Measurement of Reconfigurable Intelligent Surfaces Through the Back-Scattering Method: Demonstration at 28 GHz

This paper introduces a parameter extraction technique for characterizing and validating the performance of a Reconfigurable Intelligent Surface (RIS) unit cells (UCs) using an over-the-air back-scattering approach. This technique addresses the challenge of characterizing RIS UCs when physical access to ports is not available. Experiments were conducted at 28 GHz with a prototype featuring two RIS UCs. The results demonstrate that the UC port relfection coefficient as seen from the load termination can be estimated by measuring the back-scattered field from the RIS. The accuracy of the estimation depends on the precision of the equivalent circuit models used for the reconfigurable PIN and varactor diodes employed as variable load terminations in the UCs. This method offers a practical solution for characterizing RIS UCs that may have highly integrated components so that over-the-air characterization becomes a necessity.