wireless systems François Horlin, Frederik Petré, Eduardo Lopez-Estraviz, and Frederik Naessens

Transcription

1 antennas for 4G wireless systes François Horlin, Frederik Petré, 33Multiple Eduardo Lopez-Estraviz, and Frederik Naessens It is not the strongest species that will survive, nor the ost intelligent, but the one ost responsive to change. Charles Darwin New air interfaces New air interfaces for 4G broadband cellular networks are being developed under the auspices of the World Wireless Research Foru (WWRF) [1] and the IEEE study group for Mobile Broadband Wireless Access (MBWA) [2]. Cellular systes of the third generation (3G) are based on the recently eerged direct-sequence code-division ultiple-access (DS-CDMA) technique [3]. Intrinsically, DS-CDMA has interesting networking abilities. First, the counicating users do not need to be tie synchronized in the uplink. Second, soft handover is supported between two cells aking use of different codes at the base stations. However, the syste suffers fro intersybol interference (ISI) and ultiuser interference (MUI) caused by ultipath propagation, leading to a high loss of perforance. The use of the orthogonal frequency-division ultiplexing (OFDM) odulation is widely envisaged for wireless local area networks (WLANs) [4]. At the cost of the addition of a cyclic prefix, the tie-dispersive channel is seen in the frequency doain as a set of parallel independent flat subchannels and can be equalized at a low-coplexity. Using channel state inforation (CSI) at the transitter, the constellation on each subchannel can be further adapted to the quality, which is known as bit loading for adaptive OFDM. An alternative approach to OFDM, that benefits fro the sae low-coplexity equalization property, is single-carrier block transission (SCBT), also known as single-carrier (SC) odulation with cyclic prefix [5, 6]. As a counterpart of adaptive OFDM, it has been proposed to use a decision-feedback equalizer at the receive side of an SCBT syste. It has been shown in [7, 8] that the two approaches perfor equally well. Since the SCBT technique benefits fro a lower peak-to-average power ratio (PAPR), [9] encourages

2 684 Multiple antennas for 4G wireless systes s [j] x [n] x [n] s [j] θ N inter c [n] F H Q T u [n] Intrablock spreading Interblock spreading IFFT redundancy Figure GMCBS-CDMA transitter odel. the use of SCBT in the uplink and OFDM in the downlink in order to reduce the constraints on the analog front end and the processing coplexity at the terinal. There are potential benefits in cobining OFDM (or SCBT) and DS-CDMA. Basically the frequency-selective channel is first equalized in the frequency doain using the OFDM odulation technique. DS-CDMA is applied on top of the equalized channel, keeping the interesting orthogonality properties of the codes. The DS-CDMA signals are either spread across the OFDM carriers, leading to ulticarrier CDMA (MC-CDMA) [10, 11, 12, 13], or along the carriers, leading to ulticarrier block-spread CDMA (MCBS-CDMA) [14, 15, 16, 17, 18, 19]. The SCBT counterparts naed here single-carrier CDMA (SC-CDMA) and singlecarrier block-spread CDMA (SCBS-CDMA) have also been proposed in [20, 21] and [19, 22, 23], respectively Generic transission schee We propose a transission chain coposed of generic blocks and able to instantiate all the counication odes cobining OFDM/SCBT and TDMA/CDMA as special cases. In contrast with the transceiver proposed in [24, 25, 26] that relies on a sharing of the set of carriers to retain the orthogonality between the users, our transission schee relies on orthogonal CDMA, and thus inherits the nice advantages of CDMA related to universal frequency reuse in a cellular network, like increased capacity and siplified network planning. Furtherore, the focus is especially put on the counication odes eerging in the standards. The generalized (G) MCBS-CDMA transission schee for the th user ( 1,..., M) is depicted in Figure Since we focus on a single-user transission, the transission schee applies to both the uplink and downlink. In the uplink, the different user signals are ultiplexed at the receiver, after propagation through their respective ultipath channels. In the downlink, the different user signals are ultiplexed at the transitter, before the inverse fast Fourier transfor (IFFT) operation. The GMCBS-CDMA transission schee coprises four basic operations: intrablock spreading, interblock spreading, IFFT, and adding transit redundancy. The inforation sybols s [j] are first serial-to-parallel converted into blocks of B sybols, leading to the block sequence [ s [j]: s [jb] s [(j +1)B 1] ] T. (33.1)

