Hệ thống 3G và mạng không dây thông minh P1

Chia sẻ: Do Xon Xon | Ngày: | Loại File: PDF | Số trang:87

0
108
lượt xem
56
download

Hệ thống 3G và mạng không dây thông minh P1

Mô tả tài liệu
  Download Vui lòng tải xuống để xem tài liệu đầy đủ

Although the number of cellular subscribers continues to grow worldwide [27], the predominantly speech-, data- and e-mail-oriented services are expected to be enriched by a whole host of new services in the near future. Thus the performance of the recently standardised Code Division Multiple Access (CDMA)third-generation (3G) mobile systems is expected to become comparableto, if not better than, that of their wired counterparts.

Chủ đề:
Lưu

Nội dung Text: Hệ thống 3G và mạng không dây thông minh P1

  1. Third-Generation Systems and Intelligent Wireless Networking J.S. Blogh, L. Hanzo Copyright © 2002 John Wiley & Sons Ltd ISBNs: 0-470-84519-8 (Hardback); 0-470-84781-6 (Electronic) Third-Generation CDMA Systems K. Yen and L. Hanzo 1.1 Introduction Although the number of cellular subscribers continues to grow worldwide [27], the predom- inantly speech-, data- and e-mail-oriented services are expected to be enriched by a whole host of new services in the near future. Thus the performance of the recently standardised Code Division Multiple Access (CDMA)third-generation (3G) mobile systems is expected to become comparableto, if not better than, that of their wired counterparts. These ambitious objectives are beyond the capabilities of the present second-generation (2G) mobile systems such as the Global System for Mobile Communications known as GSM [28], the Interim Standard-95 (IS-95) Pan-American system, or the Personal Digital Cellular (PDC) system[29] in Japan. Thus, in recent years, a range of new system concepts and objectives were defined, and these will be incorporated in the 3G mobile systems. Both the European Telecommunications Standards Institute (ETSI) andthe International Telecom- munication Union (ITU) are defining a framework for these systems under the auspices of the Universal Mobile Telecommunications System (UMTS) [27,29-331 and the International Mobile Telecommunications scheme the year 2000 (IMT-2000)'[30,31,34]. in Their objectives and the system concepts will bediscussed in more detail in later sections. CDMA is the predominant multiple access technique proposed the 3G wireless commu- for nications systems worldwide. CDMA was already employed in some 2G systems, such as the IS-95 system and it has proved to be a success. Partly motivated by this successer, both the Pan-European UMTS and the IMT-2000 initiatives have opted for a CDMA-based sys- tem, although the European system also incorporates an element of TDMA. In this chapter, we provide a rudimentary introductionto a range of CDMA concepts. Then the European, American and Japanese CDMA-based3G mobile system conceptsare considered, followed by a research-oriented outlook on potential future systems. ' Formerly known as Future Public Land Mobile Telecommunication Systems 1
  2. 2 CHAPTER 1. THIRD-GENERATION SYSTEMS CDMA The chapter is organised as follows. Section 1.2 offers a rudimentary introduction to CDMA in order to make this chapter self-contained, whereas Section 1.3 focuses on the basic objectives and system concepts of the 3G mobile systems, highlighting the European, Amer- ican and Japanese CDMA-based third-generation system concepts. Finally, our conclusions are presented in Section 1.4. 1.2 Basic CDMA System CDMA is a spread spectrum communications technique that supports simultaneous digital transmission of several users’ signals in a multiple access environment. Although the de- velopment of CDMA was motivated by user capacity considerations, the system capacity provided by CDMA is similar to that of its more traditional counterparts, frequency division multiple access (FDMA), and time division multiple access (TDMA) [35]. However, CDMA has the unique property of supporting a multiplicity of usersin the same radio channel with a graceful degradation in performance due to multi-user interference. Hence, any reduction in interference can lead to an increase in capacity [36]. Furthermore, the frequency reuse factor in a CDMA cellular environment can be as high as unity, andbeing a so-called wideband sys- tem, it can coexist with other narrowband microwave systems, which maycorrupt the CDMA signal’s spectrum in a narrow frequency band without inflicting significant interference [37]. This eases the problem of frequency management as well as allowing a smooth evolution from narrowband systems to wideband systems. But perhaps the most glaring advantage of CDMA is its ability to combat or in fact to benefit from multipath fading, as it will become explicit during ourfurther discourse. In the forthcoming sections, we introduce our nomenclature, which will be usedthrough- out the subsequent sections. Further in-depth information on CDMA can be found in a range of excellent research papers [35,37,38] and textbooks [3942]. 