Combined multiuser detection and beamforming for CDMA systems: Filter structures

Code-division multiple-access (CDMA) systems are interference limited, and therefore efficient interference management is necessary to enhance the capacity of a CDMA system. In this paper, we consider combining two effective receiver-based interference management strategies: multiuser detection (temporal filtering) and receiver beamforming (spatial filtering). We formulate and examine the performance of several two-dimensional linear filter structures, which are all based on minimum mean squared error (MMSE) criterion but differ in how the MMSE problems are defined in the temporal an spatial domains, i.e., jointly or in cascade. It is shown that while the joint optimum MMSE filter achieves the maximum signal-to-interference ratio (SIR) among all possible matrix filters, the constrained optimum MMSE filter which results in a single temporal and single spatial filter, outperforms all combined single-user/multiple-user approaches and cascaded optimization approaches either uniformly or asymptotically. The constrained optimum MMSE filter is near-far resistant in all but very highly loaded systems and enjoys low complexity.