3 François Horlin et al. 685 The blocks s [j] are linearly precoded with a Q B (Q B)atrix,θ, which possibly introduces soe redundancy and spreads the sybols in s [j] with length-q codes: s [j]: θ s [j]. (33.2) We refer to this first operation as intrablock spreading, since the inforation sybols s [j] are spread within a single precoded block, s [j]. The precoded block sequence s [j] is then block spread with the length-n inter code sequence c [n] leading to N inter successive chip blocks: x [n]: s [j]c [ n jn inter ], (33.3) where j n/n inter.werefertothissecondoperationasinterblock spreading, since the inforation sybols s [j] are spread along N inter different chip blocks. The third operation involves the transforation of the frequency doain chip block sequence x [n] into the tie-doain chip block sequence: x [n]: F H Q x [n], (33.4) H where F is the Q Q IFFT atrix. Finally, the K Q (K Q) transitatrix Q T possibly adds soe transit redundancy to the tie-doain chip blocks: u [n]: T x [n]. (33.5) T With K Q + L, T T : [I, I T cp cp Q ]T,whereI is the identity atrix of size Q Q and I consists of the last L rows of I, T adds redundancy in the for of cp Q a length-l cyclic prefix (CP). The chip block sequence u [n] is parallel-to-serial converted into the scalar sequence [u [nk] u [(n +1)K 1]] T : u [n] and transitted over the air at a rate 1/T c. In the following, we will detail how our generic transission schee instantiates different counication odes and, thus, supports different eerging counication standards. We distinguish between the ulticarrier odes, on the one hand, and the single-carrier odes, on the other hand Instantiation of the ulticarrier odes The ulticarrier (MC) odes always coprise the IFFT operation and add transit redundancy in the for of a cyclic prefix (T T ). The MC odes coprise cp MC-CDMA and MCBS-CDMA as particular instantiations of GMCBS-CDMA.

4 686 Multiple antennas for 4G wireless systes MC-CDMA As we have indicated in the introduction, MC-CDMA first perfors classical DS- CDMA sybol spreading, followed by OFDM odulation, such that the inforation sybols are spread across the different subcarriers [10, 11, 12]. With Q BN intra and N intra the intrablock spreading code length, the Q B intrablock spreading atrix θ β spreads the chips across the subcarriers, where the th user s Q B spreading atrix β is defined as β : I a, (33.6) B with a : [a [0] a [N intra 1]] T the th user s N intra 1codevector and the Kronecker product. The interblock spreading operation is discarded by setting N inter 1. Since it does not preserve the orthogonality aong users in a frequency-selective channel, MC-CDMA requires advanced ultiuser detection for uplink reception in the base station, and frequency doain chip equalization for downlink reception in the obile station. MC-CDMA has been proposed as a candidate air interface for future broadband cellular systes [13] MCBS-CDMA The MCBS-CDMA transission schee is the only MC ode that coprises the interblock spreading operation N inter N. Asdetailedin[18, 19], by relying on block spreading, MCBS-CDMA retains the orthogonality aong users in both the uplink and downlink, even after propagation through a frequency-selective channel. Hence, it converts a difficult ultiuser detection proble into an equivalent set of sipler and independent single-user equalization probles. In case no CSI is available at the transitter, it perfors linear precoding to robustify the transitted signal against frequency-selective fading. In case CSI is available at the transitter, it allows to optiize the transit spectru of each user separately through adaptive power and bit loading. Note that classical MC-DS-CDMA can be seen as a special case of MCBS-CDMA, since it does not include linear precoding, but, instead, relies on bandwidth consuing forward error correction (FEC) coding to enable frequency diversity [14, 15, 17] Instantiation of the single-carrier odes The single-carrier (SC) odes eploy a fast Fourier transfor (FFT) as part of the intrablock spreading operation to annihilate the IFFT operation. For ipleentation purposes, however, the IFFT is siply reoved (and not copensated by an FFT), in order to iniize the ipleentation coplexity. The SC odes rely on cyclic prefixing (T T ) to ake the channel appear circulant. The SC odes cp coprise SC-CDMA and SCBS-CDMA as particular instantiations of GMCBS- CDMA.

