Nonlinear Distortion Issues Created by Active Reconfigurable Intelligent Surfaces

Reconfigurable intelligent surfaces (RISs) can improve the propagation conditions over wireless channels but a passively reflecting RIS must be large to be effective. Active RIS with amplifiers can deal with this issue. In this paper, we study the distortion created by nonlinear amplifiers in active RIS. We analytically obtain the directions of the reflected distortion when the desired signals arrive from specific azimuth and elevation angles. The results are demonstrated numerically and we conclude that nonlinearities can both create in-band and out-of-band distortion that is beamformed in entirely new directions.