Source-linked AI summary
The Human Body and Millimeter-Wave Wireless Communication Systems: Interactions and Implications
Ting Wu, Theodore S. Rappaport, Christopher M. Collins
TL;DR
MmWave devices raise safety-assessment challenges because near-field operation and surface-confined absorption make conventional power-density compliance difficult. The paper reviews regulations, models propagation and heating across human tissues, and evaluates a 60 GHz case, finding substantial skin reflection and proposing temperature elevation as a compliance measure.
Problem
Near-body mmWave devices create compliance challenges because near-field fields hinder reliable power-density measurement and absorption is concentrated in superficial tissues.
Method
The paper reviews exposure regulations, studies propagation and reflection using skin dielectric models, and simulates electromagnetic heating with four tissue models and varying clothing conditions.
Results
34%–42% of normally incident power is reflected at the air–skin interface at 60 GHz, while more than 90% of transmitted power is absorbed in the epidermis and dermis.
Takeaways & Limitations
Temperature elevation, potentially measured or simulated with MRI-based methods, is proposed as a more suitable mmWave exposure-compliance quantity than power density near the body.
Abstract
from arXiv · showhide
With increasing interest in millimeter wave wireless communications, investigations on interactions between the human body and millimeter wave devices are becoming important. This paper gives examples of current regulatory requirements, and provides an example for a 60 GHz transceiver. Also, the propagation characteristics of millimeter-waves in the presence of the human body are studied, and four models representing different body parts are considered to evaluate thermal effects of millimeter-wave radiation on the body. Simulation results show that about 34% to 42% of the incident power is reflected at the skin surface at 60 GHz. This paper shows that power density is not suitable to determine exposure compliance when millimeter wave devices are used very close to the body. A temperature-based technique for the evaluation of safety compliance is proposed in this paper.
I. INTRODUCTION
Millimeter-wave devices create distinctive exposure-compliance challenges because directional near-field operation can concentrate energy close to the body, while existing metrics and measurement approaches have important limits. The paper reviews regulatory requirements and motivates temperature-based safety evaluation using MRI.
- Exposure metrics: At frequencies above 6 GHz for FCC rules or 10 GHz for ICNIRP rules, power density is preferred over SAR because absorption is confined increasingly to skin surface layers.The paper describes difficulty defining a meaningful volume for SAR evaluation at higher frequencies.
- Regulatory requirements: ICNIRP sets power-density limits of 10 W/m2 for the general public and 50 W/m2 for occupational exposure between 10 and 300 GHz.These limits are averaged over 20 cm2 and a frequency-dependent time period, with spatial peak limits averaged over 1 cm2.
- Measurement challenges: Near-field operation challenges compliance assessment because mmWave handsets may be used closer than 5 cm, where reliable power-density measurements cannot normally be obtained.The paper identifies numerical FDTD or FEM modeling as an approach for devices operating closer than 5 cm.
- Motivation: MmWave handsets’ directional adaptive arrays can focus radiation toward the body and create interference patterns that alter power deposition near the surface.The paper notes that constructive and destructive electric-field combinations affect deposition within the first few millimeters at mmWave frequencies.
- Proposed approach: MRI-based thermal mapping is proposed as a potential safety-evaluation method because it can measure mmWave-induced skin heating with high spatial resolution.The paper describes MRI as wideband, three-dimensional, and faster than current SAR measurement systems.
A. Dielectric Properties of the Skin
The paper models skin dielectric behavior through complex permittivity, emphasizing frequency-dependent changes and variability across measurement studies. It argues that improved tissue measurements are needed for reliable mmWave propagation and absorption predictions.
- Skin structure: Skin comprises epidermis and dermis layers with thicknesses of 0.06–0.1 mm and 1.2–2.8 mm, respectively.These layer dimensions provide the structural basis for modeling mmWave interaction with skin.
- Dielectric representation: The imaginary component of relative complex permittivity is related to conductivity through ε′′ = σ/(2πfε0).Here, σ is conductivity, ε0 is free-space permittivity, and f is operating frequency.
- Frequency dependence: Skin relative permittivity decreases with frequency, whereas skin conductivity increases with frequency.Reported dielectric discrepancies may reflect measurement methods and differences in temperature, thickness, and sample type.
- Modeling limitation: Further dielectric measurements of skin and other tissues are needed to develop accurate human-presence mmWave propagation models.The paper notes that publicly available data reflect natural variability in biological-tissue structure and composition.
