Study of chaos-based communication systems in a multiple access environment

Abstract

Chaotic signals, having a wideband and aperiodic nature, have been proposed as carriers for spread-spectrum communications recently. It is because chaos-based systems can offer potential advantages, such as simple circuitry and enhanced security, over conventional spread-spectrum systems. Furthermore, chaotic signals have very good auto- and cross-correlation properties. These are important features in a multiple access environment because they can produce low co-channel interference and hence better system performance. Chaos-based communication systems can be broadly classified into coherent and noncoherent types. Coherent systems require an exact replica of the chaotic carrier be reproduced at the receiver. Correlator-type coherent chaos- shift-keying (CSK) system is such an example. In the first part of the thesis, we study the multiple access CSK systems in detail. We develop two approaches, namely Gaussian-approximation-based method and exact method, to computing the bit error performance of the multiple access CSK systems. Further, we investigate the use of linear and nonlinear multi-user detectors, namely decorrelating detector, minimum mean-square-error detector and parallel interference cancellation detector, to a multiple access CSK system. The objective is to reduce the interference between users during the detecting process and improve the hit error performance. The corresponding analytical techniques are also developed and presented. Robust synchronization techniques are yet to be developed, but the study of coherent chaos-based communication systems is important because it can provide very useful performance benchmarks for comparison purposes. Noncoherent systems, which do not require the reproduction of the chaotic carriers at the receiver, are more practical in real-life applications. Differential GSK and correlation-delay-shift-keying schemes are such examples. The second part of the thesis focuses on noncoherent chaos-based communication systems. Based on the analytical tools developed for the coherent systems, we evaluate the performance of a multiple access scheme derived from differential CSK. Two noncoherent modulation/demodulation schemes are also proposed and investigated. The first scheme is called adaptive-receiver multiple-access scheme, in which chaotic reference signals modulated by a binary training sequence are sent periodically. The same chaotic signals are then modulated by the binary data and transmitted. At the receiving side, an adaptive filter is employed to perform the demodulation. Multiple access is also accomplished by assigning different chaotic signals and training sequences to different users. The second noncoherent communication scheme is called generalized correlation-delay-shift-keying (GCDSK) scheme. The transmitted GCDSK signal is composed of a reference chaotic signal and a number of delayed chaotic signals, some of which are modulated by the data being sent. At the receiver, a simple correlator-type detector is employed to decode the binary symbols. The approximatc bit error rate of the scheme is also derived analytically based on Gaussian approximation.