Development of a new code for subcarrier-multiplexed optical code division multiple access system

Abstract

The growing utilization of bandwidth hungry applications of the internet and multimedia applications has lead to tremendous research development in optimizing the bandwidth capacity in optical communication, particularly in Optical Code Division Multiple Access (OCDMA). In this work, a new code in the family of Spectral Amplitude Coding (SAC) OCDMA has been developed in an attempt to overcome the limitations faced by other existing codes of the same class. This work also contributes to the development of mathematical model of hybrid subcarrier multiplexed OCDMA system based on the new code and its performance evaluations. The combination of Subcarrier Multiplexing (SCM) and OCDMA scheme can be regarded as a new research area which is still in its infancy, since most work on SCM gives privilege to the combination with Wavelength division multiplexing (WDM) scheme. This combination demonstrates an improvement in bandwidth utilization of optical communication. In particular, this thesis begins with an explicit construction of the code, namely the Recursive Combinatorial (RC) code that is designed to have a minimum cross correlation of one, with the flexibility of choosing the code’s parameter, such as the code weight and number of code sequences, with minimum code length. The simple code development are based on the matrix combinatorial method to maintain the cross correlation of one between the code sequences, that reduces the Multiple Access Interference (MAI). The mathematical derivation of this code’s performance is then generated to analyze its performance in the OCDMA system. Simulations are carried out to evaluate and optimize the parameters for best performance in the system. The study focuses on the effect of distance, bit rate, data modulation format and input power on the systems performance. Validations of theoretical results by simulations are also conducted. It is shown from the performance analysis that this codes exhibits better performance compared to other codes in the same class. It is shown in term of cardinality; this code could accommodate up to 105 users simultaneously at a permissible bit error rate of 10-9. In the hybrid SCM OCDMA using RC code, this system could accommodate up to 28 subcarrier channels for BER better than 10-9. On the other hand, the MDW code, as compared, can only accommodate up to 20 subcarrier channels. A simulation model is also developed for the SCM OCDMA system, where some comparisons are being made on its performances. In short, the significant contribution of this thesis lies in the development of a new code exhibiting a very simple code construction, which is suitable for the ease of implementation in a practical network. Adding new users in the network using this code does not require modifying the existing network; nodes can be added at the next available wavelength along the bandwidth occupied. At the same time, adding up the code weight will not change any other parameters. In other words, this code design retains the individuality of each code parameter, hence making it a more flexible code compared to others. Apart from that, the theoretical and simulation model derived for a hybrid system of subcarrier multiplexed OCDMA based on RC code has realized a potential approach in improving the bandwidth utilization efficiency of optical communication.