| The development of urban subway transit in China is progressing day by day.The communication based on train control(CBTC)system of urban subway transit puts forward severe requirements for reliability,stability and data transmission rate.The long term evolution for Metro(LTE-M)system based on leaky coaxial cable(LCX)is used with the next generation CBTC system.Multiple-input multiple-output(MIMO)technology is the core technology of physical layer in LTE system,which can improve the information transmission rate and spectral efficiency.Although the MIMO system based on LCX has been implemented in engineering applications,there are still many theories to be improved,which influence the scientific of engineering design.Therefore,this paper has carried out studies in the following aspects:1、For MIMO wireless systems based on multiple LCXs with single port feeding,a two LCXs MIMO channel model with single port feeding in tunnel scenarios is proposed.This channel model is based on the theory of geometry-based single bounce(GBSB)and the electric field distribution of LCX in tunnel,the expressions of channel impulse response(CIR)and correlation function(CF)of channel model are derived.The measurement of wireless channel was carried out in tunnel provided by Nantong Zhongtian Technology Co.,Ltd.The measurement results of channel matrix condition number(CN),channel capacity,CF for LCX-MIMO system are compared with theoretical results to verify the correctness of the channel model.At the same time,by comparing the channel matrix CN and channel capacity of LCX-MIMO system with that the dipole antennas MIMO system among the same parameters configuration,it is concluded that the MIMO system using LCX in tunnel scenarios can have higher channel capacity than dipole antennas MIMO system.Finally,the channel model theory is extended to the MIMO scenarios of multiple single port feeding LCXs,and the distribution of channel matrix CN and channel capacity are analyzed.2、Aiming at the high-mobility scenarios MIMO wireless communication system between the train and wayside equipment based on the single port feeding LCXs in tunnel,a three-dimension(3-D)spatial geometric MIMO channel model based on GBSB theory is proposed for double single port feeding LCXs with high-mobility scenarios in tunnel.According to the change of electric distribution of LCX in tunnel under the moving state of train,the expressions of CIR and space-time CF are derived theoretically.The correctness of the channel model is verified by Monte-Carlo simulation.The effects of moving speed,moving time,time delay and Rice factor(K-factor)on channel CF are analyzed.Finally,the effects of moving speed,moving time and signal-to-noise ratio(SNR)on channel capacity are analyzed.3、The MIMO system with single port feeding LCX is similar to the MIMO system of ground base station,which is a conventional method,while the MIMO technology with double ports feeding LCX is not yet practical,and it is possible to halve the number of LCXs,which is very attractive.Therefore,a MIMO channel model based on GBSB theory is proposed for double ports feeding single LCX in tunnel.The expressions of CIR of channel model are drived.The measurement of wireless channel was carried out in tunnel provided by Nantong Zhongtian Technology Co.,Ltd,and the correctness of the channel model is verified by comparing the theoretical results and measurement results of channel capacity of MIMO system.At the same time,the MIMO channel model theory of single double ports feeding LCX is extended to the MIMO scenario of multiple double ports feeding LCX,and the distribution of channel capacity is predicted.4、Aiming at the wireless channel of LCX-MIMO system,it is common that the spacing of LCX has a weak influence on the channel capacity and the channel matrix CN.In order to explore the physical reason of this phenomenon,this paper attempts to analyze this phenomenon theoretically by using the mutual coupling(MC).The expression of transmitter channel cross-correlation of MIMO system is derived.The MC impedance between LCXs is simulated and calculated by HFSS(High frequency structure simulator)software,and the influence of the MC effect of LCXs on the channel cross-correlation is analyzed.It is found that the MC effect between LCXs is the reason why the LCX spacing has a weak influence on the channel capacity,which provides a theorerical basis for the deployment of LCX engineering in narrow space.The above research work provides strong support for the LTE wireless channel research of Shanghai rail transit wireless communication laboratory and the construction of Shanghai urban rail transit,and provides a theoretical basis for engineering design. |
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