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This paper proposes a new three-dimensional (3D) wireless passive relayingsystem enabled by aerial IRS (AIRS). Compared to the conventional terrestrialIRS, AIRS enjoys more deployment flexibility as well as wider-range signalreflection, thanks to its high altitude and thus more likelihood ofestablishing 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 transmitbeamforming for the source node and the placement as well as 3D passivebeamforming for the AIRS. The formulated problem is non-convex and thusdifficult to solve. To gain useful insights, we first consider the special caseof maximizing the SNR at a given target location, for which the optimalsolution is obtained in closed-form. The result shows that the optimalhorizontal AIRS placement only depends on the ratio between thesource-destination distance and the AIRS altitude. Then for the general case ofAIRS-enabled area coverage, we propose an efficient solution by decoupling theAIRS passive beamforming design to maximize the worst-case array gain, from itsplacement optimization by balancing the resulting angular span and the cascadedchannel path loss. Our proposed solution is based on a novel 3D beam broadeningand flattening technique, where the passive array of the AIRS is divided intosub-arrays of appropriate size, and their phase shifts are designed to form aflattened beam pattern with adjustable beamwidth catering to the size of thecoverage area. Both the uniform linear array (ULA)-based and uniform planararray (UPA)-based AIRSs are considered in our design, which enabletwo-dimensional (2D) and 3D passive beamforming, respectively. Numericalresults show that the proposed designs achieve significant performance gainsover the benchmark schemes.

Aerial Intelligent Reflecting Surface: Joint Placement and Passive Beamforming Design with 3D Beam Flattening