Research And FPGA Implementation On Synchronization Technology Of Spread Spectrum Communication System Under High Dynamics

With the successful launch of China's third-generation navigation satellites,China began to build the "Beidou" global satellite navigation system officially.The navigation system has a wide range of applications in both civilian and military applications.The navigation receiver generally works in a high dynamic environment,and usually has a large motion speed and acceleration,so there is a large Doppler shift and frequency shift rate between the received signal and the local signal.At the same time,the navigation signal is relatively weak,and the spread spectrum signal is generally selected as the navigation signal.Therefore,in order to demodulate the high dynamic spread spectrum signal successfully,the frequency offset and code phase of the signal must be estimated first,so this paper focuses on the research on synchronization technology of spread spectrum sommunication system under high dynamics environment.Firstly,this paper describes the principle of spread spectrum communication technology,and then analyzes the influence of doppler shift on the spread spectrum signal.It is found that the performance of the correlation value will drop sharply in the presence of frequency offset,and the longer the spread spectrum sequence,the more sensitive it is to the frequency offset.Therefore,in high dynamic environments,conventional synchronization methods such as sliding correlation and frequency domain matching filtering will not work.In this regard,it is difficult to achieve signal acquisition under large frequency offset only in the time domain,and it is necessary to search in combination with the frequency domain.Therefore,the basic principle of the time-frequency two-dimensional search method is briefly introduced.Secondly,this paper compares several synchronization acquisition algorithms and performs matlab simulation verification.It is found that direct differential coherent accumulation is not sensitive to frequency offset but sensitive to noise.Then three kinds of synchronous matching techniques based on Partially Matched Filtering(PMF)are presented,which are PMF non-coherent accumulation,PMF differential coherent accumulation and PMF-FFT synchronous algorithms.The performance of PMF-FFT algorithm is the best.In addition,the frequency offset can be estimated while the code phase search is completed,so we choose this method as the algorithm implemented by the system.However,the PMF-FFT algorithm has the characteristics of scallop loss.In this paper,the method of zero-padding is used to improve the scallop loss.Thirdly,in the synchronous position decision,the multi-correlation peak decision is used instead of the threshold decision,which reduces the dependence on the noise estimation and effectively improves the capture probability.In order to combat greater frequency offset,a frequency serial search method is used to extend the anti-frequency offset range.At the same time,a triangulation correction method is proposed to reduce the synchronization position error,and the effectiveness of this method is verified by simulation.Finally,on the FPGA development platform based on K7410 T chip,a high dynamic spread spectrum communication system was built,which realized the normal transmission and reception of the spread spectrum signal,and the system is tested on the hardware,the results met the performance requirements.