Performance analysis of two-dimensional optical code-division multiple-access systems using novel multi-code pulse-position modulation

Previous works show that pulse-position modulation (PPM) is an effective signalling method for mitigating multiple-access interference, and hence is able to increase the number of users in two-dimensional (2-D) optical code-division multiple-access (OCDMA) systems. However, in order to achieve high bit-rates, 2-D OCDMA systems using PPM signalling require very high transmitted power because of the negative impact of dispersion. In this study, the authors propose a novel modulation technique of multi-code PPM (MCPPM), which is the combination of the PPM and multi-code modulation (MCM). As the proposed technique inherits advantages from both MCM (in mitigating the dispersion) and PPM, 2-D OCDMA systems using MCPPM signalling are able to support higher user bit-rates for a larger number of users at low transmitted powers. Numerical results show that 2-D OCDMA systems using 4-4-MCPPM can support 60 users with 5 Gbps per user at the transmitted power of 7 dBm. The power gain in this case is 11 dB compared to 2-D OCDMA systems using 4-PPM signalling. The proposed systems using 4-2-MCPPM can support as many as 36 users with the bit-rate of 10 Gbps per user and transmitted power is 2 dBm.