The space-air-ground integrated network (SAGIN) is a deep integration of satellite, aerial, and terrestrial communication systems. The satellite communication system comprises a high-, medium-, and low-orbit satellite constellation, while the aerial network consists of high-altitude platforms, civil aviation aircraft, and unmanned aerial vehicles. The terrestrial communication system is composed of the ground-based components of the satellite and aerial networks, as well as cellular mobile-communication networks. The SAGIN can overcome the limitations of each segment via collaborative planning and thereby provide large-capacity, high-rate, and seamless communication services for air traffic.