Accurate Modeling of Intelligent Reflecting Surface for Communications Systems

In the conventional sense, a passive intelligent reflecting surface (IRS) is perceived as an ideal phase shifter to the incident signal. It is assumed that the phase of the incident signal can be altered to any desired value without affecting its magnitude. In this paper, we question the veracity of this assumption which forms the basis for communication model that is widely used in scientific community. Although there exist rigorous EM based models to analyze and design metasurfaces, the same cannot be said about its successor, intelligent reflecting surface. Therefore, we attempt to present an EM based model that accurately describes intelligent scattering for any arbitrary shaped IRS. Our objective in this paper is to bridge the gap between fundamental EM formulation for an IRS to the communication model that accurately captures its functioning. We use Method-of-Moments (MoM), a computational electromagnetic approach to quantify intelligent scattering by an arbitrary shaped IRS. The proposed theoretical model is then validated with computational electromagnetic simulation in Feko. Then we adopt the general MoM based model for a special case where each IRS element is a center loaded wire. Closed form expression for pathloss and beamwidth are derived considering free space propagation. We show analytically and numerically, that the receive power predicted by the conventional model vs what is actually observed can be as much as 6 dB. Furthermore we demonstrate that the impact of optimizing an IRS using the conventional model, where each element is treated as an ideal passive phase shifter, can result in an additional 6 to 8 dB of power loss. As a concluding remark, we propose correction to the communication model that is currently used for IRS aided networks when each IRS element is a center loaded wire.