Do linear cascaded models of RIS-parametrized wireless channels violate wave physics?

Because the linear input-output relation of a linear scattering system depends, in general, in a non-linear manner on the scattering system’s structural parameters, we question the rationale behind the use of cascaded models of RIS-parametrized wireless channels that impose by construction a linear dependence of the wireless channel on the RIS configuration. In particular, using a physics-compliant end-to-end channel model, we demonstrate the importance of reverberation-induced long-range mesoscopic correlations under rich-scattering conditions that yield a clearly non-linear RIS-parametrization of the wireless channel. We further show that neglecting these correlations can yield quantitatively and qualitatively incorrect performance predictions. Our results imply that important envisioned 5G and 6G deployment scenarios of RISs at sub6 GHz or millimeter-wave frequencies (e.g., machine-type communication in smart factories) should not be planned and evaluated using the common linear cascaded models of how the wireless channel is parametrized by the RIS.