Coupling at cross, T, and L junctions in tunnels and urban street canyons

Propagation around corners in tunnels and urban street canyons is modeled using a modal approach. While the modal representation for a straight tunnel is well-known, coupling at L, T, and cross tunnels has not been studied. For the propagation into branches off the main tunnel, mode coupling at the discontinuities is obtained by hybrid ray-mode conversion. By accounting for the mode diffraction at each corner, we compute the coupling loss into cross junctions, the arms of T-junctions, and L-bends. Because the cross tunnel has four edges, they generate more corner diffracted modal fields than other structures, and thus have the strongest coupling. For low base station and mobile antennas, high rise urban street canyons can be modeled as two-dimensional dielectric waveguides so that we can apply the modal results of tunnels to the mode propagation in urban street canyons.