RIS-Aided Wireless Communications: Can RIS Beat Flat Metal Plate?

Reconfigurable Intelligent Surface (RIS) has recently been regarded as a paradigm-shifting technology beyond 5G, for its flexibility on smartly adjusting the response to the impinging electromagnetic (EM) waves. Typically, RIS can be implemented by properly reconfiguring the adjustable parameters of each RIS unit to align the signal phase on the receiver side. And it is believed that the phase alignment can be also mechanically achieved by a flat metal plate with the same physical size and shape. However, we found in the prototype experiments that, a well-rotated flat metal plate can only approximately perform as well as RIS under limited conditions even under the case of spherical wave impinging, although its scattering efficiency is relatively higher. We analyze this phenomenon with wave optics theory and propose explicit scattering models for both the metal plate and RIS in general scenarios. Finally, the models are validated by simulations and field measurements.