Abstract
This paper evaluates the performance of downlink information transmission in three-dimensional (3D) unmanned aerial vehicle (UAV) networks, where multi-tier UAVs of different types and flying altitudes employ directional antennas for communication with ground user equipments (UEs). We introduce a novel tractable antenna gain model, which is a nonlinear function of the elevation angle and the directivity factor, for directional antenna-based UAV communication. Since the transmission range of a UAV is limited by its antenna gain and the receiving threshold of the UEs, only UAVs located in a finite region in each tier can successfully communicate with the UEs. The communication connectivity, association probability as well as coverage probability of the considered multi-tier UAV networks are derived for both line-of-sight (LoS) and non-line-of-sight (NLoS) propagation scenarios. Our analytical results unveil that, for UAV networks employing directional antennas, a necessary tradeoff between connectivity and coverage probability exists. Consequently, UAVs flying at low altitudes require a large elevation angle in order to successfully serve the ground UEs. Moreover, by employing directional antennas an optimal directivity factor exists for maximizing the coverage probability of the multi-tier UAV networks. Simulation results validate the analytical derivations and suggest the application of high-gain directional antennas to improve downlink transmission in the multi-tier UAV networks.
Highlights
Unmanned aerial vehicles (UAVs) have gained increasing interest in both academia [1], [2] and industry [3], [4]
To facilitate a tractable performance analysis, we introduced a simple elevation angle based antenna pattern model to capture the antenna gain provided by directional antennas
It was revealed that the directivity factor can highly impact the connection probability, especially when the UAVs deployed at low flying heights have a large elevation angle
Summary
Unmanned aerial vehicles (UAVs) have gained increasing interest in both academia [1], [2] and industry [3], [4]. Due to the signal enhancement and interference mitigation enabled by directional antennas, which are further affected by the 3D mobility of UAVs, the impact of transmit power management on UAV communication, at different flight heights, needs to be newly investigated but has not been reported in the literature To address both challenges, in this paper, we propose a framework for modeling the downlink of UAV networks where, different from [9]–[11], [14]–[16], [18], UAVs are equipped with directional antennas. The probability distribution of communication distance for ground UEs served by the considered UAV networks is derived, where the impact of transmit output power, receiving threshold, and directivity of antenna element is revealed. All downlink transmission within the UAV network occupy the same spectrum, whereby the UAVs in networks can interfere with each other while serving the ground UEs
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