  The reconfigurable intelligent surface (RIS) is one of the promising
technologies contributing to the next generation smart radio environment. A
novel physics-based RIS channel model is proposed. Particularly, we consider
the RIS and the scattering environment as a whole by studying the signal's
multipath propagation, as well as the radiation pattern of the RIS. The model
suggests that the RIS-assisted wireless channel can be approximated by a Rician
distribution. Analytical expressions are derived for the shape factor and the
scale factor of the distribution. For the case of continuous phase shifts, the
distribution depends on the number of elements of the RIS and the observing
direction of the receiver. For the case of continuous phase shifts, the
distribution further depends on the quantization level of the RIS phase error.
The scaling law of the average received power is obtained from the scale factor
of the distribution. For the application scenarios where RIS functions as an
anomalous reflector, we investigate the performance of single RIS-assisted
multiple access networks for time-division multiple access (TDMA),
frequency-division multiple access (FDMA) and non-orthogonal multiple access
(NOMA). Closed-form expressions for the outage probability of the proposed
channel model are derived. It is proved that a constant diversity order exists,
which is independent of the number of RIS elements. Simulation results are
presented to confirm that the proposed model applies effectively to the
phased-array implemented RISs.
