Spatio-temporal array processing for aperiodic CDMA downlink transmission

We consider a DS-CDMA system employing aperiodic spreading sequences (APS) as signature sequences for different users. Multiple transmission antennas (MA) are employed for increasing the network capacity. It is assumed that partial or total knowledge of the downlink channel is available at the base-station due to either a time-division duplex (TDD) or some feedback structure in the network. Spatial and temporal dimensions can no longer be jointly exploited due to the aperiodic spreading. Hence, spatio-temporal pre-cancellation of downlink interference (zero-forcing) is no longer possible. However beamforming can be applied to maximize the signal-to-noise plus interference (SINR) ratio on a user-to-user basis. We proceed with such an approach and provide closed-form relations for the signal and interference terms at the output of a mobile station RAKE receiver. We also show that using the spatial dimension at the base station enhances the system performance. The mobile receiver can later employ on equalizer to do away with the residual intersymbol interference, thus maximizing the matched filter bound (MFB) at its output.