Adaptive Space-time Block Coded Transmit Diversity in a High Mobility Environment

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1 Adaptive Space-time Block Coded Tranmit Diverity in a High Mobility Environment Tomoyuki SAITO, Amnart BOONKAJAY and Fumiyuki ADACHI Reearch Organization of Electrical Communication, Tohoku Univerity -1-1 Katahira, Aoba-ku, Sendai, Japan aito.tmm@riec.tohoku.ac.jp, amnart@riec.tohoku.ac.jp, adachi@ecei.tohoku.ac.jp Abtract Space-time block coded tranmit diverity (-TD) with minimum mean quare error fruencydomain ualization (MMSE-FDE) and maximal-ration tranmit FDE (MRT-FDE) can improve the bit error rate (BER) performance in a fruency-elective fading. Since the MMSE-FDE/MRT-FDE weight are computed by pilot-aided channel etimation (PACE) before data tranmiion, the BER performance degrade in a high mobility environment. In thi paper, we propoe an adaptive -TD employing deciion feedback channel etimation (DFCE) for ingle-carrier (SC) uplink and orthogonal fruency-diviion multiplexing (OFDM) downlink tranmiion. Computer imulation reult how that the ue of DFCE increae an allowable maximum Doppler fruency, fdt, for keeping the BER<10 - by about 5.5 time for SC uplink and 3.8 time for OFDM downlink compared to the ue of PACE only. Keyword pace-time block coded diverity, deciion feedback channel etimation, TDD, SC, OFDM I. INTRODUCTION In the 5th generation (5G) mobile communication ytem, the enhanced mobile broadband ervice are expected [1]. However, the everely doubly-elective fading ubtantially deteriorate the bit error rate (BER). One promiing technique to improve the BER performance in a poor propagation environment i the pace time block coded tranmit diverity (-TD) which allow to ue multiple antenna at both bae tation (BS) and uer uipment (UE) [, 3] (therefore, an arbitrary high diverity order can be obtained by increaing the number of either BS or UE antenna). In thi paper, we conider -TD jointly ued with the minimum mean quare error fruency-domain ualization (MMSE-FDE) [4-6] for SC uplink tranmiion and that jointly ued with maximum ratio tranmit fruency-domain ualization (MRT-FDE) [7, 8] for OFDM downlink tranmiion. All the computationally demanded ignal proceing ruired for MMSE-FDE and MRT-FDE i implemented at BS. While limiting the number of UE antenna to N ue =, an arbitrary number N b of BS antenna can be ued to achieve larger patial diverity gain without reducing the code rate [8-10]. The BS need the channel tate information (CSI) in order to compute the MMSE-FDE weight and the MRT-FDE weight before data tranmiion. Auming time-diviion duplex (TDD) ubframe tructure hown in Fig. 1 [11], the UE tranmit the uplink fruency-diviion multiplexed (FDM) pilot for pilot-aided channel etimation (PACE) in the firt time lot (t=0) and then, the BS tranmit the FDM pilot for PACE in econd time lot (t=1). In a quai-tatic fading channel environment, the MMSE-FDE weight for SC uplink and the MRT-FDE weight for OFDM downlink computed at t=0 can be ued for ucceeding data tranmiion period (t=~13). In a high mobility environment, however, the BER performance degrade ince the channel change over the period of data tranmiion. In thi paper, we propoe an adaptive -TD in a high mobility environment. The well-known imple Alamouti encoding/decoding i conidered. The adaptive - TD employ deciion feedback channel etimation (DFCE) in addition to PACE to update the MMSE-FDE weight for SC uplink and to keep the uivalent channel gain (i.e., a concatenation of MRT-FDE and the propagation channel) cloe to that at t=1 for OFDM downlink. The effectivene of propoed adaptive -TD i confirmed by computer imulation. Thi paper i organized a follow. Sect. II propoe the adaptive -TD. The BER performance achievable with the propoed adaptive -TD i evaluated by computer imulation in Sect. III. Sect. IV offer ome concluding remark. Notation: [.] and [.] H repreent complex conjugate and Hermitian tranpoe operation, repectively. 