The capacities of optical CDMA communication channels with different code-correlation constraints

A comparison between the performance of several optical code-division multiple-access (CDMA) correlation receivers is presented. The performance is measured in terms of an uncoded throughput capacity. It is defined as the maximum data rate (in nats/chip time) that can be achieved with arbitrary small error probability. Both on-off keying (OOK) and pulse-position modulation (PPM) CDMA schemes are considered. Signature code correlations bounded by either one or two are employed. Our results reveal that the throughput capacity of the optical PPM-CDMA systems can be increased by increasing the code-correlation constraint from one to two. That of OOK-CDMA systems, however, cannot be increased. Further, the throughput capacity of PPM-CDMA systems with code-correlation constraint of two is greater than that of OOK-CDMA systems with code-correlation constraint of one or two. In fact, this improvement in the throughput of PPM-CDMA systems over that of OOK-CDMA approaches a limiting factor of 10 as the pulse-position multiplicity increases to infinity.