Universal Electromagnetic Reference Skin Model for APD Evaluation at 6–100 GHz

The increasing use of the upper part of the microwave spectrum for wireless communications requires appropriate methods and instrumentation for user exposure assessment. In this context, the IEC TC106 is developing a new international standard for user exposure compliance testing of the next generation 5G/6G wireless devices operating above 6 GHz. As a part of this initiative, the development of a universal reference skin model (RSM) is fundamental for definition of reference data to be included in specifications for body phantom design. In this study, we systematically analyze the impact of the human body near-surface tissue structure on the electromagnetic field (EMF) reflection from the skin surface in the 6–100 GHz range. A conventional multi-layer model is used to calculate skin reflectance as a function of the tissue thickness for the range of thicknesses corresponding to that of typical human skin and near-surface body tissues at four body sites concerned by the 5G/6G wireless use-case scenarios, namely: head, torso, forearm, and palm. The dominant contribution from the epidermis/dermis (ED) layer to the skin reflectance is demonstrated for all body sites in the considered frequency range. A high variation in the reflectance of the palm skin at frequencies above 20 GHz is demonstrated and explained by the matching layer effect associated with a thick stratum corneum (SC). The dry skin model, represented by a semi-infinite homogeneous medium with complex permittivity equivalent to that of the ED tissue, is shown to be an appropriate RSM both for the experimental and numerical evaluation of the absorbed power density (APD) in the 6–100 GHz range. The reference data for the antenna loading and APD at the skin surface are provided for standard reference feeds at 10 GHz, 30 GHz, 60 GHz, and 90 GHz.