consumption at increasingly higher data rates. Although massive MIMO and mmWave, seen as the key enablers for 5G, are expected to achieve dramatic spectral efficiency improvements, the deployment of large-scale antenna arrays usually results in high implementation cost and increased power consumption [@mimoover]. In addition, combining mmWave with massive MIMO for further performance improvements generally requires more sophisticated signal processing as well as more costly and energy-consuming hardware. Hence, finding green and sustainable solutions to enhance wireless network performance for higher data rates and efficiency still remains crucial.

Recently, intelligent reflecting surface (IRS) or its various equivalents have been proposed as a promising new solution to achieve the above goals [@qqmag; @qqtwc; @qqicassp; @debtwc; @mmwre; @debtwc2; @irsstatscsi; @renzomag]. Specifically, IRS is a reconfigurable planar array comprising a vast number of passive reflecting elements, each of which is able to independently induce a phase shift and/or an attenuation to the incident signal and thereby collaboratively alter the reflected signal propagation to achieve desired channel responses in wireless communications. By properly adjusting the phase shifts and attenuations of the IRS’s elements, their reflected signals can be combined with those from other paths constructively at the receiver to enhance the received signal power 