Study of chaos-based digital communications

Abstract

Chaos communications have received much attention recently because chaotic signals can provide an alternate means to spread the spectrum of signals in digital communications. As in other spread spectrum techniques such as direct sequence code division multiple access and frequency hopping, chaos communications can provide secure communications with low probability of detection as well as mitigation of multipath fading effect. In conventional communication systems, the allocated spectrum is shared by a number of users. Multiple access techniques such as frequency division multiple access, time division multiple access and code division multiple access are commonly used. Since chaos communications spreads the spectrum of the data signal over a much larger bandwidth, multiple access becomes an essential feature for practical implementation of the system. Furthermore, it is imperative that more users are included in the same bandwidth without causing excessive interference to one another. In this thesis, an in-depth study of chaos-based digital communications using chaos shifting keying (CSK) and differential chaos shift keying has been performed. Multiple access schemes have been proposed in both systems. To verify the feasibility of the schemes, a simple 1-dimensional iterative map has been used to generate the chaotic signals for the users, and computer simulations are carried out to find the bit error probabilities (BEPs). For this simple map, it is found that numerical BEPs can be derived for different number of users. Simulations are performed and the results agree well with the numerical BEPs. In practice, noise appears in any communications channel. To investigate the performance of the multiple access CSK scheme under such an environment, an additive white Gaussian noise source is added to the transmitted signal at the receiving end. It is found that a similar numerical BEP can again be derived. Simulations are performed and the simulated BEPs are consistent with the numerical ones. Although the results are not as good as conventional direct sequence spread spectrum systems using Gold codes as the spreading codes, chaos communications can provide a comparatively more secure means of communications.