Exploiting Direct Links for Secure STAR-RIS Aided Wireless Communications: Outage and Ergodic Capacity Analysis Over Nakagami-m Fading Channels

This paper examines the secrecy performance of a simultaneous transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) empowered wireless communication system, where a base station sends its confidential data through a STAR-RIS to trusted outdoor and indoor users while facing threats from outdoor and indoor eavesdroppers. We also leverage the benefits of direct links between BS and outdoor users along with the STAR-RIS link, whereas indoor users only rely on the STAR-RIS link due to the severe blockages. We derive the secrecy outage probability (SOP) and ergodic secrecy capacity (ESC) expressions for both the users over Nakagami-m fading channels. In addition, we present the asymptotic SOP expressions in high signal-to-noise ratio (SNR) and main-to-eavesdropper ratio (MER) regimes to reveal more insights into the secrecy diversity orders of both users. We then analytically discuss that the high SNR slopes of ESCs for both users are equal to zero. Additionally, we provide an analytical framework to demonstrate the impact of STAR elements on the SOP and ESC performance under two cases of interest: 1) when STAR-RIS is out of service and 2) when RIS consists of a very large number of STAR elements. A tradeoff between the energy efficiency and secrecy capacity is also discussed. Finally, numerical and simulation studies verify our analytical findings.