Far-field Wireless Power Transfer Research Articles

WiPoTS: An Application Layer Protocol With Network Protocol Stack Enhancements For Wireless Power Transfer Networks

Abstract Nowadays, a far-field wireless power transfer (WPT) system aims to deliver wireless power over a distance of a few meters. Communication among devices for the purpose of WPT in the far-field WPT system is unique as its purpose is to establish, maintain and monitor a WPT session among devices in the system. For proper functionality of a WPT system, a number of communication-, control- and management-related challenges need to be addressed. Hence, here an application layer protocol specifically designed for a WPT system is presented. The protocol provides essential control, management and communication functionalities to establish, maintain and monitor a WPT session. Some features of a WPT system require low-latency communication, hence enhancements for a networking protocol stack are also proposed. To validate the effectiveness of the presented application layer protocol along with the enhancements proposed for the networking protocol stack, a set of experiments was carried out over a real WPT system. The experimental results demonstrate that the proposed protocol possesses the features essential for a WPT system, and the proposed enhancements for the networking protocol stack provide low-latency communication along with lower control overhead compared with an existing state-of-the-art network protocol stack.

Understanding the Impact of Environmental Conditions on Zero-Power Internet of Things: An Experimental Evaluation

The booming Internet of Things is transforming all industries, making them more productive and efficient. Unfortunately, with its widespread use, a series of challenges are posed by sustainable sensing, harsh working environments, laboursome battery replacement, and the requirement for an ultra-small form factor. The concept of zero-power Internet of Things has recently been proposed to overcome these challenges by integrating farfield wireless power transfer, ambient energy harvesting, and backscatter wireless communications into networked embedded systems. However, the development of far-field wireless power transfer is still in its infancy, and how environmental conditions affect its performance in the real world remains unclear. This article fills this gap and provides an extensive experimental study to quantify the impact of environmental conditions, namely, lineof- sight propagation, multipath interference, building materials, and non-air mediums on far-field wireless power transfer. Inspired by this study, we further shed light on some future research directions.

Dual-Frequency Retrodirective Antenna Array With Wide Dynamic Range for Wireless Power Transfer

This letter presents a dual-frequency heterodyne retrodirective antenna (RDA) array with relatively large frequency ratio between receiving and transmitting for adaptive tracking in far-field wireless power transfer systems. The proposed RDA consists of an eight-channel radio frequency mixing, amplify chain and two 1 × 8 circularly polarized arrays with different operation frequency and orthogonal polarizations, receiving at 3.8 GHz with right-hand circular polarization and transmitting at 5.8 GHz with left-hand circular polarization, respectively. For received power levels from −80 to −40 dBm, each chain outputs with consistent phase and stable power about 29 dBm brought by automatic gain control and compensation phase shifter. It can transmit reversely along the direction of beacon signal from near to far distance. High system isolation is achieved through inherent isolation of different frequencies and orthogonal polarizations of the T/R arrays. Monostatic and bistatic measurements are taken, and measured results verify that the proposed RDA is of far-field tracking capability to receive and transmit from −40° to 40°.

Time-Switching Symmetric Transmitter Array to Increase Sweet Region for Far-Field Wireless Power Transmission Under Receiver Rotation and Location Variation

For wireless power transfer using linear-polarized transmitter (TX) and receiver antennas, the rotation and movement of receiving antenna cause received power variation. The received power pattern is proposed to describe the received power variation from a specific TX antenna under receiver rotation. By placing the peak of the received power pattern of additional TX antennas in null directions of the previous TX antenna, received power variation due to rotation can be compensated. With symmetrically placed TX antennas, the power variation due to location variation can be compensated. This article proposes a time-switching symmetric transmitting array combining both techniques to increase the sweet region for far-field wireless power transfer under rotation and location variation. A sweet region is an area in the 2-D case or volume in the 3-D case, which is defined as an area or a volume inside the TX array that can get guaranteed minimum received power under rotation. The received power calculation using the Friis equation with short dipole antenna pattern shows that a symmetric TX array can achieve a larger sweet region than an asymmetric TX array under the same total transmitted power. The efficiency for the receiver at the origin is 1/2 at the 2-D case and 1/3 at the 3-D case. In the experiment, a linear-polarized TX array at 915 MHz with monopole receiver antenna and power detector is used as the receiver to measure received power under rotation and location variation. Measurement results at 2-D and 3-D cases confirm the received power level predicted by calculation using the Friis equation. The power budget calculation for wirelessly powering a sensor module attached on a bee inside a beehive using the proposed 3-D TX array shows that it is feasible to deliver sufficient power to the sensor module regardless of the orientation and location of the bee inside the beehive.

