Modified adaptive LMMSE receiver for DS-CDMA systems in fading channels

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 detector depends on the channel coefficients of all users and cannot be used in frequency-selective fading channels due to fast changes of channel phases which demands for continuous detector updating. In this paper, a modified criterion for deriving LMMSE receivers in fading channels is used, which effectively converts the pathological Rayleigh fading channel to an equivalent fixed AWGN channel from the detector point of view. Thus, the detector is independent from the user's complex channel coefficients. Adaptive versions of the modified LMMSE receiver require estimation of channel delays and complex coefficients. An adaptive least mean squares (LMS) algorithm for DS-CDMA downlink receivers utilizing a pilot channel for channel estimation is presented as an example of a practical implementation of the scheme. The numerical results show that the modified criterion can be applied in downlink receivers in fading channels and the adaptive version converges close to the optimal LMMSE solution.