended CP
DL: Normal CP = 14, Extended CP = 12
UL: Normal CP = 14, Extended CP = 12
DL uses OFDM symbol whereas UL
uses SC-FDMA symbol.
DL: Normal CP = 71.8, Extended CP =
83.4, UL: Normal CP = 71.8, Extended
CP = 83.4. DL uses OFDM symbol
whereas UL uses SC-FDMA symbol.
Table 4.5: Modulation parameters for LTE and WiMAX
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4.3.6 Multiple Antenna Techniques
WiMAX and LTE use multiple antenna configurations in uplink and downlink in order to
increase capacity, diversity, data rates and efficiency as compared to single antenna systems.
As described in chapter 2, WiMAX uses three types of multiple antenna technologies such as
SAS, diversity techniques and MIMO. The MIMO systems are further subdivided into open
loop and closed loop systems. WiMAX support 1, 2, 4 antennas at the BS and 1, 2 antennas at
the MS. LTE uses multiple antenna techniques and wider spectrum to provide data rates in the
entire cell coverage area. The advanced antenna techniques used by LTE are beamforming,
Spatial Division Multiple Access (SDMA) and MIMO. The antenna configuration supported
by LTE DL is (2x2) and (4x4) having 2 or 4 antennas at eNodeB and 2 or 4 antennas at UE.
The UL of LTE supports 2x2 MIMO having 2 antennas at UE as well as at eNodeB. In
addition, the number of code words used by LTE is 2 which are independent of the antenna
configuration. Table 4.6 describes the summary of MIMO antenna configurations used by
WiMAX and LTE.
MIMO
WiMAX
LTE
Uplink
1Tx X NRx
2Tx X 2Rx
Downlink
2Tx X 2Rx
2Tx X 2Rx or 4Tx X 4Rx
Number of code words
1
2
Table 4.6: MIMO Aspects for WiMAX and LTE
4.4 Protocol Aspects of Air Interface
Protocol aspects of air interface include protocol architecture, frame structure, modulation and
physical layer control mechanisms. The detail description of these aspects with reference to
WiMAX and LTE are discussed below.
4.4.1 Protocol Architecture
Protocol architecture of WiMAX consists of physical and data link layer of the OSI model.
Data link layer is further divided into LLC layer and MAC layer where MAC layer itself
consists of three sublayers. The protocol architecture described in Figure 4.5 shows step by
step collection of data from upper layers to physical layer. The MAC layer is responsible for
assembling upper layer data into frames along with error detection and also attaches/detaches
addresses to the fields upon transmission/reception. In addition, MAC layer governs the
wireless transmission medium. The CS which is part of MAC layer takes IP or ATM packets
from upper layers through CS SAP since WiMAX supports two types of transmission modes.
In addition, CS does key processing on upper layer frames including frame compression,
addressing frames according to IEEE 802.16, transforming the QoS parameters to IEEE
802.16 and sends the MSDUs to CPS. CPS is the core part of MAC layer and it performs
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functions related to channel access, QoS requirements, connection establishment and
maintenance. Furthermore, it takes MSDUs from MAC SAP and organizes them in MAC
PDUs by doing segmentation and fragmentation. The security sublayer performs encryption,
authentication and secure key exchange functions on MPDUs and sends them to the PHY
layer for further processing. The physical layer takes MPDUs from PHY SAP and convert
them into signals in order transmit across the air interface. The protocol architecture of
WiMAX is shown in Figure 4.5 [41].
External Network (IP, ATM)
CS SAP
Service Specific Convergence
Sublayer (CS)
MAC SAP
MAC Layer
MAC Common Part Sublayer
(MAC CPS)
Security Sublayer
PHY SAP
PHY Layer
Physical Layer
Figure 4.5 Protocol Architecture of WiMAX (IEEE 802.16) [41]
LTE protocol architecture is similar to the WiMAX but it uses the first three layers of OSI
model. Data is coming in form of IP packets from the upper layers to the Packet Data
Convergence Protocol (PDCP) which performs IP header compression along with ciphering.
The data at this stage is called PDCP SDUs. PDCP attaches header information with PDCP
SDUs, to form PDCP PDUs. The header contains information about deciphering. The PDCP
PDUs are sent to the RLC for further processing. The RLC first assembles the PDCP PDUs in
RLC SDUs and then performs segmentation (or concatenation) along with header attachment.
The RLC header has information about identification of RLC PDUs in case of retransmissions
and in-sequence delivery of data in the UE. The RLC PDUs are sent to MAC layer, which
assembles them into MAC SDUs. The MAC SDUs are converted to MAC PDUs by adding
the MAC header at every MSDU. The MPDUs are sent to the physical layer for further
processing. The PHY performs encoding/decoding of data and organizes the MPDUs in
transport blocks. In addition, physical layer attaches CRC with every transport block. The
protocol architecture of LTE is shown in Figure 4.6. The logical and transport channels are
used to offer services to RLC and MAC layers respectively.
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Figure 4.6: LTE Protocol Architecture [42]
4.4.2 Modulation
LTE and WiMAX use modulation schemes such as QPSK, 16-QAM or 64-QAM in uplink
and downlink. However, the use of 64-QAM in the WiMAX uplink is optional.
