  The future 6G of wireless communication networks will have to meet multiple
requirements in increasingly demanding levels, either individually or in
combinations in small groups. This trend has spurred recent research activities
on transceiver hardware architectures and novel wireless connectivity concepts.
Among the emerging wireless hardware architectures belong the Reconfigurable
Intelligent Surfaces (RISs), which are artificial planar structures with
integrated electronic circuits that can be programmed to manipulate an incoming
ElectroMagnetic (EM) field in a wide variety of functionalities. Incorporating
RISs in wireless networks has been recently advocated as a revolutionary means
to transform any naturally passive wireless communication environment to an
active one. This can be accomplished by deploying cost-effective and easy to
coat RISs to the environment's objects (e.g., building facades and indoor
walls/ceilings), thus, offering increased environmental intelligence for the
scope of diverse wireless networking objectives. In this paper, we first
provide a brief history on wave propagation control for optics and acoustics,
and overview two representative indoor wireless trials at 2.47GHz for spatial
EM modulation with a passive discrete RIS. The first trial dating back to 2014
showcases the feasibility of highly accurate spatiotemporal focusing and
nulling, while the second very recent one demonstrates that passive RISs can
enrich multipath scattering, thus, enabling throughput boosted communication
links. Motivated by the late research excitement on the RIS potential for
intelligent EM wave propagation modulation, we describe the status on RIS
hardware architectures and present key open challenges and future research
directions for RIS design and RIS-empowered 6G wireless communications.
