Ds-cdma receivers in Rayleigh fading multipath channels: Direct vs. indirect methods

In Direct-Sequence Code Division Multiple Access (DS-CDMA) systems, the performances are limited by Multiple Access Interference (MAI) and InterSymbol Interference (ISI) due to channel effects. To increase the capacity and allow high data rates, MultiUser Detection (MUD) techniques are used. In this paper, the performances of direct and indirect MUD applied to a synchronous DS-CDMA system with multipath Rayleigh channels are reported. The direct MUD is represented by an adaptive linear FIR filter using the normalized least mean square algorithm (FIR-NLMS) while the indirect method is represented by the Minimum Mean Square Error (MMSE). The simulation results show that the optimal (perfect channel identification) indirect MMSE outperforms the adaptive FIR-NLMS. However, with imperfect channel estimation, the FIR-NLMS shows better performances. The convergence speed, and a way to fasten it, is also presented in the simulations results.