In this paper, a recently conducted measurement campaign for unmanned-aerial-vehicle (UAV) channels is introduced. The downlink signals of an in-service long-time-evolution (LTE) network which is deployed in a suburban scenario were acquired. Five horizontal and five vertical flight routes were considered. The channel impulse responses (CIRs) are extracted from the received data by exploiting the cell specific signals (CRSs). Based on the CIRs, the parameters of multipath components (MPCs) are estimated by using a high-resolution algorithm derived according to the space-alternating generalized expectation-maximization (SAGE) principle. Based on the SAGE results, channel characteristics including the path loss, shadow fading, fast fading, delay spread and Doppler frequency spread are thoroughly investigated for different heights and horizontal distances, which constitute a stochastic model.Index terms-Unmanned aerial vehicle, air-to-ground channel, path loss, delay spread, and Doppler frequency spread. I. INTRODUCTIONHistorically, unmanned aerial vehicles (UAVs) were mainly used for military operations in hostile environments for safety reasons [1], [2]. Due to the decrease in the cost and size, UAVs are being more accessible for general-purpose civil and commercial applications, such as video surveillance, weather monitoring, search and rescue operations, precision farming, wildlife monitoring, and transportation, among others [3]. The network service restoration after infrastructure damage in natural disasters or base station (BS) relaying in crowded areas (one of the key scenarios addressed by fifth generation (5G) communication systems [1], [4]) also fit well [5], [6].Different communication links can be involved in the aforementioned applications of UAVs, such as:• Air-to-ground (A2G) communication channel between a ground BS and the UAV. This communication channel can serve to many different purposes, such as to give support to control and non-payload communications (CNPC) or communication and control link (C2) (see [7]-[10]), to serve data traffic for UAV-based applications (e.g., transmitting video surveillance data from UAVs to ground) or to relay traffic of a BS. Hence, the requirements in terms of availability, quality of service, latency or throughput can be very different. Moreover, the nature of the propagation channel is very different from that of

Due to the decrease in cost, size and weight, Unmanned Aerial Vehicles (UAVs) are becoming more and more popular for general-purpose civil and commercial applications. Provision of communication services to UAVs both for user data and control messaging by using off-the-shelf terrestrial cellular deployments introduces several technical challenges. In this paper, an approach to the air-to-ground channel characterization for low-height UAVs based on an extensive measurement campaign is proposed, giving special attention to the comparison of the results when a typical directional antenna for network deployments is used and when a quasi-omnidirectional one is considered. Channel characteristics like path loss, shadow fading, root mean square delay and Doppler frequency spreads and the K-factor are statistically characterized for different suburban scenarios.

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Multimedia and data-based services experienced a nonstopping growth over the last few years. People are continuously on the move using devices to access multimedia contents or other data-based services. Due to this, railway companies are showing a great interest in deploying broadband mobile wireless networks in high-speed-trains with the aim of supporting both passenger services provisioning as well as automatic train control and signaling. Nowadays, the most widely used technology for communications between trains and the railway infrastructure is GSM for Railways (GSM-R); however, it has limited capabilities to support such advanced services. Due to its success in the mass market, Long Term Evolution (LTE) seems to be the best candidate to substitute GSM-R. In this paper, we experimentally characterize the downlink between an LTE Evolved NodeB (eNodeB) and a high-speed train in a commercial high-speed line. We consider two links: the one between the eNodeB and the antennas placed outdoors on the train roof, and the direct link between the eNodeB and a receiver inside the train. Such a characterization consists in assessing the path loss, the Signal to Noise Ratio, the K-Factor, the Power Delay Profile, the delay spread, and the Doppler Power Spectral Density.