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. Many portable wireless devices sold today operate near the accepted human SAR limit. This research examines how multiple transmitters on a portable wireless device can be modeled, analyzed, and used while subject to a SAR constraint. The work looks at how to model the constraint, and how to realize the reliability and high data-rate advantages of multiple antennas subject to the constraint.
Because portable devices are generally small, transmitters will be physically close to one another and couple electromagnetically. Hence, the research requires a blended effort in three distinct areas: (i) electromagnetics and near-field exposure modeling with multiple transmitters, (ii) wireless communication system far-field information-theoretic performance evaluation with a near-field SAR constraint, (iii) the design of SAR codes for optimum performance. Code performance will be determined by simulation and by laboratory evaluation with microwave equipment for near-field SAR and far-field error-rate measurements.