Waveform Design for Wireless Power Transfer With Power Amplifier and Energy Harvester Non-Linearities

Waveform optimization has shown its great potential to boost the performance of far-field wireless power transfer (WPT). Current research has optimized transmit waveform, adaptive to channel state information, to maximize the harvested power in WPT while accounting for the energy harvester (EH)'s non-linearity. However, the existing transmit waveform design disregards the non-linear high power amplifiers (HPA) at the transmitter. Driven by this, this paper optimizes a multi-carrier waveform at the input of HPA to maximize the harvested DC power considering both HPA's and EH's non-linearities. Two optimization models are formulated based on whether the frequencies of the input waveform are concentrated within the transmit pass band or not. Analysis and simulations show that, while EH's non-linearity boosts the power harvesting performance, HPA's non-linearity degrades the harvested power. Hence, the optimal waveform shifts from multi-carrier that exploits EH's non-linearity to single-carrier that reduces HPA's detrimental non-linear distortion as the operational regime of WPT becomes more sensitive to HPA's non-linearity and less sensitive to EH's non-linearity (and inversely). Simultaneously, operating towards HPA's non-linear regime by increasing the input signal power benefits the harvested power since HPA's DC power supply is better exploited, whereas the end-to-end power transfer efficiency might decrease because of HPA's increasing non-linear degradation. Throughout the simulations, the proposed waveforms show significant gain over those not accounting for HPA's non-linearity, especially in frequency-flat channels. We also compare the two proposed waveforms and show that the severity of HPA's non-linearity dictates which of the two proposed waveforms is more beneficial.

Closed-Loop Wireless Power Transfer With Adaptive Waveform and Beamforming: Design, Prototype, and Experiment

A closed-loop far-field wireless power transfer (WPT) system with adaptive waveform and beamforming using limited feedback is designed, prototyped, and experimented. Spatial domain and frequency domain are jointly exploited by utilizing waveform and beamforming at the transmitter in WPT system to adapt to the multipath fading channel and boost the output dc power. A closed-loop architecture based on a codebook design and an over-the-air limited feedback with low complexity is proposed. The codebook consists of multiple codewords where each codeword represents particular waveform and beamforming. The transmitter sweeps through the codebook and the receiver then feeds the optimal codeword index back to the transmitter, so that the waveform and beamforming can be adaptive for maximizing the output dc power without requiring explicit channel estimation and the knowledge of accurate Channel State Information. The proposed closed-loop WPT with adaptive waveform and beamforming using limited feedback is prototyped using a Software Defined Radio equipment and measured in two real indoor environments. It is experimentally shown that the proposed closed-loop WPT with adaptive waveform and beamforming is able to enhance the output dc power by up to 14.7 dB in comparison with conventional 1-tone 1-antenna WPT system.

Far-Field Wireless Power Transfer for the Internet of Things

A complete end-to-end far-field wireless power transfer (WPT) is proposed and studied in this paper for the application of the Internet of Things (IoT) at the industrial, scientific, and medical (ISM) band of 2.4 GHz. The radiative WPT has achieved a remarkable attraction for the capability to transfer power in the long range. We propose two approaches. In the first approach, a 2×4 microstrip patch transmitter antenna array with a high gain and a narrow beamwidth is proposed that is rotated toward the IoT device using a small stepper motor. The performance of the rectifier in the receiving circuit was separately analyzed, and 17.54% efficiency was achieved with a load of 0.6 kΩ for the circuit, while the input power was 10 dBm. The overall system test was performed and the targeted result was investigated considering the distance between the transmitter and the receiver, and an input radio frequency (RF) power of 5 dBm to 15 dBm at 2.4 GHz. The second approach uses a 1×4 transmitter antenna array fed through a Butler matrix to provide four individual beams with a 22.5∘ angular separation, and 90∘ total angular coverage. The goal was to focus the power into four angular locations and to reduce the power waste in other directions. A mobile app was developed to control the direction of the beam. A system efficiency of as much as 19% was measured for an input RF power of 0 dBm and a resistive load of 62 kΩ.

