An Overview of Direct-to-Satellite IoT: Opportunities and Open Challenges

Direct-to-Satellite (DtS) connectivity is one of the further components of the development towards the 6G communications networks. It can enable global wireless coverage for Internet of Things (IoT) applications while complementing terrestrial networks. Specifically, DtS can offer seamless connectivity for the machine applications operating in remote areas where terrestrial coverage is limited or unavailable. This paper reviews the DtS IoT networks state-of-the-art and offers an overview focusing on its fundamental components and key requirements. Particularly, we discuss the two fundamental architecture alternatives namely, DtS and indirect-to-satellite (ItS) for supporting IoT services and pinpoint their key trade-off, pros, and cons. We also present the potential DtS applications and their relevant data traffic models and requirements. Finally, we identify some open challenges including (i) link budget aspects; (ii) spectrum limitations and regulations; (iii) massive connectivity demands and interference; and (iv) synchronization problems owing to satellite mobility. This paper also presents several potential future research directions in the context of DtS IoT networks.