The Study Of Secure Communication System Based On Chaos

In the past decades, the security of the information is more and more important to people, and the chaotic communication system has been developed rapidly as a part of secure communication system. This thesis discusses the problems of the generation of the chaotic signal with proper characteristic and the improvement of the chaotic modulation method, which are focused on the basic theory of chaotic communication system, comparing with the methods of adding affixation or using more complex structure to enhance the securitiy of the system. The results can also be combined with the methods presented in literature.First in order to find a principle to design chaotic signal, this paper presents a method of producing chaotic signals with arbitrary magnitude and spectrum, which is based on the Lorenz system by introducing some parameters of the system to realize the selective magnitude and spectrum. Furthermore, this paper has shown the validity of this method by theory and simulation. In addition, this kind of generators is easily implemented, and the signal is suitable for protecting the information in communication systems. By the way, a new chaotic shift keying modulation is proposed by using the same structure systems designed by the method of producing chaotic signals with arbitrary magnitude and spectrum. The same structure systems have the same attractors and attracting region, so the bifurcation of the attractors in return maps can be eliminated. The binary information can not be attacked easily by the return map breaking method.Then the reason of the security weakness of generalized state-space observers-based approaches for secure communication is discussed which is used chaotic masking and chaotic nodulation of a Lorenz system and a method is proposed to solve this problem. Theory analysis and simulation show this improvement realized chaotic masking. By contrast with the original system, this method improves the performance of secure communication system based on chaotic masking. At the same time, in order to reduce the affect of the gusty noise to the chaotic communication system, a method is developed to enhance the synchronization ability and reduce time expenditure by adjusting the differentiation with respect to the state variables in the chaotic system. This method also has the ability to resist the gusty noise in the channel and improve the quality of the chaotic communication.Furthermore, since the chaotic modulation systems based on chaotic synchronization are very sensitive to the noise or the distortion of the channel, a new chaotic modulation method is proposed based on Lorenz system, the information signal is introduced into the modulation system as the parameter of the nonlinear part. In the receiver, the information signal is derived from the error of the partial synchronization. The system did not require the synchronization process, and the sensitivity of this modulation system to the noise is reduced. The simulation shows this system has realized secure communication and enhanced the ability to resist the noise in the channel and to secure information.In addition, a multidimentional chaotic signal generator is introduced to restrain the DC and the spectrum component in the low frequency. The n+1 dimentional system is deduced from an n dimentional system, the output signal of which is added an integration to its derivative. The characteristic of the original system is kept and the purpose is realized by analysis and simulation.At last, the study of another domain of the communication system is discussed, which is about the theory of passitive communication networks.This contained two aspects: first, a method to design cross-coupled resonator filters with parallel resonance circuits is presented in this paper. The procedure includes selecting a circuit model, introducing a scaling factor and bandwidth condition, and calculating the values of the coupling and resonance admittances of the circuit model by the coupling matrix and the given transfer function. Finally, an example is given to illustrate the designing procedure, and the simulation results show the validity of the presented approach. Second, a method used to reduce the errors in the scattering parameters measurement of N-ports based on two-port vector network analyzer (VNA) is presented, which is based on the normalization and renormalization of the scattering matrix and an optimization procedure. The method is proved by measured results.