Modified LMMSE receiver for DS-CDMA-Part I. Performance analysis and adaptive implementations

Linear minimum mean squared error (LMMSE) criterion can be used to obtain near-far resistant receivers in direct-sequence code-division multiple-access (DS-CDMA) systems. The standard version of the LMMSE receiver minimizes the mean squared error between the filter output and the true transmitted data sequence. Since the detector depends on the channel coefficients of all users, it cannot be implemented adaptively in fading channels due to severe convergence problems. In this paper, a modified criterion for deriving LMMSE receivers in fading channels is presented. The modified LMMSE receiver is independent from the users' complex channel coefficients. The performance of the LMMSE receivers in fading channels is studied via computer simulations and numerical analysis. The results show that the modified LMMSE receiver has remarkably larger capacity than the conventional RAKE receiver at the SNRs of practical interest.