| In modern communication system, the application of burst communication techniques in military and civil areas has been increasingly extensive. And the technology of directs sequence spread spectrum has excellent anti-interference capability,anti-interception ability and highly secret. These features make the technology of directs sequence spread spectrum receive more and more attention in the application of burst communication system. As the key technology of burst spread spectrum communication,synchronization technology is a research hot spot and difficulty in home and abroad. The selection of spreading sequences has an important effect on the performance of spread spectrum communication. Recent conventional spread spectrum sequence has several shortcomings of low security, bad complexity and long capture time. In order to solve these above problem and achieve better Doppler frequency estimation performance, this paper studies the performance of the chaotic sequence which is considered as spread spectrum sequence on the basis of the research result of the burst spread spectrum communication synchronization techniques. This paper also studies spreading sequences quickly capture technology under high dynamic conditions.Firstly, this paper introduces the basic theory of spread spectrum communication and spreading sequence,describes in detail the capture technique which is used mainly in the burst communication system. The system mainly includes coarse capture unit and fine capture unit.The coarse capture unit uses the parallel sweep frequency matched filtering method to realize the coarse estimation of Doppler frequency and code phase. The fine capture unit uses DFT(Discrete Fourier Transform) direct spectrum estimation algorithm to accurately estimate the Doppler frequency. The DFT direct spectrum estimation algorithm is replaced by I-Rife algorithm later.Secondly, Chebyshev mapping and modified Logistic mapping are considered as the first order and the second order of cascade chaotic system respectively. And then randomly select a weighted parameter and two cascading chaotic sequence which is mapped by different initial value for weighting processing, summation and quantization. So cascade chaotic sequence based on weighted algorithm is achieved. It is verified by MATLAB simulation that the generated chaotic sequence has the good properties of security, balance, auto-correlation and correlation. And a large number of chaotic sequences which conform with the demand of the spread spectrum sequence are selected by using optimization algorithm.Thirdly, this paper studies and analyzes the Doppler frequency capture on the basis of proposed I-Rife algorithm. The modified I-Rife algorithm has a close frequency estimation performance with I-Rife algorithm and its computational amount is smaller. Then Rife algorithm, comprehensive Rife algorithm, M-Rife algorithm, Rife algorithm and modified I-Rife algorithm are simulated and analyzed. The performance of the application of cascading chaotic sequence based on weighted algorithm and modified I-Rife algorithm in the burst spread spectrum communication system are analyzed. These all get satisfying result.Finally, this paper introduces the technical indicators, the overall design scheme and the hardware platform, and analyzes in detail the software realization of capture unit. The performance is simulated and verified by using ModelSim 6.5c simulation software. And the performance test is completed by using vector signal generator. It is known by the test that the system works fine. The cascading chaotic sequence based on weighted algorithm achieves satisfying result. It is stable and reliable and can satisfy the performance demand of spread spectrum communication system. |
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