Experimental Study of Effectiveness of Metasurface for Efficiency and Misalignment Enhancement of Near-Field WPT System

Unlikely for a far-field wireless power transfer (WPT) system, designing a metasurface is critical for a near-field WPT system. First, a robust near-field WPT system using open-loop spiral resonators and a metasurface are designed, and then, the effectiveness of the metasurface is investigated in improving the power transfer efficiency (PTE) and misalignment effects. Furthermore, a study of planar and angular misalignments is also executed experimentally to prove the WPT system immunity. The proposed WPT system offers a PTE of 30% when the Rx shifts by the lateral length of the Rx that means the Rx does not remain in front of the Tx. Moreover, the efficiency is almost unchanged due to the angular misalignment.

Development and Prospect of Wireless Power Transfer Technology Used to Power Unmanned Aerial Vehicle

Recently, unmanned aerial vehicles (UAV) have been widely used in the military and civil fields. However, the battery power is a key factor that restricts the operation range of the UAV. Using wireless power transfer (WPT) technology to power UAVs can improve the endurance of UAVs and enhance their maneuverability and flexibility. In this paper, the WPT technology is divided into three types: near-field WPT technology, far-field WPT technology and solar-powered UAV. The developments, challenges and prospects of these three types of WPT technologies used to power UAVs are summarized. For each type of WPT technology, the basic working principles are first introduced. The development of each type of WPT technology, as well as the challenges and application prospects in UAV charging, is introduced. The related works consist of academic and industry research, ranging from prototypes to commercial systems. Finally, three types of WPT technology used in UAV charging are compared and discussed, and the advantages and disadvantages of each type of WPT technology are shown. The related research showed that using WPT technology to power the UAV is a promising way to enhance the endurance of the UAV.

Information Harvesting for Far-Field Wireless Power Transfer

In today's smart world, the Internet of Things (IoT) offers a very large scale of technologies and use cases. An IoT system comprises a huge amount of low-power IoT network elements, sensors, and smart devices connected to each other. For these devices, it is important to have a sufficient level of energy resource for communication and to receive software/firmware updates periodically. In this aspect, wireless power transfer is a popular paradigm to tackle energy limitations. Therein, the battery of IoT devices can be remotely replenished by means of radio-frequency-radiated signaling, referred to as far-field wireless power transfer. Furthermore, harvesting energy from ambient communication signals available in the environment introduces simultaneous wireless information and power transfer technology. These systems face the trade-off between information capacity and energy harvesting efficiency. Accordingly, various signaling design frameworks have been proposed to comply with different system preferences between power and information. In this work, aside from existing trends, we propose a novel concept, information harvesting (IH), which introduces a novel protocol that lies in transmitting information on top of the existing wireless power transfer mechanism. Considering the diversity of IoT networks and the availability of wireless power transfer infrastructure, the proposed IH principle may turn out to be a pivotal methodology that combines physical layer security with wireless power transfer, particularly for cases where a large number of IoT devices require software/firmware updates along with periodical battery recharging needs.

Comparison Techniques for Optimization Switching Frequency In Energy Encryption Of Wireless Power Transfer System

This paper reviews the techniques for optimization switching frequency used in Wireless power transfer (WPT). WPT is one of the most useful ways to transfer power. Based on distances power transfer, the WPT system can be divided into three categories, namely, near, medium, and far fields. Inductive coupling and capacitive coupling contactless techniques are used in the near-field WPT. Magnetic resonant coupling technique is used in the medium-field WPT. Electromagnetic radiation is used in the far-field WPT. From the comparison in this paper, a model and design algorithm to optimize switching frequency for energy encryption of medium field wireless power transfer system are purposed. The maximum value of the performance are suitable to compute the optimal circuit parameter by algorithm itself.

High Performance Electrically Small Huygens Rectennas Enable Wirelessly Powered Internet of Things Sensing Applications: A Review

