  Intelligent reflecting surfaces (IRSs) are emerging as promising enablers for
the next generation of wireless communication systems, because of their ability
to customize favorable radio propagation environments. However, with the
conventional passive architecture, IRSs can only adjust the phase of the
incident signals limiting the achievable beamforming gain. To fully unleash the
potential of IRSs, in this paper, we consider a more general IRS architecture,
i.e., active IRSs, which can adapt the phase and amplify the magnitude of the
reflected incident signal simultaneously with the support of an additional
power source. To realize green communication in active IRS-assisted multiuser
systems, we jointly optimize the reflection matrix at the IRS and the
beamforming vector at the base station (BS) for the minimization of the BS
transmit power. The resource allocation algorithm design is formulated as an
optimization problem taking into account the maximum power budget of the active
IRS and the quality-of-service (QoS) requirements of the users. To handle the
non-convex design problem, we develop a novel and computationally efficient
algorithm based on the bilinear transformation and inner approximation methods.
The proposed algorithm is guaranteed to converge to a locally optimal solution
of the considered problem. Simulation results illustrate the effectiveness of
the proposed scheme compared to the two baseline schemes. Moreover, the results
unveil that deploying active IRSs is a promising approach to enhance the system
performance compared to conventional passive IRSs, especially when strong
direct links exist.
