Un-coded versus Coded QPSK-OFDM Performance
over Rayleigh Fading Channels and DL-PUSC
Subchannelization for OFDMA
Leonardo O. A. Iheme
Submitted to the
Institute of Graduate Studies and Research
in partial fulfillment of the requirements for the Degree of
Master of Science
in
Electrical and Electronic Engineering
Eastern Mediterranean University
June 2010
Gazimağusa, North Cyprus
Approval of the Institute of Graduate Studies and Research
Prof. Dr. Elvan Yılmaz
Director (a)
I certify that this thesis satisfies the requirements as a thesis for the degree of Master
of Science in Electrical and Electronic Engineering.
Assoc. Prof. Dr. Aykut Hocanın
Chair, Department of Electrical Electronic and
Engineering
We certify that we have read this thesis and that in our opinion it is fully adequate in
scope and quality as a thesis for the degree of Master of Science in Electrical and
Electronic Engineering.
Asst. Prof. Dr. Hassan Abou Rajab
Co-Supervisor
Assoc. Prof. Dr. Erhan A. İnce
Supervisor
Examining Committee
1. Assoc. Prof. Dr. Hüseyin Bilgekul
2. Assoc. Prof. Dr. Aykut Hocanın
3. Assoc. Prof. Dr. Erhan A. İnce
4. Assoc. Prof. Dr. Hasan Demirel
5. Asst. Prof. Dr. Hassan A. Rajab
iii
ABSTRACT
In this thesis, a comprehensive study of the IEEE 802.16 physical (PHY) layer was
carried out. An implementation of this standard is Wireless Interoperability for
Microwave Access (WiMAX). Using the MATLAB programming environment,
some of the mandatory parts of the PHY layer of WiMAX were simulated. Basic
blocks of the PHY layer include: A convolutional encoder and a corresponding
Viterbi decoder, a constellation mapper and an Orthogonal Frequency Division
Multiplexing (OFDM) or Orthogonal Frequency Division Multiple Access
(OFDMA) transmitter/receiver. The transmission was simulated over an Additive
White Gaussian Noise (AWGN) channel and two Rayleigh multipath fading channel
models. In order to generate small scale fading, the Jakes’ fading simulator was
adopted.
A study of subchannel permutations is un-avoidable when OFDMA is involved so a
comprehensive study of the Down Link Partial Usage of Sub-Carriers (DL-PUSC),
permutation based non-adjacent subchannelization was carried out and MATLAB
codes were written to simulate the subcarrier allocation process.
The performance of the system was assessed by link level simulations in form of Bit
Error Rate (BER) versus Signal to Noise Ratio (SNR) curves. Doppler effect as a
result of relative motion between the receiver and the transmitter was observed to
degrade the performance and also develop an error floor in multipath fading
channels. Improvement of the performance was observed after the inclusion of a rate
½ convolutional coder of constraint length and generator polynomials
iv
and . The simulation with the convolutional encoder
yielded a coding gain over the AWGN channels and a lower error floor over the
Rayleigh multipath fading channel.
Keywords: OFDM, OFDMA, DL-PUSC, Convolutional Coding, Rayleigh Fading
Channel.
v
ÖZ
Bu tezde geniş bant kablosuz iletişim standardı olan IEEE 802.16’nın fiziksel
katmanı etraflı bir şekilde incelenmektedir. Bu standardın gerçek hayata uyarlanmış
hali bugün Mikrodalga Erişim için Telsiz Birlikte İşlerlik (METBİ) sistemidir. Bu
çalışmada MATLAB programlama dili kullanılarak METBİ’nin fiziki katmanındaki
zorunlu bölümlerin benzetimleri gerçekleştirilmiştir. Fiziki katmanı oluşturan temel
bloklar; evrişimsel kodlayıcı, kodlayıcıya uygun bir Viterbi kod çözücü, bir işaret
kümesi eşleştiricisi, bir Dikgen Frekans Bölüşümlü Çoğullama (DFBÇ) veya çok
kullanıcılı DFBÇ alıcı/verici bloğu olarak sıralanabilir. Benzetim sonuçları hem
Toplanır Beyaz Gauss Gürültülü kanal hem de Jake’in sönümlemeli kanal modelini
baz alan iki farklı çokyollu sönümlemeli kanal üzerinde elde edilmiştir.
Çok kullanıcılı DFBÇ benzetimleri esnasında alt kanal permütasyon methodlarının
incelenmesi kaçınılmazdır. Bundan dolayı bu çalışmada telsiz erişim terminali yer
yönündeki alt-taşıyıcıların kısmi kullanım yöntemi (DL-PUSC) etraflı bir şekilde
incelenmiş ve alt-taşıyıcıları farklı alt-kanallara tahsis edecek MATLAB
fonksiyonları geliştirilmiştir.
Sistem başarımı link seviyesinde bit hata oranı (BHO) na karşı sinyal gürültü oranı
(SGO) eğrileri kullanılarak gösterilmiştir. Gönderici ve alıcı arasındaki bağıl
devinimden kaynaklanan Doppler etkisinin arttığı oranda çok yollu sönümlemeli
kanal üzerinde elde edilecek başarımı negatif yönde etkilediği gösterilmiştir. Bu
durumlarda hızı ½ ve kısıt uzunluğu K= 7 olan bir evrişimsel kodlayıcı kullanıldığı
takdirde (Üreteç polinomlar G1= 171oct ve G2 = 133oct) benzetim sonuçlarında
vi
iyileşme elde edilebilmektedir. Evrişimsel kodlayıcı ve Viterbi kod çözücülü
benzetimler TBGG kanala göre yüksek kazanç göstermiş Rayleigh çokyollu
sönümlemeli kanal üzerinde ise kodsuz benzetim sonuçlarına göre daha alçak bir
hata zeminine neden vermiştir.
Anahtar Kelimeler: Dikgen Frekans Bölüşümlü Çoğullama (DFBÇ), çok kullanıcılı
DFBÇ, telsiz erişim terminali yer yönündeki alt-taşıyıcıların kısmi kullanımı (DL-
PUSC), evrişimsel kodlayıcı, Rayleigh Sönümlemeli çokyollu kanal.
vii
DEDICATION
To my family:
Andee, Moji, Ije and Reni
viii
ACKNOWLEDGEMENTS
I would like to start by expressing my sincere gratitude to my supervisor, Assoc.
Prof. Dr. Erhan A. İnce for his advice and assistance all through the period of this
work. In times when I did not believe in myself he was there to encourage me and
make me believe I can do what I set my mind to. He proved to be a true supervisor
all through, showing excellent scientific and analytical skills. Words alone cannot
express how grateful and privileged I am to be his student.
I feel indebted to my instructors who impacted me with knowledge throughout my
studies here. I want to especially thank Assoc. Prof. Dr. Aykut Hocanın for
challenging me and exposing me to Mobile Communications as a subject and also as
a field of study. I acknowledge Prof. Dr. Şener Uysal and Prof. Dr. Hüseyin
Özkaramanli for their vital contributions to the successful completion of my studies
in EMU.
Thanks to my friends and colleagues for being sources of inspiration to me. Babani,
Azadeh, Mustafa and everyone else I have not mentioned, I say a big thanks. To my
family who stood by me through thick and thin, may you be richly rewarded.
To Elahi, for standing by me all the way and for her comforting words when I was at
my low states; I say thank you. You truly are my custom made love.
ix
TABLE of CONTENTS
ABSTRACT ................................................................................................................ iii
ÖZ ................................................................................................................................ v
DEDICATION ........................................................................................................... vii
ACKNOWLEDGEMENTS ...................................................................................... viii
LIST of TABLES ...................................................................................................... xiv
LIST of FIGURES ..................................................................................................... xv
LIST of SYMBOLS ................................................................................................ xviii
LIST of ABBREVIATIONS ...................................................................................... xx
1 INTRODUCTION ................................................................................................... 1
1.1 Background ...................................................................................................... 2
1.1.1 IEEE 802.16 Standards ............................................................................. 3
1.1.2 WiMAX PHY ............................................................................................ 5
1.1.3 Jakes’ Model ............................................................................................. 7
1.2 Thesis Review .................................................................................................. 7
2 OVERVIEW OF WIRELESS COMMUNICATION SYSTEMS .......................... 9
2.1 Introduction ...................................................................................................... 9
2.2 Wireless and Mobile Networks ...................................................................... 11
2.3 IEEE 802.11 ................................................................................................... 12
2.4 Broad Band Wireless Access (BWA) ............................................................ 13
2.4.1 Broadband Wireless Frequency Spectrum .............................................. 15
x
2.5 CDMA2000 .................................................................................................... 16
2.5.1 CDMA2000 Frequency Spectrum ........................................................... 17
2.5.2 CDMA Technology ................................................................................. 18
2.6 Third Generation Partnership Project (3GPP) ................................................ 20
2.6.1 3GPP Releases......................................................................................... 21
2.7 Long Term Evolution (LTE) .......................................................................... 22
2.7.1 3G LTE Technologies ............................................................................. 24
2.8 Wireless Broadband Deployment and Industry Trends ................................. 25
2.8.1 Fixed Broadband Wireless Access .......................................................... 26
2.8.2 Mobile Broadband Wireless Access ....................................................... 27
2.9 WiMAX .......................................................................................................... 28
2.10 Channel and Bandwidth Classes for WiMAX ............................................... 30
2.11 WiMAX Certification Profiles ....................................................................... 31
3 THE WIRELESS CHANNEL ............................................................................... 33
3.1 Introduction .................................................................................................... 33
3.2 Additive White Gaussian Noise Channel ....................................................... 34
3.3 Fading Channel .............................................................................................. 35
3.4 Frequency Selective Fading ........................................................................... 36
3.5 Rayleigh Fading Channel ............................................................................... 37
3.6 Generating Fading (Jakes’ Model) ................................................................. 40
3.7 Channel Models .............................................................................................. 42
3.7.1 Tapped-Delay-Line Parameters............................................................... 43
xi
4 ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING ......................... 45
4.1 Introduction .................................................................................................... 45
4.2 Inter-Symbol (ISI) and Inter-Channel Interference (ICI) ............................... 46
4.3 Multicarrier Modulation ................................................................................. 46
4.4 OFDM Basics ................................................................................................. 48
4.4.1 FEC Encoder ........................................................................................... 49
4.4.2 QAM Mapper .......................................................................................... 50
4.4.3 Discrete Fourier Transform ..................................................................... 50
4.4.4 The Cyclic Prefix .................................................................................... 51
4.5 Mathematical Description of OFDM ............................................................. 53
5 CHANNEL CODING AND DECODING ............................................................ 56
5.1 Introduction .................................................................................................... 56
5.2 Convolutional Coding .................................................................................... 56
5.2.1 Structure of the Convolutional Code ....................................................... 56
5.2.2 States of a Code ....................................................................................... 57
5.2.3 Trellis Diagram ....................................................................................... 58
5.2.4 Decoding ................................................................................................. 59
6 THE WIMAX PHYSICAL LAYER ..................................................................... 61
6.1 Introduction .................................................................................................... 61
6.2 Symbol Mapper .............................................................................................. 62
6.3 OFDM Symbol Structure ............................................................................... 63
6.3.1 Symbol Parameters .................................................................................. 63
xii
6.4 OFDMA and Subchannelization .................................................................... 64
6.5 Multiple Access Schemes ............................................................................... 65
6.6 OFDMA ......................................................................................................... 65
6.6.1 OFDMA Symbol Structure ..................................................................... 67
6.7 Subchannelization in WiMAX ....................................................................... 67
6.7.1 DL PUSC................................................................................................. 69
6.8 OFDMA Frame .............................................................................................. 76
6.8.1 OFDMA Frame Parameters .................................................................... 78
6.8.2 Data Burst Formation via Vertical Mapping ........................................... 79
7 UN-CODED vs. CODED OFDM PERFORMANCE over MULTIPATH
FADING CHANNELS .......................................................................................... 81
7.1 Introduction .................................................................................................... 81
7.2 Simulation of OFDM ..................................................................................... 82
7.2.1 Un-coded OFDM over AWGN Channel ................................................. 83
7.2.2 Coded OFDM over AWGN Channel ...................................................... 84
7.2.3 Un-coded OFDM over Multipath Rayleigh Fading Channels ................ 85
7.2.4 Coded OFDM over Multipath Rayleigh Fading Channel ....................... 91
8 CONCLUSION AND FUTURE WORK .............................................................. 93
8.1 Conclusion ...................................................................................................... 93
8.2 Future Work ................................................................................................... 94
8.2.1 Interleaved Codes .................................................................................... 94
8.2.2 MIMO...................................................................................................... 94
xiii
8.2.3 IEEE 802.16m ......................................................................................... 94
REFERENCES ........................................................................................................... 95
Appendix .................................................................................................................. 104
Appendix A: DL Subcarrier Permutation Functions ............................................ 105
xiv
LIST of TABLES
Table 1.1: IEEE 802.16 projects and standards ........................................................... 4
Table 2.1: 3GPP releases[2] ....................................................................................... 21
Table 2.2: Targets for LTE......................................................................................... 23
Table 2.3: 3G LTE specification ................................................................................ 25
Table 2.4: WiMAX Channel and Bandwidth Classes ................................................ 30
Table 3.1: Vehicular test environment, tapped-delay-line parameters[18] ................ 43
Table 3.2:Winner tapped delay-line model for scenario 2.8 (RS MS, NLOS) .......... 44
Table 6.1: Primitive parameters for OFDM symbol .................................................. 64
Table 6.2: DL PUSC Parameters ............................................................................... 69
Table 6.3: Permutation sequence ............................................................................... 74
Table 6.4: Parameters for DL PUSC example ........................................................... 75
Table 6.5: Cluster numbering(Major Group 3, DL PermBase = 10) ......................... 75
Table 6.6: Subcarrier Allocation ................................................................................ 76
Table 6.7: TDD OFDMA frame parameters .............................................................. 78
Table 7.1: OFDM Simulation Parameters .................................................................. 82
Table 7.2: Winner scenario 2.8 channel ..................................................................... 88
Table 7.3: ITU Vehicular-A channel parameters ....................................................... 89
xv
LIST of FIGURES
Figure 1.1: Evolution for 3G CDMA/UMTS Systems ................................................ 3
Figure 2.1: Basic Communication System ................................................................... 9
Figure 2.2: Anticipated Growth of Wireless Users Worldwide (Source: UMTS
Forum) .................................................................................................. 12
Figure 2.3: Typical Wireless LAN ............................................................................. 13
Figure 2.4: Channel Access Schemes ........................................................................ 19
Figure 2.5: 3GPP Arrow [3] ....................................................................................... 21
Figure 2.6: Strategic Inclination of Telecom Vendors [38] ....................................... 28
Figure 2.7: Mobile WiMAX Roadmap ...................................................................... 31
Figure 2.8: Release 1 certification profiles in Mobile WiMAX [22] ......................... 32
Figure 3.1: Multipath Scattering and Shadowing ...................................................... 34
Figure 3.2: Doppler power spectral density of Rayleigh fading with a maximum
Doppler shift of 10Hz. .......................................................................... 36
Figure 3.3: L Tap Channel Model .............................................................................. 37
Figure 3.4: PDF of Rayleigh Fading Envelope .......................................................... 39
Figure 3.5: Jakes’ Fading Simulator .......................................................................... 42
Figure 4.1: A Basic Multicarrier Transmitter ............................................................ 47
Figure 4.2: A Basic Multicarrier Receiver ................................................................. 48
Figure 4.3: OFDM Transmitter Block Diagram ........................................................ 49
Figure 4.4: Constellation Maps: (a) BPSK, (b) QPSK and (c) 16-QAM ................... 50
Figure 4.5: The OFDM Cyclic Prefix ........................................................................ 52
Figure 4.6: Examples of OFDM Spectrum (a)Five Subcarriers (b) A Single
Subcarrier ........................................................................................... 53
xvi
Figure 5.1: Convolutional Encoder CC (1, 3, 2) ........................................................ 57
Figure 5.2: Example of a Trellis Diagram Adopted from [1] .................................... 58
Figure 5.3: Rate ½ Binary Convolutional Encoder .................................................... 59
Figure 5.4: Viterbi Decoder Data Flow...................................................................... 60
Figure 6.1: Functional stages of WiMAX PHY ......................................................... 61
Figure 6.2: Subcarrier Structure in Frequency ........................................................... 63
Figure 6.3: Multiple Access Schemes. (a) FDMA (b) TDMA (c) CDMA ................ 65
Figure 6.4: OFDMA Transmission. Ref (6) ............................................................... 66
Figure 6.5: (a) OFDM (b) OFDMA ........................................................................... 67
Figure 6.6: Subchannels in the Subcarrier Structure .................................................. 67
Figure 6.7: Illustration of OFDMA Frame with Multiple Zones ............................... 69
Figure 6.8: Example of an OFDMA DL Frame ......................................................... 71
Figure 6.9: PUSC Subchannel Allocation Procedure ................................................ 72
Figure 6.10: PUSC DL Slot ....................................................................................... 73
Figure 6.11: TDD Frame Structure ............................................................................ 77
Figure 6.12: Data Burst Formation ............................................................................ 79
Figure 6.13: Data Region Showing Data Bursts for Four Users ................................ 80
Figure 7.1: OFDM Performance over AWGN Channel ............................................ 83
Figure 7.2: Coded OFDM Performance over AWGN Channel ................................. 84
Figure 7.3: Theoretical Un-coded OFDM Performance over .................................... 87
Figure 7.4: Un-coded OFDM Performance over Rayleigh Multipath Fading Channel
(Winner Scenario 2.8 Channel) ............................................................... 89
Figure 7.5: Un-coded OFDM Performance over Rayleigh Multipath Fading Channel
(ITU Vehicular-A)................................................................................... 90
xvii
Figure 7.6: Un-coded OFDM Performance over Rayleigh Multipath Fading Channels
(ITU-Vehicular A and Winner Scenario 2.8) .......................................... 91
Figure 7.7: Coded and Un-coded OFDM-QPSK over a Multipath ........................... 92
xviii
LIST of SYMBOLS
Amplitude of carrier
Bandwidth
Coherence bandwidth
Doppler spread
Carrier frequency
Doppler shift
Maximum Doppler frequency
Rayleigh probability density function
Ratio of cyclic prefix time to useful symbol time
Total number of subcarriers
Number of paths
AWGN term
Received signal
Transmitted signal
Delay spread
Symbol duration
Velocity
Multiplicative gain of the kth
path
Subcarrier frequency spacing
Phase shift of the kth
path
Wavelength of carrier frequency
Phase of carrier
xix
Random phase
Channel delay spread
Delay of kth
path
xx
LIST of ABBREVIATIONS
2G 2nd
Generation
3G 3rd
Generation
3GP 3rd
Generation Project
3GPP 3rd
Generation Partnership Project
4G 4th
Generation
ADSL Asymmetric Digital Subscriber Line
AMC Adaptive Modulation and Coding
AWGN Additive White Gaussian Noise
BER Bit Error Rate
BPSK Binary Phase Shift Keying
BS Base Station
BWA Broadband Wireless Access
CC Convolution Code
CDMA Code Division Multiple Access
CISPR Comite International Special des Perturbations Radioelectriques
CP Cyclic Prefix
CSI Channel State Information
DFT Discrete Fourier Transform
DL Downlink
DSL Digital Subscriber Line
DSSS Direct Sequence Spread Spectrum
DVB-H Digital Video Broadcasting-Handheld
EDGE Enhanced Data Rates for GSM Evolution
xxi
ETSI European Telecommunications Standards Institute
Ev-DO Evolution-Data Optimized
FCH Frame Correction Header
FDD Frequency Division Duplexing
FDM Frequency Division Multiplexing
FDMA Frequency Division Multiple Access
FEC Forward Error Correction
FFT Fast Fourier Transform
FUSC Full Usage of SubCarriers
GPRS General Packet Radio Service
GSM Global System for Mobile
HSDPA High Speed Downlink Packet Access
HSPA High Speed Packet Access
HSUPA High Speed Uplink Packet Access
ICI Inter Carrier Interference
IDFT Inverse Discrete Fourier Transform
IEEE Institute of Electrical and Electronics Engineers
IFFT Inverse Fast Fourier Transform
IMT International Mobile Telecommunications
IP Internet Protocol
IQ In-phase and Quadrature-phase
ISI Inter Symbol Interference
ITU International Telecommunications Union
LAN Local Area Network
LN Logical Number
xxii
LOS Line Of Sight
LTE Long Term Evolution
MAC Media Access Control
MAN Metropolitan Area Network
MAP Memory Allocation Processor
MG Major Group
MIMO Multiple Input Multiple Output
MS Mobile Station
NLOS Non Line Of Sight
OFDM Orthogonal Frequency Division Multiplexing
OFDMA Orthogonal Frequency Division Multiple Access
PAN Personal Area Network
PAPR Peak to Average Power Ratio
PCS Personal Communications Service
PHY Physical
PN Physical Number
PUSC Partial Usage of SubCarriers
QAM Quadrature Amplitude Modulation
QoS Quality of Service
QPSK Quadrature Phase Shift Keying
RS Reed Solomon
RTG Receive Transmission Gap
SAE Switched Access Evolution
SC Single Carrier
SFBC Space Frequency Block Coding
xxiii
SNR Signal to Noise Ratio
SS Spread Spectrum
STBC Space Time Block Coding
TDD Time Division Duplexing
TDMA Time Division Multiple Access
TTG Transmit Transition Gap
TUSC Tile Usage of SubCarriers
UL Up Link
UMTS Universal Mobile Telecommunications System
UTRA UMTS Terrestrial Radio Access
VoIP Voice over IP
WAN Wide Area Network
W-CDMA Wideband Code Division Multiple Access
WiBro Wireless Broadband
WiFi Wireless Fidelity
WiMAX Worldwide Interoperability for Microwave Access
WLL Wireless Local Loop
WSSUS Wide-Sense Stationary Uncorrelated Scattering
1
Chapter 1
1 INTRODUCTION
Communication systems seek to transmit information from source to destination at
high data rates regardless of the channel through which the signal is transmitted.
