Design and Analysis of Asynchronous Incoherent Optical-CDMA Systems Using a New Code-Shifting Technique

In this paper, a novel code-shifting technique, which can be added atop asynchronous incoherent optical code-division multiple-access systems without complex modifications, is proposed and studied. The new technique divides the time-slot (or chips), in which the optical pulses of codewords are located, into g equal-width sub-chips; every pulse is randomly and independently shifted to start at one of the g sub-chips of its own chip, where g > 1 is a positive integer. The effects of this random g-shift to the cross-correlation property and performance of asynchronous optical codes (of periodic cross-correlation functions of at most one) are formulated. Although it is found that the periodic cross-correlation functions are worsened to 3 - 2/g, our theoretical and computer-simulation results show an unconventional finding that code performance improves as g increases.