DIRECT-SEQUENCE SPREAD-SPECTRUM MULTIPLE-ACCESS COMMUNICATIONS WITH RANDOM SIGNATURE SEQUENCES - A LARGE DEVIATIONS ANALYSIS

A DS-SSMA bit-error probability analysis is developed using large deviations theory. Let m denote the number of interfering spread spectrum signals and let n denote the signature sequences length. Then the large deviations limit is as n --> infinity with m fixed. A tight asymptotic expression for the bit-error probability is proven, and in addition, recent large deviations results dealing with the importance sampling Monte Carlo estimation technique are applied to obtain accurate and computationally efficient estimates of the bit-error probability for finite values of m and n. The large deviations point of view is compared also to the conventional asymptotics of central limit theory and the associated Gaussian approximation. The Gaussian approximation is accurate when the ratio m/n is moderately large and all signals have roughly equal power. In the near/far situation, however, the Gaussian approximation is quite poor. In contrast, large deviations techniques are more accurate in the near/far situation, and it is here that these methods provide some important practical insight.