Several schemes have been designed to combat channel impairments; these schemes
could be either wired or wireless. Wireless communication systems are applied
mostly in mobile communication systems. The need for broadband access today has
gone beyond urban areas; in fact it has extended to the rural areas as well.
Deployment of wired networks that extend to hundreds of kilometres is not
economically feasible and also stands a lot of dangers in terms of natural and man-
made disasters. For this reason, wireless deployments of various size networks have
fast become the trend for realisation of broadband access around the world. This
growing demand for wireless broadband systems has brought forward many
technologically feasible solutions from different vendors.
WiMAX is a system that is resilient to channel impairments and thus provides
relatively high data rates in hostile channel conditions. Other competing technologies
like the 3GP family of broadband wireless access schemes also provide such high
data rates in similar channel conditions. There is however an intersection in the
underlying technology of these broadband wireless access schemes: mostly, OFDM
is the backbone of their various physical layer implementations.
2
Due to its popularity, OFDM has gained tremendous attention as an area of study for
researchers and developers. Using OFDM as a multiple access scheme in form of
OFDMA has proved to be more efficient and also perform better. OFDMA is the
underlying technology in mobile WiMAX which is an implementation of the IEEE
802.16e standard. Unlike OFDM, OFDMA allows multiple users to share each frame
worth of data that is to be transmitted through the channel. This is achieved by a
technique known as subchannelization in the downlink. A clear distinction between
the two technologies is made later in the thesis.
1.1 Background
The availability of a variety of solutions to the issue of high data rate delivery to
wireless subscribers has fast become a matter of the choice of technology, as there
are now a number of broadband wireless access schemes in the world. The term "3G"
is now synonymous with high speed wireless access worldwide. 3G, meaning 3rd
Generation, is a family of standards for mobile communications including Universal
Mobile Telecommunications System (UMTS) and Code Division Multiple Access
(CDMA) 2000. UMTS, sometimes referred to as WCDMA is currently the most
popular variant of cellular mobile phones even though it is widely criticized for its
large frequency spectrum usage. To improve the downlink and uplink capacity of
UMTS systems, the 3rd
Generation Partnership Project (3GPP) has developed the
High Speed Downlink Packet Access (HSDPA) and High Speed Uplink Packet
Access (HSUPA) enhancements for the downlink and uplink respectively. The latest
improvement to the CDMA2000 technology is the 1×EvolutionData Optimized
1×EVDO technology.
3
3G technologies continue to evolve in order to meet the high demand for high data
rates and a generally good Quality of Service (QoS) demanded by users. Figure 1.1
shows the evolution of 3G CDMA systems from 2004 up till 2008/9.
Figure 1.1: Evolution for 3G CDMA/UMTS Systems
1.1.1 IEEE 802.16 Standards
This group was established by the IEEE in 1998 to look at the wide area broadband
wireless access issues and to recommend air interfaces and modulation techniques.
The group gave its first recommendation in June 2001 specifying the 802.16 standard
[22]. The air interface of 802.16 was accordingly designated as wireless MAN-SC,
SC standing for Single Carrier.
There has been much development and improvement in the 802.16 standard over the
years. The first 802.16 standard was 802.16 2001. This was a fixed wireless
broadband connection which operated at a frequencies between 10 and 63GHz.
802.16.2 2001 was merely an extension to its legacy. 802.16c 2002 was used for
system profiles. 802.16a 2003 described the physical layer and MAC applications.
This standard used the 2 to 11 GHz frequency band. Some other standards and
4
projects which were withdrawn and merged overtime were P802.16b, P802.16d, and
P802.162a. The802.16 2004 and 802.16e 2005 are some superseded standards along
with 802.16f 2005 which is used as management information base for the 802.16
2004. 802.16 2004/Cor 1-2005 was published as corrections for fixed operations and
was co-published with 802.16e 2005 which is a standard for wireless broadband
access. Some other standards include 802.16K 2007, 802.16g2007 and 802.16 2009
which specifies an air interface for fixed and mobile broadband wireless access
systems.P802.16m is currently in progress and is the most recent of the 802.16
standards. Table 1.1 provides a chronological summary of the fixed and mobile IEEE
802.16 standards and projects to date.
Table 1.1: IEEE 802.16 projects and standards
Standard Description Status
802.16-2001 Fixed Broadband Wireless Access (10–
63 GHz) Superseded
802.16.2-2001 Recommended practice for coexistence Superseded
802.16c-2002 System profiles for 10–63 GHz Superseded
802.16a-2003 Physical layer and MAC definitions for 2–
11 GHz Superseded
P802.16b License-exempt frequencies
(Project withdrawn) Withdrawn
P802.16d
Maintenance and System profiles for 2–
11 GHz
(Project merged into 802.16-2004)
Merged
802.16-2004
Air Interface for Fixed Broadband Wireless
Access System
(rollup of 802.16-2001, 802.16a, 802.16c and
P802.16d)
Superseded
P802.16.2a
Coexistence with 2–11 GHz and 23.5–
43.5 GHz
(Project merged into 802.16.2-2004)
Merged
802.16.2-2004
Recommended practice for coexistence
(Maintenance and rollup of 802.16.2-2001 and
P802.16.2a)
Current
802.16f-2005 Management Information Base (MIB) for
802.16-2004 Superseded
802.16-
2004/Cor 1-
2005
Corrections for fixed operations
(co-published with 802.16e-2005) Superseded
5
802.16e-2005 Mobile Broadband Wireless Access System Superseded
802.16k-2007 Bridging of 802.16
(an amendment to IEEE 802.1D) Current
802.16g-2007 Management Plane Procedures and Services Superseded
P802.16i Mobile Management Information Base
(Project merged into 802.16-2009) Merged
802.16-2009
Air Interface for Fixed and Mobile Broadband
Wireless Access System
(rollup of 802.16-2004, 802.16-2004/Cor 1,
802.16e, 802.16f, 802.16g and P802.16i)
Current
802.16j-2009 Multihop relay Current
P802.16h Improved Coexistence Mechanisms for
License-Exempt Operation in progress
P802.16m Advanced Air Interface with data rates of 100
Mbit/s mobile & 1 Gbit/s fixed Current
1.1.2 WiMAX PHY
WiMAX is a Broadband Wireless Access scheme based on the IEEE 802.16
standard. The name "WiMAX" was created by the WiMAX Forum, which was
formed in June 2001 to promote conformity and interoperability of the standard. The
forum describes WiMAX as a ―standards-based technology‖ enabling the delivery of
last mile wireless broadband access as an alternative to cable and Digital Subscriber
Line (DSL) [48]. The IEEE specified physical layer of WiMAX is very flexible so it
has received a lot of attention from developers. It is based on the much researched
OFDM/OFDMA which can be easily implemented using the Discrete Fourier
Transform (DFT) algorithm known as Fast Fourier Transform (FFT).
Different aspects of the WiMAX physical layer have been analysed, discussed and
simulated with propositions to improve on various areas of the entire system. For
example:
In [21], a basic WiMAX physical layer model is described. In the work, they
implemented the functional stages of a fixed WiMAX model with
6
concatenated Reed Solomon and convolutional encoders rather than just a
convolutional encoder alone.
By exploiting the layered FFT structure, [51] showed that better performance
can be achieved by using a novel Quadrature OFDMA system rather than the
conventional OFDMA systems.
The capacity of a WiMAX system, like in any communication system depends on the
available channel bandwidth; in WiMAX however, the flexibility of the physical
layer extends to the fact that the channel bandwidth is scalable so that it is
proportional to the size of the FFT used during the OFDM/OFDMA block stage.
Capacity evaluation and analysis of data rate performance in [25], [40] and [13]
show the dependence of capacity and data rate on frame overhead. [25] and [40]
stress the importance of proper overhead analysis in the evaluation of capacity for
WiMAX.
1.1.2.1 Channel Coding and Decoding in WiMAX
Channel coding is an essential ingredient in communication systems especially in
multipath channel scenarios. To achieve Forward Error Correction (FEC), extra
parity bits are added to the original message to recover the corrupted information.
The results shown in [42] indicate significant improvements when FEC is applied to
the system. The WiMAX standard specifies several FEC schemes but it points out
binary convolutional coding as a mandatory scheme. The WiMAX standard specifies
an adaptive FEC scheme so that the code size adapts to the given channel condition
at that instant. In [5], a comprehensive literature review of adaptive FEC is
discussed.
7
1.1.2.2 OFDM and OFDMA
OFDM dates as far back as over forty years ago [7] but the concept has only become
very popular in the past decade. OFDM was initially used as a single user
transmission scheme but over years of development, it can now be used in
conjunction with Frequency Division Multiple Access (FDMA) or Time Division
Multiple Access (TDMA) so that it forms a multi user access scheme. In WiMAX,
one of the allowed transmission mode uses OFDM-TDMA. An OFDM-TDMA
transmission system, assumes that the total bandwidth is exclusively allocated to
each user, i.e. all subcarriers, inside a single TDMA frame, which covers some
OFDM symbols [37]. OFDM is identified as the underlying technology in the PHY
layer of the 802.16 standards. It is used as a multiple access scheme in the form of
OFDMA starting from the 802.16e 2005 standard where mobility is taken into full
consideration. In OFDMA, both time and/or frequency resources are used to separate
the multiple user signals.
1.1.3 Jakes’ Model
The Jakes’ model for generating fading has proved over years to be an effective
method for Rayleigh fading channel modelling[19] [33] [12]. It is based on summing
the sinusoids of fading waveforms with equal strength and uniformly distributed
arrival angles. Even with its wide spread use, the model has received several re-
visitations [12], [33] and [34] because it does not produce some important properties
of physical fading channels. Specifically, [12] pointed out that it is difficult to create
multiple uncorrelated fading waveforms with the classic model.
1.2 Thesis Review
In Chapter 2, an overview of wireless communication systems will be discussed.
Highlighted in Chapter 2 are the various technologies that are similar to WiMAX and
8
compete with it. The chapter will focus more on wireless broadband with WiMAX as
an implementation. Chapter 3 will focus on the wireless channel and how Rayleigh
fading can be generated using the Jakes sum of sinusoids model. A brief description
of the channel models used for simulation in this thesis will wrap up Chapter 3.
OFDM will be introduced in Chapter 4 and detailed discussion will follow, giving
descriptions of the various blocks that make up a basic OFDM system. The chapter
will end with a mathematical description of OFDM with supporting equations.
Chapter 5 will talk about channel coding using FEC in the form of convolutional
coding. The Viterbi decoder which is the most effective way of decoding short
convolutional codes will be used in this thesis and its description will conclude
Chapter 5.
The stages involved in the implementation of the physical layer of WiMAX (IEEE
802.16e 2005) will be presented in Chapter 6. Detailed discussion of the OFDMA
frame structure and DL PUSC subcarrier permutation will appear in later parts of
Chapter 6. An integral aspect of the frame structure and the DL PUSC permutation is
the data burst formation and this will be discussed in the concluding section of the
chapter.
Chapters 7 and 8 will present the results of simulation and conclusion to the thesis
respectively.
9
Chapter 2
2 OVERVIEW OF WIRELESS COMMUNICATION
SYSTEMS
2.1 Introduction
The goal of any communication system is to successfully transmit data to a receiver
with minimal errors in the received data. The case is not different for wireless
communication systems; however the channel through which the data is transmitted
may differ depending on the application of the communication system. A common
definition of wireless communication is: the transfer of information over a distance
without the use of enhanced electrical conductors [49].
A basic communication system, wireless or not is made up of three main functional
blocks, namely: transmitter, channel and receiver. The distinguishing factor in the
type of communication system is the channel; it refers to the medium through which
information or data travels from the transmitter to the receiver.
Figure 2.1: Basic Communication System
10
The challenge of a wireless communication system is in overcoming the effects of
the channel on the transmitted signal. Wireless communication, like other modes of
communication finds application in various areas such as:
Security systems
Television remote control
Cellular telephone (phones and modems)
WiFi
Wireless energy transfer
Computer Interface Devices
The most common application of wireless communication today is in the cellular
telephone system. Otherwise known as mobile phone or cell phone, cellular
telephone has been a tremendous success ever since its discovery in 1945. Statistics
show that the world's largest individual mobile operator is China Mobile with over
500 million mobile phone subscribers. The world's largest mobile operator group by
subscribers is UK based Vodafone. There are over 600 mobile operators and carriers
in commercial production worldwide. Over 50 mobile operators have over 10 million
subscribers each, and over 150 mobile operators had at least one million subscribers
by the end of 2008 (source: wireless intelligence). We can broadly classify wireless
systems as either Line Of Sight (LOS) or Non-Line of Sight (NLOS). The types of
wireless communication include:
Radio transmission
Microwave transmission
11
Infrared and Millimetre waves
Light wave transmission
The above mentioned are distinguished by their frequencies of operation and thus
their transmission range.
2.2 Wireless and Mobile Networks
Wireless network refers to any type of computer network that is wireless, and is
commonly associated with a telecommunications network whose interconnections
between nodes is implemented without the use of wires [50]. Various types of these
wireless networks exist, some of which are:
Wireless PAN
Wireless LAN
Wireless MAN
Wireless WAN
The focus of this thesis however is on the Wireless MAN (Metropolitan Area
Network) and is sometimes referred to as WiMAX covered in IEEE 802.16d and
IEEE 802.16e standards. In simple terms, a Wireless MAN can be defined as a
wireless network which connects various other wireless LANs.
