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To integrate unmanned aerial vehicles (UAVs) in future large-scaledeployments, a new wireless communication paradigm, namely, thecellular-connected UAV has recently attracted interest. However, theline-of-sight dominant air-to-ground channels along with the antenna pattern ofthe cellular ground base stations (GBSs) introduce critical interference issuesin cellular-connected UAV communications. In particular, the complex antennapattern and the ground reflection (GR) from the down-tilted antennas createboth coverage holes and patchy coverage for the UAVs in the sky, which leads tounreliable connectivity from the underlying cellular network. To overcome thesechallenges, in this paper, we propose a new cellular architecture that employsan extra set of co-channel antennas oriented towards the sky to support UAVs ontop of the existing down-tilted antennas for ground user equipment (GUE). Tomodel the GR stemming from the down-tilted antennas, we propose a path-lossmodel, which takes both antenna radiation pattern and configuration intoaccount. Next, we formulate an optimization problem to maximize the minimumsignal-to-interference ratio (SIR) of the UAVs by tuning the up-tilt (UT)angles of the up-tilted antennas. Since this is an NP-hard problem, we proposea genetic algorithm (GA) based heuristic method to optimize the UT angles ofthese antennas. After obtaining the optimal UT angles, we integrate the 3GPPRelease-10 specified enhanced inter-cell interference coordination (eICIC) toreduce the interference stemming from the down-tilted antennas. Our simulationresults based on the hexagonal cell layout show that the proposed interferencemitigation method can ensure higher minimum SIRs for the UAVs over baselinemethods while creating minimal impact on the SIR of GUEs.

Ensuring Reliable Connectivity to Cellular-Connected UAVs with Up-tilted Antennas and Interference Coordination