A Mixed Rays—Modes Approach to the Propagation in Real Road and Railway Tunnels

These days, security and efficiency of transportation networks are usually supported by wireless communication systems. In all cases, the radio-link reliability strongly depends on propagation properties and, therefore, effective prediction tools and models are requested in the design and planning phases of the radio system. In this paper, a mixed rays-modes approach to the propagation modeling in real tunnels is presented. The propagating field is computed as the superimposition of many characteristic modes, whose amplitudes are properly estimated thanks to a limited and, therefore, fast ray-tracing procedure; the geometrical optics rules are also used to model the main effects of the possible tunnel curvature. Moreover, an equivalent wall roughness is introduced in order to approximately account for the actual tunnel transversal shape and for the presence of inner elements and objects. The model is compared with some other different existing models and with measurements carried out inside an underground line in a neighborhood of Naples. The achieved performance is in line with the published scientific data.