Riding over Multiuser NOMA Carrier: A Spectrally-Efficient RIS-Enabled Symbiotic Backscatter System

This paper puts forth a proposal for a reconfigurable intelligent surface (RIS) enabled symbiotic backscatter system riding over a multiuser non-orthogonal multiple access (NOMA) carrier, where the RIS is employed as a substitute for the conventional backscatter device (BD) with the objective of enhancing both primary NOMA transmission and backscatter communications. In particular, a symbiotic mechanism is proposed to control the mutual interference by means of an adaptive adjustment of the reflection coefficient at the RIS. Moreover, a novel and effective multiuser decoding scheme is proposed, based on successive interference cancellation (SIC), with the objective of ensuring the successful transmission of the symbiotic system. To this end, analytical expressions re derived for the closed-form outage lower bounds, asymptotic outage behavior and outage error floors are derived. Furthermore, the upper bounds of the ergodic capacity and its asymptote at high signal-to-noise ratio (SNR) are developed in order to illustrate the spectral efficiency of the proposed symbiotic system. Theoretical analysis and numerical results demonstrate that a stronger backscatter channel with a greater number of elements deployed on a segmented RIS results in a reduction in transmission outage and an increase in system capacity. Additionally, numerical results illustrate the spectral efficiency advantage of the proposed symbiotic system over the conventional solutions.