1.2.1 SpreadSpectrumFundamentals In spread spectrum transmission, the original information signal, which occupies a bandwidth of B Hz, is transmitted after spectral spreading to a bandwidth N times higher, where N is known as the processing gain. In practical terms the processing gain is typically in the range of 10 - 30 dB [37]. This frequency-domain spreading concept is illustrated in Figure 1.1. The power of the transmitted spread spectrum signal is spread over N times the original bandwidth, while its spectral density is correspondingly reduced by the same amount. Hence, the processing gain is given by: B N = L B’ where B, is the bandwidth of the spread spectrum signal while B is the bandwidth of the original information signal. As we shall see during our further discourse, this unique tech- nique of spreading the information spectrum is the key to improving its detection in a mobile radio environment, and it also allows narrowband signals exhibiting a significantly higher spectral density to share the same frequency band [37]. There are basically two main types of spread spectrum (SS) systems [35]:
  3. 1.2. BASIC CDMA SYSTEM 3 Power density / p watts/Hz watts/Hz 7 - z I -- I ? Frequency B,=BxN > I- Figure 1.1: Power spectral density of signal before and after spreading. 0 Direct Sequence (DS) SS systems and 0 Frequency Hopping (FH)SS systems. 1.2.1.1 Frequency Hopping In FH spreading, which was invoked in the 2G GSM system the narrowband signal is trans- mitted using different carrier frequencies at different times. Thus, the data signal is effectively transmitted over a wide spectrum. There are two classes of frequency hopping patterns. In fast frequency hopping,the camer frequency changesseveral times per transmitted symbol, while in slow frequency hopping, the carrier frequency changes typically after a number of symbols or atransmission burst. In the GSM system each transmission burst of 114 channel- coded speechbits was transmitted on a different frequency andsince the TDMA frame dura- tion was 4.615 ms,the associated hopping frequency its reciprocal, that is, 217 hopdsec. was The exact sequenceof frequency hoppingwill be made known only to the intended receiver so that the frequency hopped pattern may be dehopped in order to demodulate the signal [37]. Direct sequence (DS) spreading more commonlyused in CDMA. Hence, our forthcoming is discussions will be in the context of direct sequence spreading. 1.2.1.2 Direct Sequence In DS spreading, the information signal is multiplied by a high-frequency signature sequence, also known as a spreading code or spreading sequence. Thisuser signature sequence facili- tates the detection of different users’ signals in order to achieve a multiple accesscapability in CDMA. Althoughin CDMA this user ‘separation’ is achieved using orthogonal spreading codes, in FDMA and TDMA orthogonal frequencyslots or time-slots are provided, respec- tively. We can see from Figure1.2 that each information symbol duration T, is broken into N , of equi-spaced subintervals of duration T,, each of which is multiplied with a different chip of
  4. 4 CHAPTER 1. THIRD-GENERATION SYSTEMS CDMA X Figure 1.2: Time-domain waveforms involvedin generating a direct sequence spread signal. m c o s w,t Figure 1.3: BPSK modulated DS-SS transmitter. the spreading sequence. Hence, N , = 9.The resulting output is a high-frequency sequence. For binary signalling T, = Tb, where Tb is the data bit duration. Hence, N , is equal to the processing gain N. However, for M-ary signalling, where M > 2, T, # Tb and hence N , # N . An understanding of the distinction between N, and N is important, since the values of N , and N have a direct effect on the bandwidth efficiency and performance of the CDMA system. The block diagramof a typical binary phase shift keying (BPSK) modulated DS-SS trans- mitter is shown in Figure 1.3. We will now express the associated signals mathematically. The binary data signal may be written as: W
  5. 1.2. BASIC 5 where Tb is the bit duration, bj E {+l, -l} denotes the jth data bit, and F T b ( t ) is the pulse shape of the data bit. In practical applications, rT(t)has a bandlimited waveform, such as a raised cosine Nyquistpulse. However, for analysis and simulation simplicity, we will assume that FT ( t )is a rectangular pulse throughout chapter, which is defined as: this 1, O S t < T , rT(t) = 0, otherwise. Similarly, the spreading sequencemay be written as M + where ah E { 1,-l} denotes the hth chip and FT, (t)is the chip-pulse with a chip duration of Tc.The energyof the spreading sequence over a bit duration of Tb is normalised according to: As seen in Figure 1.3, the data signal and spreading sequence are multiplied, and the resultant spread signal is modulated on carrier in order to produce the wideband signal s ( t ) a at the output: s ( t ) = a b ( t ) a ( t )COS w,t, (1.6) where P is the average transmittedpower. At the intended receiver, the signal is multiplied b by the conjugate of the transmitter's spreading sequence,which is known as the despreading sequence, in order to retrieve the information. Ideally, in a single-user, nonfading, noise- less environment, the original information can be decoded without errors. This is seen in Figure 1.4. In reality, however, the conditions are never so perfect. The received signal