Space-time adaptive processing for DS-CDMA communication systems

A 2D RAKE receiver for a direct-sequence spread spectrum communications system is a space-time processor that cancels strong multi-user access interference while optimally combining multipath for the desired user. A recently proposed scheme involves passing the output of each antenna through a matched filter based on the spreading waveform of the desired user, and then estimating the signal plus interference space-time correlation matrix during that portion of the bit interval where the fingers of the RAKE occur. The interference alone space-time correlation matrix is estimated during that portion of the bit interval away from the “fingers”. The weight vector yielding the optimum signal to interference plus noise ratio for bit decisions is the “largest” generalized eigenvector of the resulting matrix pencil. However, even when the number of elements comprising the array is relatively small, this space-time correlation matrix pencil is of very large dimension. In addition to detracting from the real-time applicability of the scheme, this also causes a slow convergence rate. A reduced dimension blind space-time 2D RAKE receiver is presented based on a novel frequency domain implementation of a RAKE receiver and a data adaptive transformation to a lower dimension beamspace that contains most of the desired user\u27s energy. An adaptation of the scheme for the IS-95 uplink is then presented. We present a decision directed variation of the aforementioned blind 2D RAKE receiver that reliably estimates which Walsh function was sent for every six transmission bits. Due to the high computational complexity of the eigenanalysis based algorithms, we propose a blind 2D RAKE receiver based on RLS-type space-time adaptive filtering to update the space-time weight vector on a per space-time snapshot basis as its adaptive solution. Schemes for the joint angle-of-arrival and time delay estimation of the dominant multipaths are also proposed. This information may be used for enhancing the performance of the demodulation process, “smart” downlink beamforming, and geolocation. Simulations will be presented demonstrating the efficacy of these proposed methods