The development of a WiFi chip in 2003 heralded a new dimension to the move
toward wireless services. The number of WiFi users rose to 120 million by 2005, 200
million by 2006, and was estimated to top a billion in 2008[22]
12
Figure 2.2: Anticipated Growth of Wireless Users Worldwide (Source: UMTS
Forum)
As a result of this rapid growth, WiFi which uses the IEEE 802.11 set of standards
has become synonymous with Wireless LAN. A wireless local area network
(WLAN) links devices via a wireless distribution method (typically spread-spectrum
or OFDM), and usually provides a connection through an access point to the wider
internet.
2.3 IEEE 802.11
IEEE 802.11 is a set of standards carrying out wireless local area network (WLAN)
computer communication in the 2.4, 3.6 and 5 GHz frequency bands. The 802.11
family includes over-the-air modulation techniques that use the same basic protocol.
The most popular are those defined by the 802.11b and 802.11g protocols, which are
amendments to the original standard [16].
Wi-Fi is increasingly used as a synonym for 802.11 WLANs, although it is
technically a certification of interoperability between 802.11 devices. The Wi-Fi
Alliance, a global association of companies, promotes WLAN technology and
certifies products if they conform to certain standards of interoperability. Among the
13
uses of Wi-Fi, the most important today is for internet access; another use is for
computer-to-computer communications.
Figure 2.3: Typical Wireless LAN
The relative ease of implementation of wireless LANs make them attractive but also
it has its limitations and disadvantages, the most prominent being security and range
of transmission. Among numerous limitations, some of the most obvious can be
experienced in the data rate and interference from other devices operating in the
2.4GHz frequency band. These limitations make it difficult and sometimes
impossible to implement wireless networks in nomadic rural areas. With the
development various wireless broadband schemes, it has become possible to deploy
wireless LANs as a last mile resort with a broad band wireless scheme as back haul.
2.4 Broad Band Wireless Access (BWA)
The term broadband, depending on the context of usage can have different meanings.
In telecommunication however, broadband is a signalling method that includes or
handles a relatively wide range (or band) of frequencies, which may be divided into
channels or frequency bins. Broadband is always a relative term, understood
according to its context. In data communications for example, a digital modem will
14
transmit a data rate of 56 kilobits per second (Kbit/s) over a 4 kilohertz wide
telephone line (narrowband). However when that same line is converted to a standard
twisted-pair wire (no telephone filters), it becomes hundreds of kilohertz wide
(broadband) and can carry several megabits per second (ADSL). Broadband access
not only provides faster Web surfing and quicker file downloads but also enables
several multimedia applications, such as real-time audio and video streaming,
multimedia conferencing, and interactive gaming. Broadband connections are also
being used for voice telephony using voice-over-Internet Protocol (VoIP)
technology.
Broadband wireless is about bringing the broadband experience to a wireless context,
which offers users certain unique benefits and convenience. Wireless Broadband is a
fairly new technology that provides high-speed wireless internet and data network
access over a wide area. According to the 802.16-2004 standard, broadband means
'having instantaneous bandwidth greater than around 1 MHz and supporting data
rates greater than about 1.5 Mbit/s. This means that Wireless Broadband features
speeds roughly equivalent to wired broadband access, such as that of ADSL or a
cable modem.
Both wireless and broadband have on their own enjoyed rapid mass-market adoption.
Wireless mobile services grew from 11 million subscribers worldwide in 1990 to
more than 2 billion in 2005. During the same period, the Internet grew from being a
curious academic tool to having about a billion users. This staggering growth of the
Internet is driving demand for higher-speed Internet-access services, leading to a
parallel growth in broadband adoption. In less than a decade, broadband subscription
worldwide has grown from virtually zero to over 200 million [7].
15
The International Telecommunications Union (ITU) has recognized three types of
wireless access (F.1399 recommendations).
Fixed access: Wireless access application in which the location of the end-
user termination and the network access point to be connected to the end user
are fixed.
Nomadic wireless access: Wireless access application in which the location
of the end-user termination may be in different places but it must be
stationary while in use.
Mobile wireless access: Wireless access application in which the location of
the end-user termination is mobile.
Fixed wireless broadband can be thought of as a competitive alternative to ADSL or
cable modem and it seeks to provide services similar to that of the traditional fixed-
line broadband but using wireless as the medium of transmission. The mobile
wireless broadband access on the other hand caters for portable, high speed devices
such as mobile phones, notebook computers, etc.
2.4.1 Broadband Wireless Frequency Spectrum
In many cases, the frequency assignment is as important as the broadband technology
selection; but like all other aspects of the physical world, the radio frequency
electromagnetic spectrum is subject to usage limitations. Use of radio frequency
bands of the electromagnetic spectrum is regulated by governments in most
countries, in a Spectrum management process known as frequency allocation or
spectrum allocation. A number of forums and standards bodies work on standards for
frequency allocation, including:
16
• International Telecommunication Union (ITU)
• European Conference of Postal and Telecommunications Administrations
(CEPT)
• European Telecommunications Standards Institute (ETSI)
• International Special Committee on Radio Interference (Comité
International Spécial des Perturbations Radioélectriques - CISPR)
Using the lower frequency bands is preferable for broadband-intensive network
deployments. The propagation characteristics of the lower frequency bands enable
RF transmissions to travel greater distances. The increased range provides larger
coverage areas. Fewer cell sites require fewer backhaul connections, which leads to
lower costs. The lower frequency bands also enable better in-building penetration,
better mobile performance, less power consumption and higher average data
throughputs in a NLOS environment. This is becoming progressively more important
as the bandwidth for the backhaul connections must increase to keep up with the
growing demand for mobile broadband services.
This chapter will introduce broadband wireless access schemes and take a shallow
dive into technologies implementing them. I will discuss the industry trends and
worldwide deployment of broadband wireless access solutions. The chapter will end
with an in-depth discussion about WiMAX, the IEEE 802.16 standard and how
WiMAX competes with other broadband wireless solutions.
2.5 CDMA2000
CDMA2000 represents a family of IMT-2000 (3G) standards providing high-quality
voice and broadband data services over wireless networks. CDMA2000 builds on the
17
inherent advantages of CDMA technologies and introduces other enhancements,
such as Orthogonal Frequency Division Multiplexing (OFDM), advanced control and
signalling mechanisms, improved interference management techniques, end-to-end
Quality of Service (QoS), and new antenna techniques such as Multiple Inputs
Multiple Output (MIMO) and beam forming to increase data throughput rates and
quality of service, while significantly improving network capacity and reducing
delivery cost.
Currently, CDMA2000 includes CDMA2000 1X (1X) and CDMA2000 EV-DO
(Evolution-Data Optimized) standards. CDMA2000 1X (IS-2000) supports circuit-
switched voice up to and beyond 35 simultaneous calls per sector and high-speed
data of up to 153 kbps in both directions. It was recognized by the ITU as an IMT-
2000 standard in November 1999. CDMA2000 EV-DO introduces new high-speed
packet-switched transmission techniques that are specifically designed and optimized
for a data-centric broadband network that can deliver peak data rates beyond 3 Mbps
in a mobile environment. CDMA2000 EV-DO was approved as an IMT-2000
standard (cdma2000 High Rate packet Data Air Interface, IS-856) in 2001.
CDMA2000 1X was deployed in 2000, as the first IMT-2000 standard to be
commercially available, and today, along with EV-DO, it is the leading 3G
technology serving around a half billion users worldwide. CDMA2000 systems
provide a family of related services including cellular, PCS, WLL and fixed wireless.
[9].
2.5.1 CDMA2000 Frequency Spectrum
CDMA2000 operates in a relatively small amount of spectrum, 1.25 MHz, in most of
the frequency bands designated by the International Telecommunication Union (ITU)
18
for the IMT-2000 systems. The smaller 1.25 MHz channel size enables greater
spectrum assignment flexibility to
a. incrementally assign channels as the demand for capacity increases, and
b. to facilitate in-band migration deployments which require the clearing of
spectrum
CDMA2000 1X, EV-DO Rel. 0 and Rev. A operate in a paired 2 x 1.25 MHz FDD
channel - compared to other 3G technologies which require a much larger 2 x 5 MHz
channel. By using a narrower radio channel, operators benefit from greater flexibility
and improved cost efficiencies in managing their scarce spectrum resources. EV-DO
Rev. B enables operators to aggregate multiple 1.25 MHz channels, up to 15
channels in 20 MHz of spectrum, to deliver the next-generation multi-mega-bits-per-
second data connectivity and bandwidth intensive applications more economically.
Currently, CDMA2000 network infrastructure and user devices are available in most
of the IMT-2000 frequency bands designated by the ITU, including the 450 MHz,
700 MHz, 800 MHz, 1700 MHz, 1900 MHz, AWS and 2100 MHz bands.
2.5.2 CDMA Technology
Code Division Multiple Access is the channel access method used by the
CDMA2000 standards. Unlike frequency and time access methods (FDMA &
TDMA), CDMA allocates the entire spectrum to a user and uses codes to identify
connections.
19
Figure 2.4: Channel Access Schemes
The CDMA is a digital modulation and radio access system that employs signature
codes (rather than time slots or frequency bands) to arrange simultaneous and
continuous access to a radio network by multiple users.
CDMA is a form of Direct Sequence Spread Spectrum (DSSS) communications. In
general, Spread Spectrum (SS) communications is distinguished by three key
elements:
1. The signal occupies a bandwidth much greater than that which is necessary to
send the information. This results in many benefits, such as immunity to
interference and jamming and multi-user access.
2. The bandwidth is spread by means of a code which is independent of the data.
The independence of the code distinguishes this from standard modulation
schemes in which the data modulation will always spread the spectrum
somewhat.
3. The receiver synchronizes to the code to recover the data. The use of an
independent code and synchronous reception allows multiple users to access
the same frequency band at the same time.
20
In order to protect the signal, the code used is pseudo-random. It appears random, but
is actually deterministic, so that the receiver can reconstruct the code for
synchronous detection. This pseudo-random code is also called pseudo-noise (PN)
[47].
Contribution to the radio channel interference in mobile communications arises from
multiple user access, multipath radio propagation, adjacent channel radiation and
radio jamming. The spread spectrum system’s performance is relatively immune to
radio interference; however, CDMA still has a few drawbacks, the main one being
that capacity (number of active users at any instant of time) is limited by the access
interference. Furthermore, Near-far effect requires an accurate and fast power control
scheme. More detailed information about CDMA Technology can be found in [47] &
[45].
2.6 Third Generation Partnership Project (3GPP)
3GPP is collaboration between groups of telecommunications associations, to make a
globally applicable third generation (3G) mobile phone system specification within
the scope of the International Mobile Telecommunications-2000 project of the ITU.
The original scope of 3GPP was to produce Technical Specifications and Technical
Reports for a 3G Mobile System based on evolved GSM core networks and the radio
access technologies that they support (i.e., Universal Terrestrial Radio Access
(UTRA) both frequency division duplex and time division duplex modes).
The scope was subsequently amended to include the maintenance and development
of the Global System for Mobile communication (GSM) Technical Specifications
21
and Technical Reports including evolved radio access technologies (e.g. General
Packet Radio Service (GPRS) and Enhanced Data rates for GSM Evolution (EDGE).
Figure 2.5: 3GPP Arrow [3]
3GPP was created in December 1998 by the signing of the "The 3rd Generation
Partnership Project Agreement". The latest 3GPP Scope and Objectives document
has evolved from this original Agreement [3]
2.6.1 3GPP Releases
3GPP uses a system of parallel "releases" - to provide developers with a stable
platform for implementation and to allow for the addition of new features required by
the market. So far, the group has nine releases with the tenth release in the works.
Table 2.1: 3GPP releases[2]
Version Info
Release 98 This and earlier releases specify pre-3G GSM networks
Release 99 Specified the first UMTS 3G networks, incorporating a CDMA
air interface
Release 4 Originally called the Release 2000 - added features including an
all-IP Core Network
Release 5 Introduced IMS and HSDPA
Release 6
Integrated operation with Wireless LAN networks and adds
HSUPA, MBMS, enhancements to IMS such as Push to Talk
over Cellular (PoC), GAN
22
Release 7
Focuses on decreasing latency, improvements to QoS and real-
time applications such as VoIP. This specification also focus on
HSPA+ (High Speed Packet Access Evolution), SIM high-speed
protocol and contactless front-end interface (Near Field
Communication enabling operators to deliver contactless
services like Mobile Payments), EDGE Evolution.
Release 8
LTE, All-IP Network (SAE). Release 8 constitutes a refactoring
of UMTS as an entirely IP based fourth-generation network.
Release 9 SAES Enhancements, WiMAX and LTE/UMTS Interoperability
Release 10 LTE Advanced
Current 3GPP standards incorporate the latest revision of the GSM standards. 3GPP's
plans for the future beyond Release 7 are in the development under the title Long
Term Evolution (LTE).
2.7 Long Term Evolution (LTE)
With services such as WiMAX offering very high data speeds, work on developing
the next generation of cellular technology has started. The UMTS cellular technology
upgrade has been dubbed LTE - Long Term Evolution. The idea is that 3G LTE will
enable much higher speeds to be achieved along with much lower packet latency (a
growing requirement for many services these days), and that 3GPP LTE will enable
cellular communications services to move forward to meet the needs for cellular
technology to 2017 and well beyond.
HSPA (High Speed Packet Access), a combination of HSDPA and HSUPA, and
HSPA+ are now being deployed, the 3G LTE development is being dubbed 3.99G as
it is not a full 4G standard, although in reality there are many similarities with the
cellular technologies being touted for the use of 4G. However, regardless of the
terminology, it is certain that 3G LTE will offer significant improvements in
performance over the existing 3G standards [32].
23
LTE core specifications are included in release 8. In terms of actual figures, targets
for LTE included download rates of 100Mbps, and upload rates of 50Mbps for every
20MHz of spectrum. In addition to this LTE was required to support at least 200
active users in every 5MHz cell (i.e. 200 active phone calls). Targets have also been
set for the latency in IP packet delivery. With the growing use of services including
VoIP, gaming and many other applications where latency is of concern, figures need
to be set for this. As a result a figure of sub-10ms latency for small IP packets has
been set. The LTE is an evolution of the UMTS/3GPP 3G standards and is thus
backward compatible in the sense that it:
Works with GSM/EDGE/UMTS systems
Utilizes existing 2G and 3G spectrum and new spectrum
Supports hand-over and roaming to existing mobile networks.
Unlike the earlier forms of 3G architecture, LTE uses OFDMA/SC-FDMA instead of
CDMA. This singular property of LTE makes it very similar to WiMAX.
Table 2.2: Targets for LTE
Max downlink speed
(bps) 100M
Max uplink speed
(bps) 50 M
Latency
round trip time
approx.
~10 ms
3GPP releases Rel 8
Approx. years of initial roll out 2009 / 10
Access methodology OFDMA / SC-FDMA
24
2.7.1 3G LTE Technologies
LTE has introduced a number of new technologies when compared to the previous
cellular systems. They enable LTE to be able to operate more efficiently with respect
to the use of spectrum, and also to provide the much higher data rates that are being
required.
OFDM (Orthogonal Frequency Division Multiplex): OFDM technology
has been incorporated into LTE because it enables high data bandwidths to be
transmitted efficiently while still providing a high degree of resilience to
reflections and interference. The access schemes differ between the uplink
and downlink: OFDMA (Orthogonal Frequency Division Multiple Access is
used in the downlink; while SC-FDMA (Single Carrier - Frequency Division
Multiple Access) is used in the uplink. SC-FDMA is used in view of the fact
that its peak to average power ratio is small and the more constant power
enables high RF power amplifier efficiency in the mobile.
Multiple Input Multiple Output (MIMO): One of the main problems that
previous telecommunications systems have encountered is that of multiple
signals arising from the many reflections that are encountered. By using
MIMO, these additional signal paths can be used to an advantage so that the
throughput is increased. When using MIMO, it is necessary to use multiple
antennas to enable the different paths to be distinguished. Accordingly
schemes using 2 × 2, 4 × 2, or 4 × 4 antenna matrices can be used. While it is
relatively easy to add further antennas to a base station, the same is not true
of mobile handsets, where the dimensions of the user equipment limit the
number of antennas which should be placed at least a half wavelength apart.
25
System Architecture Evolution (SAE): With the very high data rate and low
latency requirements for 3G LTE, it is necessary to evolve the system
architecture to achieve the desired improvement in. One change is that a
number of the functions previously handled by the core network have been
transferred out to the periphery. Essentially this provides a much "flatter"
form of network architecture. In this way latency times can be reduced and
data can be routed more directly to its destination.
Table 2.3: 3G LTE specification
Parameter Details
Peak downlink speed
64QAM
(Mbps)
100 (SISO), 172 (2x2 MIMO), 326
(4x4 MIMO)
Peak uplink speeds
(Mbps)
50 (QPSK), 57 (16QAM), 86
(64QAM)
Data type All packet switched data (voice and
data). No circuit switched.
Channel bandwidths
(MHz) 1.4, 3, 5, 10, 15, 20
Duplex schemes FDD and TDD
Mobility 0 - 15 km/h (optimised),
15 - 120 km/h (high performance)
Latency Idle to active less than 100ms
Small packets ~10 ms
Spectral efficiency Downlink: 3 - 4 times Rel 6 HSDPA
Uplink: 2 -3 x Rel 6 HSUPA
Access schemes OFDMA (Downlink)
SC-FDMA (Uplink)
Modulation types supported QPSK, 16QAM, 64QAM (Uplink
and downlink)
2.8 Wireless Broadband Deployment and Industry Trends
Driven by the demand for high data rates, flexible and easy-to-implement schemes
have been developed. Various companies with their expertise have been working to
achieve the best possible implemented standards and technologies. Perhaps the most
challenging aspect of BWA deployment from an engineering point of view is in the
26
indoor NLOS. This poses the problem of penetration through walls and other
obstacles. Given the wide variety of solutions developed and deployed for broadband
wireless in the past, a full historical survey of these is beyond the scope of this thesis.
