Juha Heiskala
`John Terry, Ph.D.
`
`10,079,707
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`

`Contents at a Glance
`
`Preface
`
`ix
`
`Background and WLANOverview 1
`
`Synchronization
`
`47
`
`Modulation and Coding
`
`87
`
`Antenna Diversity
`
`123
`
`RFDistortion Analysis for OFDM WLANs
`
`171
`
`Medium Access Control (MAC)for IEEE 802.11 Networks
`
`215
`
`Medium Access Control (MAC) for HiperLAN/2 Networks
`
`257
`
`Rapid Prototyping of WLANs Using FPGA=277
`Index
`299
`
`

`

`

`

`Contents
`
`Preface
`
`ix
`
`1
`
`Background and WLAN Overview 1
`Review of Stochastic Processes and Random Variables.............::::000 2
`Random Variables... ceseecesseseeeesseeteseeesecceesssesececreeteneesstesensentes 3
`
`Ensemble Averages... scesssccesessssceesesecsetceesceeescescserstenecesesenatenenss 3
`
`Review of Discrete-Time Signal Processing.....
`6
`Discrete-Time Signals .0.........cesessceesesesetereceseeresceseecessereaceceacseeteate 6
`Discrete-Time Systems..........ccccccsssscseesssssescssessectssecsectessesscesereesseens 7
`Filtering Random Processes ............ccsessssceseseseceseeseseeeeseseteseeesenseeteess 9
`Discrete Fourier Transform (DFT)..........cccccsscesscssseesereseeesseeesnesees 11
`Components of a Digital Communication System .......0..cseeeeeeeeeees 12
`Source Formatting 0.0... ees eesscseeecseseeecsesecesceetseesesacscenseneeseeseeseees 13
`Source Coding... ees eesseeseseseseeeesssaeecneeseesceeceneseaesseaseesesseeneaeees 18
`Chantne] Coding... esc seccssssscsscstessesersceessaceeassnesesseceasseseaesasseeeneses 19
`Modulation .......eeseseeeeeeeeeees
`
`Multiple Access Techniques...........:cssssccsccessssesetesteceacsesescessteesasses 25
`Channel Modeloo... eeseecsceseneecesceeesseaseesseessseseteesseeseeeeseeetes 28
`OFDM WLANOVErVieW ...0... ee eeseeeseereeetsseceeeesceseescsetereasseessceeteeeates 31
`MACfor WLAN Standards... eeseseseceeseceeeeeeseeetecereseeeesceees 32
`
`Physical Layer Specifications for WLAN Standards «0.0.0.0... 36
`Single Carrier Versus OFDM Comparison...........cccsccssesscesesteseessessereeree 38
`
`Synchronization Errors........ccccessesesseeseees
`Effects of Frequency Errors ......:.ccscesssesseccessssceescssesseescesceeseessees 41
`Bibliography .........csccsssseccssessecsesseseeseeseceneesessseeacessstecesseessecersseesssenensea 45
`
`2
`
`Synchronization 47
`Timing Estimation 00... cescecssseesserseeceseneeseeeseseereaceseeessereaceetnesetsees 49
`Packet Detection... ccsccsssesceserseseeeeecseceseceseeseeseveneteaeeeseneetes 49
`
`Symbol Timing... ee eeeseseeseeseesesseseeceeseececsseeseeesaceeseetetereseseeetes 57
`Sample Clock Tracking ......ccccccsssssssessesessesesessessssectssestssesesecseecenes 62
`Frequency Synchronization .........cccsescescssesecsecseccesrsscerececseeseesesseeseneesees 66
`
`Post DFT Approach to Frequency Error Estimation.....
`Comments on Frequency Error Estimation Algorithms.................. 72
`Alternative Techniques for Frequency Error Estimation.................. 73
`Carrier Phase Tracking ..0.......:ccesesssseecseseesecceeeeesetenetenseesercatseesessenes 73
`Channel Estimation..............ccceecesecsscsseseeeeesesceseseesnenseneeceseteesseceasenesnenses 77
`
`Frequency Domain Approach for Channel Estimation.................... 77
`Time Domain Approach for Channel Estimation... sesso 79
`Analysis of the Time Domain and Frequency Domain
`Approaches for Channel Estimation... cscsesesesseseceteseteneeeeees 80
`Enhancing the Channel Estimate... eeeeeeeseseeeeeneneeeeeeeees 81
`
`

