**2.4. Full-wave models**

In electromagnetism, two main families of numerical methods exist: frequency and time-domain methods. Nowadays, time-domain methods are becoming increasingly used [11, 31]. They are attractive because of their relative simplicity and their ability to account for arbitrary geometries, obstacles like vehicles, nonlinearities, and different materials and to determine transient response and perform wideband characterization [6, 32, 35]. Thus, full-wave methods attract more attention for the purpose of deterministic radio coverage predictions, near-field considerations, simplifications, and practical implementations [13]. However, efficient and adequate models are still needed to describe the wave propagation in tunnel environments. A modal approach is preferred because it allows one to describe the propagation by modes [1, 2, 29]. An efficient time-domain modal approach based on the transmission-line matrix (TLM) method will be introduced in the next section.
