Performance enhancement of downlink multiuser DS-CDMA detectors in multipath channels over AWGN channels

Abstract

In this paper, we have introduced the diversity scheme maximum ratio combining (MRC) as preprocessor to the adaptive multiuser-detector (MUD) in direct-sequence code-division multiple-access (DS-CDMA) downlink systems to enhance its performance in multipath channels over AWGN channels. The performance of DS-CDMA is limited by multiple-access interference (MAI). To reduce the effects of MAI, MUDs are used at the receiver side. MUDs work by taking into account the information like spreading codes of the users and system parameters, but these values are not constant and also not known beforehand. Thus MUDs were advanced to adaptive MUDs which take into account the changing parameters by employing adaptive technique for e.g. LMS or RLS algorithms. These adaptive MUDs require each transmitter to send a training sequence at startup which the receiver uses for initial adaptation. However, if there is any drastic change in interference environment, decision directed adaptation becomes unreliable, and data transmission must be temporarily suspended and yield to a fresh training sequence. Therefore MRC as preprocessor to the adaptive MUD of DS-CDMA system is used. As shown by simulation results, the BER performance achieved by the proposed detector outperforms the conventional MF detector with a good margin and is promising.