Research On Vehicle-Ground Wireless Access And Switching Technology Of Vacuum Tube High-Speed Flying Train

China's transportation technology and application has developed rapidly in the near ten years.In the meantime,China has won a number of technological achievements and made brilliant successes in the field of high-speed railway(HSR)construction,and plays an important role in leading the development trend of high-speed railway technology in the world.Vacuum tube high-speed flying train(high-speed flying train)is a novel rail transit technology.The maglev train can run at ultra-high-speed(over 1000km/h)in the low-pressure tube close to the vacuum under low mechanical friction,low air resistance and low noise mode.The wireless communication system is the key to guarantee the safety and efficiency of operation.Since the high-speed flying train operates at the speed up to 1000km/h?4000km/h,thus the requirements of communication stability and reliability are much higher than current HSR.Combining the current researches on vehicle-ground communication of HSR and the specific characteristics of high-speed flying train,this paper analyses the serious challenges of wireless communication under high-speed flying train scenario,such as serious Doppler frequency shift,specific characteristics of wireless coverage inside vacuum tube,metal wave effect and extremely frequent handoff.In view of these problems,we think that the specific design of vehicle-ground communication and system optimization based on the existing communication technology can be used to solve the extremely frequent handoff and wireless coverage problems,and thus realize wireless broadband access for high-speed flying trains.This paper mainly focuses on the problems of wireless coverage inside the vacuum tube and the extremely frequent handoff.As for the wireless coverage in vacuum tube,we designed a wireless access solution based on leaky waveguide with normal radiation characteristics.The simulation results of the field distribution inside the tube show that the leaky wave radiates in the form of cylindrical wave,and the phase along the train operation direction remains unchanged,which means that the Doppler frequency shift can be effectively suppressed from the mechanism of wave propagation.Since the time-varying channel has been changed to static channel,we proposed a directly leaky coverage scheme by installing an electromagnetic lens on the roof of the train.The phase change of the leaky wave passing through the lens can be determined by the lens'thickness.In order to transform the cylindrical wave into plane wave to achieve an uniform wireless coverage inside the train,we designed the thickness of the lens according to the equation of the relation between lens thickness and phase change.As for the extremely frequent handoff problem,the moving cell solution based on RoF network architecture has been adopted.Since the time of high-speed flying train passing through each segment of leaky waveguide is quite short,this paper proposes a switching time reservation mechanism using a position prediction algorithm.In this mechanism,firstly,the position of the train in the future time is predicted by the vehicle-ground synchronous prediction algorithm,thus then the advance execution time of cell movement operation could be predicted.At the advance execution time,the control center will perform optical layer switching and radio frequency function activation of the corresponding segments of leaky waveguide,ensuring the smooth operation of cell movement.The simulation results show that the vehicle-ground synchronization prediction algorithm can predict the position of high-speed flying train accurately with the small number of error corrections,providing a strong support for the safe and reliable operation of high-speed flying trains at ultra-high-speed.