I. Introduction
Artificial intelligence (AI) and wireless communication technology have rapidly spread in recent years. People all over the world have enjoyed the convenience brought by network services [1]. Especially in order to achieve the global coverage of wireless networks, a new type of space-air-ground integrated network (SAGIN) has been proposed to provide three-dimensional comprehensive connection services [2]. SAGIN needs to realize the seamless integration of space-based network, air-based network and ground-based network. Achieving ultra-reliable, low-latency wireless communication is an important goal of SAGIN. Satellites and unmanned aerial vehicles (UAVs) participate in network construction and provide services as air users or base stations (BSs) [3]. As an emerging wireless network architecture and new service paradigm, SAGIN can provide a reliable, stable and robust service supply for end users worldwide. A typical SAGIN scenario is shown in Fig. 1. The performance of SAGIN is largely dependent on 6G. SAGIN supported by 6G can bring benefits such as global coverage, ultra-low latency and high connection density, but 6G technology is not yet mature [4]. Especially for network operators, how to coordinate, manage and schedule the network resources of SAGIN is a severe challenge [5].