Given increased interest in using millimeter-wave (mmWave) for wireless communications, researchers at NYU Wireless are advocating for new safety metrics that are based on body temperature rather than the standard used nowadays, which is based on power density.
Their paper, titled "The Human Body and Millimeter-Wave Wireless Communication Systems: Interactions and Implications," was chosen as the best from several hundred entries at the 2015 IEEE Conference on Communications. Their study used four models representing different body parts (both clothed and unclothed) to evaluate the thermal effects of mmWave radiation on humans.
The researchers, who include NYU Polytechnic School of Engineering doctoral student Ting Wu; NYU Wireless Director Ted Rappaport; and Christopher Collins, a professor of radiology at the NYU Langone School of Medicine; give details of their study in the paper.
They conducted a simulation that showed the steady state temperature increases--even of clothed parts with less blood flow such as the forehead of a person wearing a hat--are negligible compared with the environmental temperature variations when the exposure intensity is similar to that likely to be used in a next-generation cell phone.
"Because future devices will operate on a spectrum with different properties than today's communications devices, FCC rules and regulations on safety must be reviewed and adjusted accordingly," Rappaport said in a press release. "Additionally, current safety rules regarding radio frequency exposure don't specify limits above 100 GHz, but because spectrum use will inevitably move to these bands over time, safety metrics must also be codified at these frequencies."
The paper notes that mmWave devices should be evaluated to comply with government exposure guidelines before they are introduced to the consumer market. In the U.S., the FCC uses the specific absorption rate (SAR) as a metric for exposure compliance for frequencies below 6 GHz. But at higher frequencies, energy absorption is increasingly confined to the surface layers of the skin, and it's difficult to define a meaningful volume for SAR evaluation.
The researchers also state that in recent years, the cost of operating magnetic resonance imaging (MRI) has gone down, and MRI-based systems for mapping thermal changes are becoming affordable to wireless manufacturers and regulatory bodies. MRI can accurately measure heating of the skin caused by mmWave radiations.
"MRI systems utilize electromagnetic energy at a wide range of frequencies to provide high-resolution, three-dimensional images at ever-increasing scan speeds, and researchers in our group have shown that MRI can be used to map temperature increases caused by electromagnetic fields—from the low radio frequency to millimeter wave regime. It can thus be employed to guarantee the safety of communications devices well into the future," Collins said in the release. "As an MRI engineer, I was gratified to collaborate on this project with NYU's other engineers to help ensure that when fifth-generation mobile technology becomes available to consumers, it will be safe and well-regulated."
NYU Wireless has been a pioneer in mmWave spectrum, with students conducting trials on rooftops in New York City. The mmWave spectrum historically was discounted because its tiny waves travel only short distances and were believed to be easily obstructed by rain, leaves, buildings and bodies.
But the researchers found it isn't as extreme as that. Rain and atmospheric attenuation are factors at larger distances and near 60 GHz, but those effects are less deleterious over shorter coverage distances, where propagation factors dominate path loss. Foliage and shadowing by people and objects will play a role in blocking signals, but early work indicates reflections and scattering paths from various directions may be quickly formed using electrical beamsteering, thus allowing a mobile link to be maintained at high speeds, researchers told the FCC in a filing on its proceeding that looks at using spectrum bands above 24 GHz.
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