Access timing controlled direct-detection optical CDMA systems with PPM signaling

We propose an access timing controlled direct detection optical code division multiple-access (ATC-CDMA) system with pulse position modulation (PPM) signaling to improve the performance of the direct-detection optical CDMA systems with PPM signaling without narrowing the chip width. In ATC-CDMA system, an access timing of each user to the network is controlled and the effect of channel crosstalk is reduced. We analyze the performance of the proposed system under the assumption of Poisson shot noise model for the receiver photodetector. We compare the performance of ATC-CDMA system using PPM signaling with that of conventional CDMA system using PPM signaling at the fixed bit rate and chip width. It is shown that ATC-CDMA system has better performance than conventional CDMA system. Moreover, it is shown that ATC-CDMA system with smaller M and lower access timing probability to the network has better performance at the fixed bit rate and chip width.