Performance Analysis of SAC-OCDMA Systems Adopting Overlapping PPM Schemes

A fertile technique, for increasing single-user throughput while keeping a constraint on the light pulsewidth, is proposed for spectral-amplitude-coding optical code-division multiple-access (SAC-OCDMA) systems. In this technique, two-level
M
-ary overlapping pulse-position modulation (
M
-OPPM) scheme is adopted and each user is assigned two orthogonal code sequences to represent these two levels. The code sequences are selected from a minimum cross-correlation code set. The bit error rate (BER) of the proposed system is derived, taking into account the effects of phase-induced intensity noise, shot noise, and thermal noise in addition to the multiple-access interference. The BER performance of this system is compared to other systems adopting
M
-PPM and OOK schemes under same pulsewidth constraints. Our results reveal that, while keeping the BER well below a prescribed threshold, the proposed
M
-OPPM SAC-OCDMA system achieves higher transmission rate as compared to both
M
-PPM and OOK SAC-OCDMA systems under same constraints. Specifically the transmission rate of a single user of the proposed system can be increased by about 34.44% as compared to traditional systems.