In this paper, the performance of non-orthogonal multiple access (NOMA) based cooperative spectrum sharing in hybrid satellite-terrestrial networks (HSTNs) is investigated, where the primary satellite network recruits the secondary terrestrial network as a cooperative relay. To improve the fairness and spectrum utilization under cooperative spectrum sharing (i.e. overlay paradigm of cognitive radio), the NOMA power allocation profile is determined by instantaneous channel conditioning at the

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... ond temporal phase. The closed-form outage probability and approximated ergodic capacity expressions for the primary user (PU) and the secondary user (SU) are derived by decode-and-forward (DF) relay protocols, where the generalized Shadowed-Rician fading and Nakagami-m fading are considered for satellite links and terrestrial links, respectively. Simulation results are conducted for validation of the theoretical derivation and analysis of the impact of key parameters, and prove the superiority of NOMA comparing to conventional orthogonal multiple access (OMA) schemes on cooperative spectrum sharing in HSTNs. Besides, the fairness analysis between the PU and the SU is introduced by Jain's fairness index (JFI). INDEX TERMS Non-orthogonal multiple access (NOMA), hybrid satellite-terrestrial networks (HSTNs), cooperative spectrum sharing, outage probability (OP), ergodic capacity. VOLUME 7, 2019 This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see http://creativecommons.org/licenses/by/4.0/ DAOXING GUO received the B.S., M.S., and Ph.D. degrees from the College of Communications Engineering, Nanjing, China, in 1995, 1999, and 2002, respectively. He is currently a Full Professor and a Ph.D. Supervisor with the Army Engineering University of PLA. He has authored or coauthored more than 40 conference and journal articles. His current research interests include satellite communications systems and transmission technologies, communication anti-jamming technologies, and communication anti-interception technologies, including physical layer security. He has served as a Reviewer for several journals in communication field. He holds over 20 patents in his research areas. KANG AN received the B.E. degree in electronic engineering from the Nanjing University of Aeronautics and Astronautics, Nanjing, China, in 2011, the M.E. degree in communication engineering from the PLA University of Science and Technology, Nanjing, in 2014, and the Ph.D. degree in communication engineering from Army Engineering University, Nanjing, in 2017. Since 2018, he has been with the National University of Defense Technology, Nanjing, where he is currently an Engineer. His current research interests are satellite communication, 5G mobile communication networks, and cognitive radio. ZHUNYUN CHEN received the B.S. degree from