RIS-Empowered Ambient Backscatter Communication Systems

Passive tags must harvest sufficient energy to operate. However, ensuring the tag’s energy needs and simultaneously suppressing the interference in ambient backscatter systems is challenging. To meet this, we employ a reconfigurable intelligent surface (RIS) to amplify the ambient source signal that reaches the tag. We maximize the achievable rate by optimizing the RIS phase-shifts subject to the tag’s energy harvesting constraint. Specifically, the phase-shifts are optimized to increase the tag’s received power while suppressing the direct-and-RIS-cascaded link interference. The optimization problem is non-convex, and we convexify it by means of fractional programming and a quadratic transformation. In comparison to a benchmark, our optimal phase-shift design provides ~150% and ~250% gains in harvested power and achieved rate, respectively, at 20 dB m transmit power.