Wireless Broadband can be deployed either as fixed or mobile broadband access
2.8.1 Fixed Broadband Wireless Access
Services provided using fixed broadband could include high-speed Internet access,
telephony services using voice over IP, and a host of other Internet-based
applications. Fixed wireless communication offer several advantages over traditional
wired solutions such as lower entry and deployment costs; faster and easier
deployment and revenue realization; ability to build out the network as needed; lower
operational costs for network maintenance, management, and operation; and
independence from the incumbent carriers [7].
In the United States and other developed countries with good wired infrastructure,
fixed wireless broadband is being used in rural or underserved areas, where
traditional means of serving them is more expensive. A potentially larger market for
fixed broadband exists outside the United States, particularly in urban and suburban
locales in developing economies—China, India, Russia, Indonesia, Brazil and several
other countries in Latin America, Eastern Europe, Asia, and Africa—that lack an
installed base of wire line broadband networks. National governments that are eager
to quickly catch up with developed countries without massive, expensive, and slow
network rollouts could use WiMAX to leapfrog ahead. A number of these countries
have seen sizable deployments of legacy WLL systems for voice and narrowband
data. Vendors and carriers of these networks will find it easy to promote the value of
WiMAX to support broadband data and voice in a fixed environment.
27
2.8.2 Mobile Broadband Wireless Access
By adding nomadic capabilities to fixed broadband wireless, it can be seen as a first
step towards mobility. Nomadic access may not allow for seamless roaming and
handover at vehicular speeds but would allow pedestrian-speed mobility and the
ability to connect to the network from any location within the service area. Existing
mobile operators are less likely to adopt WiMAX and more likely to continue along
the path of 3G evolution for higher data rate capabilities. Korea Telecom, however,
has begun deploying WiBro service in metropolitan areas to complement its
ubiquitous CDMA2000 service by offering higher performance for multimedia
messaging, video, and entertainment services [7]. WiBro (Wireless Broadband) is a
wireless broadband Internet technology developed by the South Korean telecoms
industry. WiBro can be seen as the South Korean service name for IEEE 802.16e
(mobile WiMAX) international standard. As operators move into entertainment with
the development of IP-TV, schemes for mobile broadband wireless access become
imperative. Figure 2.6 is a rough illustration of where different vendors are
strategically aiming, not necessarily where they are today.
28
Figure 2.6: Strategic Inclination of Telecom Vendors [38]
Despite the strategic inclinations, pretty much all vendors seem to be playing both
sides of the game. See [38] for examples of vendors’ strategies.
2.9 WiMAX
The name "WiMAX" was created by the WiMAX Forum, which was formed in June
2001 to promote conformity and interoperability of the standard. The forum
describes WiMAX [39] as "a standards-based technology enabling the delivery of
last mile wireless broadband access as an alternative to cable and DSL". ("WiMAX
Forum-Technology").
WiMAX refers to interoperable implementations of the IEEE 802.16 wireless-
networks standard (ratified by the WiMAX Forum), in similarity with Wi-Fi, which
refers to interoperable implementations of the IEEE 802.11 Wireless LAN standard
(ratified by the Wi-Fi Alliance). The WiMAX Forum certification allows vendors to
sell their equipment as WiMAX (Fixed or Mobile) certified, thus ensuring a level of
interoperability with other certified products, as long as they fit the same profile [48].
29
The IEEE 802.16 standard forms the basis of 'WiMAX' and is sometimes referred to
colloquially as WiMAX. Fixed WiMAX and Mobile WiMAX are respectively
known as802.16d and 802.16e [26]. Clarifications of the formal names are as
follows:
802.16-2004 is also known as 802.16d, which refers to the working party that
has developed that standard. It is sometimes referred to as "Fixed WiMAX,"
since it has no support for mobility.
802.16e-2005, often abbreviated to 802.16e, is an amendment to 802.16-
2004. It introduced support for mobility, among other things and is therefore
also known as "Mobile WiMAX".
Mobile WiMAX is the WiMAX incarnation that has the most commercial interest to
date and is being actively deployed in many countries. Mobile WiMAX is also the
basis of future revisions of WiMAX. As such, references to and comparisons with
WiMAX henceforth means Mobile WiMAX except otherwise stated.
WiMAX promises to substitute other broadband technologies competing in the same
segment and will become an excellent solution for the deployment of the well-known
last mile infrastructures in places where it is very difficult to get with other
technologies such as cable or DSL, and where the costs of deployment and
maintenance of such technologies would not be profitable. This way, WiMAX will
connect rural areas in developing countries as well as underserved metropolitan
areas. It can even be used to deliver backhaul for carrier structures, enterprise
campus, and Wi-Fi hot-spots. WiMAX offers a good solution for these challenges
because it provides a cost-effective, rapidly deployable solution.
30
2.10 Channel and Bandwidth Classes for WiMAX
The WiMAX Forum™
specifies the channel and FFT size combinations. The
frequency range depends on the geographical region of operation as various regions
have their operational frequency bands. For example, the Korean WiBro operates
with a nominal channel bandwidth of 7MHz and an FFT size of 1024 operating in the
2.3 - 2.4 GHz band. WiMAX however has several band classes as shown in Table
2.4
Table 2.4: WiMAX Channel and Bandwidth Classes
Band Class
Index
Frequency Range
(GHz)
Channel
Bandwidth(s)
(MHz)
FFT Size
1 2.3-2.4
5 512
10 1024
8.75 1024
2 2.305-2.320,
2.345-2.360
3.5 512
5 512
10 1024
3 2.496-2.69 5 512
10 1024
4 3.3-3.4
5 512
7 1024
10 1024
5
3.4-3.8
5 512
7 1024
10 1024
3.4-3.6
5 512
7 1024
10 1024
3.6-3.8
5 512
7 1024
10 1024
31
2.11 WiMAX Certification Profiles
The IEEE 802.16e-2005 had prescribed the frequency band of 2 to 6 GHz for Mobile
WiMAX and various options for bandwidths as well as multiplexing. The WiMAX
Forum has, however, selected a subset of these parameters for mobile WiMAX
certification profiles in Release 1 (Figure 2.8) [22].
Mobile WiMAX Rel 2(802.16m)
Mobile WiMAX Rel 1.5(802.16e Rev2)
Mobile WiMAX Rel 1(802.16e)
Mobile Broadband 70+ Mbps
2008
Mobile Broadband 125+ Mbps
2009/2010
Mobile Broadband 300+ Mbps
2010/2011
Figure 2.7: Mobile WiMAX Roadmap
Mobile WiMAX uses 512 OFDM carriers for a bandwidth of 5MHz and 1024
subcarriers for bandwidths of 7 and 10MHz. For initial certification profiles, the
WiMAX Forum has selected an FFT size of 512 carriers and a guard band of 1/8.
The frame size selected is 5ms.
32
2.3-2.4 GHz
5MHz
8.75MHz
10MHz
512
1024
1024
2.469-2.69 GHz
5MHz
10MHz
512
1024
2.305-2.32 GHz
2.345-2.36 GHz
3.5MHz
5MHz
10MHz
512
1024
1024
3.3-3.4 GHz
5MHz
7MHz
10MHz
512
1024
1024
5MHz
7MHz
10MHz
512
1024
1024
3.4-3.8 GHz
3.4-3.6 GHz
3.6-3.8 GHz
Frequency Bandwidth FFT Size Frequency Bandwidth FFT Size
2.3-2.7 GHz 3.3-2.8 GHz
Figure 2.8: Release 1 certification profiles in Mobile WiMAX [22]
Release 2 (IEEE 802.16m) of WiMAX is yet to be finalized, a revision of Release 1
(Release 1.5) is in progress and is set to be completed by the end of this year. Chip
giant Intel, a major supporter of the movement to provide mobile WiMAX wireless
broadband to Internet users around the world, expects the next major release of the
technology to be deployed starting in 2012 [30].
33
Chapter 3
3 THE WIRELESS CHANNEL
3.1 Introduction
The rapid fluctuation of the amplitude of a signal over a relatively small distance is
referred to as fading. Interference between two or more versions of the transmitted
signal can result in different propagation delays at the receiver and this is known as
multipath. Some of the causes of multipath as pointed out in [28] are: atmospheric
ducting, ionospheric reflection and refraction, and reflection from water bodies and
terrestrial objects such as mountains and buildings. Due to the relative motion
between the mobile and the base station, each multipath wave experiences an
apparent shift in frequency. The shift in received signal frequency due to motion is
called the Doppler shift, and is directly proportional to the velocity and direction of
motion of the mobile with respect to the direction of arrival of the received multipath
wave [36].
The factors influencing small scale fading are:
1. Multipath propagation
2. Speed of the mobile
3. Speed of surrounding objects
4. The transmission bandwidth of the signal
34
Figure 3.1: Multipath Scattering and Shadowing
The classification of fading is based on the relationship between the signal
parameters and the channel parameters. The channel is typically characterized by its
impulse response which contains all the necessary information required to analyse or
simulate any type of radio transmission through the channel [36].
3.2 Additive White Gaussian Noise Channel
This is a channel model in which the only impairment to communication is a linear
addition of wideband or white noise with a constant spectral density and a Gaussian
distribution of amplitude. The model does not account for fading, frequency
selectivity, interference, nonlinearity or dispersion. However, it produces simple and
tractable mathematical models which are useful for gaining insight into the
underlying behaviour of a system before these other phenomena are considered [4].
Wideband Gaussian noise comes from many natural sources, such as the thermal
vibrations of atoms in conductors, shot noise, black body radiation from the earth and
other warm objects, and from celestial sources such as the Sun.
35
Due to the limitation of this model, it is safe to say that it is not a realistic channel
model for simulating a mobile wireless communication system.
3.3 Fading Channel
The measure of how quickly the channel response de-correlates is called the
coherence time. When the coherence time is large compared to the symbol duration
of the signal, then the channel is referred to as slow fading. Fast fading is the
opposite of slow fading and occurs when the coherence time is small or comparable
to the symbol duration. Another classification of the fading process depends on the
relationship between the delay spread of the channel which is a measure of its time
depressiveness and the symbol duration. When the delay spread is much smaller than
the symbol duration the fading is classified as flat and when it is not it is termed as
frequency selective fading [35].
Doppler shift is caused by the relative motion between the receiver and the
transmitter. Doppler spread is a measure of the spectral broadening caused by the
time rate of change of the mobile radio channel and is defined as the range of
frequencies over which the received Doppler spectrum is essentially non-zero. When
a pure sinusoidal tone of frequency is transmitted, the received signal spectrum,
called the Doppler spectrum, will have components in the range to
where is the Doppler shift. The amount of spectral broadening depends
on which is a function of the relative velocity of the mobile and the angle
between the direction of motion of the mobile and the direction of arrival of the
scattered waves [36].
36
Figure 3.2: Doppler power spectral density of Rayleigh fading with a maximum
Doppler shift of 10Hz.
3.4 Frequency Selective Fading
Fading is considered to be flat when the symbol duration of the signal is much larger
than the delay spread of the channel. This is desirable for communication,
unfortunately, for high data rate applications the signal bandwidth increases and the
symbol period is on the order of a few microseconds.
The frequency selective fading channel can be modelled as an tap filter depicted in
Figure 3.3. is the number of resolvable paths provided by the channel and is a
measure of the diversity available in the channel.
37
0 Ts 2Ts 3Ts 4Ts L-1(Ts)Time
....
Figure 3.3: L Tap Channel Model
[
]
Where is the delay spread of the channel and is the symbol duration. The
impulse response of the channel can be then expressed as:
∑
The usual model assumed for frequency selective fading is Wide Sense Stationary
with Uncorrelated Scattering (WSSUS). This implies that the tap gains are
uncorrelated [35].
3.5 Rayleigh Fading Channel
The equivalent complex baseband received signal in a multipath channel can be
expressed as:
∑
38
Where , and are the multiplicative gain, phase shift and the delay of the
path, denotes the number of paths is the transmitted signal and is
theAdditive White Gaussian Noise term.
When the path delays are small compared to the symbol duration
and the received signal can be expressed as:
∑
∑
∑
From the above equation we can see that the original transmitted signal is modulated
by a random time varying scale factor . is the in-phase component and
is the quadrature component of the gain. When the number of paths is large we can
use the Central Limit Theorem to show that and are independentGaussian
39
random processes. This type of fading is known as Rayleigh fading as the envelope
of the scale factor follows a Rayleigh distribution shown in Figure 3.4.
Figure 3.4: PDF of Rayleigh Fading Envelope
The phases are uniformly distributed in the interval [ ] and independent for
each path. This type of fading is the most commonly dealt with type of fading in the
literature and is a good model for urban areas where there is no dominant or line-of-
sight path available between the transmitter and the receiver.
Frequency selective channels present opportunities as well as problems. The delay
spread in the channel being comparable or larger than a symbol period causes Inter
Symbol Interference (ISI) and additional complexity in the signal processing is
required at the receiver. On the other hand because the resolvable paths are
40
independent it is unlikely that all of them will be in a deep fade simultaneously. If the
receiver is somehow able to exploit this availability of independent signal paths and
utilize the frequency diversity in the channel it could provide a much more reliable
system than what could be achieved in a flat fading channel without frequency
diversity at the same average signal to noise ratio. This gain is called the diversity
gain achieved by the system and can be measured by the negative slope of the error
probability curve when both the error probability and the signal to noise ratio are in a
logarithmic scale of the same base [44]. There are three common approaches to
extract frequency diversity and mitigate ISI on the frequency selective channel. They
are:
• Single Carrier with Equalization
• Direct-sequence Spread-Spectrum
• Multi-carrier Systems
3.6 Generating Fading (Jakes’ Model)
From the definition of Rayleigh fading given above, it is possible for one to generate
this model by generating two independent Gaussian random variables namely:
. However, sometimes only the amplitude fluctuations are of interest.
Note that this is for link level simulations of wireless communication only. The aim
of generating Rayleigh fading is to produce a signal that has the same Doppler
spectrum shown in Figure 3.2.
Jakes’ model is based on summing sinusoids as defined by the following equations:
41
√ {[ ∑ √
]
[ ∑
√ ]}
⁄ ,
.
From the above development, the fading simulator shown in Figure 3.5 can be
constructed. There are low frequency oscillators with frequency
⁄ where
(
) where is the number of
sinusoids. The amplitudes of the oscillators are all unity except for the oscillator at
frequency which has amplitude √ ⁄ Note that Figure 3.5 implements
except for the scaling factor of √ . It is desirable that the phase of
be uniformly distributed. This can be accomplished using time
averaging described in [43].
42
2sinβ1 2cosβ1
cosω1t
2sinβM 2cosβM
cosωMt
2sinα 2cosα
1/√2cosωmt
++
g(t) = x(t) + jy(t)
y(t)x(t)
Offset oscillators
Figure 3.5: Jakes’ Fading Simulator
3.7 Channel Models
A channel can be modelled by trying to calculate the physical processes which
modify the transmitted signal. Statistically, communication channels are modelled as
a triple consisting of an input alphabet, an output alphabet, and for each pair of input
and output elements a transition probability [10]. A realistic model will be a
combination of both physical and statistical modelling. A typical example is a
wireless channel modelled by a random attenuation (fading) followed by AWGN.
The statistics of the random attenuation are decided by previous measurements or
physical simulations.
In this work, a combination of a noise model (AWGN) and a radio frequency
propagation model is used for the simulations.
43
The power delay profile gives the statistical power distribution of the channel over
time for a signal transmitted for just an instant. Similarly, Doppler power spectrum
gives the statistical power distribution of the channel for a signal transmitted at just
one frequency. While the power delay profile is caused by multipath, the Doppler
spectrum is caused by motion of the intermediate objects in the channel [19].
3.7.1 Tapped-Delay-Line Parameters
There are commonly used empirical channel models available for simulation
purposes. For the purpose of this work, two models are employed in the simulations.
These are: the ITU-A Vehicular test Environment and the Winner Scenario 2.8. In
both cases, the relative delay and the average power are the parameters of concern.
3.7.1.1 ITU-A Vehicular Test Environment
There are six taps in this model; each tap with its corresponding relative delay in (ns)
and average power in (dB). Table 3.1 shows the tapped-delay-line parameters up to
six taps.
Table 3.1: Vehicular test environment, tapped-delay-line parameters[18]
Tap
Index
Relative
Delay
(ns)
Average
Power
(dB)
1 0 0
2 310 -1
3 710 -9
4 1090 -10
5 1730 -15
6 2510 -20
3.7.1.2 Winner Multipath Fading Model
Table 3.2 shows the 20-tap Winner multipath channel model with corresponding
delays and powers.
44
Table 3.2:Winner tapped delay-line model for scenario 2.8 (RS MS, NLOS)
Tap
index
Relative
Delay
(ns)
Average
Power
(dB)
1 0 -1.25
2 10 0
3 40 -0.38
4 60 -0.1
5 85 -0.73
6 110 -0.63
7 135 -1.78
8 165 -4.07
9 190 -5.12
10 220 -6.34
11 245 -7.35
12 270 -8.86
13 300 -10.1
14 325 -10.5
15 350 -11.3
16 375 -12.6
17 405 -13.9
18 430 -14.1
19 460 -15.3
20 485 -16.3
45
Chapter 4
4 ORTHOGONAL FREQUENCY DIVISION
MULTIPLEXING
4.1 Introduction
Although the principle of OFDM has been around for several decades, it was only in
the last two decades that it started to be used in commercial systems [14]. OFDM has
developed into a popular scheme for wideband digital communication, used in
applications such as digital television, audio broadcasting, wireless networking and
broadband internet access. This is as a result of its high data rate transmission
capability with high bandwidth efficiency and its robustness to multi-path delay.
