  Future wireless networks are expected to constitute a distributed intelligent
wireless communications, sensing, and computing platform, which will have the
challenging requirement of interconnecting the physical and digital worlds in a
seamless and sustainable manner.
  Currently, two main factors prevent wireless network operators from building
such networks: 1) the lack of control of the wireless environment, whose impact
on the radio waves cannot be customized, and 2) the current operation of
wireless radios, which consume a lot of power because new signals are generated
whenever data has to be transmitted.
  In this paper, we challenge the usual "more data needs more power and
emission of radio waves" status quo, and motivate that future wireless networks
necessitate a smart radio environment: A transformative wireless concept, where
the environmental objects are coated with artificial thin films of
electromagnetic and reconfigurable material (that are referred to as
intelligent reconfigurable meta-surfaces), which are capable of sensing the
environment and of applying customized transformations to the radio waves.
Smart radio environments have the potential to provide future wireless networks
with uninterrupted wireless connectivity, and with the capability of
transmitting data without generating new signals but recycling existing radio
waves.
  This paper overviews the current research efforts on smart radio
environments, the enabling technologies to realize them in practice, the need
of new communication-theoretic models for their analysis and design, and the
long-term and open research issues to be solved towards their massive
deployment. In a nutshell, this paper is focused on discussing how the
availability of intelligent reconfigurable meta-surfaces will allow wireless
network operators to redesign common and well-known network communication
paradigms.
