High-throughput optical code-division multiple access communication systems
Doctor of Philosophy thesis
Optical communication systems offer a bandwidth orders of magnitude larger than any electric or radio-frequency system. Yet, historically, optical communication systems have been designed by adapting existing conventional low frequency systems to operate at light frequencies, thus limiting the rate of information transfer to those previously achievable at lower frequencies. The purpose of this study is to show that it is possible, through the design of the optical communication system according to its own statistical characteristics, to achieve reliable information transfer rates commensurate with the optical bandwidth, that is, orders of magnitude higher than any conventional systems. Two optical code-division multiple-access systems are proposed, one a conventional time encoded system, and one an innovative spectral amplitude encoded system. The spectral amplitude encoded system provides many advantages including the ability to implement efficient symbol detectors, insensitivity to chromatic dispersion, and availability of large codes. An optimized single-user detector, both hard-decision and soft-decision multistage detectors, and a local search detector are proposed as alternatives to the weak conventional detector and the complex optimal detector. A full performance analysis of the two systems with each detector is performed. The main analysis tool is an approximation based on large deviation theory. The approximation to the error probability is verified by comparing the results to values obtained using a characteristic function method for small problems. Some simplifying assumptions are made to evaluate the moment generating function and the validity of these is confirmed by comparing to simulation results. Both random user sequences and well-designed deterministic codes are investigated. The optimized single-user detector performed considerably better than the conventional correlation detector in all cases. All multiuser detectors error probabilities compared favorably to the theoretical lower bound. The soft-decision multistage detector showed significant improvement over the hard-decision counterpart. The local search detector outperformed them all, at the expense of added complexity. The asymptotic multiuser efficiency of the system with each detector was also computed using large deviation theory. All proposed detectors are shown to be robust against multiple-access interference.
Engineering, Electronics and Electrical