Incorporating specific absorption rate constraints into wireless signal design

Portable wireless devices used in close proximity to the human body have transmitter power constraints that are dictated in part by regulatory limits on a form of electromagnetic exposure called specific absorption rate (SAR). SAR, measured in Watts per kilogram, is a measure of electromagnetic energy absorption by the body, which can cause the heating of tissue. Some portable wireless devices sold today operate near the accepted human SAR limit. This article examines how SAR constraints can be incorporated into wireless signal design for multiple transmitters. A SAR constraint, although related to a power constraint, differs sufficiently that signals designed specifically for the constraint are needed. Although it is well known that multiple transmitters subject only to a power constraint can improve the link performance of a wireless system compared to a single transmitter, it is not clear when the device is also subject to a SAR constraint: how to model the constraint; whether performance advantages still exist; and how to realize any advantages. In this article, we introduce SAR constraints and discuss the SAR measurement, simulation, and modeling process. We then show how to incorporate the SAR constraint into system performance analysis and code design. We show how SAR codes use multiple transmit antennas to get good combined farfield error performance and near-field SAR performance, improving handset transmitter performance in the critical uplink of a communication system.