I. Introduction
With the sixth-generation (6 G) wireless communications coming, an increasing number of Internet-of-Things (IoT) applications will emerge, inevitably incurring skyrocketing hardware costs and energy consumption [1], [2], [3], [4]. In response to these issues, some technologies have attracted extensive attention, such as intelligent reflecting surface (IRS) [5], [6] and backscatter communication (BackCom) [7], [8]. IRS is an artificial metasurface consisting of copious low-cost passive elements, which can collaborate to reflect the incident electromagnetic (EM) wave in some specific directions. BackCom is an energy-efficient passive communication technology, through which a passive backscatter device (BD) equipped with no radio-frequency (RF) components can send its own information to the desired backscatter receiver (BR) by modulating and reflecting the incoming signal.