Empirical Characterization of Air-to-ground Propagation at mm-Wave Frequencies in Dense Urban Environment Enrico M. Vitucci * , Vasilii Semkin †‡ , Maximilian J. Arpaio * , Marina Barbiroli * , Franco Fuschini * , Claude Oestges ‡ , and Vittorio Degli-Esposti * * Dipartimento dell’Ingegneria Elettrica e dell’Informazione, Universit` a di Bologna, Italy † VTT Technical Research Centre of Finland Ltd., 02150 Espoo, Finland, vasilii.semkin@vtt.fi ‡ ICTEAM, Universit´ e catholique de Louvain, 1348 Louvain-la-Neuve, Belgium, claude.oestges@uclouvain.be Abstract—In the present study, a measurement setup utilizing mm-wave transceivers with steerable directive antennas, mounted on both a customized UAV and a ground station has been used to study Air-to-Ground (A2G) radio links and, more generally, full-3D mm-wave propagation in urban environment. We evaluate the double-directional characteristics of the channel by rotating the antennas, deriving Power-Angle Profiles at both link ends. Preliminary results provide useful understanding of A2G propagation, e.g. the influence of the antenna tilt angles, or the mechanisms allowing for the signal to propagate from street canyons to the air. Index Terms—urban propagation, air-to-ground measure- ments, UAV-aided wireless communications I. I NTRODUCTION The use of low-altitude, Unmanned Aerial Vehicles (UAV) in wireless communication, safety and sensing applications has been proposed by several researchers in recent years, and identified as one of the solutions for 5G key scenar- ios [1], [2]. Important advantages of UAV-aided solutions are their flexibility and the absence of a fixed infrastructure, a particularly attractive characteristic for temporary, on-demand services and for disaster-recovery applications. One of the prerequisites for the realization of UAV-aided wireless systems is the availability of reliable channel models for a large variety of environments, frequencies and UAV heights. Several experimental investigations have been carried out for the characterization of air-to-ground (A2G) propagation, especially in the last few years [3]. Among the recent studies, several of them addressed rural or open-field propagation [4], [5] and propagation in university campuses and sub-urban areas [6]–[9]. Only a few studies have addressed A2G propagation in actual urban areas, probably due to the inherent difficulties in getting authorizations to fly on densely populated zones and/or close them to public during measurements [10]– [12]. In such studies, the analysis is mainly focused on large- scale parameters such as path-loss, fading statistics and spatial correlations. Existing studies – including those carried out in university campuses - are limited to UHF or sub-mmWave frequencies, and to our best knowledge, none of them in- vestigated the double-directional characteristics of the A2G channel, probably due to the problems related to mounting and operating directive antennas or large MIMO transceivers on the UAV. The present study is aimed at filling up some knowledge gaps in the field, and in particular the characterization of low- altitude A2G propagation in urban areas, where the presence of buildings has a strong impact on propagation, with focus on the channel’s directional characteristics at two mm-wave frequencies, 27 and 38 GHz, that are quite popular for having been recently allocated to 5G systems, although only 27 GHz results will be reported in the results section for the sake of brevity. Directive antennas at mm-wave frequencies can be rel- atively small and can help overcome power-budget limitations typical of UAV-aided communications. However, a thorough knowledge of the directional characteristics of the channel at both link-ends and advanced beam-steering techniques will be required to design and operate such systems. The measurement setup of the present work is composed of a custom quadcopter equipped with a GPS plus Real-Time Kinematic (RTK) localization/navigation system, a directive horn antenna and a mm-wave portable spectrum-analyzer. An Ultra-Wide Band transceiver is also mounted on the drone to measure the channel’s time-domain characteristics, although in the lower band of 3.1-5.3 GHz. The ground station consists of the specular link-end with a directive antenna, a mm-wave generator and an UWB transceiver. Directive antennas are rotated in the azimuth or elevation planes at the ground station using a rotating positioner, while an automatic gimbal is used on the drone for 3D steering capabilities. After the description of the measurement setup, some pre- liminary results are reported in the paper for a few cases of interest, including the Power-Angle Profiles (PAP) at both the ground station and at the drone in a street canyon for different drone heights. Besides being important for the design of future UAV-aided wireless systems, this setup and these measurements can also be of great interest for the study of full 3D mm-wave urban propagation, as the drone can mimic a base station or a user arXiv:2101.01061v1 [eess.SP] 4 Jan 2021