Energy-Efficiency Maximization for STAR-RIS Enabled Cell-Free Symbiotic Radio Communications

This paper proposes a simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) enabled scheme for a cell-free symbiotic radio (SR) system. In the considered system, multiple access points (APs) are positioned on opposite sides of the STAR-RIS for cooperative primary transmission to the information receiver (IR). The STAR-RIS is set up to support the primary transmission by providing extra transmission links through simultaneous transmission and reflection. Additionally, it can send its own data to the IR by catching a ride over the primary signals. We take both perfect channel state information (CSI) and imperfect CSI scenarios into consideration. For each scenario, we formulate a primary system energy efficiency (EE) maximization problem to optimize each active beamforming constructed by the APs and the passive transmission and reflection beamforming established by the STAR-RIS. Then, a framework for alternating optimization with efficient techniques is proposed to address the non-convex problem, ensuring a nearly optimal solution. According to numerical results, the proposed scheme can improve the EE for the considered system 3.1 times greater than that of the scheme maximizing the total primary system rate. Moreover, the numerical results also reveal the negative effect of the CSI errors on the system performance.