Modeling And Optimization Of Wireless Channel In Railway Tunnels

In recent years,China's high-speed rail has developed rapidly.Because of its fast,convenient and green characteristics,it has become the most popular mode of travel,and the attendance rate of high-speed rail has increased year by year.In order to shorten the running time of trains and avoid the centrifugal force caused by the turning of trains at high speed,tunnel construction is essential.Compared with open scenes and indoor scenes,tunnels are a narrow and limited space.Electromagnetic waves will undergo multiple reflections,diffraction,transmission,etc.There will also be a keyhole effect.Radio wave propagation characteristics are significantly different.For any communication system,in order to better plan the design of the system,avoid misjudgments and lead to serious adverse results,it is necessary to establish an accurate channel model to fully grasp the propagation characteristics of electromagnetic waves in the wireless channel.Ray tracing method is a propagation model suitable for wireless signal coverage prediction in tunnel environment,which has high accuracy.This dissertation focuses on the research of wireless channel modeling and calibration in tunnel environment.Based on ray-tracing model and grey wolf algorithm,an improved grey wolf optimization algorithm based on opposing search and Levy flight strategy is proposed for tunneling model calibration.The main contents and research results are as follows:(1)Firstly,aiming at the bad condition of train operation,the radio channel model of ray tracing is established.The model is a deterministic propagation model,and its predictive rationality depends on the accuracy of geographic database and the optimization of calibration parameters.Some parameters in the model have been proved to be optimal according to empirical values and objective conditions.Empirical values can be used for parameters that have little influence on the accuracy of the model;and those that seriously affect the prediction accuracy of the model need to be calibrated to make the model more accurate.Accordingly,the parameters need to be calibrated and the indicators to measure the performance of the model are defined.(2)To solve the problem that the gray wolf optimization(GWO)algorithm is easy to fall into the local optimum and the convergence precision is poor,this dissertation proposes an improved gray wolf optimization algorithm based on opposition search and Levy flight strategy.In the initial stage of the algorithm,the opposite search strategy is adopted to narrow the range of feasible solutions.And in the process of updating location of the gray wolves,Levy flight strategy is adopted to avoid the algorithm falling into the local optimum.By four standard test functions,simulation experiments show that the proposed OLGWO algorithm is superior to the GWO algorithm in terms of convergence speed and solution accuracy,and itcan search for the optimal solution quickly and accurately.Next,based on OLGWO optimization algorithm,the ray-tracing propagation model of tunnel is calibrated,and the path losses of four different tunnel cross-section shapes(single-track rectangle,double-track rectangle,arch and circle)are compared and analyzed.The simulation results show that the calibrated model has better performances in terms of RMS and linear correlation and can achieve precise prediction of signal receiving power in railway tunnels.(3)Based on the calibrated tunnel ray tracing model,theoretical simulation experiments are carried out according to different carrier frequency,antenna polarization mode,relative position of transceiver and receiver,as well as parameters of straight tunnel and curved tunnel.The results show that the higher carrier frequency,the greater signal propagation loss;in a certain length tunnel,signal attenuation in a wider tunnel is smaller;different antenna positions and different antenna polarization modes will have a certain impact on the signal transmission in the tunnel;when the signal passes through the bend part,more reflections will occur,resulting in the signal loss in the curve section is greater than the loss of long straight tunnel section.