A Geometric View on Integrated Cascaded Channel of IRS-aided Communications

The hybrid intelligent reflecting surface (IRS) architecture is a novel technology that leverages the advantages of both passive and active IRS; the passive IRS offers a large aperture, while the active IRS provides additional power amplification. Prior studies have shown that the optimal performance of IRS-assisted wireless networks is achieved when the passive IRS is deployed near the transceivers and the active IRS is near the receiver, assuming transceivers with limited height. However, most of the prior works on hybrid IRS blindly adopted this assumption in the IRS association policy, which essentially becomes a partial selection strategy that offers analytical simplicity at the cost of sub-optimal performance. This limitation motivated us to find the globally optimal deployment strategy for all types of IRS. To this end, we first employ the geometric models for integrated path loss distance (known as Cassini oval and Ellipse for product- and sum-distance path loss laws, respectively) and use them to determine the optimal locations of the hybrid IRS. Then, we design a novel opportunistic association policy for hybrid IRS based on the integrated path loss model. Furthermore, we validate our proposed methods through simulations and show that they significantly outperform the conventional nearest association policy, especially for hybrid and active IRS.