  The recently completed 5G new radio standard is a result of several
cutting-edge technologies, including massive multiple-input multiple-output
(MIMO), millimeter (mm)-Wave communication and network densification. However,
these technologies face two main practical limitations 1) the lack of control
over the wireless channel, and 2) the high power consumption of the wireless
interface. To address the need for green and sustainable future cellular
networks, the concept of reconfiguring wireless propagation environments using
Intelligent Reflecting Surfaces (IRS)s has emerged. An IRS comprises of a large
number of low-cost passive antennas that can smartly reflect the impinging
electromagnetic waves for performance enhancement. This paper looks at the
evolution of the reflective radio concept towards IRSs, outlines the
IRS-assisted multi-user multiple-input single-output (MISO) communication model
and discusses how it differentiates from the conventional multi-antenna
communication models. We propose a minimum mean squared error (MMSE) based
channel estimation protocol for the design and analysis of IRS-assisted
systems. Performance evaluation results at 2.5 GHz operating frequency are
provided to illustrate the efficiency of the proposed system.
