  This paper proposes a new three-dimensional (3D) wireless passive relaying
system enabled by aerial IRS (AIRS). Compared to the conventional terrestrial
IRS, AIRS enjoys more deployment flexibility as well as wider-range signal
reflection, thanks to its high altitude and thus more likelihood of
establishing line-of-sight (LoS) links with ground source/destination nodes.
Specifically, we aim to maximize the worst-case signal-to-noise ratio (SNR)
over all locations in a target area by jointly optimizing the transmit
beamforming for the source node and the placement as well as 3D passive
beamforming for the AIRS. The formulated problem is non-convex and thus
difficult to solve. To gain useful insights, we first consider the special case
of maximizing the SNR at a given target location, for which the optimal
solution is obtained in closed-form. The result shows that the optimal
horizontal AIRS placement only depends on the ratio between the
source-destination distance and the AIRS altitude. Then for the general case of
AIRS-enabled area coverage, we propose an efficient solution by decoupling the
AIRS passive beamforming design to maximize the worst-case array gain, from its
placement optimization by balancing the resulting angular span and the cascaded
channel path loss. Our proposed solution is based on a novel 3D beam broadening
and flattening technique, where the passive array of the AIRS is divided into
sub-arrays of appropriate size, and their phase shifts are designed to form a
flattened beam pattern with adjustable beamwidth catering to the size of the
coverage area. Both the uniform linear array (ULA)-based and uniform planar
array (UPA)-based AIRSs are considered in our design, which enable
two-dimensional (2D) and 3D passive beamforming, respectively. Numerical
results show that the proposed designs achieve significant performance gains
over the benchmark schemes.
