Channel Measurements and Sparsity Analysis for Air-to-Ground mmWave Communications

A channel measurement system for accurate modeling of the millimeter-wave (mmWave) band air-to-ground (A2G) wireless channel is presented. The measurement system consists of a ground station on a small cart and an air station installed on a custom-made octocopter unmanned aerial vehicle (UAV). Channel sounders at 26 GHz mmWave band with a bandwidth of 1 GHz are mounted on the air station and the ground station. In contrast to the on-the-shelf drones, the custom-made UAV offers the advantages of greater payload capacity and extended flight endurance. The measurement campaign was conducted in a university campus scenario, providing valuable measurement data for further analyses of the A2G channel. To analyze multipath effects in the A2G channel, simulation result of a ray tracing (RT) model is used to identify the principal propagation effects in the actual measurements. Subsequently, the path-loss exponent and the standard deviation of shadowing are calculated based on the measurement. Furthermore, sparsity of the A2G mmWave channel is evaluated through analyses of the Gini index and the Rician
$K$
factor. By comparing with the measurement results in the different propagation scenarios presented in the related work, e.g., industrial Internet-of-Things (IIoT), vehicular urban, and laboratory room, unique characteristics of the A2G mmWave channel are discussed.