4.4.3 Frame Structure
LTE uses two types of frames, the Generic Frame Structure (GFS) and Alternative Frame
Structure (AFS). The GFS used by FDD is 10ms in duration and has 10 subframes. Every
subframe is 1ms in length and divided into two equal slots of 0.5ms. Every slot comprises 7
OFDM/SC-FDMA symbols in case of normal CP and 6 OFDM/SC-FDMA symbols in case of
extended CP. In contrast to GFS, an alternative frame structure is used by TDD. In this frame
structure, there is a certain restriction for the allocation of subframes. Subframe #1 and
subframe #6 are used for downlink synchronization. The GFS and alternative frame structure
are described in Figure 4.7(a) and 4.7(b).
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1 Frame= 10msec
1 subframe = 1 ms
1 slot = 0.5ms
#0
S#0
#1
S#1
S#2
#2
S#3
S#4
…………
#3
S#5
#18
#19
S#6
7 OFDM/SC-FDMA symbols in case of Normal CP
Figure 4.7(a): Generic Frame Structure for LTE (FDD)
1 Radio Frame = 10 ms
1 Radio Frame = 10 ms
1 subframe = 1 ms
1 slot = 0.5 ms
SF#0
DwPTS
SF#2
SF#3
SF#4
SF#5
SF#7
SF#8
SF#9
UpPTS
DwPTS
UpPTS
GP
GP
Figure 4.7(b): Alternative Frame Structure for LTE (TDD) [43]
WiMAX supports both TDD and FDD frame structures but adopted TDD frame structure due
to its flexible nature. The TDD frame is comprised of downlink subframe along with uplink
subframe. The DL subframe is further divided in to DL-PHY PDU, comprised of preamble
information, FCH and DL bursts. The preamble contains information about downlink
synchronization. Frame configurations such as modulation schemes, length of MAP messages
and usable subcarriers are provided by the Frame Control Header (FCH). The DL-Burst
consists of MAC PDU, DL-Map and UL-Map. The Map messages are used to provide user
allocations to the BS. The uplink subframe contains information about contention region,
contention bandwidth requests and UL PHY PDUs. The contention region provides
contention based access which includes periodical closed loop frequency and power
adjustments. The contention bandwidth request contains uplink bandwidth requests. The
WiMAX frame structure is flexible in nature as it supports variable length frames ranging
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from 2ms to 20ms. However, most of the WiMAX products use 5ms frames. The WiMAX
TDD frame is shown in Figure 4.8.
Frame
Downlink Subframe
Uplink Subframe
Downlink PDU
Contention (Initial
Ranging)
Preamble
DL Burst 1
FCH
Contention
Bandwidth Request
……
UL PHY PDU 1
DL Burst n
Preamble
MAC PDU
MAC PDU
UL PHY
PDU # n
.....
UL Burst
…..
Pad
.
DLFP
DL-Map, UL.Map,
DCD, UCD
MAC PDU
MAC Header
MAC Payload
CRC
Figure 4.8: WiMAX TDD Frame Structure [44]
4.5 Quality of Service
WiMAX supports connection oriented QoS mechanisms which enable end-to-end QoS
control. The QoS parameters are defined per Service Flow (SF). The SF provides multiple
flows to or from the mobile station [45]. The QoS parameters are negotiated dynamically or
statically through MAC messages and provide scheduling and transmission ordering on the air
interface. In addition WiMAX QoS mechanism supports various applications such as:
 rtPS (Real Time Polling Service): Streaming applications (Audio/Video) .
 UGS (Unsolicited Grant Service): VoIP.
 ErtPS (Extended Real Time Polling Service): Voice with Activity.
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 nrtPS (Non Real Time Polling Service): FTP (File Transfer Protocol).
 BE (Best Effort Service): Web browsing, Data Transfer.
LTE provides QoS as well as Quality of Experience (QoE). The QoS parameters defined for
LTE are as follow:
 QCI: It is a scalar quantity which is preset by the operator. It determines the
characteristics of packet forwarding.
 ARP (Allocation-Retention Priority): Contains allocation and retention priority for
SDF (Service Data Flow).
 MBR: Stands for Maximum Bit Rate. It enforces the maximum bit rate to SDF.
 GBR (Guaranteed Bit Rate): Used for determining the allocation of resources.
 AMBR (Aggregate Maximum Bit Rate): Used by non GBR flows.
4.6 Mobility
Mobile WiMAX supports idle mode and sleep mode connectivity. In idle mode, UE is not
registered with the BS whereas in sleep mode UE may scan neighboring base stations or may
power down. Mobile WiMAX supports three types of handovers; Hard Handover (HHO),
Macro Diversity Handover (MDHO) and Fast Base Station Switching (FBSS). HHOs are
mandatory in mobile WiMAX whereas FBSS and MDHO are used optionally. Mobility
speeds supported by mobile WiMAX are up to 120 km/h.
LTE supports RRC_IDLE and RRC-CONNECTED modes to provide mobility. In contrast to
WiMAX, LTE supports Inter Cell Soft Handovers and Inter RAT handovers with mobility
speeds up to 350 km/h.
4.7 Comparative Summary
The brief comparative summary of WiMAX and LTE is illustrated in Table 4.7.
