| With the progress of urbanization,the underground rail transportation network has become an important part of urban transportation.As a low-carbon and fast means of transportation,the safety of its operation is the basic requirement of transportation services,and it has important practical significance for the subway tunnel environmental monitoring.At present,the monitoring of subway tunnels is mainly to monitor some key areas,and the long-term monitoring of the entire tunnel is relatively small.Secondly,most of the subway tunnel monitoring networks are built with fiber-optic wired networks.Although the transmission speed is fast,the fiber-optic network is extremely vulnerable and the cost is high.Therefore,this article applies the LoRa technology in low-power wide area networks to subway tunnel monitoring,and studies the LoRa networking technology for subway tunnel monitoring networks.Firstly,a wireless channel model is constructed for the special wireless transmission environment of a closed and nearly linear subway tunnel.The wireless characteristics of large-scale fading and small-scale fading are analyzed in detail,and the large-scale fading and small-scale fading characteristics of subway tunnels are analyzed using wave mode theory and geometrical optics methods,respectively.Mainly analyze the two characteristics of the electromagnetic field strength and path loss of the tunnel.In the analysis of electromagnetic field strength,the Maxwell equations are used as the theoretical basis for electromagnetic wave transmission,and the Marcatelli approximation analysis method is used to deal with the boundary problem to obtain the field intensity distribution of each store in the tunnel.In the path loss analysis,first analyze the attenuation of the polarized wave,consider the influence of the roughness of the tunnel wall on the loss,and derive the total propagation loss in the rectangular empty straight tunnel through the wave mode theory.When analyzing small-scale fading,the ray tracing method is used to model the channel,and the multipath effect of the wireless channel is studied.The tent law was used in determining the propagation rays,and the multipath path length,angle of arrival,and transmission loss were calculated.Finally,the theoretical model of the wireless channel of the subway tunnel is obtained.This part of the research provides methods for the design of channel modules in subsequent software simulations.Further,this paper uses a combination of software simulation and hardware field testing to study the networking scheme of the subway tunnel monitoring network.Aiming at the problem that mathematical modeling and Matlab simulation cannot truly reflect the LoRa protocol operation process,the NS3 discrete event simulator is used for simulation experiments.According to the physical layer and MAC characteristics of the LoRa WAN protocol and the protocol communication process,LoRa communication is designed and implemented in NS3 Module.Second,a table-driven routing method is added to the MAC layer of the protocol standard,and an Ad Hoc networking module is designed to implement the construction of a multi-hop network.In this paper,five kinds of networking schemes based on centralized and multi-hop networking are designed for the linear wireless propagation links of subway tunnels.The performance of the designed networking method is tested by NS3 software simulation.The simulation shows that the transmission of the packet multi-hop network is more real-time and more reliable.In addition,LoRa chip hardware based on SX1262 is used to build a multi-hop network,which verifies the performance of the multi-hop network. |
