Taking a Sharp Turn: Modeling of Diffraction Coupling in Radio Propagation Inside Tunnels

The overall performance of a communication-based train control (CBTC) system largely depends on the performance of its data communications subsystem (DCS). The DCS network in almost all CBTC commercial system products marketed in the last decade uses radio communications in the open industrial, scientific, and medical (ISM) bands (2.4 or 5.8 GHz) to establish the bidirectional data link between central/wayside and onboard segments. Key factors in ensuring stable and sound radio communication are the number of wayside access points and locations of their antennas. Radio propagation modeling aims to provide optimal and reasonably reliable assurance of stable communication. However, the diffraction impact of sharp corners and edges in tunnels on the radio propagation process has been unaccounted for in the majority of models. The purpose of the present research is to incorporate the effect of diffraction coupling due to sharp edges in tunnel sections, including geometrical discontinuities such as cross junctions and L bends, through ray-mode conversion. The proposed modeling approach offers sufficient versatility to assimilate a variety of discontinuous geometries involving sharp edges in a tunnel environment. Numerical and empirical results suggest that the model provides an accurate tool for analyzing diffraction effects of tunnel discontinuities with sharp edges on the process of radio propagation.