1 ubframe (DL pilot lot + UL pilot lot + 1 lot) Slot t=0 Slot t=1 Slot t= Slot t=1 Slot t=13 CP Uplink pilot CP Downlink pilot P(0) P(1) P() P(3) P( max ) P( max 1) N c / max CP Subcarrier index, k CP CP No. of ubcarrier: N c=104 Subcarrier pacing:75khz Subframe length: T ub=0.09m Symbol length (104 ample): T =13.33μ CP length (18 ample): T cp= 1.67μ Slot length: T=15.00μ Fig. 1. TDD ubframe tructure (1 t pilot lot + nd pilot lot + 1 uer data lot). II. ADAPTIVE -TD We firtly decribe how to initially et the MMSE-FDE weight and MRT-FDE weight and then, decribe DFCE. A. Initial etting of MMSE-FDE and MRT-FDE BS and UE are aumed to be uipped with N b antenna and N ue = antenna, repectively. By exploiting the channel XXX-X-XXXX-XXXX-X/XX/$XX.00 0XX IEEE

2 reciprocity due to TDD, the BS can compute the initial MMSE-FDE weight for SC uplink reception and the initial MRT-FDE weight for OFDM downlink tranmiion. The initial MMSE-FDE weight and MRT-FDE weight can be computed by PACE at the firt time lot t=0 a [11] W k; n, n mme ue b 1 Nue 1 Nb 1 H k; t 0, nb, nue nue 0 nb E Nue N 0 H k; t 0, nb H kt ; 0, nb, nue for MMSE-FDE, (1) Wmrt k; nb 1Nb 1Nue 1 1 H k; t 0, nb k0 nb 0 nue 0 for MRT-FDE, () where H ( k; t 0, n b, n ue ) i the (nb, n ue)-th element of N b N ue multi-input multi-output (MIMO) channel matrix etimated at t=0. E and N 0 are repectively the ymbol energy and the ingle-ided additive white Gauian noie (AWGN) power pectrum denity. N c and k (=0~N c 1) denote repectively the number of ubcarrier and the ubcarrier index. B. -TD tranmiion Figure and 3 illutrate the -TD tranmiion ytem model for SC uplink and for OFDM downlink, repectively. Below, encoding/decoding for SC uplink with MMSE-FDE and that for OFDM downlink with MRT- FDE are briefly decribed. For detailed -TD tranmiion operation, refer to Ref.[10]. Alamouti coding i generally -block wie proceing and hence, we explain the ignal repreentation by conidering tranmiion over two conecutive lot time period t=m and t=m+1 with m=1~6. (1) SC uplink conecutive ymbol block {d(t'; t); t'=0~n c1} with t repreenting time intant in a block, to be tranformed by N c- point fat Fourier tranform () into the fruency-domain ignal {D(k; t); k=0~n c 1}, where k repreent the ubcarrier index. After Alamouti encoding [] in the fruency-domain and N c -point invere fat Fourier tranform (I), -encoded SC ignal are tranmitted from N ue = antenna and are received by N b (=arbitrary) BS antenna. The -encoded SC received ignal after N c-point are denoted by {R(k; n b, t); n b=0~n b1}. MMSE-FDE combining uing weight given by. (1) i applied to b, t); nb 0~ Nb 1} to obtained the diverity combined output { R k; nue, t; nue 0,1}. Then, decoding i applied to obtain the fruency-domain ignal { Dkt ( ; ); k0~ N c 1} a D k; t m m ;0, ;1, 1 ;1, ;0, 1 D ( kt ; m ) D k; t 1. (3) R k t m R k t m R k t m R k t m Finally, N c-point I i applied to obtain the oft-deciion ymbol block { dt ( ; t m)} and { dt ( ; tm 1)}. #N b 1 Data mod. CP CP etimation encoding I I (a) Tranmitter MMSE-FDE H ( kt ; 0) +CP +CP decoding I #N ue 1 (b) Receiver Fig.. -TD tranmiion ytem model for SC uplink with MMSE-FDE. () OFDM downlink conecutive ymbol block {d(k; t); k=0~n c1} and to be tranmitted are -encoded, mapped onto fruencydomain, and then multiplied by the MRT-FDE weight given by. (). After N c-point I, the -encoded OFDM ignal are tranmitted from N b (=arbitrary) BS antenna and are received by N ue= UE antenna. The -encoded OFDM received ignal after N c-point are denoted by ue, t); nue 0,1}. decoding and amplitudenormalization are applied to obtain the oft-deciion ymbol block { dkt ( ; m)} and { dkt ( ; m 1)} a d k; t m d ( kt ; m ) dk; t m 1 1 b 1 ue 1 1 N c N N H k; t 1, nb k0 nb 0 nue 0. (4) Nb 1Nue 1 H ( k ; t 1, n, n ) Data demod. b ue nb 0 nue 0 ( ;0, ) ( ;1, 1) R k t m R k t m Rk ( ;1, t m) R ( k;0, t m1)

