Understanding the Limits of LoRa Direct-to-Satellite: The Doppler Perspectives

The Long Range (LoRa) modulation enables low-cost and low-power communications, serving as the foundation for the widely adopted terrestrial low-power wide-area network (LPWAN) technology known as LoRaWAN. Owing to its effectiveness, this modulation scheme is emerging as a potential option to provide direct-to-satellite (DtS) connectivity supporting Internet-of-Things (IoT) applications in remote or hard-to-reach areas, and complementing existing terrestrial networks. Besides the link budget and interference, the Doppler effect is one of the main challenges in LoRa DtS connectivity. Earlier studies have extensively investigated the link budget and the network scalability aspects, confirming the feasibility of integrating LoRa with Low Earth Orbit (LEO) satellites. However, only a few studies examine the influence of the Doppler effect on LoRa DtS performance. Specifically, the majority of the available literature report empirical studies that analyze the Doppler effect solely for a specific set of communication parameters. There remains a need for extensive and comprehensive examination of LoRa DtS performance under a strong Doppler effect in the LEO scenario. In this paper, we discuss and thoroughly investigate the impact of the Doppler effect on the reliability of LoRa satellite links. In particular, we analytically study packet losses, distinguishing the effect of Doppler shift from Doppler rate, the latter being caused by the variation in the relative speed of LEO satellites with respect to a terrestrial IoT end-device. Our analysis accounts for the effects of key communications parameters and settings, such as bandwidths, carrier frequency, MAC payload, LEO satellite's orbital height, and LoRaWAN low data rate optimization (LDRO). Notably, the results identify the LoRa boundaries for direct to LEO satellite connectivity and can facilitate the selection of suitable parameters for future system designs. Specifically, our results demonstrate that the packet delivery ratio of the most vulnerable spreading factor, i.e., SF12, exceeds 82% when using 125 kHz bandwidth, 433 MHz carrier frequency, and 59 bytes payload for a satellite orbiting at 560 km height.