RIS-Enabled Symbiotic Radio: From Idea to Prototype

This article presents a reconfigurable intelligent surface (RIS)-enabled symbiotic radio system and its practical prototype implementation, advancing the technology from theory to application. This system integrates a backscatter signal modulated by an RIS with a primary signal conveyed by the transmitter, and enabling the receiver to decode both signals without relying on a direct link. However, such transmissions traditionally come at the expense of an increased hardware complexity and computational overhead. To overcome this issue, an asynchronous transmission scheme with a dedicated frame structure is proposed: the primary signal is transmitted at a low rate with one symbol per frame, while the backscatter signal is opportunistically modulated at a higher rate within the data portion of the primary signal, ensuring an interference-free decoding. In particular, a symbiotic transmission is realized in both time and frequency domains. To improve the system performance and reduce complexity, a joint constellation design based on a uniquely factorable constellation pair rule is developed, allowing concurrent demodulation of both signals through a single channel estimation and decoding process. To further improve spectral efficiency, non-orthogonal multiple access is introduced to transmit multiple information streams simultaneously on both the primary and backscatter signals. Importantly, these proposed transmission schemes are validated by a prototype system, with experimental evaluation of the symbol error rate and throughput efficiency. Finally, potential applications of the RIS-enabled symbiotic radio technology are examined alongside relevant research challenges, with several open research problems identified as avenues for future investigations.