  Intelligent reflecting surfaces (IRSs) have been introduced into wireless
communications systems due to their great potential to smartly customize and
reconfigure radio propagation environments in a cost-effective manner. Despite
the promising advantages of IRSs, academic research on IRSs is still in its
infancy. In particular, the design and analysis of IRS-assisted wireless
communication systems critically depend on an accurate and tractable modeling
of the IRS. In this article, we first present and compare three IRS models,
namely the conventional independent diffusive scatterer-based model,
physics-based model, and impedance network-based model, in terms of their
accuracy, tractability, and hardware complexity. Besides, a new framework based
on partitioning the IRS into tiles and employing codebooks of transmission
modes is introduced to enable scalable IRS optimization. Then, we investigate
the impact of the three considered IRS models on system design, where several
crucial technical challenges for the efficient design of IRS-assisted wireless
systems are identified and the corresponding solutions are unraveled.
Furthermore, to illustrate the properties of the considered models and the
efficiency of the proposed solution concepts, IRS-assisted secure wireless
systems and simultaneous wireless information and power transfer (SWIPT)
systems are studied in more detail. Finally, several promising future research
directions for IRS-assisted wireless systems are highlighted.
