Research On The Technology Of DSSS Signal Despread

Direct Sequense Spread Spectrum Communication System(DS-SS)is short for Direct Sequense Spread Spectrum Communication System(DS-SS).The System greatly widens the Spectrum of signals to be transmitted by implementing high-speed pseudo-code sequence modulation on information sequences.The significant increase in bandwidth greatly reduces the signal-to-noise ratio required by DIRECT expansion communication,resulting in the low probability of interception of direct expansion signal.In addition,direct expansion signal has the advantages of good confidentiality,strong anti-noise performance,anti-multipath and so on.Therefore,direct expansion signal is widely used in satellite communication,microwave communication and other military and civil communication fields.From the Angle of communication adversarial reconnaissance,how to realize the detection and interception,parameter estimation,deexpansion and demodulation of spread spectrum communication signal has important theoretical and application value.This thesis focuses on the estimation of carrier frequency,Pseudo Noise(PN)period and code rate,and the blind estimation of PN sequence of short/long code DSSS signal,and verifies the feasibility and effectiveness of the algorithm on simulation and real data sets.The main research content of this paper is summarized as follows:(1)DSSS signal characteristic parameter estimation.Based on the intercepted signal,the carrier frequency,pseudo-code period and code rate estimation methods of DSSS signal are studied in order to obtain PN sequence by further processing the signal and finally realize the blind despread of DSSS signal.1)Aiming at the limited range of carrier frequency estimation by existing N-fold frequency doubling method,a carrier frequency estimation method based on improved N-fold frequency doubling is proposed.Firstly,n-fold sampling rate conversion is carried out to expand the estimable spectrum range of the DSSS signal.On this basis,the carrier frequency estimation of the DSSS signal is realized by using the existing N-fold frequency method.2)The pseudo-code periodic method of DSSS signal based on time domain autocorrelation and the number rate method of DSSS signal based on cyclic spectrum are studied.Aiming at the problem that the estimation accuracy of the algorithm is not high under the low SNR,the existing algorithm is improved by using the idea of piecewise averaging,and the improved algorithm can effectively smooth the noise and accurately estimate the parameters of the DSSS signal.The effectiveness and feasibility of the proposed method are verified by experimental comparison on simulation and real data sets.(2)PN sequence estimation of synchronous short code DSSS signal.In order to solve the problem of high complexity of existing algorithms when the DSSS signal pseudo-code cycle is large,a blind PN sequence estimation algorithm based on K-means clustering is proposed.Firstly,the segmented signal is segmented twice in sequence according to the pseudo-code period and chip width of the signal.Secondly,k-means clustering algorithm is used to cluster the signals after two segmented signals respectively,and two different kinds of signals can be obtained after clustering.In order to enhance the signal-to-noise ratio of the code slices after the second segmentation,the in-phase addition and inverse subtraction of the code slices after the first clustering is carried out before segmentation.On this basis,the PN sequence estimation can be completed by assigning the values of the two kinds of signals to ±1 respectively and arranging them according to the piecewise order of signals.The effectiveness and feasibility of the proposed method are verified by experimental comparison on simulation and real data sets.(3)PN sequence estimation of unsynchronized short code DSSS signal.A PN sequence estimation method based on improved eigenvalue decomposition(EVD)was proposed to solve the problem of inaccurate estimation of out-of-step point in DSSS signals with different modulation modes under low SNR.Firstly,a correlation model between the out-of-step points of the DSSS signal and the eigenvalues of the autocorrelation matrix was constructed,and the out-of-step points of the DSSS signal were obtained based on the model,and then the synchronous operation of the DSSS signal was carried out.Finally,the signal autocorrelation matrix is eigendecomposed,and the corresponding eigenvectors of larger eigenvalues are extracted to complete the blind estimation of PN sequence.Experimental results on simulation and real data sets verify the effectiveness and feasibility of the proposed method.(4)Blind estimation of PN sequence of unsynchronized long code DSSS signal.In this thesis,the maximum norm method is also used to estimate the out-of-step point of the signal,and the problem of poor estimation performance under low SNR is improved.After the out-of-step point of the signal is estimated,the signal is synchronized.On this basis,the block matrix eigendecomposition method is used to get all the possible PN sequences of the signal.Finally,all PN sequences are screened by autocorrelation method to obtain the target sequence to complete the blind estimation of PN sequence.Experimental results on simulation and real data sets verify the effectiveness and feasibility of the proposed method.