Fixed-Wing UAV-Based Air-to-Ground Channel Measurement and Modeling at 2.7 GHz in a Rural Environment

Fixed-wing unmanned aerial vehicle (UAV) is widely considered as a vital candidate of aerial base station in beyond 5th-Generation (B5G) system due to its longer flight endurance and higher cruise altitude. Investigating and modeling the air-to-ground (A2G) wireless channel for fixed-wing UAV can better promote its development in various applications. In this article, we present a wideband A2G channel measurement campaign for fixed-wing UAV with a maximum altitude of 700 m. A comprehensive investigation of channel fading characteristics and channel modeling including path loss, shadow fading, spatial correlation, and small-scale fading is provided. Particularly, unlike terrestrial channels, a strong dependence of shadow fading and small-scale fading on UAV altitude is found. We propose to adopt Gamma distribution for modeling the shadow fading effect that has a similar fitting performance with traditional Log-normal distribution but is more robust for theoretical analysis. Spatial correlation characteristics and modeling are presented. Results reveal that the decorrelation distance of shadow fading increases with the UAV altitude. We propose to use a model combining the exponential and sinusoidal functions that fit well with the autocorrelation function for shadow fading. Further, by using Kolmogorov-Smirnov (KS) test, Cramer-von Mises (CVM) test, and Akaike information criterion (AIC) methods, the Rician distribution is found to be the best candidate for modeling small-scale fading for UAV with altitudes above 300 m. The Rician K-factor is found to be strongly depending not only on the direct distance but also on the UAV altitude. We finally propose a full-dimension empirical K-factor prediction model.