Raytracing Digital Foliage at Millimeter-Wave: A Case Study on Calibration Against 60-GHz Channel Measurements on Summer and Winter Trees

Accurate channel propagation modeling of foliage is critical to the design of wireless networks, given its pervasive nature in rural, suburban, and urban environments. Its blockage effects can be particularly devastating at millimeter-wave (mmWave) because the size of leaves and branches is comparable to the wavelength of the transmitted signal. While raytracing models are firmly based on electromagnetic principles, reliability can be attained only through calibration against measurements. In the few works that do so, foliage is represented as simple canonical shapes (cylinders, discs, etc.) and calibration is performed against measurements with foliage integrated as part of entire outdoor environments. The controlled approach that we adopt in this paper, rather, is based on measurement of single specimens of foliage, for precision characterization. To sustain this precision at mmWave, the foliage is represented digitally as a mesh of faceted leaves and branches. Raytracing predictions from the Ansys HFSS SBR+ model applied to digital twins of seven trees are calibrated against measurements–collected in summer and in winter for comprehensive analysis–with the Terragraph double-directional 60-GHz channel sounder. The tree-specific predictions, which can then be integrated as part of an entire outdoor environment, are shown to match the measurements very well.