Direct-to-satellite internet of things (DtS-IoT): a tutorial review on architectures, protocols, and future directions

The Internet of Things (IoT) is increasingly used in agriculture, logistics, smart cities, environmental monitoring, and industrial automation. However, many remote, rural, maritime, and disaster-prone regions remain disconnected because deploying terrestrial infrastructure is prohibitively expensive or technically infeasible. Direct-to-Satellite IoT (DtS-IoT) addresses this gap by leveraging Low Power Wide Area Network (LPWAN) technologies, such as adapted LoRa and Narrowband IoT (NB-IoT), which both enable direct communication between IoT devices and Low Earth Orbit (LEO) satellites and provide the long-range, low-power operation required for such links. This tutorial paper provides a structured and accessible introduction to DtS-IoT. Building on recent literature, it covers seven major themes: (i) system architectures, (ii) constellation design, (iii) LPWAN protocols, (iv) key challenges, (v) simulation tools, (vi) experimental studies, and (vii) future directions and open research issues. The goal is to guide the reader through the foundational concepts, design principles, and current technological landscape of DtS-IoT, offering a comprehensive set of references for further study.