Outage Analysis for a STAR-RIS-Segmented Symbiotic Backscatter NOMA System

Abstract

This paper proposes a simultaneous transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) segmented symbiotic backscatter non-orthogonal multiple access (NOMA) system, where the STAR-RIS is composed of an enhancing primary signal (EP) zone and a backscatter device (BD) zone. To evaluate the overall system transmission reliability, we derive a tight lower bound of the coexistence outage probability (COP), where an imperfect/realistic successive interference cancellation (SIC) is considered. It is shown analytically that the error floor of the COP in both the near field and far field coverages would appear as long as the residual interference due to SIC is non-negligible, resulting in a diversity order of zero. Such an error floor is mainly dominated by the residual interference parameters, the decoding thresholds, and the power allocation ratios. More importantly, unlike the Gamma approximation approach, the adopted Laplace approach can capture the true diversity order with perfect SIC in the near-field/far-field coverage, which is dominated by the bottleneck number of the STAR-RIS elements belonging to the EP and BD zones, regardless of the dual-hop channel statistics. In addition, it is shown that the COP performance improves with either the quantification order or the concentration parameter of the imperfect channel state information.