LMMSE receivers performance under non-ideal conditions

Linear minimum mean square error (LMMSE) receivers are capable of suppressing multiple access interference and near-far occurrences in a code division multiple access (CDMA) system operating in multipath fading wireless channels. Their relative low complexity makes them suitable for both uplink and downlink receivers. Third generation receivers are mainly based on coherent detection. Carrier frequency, carrier phase, multipath profile, path delays as well as fading characteristics are important parameters to perform a coherent detection of transmitted symbols. Several multi-user receivers for direct-sequence CDMA systems have been proposed in previous years mainly with the assumption of perfect estimation of those parameters. The aim of this paper is to analyze and investigate the impact of channel estimation and synchronization errors on the LMMSE receivers performance. In particular, two different LMMSE schemes have been considered depending on the order in which the interference suppression and the multipath combining are performed. The pre-combining and the post-combining LMMSE receivers behavior in a multipath fading wireless channel when an imperfect parameters estimation is present into the detection process has been evaluated by using the asymptotic multiuser efficiency.