The objective of this paper is to study the wideband characteristics of the radio channel in a tunnel environment, not only the delay spread, but also the angle of departure/arrival of the rays, their relative weights and their delays, which are important values for Multiple-Input Multiple-Output applications. In order to achieve this goal, a measurement campaign has been carried out in a straight arched tunnel over a frequency band extending from 2.8 to 5.0 GHz and distance varying from 50 m up to 500 m. First, the variations of the channel impulse response and of the delay spread versus the distance between the transmitter and the receiver are analyzed. Then, the bidirectional channel characteristics have been extracted from the measured channel matrices using a high resolution estimation algorithm. The main contribution of this paper is to clearly show the quantitative variation of the delay spread and the angle of departure/arrival of the rays along a real tunnel and to investigate the possibility of using the ray theory in a rectangular tunnel to interpret experimental results obtained in an arched tunnel.
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