Lmmse Receivers for DS-CDMA Systems in Frequency-Selective Fading Channels

Abstract

The linear minimum mean squared error (LMMSE) criterion can be used to obtain near-far resistant single-user receivers in direct-sequence code-division multiple-access (DS-CDMA) systems. The standard version (postcombining) of the LMMSE receiver minimizes the mean squared error (MSE) between the filter output and the actual 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. A modified criterion for deriving LMMSE receivers in fading channels is presented. The modified (precombining) LMMSE receiver is independent from users’ complex channel coefficients, and it effectively converts the pathological Rayleigh fading channel to an equivalent fixed AWGN channel from the detector updating perspective. The precombining LMMSE receiver leads to blind adaptive implementations. Some alternative blind adaptive receivers are reviewed and their performance in frequency-selective fading channels is addressed. Since the blind adaptive implementations require explicit information on multipath delays, delay estimation issue will also be considered. The blind adaptive interference suppression methods are applied furthermore to the residual interference suppression in parallel interference cancellation receivers.