will be cor- rupted by noise, interfered by both multipath fading- resulting in intersymbol interference (IS) - and by other users, generating multi-user interference. Furthermore, this signal is delayed by the time-dispersive medium.It is possibleto reduce the interference due to multi- path fadingand other users by innovative signal processing methods,which will bediscussed in more detail in later sections. Figure 1.5 shows the block diagramof the receiver for a noise-compted channel using a correlator for detecting the transmitted signal, yielding: (i+l)Tb bi = sgn{ fl J 1iTb a * ( t ) [ s ( t+ n(t)] ) coswct dt where = Tb x P is the bit energy and sgn(z) is the signum function of x,which returns b a value of 1, if x > 0 and returns a value of -1, if z < 0. In a single-user Additive White
  6. 6 CHAPTER 1. THIRD-GENERATION SYSTEMS CDMA X l- T, = N , x Figure 1.4: Time-domain waveforms involvedin decoding a direct sequence signal l cos wet a* ( t ) Signature sequence Figure 1.5: BPSK DS-SS receiver for AWGN channel. Gaussian Noise (AWGN) channel, the receiver shown in Figure 1.5 is optimum. In fact, the so performance of the DS-SS system discussed far is the same as that of a conventional BPSK modem in an AWGN channel, wherebythe probability of bit error P T b ( c ) is given by: where &(x) = l l o " eVY2/'dy (1.9) is the Gaussian Q-function. The advantages spread spectrum communications and CDMA of will only be appreciated in a multipath multiple access environment. The multipath aspects and how the so-called RAKE receiver [5,43] be used to overcome the multipath effects can will be highlighted in the next section. 1.2.2 The Effect of Multipath Channels In this section, we present an overview of the effects of the multipath wireless channels en- countered in a digital mobile communication system, which treated in depth for example was
  7. 1.2. BASIC CDMA SYSTEM 7 in [ 1l]. Interested readers may also refer to the recent articles written by Sklar in [44,45] for a brief overview on this subject, Since the mobile station is usually close to the ground, the transmitted signal is re- flected, refracted, and scattered from objects in its vicinity, such as buildings, trees, and mountains [35]. Therefore, the received signal is comprised of a successionof possibly over- lapping, delayed replicas of the transmitted signal. Each replica is unique in its arrival time, power, and phase [46]. As the receiver or the reflecting objects are not stationary, such re- flections will be imposed fading on the received signal, where the fading causes the signal strength to vary in an unpredictable manner. This phenomenon is referred to as multipath propagation [ 1l]. There are typically two types of fading in the mobile radio channel [44]: 0 Long-term fading 0 Short-term fading As argued in [l l]long-term fading is caused by the terrain configuration betweenthe base station and the mobile station, such as hills and clumps of buildings, which result in an average signal power attenuation as a function of distance. For our purposes the channel can be describedin terms of its average pathloss, typically obeying an inverse fourth power law [35] and a log-normallydistributed variation around the mean. Thus, long-termshadow fading wasalso referred to as log-normal fading in [ 11,441 . On the other hand, short-term fadingrefers to the dramatic changes in signal amplitude and phase as a result of small changes in the spatial separation between the receiver and transmitter, as we noted in [ 11,441. Furthermore, the motion betweenthe transmitter and receiver results in propagation path changes,such that the channelappears to be time-variant. The time-variant frequency- selective channel was modelled as a tapped delay line in [ 1l], where the complex low-pass impulse response can be modelled as: (1.10) where la1 ( t ) , ~ ( t ) and 7 are the amplitude, phase, and delayof the Zth path, respectively, I I 1 and L is the total number of multipath components. wasargued in [ 1l] that the rate of signal It level fluctuation is determined by the Doppler frequency,fo, which in turn is dependent on the carrier frequency, fc, and the speed of the mobile station W according to (see also page 16 of [47]): (1.11) where c is the speed of light. Typically, the short-term fading phenomenon modelled statistically by a Rayleigh,Ri- is cian, or Nakagami-m distribution [48]. The Rayleigh and Rician distributions were char- acterised for example in [l l]. There have been some contrasting views in the literature as to which of these distributions best describes the fast-fading channel statistically. Although empirical results have shown that the fading statistics are best described by a Nakagami dis- tribution [49], in most cases a Rayleigh-distributed fadingused for analysis and simulation is
