I. Introduction
Intelligent reflecting surface (IRS) has emerged as a breakthrough technology in the sixth-generation communication networks, due to its capability of reshaping radio environment. In general, the IRS is a digitally-controlled metasurface constructed of a large quantity of meta-material units, which enables proactively adjusting the phase shifts and amplitudes of the impinging signals. Moreover, the IRS allows manipulating signals without the use of power amplifiers, thereby significantly reducing power consumption and hardware cost.