Asynchronous BPPM OCDMA systems with trellis-coded modulation

The author discusses the application of spectrally efficient trellis-coded-modulation (TCM) schemes based on set partitioning of the signal set to asynchronous optical-code-division multiple-access (OCDMA) systems employing binary-pulse-position modulation (BPPM) and an optical orthogonal code (OOC). Two techniques to increase the cardinality of the signal set (number of signal points in the signal space diagram) are discussed along with the performance evaluation of OCDMA systems employing TCM based on these techniques. Suitable comparisons are reported between OCDMA systems employing TCM and those which do not. The performance evaluation is done in the presence of multi-user interference (MUI), photon-detection shot noise, detector dark-current noise and circuit thermal noise under the assumption of chip synchronous-bit/symbol asynchronous transmission. The theoretical results show that by using TCM in conjunction with the Viterbi decoding algorithm, considerable enhancement in performance is achievable in asynchronous OCDMA systems, along with a simultaneous enhancement of bandwidth efficiency.