Study On Blind Estimation Of Parameters For AltBOC Signals

With the development of satellite navigation systems,the application environment of global satellite navigation signals has become more and more severe,and most of the best frequency center points are occupied.The introduction of the Binary Offset Carrier(BOC)modulation signals effectively solves this problem.By subcarrier modulation,the BOC signal has the characteristics of spectrum splitting,which improves the mutual interference between satellite navigation signals and the problem of frequency band congestion.The Alternate Binary Offset Carrier(AltBOC)signal is a new type of signal proposed by the French space station based on BOC modulation.Compared with other types of BOC signals,the positioning accuracy and anti-interference performance have been further improved.At present,it is mainly used in the E5 a and E5 b frequency bands of the Galileo system and the B2 frequency band of the Beidou system in my country.Therefore,it is of great practical significance to accurately estimate its code sequence and various parameters.In this thesis,the modulation mode and related performance of AltBOC signal are analyzed,and the blind estimation of signal parameters and combined code sequence is studied.The main contents are as follows:(1)Aiming at the problem of blind estimation of pseudo code periodic of AltBOC signals,the quadratic spectral algorithm is studied.Firstly,the autocorrelation function of AltBOC signal model is obtained,and then the power spectrum of AltBOC signal is obtained according to the relationship between autocorrelation and power spectrum as fourier transform pair,and then the quadratic spectrum function of the signal is obtained.The quadratic spectrum has a spike pulse at the pseudo code period and its integer multiple frequency.Finally,the pseudo code period of the signal can be obtained by detecting the distance between these spectral peaks.(2)Aiming at the problem of the difficulty of blind estimation of signal parameters due to the complicated AltBOC modulation method,a cyclic spectrum blind estimation algorithm based on frequency domain accumulation is proposed.Firstly,This method obtains the cyclic autocorrelation function according to the AltBOC signal model.Then the cyclic power spectrum of the AltBOC signal is obtained based on the Wiener-Shinchin theorem.The spectrum function is composed of the cyclic power spectrum of the sub-signals corresponding to the four subcarriers modulated by the AltBOC signal.Finally,the frequency spectrum is accumulated in the frequency domain,and the generated spectrogram has a series of spikes arranged regularly.The pseudo code rate,subcarrier rate and carrier frequency of the AltBOC signal are estimated by data analysis on the corresponding coordinates of the peak at the f =0 section.(3)Aiming at the problem that it is difficult to estimate the combined code sequence of AltBOC modulated signals,the improved K-means algorithm in unsupervised learning is used to estimate the combined code blindly.First,the received AltBOC signal is divided into segments according to the combined code period window length,and the initial mean vector of each cluster is selected by the principle of similarity.Secondly,the residual signals are clustered by distance criterion,and the square error of K-means clustering is iteratively optimized to obtain an approximate solution.Finally,extract the mean vector of each cluster after stationary,which is the combined code to be estimated.Compared with the traditional K-means algorithm,this thesis has made improvements in the selection of the initial mean vector to improve the problem that the original algorithm would lead a large deviation in the estimation of the pseudo-code sequence of the signal.The experimental results show that when the signal-to-noise ratio is greater than-15 d B,the improved algorithm estimates the correct probability of the information code to approximate the cooperative despreading method.