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.

翻译：低地球轨道（LEO）卫星在泛在物联网应用中的普及，以及无人机在应急服务、农业和军事行动中的日益广泛使用，凸显了非地面网络（NTN）的变革潜力。然而，这些网络面临两大关键挑战：（1）广阔的覆盖范围导致频繁的传输碰撞；（2）移动网关引发动态链路变化，要求传输机制无需同步且具备链路感知能力。现有的随机接入方案（如ALOHA、CSMA和BSMA）在此场景下均存在局限，表现为高碰撞率、隐藏终端问题或过高的网关能耗开销。本文提出自由信号多址接入（FSMA）协议，该网关控制协议引入了一种轻量级的自由信号啁啾（FreeChirp）。FreeChirp确保节点仅在信道空闲且链路可靠时传输，从而降低碰撞率，并在无需同步或复杂调度的前提下实现链路感知接入。我们使用25个商用LoRa设备与无人机搭载的移动网关对FSMA进行评估，实验表明相较于基线方案，FSMA可实现最高2倍的吞吐量提升、2至5倍的数据包接收率改善以及5倍的能效提升。基于定制卫星物联网仿真器的大规模仿真进一步证明，FSMA可扩展至单次卫星过顶服务5000台以上设备的能力。这些结果表明FSMA为实现可扩展、高能效、可靠的非地面物联网网络迈出了切实可行的一步。