`

`
`
`vi
`
`
`
`OFDM Wire_ess LANs: A THEORETICAL AND PRACTICAL GUIDE
`
`Clear Channel Assessment............cccccccssccsssssscessceserstcesaeesresssenseceateseeeate 81
`
`we
`Signal Quality.......
`
`Bibliography 0.0... eeseseeeesceseeseeeesseesensecsessersesceesscsesceseseesseacaseetenees
`
`3 Modulation and Coding 87
`Modulation........ccssecsssssssesesesesesseseserseenssssesssaseeseasseesesesnsesessecsersenseentes 88
`Coherent Modulations...
`90
`
`
`Detection of Coherent Modulations...
`97
`
`Non-Coherent Modulations...............
`ceeeee 98
`
`Linear and Nonlinear Modulation...
`seeseeececeseseeeeseereeeeee 101
`
`Irnterleaving......... see
`eeeecssecesencesceeeseseesecoeseeseeseacenesenessceesceesaeeteceess 102
`Block Interleaver oo... cessseseesetseeesecseeesceccesecesevessceseeseasetessetere 103
`
`
`Convolutional Interleaver..........eteeseeeseeeeececeseseesceseesenesetecseeees 104
`
`Interleaving in TEEE 802.1140...
`eseseseccseseeteeeceeeeseseceesseees 104
`
`Chante] Codes... essceeeseesereesscsseesesensencnecesscenseeesssteseesseecaeeaeees 106
`Performance of TEEE 802.118... sescesseeseeseeeeeceeseceesneeecsceneees 114
`Trellis Coded Modulation 00.0.0... cceesessesssscensecteseeseeesssenenscseseseees 118
`Block Codes 00... csecssescsscsceeeecesceeeeecsenecsataeseesceeescsasaeseesesaeeseeeees 119
`TUrbo Codes oo. eee eseeecsesesseeteesecsesccsesaeseessneseceeseesesseeassneneneees 119
`
`Bibliography... eeeesscccssseeeecseeseeeeeceeeseeseesessesseeseseecsesessceseereetesees 120
`
`4 Antenna Diversity 123
`Background).......cecsscssescessesesscceecescescsecsacesessacsasesssssseesseesssassessceeseesenses 124
`Limits of Capacity in Fading Environments..........0....:cccscceseeeees 125
`Channel Model for Multiple-Input/Multiple-Output
`(MIMO)Systems.........ccccscssescscessscseessesseseceseseessssassessaseeeesseeeees 129
`Introducing Diversity ..0........ceesesssecesseeseeeesecesceseeceseeetaneateteeseeseees 130
`Receive Diversity ........ cee sseseesesssesseeceeseeesetecteseeretcatsesenseseeeeeneatees 131
`Selection Diversity 0.0.0.0... cccsesssscseecsecsseeseseeseeseceserenesesenseatensereneete 131
`Maximal Ratio Combining .......0.....cccsseeseseteseeceeeceneeeeeeseteteetateees 133
`Transmit Diversity ......cscscssesssessesesersesacenseessassesesssessserseerersereesentetens 136
`Transmit Diversity Design Criteria for Fading Channels.............. 136
`Delay Diversity 0... ccusscsssessssessesseessetsessessesesseetaresereereeseteensaee 139
`Trellis Space-Time Codes .......ccsessceessssescersesenssenerseesesenreesereenenees 140
`Layered Space-Time Codes ............cesesssssscnsseseessereeseeseecasenseneneacee 146
`Block Space-Time Codes..........csccsssssssessessssesessesesssecesesseseeseseesceees 148
`Multidimensional Space-Time Codes ..........ccccccssssssesesssssetessesseees 151
`Spherical Coding... ee eesseesececeereessececsceseestsceaeeceeteeneteeeeesees 151
`SSTC in OFDM Framework 00... seseeseecceseeeeteseseeenereeteeserseeee 152
`
`Water-Filling for Single Antenna Systems................:.cceseeceeeereees 154
`Water-Filling for Multi-Antenna Systems..............::eccesecceceeeetenes 157
`Adaptive Modulation .0.....0....ccceccsscsessesseeseseneestesesesessesnessenessensseees 158
`Bibliography ........scssssessessesccsesesessersscerenssessersesensenteenenersnecarsenersenentees 163
`
`

