On–Off Backscatter: An RIS-Enabled Symbiotic Approach in NOMA Systems

This paper investigates a segmented reconfigurable intelligent surface (RIS)-enabled backscatter communication riding over ambient non-orthogonal multiple-access (NOMA) signals. For a practical hardware, a simultaneous adjustment of the reflection coefficients of the RIS elements in phase and continuously in amplitude is physically not feasible, contradicting the conventional symbiotic approach of continuously adjusting the amplitude of the reflection coefficient from 0 to one. To address this bottleneck, a novel on–off mechanism of the RIS’s reflecting elements for symbiotic backscatter NOMA systems is proposed. To begin with, the coexistence outage probability and the ergodic capacity of the proposed system are analyzed for diverse dispersed end users and the corresponding performance boundaries in the high signal-to-noise-ratio (SNR) regime are subsequently characterized. In addition, Monte Carlo simulations are provided to verify the correctness of the proposed analytical framework. Finally, the numerical results show that the transmission effectiveness of the proposed on–off mechanism approaches that of the ideal continuous one with an increase in the number of RIS elements. The findings also reveal that the proposed on–off mechanism offers the advantage of reduced reflection coefficient control vis-à-vis the deployment of the underlying RIS-enabled symbiotic backscatter system without the need to adjust the reflection coefficient in amplitude and in phase simultaneously.