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Transversal fading analysis in straight tunnels at 2.4 GHz

In tunnel-like environments the electromagnetic waves propagate differently in comparison to free space and urban scenarios. This is because, depending on the working frequency, tunnels can act as waveguides. As a consequence of this some specific phenomena occur, such as fast and slow fadings, caused by the constructive and destructive interference due to the multipath propagation. Fadings along the longitudinal dimension of tunnels have been widely studied under different geometric conditions, antenna polarization and working frequencies. Nevertheless, fadings also take place in the transversal dimension or cross-section. This issue has been poorly treated in the literature. In this context, our research group has been investigating navigation algorithms for multi-robot teams in tunnel-like environments under communication constraints. As fadings affect the communication among robots, a study about this phenomenon is necessary. In this work we present a complete study about transversal fadings in the Somport tunnel at 2.4 GHz. Results after carrying out a measuring campaign are presented and compared to modal theory based calculations, showing that fadings present a similar behavior in each transversal half of the tunnel (with the same period and phase, and with a phase delay between both halves) but different in the center, where although no deep fadings appear, the mean value of the signal power is lower compared to the fadings maximums. This led us to define a comfort-zone for communications for robot navigation in tunnels.