**2.3. Asymptotic models**

Ray models use high-frequency expansions of Maxwell's equations and are based on optical laws to model the fields. They constitute the most commonly used deterministic techniques to analyze confined environments [32]. In ray-based methods, the waves are treated as rays propagating perpendicular to the wave fronts. This approximation is valid at positions sufficiently distant from the source. The resulting spherical waves can be approximated by a locally plane wave on a small portion of the sphere. Two methods are widely used: ray launching and ray tracing [8, 10]. Curved tunnels, arched cross sections, or bends of a tunnel can be treated by ray methods provided that radii of curvature of the surface be large compared to the wavelength [22, 33].

Current tunnel cross sections are getting smaller, and antennas have to operate close to the tunnel walls, such that these interfere with their near-fields. As a result, antenna performances cannot be guaranteed by using asymptotic methods. For this reason, full-wave models accounting for near-field effects become more relevant.