  8. 8 CHAPTER 1. THIRD-GENERATION SYSTEMS CDMA I'O 0.9 1 0.8 0.7 0 1 2 3 4 5 6 7 8 9 1 0 Time Delay [PS] Figure 1.6: COST 207 BU impulse response. because of simplicity, and it serves as a useful illustrative example in demonstrating the ef- fects of fading on transmission. Moreover, the Rayleigh distribution is a special case of the Nakagami distribution, when m, known as the fading parameter, is equal to unity (see page 48 in [5]).The Rician distribution is more applicable to satellite communication, due to the presence of a dominant signal component known as the specular component [44], than to large-cell terrestrial communication, where often there is no Line-of-Sight (LOS) path be- tween the terrestrial base station and the mobile station. However, in small microcells often the opposite is true. In our investigations in this chapter, Rayleigh-distributed frequencyse- lective fading is assumed. The delay is proportional the length of the corresponding signal path between trans- to the mitter and receiver. The delay spread due to the path-length differences between the multi- path components causes Intersymbol Interference (ISI) in data transmission, which becomes particularly dominant for high data rates. A typical radio channel impulse response isshown in Figure 1.6. This channel impulse response is known as the COST 207 bad urban (BU) impulse response[50].It can be clearly seen that the response consists of two main groups of delayed propagation paths: a main profile and a smaller echo profile following the main profile at a delay of 5 p . The main profile is caused by reflections of the signal from structures in the vicinity of the receiver with shorter delay times. On the other hand, the echo profile could be caused by several reflections from a larger but more distant object, such as a hill [51]. In either case, we can see that both profiles approximately follow a negative exponentially decaying function with respect to the time-delay. Figure 1.7 shows the impairments of the spread spectrum signal travelling over a multi- path channel with L independent paths, yielding the equivalent baseband received signal of: L (1.12)
  9. 1.2. BASIC CDMA SYSTEM 9 Figure 1.7: Multipath propagation model of the transmitted signal. where q ( t ) is the time-variant complex channel gain, which is given by J a l ( t ) l e j @ p , (in) t Equation l . 10 with a Rayleigh-distributed amplitude, uniformly distributed phase over the interval [ -7r . . .7r] and g ( t - 71) is the equivalent baseband transmitted spread spectrum signal from Equation l .6 delayed by 71. The above equation shows the Zth path is attenuated by that the channel coefficient a ( t )and delayed by 71. Without intelligent diversity techniques [ 5 ] , 1 these paths are added together at the receiver and any phase or delaydifference between the paths may result in a severely multipath interfered signal, corrupted by dispersion-induced intersymbol interference (1.31). Figure 1.8 shows the effect of a nonfading channel and a fading channel on the bit er- ror probability of BPSK-modulated CDMA. Without diversity, the bit error rate (BER) in a fading channel decreases approximately according to Prb(6) M &, where rc the av- is erage Signal-to-Noise Ratio (SNR), and hence plotted on a logarithmic scale according to log Prb(c) = - log47, we have a near-linear curve [ 5 ] . This is different from a nonfad- ing, or AWGN, channel, wherebythe BER decreases exponentially with increasing the SNR. Thus, in a fading channel, a high transmitted power is required to obtain alow probability of error. As we shall see in the next section, diversity techniques can be used to overcome this impediment. 1.2.3 RAKE Receiver As mentioned previously, spread spectrum techniques can take advantage of the multipath nature of the mobile channel in order to improve reception. This is possible due to the sig- nal's wideband nature, which has a significantly higher bandwidth than the multipath chan- nel's coherence bandwidth [52]. In this case, the channel was termed a frequency selective
  10. 10 CHAPTER 1. THIRD-GENERATION SYSTEMS CDMA Figure 1.8: Performanceof BPSK modulated CDMA overvariousRayleigh-fadingchannels.The curves were obtained using perfect channel estimation, and there was no self-interference between diversity paths. fading channel, since different transmitted frequencies faded differently if their separation was higher than the previously mentioned coherence bandwidth. Suppose that the spread spectrum has a bandwidth of B , and the channel’s coherence bandwidth is B,, such that B, > B,. Thus, the number of resolvable independent paths - that is, the paths that fade > near-independently - L R is equal to (1.13) where . is the largest integer thatless than or equal to The number of resolvable paths j1 is x. L R varies according to the environment, and it is typically higher in urban than in suburban areas, since in urban areas the coherence bandwidth is typically lower due to the typically higher delay-spread of the channel [35]. More explicitly, this is a consequence of the more dispersive impulse response, since the coherence bandwidth is proportional to the recipro- cal of the impulses responses delay spread, as it was argued in [ 5 2 ] . Similarly to frequency diversity or space diversity, these LR resolvable paths can be employed in multipath diver- sity schemes, which exploit the fact thatstatistically speaking, the different paths cannot be in deep fades simultaneously; hence, there is always at least one propagation path, which provides an unattenuated channel. These multipath components are diversity paths. Multipath diversity can only be exploited in conjunction with wideband signals. From Equation 1.13, for a narrowband signal, where no deliberate signal spreading takes place,
