  Reconfigurable intelligent surface (RIS) offers tremendous spectrum and
energy efficiency in wireless networks by adjusting the amplitudes and/or
phases of passive reflecting elements to optimize signal reflection. With the
agility and mobility of unmanned aerial vehicles (UAVs), RIS can be mounted on
UAVs to enable three-dimensional signal reflection. Compared to the
conventional terrestrial RIS (TRIS), the aerial RIS (ARIS) enjoys higher
deployment flexibility, reliable air-to-ground links, and panoramic full-angle
reflection. However, due to UAV's limited payload and battery capacity, it is
difficult for a UAV to carry a RIS with a large number of reflecting elements.
Thus, the scalability of the aperture gain could not be guaranteed. In
practice, multiple UAVs can form a UAV swarm to enable the ARIS cooperatively.
In this article, we first present an overview of the UAV swarm-enabled ARIS
(SARIS), including its motivations and competitive advantages compared to TRIS
and ARIS, as well as its new transformative applications in wireless networks.
We then address the critical challenges of designing the SARIS by focusing on
the beamforming design, SARIS channel estimation, and SARIS's deployment and
movement. Next, the potential performance enhancement of SARIS is showcased and
discussed with preliminary numerical results. Finally, open research
opportunities are illustrated.

Signal Processing