Four-slope channel model for path loss prediction in tunnels at 400 MHz

The present study analyses radio propagation at the 400 MHz frequency band inside road and railway tunnels. It proposes, on the basis of field measurements, a path loss model consisting of four segments, namely: the free space segment, the high path loss segment, the waveguide segment and, in the furthest region, the free space propagation segment. Free space propagation is characteristic in a region close to an antenna. In the next region, the near region, only a few reflected rays reach the receiver resulting in high path loss. Further away, in the far region, the waveguide effect occurs because of a set of waves reflected from the tunnel walls resulting in low path loss. In the extreme far region, the waveguide effect vanishes because of attenuation of reflected rays. The points separating the individual segments are analytically defined. Model applicability and accuracy are checked by calculating the mean error and standard deviation. The results indicate reasonable agreement between measurements and the model. This four-slope path loss channel model can be applied for rapid and simple coverage prediction of direct mode operation in TETRA systems.