Performance analysis of optical direct-detection CDMA systems using synchronous quadratic congruence optical codes with hard-limiter and PPM signaling

The performance of optical synchronous code-division multiple-access (S/CDMA) with an optical hard-limiter and M-ary pulse-position modulation (M-PPM) signaling systems using synchronous quadratic congruence (QC) optical codes as signature codes is investigated. An assumption of the Poisson shot noise model for the photo-detector is adopted. Three kinds of code sequences that are generated from QC are considered. A union upper bound on the bit error rate is derived taking into account the effect of the multiple-user interference, receiver shot noise, and an optical hard-limiter. Then the performance is compared for the average power, the length of QC sequences, and the number of simultaneous users under the throughput constraints. It is found that with longer QC sequence codes, the system has a better performance