Performance Analysis of Active Elements at Main Lobe Region on Intelligent Reflecting Surface: Deploy It at BS-Side or User-Side?

In this paper, the placement of active reflecting elements on an intelligent reflecting surface (IRS) is studied. With a hybrid IRS, the placement problem of finite active elements is vital to performance. To meet the realistic environment, we incorporate the mutual coupling effect into the reflection coefficient calculation. The reflection coefficient of each element is determined by considering the influence of neighbor elements. However, there has been little investigation into which region of the IRS should be illuminated by the main lobe and whether placing active elements in the main lobe region or outside leads to better average user SNR. This important but previously overlooked question is the main focus of this study. To the best of our knowledge, this paper is the first work that analyzes the performance of placing active elements at main lobe or side lobe regions and verifies it through comprehensive simulations. We conduct a series of simulations to validate which main lobe region on IRS yields the highest signal-to-noise ratio (SNR). Additionally, we explore IRS placement strategies to determine the optimal deployment of the hybrid IRS. Simulation results showed that active reflecting elements should be placed in the main lobe region at the center of the IRS, and deploying the hybrid IRS at the user-side achieves the highest SNR value.