3 Data mod. encoding #N ue 1 MRT-FDE H ( kt ; 0) CP CP etimation I I I CP CP etimation CP (a) Tranmitter decoding H ( kt ; 1) Amplitude normalization #N b 1 (b) Receiver Fig. 3. -TD tranmiion ytem model for OFDM downlink with MRT-FDE. C. Updating uplink receive MMSE-FDE weight and downlink receive filter by DFCE In a high mobility environment, if the MMSE-FDE weight, MRT-FDE weight, and amplitude normalization, decribed in Sect. II-A and II-B, are ued continuouly during the data tranmiion period (i.e., t=m, m+1; m=1~6), the BER performance degrade ignificantly. During the data tranmiion, MMSE-FDE weight for SC uplink can be updated by introducing DFCE, however, the MRT-FDE weight for OFDM downlink cannot be updated. Therefore, we introduce an adaptive receive filter to compenate the received ignal channel variation. Below, we decribe how the MMSE- FDE weight for SC uplink reception and the adaptive receive filter for downlink reception are updated. (1) MMSE-FDE weight updating for SC uplink The MMSE-FDE weight given by Eq. (1) can be ued for the ignal reception at t= and 3 (i.e., m=1) only. For the ignal reception at t=4 and afterward, the MMSE-FDE weight i updated by either 1 t -order linear prediction (1 t -order LP) or the nd -order linear prediction ( nd -order LP) DFCE [11]. Let u aume that decoding and ymbol deciion are completed. In DFCE, firtly, the revere modulation i applied to the fruency-domain received ignal matrix R( kt ; m) [ R( kt ; m), R ( kt ; m1)] of ize N b with m=1~6, obtained after N c -point, to etimate the channel matrix H ( kt ; m) at t=m a #1 Data demod. H H( kt ; m) R( kt ; m) D ( kt ; m) 1 1 H E, (6) D( kt ; ) D ( kt ; ) I N ue N 0 where D ( kt ; m) i the fruency-domain -encoded ignal matrix given by D( k; t m) D( k; t m) D ( k; t m 1), (7) D( k; t m 1) D ( k; t m) with { Dkt ( ; ); k0~ N c 1} being the N c -point of the t-th hard deciion data ymbol block{ d( k; t); k 0~ N c 1} in the two conecutive lot time t=m and m+1. Then, moving average operation with window ize W i applied to reduce the negative impact of the noie and deciion error and to obtain the improved channel etimate at t=m+ a W / 1 H ( kt ; m) ( kwt ; m) W w H. (8) W/ The channel etimate at t=m+ i obtained by the 1 t -order LP or nd -order LP a H ( kt ; m) t :1 -order LP H ( kt ; m ) ( kt ; m ) ( kt ; m ). (9) H H nd : -order LP The above obtained channel etimate H ( kt ; m) and H ( kt ; m1) are ued to update the MMSE-FDE weight for the ignal reception at t=m and t=m+1 (m=1~6) a follow. Wmme k; t m, nb Wmme k; t m1, nb 1 Nue 1Nb 1 H k; t m, nb nue 0 nb E Nue N 0 H k; t m, nb (10) () Adaptive receive filtering for OFDM downlink The -encoded OFDM received ignal after N c-point at t=m and m+1 are denoted by matrix R(k; t=m)=[r(k;t=m), R(k;t=m+1)] of ize N ue and can be expreed a R( kt ; m) E, (1) H ( kt ; m) X( kt ; m) N( t m) T where H (k, t=m) i the uivalent channel given by H ( kt ; m) W ( kt ; 0) H ( kt ; m). (13) mrt In a high mobility environment, H (k, t=m) become deviate from that of t=1. We want to modify R(k; t=m) to that which could be een at t=1. To achieve the above tak, we introduce the receive filtering to do thi. Firt, revere modulation to remove the modulation from the received ignal i applied to etimate the uivalent

4 channel matrix (i.e., a concatenation of MRT-FDE and the propagation channel) a ( ; ) ( ; ) ( ; ) H 1 kt m R kt m d kt m, (14) where d ( kt ; m) i a -encoded ignal matrix regenerated by uing the hard-deciion ymbol block (note that d( k; t ) denote the hard-deciion reult of dkt ( ; ) a d( k; t m) d ( k; t m1) d ( kt ; m) d( k; t m1) d ( k; t m).(15) Then, moving average operation and DFCE imilar to. (8) and (9) are applied to predict the uivalent channel matrix at t=m+. Denoting the reulting uivalent channel matrix at t=m+ by H ( kt ; m), the received ignal matrix i modified a 1 R kt ; m H ( kt ; 1) H ( kt ; m).(19) R kt ; m By uing the element of the above kt ; m R intead of ue, t); nue 0,1}, decoding and amplitudenormalization in. (4) are carried out. III. COMPUTER SIMULATION We evaluate the average uncoded BER performance of -TD for SC uplink and OFDM downlink tranmiion by computer imulation. The computer imulation parameter are ummarized in Table 1. TABLE I. Pilot tructure Tranmitter/ Receiver Propagation channel COMPUTER SIMULATION PARAMETERS. No. of pilot ubcarrier N p =16 Pilot uence Zdaff-Chu.