In [11] it was shown that a cellular mobile radio system based on OFDM using pilot
based correction would provide a large improvement in BER performance in a
Rayleigh fading environment. The flexibility and ease of equalization in OFDM
systems has also been one of the driving factors in the introduction of OFDM to the
cellular world.
The two disadvantages associated with OFDM are high Peak to Average Power
Ratio (PAPR) and frequency synchronization issues. A study of any of these
disadvantages would be out of the scope of this thesis. The next section of this
chapter would give some information about multi-carrier modulation which is a
prerequisite for understanding OFDM and OFDMA.
46
4.2 Inter-Symbol (ISI) and Inter-Channel Interference (ICI)
The delay spread can cause inter-symbol interference (ISI) when adjacent data
symbols overlap and interfere with each other due to different delays on different
propagation paths. The number of interfering symbols in a single-carrier modulated
system is given by
[
]
The maximum Doppler spread in mobile radio applications using single-carrier
modulation is typically much less than the distance between adjacent channels, such
that the effect of interference on adjacent channels due to Doppler spread is not a
problem for single-carrier modulated systems. For multi-carrier modulated systems,
the sub-channel spacing can become quite small, such that Doppler effects can
cause significant ICI. As long as all subcarriers are affected by a common Doppler
shift , this Doppler shift canbe compensated for in the receiver and ICI can be
avoided. However, if Doppler spread in the order of several per cent of the subcarrier
spacing occurs, ICI may degrade the system performance significantly [15].
4.3 Multicarrier Modulation
The principle of multi-carrier transmission is to convert a serial high-rate data stream
onto multiple parallel low-rate sub-streams. The motivation for the development of
multicarrier modulation lies in the daunting problem of ISI and the desire for high
data rates. In order to have an ISI-free channel, the symbol rate has to be
significantly larger than the channel delay spread . As a solution to this problem,
multicarrier modulation divides the high-rate transmit stream into lower rate
47
substreams, each of which has a symbol duration of and is hence ISI free.
The number of interfering symbols in a multi-carrier modulated system is given by
[
]
It is obvious from the above relationship that the condition for minimal ISI is a
symbol duration which is significantly larger than the delay spread of the channel.
The individual sub-streams can then be sent over parallel subchannels, maintaining
the total desired data rate. As a correspondence in the frequency domain, the number
of substreams is chosen to ensure that each subchannel has a bandwidth less than the
coherence bandwidth of the channel, so the subchannels experience relatively flat
fading [7].
S/P
R/L bps
R/L bps
R/L bps
+...
cos(2πfc)
cos(2πfc+Δf)
cos(2πfc+(L-1)Δf)
R bps
x(t)
Figure 4.1: A Basic Multicarrier Transmitter
Figure 4.1 depicts a high rate stream of is broken into parallel streams, each
with rate . Each individual stream is then modulated by a respective frequency.
48
In the time domain, the symbol duration on each subcarrier has increased to,
so letting grow larger ensures that the symbol duration exceeds the channel-delay
spread, , which is a requirement for ISI-free communication. In the frequency
domain, the subcarriers have bandwidth , which ensures flat fading, the
frequency domain equivalent to ISI-free communication.
Demod 1
Demod 2
Demod L
LPF
LPF
LPF
P/S R bpsy(t)
...
cos(2πfc)
cos(2πfc+Δf)
cos(2πfc+(L-1)Δf)
Figure 4.2: A Basic Multicarrier Receiver
Figure 4.2 shows the block diagram for the decoder of a multi-carrier system where
each subcarrier is decoded separately, requiring independent demodulators.
4.4 OFDM Basics
OFDM is a frequency-division multiplexing (FDM) scheme and it gets its name from
the fact that the subcarrier frequencies are chosen such that the subcarriers are
orthogonal to each other. The orthogonality allows for efficient modulator and
demodulator implementation using the FFT algorithm on the receiver side, and IFFT
on the sender side [31]. In order to totally get rid of ISI, OFDM employs the use of a
cyclic prefix which increases the length of the symbol period so that it is much
49
greater than the delay spread of the channel. Figure 4.3 shows a block diagram of an
OFDM transmitter.
FEC EncoderConstellation
Mapper
Subcarrier Mapping &
Pilot Insertion
Serial to Parallel
IFFT Add Cyclic Prefix
Binary input Data
Figure 4.3: OFDM Transmitter Block Diagram
On the receiver side, the inverse is done in order to recover the transmitted signal. In
what follows explanations are given for each block in the OFDM transmitter.
4.4.1 FEC Encoder
FEC stands for Forward Error Correction and is a scheme used for the correction of
bit errors caused by the wireless channel. FEC improves the small scale link
performance by adding redundant data bits in the transmitted message so that if an
instantaneous fade occurs in the channel, the data may still be recovered at the
receiver. The traditional role for error-control coding was to make a troublesome
channel acceptable by lowering the frequency of error events. The error events could
be bit errors, message errors, or undetected errors. The addition of FEC or coding to
an OFDM system is essential [14], especially if the transmission bandwidth is large
compared to the coherence bandwidth. Various error-coding methods can be applied
on the incoming bit stream: block codes like Reed Solomon codes and convolutional
codes are the most common ones. Also, a concatenation of a block coder, an
interleaver and a convolutional encoder is often used. Concatenating RS and CC has
the advantage of mitigating the output burst errors that are typical for convolutional
Viterbi decoders [14].
50
4.4.2 QAM Mapper
Once the signal has been coded, it enters the constellation mapper block. All wireless
communication systems use a modulation scheme to map coded bits to a form that can
be effectively transmitted over the communication channel. Thus, the bits are mapped to
a subcarrier amplitude and phase, which is represented by a complex in-phase and
quadrature-phase (IQ) vector. The IQ plot for a modulation scheme shows the
transmitted vector for all data word combinations. Types of digital modulation include
BPSK, QPSK, 16-QAM, etc. The constellation maps for BPSK, QPSK, and 16-QAM
modulations are shown in Figure 4.4.
Figure 4.4: Constellation Maps: (a) BPSK, (b) QPSK and (c) 16-QAM
The constellation mapped data is subsequently modulated onto all allocated data
carriers in order of increasing frequency offset index.
4.4.3 Discrete Fourier Transform
The Fast Fourier Transform (FFT) is an effective algorithm for the implementation
of the DFT. Forward FFT takes a random signal, multiplies it successively by
complex exponentials over the range of frequencies, sums each product and plots the
results as a coefficient of that frequency. The coefficients are called a spectrum and
represent ―how much‖ of that frequency is present in the input signal.
51
FFT can be written in sinusoids as:
∑ (
*
∑ (
*
Here, are coefficients of the sines and cosines of frequency , where is
the index of the frequencies over the frequencies and is the time index. is
the value of the spectrum for the frequency and is the value of the signal at
time . The IFFT takes a frequency spectrum and converts it to a time domain signal
by again successively multiplying it by a range of sinusoids. The equation for an
IFFT is:
∑ (
*
∑ (
*
The IFFT is used to produce a time domain signal, as the symbols obtained after
modulation can be considered the amplitudes of a certain range of sinusoids. This
means that each of the discrete samples before applying the IFFT algorithm
corresponds to an individual subcarrier. Besides ensuring the orthogonality of the
OFDM subcarriers, the IFFT represents also a rapid way for modulating these
subcarriers in parallel, and thus, the use of multiple modulators and demodulators,
which spend a lot of time and resources to perform this operation, is avoided.
4.4.4 The Cyclic Prefix
The key to making OFDM realizable in practice is the use of the FFT algorithm,
which has low complexity. In order for the IFFT/FFT to create an ISI-free channel,
the channel must appear to provide a circular convolution [7]. By having a long
52
symbol period, the robustness of an OFDM transmission against multipath delay
spread can be achieved. Figure 4.5 depicts one way to perform the cited long symbol
period, creating a cyclically extended guard interval where each OFDM symbol is
preceded by a periodic extension of the signal itself. This guard interval that is
actually a copy of the last portion of the data symbol is known as the cyclic prefix
(CP) and thus results in a longer symbol time [7].
XL-v XL-v+1 ... XL-1 X0 X1 X2 ... XL-v-1 XL-v XL-v+1 ... XL-1
Copy and pre-append last v symbols
Figure 4.5: The OFDM Cyclic Prefix
Representing such an OFDM symbol in the time domain as a length vector gives
[ ]
After applying a cyclic prefix of length , the transmitted signal is
[ ]
The cyclic prefix, although elegant and simple, is not entirely free. It comes with
both a bandwidth and power penalty. Since redundant symbols are sent, the
required bandwidth for OFDM increases from to . Similarly, an
additional symbol must be counted against the transmit-power budget [7].
53
There is a trade-off between the length of the cyclic prefix, the bandwidth and the
transmitted energy. However, the length of the cyclic prefix is in the range of one-
fourth to one-sixteenth of the symbol duration. More detail on this in [14].
4.5 Mathematical Description of OFDM
A mathematical treatment of OFDM involves
The Fourier transform
The use of the Fast Fourier Transform in OFDM
The guard interval and its implementation
Figure 4.6: Examples of OFDM Spectrum (a) Five Subcarriers (b) A Single
Subcarrier
Mathematically, each carrier can be described as a complex wave:
( )
The real signal is the real part of . Both and , the amplitude and
phase of the carrier, can vary on a symbol by symbol basis. The values of the
parameters are constant over the symbol duration period .
-8 -6 -4 -2 0 2 4 6 8-0.5
0
0.5
1
1.5
frequency
-8 -6 -4 -2 0 2 4 6 8-0.5
0
0.5
1
1.5
frequency
54
Since OFDM consists of many carriers, the modulated signal, in Figure 4.6
can be represented a:
∑
( )
This is of course a continuous signal. If we consider the waveforms of each
component of the signal over one symbol period, then the variables and
take on fixed values, which depend on the frequency of that particular carrier, and so
can be rewritten:
If the signal is sampled using a sampling frequency of , then the resulting signal
is represented by:
∑
[ ]
At this point, we have restricted the time over which we analyse the signal to
samples. It is convenient to sample over the period of one data symbol. Thus we have
a relationship:
55
If we now simplify , without a loss of generality by letting , then the
signal becomes:
∑
Now can be compared with the general form of the inverse Fourier transform:
∑ (
)
In , the function is no more than a definition of the signal in the
sampled frequency domain, and is the time domain representation. and
are equivalent if:
This is the condition that is required for orthogonality. Thus, one consequence of
maintaining orthogonality is that the OFDM signal can be defined by using Fourier
transform procedures [27].
56
Chapter 5
5 CHANNEL CODING AND DECODING
5.1 Introduction
Channel coding is used extensively in communications field in order to achieve
reliable data transfer, including digital video, mobile communication and satellite
communications. This chapter provides some brief explanation on the encoding and
decoding procedures for convolutional codes which are used in this thesis to asses
coded OFDM performance over fading channels.
5.2 Convolutional Coding
Convolutional codes are a family of error correcting codes which add redundant
information based on the block of data they are processing. Convolutionally
encoding data is basically accomplished using shift registers and associated
combinatorial logic that perform modulo-two addition. A convolutional code is
specified by , in which each information symbol to be encoded
is transformed into an symbol, where is the code rate and the
transformation is a function of the last information symbols, where is the
constraint length of the code [21].
5.2.1 Structure of the Convolutional Code
In simple terms, the structure of a convolutional encoder can be described as follows:
first, (
) boxes are drawn to represent the memory registers then modulo-two
adders to represent the output bits. The memory registers are then connected to the
57
adders using the generator polynomial. As an example, consider a convolutional
encoder specified by , the structure of this coder is shown inFigure 5.1
u1 u0 u-1
+
+
+
v1
v3
v2
(1, 1, 1)
(0, 1, 1)
(1, 0, 1)
u1
Figure 5.1: Convolutional Encoder CC (1, 3, 2)
This is a rate 1/3 code. Each input bit is coded onto 3 output bits. The constraint
length of the code is 2. The three output bits are produced by the 3 modulo-2 adders
by adding up certain bits in the memory registers. The selection of which bits are to
be added to produce the output bit is called the generator polynomial for that output
bit. The polynomials give the code its unique error protection capacity.
5.2.2 States of a Code
In Figure 5.1, the number of combinations of bits in the shaded registers are called
the states of the states of the code and are defined by . The code in our
example has states which are: . Note here that the number of
states is independent of the rate of the code.
58
5.2.3 Trellis Diagram
A convolutional encoder is often seen as a finite state machine. Each state
corresponds to some value of the encoder's register. Given the input bit value, from a
certain state the encoder can move to two other states. These state transitions
constitute a diagram which is called a trellis diagram [1].
Figure 5.2: Example of a Trellis Diagram Adopted from [1]
Each path on the trellis diagram corresponds to a valid sequence from the encoder's
output. Conversely, any valid sequence from the encoder's output can be represented
as a path on the trellis diagram. As an example, Figure 5.2 shows a possible path in
red.
The binary convolutional encoder, which as specified by the Release 1WiMAX standard
has a native rate of and a constraint length of . The generator polynomials used to
derive its two output code bits, denoted and , are specified in the following
expressions:
59
T T T T T T
+ + + +
+ + ++
1 1 1 1 1
11111
0 0
00
X=171o
Y=133o
Figure 5.3: Rate ½ Binary Convolutional Encoder
The block diagram for the binary convolutional encoder that implements the
described code is shown in Figure 5.3.
5.2.4 Decoding
There are several methods of decoding convolutional codes but they are all
categorized into two types:
1) Sequential decoding
Fano algorithm
2) Maximum likelihood decoding
Viterbi decoding
Unlike Viterbi decoding, sequential decoding has the advantage that the decoding
complexity is virtually independent of the code constraint length. For this reason,
sequential decoders are used mainly with very long codes. The main disadvantage of
sequential decoding is the unpredictable decoding latency. The decoding complexity
of Viterbi decoders grows exponentially with the code length, which makes it
suitable only for relatively short codes.
60
5.2.4.1 Viterbi Decoding
This decoder uses Viterbi algorithm for decoding a bit stream that has been encoded
using a convolutional code. It was developed by Andrew J. Viterbi and was
published in an IEEE transaction in 1967 [46]. The use of the Viterbi algorithm for
decoding covolutionally coded data has become very popular since then. According
to [1], the Viterbi algorithm consists of three major parts:
I. Branch matric calculation
Calculation of a distance between the input pair of bits and the four possible
―ideal‖ pairs (―00‖, ―01‖, ―10‖, ―11‖)
II. Path matric calculation
For every encoder state, calculate a metric for the survivor path ending in this
state (a survivor path is a path with the minimum metric).
III. Back Tracing
This step is necessary for hardware implementations that don't store full
information about the survivor paths, but store only one bit decision every
time when one survivor path is selected from the two.
These parts are depicted in Figure 5.4
Branch metric calculation
Path metric calculation
Trackbackencoded stream
decoded stream
Figure 5.4: Viterbi Decoder Data Flow
61
Chapter 6
6 THE WIMAX PHYSICAL LAYER
6.1 Introduction
The physical (PHY) layer of WiMAX was designed with much influence from Wi-
Fi, especially IEEE 802.11a. Although many aspects of the two technologies are
different due to the inherent difference in their purpose and applications, some of
their basic constructs are very similar. The WiMAX physical layer is based on
OFDM. OFDM is the transmission scheme of choice to enable high-speed data,
video, and multimedia communications and is used by a variety of commercial
broadband systems, including DSL, WiFi, Digital Video Broadcast-Handheld (DVB-
H), and MediaFLO, besides WiMAX.
Figure 6.1 shows the functional stages of the WiMAX PHY layer.
Channel Encoder
Symbol Mapper
Subcarrier Allocation
+ Pilot Insertion
IFFT
Cyclic Prefix
Input BitSequence
Figure 6.1: Functional stages of WiMAX PHY
62
The first set of functional stages is related to FEC, and includes channel encoding,
rate matching (puncturing or repeating), interleaving, and symbol mapping. The next
set of stages is related to the construction of the OFDM symbol in the frequency
domain. During this stage, data is mapped onto the appropriate sub-channels and
subcarriers. Pilot symbols are inserted into the pilot subcarriers, which allow the
receiver to estimate and track the channel state information (CSI). This stage is also
responsible for any space/time encoding for transmit diversity or MIMO, if
implemented. The final set of functions is related to the conversion of the OFDM
symbol from the frequency domain to the time domain and eventually to an analogue
signal that can be transmitted over the air.
The rest of this thesis discusses the various mandatory functional stages of the PHY
layer of WiMAX as defined by the IEEE 802.16d/e standards.
6.2 Symbol Mapper
The symbol mapping stage basically refers to a digital modulation scheme which is
used to convert the sequence of binary bits from the convolutional encoder into a
sequence of complex valued symbols. The mandatory constellations according to the
standard are QPSK and 16QAM with an optional 64QAM also defined in the
standard.
Each modulation constellation is scaled by a number c, such that the average
transmitted power is unity, assuming that all symbols are equally likely. The value of
is √ ⁄ , √ ⁄ and √ ⁄ for the QPSK, 16 QAM, and 64 QAM modulations,
respectively.
