  The idea of using a Reconfigurable Intelligent Surface (RIS) consisting of a
large array of passive scattering elements to assist wireless communication
systems has recently attracted much attention from academia and industry. A
central issue with RIS is how much power they can effectively convey to the
target radio nodes. Regarding this question, several power level models exist
in the literature but few have been validated through experiments. In this
paper, we propose a radar cross section-based received power model for an
RIS-aided wireless communication system that is rooted in the physical
properties of RIS. Our proposed model follows the intuition that the received
power is related to the distances from the transmitter/receiver to the RIS, the
angles in the TX-RIS-RX triangle, the effective area of each element, and the
reflection coefficient of each element. To the best of our knowledge, this
paper is the first to model the angle-dependent phase shift of the reflection
coefficient, which is typically ignored in existing literature. We further
measure the received power with our experimental platform in different
scenarios to validate our model. The measurement results show that our model is
appropriate both in near field and far field and can characterize the impact of
angles well.
