Design and Analysis of Optical CDMA System with Non-Coherent Light Source

Optical code division multiple access (OCDMA) systems have been investigated for about two decades. They have the advantage of providing multiple users to simultaneously access the same bandwidth with high-level security. In general, the performance of OCDMA systems is primarily affected by multi-user interference (MUI), which is unavoidable for asynchronous time-spreading OCDMA systems. In recent years, OCDMA systems with non-coherent light sources attract a lot of attention because MUI can be completely eliminated by coding. However, in these systems, due to the nature of non-coherent light sources, a phase-induced intensity noise (PIIN) is caused and the system performance is hence degraded severely.
Conventionally, there are three categories of OCDMA systems with non-coherent light sources, i.e. coherence multiplexing system, spectral-amplitude-coding (SAC) system and two-dimensional (2-D) spectral/spatial coding system. This thesis aims to improve the performance of these systems. Regarding the coherence multiplexing system, a novel system structure is proposed in this thesis to suppress PIIN and improve the system performance thereby. In the proposed system, we simply substitute the continuous wave broadband light source of the conventional coherence multiplexing system with a pulsed broadband light source to reduce the probability of beating from other users. In this way, the PIIN caused by other users is suppressed.
As for the SAC system, we propose a family of newly constructed codes, named partial modified prime (PMP) codes, and a corresponding system structure. The PMP codes are a divided version of modified prime codes and thus have in-phase cross-correlation not larger than one. Because most of the in-phase cross-correlation between the PMP codes is zero, compared with the conventional SAC system, the beating probability of the proposed system is decreased considerably and thus the PIIN is further suppressed. Moreover, for elimination of MUI, Mach-Zehnder interferometers are employed in the proposed system.
Lastly, for performance improvement of the 2-D spectral/spatial coding system, we propose 2-D perfect difference codes and its corresponding system. The 2-D perfect difference codes are derived in view of perfect different codes that originally used in a synchronous time-spreading OCDMA system. Since the 2-D perfect difference codes have a MUI cancellation property and cross-correlation much lower than that of conventional 2-D spectral/spatial codes, such as Maximal-area matrices (M-matrices) codes, the proposed system can completely eliminate the MUI and effectively suppress the PIIN. Moreover, the number of simultaneous users that can be accommodated in the proposed system can be increased almost linearly in proportion to the spatial code length of the 2-D perfect difference codes. It is unreachable for present 2-D spectral/spatial coding systems.
In accordance with analysis and numerical results, all of the three proposed systems have better performance and can accommodate more simultaneous users than the conventional systems. In addition, the numerical results of the proposed systems using PMP codes and 2-D perfect difference codes are verified by using a well-known and highly recognized software tool, “VPItransmissionMaker.”