Doppler effects in UAV-to-vehicle multipath channels under 6D mobility

Unmanned aerial vehicle (UAV) empowered vehicular networks are capable of providing flexible radio access, efficient data transmission, and secure driving operation. For these promising applications, the authors focus on channel modelling for UAV-to-vehicle (U2V) communications in this paper, which is the very first step for communication system design. Based on geometry and multi-dimensional mobility patterns, a geometry-based stochastic model framework for U2V communication is studied. Considering the practical scenario, multipath components (MPCs) are composed of a line-of-sight (LoS) path, a ground reflection (GR) path, and several random scattering (SC) paths. Specifically, the movement of a UAV in both three-dimensional (3D) translational and 3D rotational directions is considered as well as a ground vehicle capable of moving in arbitrary directions with varying velocities. Subsequently, the Doppler shift expressions for various paths and movements are presented individually, incorporating spatial angles. Comprehensive simulation results are examined, illustrating that six-dimensional (6D) motion exhibits varying effects with linear relative velocity, local angular velocity, and two types of spatial angles. Moreover, channel statistical properties such as temporal auto-correlation function (TCF) and Doppler power spectral density (PSD) are studied to show the consideration of 6D mobility is indispensable.