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

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In this article, a novel vector network analyzer (VNA)-based ultrawideband (UWB) channel sounder using radio-over-fiber (RoF) techniques is presented. Benefiting from the usage of optical fiber cable, the measurement range and the dynamic range are significantly increased compared with the conventional coaxial-based VNA system. Specifically, using RoF increases the dynamic range to a maximum of 112 dB at 30 GHz for the back-to-back connection with an optical fiber cable of length 300 m. Moreover, a state-of-the-art phase compensation scheme using optical circulators is proposed for the first time. The novel scheme renders the channel sounder immune to stochastic phase changes in the optical fiber cable due to thermal changes and mechanical stress, thus permitting the remoting of virtual antenna arrays. The proposed channel sounder is experimentally validated in back-to-back measurements, an anechoic chamber, and practical indoor scenarios. The indoor channel measurements are conducted using a virtual uniform rectangular array (URA) at the millimeter-wave (mm-wave) band from 26.5 to 30 GHz. The measured results demonstrate the developed channel sounder's capability to perform UWB large-scale antenna array measurements with a long measurement range.

Millimeter wave (mm-wave) communication with large-scale antenna array configuration is seen as the key enabler of the next generation communication systems. Accurate knowledge of the mmwave propagation channels is fundamental and essential. In this contribution, a novel complexityefficient high resolution parameter estimation (HRPE) algorithm is proposed for the mm-wave channel with large-scale uniform circular array (UCA) applied. The proposed algorithm is able to obtain the high-resolution estimation results of the spherical channel propagation parameters. The prior channel information in the delay domain, i.e., the delay trajectories of individual propagation paths observed across the array elements, is exploited, by combining the high-resolution estimation principle and the phase mode excitation technique. Fast initializations, effective interference cancellations and reduced searching spaces achieved by the proposed schemes significantly decrease the algorithm complexity. Furthermore, the channel spatial non-stationarity across the array elements is considered for the first time in the literature for propagation parameter estimation, which is beneficial to obtain more realistic results as well as to decrease the complexity. A mm-wave measurement campaign at the frequency band of 28-30 GHz using a large-scale UCA is exploited to demonstrate and validate the proposed HRPE algorithm.X. Cai and W. Fan are with the APMS section at the Index terms-Millimeter-wave, ultra-wideband, large-scale uniform circular array, spatial nonstationarity and channel parameter estimation. I. INTRODUCTIONWhile the next generation communication system (5G) is still on its ascendant, millimeter wave (mm-wave) communication has been seen as the key enabling component due to the vast amount of available spectrum [1]-[3]. However, the air attenuation, small antenna aperture etc. would result in severe power loss [4] for the mm-wave propagations, compared to the sub-6 GHz frequency bands with rich multipath components (MPCs) [5]. Nevertheless, the mm-wavelength makes it practical to pack massive antennas [6] in a small area to form large scale antenna arrays. Beamforming [7] and beam-selection [2] techniques can be exploited to compensate the power loss and to enhance the spectrum efficiency through spatial multiplexing. Furthermore, applications such as localization, tracking and surrounding environment reconstruction [8] are promising, e.g., to help vision-disabled people.To enable the advanced 5G mm-wave techniques and applications, accurate and realistic channel models are fundamental and essential. The establishment of effective mm-wave channel models relies on the comprehensive channel measurements and the channel characteristics extracted from the measured data. In the mm-wave propagations with large scale antenna array configurations, the assumptions applied for the previous generation communication systems (e.g. the Long Term Evolution system) are violated. i) The narrowband assumption [9] is invalid due to the ultrawide system...

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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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In this contribution, a recently conducted measurement campaign for indoor millimeter-wave propagation channels is introduced. A vector network analyzer (VNA)-based channel sounder was exploited to record the channel characteristics at the frequency band from 28-30 GHz. A virtual uniform circular array (UCA) with a radius of 0.25 m was formed using a rotator with 360 steps. Moreover, by taking advantage of fiber-optic technique applied in the channel sounder, measurements at 50 positions were performed from an indoor hall to an indoor corridor along a long pre-defined route. A low-complexity highresolution propagation estimation (HRPE) algorithm is exploited to estimate the propagation parameters of multipath components (MPCs). Based on the HRPE estimation results, a novel clustering identification and tracking algorithm is proposed to trace clusters. Composite channel characteristics, cluster-level characteristics and dynamic (or birth-death) behaviours of the clusters are investigated, which constitute a dynamic model for the indoor millimeter-wave channel.Index terms-Millimeter wave, cluster, dispersions, birthdeath and dynamic channel.