Tse and Viswanath:Fundamentals of Wireless Communication which base station will handle a call to or from a user and when to handoff a user from one base station to another. When another user(either wired or wireless)places a call to a given user,the reverse .First the MTSO for nd then the closes n is fo the call is set upto ugh the MTSO and the bas The wirel users is interchangeably called the downlink or the forward channel,and the link from the users to a base station is called the uplink or a reverse channel.There are usually many users connected to a single base station,and thus,for the downlink channel,the base station must multiplex together the signals to the various connected users and then broadcast one waveform from which each user can extract its own signal.For the uplink channel,each use connected to a give base station tran nits its own wave orm,and the base station receives the sum of the waveforms from the various users plus noise.The base statior must then separate out the signals from each user and forward these signals to the MTSO Older cellular systems,such as the AMPS system developed in the U.S.in the 80's,are analog.That is,a voice waveform is modulated on a carrier and transmitted without beir transformed into a digital stream. Diffe nt users in the same cel are assigned diffe erent modulatic n frequencies,and adjacent cells use different sets o frequencies. Cells sufficiently far away from each other can reuse the same set of frequencies with little danger of interference. Second-generation cellular systems are digital.One is the GSM system which was standardized in Europe but now used worldwide,another is the TDMA(time-division multiple access)standard developed in the U.S.(IS-136),and a third is CDMA(code division multipl ess)(IS-95).Since the cellula and thei r standards, wer originally de ped for telephony,the current data rates and delays in ce r system are essentially determined by voice requirements.Third-generation cellular systems are designed to handle data and/or voice.While some of the third-generation systems are essentially evolution of second-generation voice systems,others are designed from scratch to cater for the specific characteristics of data In addition to a requirement for higher rates data app cations have two features that disti inguish themf om voice 。Ma appli bursty may rem inactive periods of time but have very high demands for short periods of applications.in contrast,have a fixed-rate demand over long periods of time .Voice has a relatively tight latenc requirement of the order of 100 ms Data applications have a wide range o latency requirements;real-time applications such as gaming,may have even tighter delay requirements than voice while many others,such as http file transfers,have a much laxer requirement. In the book we will see the impact of these features on the appropriate choice of communication techniques
Tse and Viswanath: Fundamentals of Wireless Communication 15 which base station will handle a call to or from a user and when to handoff a user from one base station to another. When another user (either wired or wireless) places a call to a given user, the reverse process takes place. First the MTSO for the called subscriber is found, then the closest base station is found, and finally the call is set up through the MTSO and the base station. The wireless link from a base station to the mobile users is interchangeably called the downlink or the forward channel, and the link from the users to a base station is called the uplink or a reverse channel. There are usually many users connected to a single base station, and thus, for the downlink channel, the base station must multiplex together the signals to the various connected users and then broadcast one waveform from which each user can extract its own signal. For the uplink channel, each user connected to a given base station transmits its own waveform, and the base station receives the sum of the waveforms from the various users plus noise. The base station must then separate out the signals from each user and forward these signals to the MTSO. Older cellular systems, such as the AMPS system developed in the U.S. in the 80’s, are analog. That is, a voice waveform is modulated on a carrier and transmitted without being transformed into a digital stream. Different users in the same cell are assigned different modulation frequencies, and adjacent cells use different sets of frequencies. Cells sufficiently far away from each other can reuse the same set of frequencies with little danger of interference. Second-generation cellular systems are digital. One is the GSM system which was standardized in Europe but now used worldwide, another is the TDMA (time-division multiple access) standard developed in the U.S. (IS-136), and a third is CDMA (code division multiple access) (IS-95). Since these cellular systems, and their standards, were originally developed for telephony, the current data rates and delays in cellular systems are essentially determined by voice requirements. Third-generation