Direct satellite-to-device communications: technical routes, architecture, and enabling technologies

The rapid evolution of mega-constellation satellite-integrated Internet is shaping the landscape of sixth-generation (6G) networks, where direct satellite-to-device communication (DSDC) promises indeed seamless coverage and ubiquitous connectivity. However, the fragmented, rigid satellite architectures, and sparse, limited network resources hinder the existing satellite systems from delivering high-speed, pervasive intelligent services for DSDC in a sustainable and cost-effective manner. To address these issues, distributed satellite information networks (DSIN) have emerged as a transformative paradigm to integrate and expand current isolated space networks, enabling the envisioned broadband DSDC. This survey first provides a profound discussion on technical routes for DSDC, along with their adopted network architectures, covering transparent forwarding, partially and fully regenerative processing, laying the foundation for an innovative distributed multi-satellite collaborative transmission architecture, as well as reconfigurable satellite formations designed for DSDC. To tackle the challenges of dynamic channels, heterogeneous resources, hardware-constrained satellites and mobile terminals (MTs) within a collaborative framework, a series of enabling technologies is identified in three areas. First, adaptive enhancements of air interface technologies, encompassing channel estimation, robust multi-beamforming, channel coding, spectrum sharing, massive random access, and coherent transmission, are essential pillars for establishing high-speed connectivity between MTs and satellites. Subsequently, network management techniques developed for mobility management, resource allocation, distributed routing, and congestion control can ensure consistent, stable, and uninterrupted network services for MTs. Further, next-generation satellite payloads and MTs, including conformal phone antenna, spaceborne array antenna and onboard processing, are pivotal for unlocking the full potential of DSDC. Finally, emerging research directions and innovative technologies in realizing the DSDC vision are discussed.