In order to implement unmanned aerial vehicle (UAV)-based wireless communications, a better understanding of wireless channels and the corresponding channel characterizations are critical. In this paper, air-to-ground (AG) channel measurements are carried out at some candidate frequencies, i.e., 1 GHz, 4 GHz, 12 GHz, and 24 GHz. With measurement data, the crucial channel parameters are comprehensively analyzed. Firstly, based on the channel model in the 3rd Generation Partnership Project (3GPP), the essential coefficients for modeling path loss, including path loss exponents (PLEs) and heightdependent factors, are obtained for AG channels. Then, a novel autocorrelation model for shadow fading is proposed. Besides, the small-scale fading is statistically analyzed, where the log-logistic distribution is found as the best fit among popular distributions. Moreover, the second-order statistical characteristics, including the level crossing rate (LCR) and average fade duration (AFD), are extracted to describe indepth the fading behavior. Overall, the results and findings in this paper are essential for realizing reliable communications in AG wireless systems. INDEX TERMS Air-to-ground (AG) channel, characterizations, fading behavior, measurements, UAV.
Unmanned aerial vehicles (UAVs) have been applied to various promising applications because of their features of high‐mobility, portability, rapid deployment, and modest power consumption. Small UAVs fly at low altitudes and typically short distances from the ground controller. It is of interest and value to model the propagation channel between low‐altitude UAVs and ground stations. This article presents results of channel measurements at 3.9 GHz carried out in a suburban environment with a small‐size UAV flying at low altitudes, up to 40 m. The authors also present comparative results from ray‐tracing simulations. Height‐dependent models for path loss, root mean square (RMS) delay spread, and the number of multipath components are provided. The results illustrate that although multipath effects exist at low altitudes in this environment, the two‐ray path loss model works reasonably well in describing the channel behaviour. Initial ray tracing simulations show reasonable agreement with measurements. In addition, RMS delay spread results indicate that distant scatterers have an appreciable effect on the UAV air‐ground (AG) propagation channels, which can be different from some terrestrial channels. Authors’ results should be of use in the modelling of low‐altitude AG propagation channels and in the performance analysis of UAV‐enabled AG communication systems.
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