I. Introduction
In recent years, the space–air–ground–sea integrated wireless communication network supporting seamless, high-speed, and reliable transmission has attracted extensive attentions [1], [2]. Satellite communications have the superiority of wide-range coverage and can realize the global interconnection, thus becoming one of the hot spots in integrated network researches. The geostationary earth orbit (GEO) and low earth orbit (LEO) satellites are the main focuses of satellite communication researches. The GEO satellites with the multibeam architecture have spectrum multiplexing capabilities and high data transmission rates. They have been widely applied in civil and military fields, such as navigation, disaster relief, rescue, broadcasting, etc. [3], [4]. The LEO satellites have also gained significant attentions because of their relatively smaller path loss, less delay, and lower production costs than GEO ones [5], [6].