Exploiting Passive STAR-RIS in Symbiotic Radio Cellular Network: Joint Beamforming Optimization

In this paper, we exploit a passive simultaneously transmission and reflection reconfigurable intelligent surface (STAR-RIS) to improve the backscatter device (BD) transmission in symbiotic radio cellular network (SRCN), where the BD suffers from the cascaded channel fading and attenuation. We aim to maximize the BD rate by designing the active beamforming at the base station and the passive beamforming at the STAR-RIS with guaranteeing the minimum rate requirement of the cellular user. To address the challenge of solving the formulated problem, we proposed an alternating optimization based framework with sequential rank-one constraint relaxation, successive convex approximation and penalty dual decomposition. Simulation results show that the proposed scheme outperforms the baseline without optimized STAR-RIS and matches the independent-phase-shift STAR-RIS scheme, validating its effectiveness in enhancing the performance of the SRCN.