Analysis of path loss and delay spread at 900 MHz and 2.1 GHz while entering tunnels

Some propagation characteristics in tunnel environments at 900 MHz and 2.1 GHz are analyzed. Narrow-band parameters are given in terms of the propagation factor and linear regression slopes. Wide-band analysis is considered by calculating delay spread factors along the communication path. A ray-launching technique has been implemented, using ray optics and the uniform theory of diffraction (UTD). The proposed theoretical model has been validated by comparing the results with measurements obtained by other authors. The particular situation when a mobile antenna crosses the tunnel entrance and transmitter is outside the tunnel is studied. Our simulations show that the larger the angular separation of the transmitter with respect to the tunnel longitudinal axis is, the higher values of the delay spread. The more critical point appears around the tunnel entrance ("excitation zone"), where a signal power decay up to 40 dB may be detected. A similar value has been already found when turning a corner in microcellular urban environments.

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