Modiied 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 coeecients 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 modiied criterion for deriving LMMSE receivers in fading channels is used, which eeectively converts the pathological Rayleigh fading channel to an equivalent xed AWGN channel from detector point of view. Thus, the detector is independent from user's complex channel coeecients. Adaptive versions of the modiied LMMSE receiver require estimation of channel delays and complex coeecients. 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 modiied criterion can be applied in downlink receivers in fading channels and the adaptive version converges close to the optimal LMMSE solution.