Joint On–Off Symbiotic: An RIS-Enabled Backscatter NOMA System

This paper investigates a novel reconfigurable intelligent surface (RIS)-enabled joint symbiotic backscatter nonorthogonal multiple access (NOMA) Internet-of-Things (IoT) network, where an RIS is deployed to enhance both the primary NOMA transmissions and backscatter communications. In conventional backscatter NOMA systems, prior studies have mostly focused on achieving higher energy efficiency, ergodic rate performance, and connectivity, while giving less attention to transmission reliability. To address this issue, we first propose a control mechanism to minimize the mutual interference in multi-user scenarios based on real-time propagation conditions. Specifically, the RIS jointly controls the channel gains of both the primary and backscatter systems under a continuous mechanism that adjusts the reflection coefficient in power, and an on–off mechanism by gradually activating RIS elements. Next, we evaluate the reliability of the proposed mechanisms by analyzing the coexistence outage probability (COP) of different users for the continuous method, while defining a symbiotic map and providing integral expressions using a mathematical induction for the on–off control method. Finally, numerical results are provided to assess the validity of the proposed analytical framework and demonstrate the effective interference suppression capabilities of the proposed joint mechanism in the underlying NOMA-based system. The proposed joint mechanism eliminates the high-SNR outage floor observed in the conventional symbiotic backscatter scheme under the considered continuous configuration, thereby improving the reliability of symbiotic transmissions. In a representative setting with 12 primary and 12 backscatter RIS elements at an SNR of 30 dB, the proposed mechanism achieves a COP of approximately 5×10−6, compared with approximately 6×10−4 for the conventional symbiotic backscatter scheme with on–off control, corresponding to a reduction of about two orders of magnitude.