Abstract:
For a mobile communication system, it is important to have the capability of accommodating more than just one active user at any time. In other words, multiple users within the same system should be able to transmit and receive information simultaneously. In the last decade, the use of chaotic spreading sequences in communication systems has generated a fair amount of interest. This is due to its unique properties such as its noise-like broadband feature, sensitivity to initial condition, non-periodic nature, and promising correlation characteristics, which can enhance the security of the communication system. This thesis presents the theoretical analysis and design of a coherent multiple access chaos-based system, called the Chaos Phase Shift Keying (CPSK) scheme. The bit error rate (BER) performance of such a system in an additive white Gaussian noise (A WGN) channel was found to be slightly worse than that of the conventional Walsh-functions-based system. In comparison with the traditional Chaos Shift Keying (CSK) system with two generators, the CPSK system was found to be capable of accommodating more users for any given BER. The thesis proposes an accurate and easy way for calculating the BER of such system. The performance of CPSK in a flat fading channel was also investigated in order to compare with results presented in other research work. It was found to be unimpressive and unlike the AWGN channel case, increasing the spreading factor does not significantly improve the BER. Nonetheless, the BER expression was derived and could be evaluated using a numerical integration method. Furthermore, the anti-jamming capability of the CPSK system was investigated, with analytical BER solutions derived. As expected, since the jamming signals behave like white noise, the BER of the system degrades as the power of the jamming signal mcreases. To improve the BER performance of CPSK, the use of error correcting codes (ECC) and block interleaving schemes were investigated. For a system that is influenced by both AWGN and flat fading, the incorporation of these two schemes together into the system was shown to be capable of reducing the BER enormously. The downfalls of these two schemes are the extra digital signal processings required, which would lead to delays in the system. Three different strategies of generating new chaotic spreading functions were investigated. These were based on the combination of Walsh and chaotic signals, the addition or multiplication of two chaotic signals, and the use of interleaved chaotic sequences. The Walsh-chaotic signals combination scheme was found to have the best BER performance since the Walsh codes helped to reduce the amount of the interuser interference. The investigated CPSK system was also implemented onto the DSP hardware platform. Issues such as the loss of the aperiodic nature in the generated chaotic sequences were noticed. This particular problem was resolved by considering the use of the aperiodic segment of the sequence repeatedly. The BER performance of the prototype was found to be the same as what the theory and software simulations indicated, and the current implementation was shown to be able to support only one user at a time at the data rate that is required by the third generation CDMA system. From the results obtained and presented in this thesis, future work recommendations such as the investigation of robust synchronization technique and the hardware implementation of the system on a different platform were discussed.