`

`cressmg
`
`
`
`5
`
`RF Distortion Analysis for OFDM WLAN 171
`Components of the Radio Frequency Subsystem.......
`v 172
`
`Amplifier Classification and Distortion ...........ccccc . 173
`Predistortion Techniques for Nonlinear Distortion Mitigation........... 178
`A Decision-Aided Method for Clipping
`Amplitude Recovery.....ccccscccessssssscssssssessesssseseasessassesersacesersees 181
`Bayesian Inference Method for Clipping Amplitude Recovery..... 184
`Effects of Amplifier Nonlinearities Without Amplitude Clipping . 187
`Adaptive Predistortion Techniques.........scccsssssssssssssesseesssssesseersessere 190
`Coding Techniques for Amplifier Nonlinear Distortion Mitigation.... 197
`Partial Transmit Sequence Techniques.......0.... csc eeseeseseeseeeseeeeeete 198
`Modification of the PTS Algorithm ...........c:ccccsssseseeesesssseeseseeees 199
`Selective Mapping... ecsessesssesersescecseecseseserseeteeseesecsensseensees 202
`Phase NOiIS€
`......ccsscsesesessssssescsesersecersessserscnsasessesessenesessencsenaesenseeseseesees 205
`
`TQ Imbalance 200... ceeseeeeeceereeceseeecscestscesessenesaeasenesensesenuenseesasensess 208
`Bibliography... cc sscscscscsssssseessessssesceessescasessetsesensnseneessesensensesseeseessees 210
`
`6 Medium Access Control (MAC) for IEEE 802.11 Networks 215
`MAC OVELVIEW oo scsessesscessescseesssceeserensccerscsesensassssaracscnseceseaseeseaeseesee 216
`MACService...escesessesssessessestererscserseenscecnsseteratsessensneacenseacseneee 217
`MAC Frames ......cccccssscsesssessssecersessserecssscesseratsenecerscnteesessensnesatecsens 218
`
`MACInformation Management............ cs ecsescsseceneeseereeneesseeeessees 219
`MACSystem Architecture ........ccescseseesesseseceecesseseneneeseereessasseeetestees 219
`Basic Service Sets ....ccccscesssssssessssssessercnsesserereenerseesscessenseetenseetens 219
`Extended Service Sets .....ccsescscceccscsesssesseeseeseessseseensereseeneesenenees 221
`MACFrame Format........cscsccssssesseresccsesescrecsesensencsseseereesensseneeneeenens 222
`TEEE 802.11 Frame Format .0......c cc essesesesesseecssseneneeneateenseneneeees 222
`
`MACFrame Subtypes.....c.cccscceessseeseseseeceseesseserecsenseceseesseneseseesees 227
`Frame Fragmentation.....0..0..0.. ce ecseescseeeesseneeeeseceeseeseensseenssesesenes 229
`MACData Services... cscseesecscsseneteeseneneceaseesesesesseeseenseeesaeeseceees 231
`Distributed Coordination Function 0.0... eessetesssceseseeterseeeeesenees 231
`Point Coordination FUNCtiON...0......cssssssessssstescsessreessereessesessestes 24
`
`Frame Exchange with RTS/CTS.00.... ee eceseseseeseeseeteceeseeeceeeaes 238
`MAC Management Services .0........sesccssessesceseesccsscesesecsacsacsacsecteesereares 239
`Power Management.............cccsccsssssssscsscesessesccsencacssessessesesseeneseases 239
`SyMchronization ..........eceeccssesessessceseeneesecesssecesssescesesatssesesssesesseres 243
`Session Management ............ccssssscscessesesssseseessseeesensarsceesseesseesneens 246
`PLIVACY 0... eseesseseeeenceetecceceeerocsececeeceseseescsaenessesetsetsneatseseesetscensaneneate 250
`MAC ManagementInformation Base .............ccceeseeseeeceseesetceeeeeeneeee 251
`
`we 201
`Station ManagementAttributes........
`w+ 254
`MACAttributes... eee seeeeees
` Bibliography .0.......cccssesscssscsssessessessenssssssesseesersenersesecesseneaseeeerssataeeasens 256
`
`

