Variability of In-Body Absorbed Power Density at mmWave Frequencies

Abstract

In 5G and emerging 6G wireless networks, the operating frequencies are progressively expanding to the millimeter-wave (mmWave) range. At mmWave frequencies, the electromagnetic (EM) energy deposition is primarily confined to superficial skin layers. Dosimetric evaluation of human exposure requires accounting for anatomical variability and environmental conditions that influence the absorbed power density ($S_{\text{ab}}$). This study investigates and quantifies the individual and combined effects of anatomical site, age, skin temperature, clothing, and hydration on $S_{\text{ab}}$ at $26 \,\mathrm{GHz}$ and $60 \,\mathrm{GHz}$. Near-surface models of the head, ear, and palm are employed to evaluate the impact of site-specific structural and dielectric variations on $S_{\text{ab}}$. The results demonstrate that the palm systematically exhibits the highest $S_{\text{ab}}$ due to the impedance-matching effect of its thick stratum corneum (SC), with multilayer models yielding increases up to $24 \,\%$ at $26 \,\mathrm{GHz}$ and $40 \,\%$ at $60 \,\mathrm{GHz}$ compared to homogeneous skin models. Age-related dielectric changes induce moderate $S_{\text{ab}}$ variations (up to $8 \,\%$ at $26 \,\mathrm{GHz}$ and $7 \,\%$ at $60 \,\mathrm{GHz}$), while temperature variations lead to increases up to $7 \,\%$ at $26 \,\mathrm{GHz}$ and $10 \,\%$ at $60 \,\mathrm{GHz}$, depending on the anatomical site. Clothing introduces the most pronounced effect. When the textile thickness satisfies a quarter-wavelength matching condition, $S_{\text{ab}}$ increases by up to $37 \,\%$ at $26 \,\mathrm{GHz}$ and $33 \,\%$ at $60 \,\mathrm{GHz}$ for the head and ear. When all parameters are jointly considered, the worst-case $S_{\text{ab}}$ enhancements reach $65 \,\%$ at $26 \,\mathrm{GHz}$ and $56 \,\%$ at $60 \,\mathrm{GHz}$ compared to the reference case (adult, dry skin at 33 $^{\circ }$C). Skin hydration further impacts $S_{\text{ab}}$: at $26 \,\mathrm{GHz}$, $S_{\text{ab}}$ increases by up to $26 \,\%$ in the ear and $8 \,\%$ in the head, whereas at $60 \,\mathrm{GHz}$ the influence remains negligible in the ear and head (regions with a thin SC). In contrast, for the palm, skin hydration attenuates $S_{\text{ab}}$ at both frequencies. These findings demonstrate that anatomical site and textile-induced impedance matching dominate mmWave exposure variability (up to $37 \,\%$), while age and temperature introduce less pronounced effects (up to $10 \,\%$).