Portable wireless devices such as cell phones are becoming powerhouses of computational ability, with multiple processors, large amounts of memory, touch screens, cameras, and motion sensors all connected to WiFi, Bluetooth, 3G and 4G transceivers. Many of these transceivers can transmit concurrently, and each transmission exposes the user to some level of electromagnetic radiation. The total exposure is potentially cumulative. At the same time, regulatory agencies such as the Federal Communication Commission (FCC) are constantly being challenged to re-evaluate their thresholds for exposure to electromagnetic radiation as new science becomes available and as the public becomes more reliant on portable wireless devices. This research examines using multiple transmitters, commonly used to improve wireless performance, as a tool to minimize exposure.
This research: (i) evaluates the exposure effects of a portable device as a function of its transmitter characteristics and the timing and phasing between transmitter elements; (ii) designs communication and constrained coding techniques that minimize exposure while not sacrificing wireless performance; (iii) evaluates the success of these techniques using accepted testing procedures for exposure such as the SAR (specific absorption rate) limit of 1.6 W/kg. The research characterizes the tradeoff between minimizing near-field energy, which determines exposure, and maximizing far-field energy, which determines wireless communication system performance.
Portable wireless devices sold around the world number in the billions, and they are used daily because of their ability to provide connectivity by integrating processing power, touch screens, cameras, and motion sensors with multiple radios, many of which can run simultaneously. Portable wireless devices used in close proximity to the human body have transmitter power constraints that are dictated largely by regulatory limits on a form of electromagnetic exposure called ``specific absorption rate'' (SAR). SAR, measured in Watts per kilogram, is a measure of the rate of electromagnetic energy absorption by the body, causing heating of tissue. Many portable wireless devices sold today operate near the accepted human SAR limit. SAR constraints are inflexible and are not likely to be relaxed in the near future. The work in this project examined how multiple transmitters on a portable wireless device can be modeled, analyzed, and used while subject to a SAR constraint. Part of the effort was to design a SAR code for optimum performance that blends the requirements of the SAR constraint with the requirements for high data rates with a receiver. The result was a code with high data rates, that lowered SAR as compared with existing transmission methods. Intellectual Merit The effort: (i) modeled and analyzed the SAR of radio-frequency transmitters as a function of their characteristics such as frequency, amplitude, phase and spacing; (ii) analyzed the performance of a communication system subject to an exposure constraint; (iii) designed a communication and coding technique that obey the constraint. Broader Impacts Constraints on human exposure to electromagnetic fields affect all portable wireless devices and cannot be circumvented. With the prevalence of portable device usage in the world, the results of this work (lower SAR without compromised performance) could have a tangible and beneficial effect on the wireless industry and society.