cellular systems are designed to handle data and/or voice. While some of the third-generation systems are essentially evolution of second-generation voice systems, others are designed from scratch to cater for the specific characteristics of data. In addition to a requirement for higher rates, data applications have two features that distinguish them from voice: • Many data applications are extremely bursty; users may remain inactive for long periods of time but have very high demands for short periods of time. Voice applications, in contrast, have a fixed-rate demand over long periods of time. • Voice has a relatively tight latency requirement of the order of 100 ms. Data applications have a wide range of latency requirements; real-time applications, such as gaming, may have even tighter delay requirements than voice while many others, such as http file transfers, have a much laxer requirement. In the book we will see the impact of these features on the appropriate choice of communication techniques
Tse and Viswanath:Fundamentals of Wireless Communication 16 As mentioned above,there are many kinds of wireless systems other than cellular irst there are the broadeast systems such as AM radio,FM radio,IV,and paging stems. the data rates,the size the areas covered by each ting node e,and the fre quency ranges are very different.Next,there are wireless LANs (local area networks) These are designed for much higher data rates than cellular systems,but otherwise are similar to a single cell of a cellular system.These are designed to connect laptops and other portable devices in the local area network within an office building or sim- ilar environment.There is little mobility expected in such systems and their major fction is toallow portability The najo andards for wireless LANS are the IEEE 802.11fam ly.There are sn er scale standards like Bluetooth or a more recent one based on Ultra Wideband (UWB)communication whose purpose is to reduce cabling in an office and simplify transfers between office and hand held devices.Finally,there is another type of LAN called an ad hoc network.Here,instead of a central node (base station)through which all traffic flows,the nodes are all alike.The network organizes itself into links betwee n various pairs of nodes and develops routing tables using these links.Here the netwo ork layer s of r diss emination rol info etc.are important concerns,although problems of relaying and distributed coopera tion between nodes can be tackled from the physical laver as well and are active areas of current research. 1.3 Book Outline The central object of interest is the wireless fading channel.Chapter 2 introduces the multipath fading channel model that we use for the rest of the book.Starting from a continuous-time passband channel,we derive a discrete-time complex baseband model more suitable for analysis and design.Key physical parameters such as coherence ead and delav spread are statistical moc dels veyed Ther e have be models proposed in the literature;we will be far from exh tive here.The goal is to have a small set of example models in our repertoire to evaluate the performance of basic communication techniques we will study. Chapter 3 introduces many of the issues of communicating over fading channels in the simplest point-to-point context.As a baseline,we start by looking at the problem of detection of unc oded tran over a n band fading chann nel.We find tha the performance is very poor an an AWGN an l with the sam average signal-to-noise ratio (SNR).This is due to a significant probability that the channel is in deep fade.Various diversity techniques to mitigate this adverse effect of fading are then studied.Diversity techniques increase reliability by sending the same information through multiple independently faded paths so that the probability
Tse and Viswanath: Fundamentals of Wireless Communication 16 As mentioned above, there are many kinds of wireless systems other than cellular. First there are the broadcast systems such as AM radio, FM radio, TV, and paging systems. All of these are similar to the downlink part of cellular networks, although the data rates, the size of the areas covered by each broadcasting node, and the frequency ranges are very different. Next, there are wireless LANs (local area networks) These are designed for much higher data rates than cellular systems, but otherwise are similar to a single cell of a cellular system. These are designed to connect laptops and other portable devices in the local area network within an office building or similar environment. There is little mobility expected in such systems and their major function is to allow portability. The major standards for wireless LANS are the IEEE 802.11 family. There are smaller scale standards like Bluetooth or a more recent one based on Ultra Wideband (UWB) communication whose purpose is to reduce cabling in an office and simplify transfers between office and hand held devices. Finally, there is another type of LAN called an ad hoc network. Here, instead of a central node (base station) through which all traffic flows, the nodes are all alike. The network organizes itself into links between various pairs of nodes and develops routing tables using these links. Here the network layer issues of routing, dissemination of control information, etc. are important concerns, although problems of relaying and distributed cooperation between nodes can be tackled from the physical layer as well and are active areas of current research. 