5 François Horlin et al SC-CDMA The SC-CDMA transission schee, which cobines SCBT with DS-CDMA, can be interpreted as the SC counterpart of MC-CDMA [20, 21]. This ode is captured through our general transission schee, by setting Q BN intra. The intrablock spreading atrix θ FQ β with β defined in (33.6), perfors sybol spreading on the B inforation sybols, followed by an FFT operation to copensate for the subsequent IFFT operation. The interblock spreading operation is left out by setting N inter SCBS-CDMA The SCBS-CDMA transceiver can be considered as the SC counterpart of MCBS- CDMA. It is the only SC ode that entails the interblock spreading operation N inter N. The intrablock spreading atrix θ FQ only perfors an FFT operation to copensate for the subsequent IFFT operation. Like MCBS-CDMA, SCBS-CDMA retains the orthogonality aong users in both the uplink and downlink, even after propagation through a frequency-selective channel, and, hence, converts a difficult ultiuser detection proble into an equivalent set of sipler and independent single-user equalization probles Multiple antennas To eet the data rate and quality-of-service (QoS) requireents of future broadband cellular systes, their spectral efficiency and link reliability should be considerably iproved, which cannot be realized by using traditional single-antenna counication techniques. To achieve these goals, ultiple-input ultipleoutput (MIMO) systes, which deploy ultiple antennas at both ends of the wireless link, exploit the extra spatial diension, besides the tie, frequency, and code diensions, which allows to significantly increase the spectral efficiency and to significantly iprove the link reliability relative to single-antenna systes [27, 28, 29]. On the one hand, MIMO systes proise huge capacity gains by creating a nuber of spatial pipes, through which independent inforation streas can be siultaneously transitted at the sae frequency [30, 31]. This is called the spatial ultiplexing gain. On the other hand, they also enable huge perforance gains by creating an independently fading channel between each transit/receive antenna pair, which allows to receive any independently fading replicas of the sae signal [32, 33, 34]. This is called the spatial diversity gain. However, until very recently, the ain focus of MIMO research was on singleuser counications over narrowband channels, thereby neglecting the ultiple access aspects and the frequency-selective fading channel effects, respectively. In this section, we deonstrate the rewarding synergy between existing and evolving MIMO counication techniques and our generalized MCBS-CDMA transission technique, which allows to increase the spectral efficiency and to iprove the link reliability of ultiple users in a broadband cellular network.

6 688 Multiple antennas for 4G wireless systes s 1 [j] s 1 [j] x 1 [n] x 1 [n] d 1 [i] θ N inter c [n] F H Q T u 1 [n]. MIMO coding s N T [j] s N T [j] x N T [n] x N T [n]. d N T [i] θ N inter c [n] F H Q T u N T [n] Figure MIMO GMCBS-CDMA transitter odel. The generic transission odel is extended in Figure 33.2 to include the two types of MIMO techniques. We assue N T antennas at the transit side and N R antennas at the receive side. The inforation sybols dn T [i], which are assued independent and of variance equal to σd 2, are first serial-to-parallel converted into blocks of B sybols, leading to the block sequence [ d n T [i]: d nt [ib] dn [ ]] T T (i +1)B 1 (33.7) for n T 1,..., N T. A MIMO coding operation is perfored across the different transit antenna streas, that results into the N T antenna sequences, s n T [j], input to the generic transission schee Space-division ultiplexing On the one hand, MIMO systes create N in parallel spatial pipes, which allows to realize an N in -fold capacity increase in rich scattering environents, where N in in {N T, N R } is called the spatial ultiplexing gain [27, 28, 29]. Specifically, space-division ultiplexing (SDM) techniques exploit this spatial ultiplexing gain, by siultaneously transitting N in independent inforation streas at the sae frequency [30] (see also [31]).In [35], SDM is cobined with SC-CDMA to increase the data rate of ultiple users in a broadband cellular network. In this section, we cobine our generalized MCBS-CDMA transission schee with SDM, which allows to instantiate all cobinations of SDM with OFDM/SCBT and TDMA/CDMA as special cases. The SDM technique is ipleented by sending independent streas on each transit antenna n T,asexpressed in where j i Space-tie block coding s n T [j] d n T [i], (33.8) On the other hand, MIMO systes also create N T N R independently fading channels between the transitter and the receiver, which allows to realize an N T N R -fold

