Beamspace Modeling of Multi-mode Communications with Large Intelligent Surfaces

The use of large intelligent surfaces as antennas enables the realization of multiple communication modes even in line-of-sight conditions, thanks to the exploitation of the near-field propagation. Despite results for the number of communication modes are known for specific configurations (e.g., small antennas and paraxial scenario), ad-hoc models are needed when adopting large antennas in arbitrary configurations. This paper gives new insights into the communication between large intelligent surfaces exploiting multiple communication modes, considering a beamspace modeling capable of accounting for generic orientations among the antennas and operating conditions following inside the near field, as happens with the increasing of the antenna size and operating frequency towards mm-waves and terahertz.