WiMAX
LTE
Network Architecture
IP based, Flat
IP based, Flat
DL: OFDMA (For Mobile
WiMAX), UL: OFDMA (For
Mobile WiMAX)
DL: OFDMA,
Access Technology
UL:SC-FDMA
Channel Bandwidth (in MHz)
1.25, 3.5, 5, 10, 20
1.25, 2.5, 5, 10, 15, 20
FFT Size
128, 256, 512, 1024, 2048
128, 256, 512, 1024, 2048
Duplexing Mode
TDD and FDD; Focus: TDD
TDD and FDD; Focus: FDD
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WiMAX
LTE
Subcarrier Spacing (in kHz)
Support variable subcarrier
spacing ranges from 7 to 20
kHz. Typically 10 kHz (For
Mobile WiMAX)
15 kHz (Fixed)
Cyclic Prefix Length
Variable: 1/32, 1/16, 1/8 and
1/4.
Normal CP: 5.21 us
Extended CP: 16.67 us
Frequency Bands or
Spectrum (in GHz)
Licensed:2.3, 2.5, 3.5
Licensed Exempt: 5.8
Licensed, IMT 2000 bands
(~2GHz)
Modulation
QPSK, 16-QAM and 64QAM
QPSK, 16-QAM and 64QAM
Coding
Turbo Encoder,
Convolutional Encoder and
LDPC
Turbo Encoder,
Convolutional Encoder
Framing, TTI
Variable: 2 to 20 ms, Focus:
5 ms
Fixed: 1msec (2 slots of 0.5
ms)
Number of Symbols in
Subchannel/Physical
Resource Block
Number of symbols in a
Subchannel: 24 x 2 in PUSC
mode
Number of symbols in
Physical Resource Block:
12x7 (Normal CP)
Peak Data Rate
DL: 75 Mbps, UL: 25 Mbps
DL: 100 Mbps, UL: 50 Mbps
Cell Radius
2-7 km
5 km
Cell Capacity
100-200 users
Spectral Efficiency (in
bits/sec/Hz)
3.75
5
MIMO
DL: 2x2, 2x4, 4x2, 4x4
UL: 1x2, 1x4,
Code Words: 1
DL: 2x2, 2x4, 4x2, 4x4
UL: 1x2, 1x4, 2x2, 2x4
Code Words:2
Mobility
120 km/h
350 km/h
Handovers
Mandatory: Optimized Hard
Handover, Optional: FBSS
and MDHO.
Inter frequency Soft
Handovers are supported.
Roaming Framework
Work in process
Through existing
GSM/UMTS network
Legacy Network
IEEE 802.16a to IEEE
802.16d (For Mobile WiMAX)
GSM, GPRS, EGPRS,
UMTS, HSPA.
> 200 users (at 5 MHz),
70
>400 users (for larger
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Time Line
WiMAX
LTE
For Mobile WiMAX
Standard Completed: 2005
Initial Deployment: 2007-08
Mass Market: 2009
Standard Completed: 2007
Initial Deployment: 2010
Mass Market: 2012
Table 4.7: Comparative Summary of WiMAX and LTE
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Chapter 5: Simulation
__________________________________________________
5.1 Introduction
This chapter presents our simulation results along with underlying assumptions. In the first
part, we investigated LTE uplink and performed link level simulations of Single Carrier
Frequency Domain Equalization (SC-FDE) and SC-FDMA in comparison with OFDM. We
have used two types of multipath channels, i.e. ITU Pedestrian A and ITU Vehicular A
channels. In addition an Additive White Gaussian Noise (AWGN) channel is also used.
Furthermore, the simulation of PAPR is performed for SC-FDMA and OFDMA systems. In
the second part of this chapter, we analyzed the capacity of the MIMO system and performed
a comparison with SISO.
All simulations are performed in MATLAB 7.40 (R2007a).
5.2 Link Level Simulation of SC-FDE
SC-FDE is a frequency domain equalization technique used to minimize the frequency
selective fading effects in LTE uplink. SC-FDE has similar spectral efficiency and link level
performance as OFDM. However, it has certain advantages upon OFDM due to usage of DFT
and IDFT in the receiver. The block diagrams used in the link level simulation of SC-FDE
and OFDM are shown in Figure 5.1 and 5.2 respectively. We can see the similarity between
two block diagrams as they contain the same signal processing blocks.
The parameters used in simulation are described in Table 5.1. The parameters are chosen only
for 5 MHz transmission bandwidth of LTE system. The number of iterations used in the
Monte Carlo simulation are 10^4. A Monte Carlo simulation is a method which repeatedly
counts the number of transmitted symbols and symbol errors on every iteration.
Parameters
Assumptions
System Bandwidth
5 MHz
Sampling Rate
5 Mega-samples per second
Pulse Shaping
None
Modulation Format
QPSK
Cyclic Prefix
4 µs or 20 samples
Subcarrier Spacing
5 MHz / 512 = 9.765 kHz
IFFT Size
512 Points
Input Block Size
16 Symbols
Input FFT size
16
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Parameters
Assumptions
Channel coding
None
Number of iteration
10^4
Equalization
Minimum Mean Square Error (MMSE), Zero
Forcing (ZF)
Channel
ITU Pedestrian A, ITU Vehicular A and
AWGN.
Detection
Hard
Confidence Interval
32
Table 5.1: Simulation Parameters and Assumptions
Data block
genaration
Add CP
Channel filtering
Add and generate
AWGN
Remove CP
FFT(using 512
points)
Equalization
IFFT(using 512
points)
Detection
SC-FDE Transmitter
Channel
SC-FDE Receiver
Figure 5.1: Block Diagram of SC-FDE Link Level Simulator
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Data block
genaration
Channel filtering
IFFT(using 512
points)
Add and generate
AWGN
Add CP
Remove CP
FFT(using 512
points)
Equalization
detection
OFDM Transmitter
Channel
OFDM Receiver
Figure 5.2: Block Diagram of OFDM Link Level Simulator
The simulation results compute Symbol Error Rate (SER) for the performance measurement
of SC-FDE and OFDM in various scenarios.
