Research For Interference Suppression Techniques Based On Adaptive Filter In DSSS Communication System

The technique of spreading spectrum has been widely applied in military communication, electronic counter, mobile communication because of its inherent characters such as tenacious ability of resisting interference, information covertness, In the complex electromagnetic circumstances, it is difficult for the DSSS system to ensure a high quality of communication only by its processing gain. Therefore, the research of suppressing interferences in spread spectrum system has great theoretical and practical significance.(1) The representative time-domain adaptive filtering algorithms based on Least Mean Squares (LMS) and Recursive Least Squares (RLS) are studied. Some simulations of eliminating the sine wave noise show that these two algorithms have good effect on DSSS.(2) Adaptive heterodyne filter adopting the heterodyne_modulation conception is designed, which generates the two quadrature channel signal and makes the filter work of in low frequency. The FFT spectrum analyzer enables the filter track the interference precisely and controls the width and depth of notch. Detailed deduction and explanation are presented in this paper. Simulations demonstrate that the new structure can counter one or more narrowband jams in DSSS system and the operation has little damage on signal spectrum.(3) Based on the MMSE criterion, an iteration algorithm without division operation applied to one-order complex notch filter cell is derived. And a novel high orders complex notch filter consisting of this cell is proposed. Simulation results show that this structure can eliminate multi-narrowband interferences with fast convergence speed and remarkably improve anti-jam margin of DSSS. The FPGA design of the complex notch cell is also described in this thesis.(4) The Fractional Fourier Transform (FRFT) is a newly developed transform domain tool. for suppressing the representative broadband jam such as linear frequency modulated signal, the theory of FRFT was researched systematically including the definition, the fractionalized process of transform operator and the FRFT of typical signals. The method for eliminating multi-LFM jams in DSSS was proposed with simulation results.