`

`;_ OFDM Wire-tess LANs: A THEORETICAL AND PRACTICAL GUIDE
`
`7 Medium Access Control (MAC) for HiperLAN/2 Networks 257
`
`Network Architecture 20.0.0...
`cccccssceetsescesessesssesnessesecaeeeeesseeeeeasseeesesees 259
`DLC Functions.......sccsesscsscessersesserscncereeserscssescenseneessneeceenaeetseneensesees 260
`MAC OVEerVIEW 00... ee eesesceseseceeceeseesesecetscscesetsesnsssesecseseesecaeseetensenass 261
`
`Basic MAC Message Formats .........:..:cccccessessssessssssescescseseesecescseeeseseres 262
`Transport Channels 0.00.00... cess sescescosceeseesesesetseeaeenenseaseaeessnseesaeee 264
`PDU Trains... ee cece seceeceeneeeeceecensensescesscsetsceaeesensenseatsceeesaesecesenees 266
`
`Logical Channels 000.0... esse sesceeceseccerseceesersseeneaceseesesecaeseerseeeeses 268
`MAC Frame Structure oo... ceceeseseeecesereeceeeesereasenenesseeeesaeassnenssseeees 269
`
`Order of the Transport Channels... ecseseeeeseseseceeseeseaseneesees 271
`Schedule Addressing... ecccesscsssssessetessessecscsesesersenessesseensessensenes 271
`Building a MAC Frame... cecsssscssscetenceccseseceessecseesesessnesseaesneeees 273
`MACFrame Processing ...........cscssesssssessessseessceeescectersesessessseseeseatseesens 274
`ACCESS POINE.... sc ceccteescseesenesscnenesessessssssssesesessecesensessneetensnensecnanaees 274
`Mobile Terminal... cscsecssenseceseecseseesesenseseserseseeseeseestsenenenseas 275
`Bibliography ........ ce cscsssssescseeseerscsescenensesesecescesectsseseesaeesesetscaeseseees 275
`
`8
`
`277
`Rapid Prototyping for WLANs
`Introduction to Rapid Prototype Design.......cescsseseeseseseesesseesetensens 278
`An Example of a Rapid Prototype Design Flow...........sececeseees 280
`Good Digital Design Practices 0... sseesessssesssseeeeeecenesesteeeetseseseesens 284
`Rapid Prototyping of a WLAN System... eceseeseceseeeeeeeeeeesesees 290
`TEBE 802.1 1a Transmitter... sesceseecesereseatereecescesessecensenes 292
`TEEE 802.1 1a Receiver... sesesssseesessesceseceseececeesersensesseeeessensense 293
`TEEE 802.11a Baseband Demonstratol.............cccecssseceseseesereeesesees 295
`
`Bibliography ....... cee sseceeeecsenscsesesseeseeseasecseeseseeesaesenecseacersscetsssenaeseas 298
`
`Index 299
`
`

