Spatio-temporal array processing for FDD/CDMA/SDMA downlink transmission

We address the problem of performing optimum spatio-temporal processing when using adaptive antenna arrays at base stations for multiuser downlink transmission in DS-CDMA systems, using periodic spreading sequences and assuming partial knowledge of the channel parameters of all users. This assumption typically holds in frequency-division duplex (FDD) based mobile communication systems. We consider the SDMA strategy for using antenna arrays to gain system capacity. The channel is assumed to comprise specular multipath, and a per-path argument is pursued to design FIR transmission filters at the base station in order to maximize the signal-to-interference-plus-noise ratio (SINR) at the mobile receivers. Joint optimization of the transmitter and receiver is considered. The per path decorrelating pre-filter is introduced, and it is shown that due to the large number of degrees of freedom available because of the large processing gain (inherent oversampling with respect to the symbol rate in CDMA) and possible multiple antennas/oversampling, the downlink performance can be greatly improved in the FDD problem.