## 1 Introduction  
With the successful commercialization of fifth-generation mobile communication technology (5G), people have started exploring the next generation of mobile communication technology. This technology, 6G, aims to provide ubiquitous network coverage, enabling individuals to access the internet and enjoy network services through devices at any time and anywhere. However, current terrestrial networks are constrained by fixed base stations (BSs), which can lead to interruptions in network communication and challenging recovery when the distance is too great or failures occur. Furthermore, remote areas (e.g., mountain regions, oceans) are limited by infrastructure, making access to the network difficult [1]. To address these issues and achieve the grand vision of 6G networks, researchers are exploring networks with broader coverage, faster speeds, and more stable connections called Space-Air-Ground Integrated Networks (SAGINs). SAGINs are a fusion of terrestrial networks (TNs) and non-terrestrial networks (NTNs), incorporating terrestrial, aerial, and satellite networks with a layered structure and highly dynamic characteristics [2]. In recent years, SAGIN research has become a hotspot in the field of mobile communication. To promote SAGIN development, the 3 rd-Generation Partnership Project (3 GPP) has formulated and continuously refined key technology standards for satellite radio access networks, core NTNs, and the NTN-supported Internet of Things (IoT), among others [3]. As a crucial component of 6G technological development, SAGINs have the potential to shape future network architectures. Currently, SAGIN development faces many challenges. Due to the end-to-end (E2E) communication between ground and space (space to ground/ground to space) being sensitive to extreme weather conditions and solar storms, the connection between satellite networks and TNs remains unstable. Furthermore, in urban areas, satellite-ground communication is easily disrupted as wireless signals are absorbed and attenuated when penetrating buildings and obstacles [2,4]. The role of aerial networks is increasingly prominent in addressing these issues, though the exploration of aerial networks remains insufficient. Aerial networks, situated in three-dimensional space between terrestrial and satellite networks, often consist of various aircraft equipped with communication devices and capabilities. In recent years, thanks to breakthrough advancements in battery and communication technologies, the development of unmanned aerial vehicles (UAVs) has advanced rapidly and achieved large-scale commercialization. UAV communication technology has also seen significant progress, with ad hoc UAV networks solving numerous issues in sectors such as vehicular networks and transportation [5,6]. The highly dynamic and mobile characteristics of UAVs make them particularly suitable for SAGINs, positioning them as the main component of aerial networks. Figure 1a shows the hierarchical network structure of an SAGIN, where ground communication facilities can directly communicate with satellites or interact through an aerial network formed by UAVs. However, long-distance signal transmission is prone to interference, and the mobility and computational limitations of satellites hinder the long-term provision of stable services by satellite-assisted ground networks. To address this, a High-Altitude Platform (HAP, e.g., airship and balloon), with its altitude advantage and large size, can serve as an aerial base station to provide network support for remote users, as illustrated in Figure 1b, while collaborating with Low Earth Orbit (LEO) to enhance bandwidth and alleviate network load. Particularly in cases of ground facility failure or overload, a Low-Altitude Platform (LAP) can act as a communication relay, delivering stable regional network coverage to edge users, achieving full coverage, low latency, and stable connections, as depicted in Figure 1c. In the future, UAV-to-satellite communication is expected to become a key focus.