Study of Multicode-Keying Incoherent Optical CDMA Without the Conventional Symbol-Synchronous Assumption

In this paper, the effect of removing the symbol-synchronous assumption in the multicode-keying incoherent optical code-division multiple access (O-CDMA) systems is studied. Under true symbol asynchronism, the multicode-keying modulation can create "two hits" in the periodic cross-correlation process even though "one-hit" optical codes are used. Three pulse-erasure methods of reducing the occurrence of two hits are studied. By applying optical orthogonal codes to permute the pulses in codewords, a method of constructing one-hit optical codes that can eliminate the occurrence of two hits in "symbol-asynchronous" multicode-keying O-CDMA and enlarge code cardinality is proposed. Performance analytical models for these three two-hit reduction methods and the new optical codes are formulated. Finally, numerical examples are given to compare their performances, and computer-simulation results are presented to validate the analytical models.