  In this paper, we propose a novel wireless architecture, mounted on a
high-altitude aerial platform, which is enabled by reconfigurable intelligent
surface (RIS). By installing RIS on the aerial platform, rich line-of-sight and
full-area coverage can be achieved, thereby, overcoming the limitations of the
conventional terrestrial RIS. We consider a scenario where a sudden increase in
traffic in an urban area triggers authorities to rapidly deploy unmanned-aerial
vehicle base stations (UAV- BSs) to serve the ground users. In this scenario,
since the direct backhaul link from the ground source can be blocked due to
several obstacles from the urban area, we propose reflecting the backhaul
signal using aerial-RIS so that it successfully reaches the UAV-BSs. We jointly
optimize the placement and array-partition strategies of aerial-RIS and the
phases of RIS elements, which leads to an increase in energy-efficiency of
every UAV-BS. We show that the complexity of our algorithm can be bounded by
the quadratic order, thus implying high computational efficiency. We verify the
performance of the proposed algorithm via extensive numerical evaluations and
show that our method achieves an outstanding performance in terms of
energy-efficiency compared to benchmark schemes.
