Multi-Satellite Cooperative Communications for 6G: Fundamentals, System Design, and Applications

Recent advances in satellite technologies and non-terrestrial networks have made multi-satellite cooperative communications a promising approach for enhancing quality of service in satellite networks. To meet the ever-increasing demands for global coverage, ultra-reliable connectivity, and low-latency services, cooperation among multiple satellites is a key enabler for overcoming the limitations of single-satellite service systems. With its growing importance, both applications and opportunities are simultaneously manifesting. This paper presents a comprehensive survey of research on multi-satellite cooperative communications. In particular, we first introduce architectures for multi-satellite cooperative networks, including intralayer, inter-layer, and cross-layer architectures. Subsequently, we comprehensively analyze and discuss their operating modes, including cooperative relaying, coordinated joint transmission, cooperative reception, beamforming, and cooperative interference management. Additionally, optical inter-satellite links are being considered for cooperative communications to enhance satellite interoperability. We then introduce the challenges and corresponding solutions for implementing multi-satellite cooperative communications. Furthermore, we explore enabling technologies for multi-satellite networks, including multi-connectivity, cooperative satellite beam-hopping, multi-satellite beamforming, traffic offloading, and management of satellite topology and constellation graphs. To further enhance the evaluation of the multi-satellite network, we discuss performance metrics, analysis tools, optimization methods, and validation. Finally, we outline several compelling research directions for future multi-satellite cooperative communication networks.