63
6.3 OFDM Symbol Structure
Each OFDM symbol consists of three types of subcarriers as depicted in Figure 6.2:
1. Data subcarriers: used for carrying data symbols
2. Pilot subcarriers: used for various estimation purposes such as channel
tracking and are known a priori
3. Null subcarriers: this is further divided into two, namely the DC and the
guard subcarriers. These subcarriers have no power allocated to them; the
guard subcarriers have no power allocated to them in order to reduce the
interference with adjacent symbols.
Guard subcarriers
Pilot subcarriers
DC subcarrierGuard subcarriers
Data subcarriers
Figure 6.2: Subcarrier Structure in Frequency
6.3.1 Symbol Parameters
The primitive parameters of an OFDM symbol as defined by the standard are:
Total number of subcarriers or the FFT size
Nominal channel bandwidth,
Oversampling factor,
Ratio of cyclic prefix time to useful symbol time,
64
Table 6.1 shows a summary of these parameters with their possible values for
different scenarios
Table 6.1: Primitive parameters for OFDM symbol
Parameter Value (MHz) Definition
Variable: 1.25,
1.75, 3.5, 5, 7,
8.75, 10, 14, 15
Nominal channel bandwidth
256 for OFDM;
128, 512, 1,024,
2,048 for
SOFDMA
Number of subcarriers, including the DC
subcarrier pilot subcarriers and the guard
subcarriers
8/7, 28/25 Oversampling factor
1/4, 1/8, 1/16,
and 1/32
Ratio of cyclic prefix time to useful symbol
time
The OFDM symbol time duration is given as:
⁄
6.4 OFDMA and Subchannelization
OFDMA consists of assigning one or several subchannels to each user with the
constraint that the subcarrier spacing is equal to the OFDM frequency spacing [15].
A subchannel is defined as a group of subcarriers. Sub-channelization refers to the
process of grouping the subcarriers into subchannels. Various sub-channelization
schemes which have been defined by the WiMAX standard exist. In OFDMA,
subchannels rather than subcarriers are allocated to different users based on some
65
subcarrier permutation schemes which will be discussed later in the chapter. This is
in contrast to OFDM where all the subcarriers are allocated to a single user at a time.
OFDMA can be seen as the multiple access scheme of OFDM.
6.5 Multiple Access Schemes
Multiple access schemes provide ways in which multiple users can access the
channel. The most common way to divide the available dimensions among the
multiple users is through the use of frequency, time, or code division multiplexing. In
Frequency Division Multiple Access (FDMA), each user receives a unique carrier
frequency and bandwidth. In Time Division Multiple Access (TDMA), each user is
given a unique time slot, either on demand or in a fixed rotation. Code Division
Multiple Access (CDMA) systems allow each user to share the bandwidth and time
slots with many other users and rely on orthogonal binary codes to separate out the
users [7].
User 1
User 2
User 3
frequencypower
time
frequencypower
time
frequencypower
time(a) (b) (c)
Figure 6.3: Multiple Access Schemes. (a) FDMA (b) TDMA (c) CDMA
6.6 OFDMA
Like OFDM, OFDMA employs multiple closely spaced subcarriers, but the
subcarriers are divided into groups of subcarriers. Each group is named a subchannel.
The subcarriers that form a subchannel need not be adjacent. In the downlink, a
subchannel may be intended for different receivers. In the uplink, a transmitter may
66
be assigned one or more subchannels. Subchannelization defines subchannels that
can be allocated to mobile stations (MSs) depending on their channel conditions and
data requirements. Using subchannelization, within the same time slot a Mobile
WiMAX Base Station (BS) can allocate more transmit power to MSs with lower
SNR (Signal-to-Noise Ratio), and less power to user devices with higher SNR.
Figure 6.4: OFDMA Transmission. Ref (6)
This is illustrated in Figure 6.4. Subchannelization also enables the BS to allocate
higher power to sub-channels assigned to indoor SSs resulting in better in-building
coverage.
OFDMA is essentially a hybrid of FDMA and TDMA: Users are dynamically
assigned subcarriers (FDMA) in different time slots (TDMA) as depicted in Figure
6.5
67
Figure 6.5: (a) OFDM (b) OFDMA
6.6.1 OFDMA Symbol Structure
The OFDMA symbol structure is similar to that of OFDM. The difference lies in the
fact that subchannels rather than all subcarriers are allocated to users. Since OFDMA
is a multiple access scheme, the data for the various users is contained within a
symbol. Depending on the subcarrier permutation used, subcarriers may be adjacent
or distributed across the available channel bandwidth. Figure 6.6 is a depiction of an
OFDMA symbol showing subcarriers from different subchannels within the same
symbol.
Subchannel 1 Subchannel 2 Subchannel 3 Subchannel 4
DC
Figure 6.6: Subchannels in the Subcarrier Structure
For full OFDMA symbol specification refer to [17].
6.7 Subchannelization in WiMAX
Subchannelization refers to the process of grouping subcarriers to subchannels. The
WiMAX standard defines various types of subchannelization schemes that can be
used both in the up-link and in the down-link. A subchannel, as defined in the IEEE
802.16e-2005 standard, is a logical collection of subcarriers. The number and
distribution of the subcarriers that make up a subchannel depends on the subcarrier
68
permutation that is used. The subcarrier permutations allowed in IEEE 802.16e-2005
are:
Down-link Full Usage of Subcarriers (DLFUSC)
Down-link Partial Usage of Subcarriers (DL PUSC)
Up-link Partial Usage of Subcarriers (UL PUSC)
Tile Usage of Subcarriers (TUSC)
Band Adaptive Modulation and Coding (Band AMC)
The aforementioned subcarrier permutation schemes can be broadly classified
into two categories namely:
1. Distributed subcarrier permutation: the subcarriers are distributed
pseudo-randomly. The advantages of this type of permutation are the
exploration of frequency diversity and interference averaging [29]. On the
other hand, this type of permutation makes channel estimation difficult
since the subcarriers are distributed over the available bandwidth. PUSC,
FUSC and TUSC use the distributed subcarrier permutation.
2. Adjacent subcarrier permutation: in this mode, a subchannel is made
up of subcarriers that are adjacent in the available frequency band. It has
the advantage of easier channel estimation. This mode is used in the band
AMC permutation.
The mandatory permutation modes for up-link and downlink defined by the
WiMAX standard are:
69
PUSC, FUSC and AMC for the downlink
PUSC and AMC for the uplink
The focus of this thesis is on the Downlink PUSC. As a justification notice that in
Figure 6.7the only mandatory part of the frame is the downlink PUSC zone.
Pre
amb
le
PU
SC(f
irst
zo
ne
con
tain
s FC
H a
nd
DL-
MA
P)
PU
SC(D
L-P
erm
Bas
e X
)
FUSC
(DL-
Per
mB
ase
Y)
FUSC
(DL-
Per
mB
ase
Z)
Op
tio
nal
FU
SC
AM
C
TUSC
1
TUSC
2
PU
SC
Op
tio
nal
PU
SC
AM
C
Must appear in every frame
May appear in a frameZone switch Ies in DL-MAP
DL Subframe UL Subframe
Figure 6.7: Illustration of OFDMA Frame with Multiple Zones
6.7.1 DL PUSC
As an introduction to this section, I will start with some basic definitions. The DL
PUSC parameters are tabulated in Table 6.2
Table 6.2: DL PUSC Parameters
Parameter Value
Null Subcarriers 184
Pilot Subcarriers 120
Data Subcarriers 720
Subchannels 30
Slot: this is the minimum possible data allocation unit. It is expressed as number of
subchannels per number of OFDM symbols.
Data Region: it is a two-dimensional rectangular allocation of a group of
subchannels in a group of OFDMA symbols.
70
Segment: the set of available subchannels form a segment. There are three segments
in a frame. The concept of segmentation is used in sectorization where each segment
is allocated to one sector.
Physical Cluster: it is a set of 14 adjacent subcarriers (12 data + 2 Pilot). These
clusters are contiguous in the frequency band.
Logical Cluster: it is formed by renumbering physical clusters according to some
renumbering sequence. Adjacent logical clusters are not contiguous in the frequency
band.
Group: it is a set of logical clusters. Odd numbered groups contain half the number
of logical clusters as compared to even numbered groups. There are six groups in
total.
Perm Base: this has separate meanings in the uplink and the downlink. In the
downlink it is called DL Perm Base and is an integer ranging from 0 to 31. It
identifies the particular BS segment and is specified by the MAC layer.
Inner Permutation: this is the process of forming subchannels from the subcarriers
of the logical clusters of a group.
Outer Permutation: this is the process of renumbering physical clusters to form
logical clusters.
71
0
1
2
3
4
5
6
7
8
9...............
Sub
chan
ne
l In
de
x
Segment 0
Segment 1
Segment 2 .....
OFDM Symbol Index
K+1 K+2 K+3 K+4 K+5 K+6 K+7 K+8 K+9 K+10 K+11
PUSC Slot Data Region
Figure 6.8: Example of an OFDMA DL Frame
The process of allocating subcarriers to subchannels can be summarized as follows:
the guard and dc subcarriers are first removed after which the remaining subcarriers
(data + pilot) are renumbered and partitioned into groups of 14 subcarriers. These
groups are called physical clusters. The physical clusters are then renumbered
according to a renumbering sequence (outer permutation) so that the logical
subcarriers are formed. Pilot positions are marked for even and odd symbols then the
clusters are put into major groups according to the parity of the groups. The
remaining data subcarriers within each group are then renumbered (0 to 143 or 95)
depending on the parity of the groups. The subchannel allocation is done by
allocating subcarriers from each group to subchannels according to a permutation
formula. Figure 6.9 illustrates the steps involved in the permutation process.
72
Step 1: Divide the subcarriers into clusters of 14 subcarriers each
Ph
ysic
al C
lust
er (
PN
) 0
-59
PN 0
PN 1
PN 2
PN 3
PN 4
PN 5
PN 6
PN 7
PN 8
PN 9.......
PN
.59
Step 2: Renumber the clusters. (in this example DL Perm Base=10)
LN 30
LN 33
LN 54
LN 18
LN 10
LN 15
LN 50
LN 51
LN 58
LN 46.......
LN
32
47
Logi
cal C
lust
er (
LN)
Step 3: Gather clusters in six major groups (MG)
LN 0
– L
N 1
1LN
12
– L
N 1
9LN
20
– L
N 3
1..
.
MG 0 (even)
MG 1 (odd)
MG 2 (even)
Step 4: Allocate subcarriers to subchannels
MG X
Allocate pilots in each group
depending on the parity of the symbol
Allocate data subcarriers to
subchannels. No. Of subchannels depends on the parity of each
MG
Figure 6.9: PUSC Subchannel Allocation Procedure
PUSC is best explained in a stepwise manner:
Step 1: All the subcarriers are portioned as right guard band, left guard band, DC,
data and pilot. The data and pilot subcarriers are then grouped into sets of 14
adjacent subcarriers each. Where each set represents a physical cluster. For 1024
point FFT, there are 60 clusters (0-59).
Step 2: logical clusters are formed by renumbering physical clusters using .
Step two is essentially the outer permutation defined earlier in this section.
73
where the Renumbering sequence(j) is the jth
entry of the following vector
{6, 48, 37, 21, 31, 40, 42, 56, 32, 47, 30, 33, 54, 18,
10, 15, 50, 51, 58, 46, 23, 45, 16, 57, 39, 35, 7, 55,
25, 59, 53, 11, 22, 38, 28, 19, 17, 3, 27, 12, 29, 26,
5, 41, 49, 44, 9, 8, 1, 13, 36, 14, 43, 2, 20, 24, 52,
4, 34, 0}
Step 3: the logical clusters are gathered to form six major groups (numbered 0-5).
Even numbered groups (0, 2 and 4) contain 12 logical clusters each; while odd
numbered groups (1, 3 and 4) contain 6 logical clusters.
Step 4: pilot subcarriers are separated from the data subcarriers in this step. The
position of the pilot subcarriers depends on if the OFDM symbol is odd or even as
seen in Figure 6.10.
P P
P P
P Pilot subcarrier
Data subcarrier
Even OFDMA Symbol
Odd OFDMA Symbol
Figure 6.10: PUSC DL Slot
74
Step 5: this is the final step of the subcarrier allocation. After marking the pilot
subcarrier positions, the remaining data subcarriers are numbered from 0 to 143 or 95
depending on the parity of the major group. Subcarrier allocation is done using ,
it is worth noting however that the formula is only applied to subcarriers of a major
group.
{ [ ]
}
where is the number of subchannels in the particular major group, equal
to 4 or 6, depending on the parity of the major group; is the
subcarrier index of subcarrier varying between 0 and 23, in subchannel whose
value ranges between 0 and 143 or 95. is the subchannel index varying between 0
and 29.
where is the number of data subcarriers allocated to a subchannel in
each OFDMA symbol; [ ] is the series obtained by rotating the basic permutation
sequence (Table 6.3) cyclically to the left times.
Table 6.3: Permutation sequence
Permutation Base Sequence Major Group Parity
4 [3 0 2 1] Even
6 [3 2 0 4 5 1] odd
75
For a 1024 point FFT, there are: 1024 subcarriers, 184 null subcarriers (92 + 91 + 1),
120 pilot subcarriers and 720 data subcarriers. As an example, I will use DL PUSC
permutation to find the 24 physical (data) subcarriers of subchannel 16.
Table 6.4: Parameters for DL PUSC example
Parameter Value
DL PermBase 10
OFDMA Symbol Odd
Major Group 16
Permutation Sequence [3 0 2 1]
4
The correspondence between the logical number and the physical number of the
clusters is depicted in Table 6.5. The table also shows a correspondence between the
logical subcarrier index and the original physical subcarrier index.
Table 6.5: Cluster numbering(Major Group 3, DL PermBase = 10)
Cluster
LN
Logical Subcarrier
Index
Cluster PN
Equation (6.1)
Cluster Physical
Subcarrier Index
32 302-315 5 162-175
33 288-301 41 666-679
34 92-105 49 778-791
35 372-385 44 708-721
36 736-749 9 218-231
37 904-917 8 204-217
38 554-567 1 106-119
39 890-903 13 274-287
76
Table 6.6 depicts the logical subcarrier indexes with respect to the subchannel;
values of , logical subcarrier index in the major group. The last column in the table
shows the original physical subcarrier indexes with respect to the absolute subcarrier
scale (0-1024).
Table 6.6: Subcarrier Allocation
Logical
Subcarrier
Index in
subchannel 16
Logical subcarrier index in the
major group
Physical
subcarrier index
with respect to
the absolute
subcarrier scale
0 16 100 199
1 17 104 862
2 18 111 864
3 19 119 871
4 20 121 873
5 21 126 444
6 22 136 446
7 23 140 453
8 0 3 317
9 1 11 322
10 2 13 324
11 3 18 891
12 4 28 893
13 5 32 898
14 6 39 900
15 7 47 641
16 8 49 643
17 9 54 648
18 10 64 650
19 11 68 587
20 12 75 589
21 13 83 594
22 14 85 190
23 15 90 197
6.8 OFDMA Frame
In IEEE 802.16e-2005, both frequency division duplexing and time division
duplexing are allowed. In the case of FDD, the uplink and downlink sub-frames are
transmitted simultaneously on different carrier frequencies; in the case of TDD, the
77
uplink and downlink sub-frames are transmitted on the same carrier frequency at
different times. Figure 6.11 shows the frame structure for TDD [7]. Each DL sub-
frame and UL sub-frame in IEEE 802.16e-2005 is divided into various zones, each
using a different subcarrier permutation scheme as shown in Figure 6.7. The relevant
information about the starting position and the duration of the various zones being
used in a UL and DL sub-frame is provided by control messages in the beginning of
each DL sub frame.
DL
Fram
e P
ream
ble
FCH
DL-
MA
P
DL-
MA
PU
L-M
AP
DL
Bu
rst1
DL
Bu
rst
4D
L B
urs
t 5
DL
Bu
rst
3
DL
Bu
rst
2
UL
Bu
rst
1
UL
Bu
rst
3U
L B
urs
t 4
UL
Bu
rst
2
Ranging Subchannels
DL Subframe UL Subframe
Sub
chan
nel
s
OFDMA Symbols
TTG
k k+1 k+3 . . . k+30 . ..
Figure 6.11: TDD Frame Structure
The first OFDM symbol in the downlink sub-frame is used for transmitting the
downlink preamble. The downlink preamble is mainly used for time and frequency
synchronization and channel estimation. Following the preamble, occupied by the
initial subchannels is the Frame Correction Header (FCH). The FCH is used for
carrying system control information such as the subchannels used for ranging, the
length of the DL-MAP message and the subcarriers used (in case of segmentation).
After the FCH come the DL-MAP and UL-MAP messages respectively. They
78
specify the data regions of the various users in the DL and UL sub-frames of the
current frame. By listening to these messages, each SS can identify the subchannels
and the OFDM symbols allocated in the DL and UL for its use [7]. The gap between
the downlink and uplink sub-frames is called the Transmit Transition Gap (TTG).
6.8.1 OFDMA Frame Parameters
Table 6.7shows that at 10MHz, the OFDMA symbol time is 102.9 microseconds and
so there are 48 symbols in a 5 millisecond frame. Of these, 1 symbol is used for TTG
and RTG leaving 47 symbols. If of these are used for DL, then are
available for UL. The sub-division of the UL and DL sub-frames is done according
to the DL/UL ratio. The standard defines various ratios but for the purpose of this
study, a ratio is used. In the DL sub-frame, the overhead consists of preamble,
FCH, DL-MAP and UL-MAP [40]. The rest of the OFDMA symbols in the frame
are used to carry the data of the users. Table 6.7 shows specific values of the
parameters discussed.
