Coherent Ultrashort Light Pulse CDMA Communication Systems with Multipulse Pulse Position Modulation

We propose coherent ultrashort light pulse code-division multiple-access (CDMA) communication systems with multipulse pulse position modulation (MPPM). Coherent ultrashort light pulse CDMA is an asynchronous multiplexing technique for high-speed fiber-optic communications. MPPM is introduced to outperform both pulse position modulation (PPM) and on-off keying (OOK), when the optical bandwidths for these three modulation methods are the same. We theoretically derive the bit error rate (BER) of the proposed system. The performance characteristics are discussed with respect to a number of system parameters, and are compared with those of PPM and OOK. Our results show that MPPM is able to achieve a lower BER than that of both PPM and OOK when the bit rates for these methods are the same. We also find that there exists the optimum number of pulses in an MPPM signal block, which is inversely proportional to both the bit rate and the total number of multiple access interference (MAI) users.