## 2 Background and Key Features
### UAV-Assisted SAGINs
#### 2.1.1 Architecture of UAV-Assisted SAGINs  
Overall, an SAGIN proposes a layered network architecture that integrates aerial platforms such as satellite systems and UAVs with ground networks to build a coordinated and robust network ecosystem [2]. As shown in Figure 1a, satellites are distributed at the top level, providing critical communication and data transmission services for the entire network. Drones, balloons, and airships form the middle layer of the network, which are flexibly deployed in the air to supplement the functionality of satellites and provide services closer to the ground. The ground part is the foundation of the network, including ground stations and control centers, responsible for the management and control of the network, ensuring the stable operation and efficient coordination of the entire system [18]. This precise integration aims to achieve wide coverage, strengthen network connectivity, and drive the development of a range of applications from remote sensing monitoring to providing broadband access in inaccessible areas. 1. _Satellite Network_: The satellite network constitutes the space-based component of an SAGIN, primarily comprising satellites in diverse orbits, including LEO (e.g., Starlink and OneWeb constellations), Medium Earth Orbit (MEO), and Geostationary Earth Orbit (GEO), as well as synchronous and relay satellites. These networks enable seamless global communication, particularly in remote areas, oceans, and deserts where terrestrial base stations are unavailable. By synergizing LEO satellites (low latency, high bandwidth) with GEO satellites (wide area coverage), a multi-source data transmission system is established to meet demands for low latency and high throughput. 2. _Aerial Network_: The aerial network consists of UAVs and HAPs, serving as a flexible and dynamically adaptive intermediate layer between space and terrestrial networks. It can be rapidly deployed for emergency scenarios to act as a temporary communication relay or backup network. While low-altitude platforms are susceptible to weather and mobility constraints, seamless handover and load balancing can be achieved through coordinated operations with satellite and terrestrial networks. 3. _Ground Network_: The ground network forms the foundational layer of an SAGIN, encompassing cellular networks (5G/6G base stations), IoT devices, and fiber-optic infrastructure. It supports direct user access and delivers high-speed data transmission, catering to real-time requirements in the industrial IoT, vehicular networks, and other latency-sensitive applications. Ground base stations dynamically allocate satellite and aerial resources via Software-Defined Networking (SDN) and Network Function Virtualization (NFV) technologies to optimize the QoS requirements of E2E. However, remote regions remain reliant on satellite and aerial networks for connectivity, while densely deployed IoT devices are prone to interference. Unlike traditional networks, SAGINs are expected to abandon the \"best-effort\" design philosophy. Instead, SAGINs aim to achieve reliable network control, provide deterministic latency, and comprehensively consider QoS requirements and network stability. SDN and NFV offer new design paradigms for the SAGIN architecture, significantly influencing the development of future networks. SDN addresses the complexity caused by the lack of transparency in traditional distributed control networks through a centralized controlapproach. Its core concept is to decouple the control plane from the data plane and manage the network via programmable interfaces. SDN controllers can provide detailed network state parameters and exercise precise control over the network [19]. However, relying solely on software programming is insufficient to fully resolve network complexity. Current research efforts are focused on advancing programmable hardware chips and programming languages to further enhance the capabilities of SDN.