Optimally combined nonlinear MMSE beamforming and interference cancellation for CDMA communications

Nonlinear MMSE multiuser detection has been shown in earlier works to be equivalent to minimum probability of error multiuser detection for systems using binary or M-ary orthogonal signaling. An iterative approximation to nonlinear MMSE multiuser detection, proposed earlier for single-antenna systems, is applied in this paper to CDMA systems with random spreading and multiple receive antennas. The proposed receiver iterates between a bank of nonlinear MMSE beamformers and an interference canceler. The computational complexity of the proposed receiver, including that of the proposed adaptive array response estimation algorithm, is quadratic in the number of antennas, and linear in the number of users and the number of stages of cancellation. Simulation results demonstrate that substantial capacity improvements can be obtained using this receiver with just a single stage of cancellation, compared to receivers that use linear MMSE beamformers and no interference cancellation, and those using interference cancellation but no beamforming.