5.2.1 SER for SC-FDE and OFDM using MMSE as Equalization Scheme
We have calculated SER measurement of SC-FDE and OFDM by using three types of
channels, ITU Pedestrian A, ITU Vehicular A and AWGN channel. The equalization scheme
used to obtain the SER curves is MMSE.
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0
10
-1
10
Symbol Error Rate [SER]
-2
10
-3
10
-4
10
-5
10
-6
10
SC-FDEpadAchannel
OFDM-padAchannel
SC-FDE:vehAchaanel
OFDM:vehAchannel
SC-FDE: AWGN
OFDM:AWGN
-7
10
0
5
10
15
SNR, [dB]
20
25
30
Figure 5.3: Comparison of SC-FDE and OFDM using MMSE Equalization in Pedestrian
A, Vehicular A and AWGN Channels
OFDM
SC-FDE
Simulation results show that in case of AWGN channel SC-FDE and OFDM has similar SER
performance. However, in case of Pedestrian A and Vehicular A channel, SC-FDE
outperforms the OFDM. As we know OFDM needs additional channel coding to achieve this
performance due to its sensitive nature to carrier frequency. The comparative summary
obtained from Figure 5.3 is described in Table 5.2 and Table 5.3.
Channels
SNR (in dB)
SER
AWGN
10
0.001566
Pedestrian A
10
0.004029
Vehicular A
10
0.0577
AWGN
10
0.001566
Pedestrian A
10
0.008625
Vehicular A
10
0.09313
Table 5.2: Comparison between SC-FDE and OFDM in Various Channels Using MMSE
Equalization
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The Table 5.2 clearly shows that SC-FDE significantly reduces SER as compared to OFDM
in Vehicular A and Pedestrian A Channel.
SC-FDE
Vehicular A
OFDM
Channel
Vehicular A
SNR (in dB)
SER
16
0.001578
20
8.594e-006
16
0.02622
20
0.013
Table 5.3: Comparison between SCFDE and OFDM in Vehicular A Channel using
MMSE Equalization
Table 5.3 illustrates an important result i.e. as SNR increases the SC-FDE sharply reduces the
Symbol error rate as compared to OFDM in case of vehicular channel.
5.2.2 SER for SC-FDE and OFDM using Zero Forcing
The calculation of SER is performed using Zero Forcing as equalization scheme for the
comparison of SC-FDE and OFDM in AWGN, ITU Pedestrian A and ITU Vehicular A
channel.
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0
10
-1
10
Symbol Error Rate [SER]
-2
10
-3
10
-4
10
SC-FDE:pedAchannel
OFDM:pedAchannel
SC-FDE:vehAchannel
OFDM:vehAchannel
SC-FDE:AWGN
OFDM:AWGN
-5
10
-6
10
-7
10
0
5
10
15
SNR, [dB]
20
25
30
Figure 5.4: Comparison of SC-FDE and OFDM using Zero Forcing Equalization
OFDM
SC-FDE
Simulation results show that SC-FDE outperforms the OFDM in case of multipath channels
i.e. ITU Pedestrian A and ITU Vehicular A channel. We see that in case of Vehicular A
channel, OFDM has a continuous reduction of SER and it significantly minimizes the SER up
to certain values of SNR as compared to SC-FDE. However, SC-FDE outperforms OFDM for
higher values of SNR. The comparative summary of the results obtained from the simulation
are shown in Figure 5.2 and described in Table 5.4 and 5.5.
Channels
SNR (in dB)
SER
AWGN
10
0.001578
Pedestrian A
10
0.004428
Vehicular A
10
0.2797
AWGN
10
0.001578
Pedestrian A
10
0.008546
Vehicular A
10
0.0932
Table 5.4: Comparison of SCFDE and OFDM in various Channels using Zero Forcing
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Table 5.4 shows that SCFDE has better performance in case of AWGN and Pedestrian
channel while OFDM is better in case of vehicular channel.
OFDM
SC-FDE
Channel
Vehicular A
Vehicular A
SNR (in dB)
SER
14
0.1004
18
0.009742
22
4.492e-005
14
0.04008
18
0.01804
22
0.009223
Table5.5: Performance of SCFDE and OFDM using Zero Forcing in Vehicular A
Channel
Table 5.5 shows that OFDM gives better performance for smaller values of SNR but for
higher values, the SC-FDE significantly reduces SER as compared to OFDM system which
continuously reduces the error as the value of SNR is increased.
We observe from Figure 5.3 and 5.4 that MMSE gives better performance as compared to
zero forcing.
5.2.3 Comparison of SC-FDE and OFDM with/without CP
The comparison of SCFDE and OFDM is performed in Vehicular channel with and without
CP. The equalization scheme used in this simulation is MMSE.
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Comparison
of SC/FDE and OFDM with or without CP using Vehicular Channel
0
10
-1
Symbol Error Rate [SER]
10
-2
10
-3
10
-4
10
SC-FDE:Using CP
OFDM:Using CP
SC-FDE: No CP
OFDM: No CP
-5
10
-6
10
0
5
10
15
SNR, [dB]
20
25
30
Figure 5.5: Comparison of SC/FDE and OFDM with or without CP using Vehicular
Channel
Figure 5.5 shows that the use of CP reduces the SER as compared to the system having no CP.
