  As a means to control wireless propagation environments, the use of emerging
and novel intelligent reflecting surfaces (IRS) is envisioned to enhance and
broaden many applications in future wireless networks. This paper is concerned
with a point-to-point IRS-assisted millimeter-wave (mmWave) system in which the
IRS consists of multiple subsurfaces, each having the same number of passive
reflecting elements, whereas both the transmitter and receiver are equipped
with massive antenna arrays. Under the scenario of having very large numbers of
antennas at both transmit and receive ends, the achievable rate of the system
is derived. Furthermore, with the objective of maximizing the achievable rate,
the paper presents optimal solutions of power allocation, precoding/combining,
and IRS's phase shifts. Then it is shown that when the number of reflecting
elements at each subsurface is very large, the number of favorable and
controllable propagation paths provided by the IRS is simply equal to the
number of subsurfaces while the received signal-to-noise ratio corresponding to
each of the favorable paths increases quadratically with the number of
reflecting elements. In addition, the problem of minimizing the transmit power
subject to the rate constraint is analyzed for the scenario without direct
paths in the pure LOS propagation. Finally, numerical results are provided to
corroborate the obtained analysis.
