Closed-Loop Precoding and Capacity Analysis for Multiple-Antenna Wireless Systems With User Radiation Exposure Constraints

Mobile handsets, classified as portable devices, are regulated on the amount of user electromagnetic exposure. The widely accepted exposure measurement is the specific absorption rate (SAR). Despite the prevalence of SAR constraints throughout the world, there has been barely any work on the design and analysis of communication signals for SAR-constrained wireless systems. In this paper, we show that multiple-antenna systems greatly reduce the SAR measurements when proper precoders are used. Fifth-generation (5G) and beyond cellular systems will be expected to support high rate uplinks, making multiple transmit antennas on user equipment a necessity. Our proposed SAR-aware transmission for multiple-antenna systems can be applied in 5G handsets to reduce the SAR and increase the rate. Assuming that channel knowledge is available at the transmitter and the receiver, we perform capacity analysis for multiple-antenna systems under both transmit power and SAR constraints. Analytical and numerical results demonstrate substantial performance improvements over schemes that ignore the SAR constraint. Our work shows that SAR-constrained precoders have structures similar to precoders designed for spatially correlated channels.