Communicating with Intelligent Surfaces

This paper analyzes the fundamental communication problem involving large intelligent surfaces (LIS) starting from electromagnetic (e.m.) arguments. Since the numerical solution of the corresponding optimal eigenfunction problem is in general computationally prohibitive, simple but accurate analytical expressions for the link gain and available degrees-of-freedom (DoF) are derived. It is shown that the achievable DoF and gain offered by the wireless link are determined only by geometric factors, and that the classic Friis' formula is no longer valid in this scenario where the transmitter and receiver could operate in the near-field regime. Furthermore, results indicate that, contrarily to classic MIMO systems, when using LIS more than one DoF might be available in strong line-of-sight (LOS) channel conditions, which corresponds to a significant increase of spatial capacity density.