Reconfigurable Intelligent Surface-Enabled Two-Way Backscatter Communication in Symbiotic Radio

Abstract

Symbiotic radio (SR) is a promising technology for Internet-of-Everything (IoE), which enables the IoE backscatter devices (BDs) to be integrated with an existing primary communication system with mutualistic benefits. In this paper, we consider an SR system where a two-way BD backscatters a portion of the incident signal for the backscatter transmission and receives the remaining part for the information decoding. Due to the nature of backscattering, there however exists a conflict between these two aims for the two-way backscatter communication. To reconcile the conflict and improve the overall performance, we particularly consider that the reconfigurable intelligent surface (RIS)-enabled two-way BD, with each reflecting element achieving the two-way backscatter communication. Based on the proposed system, we first investigate the achievable rates of the interested trans-missions when the RIS transmits its messages over the incident signal with the binary phase-shift keying (BPSK) scheme. We consider the joint design of the transmit beamforming at the primary transmitter and the reflection coefficients at the RIS to minimize the transmit power when these achievable rates meet their requirements. This transmit power minimization problem is then solved with an alternating optimization (AO) algorithm. Simulation results show that under the assistance from the RIS, the proposed two-way SR system is more energy-efficient.