`

`Preface
`
`This book will address the subject of broadband communications using orthogonal frequency
`division multiplexing (OFDM). OFDMisa special case of multicarrier modulation (MCM),
`whichis the principle of transmitting data by dividing the stream into several parallel bit streams
`and modulating each of these data stream onto individual carriers or subcarriers. Although the
`origin of MCMdates back to the 1950s and early 1960s with military high frequency (HF) radio
`links, R.W. Chang in the mid 60s first published a paper demonstrating the concept we todaycall
`OFDM.Chang demonstrated the principle of transmission of multiple messages simultaneously
`through a linear band-limited channel without interchannel interference (ICI) and intersymbol
`interference (ISI). The multichannel or OFDM system developed by Changdiffered from tradi-
`tional MCMinthat the spectra of the subcarriers were allowed to overlap undertherestriction
`they were all mutually orthogonal. This characteristic of OFDM systems required the abandon-
`ment of steep bandpassfilters used in older MCM systemsto separate the spectra of the individ-
`ual subcarriers.
`
`Weinsten and Ebert werethe first to suggest using the discrete Fourier Transform (DFT) and
`inverse discrete Fourier Transform (IDFT)to perform baseband modulation and demodulation in
`1971. Currently, OFDM systemsutilize the Fast Fourier Transform (FFT) and Inverse FFT to
`perform modulating and demodulating of the information data. Saltzberg performed a perfor-
`mance analysis of OFDM,shortly after Chang published his paper, and concludedthat the domi-
`nate impairment in OFDMis ICI. To combat ICI andISI, Peled and Ruiz introduced the concept
`of a cyclic prefix (CP). Rather than using an empty guard space, a cyclic extension of the OFDM
`symbolis used instead. This effectively simulates a channel performing circular convolution as
`long as the CPis longer than the impulse response of the channel. The penalty of using a CP is
`loss of signal energy proportional to the length of the CP, yet the benefits of using a CP generally
`outweighs any loss of signal energy.
`
`Presently, OFDM appears in several standards relating to wireless communications at high data
`rates such asterrestrial digital audio broadcasting (DAB) and digital video broadcasting
`(DVB-T) in Europe. Presumably, one of the reasons OFDM waschosen as the DABstandardis
`that it is possible to deploy single frequency subnetworks within its main networks. Hence, main
`and relay broadcast transmitters may use the sameset of subcarriers. In areas with reception from
`multiple transmitters, receive diversity gains are experienced. Based on coded OFDM, DVB-Tis
`the youngest and most sophisticated of the three core DVB systems. Combining channel coding
`with OFDM permits reliable transmission over dispersing channels. Furthermore,the inherent
`structure of OFDM allows for flexible transmissionrates.
`
`Finally, WLAN,the main subject of this book, is another application for OFDM technology. For
`instance, next generation wireless LAN standards such as IEEE 802.11a, High Performance
`Local Area Network type 2 (HiperLAN/2), and Mobile Multimedia Access Communication
`
`

`

`(MMAC)system have accepted OFDMastheir physical layer specifications. These WLANsys-
`tems also incorporate coding with OFDM to combatdispersing channels. It has been shown that
`coded OFDM modulation over modest dispersing channels can improve, rather than deteriorate,
`the reliability of the transmission. This interesting counterintuitive phenomenoncanbeattributed
`to the inherent frequency diversity provided by OFDM.Arguably, this characteristic is the most
`attractive feature of OFDM.
`
`Interactive Learning
`Clearly with the growing in interest in OFDM for high data rate wireless communications, in par-
`ticular WLANs,there is a need in the technical community for a book that reviews the subject of
`OFDM WLANS. Typically, this is accomplished in a classroom setting. Unfortunately, many
`engineers and scientists today cannot afford the time required to attend classes at a university.
`Whatis neededis a tool to allow each reader to learn each of the concepts presented in the chap-
`ters at his/her own pace. We have provided that by meansofan interactive simulation environ-
`ment. Please visit our Website at http://www.samspublishing.com and search for the OFDM
`book. Morespecifically, the site contains a complete OFDM WLANphysical layer simulation
`developed in MATLAB. We developed the simulation tool to illustrate the concepts discussed in
`Chapters 2-5.
`
`To aid in the learning process, exercises are provided in each of these chapters. The exercises
`require the use of the OFDM system simulation tool and the simple programs you develop. Most
`of the examples given in this book are reproducible with the simulation program. The OFDM
`system simulation is executed through a graphicaluser interface (GUD)to facilitate system recon-
`figurability. The GUIis called from the MATLAB command window, which allowsusersto test
`quickly and easily many of the concepts in this book with a few clicks of the mouse. The novice
`and expert alike will thoroughly enjoy the endless combinationsof test conditions available to
`them. With this learning tool, readers can further improve their understanding of the concepts
`presented in this book. In addition, readers interested in testing their algorithms over a WLAN
`environmentwill save months of software development time by using the simulation program
`located at our Website.
`
`Intended Audience
`
`The primary audiences for this book are engineers and scientists without prior knowledge of
`OFDM.In the developmentof the text, we consider our primary audienceto fall within two
`broad categories of readers: novice and advanced.For the novice, we envision someone with a
`background in engineering, mathematics, and some knowledge of communication theory. For
`that audience, this book provides the basics of OFDM theory with many examplesandillustra-
`tions demonstrating concepts. An example novice reader might be a researcherin digital image
`processing, whois in interested in understanding what effects does an OFDM WLANnetwork
`
`

