Coded DS/SSMA communication systems over fading channels

The design and analytic performance evaluation of coded asynchronous direct-sequence spread-spectrum multiple-access (DS/SSMA) communication systems are studied over the additive white Gaussian noise (AWGN) or Rayleigh fading channel. For a single-user detection, a cascaded-sequence scheme is proposed to decrease the pairwise error probability (PEP) over the AWGN channel. By deriving the probability density functions (pdf\u27s) and cumulative distribution functions (cdf\u27s) of the multiple-access interference (MAI), an improved approximation of the upper union bound on the bit error probability is proposed, and is shown to be much better than the conventional Gaussian approximation. For the Rayleigh fading channel, a new set of pdf\u27s and cdf\u27s of the MAI statistic is derived, and a modified bounding technique is proposed to obtain tight bounds on the PEP. Several suboptimum decoding schemes are proposed, and their performances are shown to be very close to that of the optimum decoding scheme under certain conditions. For multiuser detection, the design of decision-feedback multiuser (DFM) receivers over mismatched Rayleigh fading channels is studied, and tight bounds on the pairwise symbol error probability are derived. It is shown that the proposed system, which takes into account the effect of mismatched fading, has some gain over the system where the estimation of fading is assumed to be exact. Finally, the cdf\u27s of the general two-dimensional MAI are derived, and some applications are illustrated