Extending the internet of things (IoT) networks to remote areas under extreme conditions or for serving sometimes unpredictable mobile applications has increased the need for satellite technology to provide effective connectivity. However, existent medium access control (MAC) protocols deployed in commercial satellite networks were not designed to offer scalable solutions for the increasing number of devices predicted for IoT in the near future, nor do they consider other specific IoT characteristics. In particular, CubeSats—a low-cost solution for space technology—have the potential to become a wireless access network for the IoT, if additional requirements, including simplicity and low demands in processing, storage, and energy consumption are incorporated into MAC protocol design for satellite IoT systems. Here we review MAC protocols employed or proposed for satellite systems and evaluate their performance considering the IoT scenario along with the trend of using CubeSats for IoT connectivity. Criteria include channel load, throughput, energy efficiency, and complexity. We have found that Aloha-based protocols and interference cancellation-based protocols stand out on some of the performance metrics. However, the tradeoffs among communications performance, energy consumption, and complexity require improvements in future designs, for which we identify specific challenges and open research areas for MAC protocols deployed with next low-cost nanosatellite IoT systems.

The Internet of Things (IoT) is enabling unprecedented applications based on the transport of small data volumes to and from constrained devices. When end-devices or sensor nodes are located in very remote zones with inaccessible topography, Direct-to-Satellite IoT (DtS-IoT) has been proposed as an appealing solution. In DtS-IoT, isolated sensor nodes can directly relay data to and from inexpensive nanosatellites (i.e., CubeSats). Because both sensor nodes and CubeSats operate on very limited energy supply and storage, the efficient management of power-hungry communication sub-systems is of the essence. Thus, specific Medium Access Control (MAC) protocols are needed to ensure the minimal overhead while considering the DtS-IoT scalability and channel dynamics. In this work, we contribute with REserveand-Send Sift-IoT (RESS-IoT), a scalable and energy-efficient DtS-IoT MAC protocol, combining LoRa physical layer with a novel link scheduling approach. Extensive simulations demonstrate that our solution provides up to four times energy savings in the satellite, and up to seven times on the sensor node on ground, with respect to state-of-the-art IoT protocols.INDEX TERMS Direct-to-Satellite IoT (DtS-IoT), Energy efficiency, LoRa, Medium access control (MAC), Nanosatellite