`

`might have on the quality of the video. Another example of the novice reader could be a radiofre-
`quency (RF) engineer, whois interested in the additional requirements imposed by OFDM mod-
`ulation on the RF subsystemsin the access point (AP) and mobile terminal (MT).
`
`An example of an advanced readeris an engineerorscientist familiar with basic OFDM con-
`cepts. For those individuals, this book is intended as a source for practical guidelines as well as
`introductory material of advanced research topics in OFDM.
`
`The secondary audiencesfor this book include individuals, such as network system engineers,
`product engineers, or managers, for whom someof the mathematical development presented in
`this text is slightly beyond their scope of understanding. For those individuals, explanatory text is
`provided throughout this book to give an intuitive feel of many of the concepts discussed.
`
`It is assumedthat the all audiences have a backgroundin calculus, physics, and random andsto-
`chastic processes. Thus, the majority of the text in this book is written at the undergraduatelevel,
`with the exception of the advanced research topics, which are written at the first-year graduate
`level. In addition, the reader will be provided in each chapterall the relevant mathematical foun-
`dations necessary to understand the OFDM principles discussed. As mentioned earlier, explana-
`tory text is also given to provide a better understanding of these OFDM principles from the
`mathematical expressions.
`
`A final point concerning the audience: to reap the fullest benefit of this book,it is advantageous
`to the reader to becomeproficient in the use of MATLAB.
`
`Weexpectthis book to attract a broad range ofreaders,as it is written to do so. Certainly, no
`book can beall things to everyone. However, no matter yourinterest level in OFDM WLANs,
`this book has someinsightto offer.
`
`Organization of this Book
`This bookis organized as follows. Chapter 1, “Background and WLANOverview,”is dedicated
`to background material as well as an overview of OFDM WLANs.The background material cov-
`ers relevant concepts in digital signal and stochastic processing.It expected that readers will refer
`to this chapter as needed to understand the concepts in latter chapters. Chapters 2-5 focus on
`the physical layer specifications of OFDM WLAN.Chapter 2, “Synchronization,” provides a
`detailed discussion of many of the popular synchronization algorithms used in OFDM networks.
`Specifically, timing synchronization algorithms, which include packet detection, symbol timing
`recovery, and sample clock tracking, are covered. Also covered are frequency, channel estima-
`tion, and clear channel assessment (CCA) algorithms. Chapter 3, “Modulation and Coding,” pro-
`vides a brief overview of modulation and coding techniques. In particular, the phase-shift keying
`(PSK) and quadrature amplitude modulations (QAM) found in OFDM WLAN standardsare cov-
`ered. With respect of channel coding, discussions on block and convolutional codes are provided.
`Performance evaluation of several operational modes of the IEEE 802.11a physical specification
`
`

`

`are given. Chapter 3 can be thoughtof as the central theme or key technology area of current
`OFDM WLAN systems.
`
`Chapter 4, “Antenna Diversity,” is dedicated to the central theme or key technology area of future
`OFDM WLAN,antennadiversity. Several popular transmit and receive diversity schemes are
`discussed in their context to OFDM systems. Examples show that drastic improvementin error
`rate performance is achievable when these techniques are deployed. Chapter 5, “RF Distortion
`Analysis for OFDM WLANs,”focuses on the system impairments of the OFDM system resulting
`from RF nonlinearities. Particularly attention is given to the peak-to-average power (PAPR) prob-
`lem present in all OFDM systems.In this chapter, a survey of the more popular techniques to
`handle this problem is analyzed. In addition, other system impairments such as phase noise and
`in-phase and quadrature (IQ) imbalances are covered.
`
`In Chapters 6 and 7, an introduction of the medium access control (MAC)layer is given. Chapter
`6, “Medium Access Control (MAC) for IEEE 802.11 Networks,” summarizes the IEEE 802.1la
`MAC,while Chapter 7, “Medium Access Control (MAC) for HiperLAN/2 Networks,” summa-
`rizes the HiperLAN/2 MAC.Both chapter details of the interaction between the MAC layer and
`the physicallayer.
`
`Interestingly, a majorcriticism of OFDM has been the complexity issues associated with real-
`time implementation of the FFT and IFFT. However, steady improvements in semiconductor
`process technology has allowedfor real-time prototyping of OFDM systems with Field
`Programmable Gate Array (FPGA)technology andcosteffective solutions with Application
`Specific Integrated Circuit (ASIC) technology. Chapter 8, “Rapid Prototying of WLANs Using
`FPGA,”is dedicated to the issues associated with real-time prototyping of an IEEE 802.11a radio
`using FPGAtechnology.
`
`