  11. 1.2. BASIC 11 the signal bandwidth B, is significantly lower than B,. In this case, the channel was termed frequency nonselectivein [52]. Hence, no resolvable diversity paths can be observed, unlike in a wideband situation, and this renders TDMA and FDMA potentially less robust in a narrowband mobile radio channel CDMA.than Multipath diversity is achieved, for example, by a receiver referred to as the RAKE re- ceiver invented by Price and Green [43]. This is the optimum receiver for wideband fading multipath signals. It inherited its name fromthe analogy of a gardenrake, whereby the fingers constitute the resolvable paths. The point where handle and fingers meet is where diversity the combining takes place. There are four basic methods of diversity combining, namely[53]: 0 Selection Combining (SC). 0 Maximal Ratio Combining (MRC). 0 Equal Gain Combining (EGC). 0 Combining of the n best signals (SCn). The performance analysis of selection combining in CDMA can be found in [54,55], while a general comparison of the various diversity combining techniques can be found in [53] for Rayleigh-fading channels. Maximal ratio combining gives the best performance, while selection combining is the simplest to implement. The number of resolvable paths that are combined at the receiver, represents the order of diversity of the receiver, which is denoted here as L P . We note, however, that in practical receivers not all resolvable multipath compo- < nents are combined dueto complexity reasons, that is, L p LR. There are two basic demodulation techniques, namely coherent noncoherent demod- and ulation [5]. We will highlight the basics of coherent demodulation in this section in the context of CDMA. However, before demodulation can take place, synchronisation between the transmitter and the intended receiver has to be achieved. Synchronisation in DS-CDMA is performed by a processknown as code acquisition and tracking. Acquisition is usually carried out by invokingcorrelation techniques between re- the ceiver’s own copy of the signature sequence andthe received signature sequence and search- ing for the displacement between them -associated with specific chip epoch that results a - in the high correlation [37,56,57]. Once acquisition has been accomplished,usually a code tracking loop [58] is employed to achieve fine alignment of the two sequences andto main- tain their alignment. Thedetails of code acquisition and trackingare beyond the scope of this chapter. Interested readers may refer to [59-62:1 and the references therein for an in-depth treatise on this subject. Hence, in this chapter, we will assume that the transmitter and the intended receiver are perfectly synchronised. For optimum performanceof the RAKE receiver using coherent demodulation,the path attenuation and phase must be accurately estimated. This estimation is performed by an- other process knownas channel estimation, which will be elaborated onin Section l .2.6. In typical low-complexity applications, known pilot symbols can be inserted in the transmit- ted sequence in order to estimate the channel’s attenuation and phase rotation. However, for now, let us assume perfect channel estimation order to assess the performance of the RAKE in diversity combiner. Figure 1.9 shows the block diagram of the HPSK RAKE receiver. The received signal is first multiplied by the estimated channel coefficients a1 ( t ) , . . , a ( t ) in each RAKE . ~ ~
  12. 12 CHAPTER 1. THIRD-GENERATION SYSTEMSCDMA a;(t) a*(t - 7 1 ) I I Figure 1.9: RAKE receiver. branch tuned to each resolvable path. For optimumperformance of the RAKE receiver using maximal ratio combining, these channel coefficient estimates should be the conjugates of the actual coefficients of the appropriate paths in order to invert the channel effects.* Note that for equal gain combining only the phase is estimated, and the various path contributions are multiplied by a unity gain before summation. The resulting signals in each RAKE branch are then multiplied by the conjugate signature sequences as wehaveseenin Figure 1.3, delayed accordingly by the code acquisition process. After despreading by the conjugate signature sequencesa*(t - T I ) , . . . , a*(t - T L ~ )the outputs of the correlators in Figure 1.9 , are combined in order obtain the decoded symbolof' to (1.14) Normally, the first term of Equation 1.14, which contains the useful information, is much larger than the despread, noise-related second term. This is because the first term is pro- portional to the sum of the absolute values of the channel coefficients, whereas the second term in Equation 1.14 is proportional to the vectorial sum of the complex-valued channel 'c~leJ91 c~le-J4I = a2 X 1' 3Here we assumed that there is no multipath interference. This interference can be considered as part of multi- user interference, which will be discussed in the next section.
