Wireless Channel Modeling And Optimization For High-speed Railway Tunnel Environments

In recent years,China's high-speed railways(HSRs)have been developed rapidly.Because of its advantages of rapid,green,high comfort and many other advantages,HSRs have become the best choice for people's travel methods,and passengers' demand for data and multimedia communications is also increasing.In many HSR scenarios,the tunnel is a typical and complex HSR terrain.As the operating mileage of HSR continues to increase,the proportion of tunnels in railway operation continues to increase,especially in the western region.Therefore,carrying out modeling and research on tunnels is the key to ensuring safe railway transportation.Unlike other scenarios,the tunnel space is long and narrow,and has a waveguide effect.At this time,the electromagnetic wave will be reflected and scattered by the tunnel wall multiple times.The received signal at the receiver is the superposition of multiple paths.The propagation of radio waves will be severely affected by many factors.Cognition of wireless channel characteristics is the basis of communication system research,and its models and simulation methods are particularly important for the optimal design of communication systems.Therefore,the establishment of an accurate wireless channel propagation model in the environment of HSR tunnels is essential for improving the capacity of HSR users,optimizing the design of wireless communication systems,and ensuring the safe operation of HSR.In this dissertation,we study the modeling and optimization of wireless channels in HSR tunnel environment,and carry out research based on finite-state Markov chain(FSMC)and wireless channel propagation characteristics,and propose a method for FSMC channel modeling with least squares fitting based on non-uniform interval.The main content and research results of the dissertation are as follows:(1)Research on wireless channel modeling method in HSR tunnel environment.The radio signal propagation characteristics in the HSR fading channels and the radio wave propagation model in the tunnel scenario are analyzed in depth,and the analysis shows that the FSMC model has good characteristics for predicting the state of the process and has been widely used in the modeling of HSR fading channels.However,the main problems of this model are: lack of verification of measured data,large error in signal-to-noise ratio(SNR)caused by the use of ideal free space path loss formula,and low accuracy of FSMC channel modeling caused by the uniform interval division of the distance between the receiver and the transmitter.(2)Research on FSMC wireless channel modeling and optimization in tunnel environment.Aiming at the problem of the large error of the received SNR caused by using the ideal free space path loss formula,a least square fitting FSMC channel path loss model is proposed.Taking different types of HSR tunnels(arch,rectangle and circle)as the research object,according to the measured data,the corresponding path loss formula is obtained based on the least square fitting.In order to solve the problem of low accuracy of FSMC modeling caused by uniform interval division,a least squares fitting FSMC channel modeling optimization method based on non-uniform interval division is proposed.It divides the communication distance between the transmitter and the receiver in the tunnel non-uniformly into several non-overlapping intervals;then,according to the path loss formulas corresponding to different types of HSR tunnels(arch,rectangle and circle)obtained by least square fitting,the channel state quantization value are determined by the Lloyd-Max quantization method,and the FSMC state transition probability matrix is obtained.(3)Simulation study of FSMC wireless channel optimization model in tunnel environment.Through the Matlab simulation experiments,the non-uniform interval division FSMC optimization model and the uniform interval division model are compared.The results show that: the non-uniform interval division method can capture the change of the channel states in time,and the mean square error(MSE)value of the model is lower than that of the uniform interval division method.At the same time,the simulation also studies the influence of the number of divided intervals and the number of Markov finite state on the accuracy of the FSMC model,and based on the comprehensive consideration of model accuracy and complexity,the reasonable values of the above two parameters were analyzed.The simulation shows that the proposed FSMC wireless channel optimization model can achieve accurate prediction of the signal received power in railway tunnel environment.