Research On Efficient Time Frequency Synchronization Technology Of User Agile Beams For Satellite Internet

Facing the demand for intelligent and ubiquitous global interconnection access in recent years,low-orbit satellites have developed rapidly in the field of communication,and it is urgent to build an efficient satellite-ground integrated Internet communication system through a space-based communication system that is not restricted by geography.Low-orbit satellite communication has the characteristics of more obvious dynamics,longer transmission distance,and faster movement speed,which leads to dynamic round-trip delay and Doppler frequency deviation of satellite wireless signals,resulting in signal reception delay in the time domain and in-frequency Compression and extension of spectrum in the domain.After receiving the data,the terminal cannot obtain the correct sampling window and sampling frequency point,and restore the original data through filtering and demodulation.The low-orbit satellites based on agile beams use the time-slicing working mode to concentrate the beam radiation energy to the target area,increase the satellite-to-ground power,maximize the use of load resources,and dynamically plan the frequency,bandwidth,duration,and order.Agile beam switching is frequent,and multiple accesses need to be completed within one communication service time,and the real-time synchronization errors of different terminals in the wave position will also cause serious residual errors.There is a complex mutual movement between the high-speed moving satellite and the terminal,which leads to high dynamics and large-scale time-frequency offsets in the received signal.It becomes an efficient and accurate timefrequency synchronization in a low signal-to-noise ratio satellite link environment.The bottleneck of satellite network access.At present,the time-frequency synchronization of the satellite Internet faces the following problems:①fixed and single frequency offset estimation strategy is difficult to meet the accuracy and range requirements of large frequency offset change estimation in complex dynamic satellite scenarios;②the timing synchronization algorithm under the condition of large frequency offset is complicated High accuracy,poor anti-frequency offset performance.Based on the above problems,this paper models and analyzes loworbit satellite synchronization based on agile beams,and proposes a dynamic game method to optimize the strategy selection problem of complex low-orbit satellite frequency synchronization,and then uses differential cross-correlation algorithm to improve rough Timing synchronization anti-frequency offset performance,and using the false alarm detector to reduce the false alarm probability of the algorithm under the condition of low signal-to-noise ratio,a singular decomposition method based on the receiving matrix is proposed to achieve efficient fine timing synchronization.The main work and innovations of this paper include:1.This paper constructs the constraint relationship between the terminal location point,the ephemeris of the serving satellite,the trajectory of the sub-satellite point of the serving satellite,the elevation angle between the terminal and the serving satellite,and the Doppler frequency offset value.The time-frequency offset problem of multi-beam satellites is analyzed,and the Doppler modeling problem of different geographic distributions of agile beams,target satellites and user positions is solved.2.For the problem that a fixed and single frequency offset estimation strategy is difficult to meet the accuracy and range requirements of large frequency offset change estimation in complex dynamic satellite scenarios.This paper proposes an efficient time-frequency synchronization method based on game theory,analyzes the constraints of Doppler frequency offset estimation accuracy,range,and complexity,and optimizes the selection strategy by calculating the utility function of the strategy.The MSE of the Doppler estimation residual is reduced to 10-5.3.Aiming at the problems of high computational complexity and poor anti-frequency offset performance of the timing synchronization algorithm under the condition of large frequency offset.This paper proposes a coarse timing synchronization algorithm that improves differential crosscorrelation,and at the same time reduces the computational complexity through Fast Fourier Transform(FFT),and improves the detection probability under low SNR conditions through a false alarm detector;A fine timing synchronization algorithm based on singular decomposition of receiving matrix is proposed.The anti-frequency offset performance of the proposed algorithm reaches 2.3 times the subcarrier spacing,the false alarm rate is increased by 37.5%compared with the traditional crosscorrelation,and the precision of fine timing synchronization is increased by 33%,avoiding the secondary synchronization sequence(Secondary Synchronization Signal,SSS).secondary correlation calculations.4.A low-orbit satellite time-frequency synchronization simulation platform and a satellite-ground link satellite test platform have been built.Realized the functions of data source import,parameter configuration,correlation peak search and demodulation equalization,realized the calculation of received data estimation residual,synchronization success rate and other parameters,and used software-defined radio to apply timefrequency synchronization algorithm to satellites on the ground test.The proposed synchronization method still has a high synchronization probability in the actual low-orbit satellite environment.The results show that the proposed scheme and process can meet the needs of the current low-orbit satellite scenario,and effectively solve the problem of difficult time-frequency synchronization in the process of loworbit satellite access networking.