A comparative analysis of an OCDMA system based on single Photodiode and spectral direct detection schemes

Abstract

The main advantages for using Optical Code Division Multiple Access (OCDMA) are the flexibility of an asynchronous access method, increased security and graceful degradation. Over the last decade, many detection techniques have been proposed for OCDMA, especially for incoherent OCDMA systems. Proper detection selection is very important for good system performance and high network scalability with low bit-error rates (BERs) of less than 10−9. OCDMA systems, however, generally, suffer from multiple access interference (MAI) noise which originates from simultaneous users and severely increases the likelihood of bit errors. To mitigate this limitation, spectral amplitude coding (SAC) OCDMA is used throughout this thesis. The advantage of SAC-OCDMA over conventional OCDMA systems is that, when using appropriate detection techniques, the MAI can be totally suppressed. This either reduces the effect of the MAI or improves the performance even in the presence of MAI. In this thesis, spectral direct detection (SDD) and single-photodiode detection (SPD) techniques based on Modified Double weight (MDW) code, are investigated. The performance analysis of the suggested detection techniques are carried out through simulation experiments using Optisystem software from OptiwaveTM. Moreover, the structure of both detection techniques is based on one photodiode per user compared with other subtraction detection techniques. The comparative analysis shows that the SPD technique is a more capable solution for efficiently restraining interference signals in the optical domain before the signals are converted to the electrical domain. For MDW codes at a data rate of 622 Mbps and a BER of 1×10-10, the results showed that SPD can support more than 60 active users than the SDD technique. Part of the work is also devoted to investigating the feasibility of utilizing different types of optical filters (optical Gaussian, thin film and Fiber Bragg Grating (FBG) filters), and different data formats non-return-to-zero (NRZ) and return-to-zero (RZ). Employing the SPD technique not only provided a BER improvement over SDD, but was used for a large number of users at multiple rates of transmission with cost-effective light sources, and for longer transmission distances.