| Summary: | Land mobile satellite communication and physical layer security are considered as the promising paradigms in beyond 5G networks. In this paper, we establish the multi-relay hybrid satellite-terrestrial systems (HSTSs) with artificial noise (AN) to investigate the reliability and security, in which the satellite sends signals to the terrestrial destination through multiple decode-and-forward relays and there exists an eavesdropper trying to overhear the useful information. In order to improve the overall system performance, two relay selection (RS) schemes: <italic>i) optimal relay-receiver pair selection (ORRPS)</italic> and <italic>ii) suboptimal relay selection (SRS)</italic> are considered. ORRPS selects an optimal relay-receiver pair and an AN signal to maximize the corresponding signal-to-interference-plus-noise ratios of <inline-formula> <tex-math notation="LaTeX">${R_{k}} \to D\left ({{R_{k} \to E} }\right)$ </tex-math></inline-formula> and the SRS refers to selecting a relay that optimize the instantaneous channel gain <inline-formula> <tex-math notation="LaTeX">${\left |{ {{h_{R_{k}D}}} }\right |^{2}}\left ({{{{\left |{ {{h_{R_{k}E}}} }\right |}^{2}}} }\right)$ </tex-math></inline-formula> among <inline-formula> <tex-math notation="LaTeX">$K$ </tex-math></inline-formula> relays, with the <italic>traditional round-robin selection</italic> scheme as a benchmark for comparison. Furthermore, the exact closed-form analytical expressions of outage probability (OP) and intercept probability (IP) under different RS schemes are derived for AN and non-AN conditions. To obtain more insights, the asymptotic analysis for high signal-to-noise-ratio regime is carried out. Based on the asymptotic analysis of OP, the diversity orders of the HSTSs are obtained. The numerical results and theoretical analysis shown that: i) The reliability of TRRS scheme is the worst among the three RS schemes, and the reliability of ORRPS scheme is the best; ii) The AN technology can significantly improve the security of the system, although it has a little adverse affect on the reliability; iii) Infrequent light shadowing has the best reliability, frequent heavy shadowing has the worst outage performance, and the change of security is opposite; iv) The OP of SRS and ORRP schemes decreased with the increase of <inline-formula> <tex-math notation="LaTeX">$K$ </tex-math></inline-formula>, while the IP changed on the contrary, and the reliability and security of TRRS scheme did not change obviously.
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