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 in 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 on the performance of the given closed-loop scheme. Although the analysis applies only in the case of the specific 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 a mobile terminal when feedback information from the receiver is applied in the transmitter.