A Survey on Random Access Protocols in Direct-Access LEO Satellite-Based IoT Communication

Low-Earth orbit (LEO) satellites can play an important role in providing seamless coverage for the Internet of Things (IoT). In satellite-based IoT (SIoT) networks, IoT devices can communicate directly with a satellite or through a gateway, which is called direct-access SIoT and indirect-access SIoT, respectively. As the number of IoT devices requiring satellite access is increasing, the role of medium access control (MAC) protocols becomes critical in reducing the latency and improving the quality of service (QoS). In this tutorial, we provide a comprehensive review of random access (RA) protocols, more specifically, grant-free RA (GFRA) protocols, which are more efficient in handling the communication requirements of SIoT networks. We discuss the challenges that arise in designing RA protocols under time-frequency resource and preamble limitations, high mobility of satellites, sporadic traffic from IoT networks, and diverse QoS requirements of IoT applications. We also highlight future research directions, including cross-layer optimization, joint activity detection and channel estimation (JAD-CE), reinforcement-learning-based solution, protocol design for dense LEO satellite networks, and reconfigurable intelligent surface (RIS) aided SIoT.