Double-Active-IRS Aided Wireless Communication With Total Amplification Power Constraint

In this letter, we consider the downlink communication for a wireless communication system aided by two active intelligent reflecting surfaces (IRSs), where a multi-antenna base station (BS) serves a single-antenna user via a double-reflection link only. An optimization problem is formulated to maximize the achievable rate by jointly designing the BS beamforming as well as the elements allocation, amplification-power allocation, and placement of the two active IRSs, subject to a total amplification power constraint. Although this problem is non-convex which is generally difficult to solve, we propose an efficient algorithm to obtain its suboptimal solution by using the approximation and block coordinate descent (BCD) methods. Moreover, we show that the considered double-active-IRS aided system attains a linear capacity scaling order only, while it can achieve superior rate performance over the benchmark systems with a single active IRS, a single passive IRS, or two passive IRSs when the number of total active elements is not too large.