Improving performance of coherent coded It can be shown that if the channel estimates are perfect ( 1 = a 1 , 2 = a? a? ELECTRONICS LETTERS 29th March 2001 Vol. 37 OFDM systems using space time transmit diversity R.A. Stirling-Gallacher and Z. Wang The performance gain of space time transmit diversity (STTD) for coded OFDM systems using space time block coding (STBC) is investigated. In particular, it is proposed to use pilot patterns, which are orthogonal in both the frequency and time directions. Therefore, to obtain channel estimates from the separate transmit antennas the receiver can use either adjacent pilots in the frequency direction or adjacent pilots in the time direction. Introduction: Orthogonal frequency division multiplexing (OFDM) is commonly used for high data rate wireless applications due to its inher- ent ability to combat inter-symbol interference (ISI). To achieve opti- mum performance with such systems, time and frequency interleaving are used with channel coding to yield the full benefits of time and fre- quency diversity. However, to obtain time diversity, the size and corre- sponding delay of the time interleaver become prohibitively large for channels with low Doppler frequencies. In such channels, the use of space diversity can yield large gains. Space diversity can be imple- mented as multiple antennas at the receiver (receiver diversity), multiple antennas at the transmitter (transmit diversity) or a combination of the two. In this Letter, we examine the use of transmit diversity using space time block coding (STBC). Space time block coding: The principle of space time block coding [1] for two transmit antennas is shown in Fig. 1. The STBC encoder receives blocks of two complex symbols, s 1 and s 2 , and for each input block produces two orthogonal output blocks each containing two com- plex symbols. These are then sent to the two respective transmitter antennas (RF components not shown). In addition to the orthogonal complex data blocks, orthogonal pilot blocks are also sent to each antenna. The pilot pattern used here is (A, A) for antenna 1 and (A, –A) for antenna 2, where A is a real number. We represent the channel trans- fer functions at a given instance in time from transmitter antennas 1 and 2 to the receiver as a 1 and a 2 , respectively. By assuming that the channel does not change from one pilot symbol to the next and that the data sym- bols are sent between the pilots, the first and second received data sym- bols, r 1 and r 2 , can be represented as and the first and second received pilot symbols p 1 , p 2 can be represented as where n 1 , n 2 , n 3 and n 4 are the respective AWGN noise terms. From the received pilot symbols we obtain channel estimates 1 and 2 : By using 1 and 2 , the sent symbols can be estimated: Fig. 1 Principle of STBC a? a? a? a? No. 7 a 2 ) diversity can be achieved: However, to achieve this diversity, it is necessary that the channel does not change significantly between received pilot symbols p 1 and p 2 , so that eqns. 5 and 6 are valid and the separate channel estimates 1 and 2 can be made. Pilot pattern: For coherent OFDM systems, traditionally pilots are placed at specified intervals in the OFDM frequency-time signal space, so that channel estimation can easily be performed for the range of Dop- pler frequencies and channel dispersions required. As it is required for Table 1: Multipath delay profile Tap number Relative time Relative power Doppler spectrum ns dB 1 0.0 0.0 classical 2 310 –1.0 classical 3 710 –9.0 classical 4 1090 –10.0 classical 5 1730 –15.0 classical 6 2510 –20.0 classical a? a? Fig. 2 Pilot patterns for transmitter antennas 1 and 2 a Transmitter antenna 1 b Transmitter antenna 2 s pilot value A l pilot value –A Fig. 3 BER results at Viterbi decoder output with and without STBC Doppler frequency is 30Hz – s – QPSK STBC off —s — QPSK STBC on – n – 64QAM STBC off —n — 64QAM STBC on STBC that the channel does not change significantly between adjacent pilots, we propose here to use pilot patterns for OFDM which are orthogonal in both the time and frequency directions. In this way, when the Doppler frequency is high, adjacent pilots in the frequency direction could be used for channel estimation and when the channel dispersion is high, adjacent pilots in the time direction could be used. The proposed patterns are shown in Fig. 2. System description and results: To evaluate the performance, an OFDM system as specified in [2] with the proposed pilot patterns shown in Fig. 2 was simulated. In particular we selected a guard interval length of T s /32 (where T s is the duration of the symbol part), 1/2 rate convolu- tional coding, modulation schemes QPSK and 64 level QAM and set the pilot value A to 4/3. The multipath channel model used is described in Table 1 and the maximum frequency of the Doppler spectrum was set to 30Hz. Channel estimation was performed using two adjacent pilots in the frequency direction. Fig. 3 shows the BER at the Viterbi decoder output with and without STBC. (In both cases the overall transmission power is the same.) As can be seen in Fig. 3, a gain of ~5dB in E b / N 0 can be obtained at a BER of 1 · 10 –4 for both modulation schemes by using STBC. Conclusions: We have shown that using space time coding a gain of ~5dB in E b / N 0 can be achieved in a multipath channel with a 30Hz Dop- pler spectrum and a simple channel estimator. Since the gain has been achieved without using a time interleaver, there is no significant process- ing delay present in the link. Furthermore, since the proposed pilot pat- terns are orthogonal in both the frequency and time directions, adjacent pilots in either direction can be utilised for channel estimation depend- ing upon the channel conditions. In situations in which the fading of adjacent pilots in both directions are uncorrelated, more advanced receiver structures such as those described in [3] could be used. Acknowledgment: This work was partially sponsored by the German Federal Ministry of Education and Research under project UMTSplus/ COMCAR [4]. ? IEE 2001 30 November 2000 Electronics Letters Online No: 20010292 DOI: 10.1049/el:20010292 R.A. Stirling-Gallacher and Z. Wang ( Telecommunication Research & Development Europe, SONY International (Europe) GmbH, Heinrich-Hertz Strasse 1, D-70327 Stuttgart, Germany ) E-mail: rasg@sony.de References 1 ALAMOUTI, S.M. : ‘A simple transmit diversity technique for wireless communications’, IEEE J. Sel. Areas Commun., 1998, 16 , (8), pp. 1452– 1458 2 Digital Video Broadcasting (DVB), ETSI EN300 744, V1.2.1, July 1999 3 VOOK, F.W. , and THOMAS, T.A. : ‘Transmit diversity schemes for broadband mobile communication systems’. IEEE Proc. VTC 2000, Boston, USA, pp. 2523–2529 4 http://www.comcar.de ELECTRONICS LETTERS 29th March 2001 Vol. 37 No. 7