Reconfigurable Intelligent Surfaces for Space-Air-Ground Integrated Network: Architecture, Opportunities, and Challenges

Space-air-ground integrated network (SAGIN) is an emerging network architecture designed to provide large-scale, flexible wireless coverage and seamless high-speed connectivity, thereby addressing the limitations of terrestrial communication systems. To ensure reliable and efficient communication in SAGIN, the advancement of complementary hardware and software technologies is critical. Reconfigurable intelligent surfaces (RIS), an innovative technology enabling precise control over electromagnetic wave propagation, hold significant potential to enhance spectral efficiency, expand coverage, and strengthen security. Integrating RIS into SAGIN is projected to improve the performance of intra-layer and inter-layer communications substantially. In this paper, we propose three RISs: satellite RIS (S-RIS), aerial RIS (A-RIS), and ground RIS (G-RIS). Adopting RIS in SAGIN represents a crucial advancement for enhancing cross-layer communication quality. Furthermore, we discuss the opportunities and challenges for future research in this field.