A Mode-Based Approach for Channel Modeling in Underground Tunnels under the Impact of Vehicular Traffic Flow

In underground tunnels with vehicular traffic flow, wireless communications experience severe fading problems due to the signal reflections and diffractions on the tunnel walls and the mobile vehicles. Currently, most of the existing tunnel channel models can only characterize the signal propagation in empty tunnels. Existing investigations on the influence of the vehicles are all limited to experimental or numerical results. This paper provides a theoretical analysis of the wireless channel in tunnels with vehicular traffic flow. First, the effects of a single vehicle on the signal propagation in a tunnel are investigated. The analytical expressions of the in-mode loss and the mode coupling coefficients are provided using the uniform theory of diffraction (UTD), Poisson summation formula, and the saddle point method. Then the channel model in a tunnel with either deterministic or random vehicular traffic flow is provided according to the analysis on a single vehicle's effect. The predicted received power given by our theoretical model has a good match with the numerical simulation results. Our channel model shows that the signal propagation in tunnels with vehicular traffic flow is influenced by multiple factors, including the number, the size and position of the vehicles, the size and the curvature of the tunnel, the vehicular traffic load, and the vehicle velocity.