Multi-RIS Empowered Symbiotic Radios for Ambient IoT: Cooperative Constellation and Beamforming Optimization

• Published in: IEEE transactions on communications p. 1
• Main Authors: Zhou, Hu, Liang, Ying-Chang, Yuen, Chau
• Format: Journal Article
• Language: English
• Published: IEEE 2025
• Subjects: ambient IoT; Array signal processing; Backscatter; constellation design; Constellation diagram; Internet of Things; Modulation; passive beamforming design; Radio frequency; Receivers; Reconfigurable intelligent surface; Reconfigurable intelligent surfaces; Signal detection; spectrum sharing; Symbiosis; symbiotic radio
• ISSN: 0090-6778; 1558-0857
• DOI: 10.1109/TCOMM.2025.3564768

Reconfigurable intelligent surface (RIS) empowered symbiotic radio (SR) holds the potential to support ambient Internet-of-Things (IoT) due to its spectrum- and energy-efficient characteristics. In this system, the RIS not only assists the primary system but also transmits IoT device information. While most existing works focus on the modulation design for single RIS scenarios, there is a lack of investigation into multi-RIS scenarios. This paper addresses this gap by considering a multi-RIS-empowered SR system, where multiple RISs backscatter the incident primary signal to transmit IoT information to the receiver. We aim to cooperatively optimize the signal constellation and phase shifts of all RISs to improve the overall symbol error rate performance for both primary and IoT transmissions. To achieve this, we formulate a problem to maximize the minimum Euclidean distance of the received noise-free signal from a signal detection perspective, subject to constraints on the peak amplitude of the IoT signal constellation and the passive reflection capabilities of the RISs. Given the non-convex nature of the problem, we propose an efficient iterative algorithm. Additionally, we sketch the structure of the optimized IoT signal constellations in the absence of a direct link to provide essential insights. We also develop a low-complexity algorithm and signal detection method by leveraging the received signal structure. Finally, simulation results demonstrate the superiority of our cooperative constellation design methodology over the traditional PSK constellation designs.