Energy-Efficient Joint Broadcast-Unicast Communications via Dual-Polarized Aerial RIS

In this contribution, a novel framework for joint broadcast-unicast downlink transmissions using dual-polarized aerial reconfigurable intelligent surface (RIS) is proposed. The objective is to maximize the energy efficiency by jointly designing the active beamforming at the base station and the passive beamforming at the RIS. The optimization problem to maximize the system’s energy efficiency is formulated and solved using a two-stage algorithm. To tackle the non-convex nature of the problem, an alternating technique, where the optimization is decomposed into two sub-problems, is proposed. In the first stage, the passive beamforming matrix of the dual-polarized aerial RIS is designed based on the Gaussian randomization method, which relaxes the inhomogeneous non-convex quadratically constrained quadratic program (QCQP) into a semi-definite program. In the second stage, the active beamforming at the base station is designed based on a dual-structured beamforming approach, in which the interior point method is used to obtain the prebeamforming matrix, and the beamforming matrices pertaining to the unicasting and broadcasting are obtained based on low-complexity zero-forcing and successive convex approximation, respectively. Numerical results and comparisons are provided. In particular, it is shown that the polarization-based communication system yields higher energy efficiency gains as compared to a system implementing layered division multiplexing.