On Models with Power Conservation in Reflective Intelligent Surfaces and their Design Implications

TL;DR

This paper investigates the importance of power conservation in models of reconfigurable intelligent surfaces (RISs), revealing that many existing models do not conserve power coherently and analyzing the implications for wireless communication channels.

Contribution

The paper introduces a comprehensive model enforcing power conservation in RISs and explores its effects on channel reciprocity and system design.

Findings

Power conservation conditions are derived for RIS models.

Existing models may violate power conservation principles.

Implications for channel reciprocity are analyzed.

Abstract

Reconfigurable intelligent surfaces (RISs) are potential enablers of future wireless communications and sensing applications and use-cases. The RIS is envisioned as a dynamically controllable surface that is capable of transforming impinging electromagnetic waves in terms of angles and polarization. Many models has been proposed to predict the wave-transformation capabilities of potential RISs, where power conservation is ensured by enforcing that the scattered power equals the power impinging upon the aperture of the RIS, without considering whether the scattered field adds coherently of destructively with the source field. In effect, this means that power is not conserved, as elaborated in this paper. With the goal of investigating the implications of global and local power conservation in RISs, work considers a single-layer metasurface based RIS. A complete end-to-end communications channel is given through polarizability modeling and conditions for power conservation and channel reciprocity are derived. The implications of the power conservation conditions upon the end-to-end communications channel is analyzed.