Performance evaluation of multi-rate DS-CDMA using frequency-domain equalization in a frequency-selective fading channel

Joint frequency-domain equalization (FDE) and antenna diversity combining is applied to the reception of multi-rate DS-CDMA signals to achieve the frequency diversity effect while suppressing interpath interference (IPI) resulting from the asynchronism of different propagation paths. At a receiver, fast Fourier transform (FFT) is applied for FDE and then inverse FFT (IFFT) is used to obtain a frequency-domain equalized DS-CDMA chip sequence for the succeeding despreading operation. An arbitrary spreading factor SF can be used for the given value of FFT window size; an extreme case is the nonspread SC system with SF=1. This property allows a flexible design of multi-rate DS-CDMA systems. Three types of FDE are considered; minimum mean square error (MMSE) equalization, maximal-ratio combining (MRC) equalization and zero-forcing (ZF) equalization. Matched filter bound analysis for achievable BER performance is presented. The improvement in the BER performance in a frequency-selective Rayleigh fading channel is evaluated by computer simulation. First, we consider the single-user case and compare the BER performances achievable with MMSE, MRC and ZF equalizations. How the fading rate and the spreading factor affect the BER performance is also evaluated. Furthermore, the BER performance comparison between FDE and rake combining is presented for various values of SF and also performance comparison between DS-CDMA and SC signal transmissions, both using FDE, is presented. Finally, we extend our evaluation to the multi-user case. Both downlink and uplink are considered and how the BER performances of downlink and uplink differ is discussed.