PLS Analysis of NOMA-Aided STAR-RIS Network Under Colluding and Non-Colluding Eavesdroppers

To overcome signal blockage and extend coverage to both indoor and outdoor users, simultaneous transmitting and reflecting reconfigurable intelligent surfaces (STAR-RIS) are employed. In addition, Non-Orthogonal Multiple Access (NOMA) is integrated to improve spectral efficiency and support multiple-user access. Given this setup, it is essential to investigate the Physical Layer Security (PLS) performance of STAR-RIS-assisted NOMA systems, particularly with multiple eavesdroppers. To evaluate the signals’ interception risks, we derive closed-form expressions for the Secrecy Outage Probability (SOP) and the Intercept Probability (IP) for both system setups at the near users and colluding/non-colluding eavesdroppers.1