Secure Performance of STAR-RIS Aided NOMA System With Imperfect SIC

This letter investigates the secrecy performance of a non-orthogonal multiple access (NOMA) system assisted by simultaneous transmitting and reflecting reconfigurable intelligent surface (STAR-RIS), where the imperfect successive interference cancelation (SIC) is considered. The cumulative distribution function of the signal-to-interference-noise ratio (SINR) of the legitimate users and the eavesdropping user are respectively deduced by using the central limit theorem. With this result, the theoretical secrecy rate (SR) and its approximate expressions are respectively derived by means of the Gauss-Chebyshev quadrature and Jensen's inequality. Furthermore, the asymptotic SR analysis and corresponding expression are provided. The results indicate that the number of STAR-RIS elements and the imperfect SIC of NOMA users can affect the secrecy rate performance of the system. The simulations are offered to verify the effectiveness of theoretical analysis and the accuracy of approximate expressions.