Research On Carrier Synchronization Technology In Large Frequency Offset Variation And Weak Environment

Synchronization is one of the most important problems in communication systems.Symbol synchronization and frame synchronization need to be based on carrier synchronization.Therefore,research on carrier synchronization is of great significance.In the environment with large frequency offset variation,the signal has large doppler frequency offset and rate of first-order frequency offset.Under weak environment,the signal-to-noise ratio of the receiving end is low and there is interference.The traditional carrier synchronization is not applicable.In this paper,the advantages of data-assisted carrier synchronization and non-coded auxiliary carrier synchronization are combined and applied to carrier synchronization in large frequency offset variation and weak environment.Aiming at the problems of high algorithm complexity,inaccurate capture accuracy and poor tracking loop performance,two improved algorithms are proposed.The main research contents of the thesis are as follows:Firstly,this paper studies the signal processing flow of the receiver,and divides the carrier synchronization into two processes:carrier acquisition and carrier tracking.The carrier acquisition part obtains the rough estimate of the doppler frequency offset and rate of first-order frequency offset at a certain moment,and in the carrier tracking part,the initial value is the rough estimation transmitted by carrier acquisition,and the accurate estimation of carrier parameters at all sampling time is given in real time sequential.Secondly,this paper studies four kinds of data-assisted carrier acquisition algorithms:first-order frequency approximation algorithm,average periodogram algorithm,modified discrete chirp Fourier transform algorithm and fractional Fourier transform peak search algorithm;the performance and complexity of the carrier acquisition of the four kinds of algorithms are compared.The experimental results show that the modified discrete chirp Fourier transform algorithm has the best performance for estimating rate of first-order frequency offset.The fractional Fourier transform peak search algorithm has the best performance for doppler frequency offset variation estimation and has minimal algorithm complexity.Thirdly,this paper improves the fractional Fourier transform peak search algorithm.The parameters of the fractional Fourier transform are interpolated in two steps to improve the estimation accuracy of the parameters.Then the discrete polynomial phase transformation algorithm is used to estimate rate of first-order frequency offset and narrow the search interval of the fractional-order domain.When the length is 0.001,the calculation time is only one-fifth of the original fractional Fourier transform peak search algorithm.Finally,an improved variational Bayesian adaptive extended Kalman filter algorithm is proposed for non-coding assisted carrier tracking.Two kinds of carrier tracking algorithms are studied.The tracking accuracy and convergence speed of the extended Kalman filter algorithm are higher than the second-order frequency-locked loop assisted third-order phase-locked loop algorithm.However,the extended Kalman filter algorithm has the problem of poor anti-interference.The improved algorithm proposed in this paper can accurately track the variation of the observed noise covariance value,and can obtain higher tracking accuracy under the interference condition.