On the performance of coded low spreading gain DS-CDMA systems with random spreading in sequences multipath Rayleigh fading channels

Next generation wireless systems based on variable spreading gain (VSG) DS-CDMA offer variable data-rates while keeping constant the signal envelope and bandwidth. For high data-rate services the SG can be as low as 2 chips per transmitted (coded) symbol, which deteriorates the system performance because it enhances the inter-path interference (IPI). Different self-interference cancellation (SIC) approaches have been examined in uncoded VSG-CDMA systems to combat IPI. We examine the effect of low SG in practical VSG-CDMA systems, which use convolutional and turbo coding and concatenated short orthogonal and long pseudo-random spreading. We show that the performance of the conventional RAKE receiver degrades only moderately for SG as low as 4 chips per transmitted symbol for BER⩾10^-6 and for different multipath profiles. In this range of interest the error floor typical to turbo codes is observed due to the free distance properties of these codes rather than the effect of IPI. This suggests that the increased complexity of SIC schemes is justified only for very high data-rates