Reconfigurable Intelligent Surface Empowered Symbiotic Radio over Broadcasting Signals

This paper studies reconfigurable intelligent surface (RIS) empowered symbiotic radio over broadcasting signals, i.e., a base station (BS) broadcasts to multiple primary receivers (PRs) under the assistance of a RIS, while the RIS also transmits information to an Internet-of-Things receiver (IR) by modulating the incident broadcasting signals. We formulate a problem to minimize the BS's transmit power by jointly optimizing the BS's active beamforming and the RIS's phase shifts (i.e., passive beamforming), under the signal-to-noise-ratio constraints of the primary and IoT transmission as well as the RIS's phase-shift constraints. However, the problem is challenging to be solved optimally, since the optimization variables are coupled and the constraints are non-convex. An efficient iterative algorithm based on the block coordinated descent and modified semidefinite relaxation techniques is proposed to solve this problem for both discrete and continuous phase shift scenarios. The convergency of the algorithm is proved and the complexity of the algorithm is analyzed. Numerical results validate that the proposed system outperforms the benchmark of traditional broadcasting system without RIS.