Direct-detection synchronous O-CDMA system with interference estimation and cancellation

An M-ary coded synchronous optical code-division multiple-access (O-CDMA) system with pulse-position modulation (PPM) is investigated. One novel class of optical spreading codes based on combinatorial designs is adopted in the multiplexing process. Their ideal correlation properties facilitate the discrimination between desired signals and jamming. However, the multiple-access interference (MAI) with high intensity significantly deteriorates the system performance even if the number of interferers is small. In this paper, we present an interference reduction technique for direct-detection O-CDMA to suppress the noise effect and increase the system capacity. The MAI from reference signals can be estimated by utilizing the uniform cross-correlation (CC) among its sequences and considerably cancelled out after the photodetection process. The upper bound on the error probability of optical synchronous PPM-CDMA is then derived. The proposed system is shown to be effective to improve the bit error performance and to alleviate the error floor when the number of simultaneous users and the received optical power are not appreciably small.