Optical spectral amplitude coding CDMA systems using perfect difference codes and interference estimation

Abstract

The spectral amplitude coding (SAC) technique is applied to a synchronous optical code-division multiple-access (OCDMA) system, using perfect difference code (PDC). The use of the SAC technique removes the codeword synchronisation requirement of time-spreading OCDMA systems using PDCs. In the proposed system, the unity cross-correlation property of PDCs is exploited by the interference estimator of the proposed system to estimate the number of active users simultaneously transmitting bit 1. This estimate is then employed to modify the threshold used to differentiate between transmitted bits 1 and 0. In contrast to the variance expansion of the decision statistics used in synchronous OCDMA systems with interference cancellation, the proposed system preserves the characteristics of the received signal statistics, thereby avoiding data decision errors. As the estimation rules and the decision thresholds are important factors influencing the estimates and the bit-error rates, the effects of these two factors are thoroughly investigated. A second SAC-PDC OCDMA system is developed, which mitigates multiple-access interference (MAI) by means of an interference cancellation technique and makes data decisions based on the estimated MAI. The numerical results reveal that the two systems provide a better performance and a larger capacity than the synchronous OCDMA system.