Transmit covariance optimization with a constraint on user electromagnetic radiation exposure

With the continuous evolution of cellular systems, portable wireless communication devices, such as mobile phones, are becoming more and more powerful, complex, and essential to our daily life. However, the debate over the health effects from using portable devices in close proximity has still not been settled. To limit the amount of user exposure, regulatory agencies in most countries set exposure threshold in terms of the specific absorption rate (SAR), measured in Watts per kilogram. SAR is a measure of the rate of electromagnetic energy absorption by the human body. Surprisingly, portable devices are often designed with little attention to the SAR thresholds, instead focusing on transmit power constraints. In this paper, we propose SAR-aware transmission that considers constraints on both transmit power and SAR with multiple antennas. We discover that the effect of a SAR constraint can be regarded in a way similar to a transmitter-side spatial correlation. Numerical results demonstrate the proposed method increases the capacity by 1.75 folds under the regulatory SAR limitation of 1.6 W/kg over schemes that only consider the power constraint.