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 direct-detection optical CDMA systems with PPM signaling without narrowing the chip width. In the ATC-CDMA system, each user is allowed to access the network T-u times out of T-t times according to the scheduled access timing pattern, and the number of users accessing the network simultaneously is decreased; the effect of the channel crosstalk is thus reduced. We analyze the performance of the proposed system under the assumption of Poisson shot noise model for the receiver photodetector including the noise due to the detector dark currents. We compare the performance of the ATC-CDMA system using PPM signaling with that of a conventional CDMA system using PPM signaling under a fixed bit rate, almost the same chip width and a constraint on the transmitted energy per pulse. It is shown that the ATC-CDMA system has better performance than the conventional CDMA system. Moreover, it is shown that the ATC-CDMA system with fewer slots per symbol and lower access timing probability to the network has better performance at the fixed bit rate and the chip width. Therefore, controlling an access timing of each user to the network is shown to be more effective for improving the bit error probability performance than the pulse position multiplicity with the increase in the number of slots per symbol under the constraint on the bit rate and the chip width on direct-detection optical CDMA systems with PPM signaling.