Chip-Asynchronous Binary Optical CDMA: An Optimum Signaling Scheme for Random Delays

Most optical code division multiple access (OCDMA) research literature has focused on chip-synchronous OCDMA. However, practical OCDMA networks are inherently chip-asynchronous since users transmit their signals from different locations without any coordination. This paper focuses on binary chip-asynchronous OCDMA with time spreading, and the main result is that appropriate nonuniform signaling can achieve a data throughput of 2.5 bits per OCDMA chip. This result shows that the presence of chip asynchronism leads to approximately a threefold capacity gain over chip-synchronous OCDMA capacity. Additional results are presented to illustrate that the near capacity multiuser interference has a low variance and a non-Gaussian distribution. Finally, the robustness of the achieved capacity gains is verified in the presence of additive channel noise.