Space-time Rake receiver with optimal beamforming for the uplink of CDMA-based wireless systems

In this paper a new spatial filtering approach for the uplink space-time Rake receiver is investigated. Recent field trials have shown that classical beamforming algorithms are not always reliable in the presence of strong interference and rapidly time-varying channels. More robust beam pattern control can be achieved by combining nullsteering and directivity optimization. The necessary directional information can be extracted from the covariance matrix, e.g. by code filtering. The main focus of this work is the analytical investigation of the capability of the beam pattern to suppress self-interference and multiple access interference (MAI). It is proved that for a large number of equally spaced interferers and under assumption of ideal power control, the SNIR is optimized by maximizing the directivity of the beam pattern. In case of less ideal scenarios, additional nulls of the beam pattern may be placed. This approach can also be extended by an additional pilot signal, if available.