Minimizing exposure to electromagnetic radiation in portable devices

Portable 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 the radios can run simultaneously, and each radio exposes the user to some level of electromagnetic radiation. The exposure levels are potentially cumulative in the number of transmitters and number of carrier frequencies used. Fourth-generation (4G) device designers are looking forward to placing four separate transmitting elements for the 4G radio alone. Traditionally, system designers have focused on designing wireless systems assuming one or more transmitter power constraints. However, today\´s wireless devices that are used in close proximity to the human body are almost universally subjected to a measure of electromagnetic exposure testing called specific absorption rate (SAR). SAR, typically measured in Watts per kilogram, is a measure of the rate of energy absorption by the body (or "heating effect") in the presence of an electromagnetic field. In many portable wireless devices SAR is the limiting constraint that determines how the device will perform as a transmitter, yet this constraint is not generally considered until the end of the design cycle of the device. We examine how gains in system performance may be achieved by incorporating SAR constraints early in the device design cycle. For example, multiple transmitting elements, commonly used to improve link performance, may be used as a tool to reduce SAR. We present some models and examples of performance criteria that target the goals of exposure modeling and minimization.