  13. 1.2. BASIC SYSTEM 13 Figure 1.10: CDMA system model. coefficients. Hence, the real part of the first term is typically larger than that of the second term. Thus, the RAKE receiver can enhance the detection of the data signal in a multipath environment. Refemng back to the BER curves of Figure 1.8, we can see that the performance of the system is improved when multipath diversity is used. Better performance is observed by increasing the number of diversity paths LP. However, this also increases the complexity of the receiver, since the number of correlators has to be increased, which is shown in Figure 1.9. 1.2.4 Multiple Access So far, only single-user transmission was considered. The system is simple and straight- forward to implement. Let us now consider how multiple user transmission can affect the performance of the system. Multiple accessis achieved in DS-CDMA by allowing different users to share a common frequency band. Each transmitter and its intended receiver are assigned adistinct user signa- ture sequence. Onlythe receivers having the explicit knowledge of this distinct sequence are capable of detecting the transmitted signal. Consider a CDMA scenariowith K number of active users, transmitting simultaneously. The baseband equivalent system model is shown in Figure l. 10. For simplicity, it is assumed that there is no multipath propagationand perfect power control is maintained. The mathematical representation of the kth user’s data signal is similar to that shown in Equation 1.2, except for an additional superscript, denoting multi-usertransmission. Hence, it is written as: (1.15) + where b i k ) E { 1,-l}. There is a distinct user signature sequence ( t )associated with the kth user, which is similar to that of Equation 1.4, with the exception of a superscript,
  14. 14 CHAPTER 1. THIRD-GENERATION SYSTEMSCDMA differentiating between users: (1.16) h=-m The lcth user’s data signal b(”(t) and signature sequence a(”(t) are multiplied in order to produce an equivalent baseband wideband signal, namely, (1.17) where Pi’) is the average transmit power of the lcth user’s signal. The compositemulti-user baseband received signal is: where d k ) is the propagation delay plus the relative transmission delay of the lcth user with respect to other users, and n ( t )is the AWGN with a double-sided power spectral density of + WMZ. 1.2.4.1 Downlink Interference In the downlink (base station to mobile), the base station is capable of synchronising the transmission of all the users’ signals, such that the symbol durations are aligned with each other. Hence the composite signal is received at each mobile station with d l i ) = 0 for IC = l,2, . . . , K . This scenario is also known as symbol-synchronous transmission. Using the conventional so-called single-user detector, each symbol of the jth user is retrieved from the received signal r ( t ) by correlating it with the jth user’s spreading code in order to give: (1.19)
  15. 1.2. BASIC CDMA SYSTEM 15 Substituting Equation 1.18 into Equation 1.19 yields: = sgn -- 5 @b!j) wanted signal + k=l k#j &,k)bik)Rjk noise multiple access interference white (1.20) where R j k is the cross-correlation of the spreading codes of the kth and jth user for iTb 5 + t 5 (i l)Tb, which is given by: (1.21) There will be no interference from the other users if the spreading codesare perfectly orthog- onal to each other. That is, R j k = 0 for all k # j . However, designing orthogonal codes a for large number of users is extremely complex. The so-called Walsh-Hadamard codes [63] used in the IS-95 system excel terms of achieving orthogonality. in 1.2.4.2 Uplink Interference In contrast to the previously considered downlinkscenario, in practical systems perfect or- thogonality cannotbe achieved in the uplink (mobileto base station), since there is no coor- dination in the transmission of the users' signals. In CDMA, all users transmit in the same frequency band in an uncoordinated fashion. Hence, d k ) # 0, and the corresponding sce- nario is referred to as an asynchronous transmission scenario. In this case, the time-delay d k ) , k = 1, ..., K , has to be included in the calculation. Without loss of generality, it can be assumed that = 0 and that 0 < T ( ~ < d 3 ) < ... < d K ) < T b . In contrast to the ) synchronous downlink scenario Equation 1.19, the demodulation of the ith symbol of the of jth user is performed by correlating the received signal r ( t ) with a ( j ) * t )delayed by #, (
  16. 16 CHAPTER 1. THIRD-GENERATION SYSTEMSCDMA yielding: (1.22) where i ( j ) is the estimated time-delay at receiver. the Substituting Equation1.18 into Equation 1.22 assuming perfect code acquisition and and tracking yield:4 4For perfect code acquisition and tracking, ? ( j ) = ~ ( j ) .
