Capacity analysis for MIMO systems with user exposure constraints

Mobile handsets and other portable devices are already an essential part of our daily life. All such wireless portable devices are regulated on the amount of radio-frequency electromagnetic radiation to their users. The specific absorption rate (SAR) is a widely accepted user exposure measurement used by most wireless communication regulatory agencies throughout the world. Previous research demonstrates that the SAR measurement varies significantly with different kinds of transmit signals in a multiple antenna system, meaning that the near-field exposure constraints can greatly affect the far-field rate performance in a wireless MIMO system. Fifth-generation (5G) and beyond cellular systems are expected to support multiple transmit antennas on the user equipment. Therefore, 5G systems should no longer be regarded as only power-constrained systems but also SAR-constrained systems. Our work presents transmit signal design guidelines and capacity analysis results for SAR-constrained wireless channels. SAR-aware transmission strategies using both spatial and temporal diversity are presented to maximize the rate performance, as well as maintain the SAR-level below the regulation limits. Our analytical and numerical results demonstrate substantial performance improvements of the proposed SAR-aware transmission over conventional transmission schemes in a SAR-constrained channel. Our work shows that the SAR constraints have a mathematically similar impact on system design to that of spatially transmit correlation on uplink channels