Intelligent Reflecting Surface-Assisted Symbiotic Radio Systems: A Double-Reflection Covert Communication Design

We investigate covert communication in an intelligent reflecting surface (IRS)-assisted symbiotic radio (SR) system under the parasitic SR (PSR) and the commensal SR (CSR) cases, where an IRS is exploited to create a double reflection link for legitimate users and degrade the detection performance of the warden (W). Specifically, we derive an analytical expression for the average detection error probability of W and design an optimal strategy to determine the transmit power and backscatter reflection coefficient. To further enhance the covert performance, the joint optimization of the source transmit power, backscatter device (BD) reflection coefficient, and IRS phase-shifter is formulated as an expectation-based quadratic-fractional (EQF) problem. By reformulating the original problem into a fraction-eliminated backscatter power leakage minimization problem, we further develop the phase alignment pursuit and the power leakage minimization algorithms for the PSR and the CSR cases, respectively. Numerical results confirm the accuracy of the derived results and the superiority of our proposed strategy in terms of covertness.