In addition, it is clearly shown that SC-FDE system gives low SER as compared to OFDM.
Table 5.6 summarizes the comparison obtained from simulation.
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OFDM
SC-FDE
Channel
With CP
Without CP
SNR (in
dB)
SER
SNR (in
dB)
SER
16
0.00155
16
0.003771
18
0.0001684
18
0.002033
20
7.813e-006
20
0.001675
16
0.02626
16
0.02895
18
0.01823
18
0.02083
20
0.01292
20
0.01611
Vehicular A
Vehicular A
Equalization
MMSE
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Table 5.6: Comparison of SC-FDE and OFDM With and Without CP
5.3 Link Level Simulation of SCFDMA
The simulation flow for SCFDMA is shown in Figure 5.6. We have investigated two types of
subcarrier mapping schemes for SCFDMA and compared their performance in terms of SER
and SNR. The types of subcarrier mapping schemes are Interleaved FDMA (IFDMA) and
Localized FDMA (LFDMA). Parameters used in simulation are given in Table 5.7.
Parameters
Assumptions
System Bandwidth
5 MHz
FFT Size
512
Block Size
16 symbols
CP Length
20 samples
Range of SNR
0 to 30 dB
Modulation
QPSK
Number of iteration
10^4
Channel
AWGN, Pedestrian A and Vehicular A.
Equalization
MMSE
Confidence Interval
32
Table 5.7: Simulation Parameters of SC-FDMA
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Data
Genaration(QPSK)
Channel filtering
FFT(using 16
points)
Add and generate
AWGN
Remove CP
FFT(using 512
points)
Mapping of
subcarrier
demapping of
subcarrier
IFFT(using
512points)
Equalization
Add CP
IFFT (using 16
points)
Detection
Transmitter
Channel
Receiver
Figure 5.6: System Model of SC-FDMA
0
10
-1
Symbol Error Rate [SER]
10
-2
10
-3
10
IFDMA:pedAchannel
LFDMA:pedAchannel
IFDMA:vehAchannel
LFDMA:vehAchannel
IFDMA:AWGN
LFDMA:AWGN
-4
10
-5
10
0
2
4
6
8
10
SNR, [dB]
12
14
16
18
Figure 5.7: Comparison of SER with Various Subcarrier Mapping Schemes
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Figure 5.7 presents the performance of SC-FDMA system using subcarrier mapping schemes
IFDMA and LFDMA for various channels. It is clear from the simulation that LFDMA
outperforms the IFDMA in all channel conditions and gives better performance.
0
10
Pedestrian
A Channel
-1
Symbol Error Rate [SER]
10
-2
10
-3
10
-4
10
IFDMA:subband0
LFDMA:subband0
IFDMA:subband15
LFDMA:subband15
IFDMA:AWGN
LFDMA:AWGN
-5
10
-6
10
0
2
4
6
8
10
SNR, [dB]
12
14
16
18
Figure 5.8: SER Performance of SC-FDMA System Using Various Subcarrier Mapping
Schemes
Figure 5.8 presents the SER performance of SC-FDMA system in AWGN and Pedestrian A
channel using two subcarrier mapping schemes. In case of AWGN channel, we see that
IFDMA and LFDMA have similar performance whereas for Pedestrian A channel the two
subcarrier schemes have different SER performance. In addition, it is clearly shown that the
performance of IFDMA system does not depend on location of subband and gives
approximately similar SER curves for subband 0 and subband 15. This is due to the inherent
characteristic of frequency diversity of the IFDMA scheme. As for LFDMA, the performance
of SC-FDMA is better in case of subband 0 and worst in case of subband 15. This is because
of channel gain which is higher than average at subband 0 and below to average at subband
15.
5.4 Peak -to- Average Power Ratio
Peak to average power ratio is defined as “the ratio of peak signal power to the average signal
power”.
𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃 =
𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃 𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆 𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃
𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴 𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆 𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃
Mathematically, PAPR can be written as
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max
2
� |𝑥𝑥 (𝑡𝑡)|
0
≤
𝑡𝑡
≤
𝑁𝑁𝑇𝑇
𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃 =
1 𝑁𝑁𝑇𝑇�
∫ |𝑥𝑥(𝑡𝑡)|2 𝑑𝑑𝑑𝑑
𝑁𝑁𝑇𝑇� 0
Where
𝑥𝑥(𝑡𝑡) = 𝑒𝑒
𝑗𝑗 𝑤𝑤 𝑐𝑐 𝑡𝑡
𝑁𝑁−1
� 𝑥𝑥�𝑛𝑛 𝑝𝑝(𝑡𝑡 − 𝑛𝑛𝑇𝑇�) 𝑎𝑎𝑛𝑛
𝑛𝑛=0
𝑥𝑥�𝑛𝑛 : n =0, 1, …, N-1 are the time domain symbols that come after the IDFT.
𝑤𝑤𝑐𝑐 = Carrier Frequency
�= 𝑥𝑥�𝑛𝑛 symbol duration, and
T
p (t) = Baseband Pulse.
The simulation model for calculating PAPR of SC-FDMA system is shown in Figure 5.9.