1.3 Book Outline The central object of interest is the wireless fading channel. Chapter 2 introduces the multipath fading channel model that we use for the rest of the book. Starting from a continuous-time passband channel, we derive a discrete-time complex baseband model more suitable for analysis and design. Key physical parameters such as coherence time, coherence bandwidth, Doppler spread and delay spread are explained and several statistical models for multipath fading are surveyed There have been many statistical models proposed in the literature; we will be far from exhaustive here. The goal is to have a small set of example models in our repertoire to evaluate the performance of basic communication techniques we will study. Chapter 3 introduces many of the issues of communicating over fading channels in the simplest point-to-point context. As a baseline, we start by looking at the problem of detection of uncoded transmission over a narrowband fading channel. We find that the performance is very poor, much worse than an AWGN channel with the same average signal-to-noise ratio (SNR). This is due to a significant probability that the channel is in deep fade. Various diversity techniques to mitigate this adverse effect of fading are then studied. Diversity techniques increase reliability by sending the same information through multiple independently faded paths so that the probability
Tse and Viswanath:Fundamentals of Wireless Communication of successful transmission is higher.Some of the techniques which will be studied include: interleaving of coded symbols over time; inter-symbol equalization,multipath combining in spread-spectrum systems and coding over sub-carriers in orthogonal frequency division multiplexing (OFDM) systems to obtain frequency diversity: use of multiple transmit and/or receive antennas,via space-time coding. In some scenarios.there is an interesting interplay between channel uncertainty and the diversity gain:as thember of diversity pe of e to the e dive gain but the rate branches In Chapter 4 the focus is shifted from point-to-point communication to studying cellular systems as a whole.Multiple access and inter-cell interference management are the key ssu that come to the forefront.We explain how existing digital wireles systems deal with these i use and are disc cepts of fr quer orizatio ed, and we co tra narrowt and system s such as GSM and IS-136 he users within the same cell are kept orthogonal and frequency is reused only in cells away.and CDMA systems.such as Is-95.where the signals of users both within the same cell and across different cells are spread across the same spectrum,i.e.,frequency reuse factor of 1.Due to the full reuse,CDMA systems have to manage intra-cell and interterencnn addition to the diver interleaving. oath combini ng and soft handoff ower e averaqing are the key interference management mechanisms.All th ive techniques strive toward the same system goal:to maintain the channel quality of each user,as measured by the signal-to-interference-and-noise ratio(SINR),as constant as possible This chapter is concluded with the discussion of a wideband OFDM system,which combines the advantages of both the CDMA and the narrowband systems. Chapter 5 studies the capacity of wireless channels.This provides a higher level view of the tr Heoffs involved in the earlier as s the fo oundati for understanding he more me dern developments in the subseque Th performance over the(non-faded)additive white Gaussian noise(AWGN)channel,as a baseline for comparison.We introduce the concept of channel capacity as the basic performance measure.The capacity of a channel provides the fundamental limit of communication achievable by any scheme.For the fading channel,there are several capacity measures.relevant for different scenarios.Two distinct sc cenarios provide par ticulr insight: 1)the slow fa nnel,wher the ays the (rando value)over the entire time se e f communication,and 2)the fast fading channe where the channel varies signifcantly over the time scale of communication