Far-field wireless power transfer (WPT) is a major breakthrough technology that will enable the many anticipated ubiquitous Internet of Things (IoT) applications associated with fifth generation (5G), sixth generation (6G), and beyond wireless ecosystems. Rectennas, which are the combination of rectifying circuits and antennas, are the most critical components in far-field WPT systems. However, compact application devices require even smaller integrated rectennas that simultaneously have large electromagnetic wave capture capabilities, high alternating current (AC)-to-direct current (DC) (AC-to-DC) conversion efficiencies, and facilitate a multifunctional wireless performance. This paper reviews various rectenna miniaturization techniques such as meandered planar inverted-F antenna (PIFA) rectennas; miniaturized monopole- and dipole-based rectennas; fractal loop and patch rectennas; dielectric-loaded rectennas; and electrically small near-field resonant parasitic rectennas. Their performance characteristics are summarized and then compared with our previously developed electrically small Huygens rectennas that are proven to be more suitable for IoT applications. They have been tailored, for example, to achieve battery-free IoT sensors as is demonstrated in this paper. Battery-free, wirelessly powered devices are smaller and lighter in weight in comparison to battery-powered devices. Moreover, they are environmentally friendly and, hence, have a significant societal benefit. A series of high-performance electrically small Huygens rectennas are presented including Huygens linearly-polarized (HLP) and circularly-polarized (HCP) rectennas; wirelessly powered IoT sensors based on these designs; and a dual-functional HLP rectenna and antenna system. Finally, two linear uniform HLP rectenna array systems are considered for significantly larger wireless power capture. Example arrays illustrate how they can be integrated advantageously with DC or radio frequency (RF) power-combining schemes for practical IoT applications.

Optimal Path Planning for Wireless Power Transfer Robot Using Area Division Deep Reinforcement Learning

This paper aims to solve the optimization problems in far-field wireless power transfer systems using deep reinforcement learning techniques. The Radio-Frequency (RF) wireless transmitter is mounted on a mobile robot, which patrols near the harvested energy-enabled Internet of Things (IoT) devices. The wireless transmitter intends to continuously cruise on the designated path in order to fairly charge all the stationary IoT devices in the shortest time. The Deep Q-Network (DQN) algorithm is applied to determine the optimal path for the robot to cruise on. When the number of IoT devices increases, the traditional DQN cannot converge to a closed-loop path or achieve the maximum reward. In order to solve these problems, an area division Deep Q-Network (AD-DQN) is invented. The algorithm can intelligently divide the complete charging field into several areas. In each area, the DQN algorithm is utilized to calculate the optimal path. After that, the segmented paths are combined to create a closed-loop path for the robot to cruise on, which can enable the robot to continuously charge all the IoT devices in the shortest time. The numerical results prove the superiority of the AD-DQN in optimizing the proposed wireless power transfer system.

Future Networks With Wireless Power Transfer and Energy Harvesting [Scanning the Issue]

This month’s special issue provides an overview of the state-of-the-art technology and theory for future networks with wireless power transfer and energy harvesting. Wireless communication using radio frequency (RF) radiation has been around for over 100 years and has shaped our society significantly over the past 40 years. However, wireless is not just about communications. For very short ranges, wireless power supply by inductive power transfer has become a reality through several commercial products and standards (Wireless Power Consortium, Power Matters Alliance, Alliance for Wireless Power, and Rezence). Far-field wireless power transfer (WPT) and wireless energy harvesting (WEH) via RF (as in wireless communications) on the other hand could be used for a longer range. Although it has long been seen as a potential for powering low-power devices, it is only recently that wireless (via RF) power has been recognized as a promising technology to cope with the explosion of low-power devices in future networks. Driven by the reduction in the energy needs of electronic devices (remember Koomey’s law according to which, in 20 years, a device will require 10 000 times less energy to compute a given task) and the advent of trillions of internet device objects [Internet of Things (IoT)], there is a need to rethink the design of the network of the future so that wireless can reach its full potential not only to carry information but also to transfer energy.

Ka-Band Rotman Lens-Based Retrodirective Beamforming System for Wireless Power Transfer

A retrodirective beamforming system (BFS) based on a Rotman lens is proposed for far-field wireless power transfer at Ka-band. The true-time-delay property of the Rotman lens allows for a wideband operation covering 28–38 GHz. The designed BFS comprises a Rotman lens with nine beam ports connected to a nine-element linear Vivaldi array. The proposed BFS is implemented using PCB technology for ease of manufacturing and low-cost processing. The simulated and measured results demonstrate that the proposed BFS can generate nine discrete beams over a scan range of ±45° with a wide impedance bandwidth.

