Research On Blind Despreading And Blind Demodulation In Non-cooperative DSSS Systems

It is always been the focus of research in the area of communications confrontation for DSSS signals.In recent decades,the emphasis has been placed on DSSS signal parameter estimation such as chip rate estimation,PN code period estimation,carrier frequency estimation and pseudo-random sequence estimation.However,The basis of existing research is basically based on the signal has been demodulated,and then the signal is modeled on the baseband,the actual situation is the DSSS signal need to be pseudo-code despreading to improve the signal to noise ratio before demodulation,which leads to This article studies the key and difficult.This article discusses the object is the short code DS signal,the study is the IF DSSS signal,the main contents are as follows:First of all,the basic principle of signal modulation is introduced.The mathematical models of the common 7 kinds of modulation signals(AM,FM,2ASK,4ASK,2FSK,4FSK and 2PSK)are respectively established to analyze the characteristics of the signal in time domain and frequency domain.Then,the principle of signal modulation is analyzed.Six kinds of characteristic parameters are distinguished according to the instantaneous amplitude,instantaneous phase and instantaneous frequency.Then based on this characteristic parameter,a signal modulation recognition decision tree model is established,and the recognition rate of the common 5 kinds of modulated signals is simulated and analyzed.Secondly,we analyzed the autocorrelation of pseudo-random codes using the most common m-sequences as the object,and then analyzed the pseudo-code period estimation method based on the delay-dependent cumulant algorithm.Then the eigen decomposition theory of the signal is introduced.Based on the analysis,the blind estimation of the code sequence based on the signal subspace decomposition is analyzed.The received IF short-code DS signal is divided into two periods according to the estimated PN code period to form a received signal matrix.The signal covariance matrix is constructed and the matrix is decomposed to find the eigenvector corresponding to the largest eigenvalue.Taking the estimated pseudo code period as the window,the maximum 2 norm search is performed on the eigenvector.The starting position corresponding to the maximum 2 norm is the estimated pseudo code synchronization point.The eigenvector of the obtained period length is the intermediate frequency pseudo-carrier with carrier Code waveform.In accordance with the estimated pseudo-code waveform and pseudo-code synchronization point,the original direct-sequence extended desynchronization,low-pass filtering,sampling decision,and finally achieve the direct sequence extended code extraction.Finally,the common structure of general communication signal processing platform is analyzed.Based on DSP and FPGA,a hardware algorithm program of signal modulation identification and a hardware algorithm of pseudo-code period estimation are prepared.The software is programmed by CCS3.3 and ISE14.7.The foundation passes the ModelSim and ChipScope simulation,debugs and realizes the signal modulation recognition and the pseudo-random sequence period estimation algorithm in this article.