Efficient Multi-Rate Multi-User Detection for the Asynchronous WCDMA Uplink

Sub-optimal multi-user joint-detection schemes such as the zero-forcing and minimum mean-square block-linear equalizers (and their decision feedback counterparts) are effective technique for combating the near-far problem experienced in CDMA communication systems. The implementation of these schemes, however, is accompanied by a prohibitive computational complexity when supporting large numbers of asynchronous users and channels with long impulse responses. Two approaches for the reduction of this complexity are presented in this paper. The first is based on an approximate Cholesky factorization which exploits the pseudo block-Toeplitz nature of the triangular factor of the code-channel correlation matrix. The second approach is based on the method of conjugate gradients and solves the joint-detection problem via iterative minimization of a quadratic cost function. The two approaches result in significant reductions in computational complexity at the expense of little degradation in performance. The results are demonstrated for the asynchronous multi-rte environment of the WCDMA uplink.