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

Abstract

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. 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 multi-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.Index Terms-Multiple antenna array, specific absorption rate, precoding optimization 1536-1276 (c)

On the Energy Efficiency of MIMO Hybrid Beamforming for Millimeter-Wave Systems With Nonlinear Power Amplifiers

“…However, in this latter paper the impact of hardware impairments are not taken into account. Additionally, the impact of regulatory electromagnetic exposure constraints has also been taken into account when designing multiple transmit antenna signals in [37]- [39]. Recently, the interplay between waveforms, amplifier efficiency, distortion and performance in the massive MIMO downlink was studied in [40].…”

Fast Specific Absorption Rate Aware Beamforming for Downlink SWIPT via Deep Learning

“…As discussed, wireless communication devices are subject to SAR limitations. Previous reported results such as [15] and [16] have shown that the pointwise SAR value with multiple transmit antennas can be modeled as a quadratic form of the transmitted signal, and the SAR matrices fully describes the SAR measurement's dependence on the transmitted signals and are positive-definite conjugate-symmetric matrices.…”

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“…In [20], two system models were developed where one used transmit (Tx) channel state information (CSI) to perform beamforming at the receiver and the other, which did not have Tx CSI, but was able to be 'SAR aware', i.e., could exploit knowledge of SAR in order to boost Tx power but stay in SAR limits. A beamforming approach based on Tx CSI approach was then further developed in [21] where a dual information theoretical optimal and SAR reduction precoder was proposed. Using a rigorous mathematical analysis, it was shown that the waterfilling of singular value decomposition (SVD) based eigenmodes could be altered to make the dual accommodation of maximized data throughput and SAR reduction by implementing a precoder based on the SVD itself in conjunction with other precoder structures similar to the ones used in MIMO systems operating in spatially correlated channels.…”