| Low earth orbit(LEO)Internet satellite system can not only work stably in the area that cannot be covered by the ground communication network,but also has the characteristics of large data service bandwidth,high signal transmission rate and low beam receiving delay.In recent years,more and more attention has been paid by the communication industry at home and abroad.However,it is different from the ground communication network,with the particularity of wide coverage,relatively fast speed,large number of access,frequent dynamic switching and so on,there are still many key technologies to be solved.Time frequency synchronization is not only an indispensable step in the initial access of LEO satellite system,but also one of the key technologies.Because of the large Doppler frequency shift of LEO satellite to the earth,the commonly used synchronization algorithm based on Zadoff-Chu sequence is vulnerable to the interference of frequency offset,which affects the accuracy of time synchronization.Taking the LEO Internet satellite system as the background,this thesis analyzes the characteristics of the LEO Internet satellite system,puts forward the method to solve the key technology of time-frequency synchronization,and designs the LEO Internet satellite payload verification scheme by using software defined radio.The main contents of this thesis are as follows:(1)For timing synchronization,this thesis proposes an improved algorithm using unique word(UW)with filtering redundancy as training sequence.In the process of baseband signal modulation,because the root raised cosine filter has long memory characteristics,the UW code after interpolation and filtering will produce redundant data in the frame head,and has strong autocorrelation.According to this feature,the UW code with redundant data in the front end is used as the local timing synchronization sequence,and the received signals are cross correlated to achieve timing synchronization.Simulation results show that the improved algorithm can improve the accuracy of time synchronization.For carrier synchronization,a simplified Doppler frequency shift estimation algorithm in earth centered earth fixed(ECEF)is derived by using orbit parameters,and a Doppler frequency offset compensation method based on real-time mixing with frequency synthesizer is proposed.The frequency offset estimation and compensation algorithm based on UW code as training sequence has limitations.It can only estimate the fractional frequency offset,but cannot estimate when the frequency offset exceeds the angle range(-π,π).Therefore,in order to adapt to the characteristics of large Doppler frequency shift of LEO satellite system,this thesis proposes two methods of frequency offset estimation and compensation,which can not only estimate the larger Doppler offset,but also estimate the fractional frequency offset more accurately and reduce the error of frequency offset estimation.(2)This thesis also uses the software defined radio module USRPX310 to design a satellite payload verification scheme,and designs the frame format and data processing flow of the satellite link.In the load verification scheme of feed link,in order to improve the efficiency of frame synchronization and reduce the complexity and computation of the system,a hardware platform synchronization algorithm based on the combination of coarse synchronization based on threshold judgment and fine synchronization based on training sequence is proposed,which greatly reduces the search time of the frame header.In the load verification scheme of user link,the physical layer processing flow and frame structure of each logical channel are designed respectively.The work of this thesis can provide a theoretical reference for the research of key technologies and load verification of Leo Internet satellite. |
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