Data Genaration
(M-size)
Up-sampling
FFT(using Mpoints)
Filtering (Pulse
Shape)
PAPR calculations
Mapping of
subcarrier
IFFT(using
512points)
Modulator
Pulse shaper (RC or RRC)
PAPR for SCFDMA
Figure 5.9: Simulation Model of PAPR Calculations for SCFDMA
For pulse shaping we used Raised Cosine (RC) and Square Root Raised Cosine (RRC) filters
because they make the receiver robust against timing synchronization errors. The parameters
used for the calculation of PAPR are illustrated in Table 5.8. For the calculation of PAPR we
use Complementary Cumulative Distribution Function (CCDF). The CCDF is defined as the
probability for which PAPR is greater than any PAPR value i.e. PAPR0.
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CCDF: Pr (PAPR >PAPR0)
Parameters
Assumptions
System Bandwidth
10 MHz
Number of Subcarriers (N)
512
Number of Symbols (M)
128
Spreading Factor for IFDMA (Q)
Q= N/M=4
Spreading Factor for LFDMA
2
Roll of Factor
0.25
Over Sampling Factor
4
Number of iteration
10^4
Subcarrier Mapping Schemes
IFDMA,DFDMA,LFDMA
Confidence Interval
32
Table 5.8: Parameters used in the simulation of PAPR calculation for SCFDMA
5.4.1 PAPR-SCFDMA Calculation Using QPSK
The PAPR calculation using various subcarrier mapping schemes for SCFDMA system is
shown in Figure 5.10. The modulation scheme used for the calculation of PAPR is QPSK.
0
10
-1
Pr (PAPR>PAPRo)
10
-2
10
-3
10
-4
10
0
IFDMA:RC
IFDMA:RRC
IFDMA: No PS
LFDMA:RC
LFDMA:RRC
LFDMA:No PS
DFDMA:RC
DFDMA:RRC
DFDMA:No PS
2
4
6
8
PAPRo [dB]: Modulation QPSK
10
12
Figure 5.10: Comparison of CCDF of PAPR for DFDMA, IFDMA and LFDMA using
QPSK
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Figure 5.10 show that IFDMA gives lowest PAPR values as compared to other subcarrier
mapping schemes (DFDMA and LFDMA).
5.4.2 PAPR-SCFDMA Calculation Using 16-QAM
The PAPR calculation using various subcarrier mapping schemes for SC-FDMA system is
shown in Figure 5.11. The modulation scheme used for the calculation of PAPR is 16-QAM.
0
10
-1
Pr (PAPR>PAPRo)
10
-2
10
IFDMA:RC
IFDMA:RRC
IFDMA: No PS
LFDMA:RC
LFDMA:RRC
LFDMA:No PS
DFDMA:RC
DFDMA:RRC
DFDMA:No PS
-3
10
-4
10
1
2
3
5
6
7
4
PAPRo [dB]: Modulation: 16QAM
8
9
10
Figure 5.11: Comparison of CCDF of PAPR for IFDMA, DFDMA and LFDMA using
16-QAM
Figure 5.11 show that IFDMA has lowest value of PAPR at 3.2dB which is 0dB in case of
QPSK as modulation technique. We can also observe from the figure that we get higher
values of PAPR by using 16-QAM which is undesirable because they cause non linear
distortions at the transmitter.
5.4.3 PAPR Calculation for OFDMA
We know theoretically that OFDMA gives higher PAPR values as compared to SCFDMA due
to its multicarrier nature. In addition, there is no pulse shaping filter used in OFDMA. The
simulation model for the calculation of PAPR for OFDMA system is shown in Figure 5.12.
Data Genaration
(M-size)
Modulation (using
512 subcarriers)
Calculate PAPR
Figure 5.12: Simulation Model of PAPR Calculations for OFDMA
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The simulation parameters used in the simulation are described in Table 5.9.
Parameters
Assumptions
System Bandwidth
5 MHz
Number of Subcarriers (N)
512
Number of Symbols (M)
128
Over Sampling Rate
4
Number of Iterations
10^4
Confidence Interval
32
Table 5.9: Parameters Used in the Simulation of PAPR-Calculation for OFDMA
Figure 5.13 shows the PAPR calculation of OFDMA system using QPSK and 16-QAM
modulation techniques. The graph shows that the PAPR value of OFDMA system is much
higher than SC-FDMA system. We can also observe that the behavior of CCDF is quite
similar in case of QPSK and 16-QAM.
0
10
QPSK
16QAM
-1
Pr (PAPR>PAPRo)
10
-2
10
-3
10
-4
10
0
2
4
6
PAPRo [dB]
8
10
12
Figure 5.13: Comparison of CCDF of PAPR for OFDMA using QPSK and 16-QAM
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5.5 Capacity of MIMO System
MIMO system consists of multiple transmit and receive antennas interconnected with multiple
transmission paths. MIMO increases the capacity of system by utilizing multiple antennas
both at transmitter and receiver without increasing the bandwidth.
𝑟𝑟
Capacity of MIMO= �
𝑖𝑖=1
Where,
𝑙𝑙𝑙𝑙𝑙𝑙2 (1 +
𝜌𝜌
𝑀𝑀
𝜆𝜆𝜆𝜆)
r = rank of matrix
λ= Positive eigenvalues of HHH (as HH is the conjugate of H)
ρ= SNR
Capacity of SISO= log2 (1+ ρh2)
40
nt
nt
nt
nt
nt
35
Capacity bits/s/Hz
30
=
=
=
=
=
1,
2,
3,
4,
8,
nr =
nr =
nr =
nr =
nr =
1
2
2
4
8
25
20
15
10
5
0
-10
-5
0
5
SNR in dB
10
15
20
Figure 5.14: Comparison of MIMO and SISO system in terms of Capacity
Figure 5.14 shows the comparison between MIMO and SISO systems in terms of capacity.
