[1] [1] VAEZI M, AZARI A, KHOSRAVIRAD S R, et al. Cellular, wide-area, and non-terrestrial IoT: a survey on 5G advances and the road toward 6G[J]. IEEE Communications Surveys & Tutorials, 2022, 24(2): 1117-1174.

[2] [2] ALSABAH M, NASER M A, MAHMMOD B M, et al. 6G wireless communications networks: a comprehensive survey[J]. IEEE Access, 2021, 9: 148191-148243.

[3] [3] MARINI R, MIKHAYLOV K, PASOLINI G, et al. Low-power wide-area networks: comparison of LoRaWAN and NB-IoT performance[J]. IEEE Internet of Things Journal, 2022, 9(21): 21051-21063.

[4] [4] LU Y, ZHENG X R. 6G: a survey on technologies, scenarios, challenges, and the related issues[J]. Journal of Industrial Information Integration, 2020, 19: 100158.

[5] [5] VAHIDI S, GHAFOURI M, AU M, et al. Security of wide-area monitoring, protection, and control(WAMPAC)systems of the smart grid: a survey on challenges and opportunities[J]. IEEE Communications Surveys & Tutorials, 2023, 25(2): 1294-1335.

[6] [6] dos REIS BEZERRA W, do NASCIMENTO BOING R, de SOUZA C A, et al. An experimentation on CoAP multi factor authentication mechanism with reputation for Internet of Things constrained devices and low power wide area network[C]//Proceedings of the 2023 International Conference on Information Networking(ICOIN), January 11-14, 2023, Bangkok, Thailand. New York: IEEE Xplore, 2023: 67-72.

[7] [7] MEO M, RENGA D, SCARPA F. Integrating Aerial Base Stations for sustainable urban mobile networks[C]//Proceedings of the 2022 IEEE Global Communications Confe-rence, December 4-8, 2022, Rio de Janeiro, Brazil. New York: IEEE Xplore, 2022: 1727-1733.

[9] [9] LIU C X, FENG W, CHEN Y F, et al. Cell-free satellite-UAV networks for 6G wide-area Internet of Things[J]. IEEE Journal on Selected Areas in Communications, 2021, 39(4): 1116-1131.

[10] [10] GIORDANI M, ZORZI M. Non-terrestrial networks in the 6G era: challenges and opportunities [J]. IEEE Network, 2021, 35(2): 244-251.

[11] [11] ABDELSADEK M Y, CHAUDHRY A U, DARWISH T, et al. Future space networks: toward the next giant leap for human kind[J]. IEEE Transactions on Communications, 2023, 71(2): 949-1007.

[12] [12] AMMAR S, PONG LAU C, SHIHADA B. An in-depth survey on virtualization technologies in 6G integrated terrestrial and non-terrestrial networks[J]. IEEE Open Journal of the Communications Society, 2024, 5: 3690-3734.

[13] [13] RIERA-PALOU F, FEMENIAS G, CAUS M, et al. Enhancing cell-free massive MIMO networks through LEO satellite integration [C]//Proceedings of the 2021 IEEE Wireless Communications and Networking Conference Workshops(WCNCW), March 29, 2021, Nanjing, China. New York: IEEE Xplore, 2021: 1-7.

[14] [14] KOTA S, GIAMBENE G. 6G integrated non-terrestrial networks: emerging technologies and challenges [C]//Proceedings of the 2021 IEEE International Conference on Communications Workshops(ICC Workshops), June 14-23, 2021, Montreal, QC, Canada. New York: IEEE Xplore, 2021: 1-6.

[15] [15] DIAZ-VILOR C, LOZANO A, JAFARKHANI H. Cell-free UAV networks with wireless fronthaul: analysis and optimization[J]. IEEE Transactions on Wireless Communications, 2024, 23(3): 2054-2069.

[17] [17] ABDELSADEK M Y, YANIKOMEROGLU H, KURT G K. Future ultra-dense LEO satellite networks: a cell-free massive MIMO approach[C]//Proceedings of the 2021 IEEE International Conference on Communications Workshops(ICC Workshops), June 14-23, 2021, Montreal, QC, Canada. New York: IEEE Xplore, 2021: 1-6.

[18] [18] MA T, QIAN B, QIN X H, et al. Satellite-terrestrial integrated 6G: an ultra-dense LEO networking management architecture [J]. IEEE Wireless Communications, 2024, 31(1): 62-69.

[19] [19] WANG P F, DI B Y, SONG L Y. Mega-constellation design for integrated satellite-terrestrial networks for global seamless connectivity[J]. IEEE Wireless Communications Letters, 2022, 11(8): 1669-1673.

[20] [20] GAO Q L, JIA M, GUO Q, et al. Jointly optimized beam-forming and power allocation for full-duplex cell-free NOMA in space-ground integrated networks [J]. IEEE Transactions on Communications, 2023, 71(5): 2816-2830.