(i=1) No. of ubcarrier N c=104 CP length N cp=18 No. of BS antenna N b=4 No. of UE antenna N ue= Fruency-elective block Rayleigh fading Power delay profile (PDP) hape 16-path uniform Maximum delay time max=16 Fig. 3 plot the average uncoded BER due to fading Doppler hift for SC uplink and OFDM downlink a a function of normalized maximum Doppler fruency, f D T. The average E /N 0 i et to 40dB and the BER i produced by Doppler hift only. It can be een that the ue of the nd -order improve ignificantly the BER performance in a high mobility environment. The ue of nd -order can increae the allowable maximum f D T for keeping BER<10 - about 5.5 time and 3.8 time for SC uplink and OFDM downlink, repectively, compared to the ue of PACE only. Auming the ubcarrier pacing of 75 khz and 5GHz carrier fruency, the allowable maximum travelling peed can be increaed to about 500km/h and 340km/h for SC and OFDM tranmiion, repectively. Average uncoded BER Average uncoded BER 1.E+00 1.E-01 1.E-0 1.E-03 SC uplink -TD, 16QAM Average E /N 0 =40dB no DFCE w/ 1 t -order w/ nd -order 1.E-04 1.E-03 1.E-0 1.E-01 1.E+00 1.E-01 1.E-0 1.E-03 OFDM downlink -TD, 16QAM Average E /N 0 =40dB no DFCE f D T (a) SC uplink 1.E-04 1.E-03 1.E-0 1.E-01 f D T w/ nd -order w/ 1 t -order (b) OFDM downlink Fig. 3. f DT v BER performance. IV. CONCLUSION In thi paper, we propoed an adaptive -TD for SC uplink and OFDM downlink tranmiion in a high mobility environment. It wa confirmed by computer imulation that the propoed adaptive -TD with DFCE increae the allowable maximum Doppler fruency for keeping BER<10 - about 5.5 time and 3.8 time for SC uplink and OFDM downlink, compared to conventional -TD with PACE only. ACKNOWLEDGMENT Thi paper include a part of reult of The reearch and development project for realization of the fifth-generation mobile communication ytem commiioned to Tohoku Univerity by The Minitry of Internal Affair and Communication (MIC), Japan. REFERENCES [1] C. X. Wang, F. Haider, and X. Gao, Cellular architecuture and key technologie for %G wirele communication network, IEEE Commun. Mag., Vil. 5, Iue, pp , Feb [] S.M. Alamouti, A imple tranmit diverity technique for wirele communication, IEEE J. Sel. Area. Commun., vol.16, no.8, pp , Oct

5 [3] V. Tarokh, H. Jafarkhani and A. R. Calderbank, Space-time block code from orthogonal deign, IEEE Tran.on Inform. Theory, Vol. 45, No. 5, pp , July [4] F. Adachi, H. Tomeba, and Kazuki Takeda, Introduction of fruencydomain ignal proceing to broadband ingle-carrier tranmiion in a wirele channel, IEICE Tran. Commun., Vol.E9-B, No.09, pp , Sep [5] K. Takeda, T. Itagaki, and F. Adachi, Application of pace-time tranmit diverity to ingle-carrier tranmiion with fruency-domain ualization and receive antenna diverity in a fruency-elective fading channel, IEE Proc. Commun., Vol. 151, No. 6, pp , Dec [6] J. M. Auffray and J. F. Helard, Performance of multicarrier CDMA technique combined with pace-time block coding over Rayleigh channel, Proc. IEEE Seventh International Sympoium on Spread Spectrum Technique and Application, Vol., pp , Sept. 00. [7] J. K. Caver, Single-uer and multiuer adaptive maximal ratio tranmiion for Rayleigh channel, IEEE Tran. Vehi. Technol., Vol. 49, No.6, pp , Nov [8] H. Tomeba, K. Takeda, and F. Adachi, Space-time block coded-joint tranmit/receive antenna diverity uing more than 4 receive antenna, Proc. 008 IEEE 68th Veh. Technol. Conf. (VTC008-Fall), Calgary, Canada, 1-5 September 008. [9] R. Matukawa, T. Obara, and F. Adachi, Fruency-domain pace-time block coded tranmit/receive diverity for ingle-carrier ditributed antenna network, IEICE Commun. Expre (ComEX), Vol., No. 4, pp , 15 April, 013. [10] F. Adachi, A. boonkajay, Y. Seki, T. Saito, S. Kumagai and H. Miyazaki, Cooperative Ditirbuted Antenna Tranmiion for 5G Mobile Communication Network, IEICE Tran. Commun., Vol. 100-B, No. 8, Aug [11] F. Adachi, A. Boonkajay, Y. Seki and T. Saito, MIMO Etimation for Time-Diviion Duplex Ditributed Antenna Cooperative Tranmiion, Proc. Int. Wirel. Commun. and Mobile Comput. Conf. (IWCMC 017), Valencia, Spain, Jun. 017.