Propagation mechanism modelling in the near region of circular tunnels

Along with the increase in operating frequencies in advanced radio communication systems utilised inside tunnels, the location of the break point is further and further away from the transmitter. This means that the near region lengthens considerably and even occupies the whole propagation cell or the entire length of some short tunnels. To begin with, this study analyses the propagation loss resulting from the free-space mechanism and the multi-mode waveguide mechanism in the near region of circular tunnels, respectively. Then, by conjunctive employing the propagation theory and the three-dimensional solid geometry, a general analytical model of the dividing point between two propagation mechanisms is presented for the first time. Moreover, the model is validated by a wide range of measurement campaigns in different tunnels at different frequencies. Finally, discussions on the simplified formulae of the dividing point in some application situations are made. The results in this study can be helpful to grasp the essence of the propagation mechanism inside tunnels.