MIMO System Performance Using Leaky Coaxial Cable In Tunnel Environment

In recent years,the problem of wireless mobile communication in underground confined space scenarios of mines,tunnels and shopping plazas has become prominent.Communication-based train control(CBTC)in modern urban rail transit system is gradually using Long Term Evolution for Metro(LTE-M)communication system to transmit train control signals.It is required to have high reliability.However,the study of using leaky cables to transmit signals in LTE-M system of underground tunnels is not perfect.For this reason,this paper is devoted to studying the characteristics of wireless transmission in different metro tunnels which are normally different from ground environments,and to establishing the basic theory of 1.8GHz multiple-input multipleoutput(MIMO)system based on leaky cables in Metro tunnels.The main work of this paper is as follows:Firstly,based on electromagnetic field theory and communication theory,this paper innovatively deduces the electric field radiation channel model of leaky cable in limited space,gives the expression of MIMO system performance based on leaky cable in confined space,analyses the efficiency of MIMO system of leaky cable using radiation field distribution,and gives the analysis method of spatial characteristics of broadband wireless channel of leaky cable in tunnel scenario.Since the radiation field is the basis of communication transmission,the distribution of the radiation field of the test tunnel and the non-line-of-sight diameter of Nantong Zhongtian Company are determined in this paper based on the actual measurement and ray tracing simulation.It is found that the smaller the slotting period of leaky cable is,the better the radiation field characteristics are.Horizontal polarized leaky cable can provide better performance than vertical polarization.The fluctuation of radiation field can be reduced by properly adjusting the slotting period of the leaky cable and the position of the leaky cable relative to the receiver.These studies are the basis for further research on MIMO performance.According to the actual project deployment requirements of MTR LTE-M,this paper studies the Power delay profile(PDP)of 2x2 MIMO system under different conditions when the leakage cable is placed.The results show that the performance of MIMO system does not depend on the relative spacing of leaky cables,and the horizontal leaky cables have better performance.In addition,this paper investigates the channel capacity and the number of conditions in different receiving regions.These results can directly guide the laying of leaky cables.Taking Nantong Tunnel in China and 2x2 MIMO and 3x4 MIMO systems based on leaky cables in large central tunnels in France as objects,this paper combines the actual measurement results with the simulation calculation using Wireless Insite.The effects of antenna spacing,leaky cable spacing and height on MIMO performance are further studied.The channel capacity,correlation coefficients between channels and root mean square delay spread are emphasized.The research verifies that the performance of 2x2 MIMO system is better than that of traditional SISO(single input single-output)system.It is found that the correlation coefficient of 3x4 MIMO system is lower than that of 2x2 MIMO system,and the channel capacity is higher.It provides suggestions for improving the MIMO communication capacity in tunnels.The spatial characteristics of MIMO channels with different polarizations of leaky cables are further studied by ray tracing method.It is found that the power angle spectra of both horizontal and vertical polarizations can be fitted by truncating the Gauss distribution.It is found that due to the existence of metal doors at the bottom of the tunnel and some metal boxes,the middle part of the scene of Nantong Tunnel has a larger angle expansion than the two ends.In addition,the power angle spectra and spatial correlation coefficients under two polarization modes are estimated.The results show that the angle expansion decreases with the increase of the transmitter distance,and the spatial correlation coefficient is proportional to the increase of the axis distance.When the angle spread increases,the power distribution increases with the increase of the angle,but then it will leads to the decrease of spatial correlation coefficient.Therefore,this paper proposes an optimal system design model,which has the advantage of improving system performance.The authors of the above research published three papers in SCI journal,and some of the conclusions have been adopted by the LTE-M Engineering Design Institute of Shanghai Railway.