Sparse Satellite Constellation Design for Global and Regional Direct-to-Satellite IoT Services

In this article, we introduce and design sparse constellations for direct-to-satellite Internet of Things (DtS-IoT). DtS-IoT does not require a ground infrastructure, because the devices are directly connected to low earth orbit satellites acting as orbiting gateways. The key idea of sparse constellations is to significantly reduce the number of in-orbit DtS-IoT satellites by a proper dimensioning of the delivery delay anyway present in resource-constrained IoT services and an optimal positioning of the orbiting gateways. First, we analyze long-range modulation (LoRa)/LoRaWAN and narrowband Internet of Things (NB-IoT) standards and derive realistic constraints on the maximum gap time between two consecutive passing-by satellites. Then, we introduce and optimize an algorithm to design quasi-optimal topologies for sparse IoT constellations. Finally, we apply our design to both global and regional coverage and we analyze the tradeoff between latency, number of orbit planes, and total number of satellites. Results show that sparse constellations can provide world-wide IoT coverage with only 12.5 and 22.5% of the satellites required by traditional dense constellations considering 3 and 2-h gaps. Also, we show that region-specific coverage of Africa and Europe can be achieved with only four and three satellites for LoRa/LoRaWAN and NB-IoT, respectively.