  Future wireless networks are expected to evolve towards an intelligent and
software reconfigurable paradigm enabling ubiquitous communications between
humans and mobile devices. They will be also capable of sensing, controlling,
and optimizing the wireless environment to fulfill the visions of low-power,
high-throughput, massively-connected, and low-latency communications. A key
conceptual enabler that is recently gaining increasing popularity is the
Holographic Multiple Input Multiple Output Surface (HMIMOS) that refers to a
low-cost transformative wireless planar structure comprising of sub-wavelength
metallic or dielectric scattering particles, which is capable of impacting
electromagnetic waves according to desired objectives. In this article, we
provide an overview of HMIMOS communications by introducing the available
hardware architectures for reconfigurable such metasurfaces and their main
characteristics, as well as highlighting the opportunities and key challenges
in designing HMIMOS-enabled communications.