B. Reflection and Transmission at the Surface of the Skin
The paper models millimeter-wave interaction with skin using plane-wave reflection, transmission, and penetration analyses across frequency and dielectric-property assumptions. At 60 GHz, skin reflection varies substantially across models, while penetration becomes shallow and is concentrated in the epidermis and dermis.
- Surface model: A semi-infinite flat skin surface is used to model a plane wave incident on the body because millimeter-wave wavelengths are much shorter than the human body.The analysis considers normal incidence and separates parallel and perpendicular polarization cases at the air–skin boundary.
- Surface model: Parallel and perpendicular polarizations define the reflection behavior at the air–skin interface, with power transmission given by one minus the corresponding power reflection coefficient.The electric field is respectively parallel or perpendicular to the plane of incidence.
- Reflection results: 34%-42% of normally incident power is reflected at the skin surface at 60 GHz across the evaluated skin dielectric models.The power reflection coefficients vary by 20% when different dielectric model parameters are applied.
- Reflection results: Brewster angles between 65° and 80° correspond to conditions where almost all incident energy is absorbed.The reported reflection behavior is evaluated for both polarization components using different skin model parameters.
- Penetration results: More than 90% of transmitted electromagnetic power is absorbed within the epidermis and dermis, and penetration depth decreases rapidly as frequency increases.Because little power penetrates into deeper tissues, a single-layer skin model appears sufficient for reliable millimeter-wave energy-distribution evaluation.
III. MILLIMETER-WAVE HEATING OF THE SKIN
The paper models millimeter-wave heating in four one-dimensional tissue configurations and solves the bioheat response under steady-state conditions. At 60 GHz, heating depends on body-part structure, clothing, and power transmission rather than incident power density alone.
- Tissue models: Four one-dimensional models represent naked body tissue, naked forehead, clothed body, and a forehead covered by clothing such as a hat.The models use normally incident continuous plane waves and are infinite in the xy-plane and semi-infinite along the z-axis.
- Electromagnetic model: The electromagnetic fields in each tissue layer are determined from the layer’s complex permittivity, magnetic permeability, propagation constant, and wave impedance, with interface continuity conditions.The electric and magnetic fields are matched across interfaces by enforcing continuity of both E and H.
- Thermal model: The one-dimensional Pennes bioheat equation models temperature using thermal conduction, blood perfusion, metabolism, and the volumetric SAR heat source.For steady-state temperature elevation, the bioheat equation is simplified to an ordinary differential equation and solved analytically subject to boundary conditions.
- Simulation results: 0.8 ℃ is the skin-surface temperature elevation at 50 W/m2 in naked skin, below the 1 ℃ IEEE temperature threshold.The simulated temperature elevation is proportional to incident power density across the evaluated values.
- Simulation results: 0.16 ℃ is the lowest steady-state elevation for naked skin at 10 W/m2, whereas 0.3 ℃ is the highest for a hat-covered forehead.The differences arise from air and blood-flow cooling, bone’s poor heat conduction, and clothing that restricts heat transfer.
- Simulation results: Power transmission through clothing varies with thickness, with local peaks every half wavelength and an overall decrease caused by clothing attenuation.At 60 GHz, the wavelength in the modeled clothing is about 3.95 mm.
- Implications: Different body locations can have different steady-state temperature elevations under the same radiation intensity because power density omits reflection and transmission across boundaries.The paper therefore proposes MRI-based temperature measurement as a possible dosimetric quantity for near-field safety assessment.
IV. CONCLUSION
The paper concludes that dielectric-model variation affects predicted reflection at 60 GHz, while penetration and thermal analyses support different skin-model choices. It proposes temperature elevation as a more direct compliance measure for close-body mmWave exposure.
- IV. CONCLUSION: 34%-42% of normally incident power is reflected at the air/skin interface at 60 GHz, depending on dielectric parameters.The paper emphasizes that a sound dielectric database is important for reliable reflection and transmission predictions.
- IV. CONCLUSION: More than 90% of transmitted power is absorbed in the epidermis and dermis, supporting a single-layer skin model for electromagnetic evaluation.Thermal modeling still requires multiple layers because surface heat conducts through underlying tissues such as SAT and muscle.
- IV. CONCLUSION: Four one-dimensional tissue models were used to study thermal heating, electromagnetic penetration, and the effects of clothing thickness.The analyses examined how clothing thickness affects power transmission and steady-state temperature elevation.
- IV. CONCLUSION: Temperature elevation in the head or body is proposed as a valid mmWave exposure-compliance measure because it relates more directly to safety than power density.The paper notes that temperature measurements or simulations are already accepted for demonstrating compliance with radio-frequency exposure limits in MRI.