Table 6.7: TDD OFDMA frame parameters
Parameters Values
Channel Bandwidth 10 MHz
Frame duration 5 ms
Number of OFDMA Symbols/Frame 48
Total Number of OFDMA Overhead
Symbols 10
Number of OFDMA symbols for TTG
and RTG 1
Total Number of OFDMA Data
Symbols 37
Symbol Duration 102.9 μs
DL:UL
3:1
DL OFDMA Data Symbols 28
UL OFDMA Data Symbols 9
79
6.8.2 Data Burst Formation via Vertical Mapping
The data burst(s) in the PUSC zone of the DL sub-frame is formed by allocating slot
by slot downwards across the subcarriers of the subchannels until all the subchannels
of that time period are filled; then the same process is repeated for the adjacent
OFDMA symbols until the entire DL sub-frame fills up or when the data is
exhausted. The allocation is done user by user so that each user’s data is contained
within that user’s data burst. Figure 6.12 depicts this process.
0
1
2
3
4
5
6
7
8
9...........
Sub
chan
ne
l In
de
x
Segment 0
Segment 1
OFDM Symbol Index
Figure 6.12: Data Burst Formation
This is illustrated with an example. Assume that there are four users with data
symbols from the QAM mapper that are to fit into segment 0 of the DL sub-frame.
There are 10 subchannels in each segment and assume the length of the DL sub-
frame data region is 10 OFDMA symbols (5 slots). Each slot will contain 56 bits of
data. The data regions of the DL sub-frame is depicted in Figure 6.13
80
OFDMA Symbols
0
1
2
3
4
5
6
7
8
9
Sub
chan
nel
s
User 1User 2User 3User 4
Slot
Time interval
Data exhausted
Figure 6.13: Data Region Showing Data Bursts for Four Users
81
Chapter 7
7 UN-CODED vs. CODED OFDM PERFORMANCE
OVER MULTIPATH FADING CHANNELS
7.1 Introduction
Link level (LL) simulations, model the behaviour of a link over a short period of
time and usually involve modelling parts of the physical layer and some aspects of
the MAC layer. The simulations are then used to arrive at theoretical results that
model the behaviour of the single link under given channel conditions. The results
are generally presented in terms of Bit Error Rate (BER) as a function of the Signal
to Noise Ratio (SNR).
The aim of this chapter is to show the BER performance of un-coded and coded
OFDM over non-fading and fading channels. The two fading channel models used
are the Winner Scenario 2.8 channel model and the ITU-A Vehicular channel. The
Winner channel model has been structured for indoor and outdoor environments for
the 5 GHz frequency range. The Winner model is based on the widely accepted
modelling approach presented in [53]. Another commonly used set of empirical
channel models is that specified in ITU-R recommendation [18]. The
recommendation specifies three different test environments: indoor office, outdoor to
indoor pedestrian and vehicular to high antenna. Since the delay spread can vary
significantly, the recommendation specifies two different delay spreads for each test
environment: low delay spread (ITU-A) and medium delay spread (ITU-B). In all
82
there are 6 different scenarios and for each of these cases, a multipath tap delay
profile is specified [18].
In all ITU channel models each multipath component is modelled as an independent
Rayleigh fading, and the correlation in the time domain is due to the Doppler shift
that is related to the speed the mobile is moving with.
7.2 Simulation of OFDM
The parameters used in the simulation of OFDM in this thesis are summarized in
Table 7.1. The performance of OFDM was simulated over different channel
conditions. The effect of the speed of the receiver was also taken into consideration
while simulating under multipath conditions.
Table 7.1: OFDM Simulation Parameters
Parameter Value
FFT Size 1024
Constellation Mapping QPSK
Symbol Duration 102.4µs
Length of Cyclic Prefix 1/8 of Symbol duration (12.8µs)
Channel Coding R ½ CC and Viterbi Decoding
Multipath Channels ITU A Vehicular Channel
Winner Channel (Scenario 2.8)
Theoretical and simulated results were compared in order to show conformance of
the simulation to already developed theory. The BER is the number of received bits
that have been altered due to noise, interference and distortion, divided by the total
number of transferred bits during a studied time interval [8]. The BER is expressed
as a function of the normalized carrier-to-noise ratio measure denoted ,
(energy per bit to noise power spectral density ratio).
83
7.2.1 Un-coded OFDM over AWGN Channel
The performance of OFDM-QPSK over an AWGN channels is shown in Figure 7.1.
The graph shows the theoretical as well as the experimental performance of the
system plotted as BER against . For QPSK the theoretical BER is given by:
√ ⁄
Figure 7.1: OFDM Performance over AWGN Channel
Figure 7.1 shows conformity of the simulated performance to the theoretically
obtained BER performance in terms of the shape of the curves. The observed SNR
loss of approximately in the experimental BER curve is as a result of the
cyclic prefix introduced by OFDM [14]. The SNR loss is given by:
(
*
where denotes the length of the cyclic prefix and is the length of
the transmitted symbol. With μ and μ , using equation (7.2)
0 1 2 3 4 5 6 7 8 9 1010
-6
10-5
10-4
10-3
10-2
10-1
Eb/N0 (dB)
BE
R
Performance of QPSK modulated OFDM Transmission over an AWGN Channel
experimental ber
theoretical ber
84
is found to be . This loss is uniform throughout the performance
and can be seen as one of the costs of OFDM. For a longer cyclic prefix, it is
expected that there will be a greater . The result obtained is in parallel to
what is shown on page 45 of [14].
7.2.2 Coded OFDM over AWGN Channel
In this simulation FEC is added to the system in order to improve its performance.
The data is coded using a rate ½ convolutional encoder with a constraint length of 7
and decoded using a corresponding Viterbi decoder with a track back length of 32
(approximately 5 times the constraint length).
Figure 7.2: Coded OFDM Performance over AWGN Channel
The simulation was repeated 300 times for 15 OFDM symbols and the results were
averaged. Note from Figure 7.2 that, the system BER performance for coded QPSK-
OFDM would reach a much lower bit error rate at an earlier (lower) signal to noise
0 1 2 3 4 5 6 7 8 9 1010
-6
10-5
10-4
10-3
10-2
10-1
100
Eb/N0 (dB)
BE
R
Performance of Rate 1/2 Convolutionally Coded OFDM-QPSK OFDM Transmission over
an AWGN Channel
Coded BER
Uncoded BER
85
ratio. For example the coded system will achieve a target BER of at
whereas the un-coded system will achieve the same BER at .
In order to have a lower BER one must further increase the number of bits or
symbols in the frame to transmit. However, since is a good BER, this was not
done in this study. The significant reduction in the SNR in order to achieve a
required BER is known as coding gain [20]. Figure 7.2 shows that with convolutional
coding of rate R = ½ and constraint length of k = 7, at a BER of there is a
coding gain of over the un-coded performance.
7.2.3 Un-coded OFDM over Multipath Rayleigh Fading Channels
Again OFDM-QPSK was simulated and presented as BER as a function of the SNR.
It was expected that in a multipath channel, the performance of the system as
compared to that in an AWGN channel would be worse. Since in a real life situation
the receiver is mobile, mobility was also taken into consideration. The relationship
between Doppler frequency and velocity was used for this purpose. The simulations
in this section compare the performance of the system in the Winner and ITU-R
specified (ITU-Vehicular A) channel models at different Doppler frequencies while
using theoretical results as a benchmark. The performance was observed to degrade
with increasing Doppler frequencies. The simulated BER is in close conformity with
theoretical results obtained in [14]. The plots in Figure 7.3 show what is obtainable
theoretically for various speeds of the receiver. Even though the profile information
is not specified in [14], it has been made clear that the channel taps are
approximately Rayleigh distributed. Thus, equation (7.3) and the curve obtained by
equation (7.6) have been used for comparison. In a multipath Rayleigh fading
channel, the probability of symbol error is given by:
86
√
(
√
)
where
is the signal to noise ratio, is the maximum Doppler frequency and is the
subcarrier frequency spacing. According to page 84 of [52], the relationship between
Symbol Error Rate (SER) and Bit Error Rate (BER) is:
where is the number of bits per symbol. For QPSK, there are two bits per symbol.
Therefore equation (7.5) becomes:
Then the BER is:
The expression for the probability of bit error can therefore be derived from equation
(7.3) and is given by:
(
√
(
√
))
For detailed development of equation (7.3), refer to [14].
For multipath channels which have high delay spreads compared to the symbol
duration, the channel coefficients might not be constant over neighbouring
subcarriers. Therefore, the orthogonality of adjacent subcarriers is no longer
87
preserved. This causes an error floor in the BER performance due to the interference
which comes from the neighbouring symbols. The resulting inter-symbol
interference creates an irreducible error floor which is clearly visible in the curves of
Figure 7.3.
Figure 7.3: Theoretical Un-coded OFDM Performance over
Rayleigh Multipath Fading Channel
In the following sections, a comparison was made between two channel models: ITU
Vehicular-A channel model and the Winner Scenario 2.8 channel model. The Winner
channel models were developed before the ITU channel models and they were used
for evaluation of 3G systems but the ITU channels which present more adverse
conditions were developed in order to be used for evaluation of IMT-Advance
systems (4G). It was expected that the performance of OFDM will be better in the
Winner channel when compared to the ITU channel because the ITU channel has
larger relative delays.
0 5 10 15 20 25 30 35 4010
-5
10-4
10-3
10-2
10-1
100
Eb/N0 (dB)
BE
R
Theoretical OFDM-QPSK performance over
a multipath Rayleigh fading channel
fd = 100 Hz
fd = 400 Hz
fd = 833 Hz
88
The OFDM parameters specified in Table 7.1 earlier were used for simulating both
channels. Each simulation was repeated 3000 times for 15 OFDM symbols and the
results were averaged. For fair comparison of the performance between the two
channels, the same length of cyclic prefix was used.
7.2.3.1 Un-coded OFDM performance over Winner Scenario 2.8 Channel
Using the relative delay and power values in Table 7.2 and Jakes sum of sinusoids
model earlier discussed in section 3.6, the following performance curves were
obtained for Doppler shifts of , and corresponding to speeds
of approximately , and respectively. Again it is in
agreement (in terms of the shape of the curve) with the performance curves obtained
after plotting equation (7.8).
Table 7.2: Winner scenario 2.8 channel
Tap
index
Relative
Delay
(ns)
Average
Power
(dB)
1 0 -1.25
2 10 0
3 40 -0.38
4 60 -0.1
5 85 -0.73
6 110 -0.63
7 135 -1.78
8 165 -4.07
9 190 -5.12
10 220 -6.34
11 245 -7.35
12 270 -8.86
13 300 -10.1
14 325 -10.5
15 350 -11.3
16 375 -12.6
17 405 -13.9
18 430 -14.1
19 460 -15.3
20 485 -16.3
89
Figure 7.4: Un-coded OFDM Performance over Rayleigh Multipath Fading Channel
(Winner Scenario 2.8 Channel)
7.2.3.2 Un-coded OFDM performance over ITU Vehicular-A Channel
The same Doppler shifts and thus MS velocities were used with the delay and power
values in Table 7.3 to obtain the performance curves in Figure 7.5.
Table 7.3: ITU Vehicular-A channel parameters
Tap
Index
Relative
Delay
(ns)
Average
Power
(dB)
1 0 0
2 310 -1
3 710 -9
4 1090 -10
5 1730 -15
6 2510 -20
It is clear from the performance curves of Figure 7.5 that as the velocity of the MS
increases so does the Doppler shift. The observed error floors for the various Doppler
0 5 10 15 20 25 30 35 40
10-4
10-3
10-2
10-1
100
Eb/N0 (dB)
BE
R
Performance of Uncoded OFDM-QPSK Transmission over
Winner Sc. 2.8 multipath Rayleigh fading Channel
fd = 100Hz
fd = 400Hz
fd = 833Hz
fd = 100 Hz
fd = 400 Hz
fd = 833 Hz
Theoretical
Experimental
90
frequencies also increased with the Doppler frequency as expected. Therefore, for a
mobile observing a maximum Doppler shift of , the error floor is significantly
lower than that of one observing a Doppler shift of . This is attributed to the
fact that communication is more reliable when there is no relative motion between
the transmitter and the receiver.
Figure 7.5: Un-coded OFDM Performance over Rayleigh Multipath Fading Channel
(ITU Vehicular-A)
From the performance curves in Figure 7.6, it can be seen that OFDM has a better
performance in the Winner Scenario 2.8 channel than in the ITU-Vehicular A
channel. This is as a result of the large delays found in the ITU-Vehicular A channel:
the maximum delay in the ITU-Vehicular A channel is while that in the
Winner Scenario 2.8 channel is . This observation has also shown that the
error floor is dependent on the delay spread of the channel. For a Doppler frequency
of , the error floor of the OFDM performance over the Winner channel started
0 5 10 15 20 25 30 35 40
10-4
10-3
10-2
10-1
100
Eb/N0 (dB)
BE
R
Performance of Uncoded OFDM-QPSK Transmission over
ITU-Vehicular A multipath Rayleigh FadingChannel
fd = 100Hz
fd = 400Hz
fd = 833Hz
fd = 100 Hz
fd = 400 Hz
fd = 833 Hz
Experimental
Theoretical
91
to form at while for the ITU channel the error floor formation started at
.
Figure 7.6: Un-coded OFDM Performance over Rayleigh Multipath Fading Channels
(ITU-Vehicular A and Winner Scenario 2.8)
7.2.4 Coded OFDM over Multipath Rayleigh Fading Channel
As indicated in [14], coding is essential in order to mitigate the effects of the
multipath channel in a wireless OFDM transmission. The same convolutional code
configuration was applied to analyse the improved performance of the system in a
fading channel. The channel used for this simulation is the ITU-Vehicular A channel
(parameters are shown in Table 7.3) and a Doppler frequency of
(corresponds to speed of ).
0 5 10 15 20 25 30 35 40
10-4
10-3
10-2
10-1
100
Eb/N0 (dB)
BE
R
Performance of Uncoded OFDM-QPSK Transmission over
ITU-Vehicular A and Winner Scenario 2.8 Multipath Rayleigh Fading Channels
fd = 833 Hz
fd = 400 Hz
fd = 100 Hz
fd = 100Hz
fd = 400Hz
fd = 833Hz
ITU-Vehicular A Channel
Winner Scenario 2.8 Channel
92
Figure 7.7: Coded and Un-coded OFDM-QPSK over a Multipath
Rayleigh Fading Channel (ITU-Vehicular A)
Figure 7.7 shows the improved performance of the system when coding was applied.
Notice that the error floor of the transmission was lowered from about to
around as a result of coding. There would be significant improvement in
the performance if the convolutional encoder is concatenated with a Reed Solomon
encoder; where the Reed Solomon encoder is an outer encoder and the convolutional
encoder will be an inner encoder.
0 5 10 15 20 25 30 35 4010
-4
10-3
10-2
10-1
100
Eb/N0 (dB)
BE
R
Performance of Uncoded and Rate 1/2 Convolutionally Coded OFDM-QPSK transmission over
ITU-Vehicular A multipath Rayleigh fading Channel (fd = 400 Hz)
Coded BER
Uncoded BER
93
Chapter 8
8 CONCLUSION AND FUTURE WORK
8.1 Conclusion
Although a complete system level simulation is beyond the scope of this thesis, a
comprehensive study and analysis of the mandatory parts of the PHY layer of IEEE
802.16e was carried out. Particular attention was paid to OFDM, OFDMA,
convolutional coding and Viterbi decoding and the structure of the DL frame of the
standard. The performance of the system seen in Chapter 7 agrees with theoretical
results and I have shown in my simulations that improvement can be made by the
inclusion of FEC in the system.
Real life channel model parameters were used for the simulation in order to obtain
realistic performance figures. An error floor of about was obtained for OFDM-
QPSK transmission in a multipath Rayleigh fading channel. The small scale fading
was heralded by the use of sums of sinusoids in Jakes’ fading channel model.
A successful simulation of the DL PUSC permutation in Chapter 6 showed how
frequency diversity is exploited in the DL OFDMA frame. This permutation and
allocation of subcarriers to users within the same frame is the main distinguishing
factor between OFDM and OFDMA. The various building blocks of the system,
when put together for the mandatory parts of the PHY layer of WiMAX.
94
8.2 Future Work
8.2.1 Interleaved Codes
It is possible to improve the performance of the FEC scheme by interleaving and
puncturing the coded data before sending it to the constellation mapper. The
interleaver serves to reduce the correlation between the fades experienced by
successive source symbols that are transmitted over the channel (Stüber, 2002).
8.2.2 MIMO
The addition of multiple antennas to both the transmitter and receiver has proved to
improve performance of OFDM and OFDMA systems. This can be done by
incorporating Space Time or Frequency Block coding (SFBC or STBC) to the
system. Space time block coding has emerged as an efficient means of achieving
near optimal transmitter diversity gain [6]. SFBC outperforms STBC in a fast fading
channel as seen in [24].
8.2.3 IEEE 802.16m
The IEEE 802.16m will build upon the existing IEEE 802.16e standard technology.
It promises to deliver higher data rates and a generally better performance than the
present standard. A 2048 FFT size and a nominal channel bandwidth of 20 MHz will
be used in this system. It also has support for scalability and multiple antennas at
both the transmitter and receiver. It is expected to compete with the 3GPP LTE
technology (4G) with data rates of up to 100 Mbit/s for mobile and 1 Gbps for fixed
applications.