`

`About the Authors
`
`John Terry, Ph.D., is a senior research engineer at Nokia Research Center in Dallas, TX. Heis
`currently managing the OFDM modulation and coding project in the High Speed Access (HSA)
`group. Dr. Terry has published several conference and journal papers, given numerouspresenta-
`tions on wireless communications, and generated six pending patents related to OFDM wireless
`LANs. Hehas 12 years of experience working in wireless communications, including tenures at
`NASA Glen Research Center and Texas Instruments,Inc. In addition, Dr. Terry is the vice-chair
`of the IEEE 802.11 Task Group g and servesas a technical reviewer for several conference and
`journal publications of the IEEE in wireless communications. Dr. Terry wasthe recipient of the
`2002 Black Engineer of the Year Award for Outstanding Technical Contributions in Industy.
`
`Juha Heiskala is a senior research engineer at Nokia Research Centerin Dallas, TX. Heis active
`in the IEEE 802.11 standards bodies and has been tasked with developing the 802.11a system
`simulation on several software platforms. He is the inventor/co-inventorof three pending patents
`in the area of OFDM LANsand co-designed with Dr. John Terry the modulation and coding
`schemefor achieving 100 Mbps speeds within currently allocated bandwidth specifications for
`OFDM wireless LANs.
`
`

`

`About the Technical Editors
`
`Douglas Williams received B.S.E.E., M.S., and Ph.D. degrees in electrical and computerengi-
`neering from Rice University, Houston, Texas, in 1984, 1987, and 1989, respectively. In 1989, he
`joined the faculty of the School of Electrical and Computer Engineering at the Georgia Institute
`of Technology, Atlanta, Georgia, where he is currently an Associate Professor. There he is also
`affiliated with the Center for Signal and Image Processing and teaches coursesin signal process-
`ing and telecommunications. Dr. Williams has served as an Associate Editor of the [EEE Trans-
`actions on Signal Processing and is a memberof the IEEE Signal Processing Society’s Signal
`Processing Theory and Methodstechnical committee. He was co-editor of the Digital Signal Pro-
`cessing Handbook published in 1998 by CRC Press and IEEEPress. Dr. Williams is a member of
`the Tau Beta Pi, Eta Kappa Nu, and Phi Beta Kappa honorsocieties.
`
`Anthony Reid received his B.S.E.E. (cum laude), from Rensselaer Polytechnic Institute, Troy,
`NewYork, M.S.E.E., from Stanford University, Palo Alto, California and Ph.D., from Southern
`Methodist University (S.M.U.), Dallas, Texas. Dr. Reid is currently a Principal Research Scientist
`at Nokia Research Center doing applied research in modulation, coding and equalization for 3-G
`(third generation) wireless systems. He also worked at Nortel Networks on Advanced Network
`Architectures for 3-G wireless networks. Before joining Nortel and Nokia, Tony was an Engi-
`neering Fellow and Branch Manager of Advanced Signal Processing in the Systems Technology
`Center in Raytheon Systems Corp. (RSC). This group developed broadband communications
`systems capability for both military and commercial applications. In the commercial arena capa-
`bilities were developed for high-speed SATCOM modems.In the past, this group developed
`image and signal processing algorithms for autonomousdetection/tracking of military targets in
`infrared imagery andradar.
`
`

`

`Dedication
`
`To my son Amiel Terry and the fond memory of my grandfather, John D. Terry, both of
`whom madeit possible for this work to be completed.
`—John
`
`To my parents.
`—Juha
`
`

