Optimal Source Matching in Optical Code Division Multiple Access

The capacity of a discrete noisy channel is achieved when the transmission source is properly matched to the channel, in the sense that the distribution of the transmitted bits maximizes the channel's mutual information. Traditional source matching assumes independency between the source and the channel, while the case of source dependent channel is usually ignored. In this article, the source dependant channel of Optical Code Division Multiple Access (OCDMA), dominated by Multiple Access Interference (MAI) is analyzed. It is shown that source dependency of MAI reverses the expected behavior of the OCDMA channel, and also that for a given value of channel condition, or mutual information, the number of concurrent users can be increased when the source is properly matched to the channel. A simple technique, which allows combining source matching with practical bit stream methods such as synchronization, error correction codes and the utilization of different detection methods, is also presented.