EFFECTS OF LASER PHASE DRIFT ON COHERENT OPTICAL CDMA

Code division multiple access (CDMA) has been proposed for use in fibe r high speed point-to-point systems. Previous research into CDMA has c entered 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, semicondu ctor lasers suffer from serious phase variation that makes tracking of all phases difficult to accomplish. This paper examines the effects o f phase drift on several multiuser detectors for binary phase shift ke ying (BPSK), as well as two detectors for binary frequency shift keyin g (BFSK). It is shown that when the desired user's phase can be tracke d, BPSK match filtering offers better bit error rates than BFSK, and c losed form solutions for the asymptotic error probabilities are derive d. When there is no phase tracking, it is shown that the probability o f error of the multiuser detectors approaches one half. The analysis i s based on a proof of the asymptotic normality of the interference cau sed by even a single undesired user. All asymptotic results are for in finitely long spreading codes that can be modeled as a random sequence of plus and minus ones. Whether or not the desired user's phase is tr acked, all receivers examined suffer from the near-far problem.