7 François Horlin et al. 689 diversity increase, where N T N R is called the ultiantenna diversity gain. Specifically, space-tie coding (STC) techniques exploit diversity and coding gains, by encoding the transitted signals not only over the teporal doain but also over the spatial doain [32, 33, 34]. Space-tie block coding (STBC) techniques, introduced in [33] forn T 2 transit antennas, and later generalized in [34] for any nuber of transit antennas, are particularly appealing because they facilitate ML detection with siple linear processing. However, these STBC techniques have originally been designed for frequency-flat fading channels exploiting only ultiantenna diversity of order N T N R. Therefore, tie-reversal (TR) STBC techniques, originally proposed in [36] for single-carrier serial transission, have been cobined with SCBT in [37, 38] for signaling over frequency-selective fading channels. While [37] only exploits ultiantenna diversity, [38] exploits ultiantenna as well as ultipath diversity, and achieves axiu diversity gains of order N T N R (L +1)overfrequencyselective fading channels, where L is the order of the underlying ultipath channels. In [39, 40], the TR-STBC technique of [37] is cobined with SC-CDMA to iprove the perforance of ultiple users in a broadband cellular network. Although this technique enables low-coplexity chip equalization in the frequency doain, it does not preserve the orthogonality aong users and, hence, still suffers fro ultiuser interference. The space-tie coded ultiuser transceiver of [19, 41], which cobines the TR-STBC technique of [38] with SCBS-CDMA, preserves the orthogonality aong users as well as transit streas, regardless of the underlying ultipath channels. This allows for deterinistic ML user separation through low-coplexity code-atched filtering as well as deterinistic ML transit strea separation through linear processing. Moreover, applying ML Viterbi equalization for every transit strea separately guarantees sybol recovery. Therefore, axiu diversity gains of N T N R (L+1) can be achieved for every user in the syste, irrespective of the syste load. Another alternative to reove MUI deterinistically in a space-tie coded ultiuser setup [42, 43] cobines generalized ulticarrier (GMC) CDMA, originally developed in [24], with the STBC techniques of [34] but ipleented on a per-carrier basis. In this section, we cobine our generalized MCBS-CDMA transission schee with STBC, which encopasses the previously discussed space-tie coded ultiuser transceivers as special cases. For conciseness, we liit ourselves to the case of N T 2 transit antennas. The STBC coding is ipleented by coding the two antenna streas across two tie instants, as expressed in [ s ] [ 1 [j] d ] 1 [i] s 2 [j] d, 2 [i] [ s ] 1 [j +1] s 2 [j +1] χ d 1 [i] d 2 [i], (33.9)

8 690 Multiple antennas for 4G wireless systes where i j/2 and χ : χ χ N T B with χ N T : [ ] 0 1. (33.10) 1 0 In the case of the MC odes, the STBC coding is applied in the frequency doain on a per-carrier basis, so that χ : I. (33.11) B B In the case of the SC odes, the STBC coding is applied in the tie doain by further peruting the vector eleents, so that T χ : F F B B B (33.12) is a B B perutation atrix ipleenting a tie reversal. It is easily checked that the transitted block at tie instant j + 1 fro one antenna is the tie-reversed conjugate of the transitted sybol at tie instant j fro the other antenna (with possible perutation and sign change). As we will show later, this property allows for deterinistic transit strea separation at the receiver, regardless of the underlying frequency-selective channels Receiver design Cyclo-stationarization of the channels Adopting a discrete-tie baseband equivalent odel, the chip-sapled received signal at antenna n R (n R 1,..., N R ), v nr [n], is the superposition of a channeldistorted version of the MN T transitted user signals, which can be written as v nr [n] M N T L 1 n T 1 l0 h n R,n T [l]u n T [n l]+w nr [n], (33.13) where h n R,n T [l] is the chip-sapled finite ipulse response (FIR) channel of order L that odels the frequency-selective ultipath propagation between the th user s antenna n T and the base station antenna n R, including the effect of transit/receive filters and the reaining asynchronis of the quasi-synchronous users, and w nr [n] is additive white Gaussian noise (AWGN) at the base station antenna n R with variance σ 2 w. Furtherore, the axiu channel order L, that is, L ax {L }, can be well approxiated by L (τ ax,a + τ ax,s )/T c +1,where τ ax,a is the axiu asynchronis between the nearest and the farthest users of the cell, and τ ax,s is the axiu excess delay within the given propagation environent. The received sequence v nr [n] is serial-to-parallel converted into the block sequence v nr [n]:[v nr [nk] v nr [(n +1)K 1]] T, assuing perfect tie and

