Research On Blind Parameter Estimation Of Multi-user Long Code Direct Spread Signal

Direct Sequence Spread Spectrum(DSSS)is a very important method in spread-spectrum communication.Due to the widening of the spectrum of the signal after being modulated by the pseudo-code sequence,the signal can be transmitted at a lower signal-to-noise ratio,so the DSSS signals have a low interception probability and strong anti-interference ability,and are widely used in military communications.However,the actual signals are complex direct-spread signals,the receiving end cannot estimate the pseudo-code period,the number of source and the direction of arrival precisely,which makes it impossible to obtain the transmitted information sequence.Thus,the study of blind parameter estimation of Multi-user Long Code Direct Sequence Spread Spectrum(LC-DS-CDMA)signals and highly dynamic asynchronous LC-DS-CDMA signals has an important practical meaning.This thesis studies the parameters(the number of sources,direction of arrival,and pseudo-code period)estimation of LC-DS-CDMA signals.The main contents are divided into the following three aspects:(1)Aiming at the problem of estimating the number of sources of LC-DS-CDMA signals under low SNR condition,a method for estimating the number of sources based on array reception is proposed.This method first uses an array antenna to receive the LC-DS-CDMA signal.Then,after feature decomposition of the received signal's covariance matrix,the information theory criterion,the Gauss-circle algorithm and the smooth rank algorithm are used to estimate the number of sources.Experimental results show that number of true sources of the signal can be accurately estimated by the proposed method at a very low SNR.Among them,the accuracy rate of the number of sources estimated by the MDL criterion when the signal-to-noise ratio is greater than-10 d B reaches 95%,which is better than other algorithms.(2)Aiming at the difficulty of estimating the direction of arrival(DOA)of asynchronous LC-DS-CDMA signals,a method using coherent signal subspace and the high-order cumulants algorithm is proposed to estimate the DOA of asynchronous LC-DS-CDMA signals.Firstly,Fast Fourier Transform(FFT)is used to transform the received signal into the frequency domain,and the unitary focus transform is used to focus these signals at the same reference frequency point.Then,after calculating the covariance matrix of the focused signal,both feature decomposition and the result of fourth-order cumulant matrix are used to construct the noise subspace.Finally,the Multiple Signal Classification(MUSIC)algorithm is used to search the spectral peaks for the estimation of DOA.Experimental results show that the proposed method can accurately estimate the DOA of LC-DS-CDMA signals,and fourth-order cumulant matrix algorithm achieves 100% estimation accuracy when SNR is greater than-15 d B,which is better than the coherent signal subspace algorithm.(3)To solve the problem of pseudo-code period estimation for highly dynamic asynchronous LC-DS-CDMA signals,a method combine polynomial phase transformation with the secondary power spectrum for pseudo-code period estimation is proposed.Firstly,the signal is squared to eliminate the abrupt change of the pseudo-code phase.Then,the polynomial phase transformation is used to find the frequency modulation slope and the original signal is degraded to eliminate the second-order exponential term to obtain an asynchronous LC-DS-CDMA signal with frequency offset.Finally,the pseudo-code period of the original signal is estimated according to the phenomenon that the secondary power spectrum algorithm will generate a spike at an integer multiple of the pseudo-code period of the signal.Experimental results show that,compared with directly using the secondary power spectrum,the proposed method can obtain better pseudo code period estimation performance when the SNR is-7d B.