Special Issue on High Speed Point-to-point Optical Communications Systems Eeects of Laser Phase Drift on Coherent Optical Cdma

| Code division multiple access (CDMA) has been proposed for use in ber high speed point-to-point systems. Previous research into CDMA has centered on completely coherent detection, that is, when the phases of all users in the system are known or tracked. While coherent detection is a reasonable assumption in many radio frequency systems, semiconductor lasers suuer from serious phase variation that makes tracking of all phases diicult to accomplish. This paper examines the eeects of phase drift on several multiuser detectors for binary phase shift keying (BPSK), as well as two detectors for binary frequency shift keying (BFSK). It is shown that when the desired user's phase can be tracked, BPSK match ltering ooers better bit error rates than BFSK, and closed form solutions for the asymp-totic error probabilities are derived. When there is no phase tracking, it is shown that the probability of error of the multiuser detectors approaches one half. The analysis is based on a proof of the asymptotic normality of the interference caused by even a single undesired user. All asymptotic results are for innnitely long spreading codes that can be mod-eled as a random sequence of plus and minus ones. Whether or not the desired user's phase is tracked, all receivers examined suuer from the near-far problem.