Space-Air-Ground Network for Direct-to-Cell Communication

The growing demand for ubiquitous and resilient wireless connectivity, particularly in remote or disaster-stricken areas, highlights the limitations of traditional Direct-to-Cell (D2C) communication systems, which suffer from capacity bottlenecks, high propagation losses, and unreliable coverage. These challenges are critical barriers to achieving seamless global communication and supporting applications such as emergency response, smart infrastructure, and industrial automation, where robust connectivity directly impacts safety, operational efficiency, and societal resilience. To this end, we propose a UAV-enabled Space-Air-Ground Integrated Network (SAGIN) framework, in which UAVs operate as adaptive relays between low-earth-orbit satellites and user terminals. Our system leverages altitude-optimized UAV placement and environment-aware channel modeling, incorporating realistic ionospheric, atmospheric, multipath, and fading effects to dynamically mitigate propagation losses and optimize link reliability. We validate our approach using the NVIDIA Sionna simulator by conducting experiments across diverse environments and disaster-response scenarios. Our results demonstrate that the proposed SAGIN framework achieves a 91.7% improvement in downlink throughput compared to baseline D2C model, with a graceful degradation of 12ms end-to-end latency.