9 François Horlin et al. 691 frequency synchronization. Fro the scalar input/output relationship in (33.13), we can derive the corresponding block input/output relationship v nr [n] M N T 1 n T 1 ( ) H [0] u n T [n]+h [1] u n R,n T n T [n 1] + w nr [n], n R,n T (33.14) where w nr [n]:[w nr [nk] w nr [(n +1)K 1]] T is the corresponding noise block sequence, H [0] is a K K lower triangular Toeplitz atrix with entries n R,n T [H h n R,n T [0]]p,q n R,n T [p q], and H [1] is a K K upper triangular Toeplitz n R,n T atrix with entries [H h n R,n T [1]]p,q n R,n T [K + p q] (see, e.g., [24] foradetailed derivation of the single-user case). The delay-dispersive nature of ultipath propagation gives rise to so-called interblock interference (IBI) between successive blocks, which is odeled by the second ter in (33.14). The Q K receive atrix R reoves the redundancy fro the chip blocks, that is, y nr [n] : R v nr [n]. With R R [0, I ] in which 0 is a atrix cp Q L Q Q L of zeros of size Q L, R again discards the length-l cyclic prefix. The purpose of the transit/receive pair is twofold. First, it allows for siple block-by-block processing by reoving the IBI, that is, R H [1] T 0, provided the CP length is at n R,n T least the axiu channel order L. Second, it enables low-coplexity frequency doain processing by aking the linear channel convolution appear circulant to the received block. This results in a siplified block input/output relationship in the tie doain: y nr [n] M N T 1 n T 1 Ḣ n R,n T x n T [n]+z nr [n], (33.15) where Ḣ n R,n T R H [0] T is a circulant channel atrix, and z n R,n T nr [n] R w nr [n] is the corresponding noise block sequence. Note that circulant atrices can H be diagonalized by FFT operations, that is, Ḣ F Λ F,whereΛ n R,n T Q n R,n T Q n R,n T is a diagonal atrix coposed of the frequency doain channel response between the th user s antenna n T and the base station antenna n R Matrical odel The generalized input/output atrix odel that relates the MIMO coded sybol vector defined as s[j]: [s 1 [j] T s M [j] T] T (33.16)

10 692 Multiple antennas for 4G wireless systes with [ s [j]: s 1 [j] T s N T [j] T] T, (33.17) for 1,..., M, to the received and noise vectors defined as [ y[j]: y1 [j] T y NR [j] T] T, [ z[j]: z1 [j] T z NR [j] T] (33.18) T with [ [ ] y [j]: jninter nr (ynr ) T [ (znr[ ] z nr [j]: ) T jninter ( ynr[ (j +1)Ninter 1 ]) T ] T, ( [ znr (j +1)Ninter 1 ]) ] T T, (33.19) for n R 1,..., N R,isgivenby y[j] C F H Λ θ s[j]+z[j], (33.20) where the channel atrix is 1 Λ 0 1 Q..... M 0 Λ Q 1 Λ :... 1 Λ 0 N R Q M 0 Λ Q N R (33.21) with [ Λ : Λ n R n R,1 Λ n R,N T ], (33.22)

11 François Horlin et al. 693 for 1,..., M and n R 1,..., N R,and 1 I θ N T θ. 0 N TQ N T B F I NRM F Q, 0 NTQ NT B.... M, I θ NT (33.23) [ 1 C I C NR C M ] in which C : c I Q (33.24) with c : [c [0],..., c [N inter 1]] T Space-division ultiplexing Taking (33.8) into account, the odel (33.20) is extended to the SDM input/output atrix odel ( ) H y sd [i] C F Λ θ χ d[i]+z sd sd sd sd sd [i], (33.25) sd where the vector of transitted sybols is defined as d[i]: [d 1 [i] T d M [i] T] T (33.26) with d [ [i]: d 1 [i] T d N T [i] T] T, (33.27) for 1,..., M, and the received and noise vectors are defined as y sd [i]: y[j], z sd [i]: z[j], χ : I MNTB, sd θ : θ, sd Λ : Λ, sd F : F, sd C : C. sd (33.28)

