A comparison of long versus short spreading sequences in coded asynchronous DS-CDMA systems

The performance of turbo-coded asynchronous direct sequence code division m ultiple access (DS-CDMA) using long and short spreading sequences is compar ed by both analysis and simulation. For coded systems with a conventional m atched filter (MY) receiver, three analytical methods with different comple xity are compared: the standard Gaussian approximation, the improved Gaussi an approximation (IGA), and the density function approach. It is shown that while the standard Gaussian approximation is fairly accurate for the long sequences, it is too optimistic for the short sequences. For the short-sequ ence systems, the IGA gives an accurate estimate for the performance with m uch less complexity than the density function approach. The analysis shows that for either the additive white Gaussian noise (AWGN) channel or the fla t Rayleigh fading channel and a NIF receiver, there is a degradation in the average performance of the turbo-coded short-sequence systems compared to the long-sequence systems due to the fact that the cross-correlations are n ot time-varying. However, the short-sequence systems are amenable to the us e of an interference suppression technique designed to minimize the mean sq uare error. Such a minimum mean square error (MMSE) receiver in the turbo-c oded system is shown to outperform the long-sequence system with the MF rec eiver, especially when there is a near-far problem, as previously observed in a convolutionally-coded system. Finally, similar results are obtained by computer simulations for the turbo-coded CDMA systems on a frequency-selec tive Rayleigh fading channel.