Joint IoT Device Selection and Health-Aware Beamforming Design for MIMO-WPT

Wireless power transfer (WPT) has emerged to enhance the robustness of the energy harvesting Internet of Things (EH-IoT), whereas beamforming has been leveraged to significantly boost the efficiency of far-field WPT. Nevertheless, potential negative impacts due to high electromagnetic fields (EMF) exposure for radiation-susceptible users have not been thoroughly considered in the design of WPT for EH-IoT with IoT application-level requirements (e.g., coverage). In this article, we explore the health-aware beamforming and IoT selection problem under the EH and human safety constraints. First, we formulate a new optimization problem Health-Aware Beamforming and IoT Selection (HABIS) and prove the NP-hardness. Second, we design an approximation algorithm, named Minimum Radiation Exposure and Maximum IoT Coverage (MREMIC), to exploit the EH-health dependency (EHHD) graph for properly addressing the trade-off between EH efficiency and potential EMF radiation exposure to human bodies. We also discover the optimal health-aware beamforming to minimize the total radiation energy absorption of humans. Simulation results show that MREMIC can effectively charge IoT devices and significantly outperforms existing EH approaches by more than 200% regarding human safety.

I. Introduction

Internet of Things (IoT) is the cornerstone for various important services, such as smart cities and environmental detection, where achieving high coverage is a primary objective for IoT services [1]. However, the limited battery capacity has hindered widespread IoT deployment. To address this critical issue, many companies have developed wireless power transfer (WPT) technologies to charge IoT devices¹

For simplicity, we use “IoT” to represent “IoT device” in this paper.

to prolong their lifetime, and the notion of energy harvesting-IoT (EH-IoT) emerged [2], [3], [4]. For example, Powercast (http://www.powercastco.com/) and Cota (http://www.ossia.com/cota/) developed power transmitters and receivers that can be integrated into IoTs to achieve EH-IoT. With recent advancements in multiple-input multiple-output (MIMO), many researchers have explored its potential to significantly boost the efficiency of far-field WPT [4], [5], [6], [7], [8], [9], [10]. MIMO enables power delivery from multiple transmitters to receivers, providing a unique ability to focus energy toward the target receivers. The design of beamforming is critical and has been investigated in various forms to optimize EH efficiency and throughput for data transmission. A multi-objective optimization framework was proposed to study the fundamental trade-off between the data rate and harvested energy maximization [11] for jointly optimizing the energy and information beamforming vectors. The problem is first converted to a single-objective optimization problem via the $\epsilon$-constraint method and then solved by majorization minimization and inner approximation techniques. Nevertheless, the potential radiation absorbed by nearby IoT users, which may lead to lethal pathology (e.g., cancers), has not been observed in existing research. In this paper, we jointly select IoTs and design beamforming for multiple base stations (BSs) to safely charge IoTs under individual health constraints to avoid excessive electromagnetic field (EMF) exposure.