Direct Sequence Spread Spectrum System Of Linear Fm Interference Rejection Techniques

Spread spectrum communication is a sort of new high-tech communication technique, it has a number of internal advantages, such as large capacity, interference immunity, low probability of intercept, code division multiple access (CDMA) etc, which make it get broad applications in civilian as well as military environments, and become the technical groundwork of next generation mobile communications. Compared with narrowband communication systems, DSSS systems have remarkable interference suppressing ability, whose interference tolerance depends on systemic spreading gain (bandwidth). On account of the restriction of frequency resource, various DS system spreading gains can't be improved arbitrarily. Therefore, when outside interference might be more powerful than accepted tolerance, it is necessary to introduce corresponding interference suppressing measures for improving system performance. At present, most of the research achievements concentrate on suppressing narrowband interference. In recent years, the effect of broadband non-stationary interference on spread spectrum systems gets more and more attention, of which general type is linear frequency modulated (LFM) interference, so in this thesis we concentrate on performance analysis of DSSS systems' immunity to LFM interference and corresponding suppressing measures.In this thesis, first the interference suppressing measures in time-frequency(t-f) domain in DSSS communication systems are summarized; then a detailed analysis of the interference suppressing ability of DSSS systems without any suppressing measures is carried out, the closed form of bit-error-rate (BER) is educed, and the simulation of BER changing with the interference parameters are simulated; at last, an novel adaptive interference suppressing algorithm based on discrete Chirp-Fourier transform is proposed and the interference detection, estimation and suppression are carried out in DCFT domain.. The performance analyses have shown that the proposed algorithm distinctly can obtain the better BER performance and overcomes the shortcoming of other transform domain algorithms that are disable to powerful interference. BecauseChirp-Fourier transform can be implemented by FFT, comparing with other filtering algorithms based two-dimensional t-f tool, it is easy in computation and simple in implementation.