Optimization of Channel Fading and Channel Hardening for RIS Assisted NOMA communication

An accurate evaluation of the performance of a wireless technology system requires a channel model that captures its unique characteristics. The independent and identically distributed (i.i.d.) Rayleigh fading model has been widely used in theoretical studies of multiple antenna systems in scattering environments. However, when applied to a rectangular-shaped Reconfigurable Intelligent Surface (RIS), this model is not physically realistic. This letter proposes an alternative physical Rayleigh fading model specifically designed for RIS, which can be used as a benchmark for evaluating RIS-enhanced communication systems. Using this model, we revisit important RIS properties such as the rank of spatial correlation matrices and channel hardening. We also explore the integration of Non-Orthogonal Multiple Access (NOMA) with RIS. Through NOMA, leveraging the power-domain multiplexing of multiple users is enabled simultaneously to be served under the same frequency band. Increasing the spectral efficiency of an RIS-assisted system and thereby its performance via interference mitigation may be achieved if NOMA can be combined with RIS under realistic wireless environments.This work lays the foundation for future research on the performance evaluation of RIS-enabled NOMA systems.