Modal theory for interpreting path loss and MIMO capacity in a tunnel excited by an outside antenna

Theoretical and experimental approaches of the propagation in tunnels have been widely described in the literature, mainly focusing on the possibility of using multiple-input multiple-output (MIMO) transmission schemes, despite the small angular spread both at the transmitting and at the receiving site. Nevertheless, all these studies assume that the fixed base station is situated inside the tunnel. The objective of this paper is to generalize this case by considering the excitation of a tunnel by an outside antenna in order to know if, inside the tunnel, the diversity remains rich enough to get profit from MIMO techniques. The modal theory of the electromagnetic wave propagation will be used for interpreting the variation of path loss, correlation between array elements and capacity, as a function of the offset angle characterizing the direction of the fixed antenna array referred to the tunnel axis.