FSMA: Scalable and Reliable LoRa for Non-Terrestrial Networks with Mobile Gateways

TL;DR

Non-terrestrial IoT over LoRa faces severe collisions over vast satellite footprints and rapidly changing link conditions due to moving gateways. FSMA introduces a synchronization-free, gateway-controlled MAC that uses a single FreeChirp as a control signal to coordinate channel access and employs link-aware transmissions via channel reciprocity, reducing collision windows and enabling decoding through the capture effect. Hardware experiments with 25 off-the-shelf LoRa devices and a drone gateway show up to 2× throughput, 2.5–5× PRR gains, and up to 5× energy efficiency improvements; large-scale NTNLoRa simulations demonstrate scalability to 5000+ devices per satellite pass with substantial throughput and reliability gains. Together, FSMA offers a practical, energy-efficient solution for scalable NTN IoT that maintains compatibility with existing devices and firmware updates.

Abstract

The proliferation of Low Earth Orbit (LEO) satellites for universal IoT applications and the growing use of drones in emergency services, agriculture, and military operations highlight the transformative potential of non-terrestrial networks (NTN). However, these networks face two key challenges: (1) large coverage footprints that create frequent collisions and (2) moving gateways that cause dynamic links and demand synchronization-free, link-aware transmissions. Existing random access schemes such as ALOHA, CSMA, and BSMA fail in this setting, suffering from high collision rates, hidden terminals, or excessive gateway energy overhead. We propose Free Signal Multiple Access (FSMA), a gateway-controlled protocol that introduces a lightweight free signal chirp (FreeChirp). FreeChirp ensures that nodes transmit only when the channel is idle and when links are reliable, thereby reducing collisions and enabling link-aware access without the need for synchronization or complex scheduling. We evaluate FSMA using 25 commercial LoRa devices with a drone-mounted moving gateway and demonstrate up to 2x higher throughput, 2x to 5x better packet reception ratio, and 5x improved energy efficiency compared to the baselines. Large-scale simulations with a custom Satellite IoT Simulator further show that FSMA scales to 5000+ devices per satellite pass. These results establish FSMA as a practical step toward scalable, energy-efficient, and reliable NTN IoT networks.

Figures (19)

• Figure 1: Non-terrestrial LoRa networks experience packet loss from both collisions and link failures: In (a), Satellite's vast footprint causes numerous devices to compete for the same channel, leading to collisions. In both (a) and (b), gateway mobility creates dynamic links that alternate between high SNR, low SNR, and outage phases. FSMA addresses these challenges by reducing collisions and enabling link-aware transmissions through gateway-controlled access.
• Figure 2: Collision window comparison. In baseline LoRa (ALOHA, CSMA), collisions span the entire packet duration (20–400 symbols). FSMA confines contention to the sensing window (<4 symbols), reducing the collision window by 5$\times$-100$\times$, depending on packet length.
• Figure 3: Link-aware transmissions in FSMA. A low-SF FreeChirp enables only nodes with strong links to transmit at higher SFs, leveraging channel reciprocity for reliable uplinks. Unlike static terrestrial gateways, the moving gateway shifts coverage over time, ensuring fair access across devices.
• Figure 4: Quantifying the large footprint and dynamic link behaviour challenges using data packets of Norby, Fossa, and Tianqi satellites, received on tinyGS ground stations tinyGSNorby2Packet2025April22tinyGSsatellites. (a) shows coverage exceeding 3000km, (b) demonstrates varying active nodes with time, a 100x drop in 5-minute window, and (d) illustrates varying link and outage phases.
• Figure 5: The figure demonstrates three random access MAC protocols for LoRa: ALOHA, CSMA, and BSMA.