A Survey on Direct-to-Device Satellite Communications: Advances, Challenges, and Prospects

Direct-to-Device (D2D) communication in satellite networks represents a significant advance in telecommunications, enabling seamless connectivity without relying on terrestrial infrastructure. This survey aims to provide a detailed overview of D2D communication technologies, protocols, applications, and recent advances based on the current state of D2D. We categorize the literature on D2D into a new taxonomy. This taxonomy covers technological foundations, protocols and algorithms, use cases, challenges, and future trends. We emphasize the key challenges and pinpoint research gaps within each category, offering a well-structured domain overview.