The graph depicts that the capacity of system can be increased by increasing the number of
antennas at transmitter and receiver. The graph also show that 8x8 MIMO system has larger
capacity whereas SISO system as lowest capacity.
Table 5.10 summarizes simulation results obtained from Figure 5.13.
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For SNR= 5dB
Antenna Configuration
Capacity (bits/s/Hz)
SISO
2.589
MIMO (2x2)
4.589
MIMO (3x2)
5.325
MIMO (4x4)
7.907
MIMO (8x8)
13.7
Table 5.10: Comparison between MIMO and SISO System with SNR=5 dB
For SNR= 14dB
Antenna Configuration
Capacity (bits/s/Hz)
SISO
4.89
MIMO (2x2)
8.485
MIMO (3x2)
9.941
MIMO (4x4)
14.83
MIMO (8x8)
27.48
Table 5.11: Comparison between MIMO and SISO System with SNR=14 dB
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Chapter 6: Conclusion and Future Work
__________________________________________________
6.1 Conclusion
We conclude that both WiMAX and LTE are technically similar standards. However, there
are some differences present in the uplink access method used by both technologies. LTE uses
SC-FDMA whereas WiMAX uses OFDMA as an access method. The adaptation of SCFDMA in the uplink gives edge to LTE over WiMAX because it resolves the PAPR problem
of OFDMA due to its single carrier nature.
We also conclude that, LTE gives better data rates in the uplink and downlink due to support
of MIMO system as compared to WiMAX which only supports MIMO in the downlink
direction.
From a market prospective, WiMAX has edge on LTE due to its early deployments. WiMAX
was first deployed in 2007-08 whereas LTE is not yet deployed. Due to timeline advantages
of WiMAX over LTE, we also conclude that new and existing service providers will go for
mobile WiMAX in order to provide mobile services to subscribers. We also conclude that the
service providers of GSM and CDMA 2000 in developing countries will naturally go for
mobile WiMAX for broadband wireless services, whereas service providers of UMTS/HSPA
will go for 3GPP-LTE.
We conclude from our simulations that SC-FDE has low SER as compared to OFDM in all
channel conditions. Also, the use of LFDMA as a subcarrier mapping scheme in SC-FDMA
gives better SER performance when compared to IFDMA in all channel conditions (ITU
Pedestrian A, ITU Vehicular A, AWGN).
IFDMA gives lowest PAPR as compared to LFDMA and DFDMA subcarrier mapping
schemes. The use of QPSK further reduces the PAPR as compared to 16-QAM.
We also conclude that OFDMA gives high PAPR values as compared to SC-FDMA due to
the use of multiple subcarriers.
6.2 Future Work
In future, implementation of WiMAX and LTE on a single chip could be done to facilitate the
advantages of the two technologies in one system. Practical implementation of multiple
antenna techniques on LTE can be tested in future to verify our theoretical results.
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References
[1]
Tejas Bhandare, “LTE and WiMAX Comparison”, Santa Clara University, 2008, White
Paper
[2]
Jeffrey G.Andrews, Arunabha Ghosh, Rias Muhamed, Fundamentals of WiMAX,
Prentice Hall Communications Engineering and Emerging Technology Series, 2007.
[3]
Syed Ahson, Mohammad Ilyas, “WiMAX Applications”, pp, 3, CRC Press, 2008
[4]
IEEE 802.162004,” IEEE Standard for Local and Metropolitan Area Networks Part
16: Air Interface for Fixed Broadband Wireless Access Systems”, 1 October, 2004
[5]
Wu, Zhongshan, “MIMO OFDM Communication Systems: Channel Estimation and
Wireless Location”, PhD Thesis, Dept. of Electrical & Computer Engineering,
Louisiana State University, USA, May 2006
[6]
M. Rahman, S. Das , F. Fitzek, “OFDM based WLAN systems”, Technical Report,
Aalborg University, Denmark, February 2005
[7]
http://www.WiMAX.com/commentary/WiMAX_weekly/2-3-6-adaptive-modulationand-coding-in-WiMAX/?searchterm=Adaptive%20modulation%20and%20coding
[8]
Syed Ahson, Mohammad Ilyas, “WiMAX Applications”, CRC Press, 2008
[9]
“IEEE 802.16a Standard and WiMAX”, Igniting Broadband Wireless Access,
White Paper
[10] “Multiple Antenna Systems in WiMAX”, Airspan’s WiMAX Product Line, White
Paper
[11] “Smart Antenna Systems,” 2003 International Engineering Consortium. White Paper
http://www.iec.org/online/tutorials/smart_ant/topic01.html
[12] Jeffrey G. Andrews, Arunabha Ghosh, Rias Muhamed, “Fundamentals of WiMAX:
