RIS-Assisted Hybrid Symbiotic Radio Network: From Practical End-to-End Model to Multi-Objective Optimization

A symbiotic radio (SR) network is built to address the situations of limited spectrum and energy resources. Particularly, a hybrid SR network is proposed based on simultaneous wireless information and power transfer (SWIPT). Specifically, the backscatter devices (BDs) will have the capability to provide active communications for the primary receiver (PR) with the harvested energy from SWIPT. Within the network, the BDs and primary transmitter (PT) based transmissions will achieve symbiotic status. In detail, the whole network will operate in three phases, which are assisted by a reconfigurable intelligent surface (RIS). Further, an end-to-end S-parameter based model is introduced to reveal the hardware characteristics. Under these settings, a multi-objective problem with energy efficiency (EE) and sum rate maximization is proposed. Additionally, the constraints are related to the communication demands and physical restrictions. To resolve the problem, the $\epsilon $ -constraint approach is utilized to transform it into a single-objective problem. Then the coupled variables are separated into three parts to allow an iterative solution. Specifically, an element-wise approach based on the Sherman-Morrison transformation is adopted to optimize the subproblem for RIS with the double-inverse form. Simulation results confirm the significance of the hardware features and the effectiveness of the proposed scheme.