`

`Acknowledgments
`The authors of this book will like to thank the reviewers, Dr. Tony Reid from Nokia Research
`Center in Dallas and Prof. Douglas Williams from Georgia Institute of Technology, for their
`invaluable comments, which greatly improved the content of this book. Finally, the authors
`would like to express their deepest gratitude to Nokia, Inc. for providing the opportunity to work
`in this new and exciting area of wireless communications.
`
`

`

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`
`
`

`

`f
`
`2
`
`| Background and WLAN Overview
`| CHAPTER1
`
`Before delving into the details of orthogonal frequency division multiplexing (OFDM),relevant
`background material must be presentedfirst. The purpose of this chapter is to provide the neces-
`sary building blocks for the development of OFDMprinciples. Included in this chapter are
`reviews of stochastic and random process, discrete-time signals and systems, and the Discrete
`Fourier Transform (DFT). Tooled with the necessary mathematical foundation, we proceed with
`an overview of digital communication systems and OFDM communication systems. We con-
`clude the chapter with summaries of the OFDM wireless LAN standards currently in existence
`and a high-level comparison of single carrier systems versus OFDM.
`
`The main objective of a communication system is to convey information over a channel. The sub-
`ject of digital communications involves the transmission of information in digital form from one
`location to another. The attractiveness of digital communications is the ease with whichdigital
`signals are recovered as comparedto their analog counterparts. Analog signals are continuous-
`time waveforms and any amountofnoise introducedinto the signal bandwidth can not be removed
`by amplification orfiltering. In contrast, digital signals are generated from a finite set of discrete
`values; even when noise is present with the signal, it is possible to reliably recover the informa-
`tion bit stream exactly. In the sections to follow, brief reviews of stochastic random processes and
`discrete-time signal processing are given to facilitate presentation of concepts introducedlater.
`
`Review of Stochastic Processes and
`Random Variables
`
`The necessity for reviewing the subject of random processesin this text is that many digital com-
`munication signals [20, 21, 22, 25] can be characterized by a random orstochastic process. In
`general, a signal can be broadly classified as either deterministic or random. Deterministic sig-
`nals or waveforms can be knownprecisely at instant of time, usually expressed as a mathematical
`function of time. In constrast, random signals or waveforms always possess a measure of uncer-
`tainty in their values at any instant in time since random variables are rules for assigning a real
`numberfor every outcome € of a probabilistic event. In other words, deterministic signals can be
`reproduced exactly with repeated measurements and random signals cannot.
`
`A stochastic or random processis a rule of correspondence for assigning to every outcome & to a
`function X(t,&), where t denotes time. In other words, a stochastic processis a family of time-
`functions that depends on the parameter €. When random variables are observed over very long
`periods, certain regularities in their behavior are exhibited. These behaviors are generally
`described in terms of probabilities andstatistical averages such as the mean, variance, and corre-
`lation. Properties of the averages, such as the notion of stationarity and ergodicity, are briefly
`introducedin this section.
`
`

`

` Background and WLAN Overview
`CHAPTER 1
`|
`
`3
`
`Random Variables
`
`A random variable is a mapping between a discrete or continuous random event and a real num-
`ber. The distribution function, F(t), of the random variable, X, is given by
`
`or equal to a real number @. The distribution function Fy (a) has the following properties:
`
`where Pr(X < &) is the probability that the value taken on by the random variable X is less than
`
`Fy(a@) = Pr(X < a)
`
`(1.1)
`
`* 0< (a) <1
`° Fy(a)< Fy (B)ifas B
`° Fy(-e) =0
`
`° Fy(tee)=1
`
`Anotheruseful statistica] characterization of a random variableis the probability densityfunction
`(pdf), fy(@), defined as
`
`_ 2
`fx(@) = Bor Fy (@)
`
`(1.2)
`
`Based on properties of F(a) and noting the relationship in Equation 1.2, the following proper-
`ties of the pdf easily deducted:
`
`°
`fx(a) 20
`+ [Felder = Fy(400) — Fy(-20) = 1
`Thus, the pdf is always a nonnegative function with unit area.
`
`Ensemble Averages
`In practice, complete statistical characterization of a random variable is rarely available. In many
`applications, however, the average or expected value behavior of a random variable is sufficient.
`In latter chapters of this book, emphasis is placed on the expected value of a random var