Understanding Broadband Wireless Networking”, Prentice Hall, 2007
[13] Pierre Lescuyer, Thierry Lucidarme, “Evolved Packet System: The LTE and SAE
Evolution of 3G UMTS”, John Wiley & Sons Ltd, 2008
[14] Jim Zyren, “Overview of the 3GPP Long Term Evolution Physical Layer”, 2007, White
Paper
[15] Online Available: http://wiki.hsc.com/LTE__PHY#LTEphysicallayerlayer1
[16] “3GPP LTE Channels and MAC Layer”, EventHelix.com Inc., 2009, White Paper
90
MEE09:37
BTH, COM
[17] Hyung G. Myung, “Technical Overview of 3GPP LTE”, 2008
[18] Harri Holma, Antti Toskala, “LTE for UMTS-OFDMA and SC-FDMA Based Radio
Access”, John Wiley & Sons Ltd, 2009
[19] Stefania Sesia, Issam Toufik, Matthew Baker, “LTE – The UMTS Long Term Evolution:
From Theory to Practice”, John Wiley & Sons Ltd, 2009
[20] Harri Holma, Antti Toskala, “LTE for UMTS-OFDMA and SC-FDMA Based Radio
Access”, John Wiley & Sons Ltd, pp. 70, 2009
[21] Harri Holma, Antti Toskala, “LTE for UMTS-OFDMA and SC-FDMA Based Radio
Access”, pp. 71, John Wiley & Sons Ltd, 2009
[22] Erik Dahlman, Stefan Parkvall, Johan Sköld, Per Beming, “3G Evolution HSPA and
LTE for Mobile Broadband”, 2nd ed., pp. 328, Academic Press, 2007
[23] 3GPP TS 36.300 V8.0.0, E-UTRA and E-UTRAN
http://www.3gpp.org/ftp/Specs/archive/36%5Fseries/36.300/
Overall
Description,
[24] Erik Dahlman, Stefan Parkvall, Johan Sköld, Per Beming, “3G Evolution HSPA and
LTE for Mobile Broadband”, 2nd ed., pp. 331, Academic Press, 2007
[25] Pierre Lescuyer, Thierry Lucidarme, “Evolved Packet System: The LTE and SAE
Evolution of 3G UMTS”, pp. 159, John Wiley & Sons Ltd, 2008
[26] Hughes Systique, “Uplink Physical Channels”, Available; http://wiki.hsc.com/ online:
http://wiki.hsc.com/LTE__PHY#UplinkPhysicalChannels
[27] Hughes Systique, “Uplink Physical Signals”, Available; http://wiki.hsc.com/ online:
http://wiki.hsc.com/LTE__PHY#UplinkPhysicalSignals
[28] Jim Zyren, “Overview of the 3GPP Long Term Evolution Physical Layer”, freescale,
2007, white Paper
[29] Hyung G. Myung, David J. Goodman,”A New Air Interface For Long Term Evolution”,
2nd ed., John Wiley & Sons Ltd, 2008
[30] Erik Dahlman, Stefan Parkvall, Johan Sköld, Per Beming, “3G Evolution HSPA and
LTE for Mobile Broadband”, 2nd ed., pp. 414, Academic Press, 2008
[31] “UMTS Long Term Evolution (LTE) Technology Introduction”, Application Note
1MA111, Rohde & Schwarz Products, 2007
[32] 3GPP TS 36.300 V8.0.0, E-UTRA and E-UTRAN
http://www.3gpp.org/ftp/Specs/archive/36%5Fseries/36.300/
Overall
Description,
[33] Stefania Sesia, Issam Toufik, Matthew Baker, “LTE – The UMTS Long Term Evolution:
From Theory to Practice”, Ist ed., pp. 102, John Wiley & Sons Ltd, 2009
91
MEE09:37
BTH, COM
[34] Stefania Sesia, Issam Toufik, Matthew Baker, “LTE – The UMTS Long Term Evolution:
From Theory to Practice”, Ist ed., pp. 109, John Wiley & Sons Ltd, 2009
[35] Tejas Bhandare, “LTE and WiMAX Comparison”, Santa Clara University, 2008, White
Paper
[36] Jeffrey G. Andrews, Arunabha Ghosh, Rias Muhamed, “Fundamentals of WiMAX:
Understanding Broadband Wireless Networking”, pp. 57, Prentice Hall, 2007
[37] Jeffrey G. Andrews, Arunabha Ghosh, Rias Muhamed, “Fundamentals of WiMAX:
Understanding Broadband Wireless Networking”, pp. 338, Prentice Hall, 2007
[38] Stefania Sesia, Issam Toufik, Matthew Baker, “LTE – The UMTS Long Term Evolution:
From Theory to Practice”, Ist ed., pp. 24, John Wiley & Sons Ltd, 2009
[39] Stefania Sesia, Issam Toufik, Matthew Baker, “LTE – The UMTS Long Term Evolution:
From Theory to Practice”, Ist ed., pp. 28, John Wiley & Sons Ltd, 2009
[40] Erik Dahlman, Stefan Parkvall, Johan Sköld, Per Beming, “3G Evolution HSPA and
LTE for Mobile Broadband”, 2nd ed., pp. 498, Academic Press, 2008
[41] Loutfi Nuaymi, “WiMAX: Technology for Broadband wireless Access”, John Wiley &
Sons, 2007, ISBN: 9780470028087
[42] Erik Dahlman, Stefan Parkvall, Johan Sköld, Per Beming, “3G Evolution HSPA and
LTE for Mobile Broadband”, Ist ed., pp. 300, Academic Press, 2007
[43] Hyung G. Myung, “Technical Overview of 3GPP LTE”, 2008
[44] “Long Term Evolution Overview”, freescale, 2008, White Paper
[45] Erik Dahlman, Stefan Parkvall, Johan Sköld, Per Beming, “3G Evolution HSPA and
LTE for Mobile Broadband”, Ist ed., pp. 426, Academic Press, 2007
92
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