  Unmanned aerial vehicles (UAVs) are widely applied in multiple fields, which
emphasizes the challenge of obtaining UAV flight information to ensure the
airspace safety. UAVs equipped with automatic dependent surveillance-broadcast
(ADS-B) devices are capable of sending flight information to nearby aircrafts
and ground stations (GSs). However, the saturation of limited frequency bands
of ADS-B leads to interferences among UAVs and impairs the monitoring
performance of GS to civil planes. To address this issue, the integration of
the 5th generation mobile communication technology (5G) with ADS-B is proposed
for UAV operations in this paper. Specifically, a hierarchical structure is
proposed, in which the high-altitude central UAV is equipped with ADS-B and the
low-altitude central UAV utilizes 5G modules to transmit flight information.
Meanwhile, based on the mobile edge computing technique, the flight information
of sub-UAVs is offloaded to the central UAV for further processing, and then
transmitted to GS. We present the deterministic model and stochastic geometry
based model to build the air-to-ground channel and air-to-air channel,
respectively. The effectiveness of the proposed monitoring system is verified
via simulations and experiments. This research contributes to improving the
airspace safety and advancing the air traffic flow management.
