SINR maximizing space-time filtering for asynchronous DS-CDMA

Direct sequence code division multiple access (DS-CDMA) is a core technology for wireless access that is proposed for future generation (3G) mobile and personal communication systems. The use of a base-station antenna array is strongly endorsed in these systems, because of its capabilities of filtering out the interference in the space domain. This paper considers space-time one shot single user receivers for DS-CDMA, based on a multiple input single output time invariant linear filter. The optimization of the filter response is performed jointly in the space and time domains yielding maximal signal-to-interference-plus-noise ratio. Using this framework, a space-time noise whitening matched filter (ST-NWMF) for DS-CDMA demodulation is introduced. It is shown that combined spatio-temporal filtering, taking advantage of the fact that the multiple access interference is colored in the space as well as time domains, can reduce the effects of strong interference components and improves the performance. Simpler suboptimal space-time linear receivers, consisting of a single temporal LT filter following the antenna array, are also considered. It is shown that such a filter approaches the ST-NWMF in the limit, when the M chip waveform has a flat spectrum with no excess bandwidth and N the observation interval is infinite. These schemes, optimal as well as suboptimal, for large observation intervals are suitable also for DS-CDMA systems employing long sequence spreading.