  Reconfigurable intelligent surfaces (RISs) comprised of tunable unit cells
have recently drawn significant attention due to their superior capability in
manipulating electromagnetic waves. In particular, RIS-assisted wireless
communications have the great potential to achieve significant performance
improvement and coverage enhancement in a cost-effective and energy-efficient
manner, by properly programming the reflection coefficients of the unit cells
of RISs. In this paper, free-space path loss models for RIS-assisted wireless
communications are developed for different scenarios by studying the physics
and electromagnetic nature of RISs. The proposed models, which are first
validated through extensive simulation results, reveal the relationships
between the free-space path loss of RIS-assisted wireless communications and
the distances from the transmitter/receiver to the RIS, the size of the RIS,
the near-field/far-field effects of the RIS, and the radiation patterns of
antennas and unit cells. In addition, three fabricated RISs (metasurfaces) are
utilized to further corroborate the theoretical findings through experimental
measurements conducted in a microwave anechoic chamber. The measurement results
match well with the modeling results, thus validating the proposed free-space
path loss models for RIS, which may pave the way for further theoretical
studies and practical applications in this field.
