The Study On Transform-Domain Narrowband Interference Suppression In DSSS Communication Systems

The SSC(Spread Spectrum Communication: SSC) technique with ability of anti-jammer is extensively used in military and civil communication. The capacity of reject interference mainly relys on the spreading gain, and theoretically, by increasing the spreading gain, the system can supress any powerful interference. But in practice, with the limits of the device cost and the available of the spectrum width, the spreading gain is restricted. While the power of the narrowband interference(NBI) exceeding the tolerance of the system, the performance of the SSC system is seriously declined. In order to enhance the ability of anti-jammer, there must take some effective measures in the SSC system.The thesis has investigated the interference suppression method in the SSC system, and takes mainly emphasis on the Transform-domain Interference Suppressing method, which is a very valueable method for researching and with much merit such as faster convergence speed, more effective in time-varying NBI,etc. There are two modes in the transform-domain technique: the uniform spectrum dividing based on M-channel filterbank and the nonuniform spectrum dividing based on two-channel wavelet filterbank. Because of the characteristic of non-perfect in the filterbank, the power of the NBI will diffuse partly in neighboring subbands by the phenomena of spectrum leakage. For alleviating the NBI power leakage, on one hand, the thesis based on the design theory of the multirate filterbank, has researched the design of the M-channel cosine-modulated filterbank and analysed the limitation of the lapped transform filterbank which has been widely used in transform-domain technique, and finally proposed a transform method based on Near Perfect Reconstruction(NPR) cosine-modulated filterbanks. Because of the high stopband attenuation in Chebyshev window function, The NPR filterbank designed with the Chebyshev window function can effectively overcome the phenomena of spectrum leakage. On the other hand, for the transform-domain technique based on the nonuniform spectrum dividing, the thesis has investigated the theory of a structual perfect reconstruction two-channel IIR wavelet filterbank and the theory of the multi-resolution in the wavelet packet, and finally proposed a transform method by using the IIR filterbank rather than Fir filterbank. The IIR wavelet filterbank designed with a procedure of optimization, has lower computational complexity and higher performance of the spectrum dividing than the conventional Fir wavelet filterbank.Simulating results show that, the M-channel NPR cosine-modulated filterbank and the IIR wavelet filterbank both can effectively overcome the phenomena of spectrum leakage. The transfom-domain technique proposed in the thesis can obviously improve the performance of the transform-domain interference suppression method.