A Novel Physics-Based Channel Model for Reconfigurable Intelligent Surface-Assisted Multi-User Communication Systems

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. In the model, the signal reflected through the scatters and the RIS elements are jointly studied as multipath components of the overall received envelope. This novel strategy simplifies the mathematical structure of the channel gain and is able to compactly derive the distribution of the overall channel. 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 discrete phase shifts, the distribution further depends on the number of phase quantization levels. 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 element-based RISs.