I. Introduction
With the rapid development of wireless communications, 6G technologies is being researched and developed [1]. For 6G technology, satellite communication system is one of the most promising development directions at present [2], [3]. Because of the relatively small propagation delay, small path loss, and low transmission power in LEO satellite communications, LEO satellite network has become one promising solution for providing global coverage for Internet-of-Things (IoT) services, wireless sensor networks (WSNs), and other technologies of 6G mobile communication [4], [5].