The effect of feedback delay to the closed-loop transmit diversity in FDD WCDMA

Transmit diversity techniques provide attractive solutions for increasing downlink capacity in 3G systems within low-mobility environments. Open-loop and closed-loop transmit diversity modes with two transmit antennas have already been included into 3GPP WCDMA Release 4, and at the moment schemes exploiting a larger number of transmit antennas are being developed. This paper analyzes the performance of a simple closed–loop transmit diversity scheme suitable for 3GPP WCDMA. The goal is to study the effect of different feedback delays to the performance of the given closed– loop scheme. Although the analysis applies only in the case of the speci£c algorithm (phase–only feedback) it is expected that the performance of the more general magnitude and phase feedback shows similar properties. We use signal-to-noise ratio (SNR) gain as a performance measure, which corresponds to the gain in expected received SNR in mobile terminal when feedback information from the receiver is applied in the transmitter.