Tse and Viswanath: Fundamentals of Wireless Communication 17 of successful transmission is higher. Some of the techniques which will be studied include: • interleaving of coded symbols over time; • inter-symbol equalization, multipath combining in spread-spectrum systems and coding over sub-carriers in orthogonal frequency division multiplexing (OFDM) systems to obtain frequency diversity; • use of multiple transmit and/or receive antennas, via space-time coding. In some scenarios, there is an interesting interplay between channel uncertainty and the diversity gain: as the number of diversity branches increases, the performance of the system first improves due to the diversity gain but then subsequently deteriorates as channel uncertainty makes it more difficult to combine signals from the different branches. In Chapter 4 the focus is shifted from point-to-point communication to studying cellular systems as a whole. Multiple access and inter-cell interference management are the key issues that come to the forefront. We explain how existing digital wireless systems deal with these issues. The concepts of frequency reuse and cell sectorization are discussed, and we contrast narrowband systems such as GSM and IS-136, where users within the same cell are kept orthogonal and frequency is reused only in cells far away, and CDMA systems, such as IS-95, where the signals of users both within the same cell and across different cells are spread across the same spectrum, i.e., frequency reuse factor of 1. Due to the full reuse, CDMA systems have to manage intra-cell and inter-cell interference more efficiently: in addition to the diversity techniques of timeinterleaving, multipath combining and soft handoff, power control and interference averaging are the key interference management mechanisms. All the five techniques strive toward the same system goal: to maintain the channel quality of each user, as measured by the signal-to-interference-and-noise ratio (SINR), as constant as possible. This chapter is concluded with the discussion of a wideband OFDM system, which combines the advantages of both the CDMA and the narrowband systems. Chapter 5 studies the capacity of wireless channels. This provides a higher level view of the tradeoffs involved in the earlier chapters as well as lays the foundation for understanding the more modern developments in the subsequent chapters. The performance over the (non-faded) additive white Gaussian noise (AWGN) channel, as a baseline for comparison. We introduce the concept of channel capacity as the basic performance measure. The capacity of a channel provides the fundamental limit of communication achievable by any scheme. For the fading channel, there are several capacity measures, relevant for different scenarios. Two distinct scenarios provide particular insight: 1) the slow fading channel, where the channel stays the same (random value) over the entire time scale of communication, and 2) the fast fading channel, where the channel varies significantly over the time scale of communication
Tse and Viswanath:Fundamentals of Wireless Communication 18 In the slow fading channel,the key event of interest is outage:this is the situation when the channel is so poor that no scheme can communicate reliably at a certain fixed .The larges ate of reliable co nication at a certain ed the ontage copacity:oding sceme tht a outage pr babilit ves th outage cap to be universal since it communicates reliably over all slow fading channels that are not in outage.In the fast fading channel.in contrast.outage can be avoided due to the ability to average over the time variation of the channel,and one can define a positive capacity at which arbitrarily reliable communication is possible.Using these capacity measures,several resources associated with a fading channel are defined:1)diversity 2)number of de grees of eedom;3)received po er These thre resou es form a basis for ass ng the nature of performance gain by the various communication schemes studied in the rest of the book. Chapters 6 to 10 cover the more recent developments in the field.In Chapter 6 we revisit the problem of multiple access over fading channels from a more fundamental ahte the total ests that if both the transmitters and the system th to w only the rith the b channel to t at time.A simil rstrategy is also optimal for the downlink(one-to-many).Opportunisti strategies of this type vield a system-wide multiuser diversity gain:the more users in the system,the larger the gain,as there is more likely to have a user with a very strong channel.To implement this concept in a real system,three important considerations are:fairness of the resource allocation across users,delay experienced by the individual aiti its ch al to be e and delaz in feeding ack the chanr ar third-generation wireless data system. A wireless system consists of multiple dimensions:time,frequency,space and users Opportunistic communication maximizes the spectral efficiency by measuring when and where the channel is good and only transmits in those degrees of freedom.In this con- text s be neficial in the that the ation of the the degree of freedom ens sures that here will be sor degrees o freedom in whi the channel is very good.This is in sharp contrast to the diversity-based approach in Chapter 3,where channel fluctuation is always detrimental and the design goal is to average out the fading to make the overall channel as constant as possible.Taking this philosophy one step further.we discuss a technique.called opportunistic beamform- ing.in which channel fluctuation can be induced in situations wl nen the natural fading has small dynamic range d/or is slow o血the point of iew,this technique also increas uctuations of the interferen imparted on adjacent and presents an opposing philosophy to the notion of interference averaging in CDMA systems. Chapters 7,8,9 and 10 discuss multi-input multi-output(MIMO)communication
