Research On High Sensitivity Asynchronous Spread Spectrum Demodulation Technology

In China,asynchronous spread spectrum measurement and control system is widely used in the field of satellite navigation and ranging,and the key point of the research is how to actualize the demodulation of asynchronous spread spectrum signal.In the demodulation process of asynchronous spread spectrum signal,it is roughly divided into four parts:the acquisition of PN code phase and Doppler frequency offset,PN code phase tracking,carrier frequency acquisition and synchronization,symbol synchronization,etc.In satellite communications,the spread spectrum signal acquisition and tracking is more difficult than that of ground communication.There are two main reasons:firstly,the channel environment is complex,the SNR of the received signal can hardly be guaranteed;secondly,the carrier of communication devices usually have high velocity and acceleration,which brought high Doppler shift and frequency shift rate,greatly increased the difficulty.According to the requirements of the project,an acquisition and tracking algorithm of asynchronous direct spread spectrum signal is designed,based on the research of the existing common acquisition and tracking methods,and the algorithm can work in low SNR and high dynamic.In addition,the simulation and implemented on FPGA is also presented.The main contents of the paper includes:elaborated several common methods of PN code acquisition in SS communication field,and analysed their principle and performance,pointed out their advantages and disadvantages,and used simulation results of Matlab as support;proposed an improved frequency domain parallel acquisition method,not only can adapt to larger input SNR range and Doppler range is,but also reduced the amount of computation,shorten the acquisition time;use the carrier to aid tracking accuracy of the DLL code tracking loop architecture;design a double loop carrier phase tracking method,which can complete the tracking loop in a large dynamic;designed a symbols synchronization method based on the early-late gate.Finally,aiming at a space engineering application,this paper implemented the algorithm in hardware.