Hierarchical Codebook and Beam Alignment for UAV Communications

Base station (BS) to unmanned aerial vehicles (UAV) communications is an enabling technology that supports emerging UAV-related applications. In this paper, we study a hybrid beamforming technology for BS-to-UAV backhaul communications. First, we propose an efficient beam tracking scheme that is tailored for UAV system with high mobility. The proposed scheme does not require any knowledge of the moving pattern and/or trajectory of UAV, but only needs a few training overhead. Based on our proposed beam tracking scheme, the wider beamwidth requires less training overhead but results in lower beamforming gain. We then investigate this trade-off effect by optimizing the beamwidth to provide the maximum flying range for UAV. We show that in most practical scenarios, narrower beam allows larger flying range of UAV with a fixed speed, which means the maximum flying range of UAV can be achieved with the narrowest beamwidth. Simulation results are provided to validate our analysis.