12 694 Multiple antennas for 4G wireless systes Space-tie block coding Taking (33.9) into account, the odel (33.20) is extended to the STBC input/output atrix odel ( ) H y stbc [i] C F Λ θ χ d[i]+z stbc stbc stbc stbc stbc [i], (33.29) stbc where the vector of transitted sybols is given in (33.27) assuing N T 2, the received and noise vectors are defined as [ ] y[j] y [i]: stbc y[j +1], [ ] z[j] z stbc [i]: z[j +1], I MN χ : TB I stbc χ, M θ 0 θ : MNTQ MN T B, stbc 0 θ MN TQ MN TB Λ 0 Λ : NRMQ MN TQ, stbc 0 Λ N RMQ MN TQ F 0 F : NRMQ NRMQ, stbc 0 F N RMQ N RMQ C 0 C : NRNinterQ N RMQ. stbc 0 C N RN interq N RMQ (33.30) Multiuser joint detector In order to detect the transitted sybol block of the pth user d p [i], based on the received sequence of blocks within the received vector, y ode [i] ( ode stands for SDM or STBC ), a first solution consists of using a single-user receiver, that inverts successively the channel and all the operations perfored at the transitter. The single-user receiver relies iplicitly on the fact that CDMA spreading has been applied on top of a channel equalized in the frequency doain. After CDMA despreading, each user strea is handled independently. However the single-user receiver can fail in the uplink where ultiple channels have to be inverted at the sae tie.

13 François Horlin et al. 695 The optial solution is to jointly detect the transitted sybol blocks of the different users within the transitted vector, d[i], based on the received sequence of blocks within the received vector, y ode [i]. The optiu linear joint detector according to the iniu ean square error (MMSE) criterion is coputed in [44]. At the output of the MMSE ultiuser detector, the estiate of the transitted vector is ( d[i] σ 2 w I + G MN TB σ 2 d ) 1 H G H G y ode ode ode ode [i], (33.31) where ( ) H G : C F Λ θ χ. (33.32) ode ode ode ode ode ode The MMSE linear joint detector consists of two ain operations [44, 45]. (i) First, a filter atched to the coposite ipulse responses ultiplies the received vector in order to iniize the ipact of the white noise. The atched filter consists of the CDMA interblock despreading, the FFT operator to ove to the frequency doain, the axiu ratio cobining (MRC) of the different received antenna channels, the CDMA intrablock despreading, the IFFT to go back to the tie doain in case of the SC-odes, and the STBC de-coding. (ii) Second, the output of the atched filter is still ultiplied by the inverse of the coposite ipulse response autocorrelation atrix of size MN T B that itigates the reaining intersybol, interuser, and interantenna interference. When no linear precoding is considered in the syste, it can be easily checked that the linear MMSE estiate in (33.31) exactly reduces to the single-user receiver estiate for the MC and SCBS-CDMA systes. In the case of MC and SC-CDMA, however, the linear MMSE receiver is different fro the single-user receiver and suffers fro a higher coputational coplexity. Fortunately, both the initialization coplexity, which is required to copute the MMSE receiver, and the data processing coplexity can be significantly reduced for MC and SC-CDMA, by exploiting the initial cyclo-stationarity property of the channels. Based on a few perutations and on the properties of the block circulant atrices given in [20], it can be shown that the initial inversion of the square autocorrelation atrix of size MN T B can be replaced by the inversion of B square autocorrelation atrices of size MN T Results In this section, a perforance coparison between the different counication odes is ade, which can serve as an input for an optial ode selection strategy. We consider a static cellular syste, which operates in an outdoor urban icrocell propagation environent. According to the 3GPP TR spatial channel odel, this propagation environent is characterized by a specular ultipath

