Abstract
Smart radio environments aided by reconfigurable intelligent reflecting
surfaces (RIS) have attracted much research attention recently. We propose a
joint optimization strategy for beamforming, RIS phases, and power allocation
to maximize the minimum SINR of an uplink RIS-aided communication system. The
users are subject to constraints on their transmit power. We derive a
closed-form expression for the beam forming vectors and a geometric
programming-based solution for power allocation. We also propose two solutions
for optimizing the phase shifts at the RIS, one based on the matrix lifting
method and one using an approximation for the minimum function. We also propose
a heuristic algorithm for optimizing quantized phase shift values. The proposed
algorithms are of practical interest for systems with constraints on the
maximum allowable electromagnetic field exposure. For instance, considering
$24$-element RIS, $12$-antenna BS, and $6$ users, numerical results show that
the proposed algorithm achieves close to $300 \%$ gain in terms of minimum SINR
compared to a scheme with random RIS phases.