I. Introduction
Intelligent reflecting surface (IRS), consisting of nearly passive, low-cost, reflecting elements with reconfigurable parameters, has been shown to significantly improve the spectrum and energy efficiency of wireless transmissions by reconfiguring the wireless propagation environment [2], [3], [4], [5], [6]. Existing beamforming and phase shift designs for IRS-assisted systems can be classified into two categories, i.e., instantaneous channel state information (ICSI)-adaptive designs [7], [8], [9] and statistical CSI (SCSI)-adaptive (also termed quasi-static) designs [2], [3], [4], [5], [6]. The former adapts to the ICSI and changes over fading blocks according to the ICSI, whereas the latter adapts to the CSI statistics and does not change over fading blocks during a specific period.