Low-Complexity Transmission for STAR-RIS-Assisted Symbiotic Radio Using OOK

This paper proposes a simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS)-assisted symbiotic radio (SR) system, in which the transmission functionality of STAR-RIS is used to transmit extra Internet of Things (IoT) data and the reflection functionality of STAR-RIS is adopted to enhance the downlink transmission. STAR-RIS utilizes ON-OFF keying for IoT data modulation, which allows the IoT receiver to decode data using a low-complexity energy detector. The transmit beamforming at the base station and the transmission and reflection beamforming at the STAR-RIS are jointly optimized to maximize the weighted sum-rate (WSR) of downlink users, subject to the received power constraint of IoT transmission. Considering the practical coupled phase shift constraint of STAR-RIS, an efficient penalty dual decomposition-based algorithm is proposed to solve the formulated highly non-convex problem. Based on the obtained solution, the optimal IoT receiver and the theoretical bit error rate are further derived. Simulation results verify the effectiveness of the proposed STAR-RIS-assisted SR system.