New results on low complexity detectors for over-saturated CDMA systems

This paper analyzes two different low-complexity multiuser detection schemes for the oversaturated synchronous CDMA systems, ie, systems which can accommodate more users than the processing gain of the system. The first scheme is multistage iterative alternative detection of the users of the two groups combined with interference cancellation. The bit-error rate performance analysis of this scheme using Gaussian approximation is also derived and it is shown that this scheme is the realization of the space altering generalized expectation maximization algorithm known in signal processing literature. As an improvement for the shortcomings of the first scheme in near-far situations, a group orthogonal sequence allocation scheme is introduced. It is demonstrated that it can lead to efficient optimal multiuser detection. It is also shown that if all groups of sequences attain the Welch bound for the sum of squared sequence correlation coefficients then the optimal joint multiuser detection maximizes the sum capacity of the channel.