Research On Doppler Shift And Asynchronous Space-time Cooperative Coding In LEO Satellite Communication System

Low Earth Orbit(LEO)not only shortens the transmission time delay and reduces the path loss,but also has flexible networking and fast network construction.As an important part of air-ground integrated system,LEO satellite communication system has become a hot spot in all countries.However,in the LEO communication system,the Doppler shift phenomenon caused when the satellite signal is transmitted through the channel causes the frequency offset and the carrier interference when the signal arrives at the receiver.At the same time,in the satellite cooperative communication system,the problem of asynchronous reception of different relay signals can seriously affect the system performance.In the existing algorithms,the Doppler frequency shift estimation is mainly aimed at the fixed receiving end on the ground and has a high computation time complexity.Meanwhile in the asynchronous cooperative system,a scheme is mainly provided for the case that the delay difference is an integer multiple of the symbol period.In view of the above problems,this paper mainly studies the low complexity and high accuracy satellite Doppler shift estimation algorithms for LEO satellite communication and the anti-asynchronous delay method for multi-LEO satellite cooperative network communication.The main work and innovation of this paper are as follows:1.Aiming at the time complexity of Doppler shift in elliptic orbit LEO satellite communication system,a fast Doppler shift estimation algorithm is proposed in this paper.According to the characteristics of LEO satellite traveling around the earth with elliptic orbits,the algorithm estimates the Doppler frequency shift by using parameters such as the semi-major axis and eccentricity of the satellite orbit,and simplifies the complicated angle iteration in the calculation.The proposed algorithm has lower time complexity and is suitable for real-time estimation and compensation of Doppler shift in satellite communication system.The simulation results show that the proposed algorithm has higher accuracy than the method based on circular orbit in elliptical orbits with different eccentricities and compared with the satellite Doppler shift calculation method based on SGP4 / SDP4 model,the proposed algorithm has lower time complexity.2.Aiming at the precision of Doppler shift in LEO broadband mobile satellite communication system,the paper propose a modified method of Doppler shift which compares the prediction results of Doppler shift under elliptical orbit satellite with the cyclic prefix(CP)before OFDM symbol in mobile communication system.The simulation results show that compared with the existing algorithms,the proposed method improves the accuracy of Doppler shift in satellite channel and additive white Gaussian noise channel,and largely eliminates the influence of high-speed moving at the receiver on the accuracy of the algorithm.3.Aiming at the problem of dynamic delay at the receiver in cooperative LEO satellite communication system,the paper proposes a double sampling differential space-time Orthogonal Frequency Division Multiplexing Program.The proposed scheme constructed a dual-sampling method at the receiver based on the D-OFDM coding with delay tolerance when the satellite channel condition was unknown.The method added another sample to the current main lobe interval and equal gain merge the two sampling results.Comparing with the traditional scheme when the fractional delay is present,the D-OFDM2 scheme can obtain higher signal-to-noise ratio than single sampling method,and improve the system error performance.