Improving performance of coherent coded It can be shown that if the channel estimates are perfect(a1=a1, a2 OFDM systems using space time transmit a2)diversity can be achieved diversity (a1l+la2F)s1 +noise terms A. Stirling-Gallacher and Z. wang However, to achieve this diversity, it is necessary symbols ne channel does The performance gain of space time transmit diversity (STID)for that eqns. 5 and 6 are valid and the separate channel estimates &, and coded OFDM systems using space time block coding (STBC)is &z can be made investigated. In particular, it is proposed to use pilot patterns, which are in both the frequency and time directions. Therefore, to btain channel estimates from the separate transmit antennas the receiver can use either adjacent pilots in the frequency direction or 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 corm sponding delay of the time interleaver become prohibitively large 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 wo. In this Letter, we examine the use of transmit diversity using space Hann time block coding(STBC) D5682 Fig. 2 Pilot STBC a Transmitter antenna I pilot value -d transmitte ath j Table 1: Multipath delay profile Fig. 1 Principle of sTBC Tap number Relative time Relative power Doppler spectrum block coding: The principle of space time block coding [1 for two transmit antennas is shown in Fig. 1. The STBC encoder 0.0 0.0 classical receives blocks of two complex symbols, s, and S,, and for each input block produces two orthogonal output blocks each containing two com- 9.0 plex symbols. These are then sent to the two respective transmitter tenna(RF components not shown). In addition to the orthogonal omplex data blocks, orthogonal pilot blocks are also sent to each 1730 ntenna. The pilot pattern used here is(A, A)for antenna I and(A, -4 10 200 classic for antenna 2, where A is a real number. We represent the channel tra fer functions at a given instance in time from transmitter antennas I an 2 to the receiver as a, and a,, respectively. By assuming that the channel 101 t change from one pil to the next and that the bols are sent between the pilots, the first and second received data sym- 1=1a1-8202+1 (1) 72=S2a1+S102+ and the first and second received pilot symbols pi, p, can be represented Pi= A0 + Ae Ao -Ao%+ny where n, n2, n, and na are the respective AWGn noise terms. From the received pilot symbols we obtain channel estimates a and a, Eb/No, dB Fig. 3 BER results at Viterbi decoder output with and without STBC By using ai and a2, the ser bols can be estimated Pilot pattern: For coherent OFDM systems, traditionally pilots are =rIai + rC2 placed at specified intervals in the OFDM frequency-time signal space so that channel estimation can easily be performed for the range of Dop- 82=2a1-102 pler frequencies and channel dispersions required. As it is required for ELECTRONICS LETTERS 29th March 2001 Vol 37 No. 7
ELECTRONICS LETTERS 29th March 2001 Vol. 37 No. 7 Improving performance of coherent coded 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 inherent ability to combat inter-symbol interference (ISI). To achieve optimum performance with such systems, time and frequency interleaving are used with channel coding to yield the full benefits of time and frequency diversity. However, to obtain time diversity, the size and corresponding 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 implemented 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, s1 and s2, and for each input block produces two orthogonal output blocks each containing two complex 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 transfer functions at a given instance in time from transmitter antennas 1 and 2 to the receiver as α1 and α2, respectively. By assuming that the channel does not change from one pilot symbol to the next and that the data symbols are sent between the pilots, the first and second received data symbols, r1 and r2, can be represented as and the first and second received pilot symbols p1, p2 can be represented as where n1, n2, n3 and n4 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: It can be shown that if the channel estimates are perfect ( 1 = α1, 2 = α2) diversity can be achieved: However, to achieve this diversity, it is necessary that the channel does not change significantly between received pilot symbols p1 and p2, 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 Doppler frequencies and channel dispersions required. As it is required for Fig. 1 Principle of STBC αˆ αˆ αˆ αˆ 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 αˆ αˆ αˆ αˆ Fig. 2 Pilot patterns for transmitter antennas 1 and 2 a Transmitter antenna 1 b Transmitter antenna 2 s pilot value A ● pilot value –A Fig. 3 BER results at Viterbi decoder output with and without STBC Doppler frequency is 30Hz – ▲ – QPSK STBC off —▲— QPSK STBC on – ■ – 64QAM STBC off —■— 64QAM STBC on
STBC that the channel ot change significantly between adjacent thogonal in both the frequency and time direction ns. arace pilots, we pr tterns for OFDM which are ither directio stimation depend orthogonal in both the directions. In this way when on the channel conditions. In situ the Doppler frequency is high, adjacent pilots in the frequency direction pilots in both directions are 站 hich the fading of more advanced could be used for channel estimation and when the channel dispersion is receiver structures such as those described in 3] could be used. high, adjacent pilots in the time direction could be used. The proposed patterns are shown in Fig. 2 Acknowledgment: This work was partially sponsored by the German Federal Ministry of Education and Research under project UMTSplus/ System description and results: To evaluate the performance, an OFDM COMCAR 141 ystem as specified in [2] with the proposed pilot patterns shown in Fig. 2 was simulated. In particular we selected a guard interval length O 2001 30 November 2000 tional coding, modulation schemes QPSK and 64 level QAM and set the DoL: 0 CS 49/el: 2001029220: 20010292 R.A. Stirling-Gallacher and Z. Wang (Telecommunication Research 30Hz. Channel estimation was performed using two adjacent pilots in Srasse f, a,a xo sunart, Germany) Table I and the maximum frequency of the Doppler spectrum was set to SONY International(Europe) GmbH, Heinrich the frequency direction. Fig. 3 shows the BER at the Viterbi decoder E-mail: rasg asony. de with and without STBC. (In both cases the overall transmission is the same )As can be seen in Fig. 3, a gain of-5dB in ENo References obtained at a BER of 1 x 10- for both modulation schemes by using stbc 1 ALAMOUTL, SM diversity technique for Comm,1998,16,(8)pp.1452 5dB in ENo can be achieved in a mu with a 30 2 Digital Video Broadcasting (DVB), ETSI EN300 744, V1. 2.1, July 1999 3 VOOK, E.W., and THOMAS, T.A.: " Transmit diversity schemes for broadband pler spectrum and a simple channel the gain mobile communication systems'. IEEE Proc. VTC 2000, Boston, USA, achieved without using a time interleaver, there is no significant process- pp.2523-2529 ingdelaypresentinthelinkFurthermoresincetheproposedpilotpat-4http://www.comcar.de ELECTRONICS LETTERS 29th March 2001 Vol 37 No. 7
ELECTRONICS LETTERS 29th March 2001 Vol. 37 No. 7 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 Ts/32 (where Ts is the duration of the symbol part), 1/2 rate convolutional 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 Eb/N0 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 Eb/N0 can be achieved in a multipath channel with a 30Hz Doppler spectrum and a simple channel estimator. Since the gain has been achieved without using a time interleaver, there is no significant processing delay present in the link. Furthermore, since the proposed pilot patterns are orthogonal in both the frequency and time directions, adjacent pilots in either direction can be utilised for channel estimation depending 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
