Active Reconfigurable Intelligent Surface-Based Symbiotic Radio: Boosting Mutual Benefits

In this paper, we investigate a novel active reconfigurable intelligent surface (RIS)-based multiple-input single-output (MISO) symbiotic radio (SR) system where an active RIS leverages its controllable reflecting elements (REs) to assist the legacy and backscatter transmissions via enhanced reflections. Compared to a traditional passive RIS, an active RIS enables the REs to reflect the incident signal with a phase adjustment and an amplitude gain, and thus strengthens the backscatter link at the expenses of a moderate increase in the power budget. In this setup, we aim to maximize the achievable rate of the legacy transmission subject to a minimum signal-to-noise ratio (SNR) requirement for the backscatter transmission. Besides, in the presence of several non-SR users, we discuss the interference management of the proposed SR system, where the interference caused by the SR transmitter and the active RIS is required to be below an interference temperature threshold. To this end, we propose an alternating optimization algorithm to solve the rate maximization problem. Specifically, we obtain the transmit beamforming by solving several weighted minimum mean-square-error problems and the RIS beamforming with the aid of fractional programming. Our extensive simulations highlight the significant benefits of utilizing active RIS in SR systems. The results show that, compared to nearly-passive RIS-based systems, the proposed active RIS-based SR system exhibits superior performance under similar surface sizes or power budgets. Notably, even with strict interference temperature constraints, the proposed SR system results in more mutual benefits for the legacy and backscatter transmissions thanks to the use of an active RIS.