BeiDou Signal Acquisition based on a New Double Block Zero-padding Method
Shan Xu, Falin Wu, Chenghao Geng, Beihang University, P.R. China
BeiDou navigation data transmitted on B1 frequency band has been modulated with NH code to provide more accurate positioning service, which causes the bit sign transition may occur even in every 1 ms received signal. Frequent bit sign transitions limit the extension of coherent integration time which is disadvantageous for weak signal acquisition under harsh environments such as indoor and jungle. This paper analyzed the side effect of bit sign transitions by formulas derivation and proposed a BeiDou weak signal acquisition method based on a new modified double block zero-padding to overcome the side effect, extend coherent integration time and reduce acquisition time simultaneously. Because the side effect of bit sign transitions on acquisition is negligible in the code phase domain, the proposed method takes the first 1 ms received signal to implement traditional parallel code-phase acquisition, and get coarse code phase, then generate local replicate C/A code. The sample point of the acquired coarse code phase indicate the bit sign transition since the navigation data, NH code and C/A code are modulated aligned at every start point of 1 ms signal. Then the proposed method modified the local replicate C/A codes by modulating one period NH code on 20 periods’ local replicate C/A codes to generate the local PRN-NH intermediate codes. Twenty milliseconds received signals were taken from the sample point of the acquired code phase and it assumed that the local PRN-NH intermediate codes aligned with received signal. And then the local PRN-NH intermediate codes were circularly shifted 1 ms, and signal acquisition was implemented with DBZP method each time. The DBZP method mainly contained four parts. Firstly, twenty milliseconds received signals and generated local PRN-NH intermediate codes were divided into same number of blocks, respectively, and each block contained same amount of sample points. Secondly, operations of double block and zero-padding were implemented on twenty milliseconds received signals and local PRN-NH intermediate codes, respectively. And each new double block also contained same amount of data points which was twice as large as the number of data points in one block received from the first part. Thirdly, correlation operation of ring shift left was implemented between received signals and local PRN-NH intermediate codes, and the twenty milliseconds received signals were shifted left one block each time. And the result of correlation operation of ring shift left was given in form of a matrix. The last part of DBZP was to implement FFT on the columns of the matrix so as to find acquisition peak. After 19 times of shift operations, 20 acquisition peaks and their corresponding locations in the acquisition result matrix could be got each time. The Doppler frequency shift and code phase will be the row and column of corresponding locations of the maximum peak among 20 acquisition peak in the acquisition result matrix, respectively. To validate the proposed method, this paper designed simulation experiment to compare acquisition performance of traditional parallel code phase method, traditional DBZP method and the proposed method under different SNR environments. The simulation results demonstrated that the proposed method could overcome bit sign transition problem caused by NH codes, and had better acquisition performance in comparison with other two traditional methods.