Iterative nonlinear MMSE multiuser detection

This paper introduces the notion of nonlinear MMSE multiuser detection and shows that MMSE signal estimates followed by single user detectors yield MAP (or minimum probability of error) decisions for CDMA signals. Iterative solutions are proposed for nonlinear MMSE estimation. If interferers' codes are assumed to be known by the signal estimators for each user, the MMSE solution is shown to be a fixed point which is reached in one iteration, but is computationally intractable. If interferers' codes are not assumed to be known, the resulting iterative multiuser detector (called random-code NMIC) is shown to be of the same order of complexity as a conventional multistage interference canceler. Furthermore, the nonlinear MMSE solution is shown to be a fixed point of the random-code NMIC. Particular solutions are presented for the cases of BPSK and M-ary orthogonal spread spectrum systems.