A near-optimum equal-power transmit diversity scheme

Spatial diversity techniques are now used in most wireless communications systems to improve robustness to signal fading. On the downlink of cellular systems, low-cost requirements of the user terminal tend to favor transmit diversity over receive diversity, and Alamouti´s transmit diversity technique appears today in most wireless communications systems standards. But in terms of the total transmit power, Alamouti´s technique loses 3 dB with respect to optimum transmit diversity which requires perfect channel state information at the transmitter. In this paper, we introduce a simple transmit diversity technique which performs significantly better than both Alamouti´s transmit diversity and switched diversity schemes and comes very close to optimum transmit diversity performance. In this technique, the same power is transmitted from the two antennas and the phase difference of the signals received from the two channels is constrained to be smaller than π/4. Its performance is studied in both narrowband fading channels and in OFDM systems operating on frequency- selective channels.