Space-Time Adaptive MMSE Multiuser Receiver Structures for Wideband CDMA Systems

Abstract

In this paper we propose computationally efficient space-time multiuser adaptive minimum mean square error (MMSE) receivers using affine projection adaptive (APA) filtering algorithm for asynchronous wideband code division multiple access systems (W-CDMA) in multipath channels. It is found that APA algorithm outperforms normalized least mean square (NLMS) algorithm in convergence characteristics with significant reduction in computational complexity as compared to recursive least square (RLS) algorithm. Three adaptive multiuser receiver architectures are considered, all employ chip-matched filter (CMF) at their front end. In the first two architectures perfect estimation of time delay of individual user is assumed and one CMF is employed per user. The difference between two architectures is that, in the first the samples collected from CMFs are cross-coupled and in the second architecture samples collected for individual users are not combined. In the third architecture, there is no need to estimate the time delay, but fractional sampling is used. It is shown that first architecture outperforms the other two architectures by virtue of its diversity advantage. It is also shown that by exploiting the spatial diversity (using multiple antennas at the receiver) significant improvement can be achieved in the output signal-to-interference-and-noise ratio (SINR).