Multiuser CDMA with a chip-level interference cancellation technique

Abstract

Multiple-access interference (MAI) limits the performance of CDMA systems using the conventional receiver, but the optimal receiver is too complex for practical implementation. This paper demonstrates a numerically efficient technique for reducing MAI in CDMA systems. The technique, a nonlinear approach inspired by the Kalman filter, uses decoupled filters to estimate symbols for each user while accomplishing interference cancellation in the innovation term. The filter technique developed here provides a computationally practical approach to multiuser detection for asynchronous long code systems, which significantly outperforms the conventional receiver. Kalman filter approaches to multiuser detection have been applied before (Lim and Ma 2000, Lim et. al. 1998) to binary phase shift keying through implementation of a standard Kalman filter approach. Unfortunately, this leads to high computational complexity. Here, we instead consider a formulation in which the spreading codes are viewed as random and their properties are used to decouple the Kalman filter for each user. Interference cancellation is then implemented through the innovations. A binary minimum mean squared error estimate is made, which results in a nonlinear estimator. The new nonlinear technique is shown to significantly outperform an optimized partial parallel interference cancellation approach, which is the current state of the art technique for asynchronous long code systems. System capacity is then significantly increased.