Tse and Viswanath: Fundamentals of Wireless Communication 18 In the slow fading channel, the key event of interest is outage: this is the situation when the channel is so poor that no scheme can communicate reliably at a certain fixed data rate. The largest rate of reliable communication at a certain outage probability is called the outage capacity; a coding scheme that achieves the outage capacity is said to be universal since it communicates reliably over all slow fading channels that are not in outage. In the fast fading channel, in contrast, outage can be avoided due to the ability to average over the time variation of the channel, and one can define a positive capacity at which arbitrarily reliable communication is possible. Using these capacity measures, several resources associated with a fading channel are defined: 1) diversity; 2) number of degrees of freedom; 3) received power. These three resources form a basis for assessing the nature of performance gain by the various communication schemes studied in the rest of the book. Chapters 6 to 10 cover the more recent developments in the field. In Chapter 6 we revisit the problem of multiple access over fading channels from a more fundamental point of view. Information theory suggests that if both the transmitters and the receiver can track the fading channel, the optimal strategy to maximize the total system throughput is to allow only the user with the best channel to transmit at any time. A similar strategy is also optimal for the downlink (one-to-many). Opportunistic strategies of this type yield a system-wide multiuser diversity gain: the more users in the system, the larger the gain, as there is more likely to have a user with a very strong channel. To implement this concept in a real system, three important considerations are: fairness of the resource allocation across users, delay experienced by the individual user waiting for its channel to become good, and measurement inaccuracy and delay in feeding back the channel state to the transmitters. We discuss how these issues are addressed in the context of IS-865 (also called HDR or CDMA 2000 1x EV-DO), a third-generation wireless data system. A wireless system consists of multiple dimensions: time, frequency, space and users. Opportunistic communication maximizes the spectral efficiency by measuring when and where the channel is good and only transmits in those degrees of freedom. In this context, channel fading is beneficial in the sense that the fluctuation of the channel across the degrees of freedom ensures that there will be some degrees of freedom in which the channel is very good. This is in sharp contrast to the diversity-based approach in Chapter 3, where channel fluctuation is always detrimental and the design goal is to average out the fading to make the overall channel as constant as possible. Taking this philosophy one step further, we discuss a technique, called opportunistic beamforming, in which channel fluctuation can be induced in situations when the natural fading has small dynamic range and/or is slow. From the cellular system point of view, this technique also increases the fluctuations of the interference imparted on adjacent cells, and presents an opposing philosophy to the notion of interference averaging in CDMA systems. Chapters 7, 8, 9 and 10 discuss multi-input multi-output (MIMO) communication
Tse and Viswanath:Fundamentals of Wireless Communication 19 It has been known for a while that the uplink with multiple receive antennas at the base station allow several users to simultaneously communicate to the receiver.The multiple antennas in effect increase the number of degrees of freedom in the system hopargtion of the sin rom the difcntIt haso effe toforpont-to-pontethtip and receive antenna i.e.,even when the antennas of the multiple users are co-located This holds provided that the scattering environment is rich enough to allow the receive antennas separate out the signal from the different transmit antennas,allowing the spatial multiplering of information.This is yet another example where channel fading is beneficial to communication.Chapter 7 studies the properties of the multipath environment that determine the amount of spatial multiple ing po ossible and defines an angular domai 1 properties are seen mos 1 with a class of statistical MIMO channel models,based in the angular domain,which will be used in later chapters to analyze the performance of communication techniques on MIMO channels. Chapter 8 discusses the capacity and capacity-achieving transceiver architectures for MIMO channels,focusing on the fast fading scenario.It is demonstrated that the fast fading capacity inc eases linearly with the minim m of the