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This paper presents an experimental study of the air-to-ground (AG) propagation channel through ultrawideband (UWB) measurements in an open area using unmanned-aerial-vehicles (UAVs). Measurements were performed using UWB radios operating in the frequency range of 3.1 GHz-4.8 GHz and UWB planar elliptical dipole antennas having an omni-directional pattern in the azimuth plane and typical donut shaped pattern in the elevation plane. Three scenarios were considered for the channel measurements: (i) two receivers (RXs) at different heights above the ground and placed close to each other in line-of-sight (LOS) with the transmitter (TX) on the UAV and the UAV is hovering; (ii) RXs in obstructed line-of-sight (OLOS) with the UAV TX due to foliage, and the UAV is hovering; and, (iii) UAV moving in a circular path. Different horizontal and vertical distances between the RXs and the TX were used in the measurements. In addition, two different antenna orientations were used on the UAV antennas (vertical and horizontal) to analyze the effects of antenna radiation patterns on the UWB AG propagation. From the empirical results, it was observed that the received power depends mainly on the antenna radiation pattern in the elevation plane when the antennas are oriented in the same direction, as expected for these omni-azimuth antennas. Moreover, the overall antenna gain at the TX and RX can be approximated using trigonometric functions of the elevation angle. The antenna orientation (polarization) mismatch increases path loss, and produces a larger number of weak multipath components (MPCs) than when aligned. Similarly, additional path loss and a larger number of MPCs were observed for the OLOS scenario. In the case of the UAV moving in a circular path, the antenna orientation mismatch has smaller effects on the path loss than when the UAV is hovering, because a larger number of cross polarized components are received during motion. A statistical channel model for UWB AG propagation is built from the empirical results.

UWB Air-to-Ground Propagation Channel Measurements and Modeling Using UAVs