14 696 Multiple antennas for 4G wireless systes with a ean excess delay of 251 nanoseconds, a ean angle spread at the base station of 19 degrees and a ean angle spread at the obile terinals of 68 degrees. The syste operates at a carrier frequency of 2 GHz, with a syste bandwidth of 5 MHz. We assue an antenna spacing of 2 wavelengths at the obile terinals and of 10 wavelengths at the base station. Monte-Carlo siulations have been perfored to average the bit error rate (BER) over 500 stochastic channel realizations and to copute the corresponding goodput defined as the actual throughput offered to the user assuing a retransission of the erroneous packets. The inforation bandwidth is spread by a spreading factor (SF) equal to 8. The user signals are spread by periodic Walsh-Hadaard codes for spreading, which are overlayed with an aperiodic Gold code for scrabling. QPSK, 16-QAM, or 64- QAModulationisusedwithQ 128 subchannels and a cyclic prefix length of L 32. We assue a packet size of 512 sybols (4 blocks of 128 sybols in case of MC/SCBS-CDMA or 32 blocks of 16 sybols in case of MC/SC-CDMA). Convolutional channel coding in conjunction with frequency doain interleaving is eployed according to the IEEE a/g standard. The code rate is 1/2, 2/3, or 3/4. At the receiver, soft-decision Viterbi decoding is used. We distinguish between the uplink and the downlink. In the uplink, transit power control is applied, such that the received sybol energy is constant for all users. The power transitted by each terinal depends on the actual channel experienced by it. The BER (or the goodput) is deterined as a function of the received bit energy or, equivalently, as a function of the transit power averaged over the different channel realizations. In the downlink, no transit power control is applied. For a constant transit power at the base station, the received sybol energy at each terinal depends on the channel under consideration. The BER (or the goodput) is deterined as a function of the transit power or, equivalently, as a function of the received bit energy averaged over the channel realizations. For a given received sybol energy, the required transit powers for the different counication odes appear to be very siilar. Rather than coparing all possible odes for the two link directions, we only consider the relevant odes for each direction. For the uplink, SC deonstrates two pronounced advantages copared to OFDM [9]. First, SC exhibits a saller PAPR than OFDM odulation, which allows to reduce the power aplifier backoff and, thus, leads to increased power efficiency. Second, SC allows to ove the IFFT at the transitter to the receiver, which results in reduced terinal coplexity. No coputational effort is needed at the transitter. For the downlink, OFDM is the preferred odulation schee, since it only incurs a single FFT operation at the receiver side (as opposed to two FFT operations for SC), which leads to reduced terinal coplexity. Figure 33.3 illustrates the gain obtained by the use of different ultiple-antenna techniques in the downlink of an MCBS-CDMA-based syste. Three different syste configurations have been considered (1 1, 1 2, 2 2). A full user load is assued (nuber of users is equal to the SF). A 16-QAM constellation is used and the coding rate is equal to 1/2. A gain of axiu 4 db is achieved by cobining the signals received through 2 antennas at the obile terinal (MRC

15 François Horlin et al. 697 Goodput (Mbps) E b /N 0 (db) SISO (1 1) MRC (1 2) STBC + MRC (2 2) SDM (2 2) Figure Multiple-antenna gain in the downlink of an MCBS-CDMA-based counication syste. technique), while a suppleentary gain of axiu 1 db can be achieved by further perforing STBC coding across two antennas at the base station (STBC and MRC techniques together). On the other hand, SDM suffers fro a high 15 db loss in the goodput regions that can also be reached by a single-antenna syste. One can only achieve an increase of capacity by the use of ultiple-antennas at very high signal-to-noise-ratio (SNR) values. Due to the liited angle spread that characterizes the outdoor channels, the gain obtained by the use of ultiple antennas is on the overall quite low. In the next siulation results, one directive antenna will be assued at the base station to increase the cell capacity and two antennas will be assued at the obile terinal to iprove the link reliability. The MRC cobining technique is perfored in the downlink, while the STBC coding schee is applied in the uplink. Figure 33.4 illustrates the total goodput of the MCBS-CDMA counication ode in the downlink for different cobinations of the constellation sizes and code rates. A full user load has been assued. Looking to those cobinations that give the sae asyptotic goodput (16-QAM and CR 3/4 copared to 64-QAM and CR 1/2), it is always preferable to cobine a high constellation size with a low code rate. The sae conclusion holds in order to get the optial trade-off for each SNR value. The envelope is obtained by progressively eploying QPSK, 16-QAM, 64-QAM constellations, and a code rate equal to 1/2, and then by increasing the code rate progressively to the values 2/3, 3/4 while keeping the constellation size fixed to 64-QAM. Figure 33.5 copares the user goodput of MC-CDMA versus MCBS-CDMA in the downlink for a varying load (nuber of users is equal to 1, 5, and 8). Again a 16-QAM constellation and a coding rate equal to 1/2 are selected. The MMSE ultiuser receiver for MCBS-CDMA reduces to an equivalent but sipler

