RIS-Enabled Full-Duplex Backscatter Communication in Multi-User Symbiotic Radio

In this paper, we investigate reconfigurable intelligent surface (RIS)-enabled full-duplex backscatter communications in multi-user symbiotic radio (SR) systems. Specifically, an RIS is integrated with a primary transmission in which the primary transmitter (PT) broadcasts common messages to multiple primary receivers (PRs) and the RIS. Thanks to the full-duplex backscatter communication nature, the RIS absorbs part of the PT signals to decode the PT messages and, meanwhile, reflects the remaining part to convey its own messages for the PRs. By doing so, the RIS and the PRs can receive common messages, like the pairing messages, from the PT, and simultaneously establish information links between them without requiring additional spectrum and radio resources. However, it brings a challenging task on how to properly design the reflection matrix to resolve the conflicts between the signal absorption and reflection in the RIS-enabled full-duplex backscatter communication. Towards this end, we formulate an optimization problem that aims to jointly optimize the PT transmit beamforming vector and the RIS reflection matrix subject to the transmission rate constraints for the PT and the RIS. The problem is solved by the proposed alternating optimization (AO) method combined with difference-of-convex (DC) algorithm. Simulation results are presented to show that the RIS-enabled full-duplex backscatter communication can efficiently save the transmit power as compared to its half-duplex counterpart.