Effects of hard-limiter and error correction coding on performance of direct-detection optical CDMA systems with PPM signaling

The effect of an optical hard-limiter on the performance of direct-detection optical synchronous code-division multiple-access (CDMA) systems with M-ary pulse position modulation (PPM) signaling is analyzed. Moreover, the effect of the error correction coding on the performance of direct-detection optical synchronous CDMA systems with PPM signaling is analyzed: Reed-Solomon (RS) codes and convolutional codes (CC's) with soft-decision Viterbi decoding are employed. We analyze the performance under the assumption of Poisson shot noise model for the receiver photodetector and the noise due to the detector dark currents is considered. We analyze the performance under average power and bit rate constraints. Our results show that the optical hard-limiter is not effective for improving the performance of the optical CDMA systems with PPM signaling. Moreover, RS codes are shown to be more effective than CC's with soft-decision Viterbi decoding to reduce an asymptotic floor to the error probability of the system with large M, while CC's with soft-decision Viterbi decoding is more effective than RS codes for the system with small M. Furthermore, we show that as the code rate of the error correction code increases, the required average energy to achieve the bit error probability P_b = 10^-5 for the RS coded PPM/CDMA system appreciably increases compared with that for the convolutional coded PPM/CDMA system when M = 16.