Small Triple-Band Meandered PIFA for Brain-Implantable Biotelemetric Systems: Development and Testing in a Liquid Phantom

We present a meandered triple-band planar-inverted-F antenna (PIFA) for integration into brain-implantable biotelemetric systems. The target applications are wireless data communication, far-field wireless power transfer, and switching control between sleep/wake-up mode at the Medical Device Radiocommunication Service (MedRadio) band (401–406 MHz) and Industrial, Scientific and Medical (ISM) bands (902–928 MHz and 2400–2483.5 MHz), respectively. By embedding meandered slots into the radiator and shorting it to the ground, we downsized the antenna to the volume of 11 × 20.5 × 1.8 mm3. We optimized the antenna using a 7-layer numerical human head model using full-wave electromagnetic field simulation. In the simulation, we placed the implant in the cerebrospinal fluid (CSF) at a depth of 13.25 mm from the body surface, which is deeper than in most works on implantable antennas. We manufactured and tested the antenna in a liquid phantom which we replicated in the simulator for further comparison. The measured gain of the antenna reached the state-of-the-art values of −43.6 dBi, −25.8 dBi, and −20.1 dBi at 402 MHz, 902 MHz, and 2400 MHz, respectively.

A performance predictor of beamforming versus time-reversal based far-field wireless power transfer from linear array

For far-field wireless power transfer (WPT) in a complex propagation environment, a time-reversal (TR) based WPT that can overcome the drawbacks of conventional beamforming (BF) by taking advantage of multipath has been recently proposed. However, due to the WPT performance of BF and TR depending on the complexity of the propagation environment, the performance prediction between BF versus TR would be required. We present a detailed and generalized analysis of the recently proposed performance metric referred to as the peak received power ratio (PRPR) for linear array-based WPT. Here, the effectiveness of PRPR is verified via measurement for free space and indoor scenarios. The results demonstrate that PRPR is directly related to the complexity of the propagation environment and the corresponding power transmission capability of BF and TR. That is, the higher the complexity, the greater the value of PRPR and TR outperforms BF with higher peak power given the same average transmit power and vice versa. The mode decision between BF and TR based on PRPR potentially promises efficient far-field WPT even in a dynamic propagation environment.

In Situ MIMO-WPT Recharging of UAVs Using Intelligent Flying Energy Sources

Unmanned Aerial Vehicles (UAVs), used in civilian applications such as emergency medical deliveries, precision agriculture, wireless communication provisioning, etc., face the challenge of limited flight time due to their reliance on the on-board battery. Therefore, developing efficient mechanisms for in situ power transfer to recharge UAV batteries holds potential to extend their mission time. In this paper, we study the use of the far-field wireless power transfer (WPT) technique from specialized, transmitter UAVs (tUAVs) carrying Multiple Input Multiple Output (MIMO) antennas for transferring wireless power to receiver UAVs (rUAVs) in a mission. The tUAVs can fly and adjust their distance to the rUAVs to maximize energy transfer gain. The use of MIMO antennas further boosts the energy reception by narrowing the energy beam toward the rUAVs. The complexity of their dynamic operating environment increases with the growing number of tUAVs and rUAVs with varying levels of energy consumption and residual power. We propose an intelligent trajectory selection algorithm for the tUAVs based on a deep reinforcement learning model called Proximal Policy Optimization (PPO) to optimize the energy transfer gain. The simulation results demonstrate that the PPO-based system achieves about a tenfold increase in flight time for a set of realistic transmit power, distance, sub-band number and antenna numbers. Further, PPO outperforms the benchmark movement strategies of “Traveling Salesman Problem” and “Low Battery First” when used by the tUAVs.

Recycling Ambient RF Energy: Far-Field Wireless Power Transfer and Harmonic Backscattering

It all started with a spark. Let us rewind back to 1887, in Germany. The days were getting cooler in the garden of the Technische Hochschule in Karlsruhe as Heinrich Rudolf Hertz was setting up the first-ever far-field wireless power transmission with increasingly more power [1]. He was striving to demonstrate the wireless nature of electromagnetic waves and propagation. Moreover, as no high-frequency (nearly 100 MHz) voltmeter was available at that time for his experiment, he had to transmit enough power to generate a spark-hundreds of volts!-at the receiver to validate the famous theory of James Clerk Maxwell.

Graded refractive index metamaterial for enhanced far-field wireless power transfer efficiency in S-band

A graded refractive index metamaterial (MM) exhibits a magnetic dipole designed and demonstrated experimentally for 2.45 GHz operating frequency. The system efficiency of the far-field wireless power transfer technique increased to 32.6% with MM. With 16° reduction in half power beamwidths, 9.78 dBm is received at 1 m with 30 dBm input when MM is placed at an optimum distance of 2.5λ0 from transmitting antenna aperture. The method is studied experimentally, and results are compared with the Friis transmission equation, thus validating experimental and theoretical analysis. The system efficiency is always higher than the system with no MM, and hence the proposed method is suitable for long-range wireless power transfer applications maximizing the theoretical power received.