Analysis and Design of Reliable and Secure Chaotic Communication Systems for Optical and Wireless Links

Abstract

There has been growing interest in the use of chaotic techniques for enabling secure communication in recent years. A number of researchers have focused their energies to develop communication strategies based on the discipline of chaotic mechanics. This need has been motivated by the emergence of a number of wireless services which require the channel to provide very low Bit-Error-Rate (BER), high bandwidth efficiency along with information security. Simultaneous provision of these three conflicting requirements is difficult to achieve with conventional communication strategies. This has motivated researchers in the Communication Engineering community to explore new domains in their search for efficient and secure communication techniques. This work reported in this thesis has aimed at the study, design and validation (via analysis and simulation) of techniques derived from chaotic mechanics to enhance security and BER performance at physical layer for wireless communication. Both RF and Optical Wireless system domains have been included in our study. Conventional techniques aiming to provide security enhancement at the physical layer have employed spreading sequences. The use of these techniques requires bandwidth expansion, and the amount of security is limited. Further, the security provided by these techniques comes with a penalty in BER performance and bandwidth efficiency. As a consequence of rapidly increasing demand for wireless services and limited licensed bandwidth, there is a strong need for bandwidth efficient secure systems. In our work, we have designed and verified (by analysis and simulation) chaos-based systems with enhanced BER performance and bandwidth efficiency similar to that offered by conventional PN sequence based systems. We have also proposed techniques that are applicable to the emerging domain of Free Space Optical (FSO) communication because this technology has the potential of providing fiber like unlicensed bandwidth for high speed short distance communication links. We have started the discussion with a study of the issues involved in synchronization between master and slave chaotic systems. We have suggested the use of Low Densityv Parity Check (LDPC) error correcting code in the system to reinforce the ability of the system resist noise and facilitate the synchronization between master and slave systems in presence of AWGN. In addition, it is shown that synchronization can be achieved even when the spreading factor is decreased to low values ( ). We have proposed a dual chaotic encryption algorithm to solve the dynamical degradation problem. An important feature in the analysis of the dynamical systems is system stability, which can be determined using the Lyapunov Exponent (LE). We have computed the LE for the single and dual chaotic maps. We have also investigated the BER for different types of dual and single chaotic maps by employing Chaos Shift Keying (CSK) modulation scheme with Multiple-InputMultiple-Output (MIMO) communication system under AWGN channel. Simulation results indicate that the single tent map gives acceptable security and superior BER performance as compared to dual tent map which gives the superior security but with relatively lesser BER performance. Although the chaotic sequences are more secure as compared to PN sequences, they are inferior in terms of bandwidth efficiency and BER performance. In order to overcome this limitation, we have proposed the use of a chaotic modulation schemes in MIMO channels. The BER performance of coherent and non-coherent chaotic modulation schemes combined with and Alamouti schemes over AWGN channel and Rayleigh fading channel have been evaluated and compared. Continuing further in our efforts to propose superior communication strategies, we have proposed a concatenated scheme involving the combination of LDPC and MIMO schemes based on chaotic technique. The security and BER performance of this Chaotic-LDPC scheme with two transmit antennas and two receive antennas under various channel models has been evaluated. We have discussed the theory and carried out detailed analysis pertaining to encoding/decoding of chaotic modulation schemes, the use of suitable LDPC codes and MIMO schemes for providing secure and reliable communication over the AWGN channel, the Rayleigh fading channel and the Gamma-Gamma fading channel.vi To improve security and reliability with enhanced throughput, we have proposed a Quadrature Chaos Shift Keying (QCSK) modulation scheme with high rate STBC. The bandwidth efficiency of chaos based communication schemes is inferior to that of the traditional communication schemes. To address this problem, we have designed a rate- and rate- full diversity orthogonal STBC for QCSK and 2 transmit antennas and 2 receive antennas. Simulation results indicate that these high rate codes achieve better throughputs in the high SNR region. It is seen that a rate- code achieves a 25% improvement in information rate and - code achieves a 50% improvement in information rate increase compared to the traditional Alamouti scheme for Differential Chaos Shift Keying (DCSK). To evaluate the performance of these techniques in multi-user environment, we have analyzed and evaluated the anti-jamming performance of CSK in a MIMO channel. The BER performance analysis for three common types of jamming, namely singletone jamming, pulsed sinusoidal jamming and multi-tone jamming under different levels of noise power over AWGN channel has been derived and evaluated. We have also discussed the design and evaluated the performance of a communication system that combines a MIMO scheme with a chaotic sequence based Direct Sequence Code Division Multiple Access (DS-CDMA) scheme. In the last part of our work, we have considered the application of the chaotic techniques in the Free-Space Optical (FSO) communication system. The design analysis, simulation and BER performance evaluation of different optical chaotic modulation schemes with MIMO-FSO communication system are presented. Simulations were carried out using available simulators from Rsoft, OPTSIM version 5.2. The main aim of this work is to assess the feasibility of employing Space-Time Coded chaotic communications over MIMO communication channels (both RF and Optical). Our analyses and simulations show that it is feasible to develop reliable and secure communication systems based on chaotic modulation schemes combined with MIMOvii and channel codes. These systems can provide the benefits of information integrity, security and enhanced throughput. It is hoped that the use of tools from chaotic mechanics will enable communication engineers to devise strategies that will allow wide dissemination of wireless services to all of humankind.