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. 7ELECTRONICS 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 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 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 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