Unmanned Aerial Vehicle (UAV) communication is known to suffer from significant interference due to the clearance of the radio paths with ground base stations. Multi-antenna receive combining has the promise of alleviating the impact of interference, translating to improved connectivity performance. In this paper, we evaluate the performance of Conventional Beamforming (CB) and Maximum Ratio Combining (MRC) receivers for UAV communication based on live Long Term Evolution (LTE) networks. Our measurement setup consists of nine Universal Software Radio Peripheral (USRP) boards and a circular antenna array with sixteen elements. The LTE signals are recorded at different UAV flight heights in urban environments, and processed offline. Results show similar Signal-to-Interference-plus-Noise Ratio (SINR) performance by MRC and CB, with CB slightly outperforming MRC provided knowledge of LTE signal structure is used for the beam selection. No significant dependency from the flight height has been observed. The outage probability analysis further emphasizes the benefits of using CB in the studied scenarios.

High-throughput unmanned aerial vehicle (UAV) communication may unleash the true potential of novel applications for aerial vehicles but also represents a threat for cellular networks due to the high levels of generated interference. In this article, we investigate how a beamforming system installed on board a UAV can be efficiently used to ensure high-throughput uplink UAV communications with minimum impact on the services provided to users on the ground. We study two potential benefits of beamforming, namely, spatial filtering of interference and load balancing, considering different beam switching methodologies. Our analysis is based on system-level simulations followed by a series of measurement campaigns in live Long-Term Evolution (LTE) networks. Our results show that using UAV-side beamforming has a great potential to increase uplink throughput of a UAV while mitigating interference. When beamforming is used, even up to twice as many UAVs may be served within a network compared with UAVs using omni-directional antennas, assuming a constant uplink throughput target. However, to fully exploit the potential of beamforming, a standardized solution ensuring alignment between network operators and UAV manufacturers is required.

Cellular Vehicle to Everything (C-V2X) communications with its safety and infotainment services will require a high performance receivers to cope with challenging throughput, latency and reliability requirements. With increasing levels of interference due to cell densification and introduction of the roadside units, single antenna receivers may not be able to provide the required quality of service. In this work we experimentally study the performance of multi antenna receivers based on more than 150 km of data recorded during experiments using a customized software defined radio testbed. The performance of sixteen antennas Maximum Ratio Combiner (MRC) is compared with the receive beamforming technique for the live cellular signals in the 1.8 GHz band. This study is followed by an analysis of the impact of interference and measurement environment on the receiver's performance. The results show that receive beamforming can outperform MRC in low-interfered scenarios with high Line of Sight (LoS) probability, like highways or rural areas, while ensuring comparable performance even in dense urban scenarios where LoS communication cannot be guaranteed.

This paper presents a design of a Software Defined Radio (SDR) multi-antenna testbed able to record live cellular signals from multiple sites. This measurement setup based on Universal Software Radio Peripheral (USRP) boards, is used to record live Long Term Evolution (LTE) signals in sub-6 GHz frequency bands. Due to recording of raw I&Q samples, this fully digital testbed is suitable for variety of research activities spanning channel characterization and beamforming performance evaluation. We propose a phase calibration method based on transmission of a single out of band tone to overcome the uncertainty introduced by the USRP's lack of phase alignment. We demonstrate two use cases where the proposed testbed can be used and we validate its performance during two measurement campaigns with self-generated and real cellular signals. INDEX TERMS Antennas and propagation, beam steering, measurement techniques, multi-antenna, SDR, USRP.

Cellular-connected unmanned aerial vehicles (UAVs) have recently attracted a surge of interest in both academia and industry. Understanding the air-to-ground (A2G) propagation channels is essential to enable reliable and/or highthroughput communications for UAVs and protect the ground user equipments (UEs). In this contribution, a recently conducted measurement campaign for the A2G channels is introduced. A uniform circular array (UCA) with 16 antenna elements was employed to collect the downlink signals of two different Long Term Evolution (LTE) networks, at the heights of 0-40 m in three different, namely rural, urban and industrial scenarios. The channel impulse responses (CIRs) have been extracted from the received data, and the spatial, including angular, parameters of the multipath components in individual channels were estimated according to a high-resolution-parameter-estimation (HRPE) principle. Based on the HRPE results, clusters of multipath components were further identified. Finally, comprehensive spatial channel characteristics were investigated in the composite and cluster levels at different heights in the three scenarios.Index terms-UAV, cellular networks, air-to-ground, spatial channels, angular characteristics, and clusters. I. INTRODUCTIONRecently, unmanned aerial vehicles (UAVs) have been shifting their use from purely military operations to a more generalpurpose scope with rapidly increasing popularity, due to the continuous reduction of cost, size, weight and consumption. A huge market is foreseen with many new civilian and commercial applications such as forest monitoring, goods delivery or search and rescue in hostile environments [1]-[3].Temporary network access provided by UAVs as movable aerial base stations (BSs) in emergency situations or saturated communication environments has become a key scenario addressed for