Research On Uplink Channel Estimation And Random Access Of Low-orbit Satellite Mobile Communication

Low-orbit satellites have the advantages of wide coverage,small signal attenuation,and short delay.They are integrated with 5G to form a global communication network,which is an important direction for future communication development.Based on the 5G low-orbit satellite mobile communication system,the uplink channel estimation method and the random access procedure applicable to satellite systems are studied.Because the 5G low-orbit satellite pilot uses the combination of frequency division and code division multiplexing,channel estimation does not find complete channel information,and the random access of the communication system has long delay due to the rapid movement of the satellite.The problem of rapid change in uplink timing.This paper has achieved the following results in the work on the above issues.Firstly,The channel estimation algorithm for low-orbit satellite communication is studied.In the loworbit satellite system model,the pilot signals for different antenna ports in the 5G protocol are combined by frequency division multiplexing and code division multiplexing.For the pilot signals using the same time-frequency,code division multiplexing is used.Differentiate,for different time-frequency signals,use frequency division multiplexing to distinguish.In the channel estimation using the least squares estimation method and the least mean square error estimation method,the complete channel information is not used.Therefore,a method of group interpolation is proposed to improve the accuracy of channel estimation,and finally the minimum two improved by this method.Both the multiplication estimation and the minimum mean square error estimation performance are improved.On this basis,the transform domain channel estimation method not only reduces the computational complexity but also ensures the accuracy of channel estimation.Then,The compressed sensing channel estimation method for low-orbit satellite communication is studied.Since the signal obtained by the traditional Nyquist sampling method contains redundant information,it is not conducive to compression transmission in the system,and because less impulse response has most of the energy of the channel,such a satellite channel is sparse and can be utilized.Compressed sensing algorithm for channel estimation research.Firstly,the three main contents of compressed sensing are introduced: signal thinning,observation matrix establishment and signal reconstruction.Then use different greedy iterative algorithms for signal reconstruction research.The simulation results show that the compressed sensing method can obtain more accurate estimation response than the least squares estimation method while using less pilot signals.Finally,the random access process of low-orbit satellites is studied.Due to the rapid movement of satellites,the whole communication system will have two problems of long delay and fast change of uplink timing,which makes the existing terrestrial mobile communication system unable to be directly used in the satellite system.In this paper,a random access sequence suitable for satellite systems is proposed by using polar extension,which solves the long delay problem of the system.Then,the proposed downlink synchronization tracking solves the problem of rapid uplink timing change.The final simulation shows that the improved method ensures the effective implementation of the random access process.