Closed-loop transmit diversity for FDD WCDMA systems

Transmit diversity techniques provide attractive solutions for increasing the 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. System capacity can be increased from that of open-loop modes if the transmitter is equipped with additional side information of the downlink channel. In an FDD system this means that the receiver has to provide the information through some feedback mechanism. We give a general algorithm by which it is possible to design a large variety of different closed-loop schemes depending on the feedback capacity of the system. The quantization of the transmit weights can be reduced to an eigenvalue problem by using linear functions of order statistics of the underlying probability distribution. This provides a simple analytical tool for the design of feedback algorithms. Furthermore, we provide analytical results for the transmit weights in the case of single-path Rayleigh fading channels.