  The intelligent reflecting surface (IRS) is emphasized as a controlled
scattering cluster. To this end, scatterers and traveling paths of multipath
components are classified to build a new channel model. Unlike the conventional
modeling, where the channels between system units are modeled independently,
the new model considers the channel as a whole and decomposes it based on the
traveling paths. The model shows clearly how IRS, in the beam-space context,
converts the channel from a problem into a design element. After investigating
IRS as a scattering cluster, based on a proposed segmentation scheme, the
beamforming problem is considered with a focus on first-order reflections.
Passive beamforming at IRS is shown to have two tiers; at the scatterer and
antenna levels. A segment-activation scheme is proposed to maximize the
received signal power, where the number of transmitting antenna elements to be
used is given as a function of IRS positioning and beamforming at the receiver.
The results show that while using more transmitting antenna elements to get
narrower beams is possible, using fewer elements can give better performance,
especially for larger IRS at close distances. The developed model also proves
useful in addressing emerging issues in massive MIMO communication, namely,
stationarity and spherical wavefronts.
