Multiple-access Interference Resistant Acquisition for Cdma Systems with Long Spreading Sequences

The problem of estimating the propagation delay of a new user in a (coded) DS/CDMA system in the presence of multiple access interference (MAI) is considered. MAI resistant acquisition schemes are developed for a general CDMA system without the constraint that the spreading sequences of the users repeat every symbol period. It is assumed that the spreading sequences and delays of the interfering users are known. However, knowledge of their amplitudes (which would need estimation) is not assumed, and their unreliable code-symbol estimates are not used. Under this scenario, the optimum maximum-likelihood (ML) scheme is derived. An approximation to the ML scheme, the AML scheme, is also presented for the situation where the chip timing is known a priori. The performance of the AML scheme is analyzed using a Gaussian approximation. Simulations show that the approximation is reasonably accurate, and that the AML performance is close to that of the ML scheme. Comparisons are also made with the conventional parallel acquisition scheme that treats the interference as random noise.