16 698 Multiple antennas for 4G wireless systes Goodput (Mbps) E b /N 0 (db) QPSK, 1/2 QPSK, 2/3 QPSK, 3/4 16 QAM, 1/2 16 QAM, 2/3 16 QAM, 3/4 64 QAM, 1/2 64 QAM, 2/3 64 QAM, 3/4 Figure Trade-off channel coding rate and constellation for the downlink of an MCBS-CDMAbased counication syste perforing MRC cobining Goodput (Mbps) E b /N 0 (db) IE x users IA 1 user IA 5 users IA 8 users Figure Coparison of MCBS-CDMA and MC-CDMA in the downlink of a counication syste perforing MRC cobining (IA stands for intrablock spreading and IE stands for interblock spreading). single-user receiver, which perfors channel-independent block despreading followed by MMSE single-user equalization. The perforance of MC-CDMA increases for a decreasing nuber of users, since it is liited by MUI. On the other hand, MCBS-CDMA is an MUI-free transceiver, such that its perforance reains unaffected by the user load. Nevertheless, MCBS-CDMA always perfors

17 François Horlin et al Goodput (Mbps) E b /N 0 (db) IE x users IA 1 user IA 5 users IA 8 users Figure Coparison SCBS-CDMA and SC-CDMA in the uplink of a counication syste perforing STBC coding (IA stands for intrablock spreading and IE stands for interblock spreading). worse than MC-CDMA, because the latter benefits fro the diversity offered by spreading. Figure 33.6 copares the user goodput of SC-CDMA versus SCBS-CDMA in the uplink for a varying load (nuber of users is equal to 1, 5, and 8). As for MC- CDMA, MMSE ultiuser reception is needed for SC-CDMA, since a single-user receiver cannot get rid of the MUI, and features a BER curve flattening already at low SNRs. For the SCBS-CDMA ode, the MMSE ultiuser receiver also reduces to a set of independent and low-coplexity MMSE single-user receivers, which perfor channel-independent block despreading followed by MMSE single-user equalization. The perforance of SC-CDMA increases for a decreasing nuber of users, since it is liited by MUI. This effect is ore pronounced than for the downlink MC-CDMA since the signals propagate through different channels, which is ore difficult to copensate for. On the other hand, since SCBS-CDMA is an MUI-free transceiver that preserves the orthogonality aong users (even after propagation through frequency-selective channels), its perforance is independent of the user load Suary Because of the liited frequency bandwidth on one hand and the potential liited power of terinal stations on the other hand, spectral and power efficiency of future systes should be as high as possible. New air interfaces need to be developed to eet the new syste requireents. Cobinations of the ulticarrier (MC) and spread-spectru (SS) odulations, naed ulticarrier-spread-spectru techniques, could be interesting candidates. They ight benefit fro the ain advantages of both MC and SS schees such as high spectral efficiency, ultiple-access capabilities, narrowband interference rejection, siple one-tap equalization, and

18 700 Multiple antennas for 4G wireless systes so forth. The huge potential of ultiple antenna technologies is now widely recognized. One could rely on the diversity offered by the wireless channels to iprove significantly the link reliability. Another possibility is to create artificially independent channel pipes between the transit and receive antenna arrays so that the link capacity is highly increased. Different flavors exist to ix the MC and SS odulations, that copleent each other and allow to ake an optial trade-off between the spectral and power efficiency according to the user requireents, channel propagation characteristics (tie and frequency selectivity), and terinal resources. In this chapter, we proposed a generic transission schee that allows to instantiate all the cobinations of orthogonal frequency-division ultiplexing (OFDM) and cyclic-prefixed single-carrier (SC) odulations with direct-sequence code-division ultiple access (DS-CDMA). The space-division ultiplexing (SDM) and spacetie block coding (STBC) ultiple-antenna techniques are next integrated in the generic transission schee. For each resulting ode, the optial linear iniu ean square error (MMSE), ultiuser receiver is derived. An optial strategy for the selection of the counication ode is proposed. It is shown that an adaptive transceiver is interesting to support different counication odes and to optially track the changing counication conditions. Acknowledgents Before closing this chapter, we would like to acknowledge all our colleagues who have contributed to this work. We thank especially Liesbet Van der Perre for having enthusiastically launched and sustained this activity. Abbreviations AWGN BER CIR CSI CDMA DS-CDMA FEC FIR FFT GMCBS-CDMA IBI IFFT MC MC-CDMA MCBS-CDMA MBWA MIMO MMSE Additive white Gaussian noise Bit error rate Channel ipulse response Channel state inforation Code-division ultiple access Direct-sequence code-division ultiple access Forward error coding Finite ipulse response Fast Fourier transfor Generalized ulticarrier block-spread code-division ultiple access Interblock interference Inverse fast Fourier transfor Multicarrier Multicarrier CDMA Multicarrier block-spread CDMA Mobile broadband wireless access Multiple-input ultiple-output Miniu ean square error

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