Abstract

The reconfigurable intelligent surfaces (RISs) are expected to be a cheap way to extend service areas of base stations. This is especially promising in the millimeter wave and THz bands (from 30 GHz to +300 GHz) where base station coverage is expected to be modest and suffer greatly from blockages. As the RISs can potentially be large (physically and via number of sub-elements), there is a good change that a user is in the near field of the RIS. This paper considers RIS near field propagation and achievable power levels close to these surfaces. Ideal energy levels are looked into among with the impact of beamforming and beam squinting. Human safety issues close to these surfaces are also analyzed from the energy density point of view. It is shown that the achievable received power in the near field are very good, but the beam squinting may have a significant impact on the received power and frequency response. We also conclude that RISs are safe for humans even at close proximity due to relatively large channel losses in the reflected channels and hence low power densities in the air.