A Research On Interference Suppression In DSSS System

Because spread spectrum communication has characteristics of strong anti-interference ability,low probability of intercept,good secrecy and etc.,it is widely used in the military,electronic warfare and other fields.Now the electromagnetic environment is more and more complex,the communication system is vulnerable to various types of interference,resulting in the interference tolerance of the spread spectrum communication system.Therefore,the research of spread spectrum system interference suppression technology has practical significance.The thesis introduces the development of interference suppression technology in DSSS system at home and abroad briefly,then gives common types of interference signals according to the characteristics of Direct Sequence Spread Spectrum(DSSS)system,and proposes the right interference suppression method in order to suppress different interference.In the spread spectrum system,narrowband interference is the most common interference.According to the different characteristics of the narrowband interference signal and the spread spectrum signal in frequency domain,the thesis adopts the narrowband interference suppression in transform domain,and analyses the theory and the realization process;and in the process of implementation,through setting threshold to judge the location of interference,the thesis puts forward adaptive threshold algorithm to solve the fixed threshold dependence on the environment;finally,according to the Fast Fourier Transform(FFT)spectrum leakage problems and the characteristics of window function in the frequency domain and time domain,the thesis gives a method of overlap window,presents the final narrowband interference suppression algorithm,and gives the simulation result.For the sweep frequency signal interference,researching the interference suppression method based on Short-time Fourier Transform(STFT),Fractional Fourier Transform(FRFT)and Discrete Chirp-Fourier Transform(DCFT),we can achieve DCFT used FFT,which can reduce the complexity,so the thesis adopts the suppression method based on DCFT.But because of the large signal constraints of DCFT transform,we modify the DCFT definition into Modified Discrete Chirp-Fourier Transform(MDCFT)suppression based on MDCFT,and after analysis of MDCFT,the complexity is too high,in order to reduce the complexity,we divide the search process into two steps,including coarse search and fine search,which can reduce the number of MDCFT transform.Finally,the interference effect of the sweep interference suppression method based on MDCFT is verified by simulation.The spread spectrum system is an interference limited system,namely,its capacity and performance are limited by the multiple access interference,so it is necessary to adopt the suppression method to reduce the multiple access interference,so as to reduce the error rate of the target user.If the receiving terminal knows the user situation,the thesis adopts interference cancellation to suppress multiple access interference,and through the simulation to verify the reliability of the algorithm;conversely,that is to say,the user is unknown in the received signal,through the research on blind multiuser detection algorithm,the thesis gives the theoretical and simulation results of Least Mean Square(LMS)algorithm,Recursive Least Square(RLS)algorithm and Kalman filter algorithm,and improves the existing LMS algorithm,namely variable step LMS algorithm,making up the difference in the performance of different interference environment greatly,and through the simulation results to verify the effect of interference suppression.