Sixth-generation (6G) communication systems are expected to support direct-to-device (D2D) connectivity, enabling standard user equipment (UE) to seamlessly transition to non-terrestrial network (NTN), particularly satellite communication mode, when operating beyond terrestrial network (TN) coverage. This D2D concept does not require hardware modifications to conventional UEs and eliminates the need for dedicated satellite ground terminals. D2D-capable UEs can be mounted on both manned and unmanned aircraft, however, they are especially well-suited for low-altitude unmanned aircraft due to their compact form factor, lightweight design, energy efficiency, and TN-NTN roaming capabilities. D2D can also enable beyond-visual-line-of-sight operation by providing NTN support for Communications, Navigation, and Surveillance (CNS) services during TN outages or congestion. This paper investigates the capabilities and limitations of D2D connectivity for low-altitude unmanned aircraft operating in urban environments. We analyze the variation in line-of-sight probability for both TN and NTN links as a function of aircraft altitude. We further compute path loss and received signal strength while accounting for a representative TN deployment with down-tilted antennas. The results show that the TN and NTN links complement each other, significantly improving the availability of the CNS service at low altitudes. These findings provide insights to support the design and optimization of future 6G-enabled integrated CNS services.

翻译：第六代（6G）通信系统有望支持面向设备（D2D）连接，使标准用户设备（UE）在超出地面网络（TN）覆盖范围时，能够无缝切换至非地面网络（NTN），尤其是卫星通信模式。该D2D概念无需对传统UE进行硬件改造，也无需配备专用卫星地面终端。具备D2D能力的UE可搭载于有人及无人飞行器上，尤其适用于低空无人机——因其紧凑的外形、轻量化设计、高能效以及TN-NTN漫游能力。D2D还能在TN中断或拥塞期间，为通信、导航与监视（CNS）服务提供NTN支持，从而实现超视距运行。本文研究了城市环境下低空无人机D2D连接的能力与局限性。我们分析了作为飞行高度函数的地面链路与NTN链路的视距概率变化，并进一步在考虑典型地面网络部署（采用下倾天线）的前提下，计算了路径损耗与接收信号强度。结果表明，地面链路与NTN链路相互补充，显著提升了低空环境下CNS服务的可用性。这些发现为未来6G赋能的一体化CNS服务的设计与优化提供了参考依据。