Research On Pseudo-code Acquisition Technology In DSSS Communication

Spread spectrum communication is one of the most popular and thoroughly studied communication systems in the communication field in 20th century. At present, more attention is paid to Spread Spectrum (SS) communication because of its numerous advantages. However, technical problems of SS communication are much more complicated than those of common communication systems, which lead to some constraints to the development of SS communication. For instance, synchronization of PN code is an important and challenging technique in SS communication, which can be achieved by two steps: acquisition and tracking.A key problem in Direct Spread Spectrum Communication is the implementation of PN synchronization, with an emphasis on PN acquisition. The commonly used methods have disadvantages of long acquisition time and complicated circuit structure, and they can't work in the condition of low SNR or large frequency offset. This paper firstly discusses PN acquisition methods commonly used in the DS system—sliding correlation method and matched filter method, 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 low SNR that the sending and local PN codes are firstly adopted FFT operation respectively, and then multiplied with each other in frequency domain, finally adopted IFFT to complete correlation of codes. 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.The other 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. This paper firstly analyzes this method's principle, then studies the impact of the number of section and FFT points on acquisition performance. Finally, the paper gives a concrete realization of this method and uses MATLAB simulation to validate its feasibility.