  17. 1.2. BASIC CDMA SYSTEM 17 - wanted signal k=L p multiple access interference k=l k=.7+l v multiple access interference K - (1.24) k=j+l ~ white-noise I multiple access interference where R j k ( i ) and f i j k ( i ) ,i E { + l ,0 , - l } represent the cross-correlation of the spreading codes due to asynchronous transmissions, which are given by [64]: (1.25) and and is limited to +l,0 , - 1, since the maximum path delay is assumed to be limited to one symbol duration, asmentioned in Section 1.2.2. Equations 1.24 and 1.20 represent the estimated demodulated data symbol of the jth user at the base station and mobile station, respectively. Both contain the desired symbol of the j t h user. However, thisis corrupted by noise and interference from the other users. This inter- ference is known as multiple access interference (MAI). It contains the undesired interfering signals from the other ( K - 1 ) users. The MA1 arises due to the nonzero cross-correlation of the spreading codes. Ideally, the spreading codes should satisfy the orthogonality property such that (1.27) However, it is impossible to design codes thatare orthogonal for all possible time offsets im- posed by the asynchronous uplink transmissions. Thus there will always be MA1 in the up- link. These observations are augmented by comparing the terms of Equations 1.20 and 1.24. On the other hand, multipath interference is always present in both the forward and re- verse link. Multipath interference is due to the different arrival times of the same signal via
  18. 18 CHAPTER 1. THIRD-GENERATION SYSTEMSCDMA the different paths at the receiver. This is analogous to the signals transmitted from other users; hence, multipathinterference is usually analysed in the same way as MAI. As the number of users increases, the MA1 increases too. Thus, the capacity of CDMA is known to be interference limited. CDMA is capable accommodating additional users at of the expense of a gradual degradationin performance in a fixed bandwidth, whereas TDMA or FDMA would require additional bandwidth to accommodate additional users. Intensive research has been carried out to find ways of mitigating the effects of MAI. Some of the methods include voice activity control, spreading code design, power control schemes, and sectored/adaptive antennas[65]. These methods reduce MA1 to a certain extent. the The most promising uplink method so far has been in the area of multi-user detection, which was first proposed by Verdii [66]. Multi-user detection [6749], which will be dis- cussed in more depth in the next chapter, invokes the knowledge of all users’ signature se- quences and all users’ channel impulse response estimates order to improve the detection in of each individual user. The employment of this algorithm is more feasible for the uplink, because all mobiles transmit to the base station and the base station has to detect all the users’ signals anyway. The topic of multi-user detection is however beyondthe scope of this chapter, since it will be discussed in a little more detail in the next chapter, namely in Chap- ter 2. For a more indepth treatment the interested readers are referred to Verdu’s excellent book [70], which provides a comprehensive discussion on topic. A generalreview of the the various multi-user detection schemes further references can also be found, for example, and in Moshavi’s contribution[65]. Another shortcomingof CDMA systemsis their susceptibility to the near-far problem to be highlighted below. If all users transmit at equal power, then signals from users near the base station are received at a higher power than those from users at a higher distance due to their different path-losses. The effects of fading highlighted in Section 1.2.2 also contribute to the power variation. Hence, accordingto Equation 1.24, if the jth user is transmitting from the cell border and all other users are transmitting near the base station, then the desired jth user’s signal will be masked by the other users’ stronger signals, which results in a high bit error rate. In order to mitigate this so-called near-far problem, power control is used to ensure that all signals from the users are received at near-equal power, regardless of their distance from the base station. There are typically two basic types of power control [38]: 0 open-loop power control 0 closed-loop power control Open-loop power control is usually used to overcome the variation in power caused by pathloss. On the other hand, closed-loop power control is used to overcome shadow fad- ing caused by multipath. The details of the various power control techniques will not be elaborated onin this chapter. Readers may refer to [7l] for more information. 1.2.4.3 Gaussian Approximation In order to simplify any analysis involving multi-user transmission in CDMA, the MA1 is usually assumed to be Gaussian distributed by virtue of the central limit theorem [72-741. This assumption is fairly accurate even for K < 10 users, when the BER is lop3 or higher.
