Measurement-Based High-Altitude UAV Air-to-Ground Channel Characterization and Modeling for Different Flight Maneuvers at 28 GHz

As one of the components of the Low Altitude Platforms (LAPs) and High Altitude Platforms (HAPs) for Non-Terrestrial Networks (NTNs), Unmanned Aerial Vehicles (UAVs) have become a key player for future six-generation (6G) networks. Understanding the Air-to-Ground (A2G) propagation channels is essential to enable reliable wireless communication systems between Ground Stations (GSs) and UAVs. In particular, due to the scarcity of available bandwidth, higher frequency bands, such as millimeter wave (mmWave), are essential. However, in previous studies, most of the high-altitude and highspeed UAV channel models in mmWave bands are solely based on simulations. To fill the gap, in this paper, A2G channels have been evaluated by means of a fixed-wing UAV measurement campaign considering three different heights (1500 m, 2000 m, 2400 m) and two different maneuvers, namely cruising and turning, in a rural scenario at Ka-band (28 GHz) and with 420MHz bandwidth. The multipath components (MPCs) are extracted by the space-alternating generalized expectation-maximization (SAGE) algorithm and used as the basis for developing a complete stochastic channel model. This model explicitly characterizes key parameters, including Path Loss (PL) with its exponent, Rician K-factor, Root-Mean-Square Delay Spread (RMS-DS), and Root- Mean-Square Doppler Frequency Spread (RMS-DFS) for different flight altitudes and aircraft attitudes. Due to large bandwidth considered, the increased delay resolution allowed us to provide through analyses of the exhibited propagation effects, such as the MPCs caused by the airframe. Furthermore, the models specifically include the effects of the attitude characteristics for each flight maneuver. For example, the PL model incorporated correction factors related to the changing flight attitude of the aircraft. As a result, the provided models constitute a complete framework for the simulation and evaluation of high-altitude and high-speed A2G channels, which are of utmost importance for the design and deployment of 6G NTN systems.