Intelligent Reflecting Surface-Assisted Bistatic Backscatter Networks: Joint Beamforming and Reflection Design
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The passive communication of backscatter devices is a major limiting factor against its widespread deployment, as it requires the availability of a strong carrier signal. The recent uptake of intelligent reflecting surfaces (IRS), capable of significantly enhancing the performance of a range of communication systems, presents an opportunity to improve the flexibility of backscatter communication (BackCom). In this paper, we introduce a novel system setup where an IRS is utilized to assist a bistatic BackCom network consisting of a carrier emitter, one or more tags, and a reader. We study the transmit power minimization problem at the carrier emitter, wherein its transmit beamforming vector is jointly optimized with the IRS phase shifts, whilst guaranteeing a required BackCom performance. A unique feature in this system setup is the multiple IRS reflections experienced by signals traveling from the carrier emitter to the reader, which renders the optimization problem highly non-convex. Therefore, we propose algorithms based on the minorization-maximization and alternating optimization techniques to obtain approximate solutions for the joint design. We also propose low-complexity algorithms based on successive optimization of individual phase shifts. Our results indicate considerable transmit power savings for both single-tag and multi-tag systems, even with modest IRS sizes.
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