Assessing the Third-Order Intermodulation of Reconfigurable Intelligent Surface With Embedded Varactor Diode

The reconfigurable intelligent surface (RIS) is a promising route for the next generation of communication and has been widely investigated. The research on its nonlinear electromagnetic (EM) phenomenon has been reported extensively. However, few researchers focus on the quantitative analysis of the nonlinear distortion of the RIS due to the active components serving as the tuning devices in its unit cells. A method of assessing the third-order intermodulation of RIS is proposed in this article, which is implemented with a combination of the nonlinear active device modeling and full-wave simulation of the passive part of the RIS. Considering that the varactor diode is widely utilized, a nonlinear equivalent circuit model is proposed in this article. It contains an improved capacitance–voltage model with an m expression acquired with the nonlinear current calculation. The nonlinear diode model is used to form the meta-atoms in the RIS system to realize nonlinearity assessment. Several characteristics of the diode are measured, and the measurement data are employed to verify the accuracy of the nonlinear modeling work for the varactor diode. More importantly, consistent comparison results are also obtained from the nonlinear assessment of the RIS system, which indicates that the proposed method is precise. With the nonlinear modeling work of the RIS system, much analysis can be carried out. For example, we can study the nonlinear characteristics of each meta-atom to improve the linearity of the RIS system. Consequently, it demonstrates that the proposed assessment method is significant.