Research On Key Technology In DSSS Communication

PN synchronization is an important and challenging technique in SS communication,with an emphasis research in DSSS communication at present. PN code can be achieved by two steps: acquisition and tracking. Acquisition is a coarse synchronization, which the key point of synchronization. Tracking is a fine synchronization, which can further reduced deviation of code phase. Acquisition is the key point to judge synchronization which is accomplished successfully or not.This paper firstly discusses PN acquisition methods commonly used in the DS system—parallel searching method, matched filter method and adding method based on matched filter, which acquisition time has been theoretically derivated and acquisition performance has been simulated. Analysis shows that they are simple in principle, easy in implementation, but they couldn't work in the condition of very low SNR and large Doppler frequency shift, because they are based on time-domain correlation to complete PN acquisition.Given the problems resulted from time-domain correlation, this paper finally researches two improved methods with FFT to complete PN acquisition in harsh conditions. One is the improvement for large frequency offset that the received signal and the local PN cycle are firstly divided into sections of same length, and then corresponded sections are correlated partially, finally FFT is adopted to eliminate the impact of frequency offset on correlation value. The other is the improvement for low SNR that the sending and local PN codes are firstly adopted FFT operation respectively, With the FFT algorithm's efficiency, the method can reduce acquisition time, more importantly, it can improve the correlation peak by coherent and non-coherent accumulation of several adjacent codes, thus working under a much lower SNR condition.Finally, the system is implemented with FPGA and can work properly.