  The performance of wireless communication systems is fundamentally
constrained by the random and uncontrollable wireless channel. By leveraging
the recent advance in digitally-controlled metasurface, intelligent reflecting
surface (IRS) has emerged as a promising solution to enhance the wireless
network performance by smartly reconfiguring the radio propagation environment.
Despite the substantial research on IRS-aided communications, this article
addresses the important issue of how to deploy IRSs in a wireless network to
achieve its optimum performance. We first compare the two conventional
strategies of deploying IRS at the side of base station or distributed users in
terms of various communication performance metrics, and then propose a new
hybrid IRS deployment strategy by combining their complementary advantages.
Moreover, the main challenges in optimizing IRS deployment as well as their
promising solutions are discussed. A case study is also presented to compare
the performance of different IRS deployment strategies and draw useful insights
for practical design.