number of transmit and receive antennas at all values of SNR.At high SNR,the linea e is du to the increase in degrees of freedom due to spatial multiplexing.At low SNR,the linear increase is due to a power gain due to receive beamforming.At intermediate SNR ranges,the linear increase is due to a combination of both these gains.Next. we study the transceiver architectures that achieve the capacity of the fast fading channel.The focus is on the V-BLAST architecture which multiplexes independent data on t smit ant na anr A ariety structures are these include the decorrel tor and the linear】 um mea square-erro (MMSE)receiver.The performance of these receivers can be enhanced by successively canceling the streams as they are decoded:this is known as successive interference cancellation (SIC).It is shown that the MMSE-SIC receiver achieves the capacity of the fast fading MIMO channel. The V-BLAST architecture is very suboptimal for the slow fading MIMO channel cod the thus the diversity by that obtained vith the rooelv antemm array.A modifcaton.am D-BLAS where the data streams are interleaved across the transmit antenna array,achieves the outage capacity of the slow fading MIMO channel.The boost of the outage capacity of a MIMO channel as compared to a single antenna channel is due to a combination of both diversity and spatial multiplexing gains.in chapter 9.we study a fundamental tradeoft n the di s that an be sim ults d ove a slow fad ing MIMO channe This for ed as a unified fram ework to assess both the diversity and multiplexing performance of several schemes which have appeared earlier in the book.This framework is also used to motivate the construction
Tse and Viswanath: Fundamentals of Wireless Communication 19 It has been known for a while that the uplink with multiple receive antennas at the base station allow several users to simultaneously communicate to the receiver. The multiple antennas in effect increase the number of degrees of freedom in the system and allow spatial separation of the signals from the different users. It has recently been shown that a similar effect occurs for point-to-point channel with multiple transmit and receive antennas, i.e., even when the antennas of the multiple users are co-located. This holds provided that the scattering environment is rich enough to allow the receive antennas separate out the signal from the different transmit antennas, allowing the spatial multiplexing of information. This is yet another example where channel fading is beneficial to communication. Chapter 7 studies the properties of the multipath environment that determine the amount of spatial multiplexing possible and defines an angular domain in which such properties are seen most explicitly . We conclude with a class of statistical MIMO channel models, based in the angular domain, which will be used in later chapters to analyze the performance of communication techniques on MIMO channels. Chapter 8 discusses the capacity and capacity-achieving transceiver architectures for MIMO channels, focusing on the fast fading scenario. It is demonstrated that the fast fading capacity increases linearly with the minimum of the number of transmit and receive antennas at all values of SNR. At high SNR, the linear increase is due to the increase in degrees of freedom due to spatial multiplexing. At low SNR, the linear increase is due to a power gain due to receive beamforming. At intermediate SNR ranges, the linear increase is due to a combination of both these gains. Next, we study the transceiver architectures that achieve the capacity of the fast fading channel. The focus is on the V-BLAST architecture which multiplexes independent data streams on to the transmit antenna array. A variety of receiver structures are considered: these include the decorrelator and the linear minimum mean-square-error (MMSE) receiver. The performance of these receivers can be enhanced by successively canceling the streams as they are decoded; this is known as successive interference cancellation (SIC). It is shown that the MMSE-SIC receiver achieves the capacity of the fast fading MIMO channel. The V-BLAST architecture is very suboptimal for the slow fading MIMO channel: it does not code across the transmit antennas and thus the diversity gain is limited by that obtained with the receive antenna array. A modification, called D-BLAST, where the data streams are interleaved across the transmit antenna array, achieves the outage capacity of the slow fading MIMO channel. The boost of the outage capacity of a MIMO channel as compared to a single antenna channel is due to a combination of both diversity and spatial multiplexing gains. In Chapter 9, we study a fundamental tradeoff between the diversity and multiplexing gains that can be simultaneously harnessed over a slow fading MIMO channel. This formulation is then used as a unified framework to assess both the diversity and multiplexing performance of several schemes which have appeared earlier in the book. This framework is also used to motivate the construction