  19. 1.2. BASIC 19 We will use the standard Gaussian approximation theory presented Pursley [72] to repre- by sent the MAI. When the desired user sequence is chip- and phase-synchronous with all the interfering sequences, wherethe phase-synchronous relationship is defined as in the absence of noise, the worst-case probability of error Prb ( E ) performance was given by Pursley [72] as: (1.28) where Q(.) is the Gaussion Q-functionof Equation 1.9, since the synchronous transitions do not generate pure random Gaussian-like impairments. This formula would be characteristic of the synchronous downlink scenario Section l .2.4.1. However, in practical uplink situa- of tions as augmented in Section 1.2.4.2, there is always some delay among users, and each the received signal will be phase-shifted independently. In this case, according to Pursley, the probability of error in the absence of noise will be [72]: (1.29) Equation 1.29 represents the best performance corresponding to Gaussian-like impair- ments. In between these two extremes are situations whereby, in the first case, the desired sequence andthe interfering sequence are chip synchronousbut not phase synchronous. The probability of error in the absence of noise is given by [72]: (1.30) In the second case, the desired sequence and interfering sequence are phase synchronous but not chip synchronous. Hence, the probability of error in the absence of noise is given by [72]: (1.31) Analysing the above equations,it can be seen that by increasing the number of chips N , per symbol,the performance of the system will be improved. However, there is alimitation to the rate of the spreading sequence based on Digital Signal Processing (DSP) technology. Fig- ure 1.1 1 compares the simulated results with the numerical results given by Equations 1.28 to I .3 1 for a binary systemwith a processing gain 7. The figure shows that the assumption of of Gaussian distributed MA1 is valid, especially for a high number of users. It also demon- strates that CDMA attains its best possible perfolmance an asynchronous multi-user trans- in mission system. This is an advantage over TDMA and FDMA because TDMAand FDMA require some coordination amongthe transmitting users, which increases the complexity of the system.
  20. 20 CHAPTER 1. THIRD-GENERATION SYSTEMSCDMA A Chip & phase sync 0 Phase sync 0 Chipsync 0 Async ,n-4 I I Figure 1.11: Probability of error against number of users using Equations 1.28, 1.29, 1.30, and 1.31. Markers: Simulation;solid line: Numerical computation. The processing gain is 7. 1.2.5 Spreading Codes As seen previously, the choice of spreading codes plays an important role in DS-CDMA. The a main criteria for selecting particular setof user signature sequences in CDMA applications are that the number possible differentsequences in the set forany sequence length must be of high inorder toaccommodate a high number of users in a cell. The spreading sequences must also exhibitlow cross-correlations for the of reducing the multi-user interference during sake demodulation. A high autocorrelation main-peak to secondary-peak ratio - as indicated by Equation 1.27 - is also essential, in order to minimise the probability of so-called false alarms during code acquisition. This also reduces the self-interference among the diversity paths. Below we provide a brief overview of a few differentspreading sequences. 1.2.5.1 m-sequences Perhaps the most popular set of codes known are the m-sequences[ 5 ] . An m-sequence with a periodicity of n = 2" - l can be readily generated by an m-stage shift register with linear feedback, as shown in Figure 1.12. The tap coefficients c l , c2, . . . , c, can be either 1 (short circuit) or 0 (open circuit). In- formation on the shiftregister feedback polynomials, describing the connections between the register stagesand the modulo-2 adders can be found, for example, in [5]. Note that in spread spectrum applications, theoutput binary sequences of 0,l are mapped into a bipolar sequence
Đồng bộ tài khoản