95
REFERENCES
[1] 1-CORE Technologies. (2008, November). Viterbi Algorithm for Decoding
of Convolutional Codes. Retrieved May 2010, from 1-Core Technologies:
http://www.1-core.com/library/comm/viterbi/viterbi.pdf
[2] 3GPP. (n.d.). Retrieved April 2010, from Wikipedia: http://en.wiki-
pedia.org/wiki/3GPP
[3] 3GPP - About 3GPP. (n.d.). (European Telecommunications Standards
Institute (ETSI)) Retrieved April 2010, from 3GPP: http://www.3gpp-
.org/About-3GPP
[4] Joseph, B. (n.d.). Channel Model. Brian Joseph - Home. Retrieved April 15,
2010, from http://www.brianjoseph.com/viterbi/doc/chanmodel.html
[5] Ahmad, I.; Habibi, D.;, "A proactive forward error control scheme for mobile
WiMAX communication," Communication Systems, 2008. ICCS 2008. 11th
IEEE Singapore International Conference on, vol., no., pp.1647-1649, 19-21
Nov. 2008.
[6] Alamouti, S.M.;, "A simple transmit diversity technique for wireless
communications," Selected Areas in Communications, IEEE Journal on ,
vol.16, no.8, pp.1451-1458, Oct 1998.
96
[7] Andrews, J. G., Ghosh, A., & Muhamed, R. ―Fundamentals of WiMAX:
Understanding Broadband Wireless Networking (Prentice Hall
Communications Engineering and Emerging Technologies Series)‖, Upper
Saddle River: Prentice Hall PTR, 2007.
[8] Bit error rate. (2010). Retrieved May 2010, from Wikipedia:
http://en.wikipedia.org/wiki/Bit_error_rate
[9] CDMA Development Group. (2010). CDG: Technology: CDMA2000.
(CDMA Development Group) Retrieved April 2010, from CDMA
Development Group: http://www.cdg.org/technology/cdma2000.asp
[10] Channel (Communications). (2010). Retrieved May 2010, from Wikipedia:
http://en.wikipedia.org/wiki/Channel_(communications)#Channel_models
[11] Cimini, L., Jr.;, "Analysis and Simulation of a Digital Mobile Channel Using
Orthogonal Frequency Division Multiplexing, “Communications, IEEE
Transactions on, vol.33, no.7, pp. 665- 675, Jul 1985.
[12] Dent, P.; Bottomley, G.E.; Croft, T.;, "Jakes fading model
revisited," Electronics Letters, vol.29, no.13, pp.1162-1163, 24 June 1993.
[13] Eira, J. P., & Rodrigues, A. J., ―Analysis of WiMAX Data Rate Performance‖
Lisbon: Instituto de Telecomunicações/Instituto Superior Técnico, Technical
University of Lisbon, April 2009.
97
[14] Engels, M., ―Wireless OFDM Systems: How to make them work? (The
Springer International Series in Engineering and Computer Science)‖. New
York: Springer, July 2002.
[15] Fazel, K., & Kaiser, S. ―Multi-Carrier and Spread Spectrum Systems 2nd
Edition‖, New York, NY: Wiley, November 2008
[16] IEEE 802.11. (2008). Retrieved April 2010, from Wikipedia:
http://en.wikipedia.org/wiki/IEEE_802.11
[17] "IEEE Standard for Local and Metropolitan Area Networks Part 16: Air
Interface for Fixed and Mobile Broadband Wireless Access Systems
Amendment 2: Physical and Medium Access Control Layers for Combined
Fixed and Mobile Operation in Licensed Bands and Corrigendum 1," IEEE
Std 802.16e-2005 and IEEE Std 802.16-2004/Cor 1-2005 (Amendment and
Corrigendum to IEEE Std 802.16-2004), vol., no., pp.0_1-822, 2006.
[18] ITU-R Recommendation M.1225, ―Guidelines for evaluation of radio
transmission‖, 1997
[19] Jain, R. ―Channel Models: A Tutorial‖, 2007
[20] Jay M. & Jacobsmeyer, P. ―Introduction to Error-Control Coding‖, Pericle
Communications Company, 1996.
98
[21] Khan, M.N.; Ghauri, S.;, "The WiMAX 802.16e physical layer
model," Wireless, Mobile and Multimedia Networks, 2008. IET International
Conference on, vol., no., pp.117-120, 11-12 Jan. 2008.
[22] Kumar, A. ―Mobile Broadcasting with WiMAX: Principles, Technology, and
Applications (Focal Press Media Technology Professional Series)‖.
Amsterdam: Focal Press, 2008.
[23] Langton, C., (2002). Tutorials in Digital Communications. Retrieved May
2010, from Complex2Real: http://complextoreal.com/chapters/convo.pdf
[24] Lee, K. F., ―Space-Time and Space-Frequency Coded OFDM Transmitter
Diversity Techniques‖, Georgia: Georgia Institute of Technology CSIP, 2000.
[25] Mach, P.; Bestak, R., "WiMAX Performance Evaluation," Networking, 2007.
ICN '07. Sixth International Conference on, vol., no., pp.17-17, 22-28 April
2007.
[26] Marks, R., ―IEEE 802.16 Wireless MAN Standard: Myths and Facts‖,
Washington, DC: IEEE, 2006.
[27] Matic, D. (1999). Mathematical Description of OFDM. Retrieved May 2010,
from Wireless Communication: http://www.wirelesscommunication.nl/-
reference/chaptr05/ofdm/ofdmmath.htm
99
[28] Multipath Propagation. (2009, February). Retrieved May 2010, from
Wikipedia: http://en.wikipedia.org/wiki/Multipath_propagation
[29] Nuaymi, L., ―WiMAX: Technology for Broadband Wireless Access‖, New
York, NY: Wiley, 2007.
[30] Nystedt, D. (2009, March). ―Intel Sees 2012 Deployment for Mobile
WiMAX Release 2‖, Retrieved April 2010, from PC World:
http://www.pcworld.com/businesscenter/article/191059/intel_sees_2012_depl
oyment_for_mobile_wimax_release_2.html%20by%20Dan%20Nystedt
[31] Orthogonal Frequency Division Multiplex Over Copper Wire Versus Fiber |
Sea Technology | Find Articles at BNET. (n.d.). Find Articles at BNET |
News Articles, Magazine Back Issues & Reference Articles on All Topics.
Retrieved April 16, 2010, from http://findarticles.com/p/articles/mi-
_qa5367/is_200905/ai_n32127700/?tag=content;col1
[32] Poole, I. (Ed.). (n.d.). 3G LTE Tutorial - 3GPP Long Term Evolution. (Adrio
Communications Ltd) Retrieved April 2010, from Radio-Electronics.com:
http://www.radioelectronics.com/info/cellulartelecomms/ltelongtermevolutio
n/3gltebasics.php
100
[33] Pop, M.F.; Beaulieu, N.C., "Limitations of sum-of-sinusoids fading channel
simulators," Communications, IEEE Transactions on, vol.49, no.4, pp.699-
708, Apr 2001.
[34] Pop, M.F.; Beaulieu, N.C., "Statistical investigation of sum-of-sinusoids
fading channel simulators," Global Telecommunications Conference, 1999.
GLOBECOM ‘99, vol.1A, no., pp.419-426 vol. 1a, 1999.
[35] Rajkumar, S. PhD. Dissertation, ―Modelling Of Multipath Fading Channels
For Network Simulation‖, Texas: Texas A&M University,2007
[36] Rappaport, T., & Rappaport, T. S., ―Wireless Communications: Principles
and Practice (2nd Edition)‖, (2 ed.), Upper Saddle River: Prentice Hall PTR,
2001.
[37] Rohling, H.; Grunheid, R., "Performance of an OFDM-TDMA mobile
communication system," Vehicular Technology Conference, 1996. 'Mobile
Technology for the Human Race’, IEEE 46th, vol.3, no., pp.1589-1593 vol.3,
28 Apr-1 May 1996.
[38] Samiseppo Aarnikoivu, J. W. (2006, May). Retrieved April 2010, from Merit:
http://www.merit.org.uk/kbasedocs/0002%20Mobile%20Broadband%20Wire
less%20Access.pdf
[39] Sharma, P. (2009, June). WiMAX in detail. Retrieved April 2010, from
TechPluto: http://www.techpluto.com/wimax-in-detail/
101
[40] So-In C., Jain R., & Tamimi A-K., ―Capacity Evaluation for IEEE 802.16e
Mobile WiMAX,‖ Journal of Computer Systems, Networks, and
Communications, vol. 2010, Article ID 279807, 12 pages, 2010.
[41] Sorensen, T.B.; Mogensen, P.E.; Frederiksen, F., "Extension of the ITU
channel models for wideband (OFDM) systems, "Vehicular Technology
Conference, 2005. VTC-2005-Fall. 2005 IEEE 62nd, vol.1, no., pp. 392- 396,
28-25 Sept., 2005.
[42] Stewart, B.G.; Vallavaraj, A., "BER Performance Evaluation of Tail-Biting
Convolution Coding Applied to Companded QPSK Mobile
WiMAX," Parallel and Distributed Systems (ICPADS), 2009 15th
International Conference on, vol., no., pp.734-739, 8-11 Dec. 2009.
[43] Stüber, G. L, ―Principles of Mobile Communication‖, New York: Springer,
2002.
[44] Tse, D., & Viswanath, P., ―Fundamentals of Wireless Communications‖,
New York, NY: Cambridge University Press, 2005.
[45] University of Plymouth. (n.d.). CDMA Introduction. Retrieved April 2010,
from http://www.tech.plym.ac.uk/see/research/CDMA/CDMAIntro.htm
102
[46] Viterbi, A.; , "Error bounds for convolutional codes and an asymptotically
optimum decoding algorithm," Information Theory, IEEE Transactions on ,
vol.13, no.2, pp. 260- 269, Apr 1967
[47] Viterbi, A. J, ―CDMA: Principles of Spread Spectrum Communication‖
Upper Saddle River: Prentice Hall PTR, 1995
[48] Sharma, P. (2009, June 20). WiMAX in detail | TechPluto. Startup reviews,
Tech news, tech events, tech tips. Retrieved June 15, 2010, from
http://www.techpluto.com/wimax-in-detail/
[49] Anderson, H. R., ―Fixed Broadband Wireless System Design‖. New York,
NY: Wiley,2003
[50] Wireless Network. (2010). Retrieved April 2010, from Wikipedia:
http://en.wikipedia.org/wiki/Wireless_network
[51] Zhang, J.A.; Lin Luo; Zhenning Shi, "Quadrature OFDMA systems based on
layered FFT structure," Communications, IEEE Transactions on , vol.57,
no.3, pp.850-860, March 2009.
[52] Nee, R. V., ―OFDM for Wireless Multimedia Communications (Artech
House Universal Personal Communications)‖, Norwood: Artech House
Publishers, 1999.
103
[53] 3GPP TR 25.996, ―3rd
Generation Partnership Project; technical specification
group radio access network; spatial channel model for MIMO simulations
(Release 6)‖, V6.1.0.
104
9 Appendix
105
Appendix A: DL Subcarrier Permutation Functions
This appendix presents the subcarrier permutation MATLAB functions developed for
this thesis. All the functions are self-explanatory and there is an example to illustrate
how the functions work.
(1)
function out = nk(k,s)
% k is the subcarrier index wrt subchannel between 0 & 27 (with
pilots) % s is the subchannel index between 0 & 29 for 1024pt fft
% Written By: Leonardo Obinna A. Iheme % Date: 12-04-2010 % Supervised by: Dr. Erhan Ince
data_subcarrier_subchannel_index = 0:27;
subchannel_index = 0:29;
Nsubcarriers = 28;
k = data_subcarrier_subchannel_index([k+1]);
s = subchannel_index([s+1]);
out = mod((k+13*s),Nsubcarriers);
(2)
The outer permutation described in Chapter 6. This function does the renumbering of
the physical clusters
function CL_Logical_No = Outer_permutation(CL_PHY_No, DL_PermBase)
% this function gives an output of the cluster logical number % it is possible to input CL_PHY_No as a vector of maximum length 60
for
106
% 1024pt fft
% the physical numbered clusters are renumbered according to the % renumbering sequence and the DL_PermBase
% Written By: Leonardo Obinna A. Iheme % Date: 12-04-2010 % Supervised by: Dr. Erhan Ince
Re_num_sequence = [6 48 37 21 31 40 42 56 32 47 30 33 54 18 10 15 50
51 58 ... 46 23 45 16 57 39 35 7 55 25 59 53 11 22 38 28 19 17 3 27 12 29
26 5 41 49 44 9 8 1 13 36 14 43 2 20 24 52 4 34 0];
if CL_PHY_No ~= 0:59 error ('CL_PHY_No must be between 0 and 59') end Nclusters = 60;
CL_Logical_No =
Re_num_sequence([mod(((CL_PHY_No)+13*DL_PermBase),Nclusters)]+1);
(3)
function phy_indexes = subcarrier_indexes(CL_PHY_No)
% this function gives the subcarrier index (given the physical
cluster number) % wrt absolute subcarrier index for 1024pt fft
% Written By: Leonardo Obinna A. Iheme % Date: 12-04-2010 % Supervised by: Dr. Erhan Ince
subcarrier_index = 92:931;
if CL_PHY_No ~= 0:59 error ('CL_PHY_No must be between 0 and 59') end
[phy_indexes] =
subcarrier_index((14*CL_PHY_No)+1:(14*CL_PHY_No+13)+1)
107
(4)
Reshuffles the Subcarriers
function subcarriers = reshufled_Sc(CL_PHY_No,DL_PermBase)
% reshuffles the subcarriers according to the renumbering sequence
and % outputs subcarrier indexes wrt the absolute subcarrier index
% Written By: Leonardo Obinna A. Iheme % Date: 12-04-2010 % Supervised by: Dr. Erhan Ince
for ii = Outer_permutation(CL_PHY_No,DL_PermBase)
subcarriers = subcarrier_indexes(ii);
end
(5)
The inner permutation described in Chapter 6
{ [ ] }
function out = Inner_permutation(k,s,group_index,DL_PermBase)
% this function gives the output of the subcarrier index wrt % the group indexes. % it is possible to specify k as a vector of subcarrier indexes
% k = the subcarrier index within the subchannel s % s = the subchannel index
% Written By: Leonardo Obinna A. Iheme % Date: 12-04-2010 % Supervised by: Dr. Erhan Ince
P_s_even = [3 0 2 1];
P_s_odd = [3 2 0 4 5 1];
ifgroup_index == (1|3|5) % if the group is even
Nsubchannels = 4;
108
P_s = P_s_even;
else
Nsubchannels = 6;
P_s = P_s_odd;
end
n_k = nk(k,s);
Ps = shiftleft(P_s,s);
Pj = Ps([mod(n_k,Nsubchannels)+1]);
out = Nsubchannels*n_k+mod(Pj+DL_PermBase,Nsubchannels);
(6)
function out = PHY_Sc_indexes(s,DL_PermBase)
% this function gives an output of the physical reshuffled
subcarrier % indexes. % s is the subchannel index from 0-29 for 1024pt fft
% Written By: Leonardo Obinna A. Iheme % Date: 12-04-2010 % Supervised by: Dr. Erhan Ince
sclist =[];
for n = 0:59
subcarriers = reshufled_Sc(n,1);
sclist = [sclist subcarriers]; end
% Physical subcarrier indexes in groups
group0 = sclist([1:168]); group1 = sclist([169:280]); group2 = sclist([281:448]); group3 = sclist([449:560]); group4 = sclist([561:728]); group5 = sclist([729:840]);
m = 28; % no. of subcarriers in a subchannel (0:(m-1))
% permutation based on the parity of the groups
109
% *******Checking which group s belongs to********** if s <= 5
group_index = 0;
subcarrier_logical_indexes = Inner_permutation(0:(m-
1),s,group_index,DL_PermBase);
out = group0(subcarrier_logical_indexes+1);
elseif 6<=s<=9
group_index = 1;
subcarrier_logical_indexes = Inner_permutation(0:(m-
1),s,group_index,DL_PermBase);
out = group1(subcarrier_logical_indexes+1);
elseif 10<=s<=15
group_index = 2;
subcarrier_logical_indexes = outer_permutation(0:(m-
1),s,group_index,DL_PermBase);
out = group2(subcarrier_logical_indexes+1);
elseif 16<=s<=19
group_index = 3;
subcarrier_logical_indexes = outer_permutation(0:(m-
1),s,group_index,DL_PermBase);
out = group3(subcarrier_logical_indexes+1);
elseif 20<=s<=25
group_index = 4;
subcarrier_logical_indexes = outer_permutation(0:(m-
1),s,group_index,DL_PermBase);
out = group4(subcarrier_logical_indexes+1);
else 26<=s<=29;
group_index = 5;
subcarrier_logical_indexes = outer_permutation(0:(m-
1),s,group_index,DL_PermBase);
110
out = group5(subcarrier_logical_indexes+1); end end
(7)
This was used for circularly shifting the basic permutation sequence to the left
depending on the group number. The detailed explanation is in Chapter 6
function [out] = shiftleft(X,k)
% shifts elements of X circularly k times
out = X( mod((1:end)+k-1, end)+1 );
(8)
This script is an example that demonstrates the usage of the functions.
% Example illustrating how the function, PHY_Sc_indexes, works
% Written By: Leonardo Obinna A. Iheme % Date: 12-04-2010
Clear all clc
subcarrier_phy_numbers = [];
n = 30; % number of subchannels
DL_PermBase = input ('What is your DL _PermBase? ');
if DL_PermBase ~= 0:31 error ('DL_PermBase must be between 0 and 31') end
subchannel_range = input('What is the range of subchannels?
');
for s = subchannel_range sub = PHY_Sc_indexes(s,DL_PermBase); subcarrier_phy_numbers = [subcarrier_phy_numbers sub]; end
111
display('Concatenated PHY indexes for n subchannels'); disp(subcarrier_phy_numbers)
Top Related