Machine-type Direct-to-Satellite Communications: Modeling and Performance Analysis

Despite the massive progress in 5G and beyond systems, the deployed terrestrial networks do not provide ubiquitous coverage even on land; the situation is even worse in the open sea, which covers 70% of the Earth's surface. This disparity illustrates a significant digital divide. To address this vital need, global wireless coverage stands as one of the most crucial requirements for the upcoming 6G wireless networks to offer connectivity to both human and IoT machines.

This thesis advocates massive machine-type communication (mMTC) and low Earth orbit (LEO) satellite integration to enable direct connectivity from low-cost, low-power end devices to satellites without relying on a terrestrial network. This emerging form of new connectivity, machine-type direct-to-satellite (DtS) communications, promises to bridge the digital divide. However, DtS presents exciting challenges which need to be carefully examined and addressed. Among these challenges is LEO satellite mobility, which introduces a strong Doppler effect and time-varying channel conditions. Similarly, the long link distance ranging from hundreds to thousands of kilometers and the wide satellite footprint contributes to high propagation losses and massive interference, respectively. Additionally, terrestrial end devices possess limited energy resources. Therefore, successful DtS operation requires high energy efficiency, allowing the low-power signals to reach satellites orbiting thousands of kilometers away from Earth.

Motivated by the low-power and long-range capabilities of LoRaWAN, this thesis selected LoRa and LR-FHSS-based solutions for the modeling. This thesis develops novel Monte Carlo simulation and analytical models for DtS communications performance analysis. The results confirmed the feasibility and potential of both LoRa and LR-FHSS for DtS communications and highlighted the trade-off of different parameter configurations. In summary, LR-FHSS reveals better performance compared to LoRa, primarily due to its high sensitivity, robust Doppler resistance, and increased capacity, making it a suitable choice for DtS communications.

LR-FHSS end devices are expected to be powered by battery, therefore, it is vital to investigate the energy consumption of LR-FHSS. This work conduct experiments using real-life end devices and measures the LR-FHSS air-time and current consumption. We leverage these empirical measurement results to develop analytical models which give us the energy efficiency and battery lifetime of LR-FHSS end devices. LR-FHSS air-time model is highly important for accurate collisions modeling and scalability analysis.