Spatio-temporal array processing for DS-CDMA downlink transmission

We address the problem of optimal downlink transmission in a DS-CDMA system where periodic orthogonal Walsh-Hadamard sequences spread different users´ symbols followed by scrambling by a symbol aperiodic cell specific overlay sequence. We assume that base-stations have multiple-antenna transceivers and each user´s receiver consists of a RAKE. Multiple transmit antennas can reduce the interference and increase the signal-re-noise ratio at mobile receivers. It is assumed that at least partial knowledge of the intracell users´ downlink channels is available at the base-station. This information combined with the knowledge of the type of mobile receiver, suffices to perform optimal spatio-temporal downlink transmission at base-stations. Although spacial and temporal dimensions can no longer be jointly exploited due to the aperiodic overlay scrambling, pure spatial processing or beamforming, can still be performed in order to optimize the whole spatio-temporal processing through the transmitter-channel-receiver cascade. We provide closed-form relations for the signal, interference and noise terms at the output of a mobile station RAKE receiver as functions of the transmit power allocation and beamforming weight vectors. We show that analytical solutions to two optimization problems can be found for the multiuser transmit beamformers, relying on the spatio-temporal structure of the propagation channel.