Reserch And Implementation Of Fast Pn Code Acquisition Based On PMF-FFT Algorithm

In recent years, low earth orbit (LEO) satellite communication has been widely applied, because of its small delay and small path loss. It is also considered as the most promising and the latest satellite communication system. However, the LEO satellite is running with a very high speed, there will be facing the case of the large Doppler frequence offset between the satellite and the ground. Spread spectrum communication Technology is the most useful method to overcome the electromagnetic interference, shadow effect and multipath fading of the LEO satellite communication systems. PN code synchronization is the prerequistite for the spread spectrum communication, due to Doppler shift, can result in a significant reduction of a correlation peak which is used to indicate code alignment. As a result, Doppler or local oscillator offset can lead to frequency uncertainties in the receiver which make the task of code acquisition particularly difficult. In the present paper we discuss the use of a set of partial correlators for code phase acquisition, combined with a fast Fourier transform (FFT) for the purpose of simultaneous Doppler estimation.Firstly, this paper analyzed the theory source of spread spectrum communication technology, and the direct sequence spread spectrum communication system (DS-SS). Then some classical methods for PN code aquusition of DSSS has been discussed, Of these, the method based on matched filter (MF) correlators has a faster acquisition speed at the expense of a larger system complexity for alarge PN code period, this paper has a rigorous mathematical derivation for MF, and study the reduction of a correlation peak bringed by the Doppler shift.Secondly, this paper studied the basic principles of the rapid PN acquisition method based on the PMF-FFT architecture, which has been proved with the mathematical methods. The approach outlined in this paper utilises an FFT to simultaneously searchall possible code Doppler offsets at one time, thus reducing the 2-dimensional search problem to a 1-dimensional code phase search. Though the FFT corrected the phase rotations due to the Doppler offset at chip level and no coherent combining loss occurs, still the scalloping loss remained. Two techniques to reduce the scalloping loss were discussed. First the effect of using a zero-padded FFT was examined, and then the paper focued on the method of windowing of the FFT input and the PMF input. The structure of window function was improved to eliminat the impact of scallop loss. Furthermore, An adaptive threshold method was employed to further improve the performance of the acquisition algorithm.Finally, Based on the improved PMF-FFT synchronization acquisition algorithm, combined with the rapid development of FPGA digital technology, the paper designed the PMF-FFT acquisition structure which can achieve under the premise trade-off between the hardware resources and the acquisition time. Very detailed study of the PMF, FFT and other important sub-module FPGA-specific design, each module will constitute the synchronous capture the organic system.