Ergodic capacity analysis of multi-user STAR-RIS–NOMA systems over nakagami-m fading under hardware impairments

Abstract Reconfigurable intelligent surfaces capable of simultaneous signal transmission and reflection (STAR-RIS) have recently attracted attention as a means to extend coverage and improve spectral efficiency in post-5G wireless networks. When integrated with non-orthogonal multiple access (NOMA), STAR-RIS enables concurrent service to multiple users located in both transmission and reflection regions. This paper investigates the ergodic capacity of a multi-user STAR-RIS–NOMA system over Nakagami-m fading channels by jointly accounting for imperfect channel state information (CSI), residual successive interference cancellation (SIC) errors, and co-channel interference. In contrast to existing studies that typically consider these effects individually or restrict the analysis to two-user scenarios, a unified analytical framework is developed for a realistic multi-user setting. Closed-form approximations of the ergodic capacity are derived using a moment-generating-function-based approach combined with Gauss–Laguerre polynomial expansion and are validated via Monte Carlo simulations. Numerical results show that STAR-RIS–NOMA achieves ergodic capacities of approximately 10–10.5 bits/s/Hz for near users and 4.5–5 bits/s/Hz for far users at 40 dB SNR, whereas orthogonal multiple access remains below 1.5 bits/s/Hz. The results further indicate that far users are particularly sensitive to imperfect CSI and residual SIC errors, underscoring the importance of accurate channel estimation and robust interference cancellation. These findings confirm the effectiveness of STAR-RIS–NOMA and provide useful design insights for beyond-5G wireless systems.