Modulation and detection schemes based on chaotic attractors properties : application to wideband transmissions

Au cours des vingt dernières années, les systèmes de communications basés sur le chaos ont été étudiés, avec pour objectif la possibilité de générer les signaux large-bande par des circuits électroniques simples, permettant une faible complexité des circuits émetteurs-récepteurs. Cette thèse concerne l’étude de systèmes de transmissions large-bande basés sur le chaos, en utilisant certaines propriétés des attracteurs chaotiques. Tout d’abord, un système dynamique a été choisi et étudié, permettant de générer des signaux chaotiques qui possèdent des composantes périodiques. L’analyse de ces attracteurs chaotiques cycliques (CCA) met en évidence des propriétés spécifiques en lien avec leur période. Ensuite, deux schémas de modulation basés sur les CCAs sont proposés. Les détections non-cohérentes associées sont réalisées par l’observation des propriétés spécifiques des signaux rec¸us. L’évaluation des performances des systèmes basés sur les CCAs dans le cas d’un canal de bruit additif Gaussien montre des performances meilleures que celles des systèmes dits ”differential chaos shift keying (DCSK)”, en bas débit de symboles. En outre, les performances dans le cas multi-trajet sont comparables dans la bande de 2,4 GHz. ------------------------------------------------------------------------------------------------------------------------------------------- In the past twenty years, chaos-based communication systems have been studied, considering the possibility of generating wideband signals by simple electronic circuits, hence low complexity in transceiver. The aim of this thesis is to study the chaos-based wideband transmission systems relying on the properties of chaotic attractors. Firstly, a dynamical system is selected and studied, allowing to generate the chaotic signals with a periodic component. The analysis of such chaotic cyclic attractors (CCA) shows the specific properties. Then, two CCA-based modulation schemes are proposed, with the simple noncoherent detections realized by observing the specific properties of the received signals. The performance evaluation of CCA-based systems in the additive white Gaussian noise (AWGN) channel shows a better noise performance with long symbol duration, compared to the one of differentially chaos shift keying (DCSK). In addition, they have a comparable multipath performance in the 2.4 GHz ISM environmen

Techniques in secure chaos communication

In today's climate of increased criminal attacks on the privacy of personal or confidential data over digital communication systems, a more secure physical communication link is required. Chaotic signals which have bifurcation behavior (depending on some initial condition) can readily be exploited to enhance the security of communication systems. A chaotic generator produces disordered sequences that provide very good auto- and cross- correlation properties similar to those of random white noise. This would be an important feature in multiple access environments. These sequences are used to scramble data in spread spectrum systems as they can produce low co-channel interference, hence improve the system capacity and performance. The chaotic signal can be created from only a single mathematical relationship and is neither restricted in length nor is repetitive/ cyclic. On the other hand, with the progress in digital signal processing and digital hardware, there has been an increased interest in using adaptive algorithms to improve the performance of digital systems. Adaptive algorithms provide the system with the ability to self-adjust its coefficients according to the signal condition, and can be used with linear or non-linear systems; hence, they might find application in chaos communication. There has been a lot of literature that proposed the use of LMS adaptive algorithm in the communication arena for a variety of applications such as (but not limited to): channel estimation, channel equalization, demodulation, de-noising, and beamforming. In this thesis, we conducted a study on the application of chaos theory in communication systems as well as the application of adaptive algorithms in chaos communication. The First Part of the thesis tackled the application of chaos theory in com- munication. We examined different types of communication techniques utilizing chaos theory. In particular, we considered chaos shift keying (CSK) and mod- ified kind of logistic map. Then, we applied space-time processing and eigen- beamforming technique to enhance the performance of chaos communication. Following on, we conducted a study on CSK and Chaos-CDMA in conjunction with multi-carrier modulation (MCM) techniques such as OFDM (FFT/ IFFT) and wavelet-OFDM. In the Second Part of the thesis, we tried to apply adaptivity to chaos com- munication. Initially, we presented a study of multi-user detection utilizing an adaptive algorithm in a chaotic CDMA multi-user environment, followed by a study of adaptive beamforming and modified weight-vector adaptive beam- forming over CSK communication. At last, a study of modified time-varying adaptive filtering is presented and a conventional adaptive filtering technique is applied in chaotic signal environment. Twelve papers have been published during the PhD candidature, include two journal papers and ten refereed conference papers

Noise-suppressing chaos generator to improve BER for DCSK systems

In this paper, we propose a noise suppressing chaos generator to improve the bit error rate (BER) performances for the high-order differential chaos shift keying (DCSK) systems. The presented chaos generator outputs the overlapping chaotic sequences for the information modulation, which use a buffer for temporary storage, and the stored chaotic sequence is used to modulate the information bit while the second element of the chaotic sequence is fed back to serve as the initial value to generate chaotic sequence for the modulation of the next information bit. Namely, the different information bit may share the same chaotic segments of the whole chaotic sequence. Thus, the repetitive transmission of the same reference chaotic segments suppress the channel noise, and the BER performance of high-order DCSK systems are improved. We derive the theoretical BER expression for the proposed scheme over additive white Gaussian noise (AWGN) channel. The simulation results match the theoretical BER, and demonstrate that the BER performances have been improved with the use of noise-suppressing chaos generator

Noise-suppressing chaos generator to improve BER for DCSK systems

In this paper, we propose a noise suppressing chaos generator to improve the bit error rate (BER) performances for the high-order differential chaos shift keying (DCSK) systems. The presented chaos generator outputs the overlapping chaotic sequences for the information modulation, which use a buffer for temporary storage, and the stored chaotic sequence is used to modulate the information bit while the second element of the chaotic sequence is fed back to serve as the initial value to generate chaotic sequence for the modulation of the next information bit. Namely, the different information bit may share the same chaotic segments of the whole chaotic sequence. Thus, the repetitive transmission of the same reference chaotic segments suppress the channel noise, and the BER performance of high-order DCSK systems are improved. We derive the theoretical BER expression for the proposed scheme over additive white Gaussian noise (AWGN) channel. The simulation results match the theoretical BER, and demonstrate that the BER performances have been improved with the use of noise-suppressing chaos generator