Phase drift effects in optical CDMA

Fiber optic communication systems using CDMA have been proposed. While coherent detection is a reasonable assumption in many RF systems, semiconductor lasers for fiber applications have been shown to suffer from more serious phase variation that precludes fully coherent detection, the focus of previous CDMA research. The authors examine the effect of phase drift on several multiuser detectors for BPSK, as well as two detectors for BFSK. They show that when the desired user's phase can be tracked, BPSK match filtering offers better bit error rates than BFSK, and the authors derive closed form solutions for the error probabilities in the limit of infinitely long spreading codes modeled as a random sequence of plus and minus ones. When there is no phase tracking, they show that the probability of error of the multiuser detectors approaches one half.