Performance of optical direct-detection CDMA systems using prime sequence codes

The performance of optical synchronous code-division multiple-access (CDMA) with on-off keying (OOK) signaling (OOK/CDMA) and CDMA with M-ary pulse position modulation (PPM) signaling (PPM/CDMA) systems using prime sequence codes as signature codes is analyzed under the assumption of Poisson shot noise model for the photodetector. The bit error probability of OOK/CDMA is evaluated, and the upper bound on the bit error probability of PPM/CDMA is derived. Further, the performance is compared for some values of the average signal count per second, the length of prime sequences, and the number of simultaneous users under the constraint on throughput. It is found that PPM/CDMA performs better than OOK/CDMA by the pulse position multiplicity. It is also found that as the number of slots per frame of PPM increases, the bit error probability performance is improved. Moreover, it is found that CDMA with longer prime sequence codes has better performance. Therefore, we show that in order to achieve the low bit error probability and many simultaneous users, we should employ PPM/CDMA with larger number of slots and longer prime sequence codes.