MULTIUSER DETECTION FOR OPTICAL CODE-DIVISION MULTIPLE-ACCESS SYSTEMS

This paper considers a multiuser detection scheme for optical direct sequence code division multiple access (OCDMA) systems, referred to as the multistage detector. Previous works in this area have proposed two detectors: the correlation detector that is simple, but has poor performance for large number of users, and the optimal (minimum probability of error) detector that has exponential complexity in the number of users. Efficient multiuser detection algorithms consider the interfering user codes at the expense of electronic speed processing, unlike the optical processing achievable with the correlation detector. The multiple access system using the proposed multistage detector is shown to be of high performance and low complexity, compared to the conventional correlation detector and the optimal detector. The model studied includes multiple access interference as well as the Poisson characteristics of the optical direct detection process. An approximation to the probability of error is derived for the multistage detector, and it is compared to the actual error probability of the correlation detector, to a lower bound to the error probability of the optimal detector, and to simulation results for the multistage detector. The probability of error is calculated using a characteristic function method. Results are presented for a random code case that show a significant improvement in the performance of the OCDMA system using this detector over the correlation detector.