CO-DESIGN OF FLEXIBLE RF/ANTENNA SYSTEMS FOR EFFICIENT WIRELESSLY POWERED BACKSCATTER COMMUNICATION PLATFORMS

The energy efficiency paradigm is a major bottleneck for the development of wireless sensor networks (WSNs) and Internet of Things (IoT) architectures and technologies. This edited book presents comprehensive coverage of energy harvesting sources and techniques that can be used for WSN and IoT systems

Since the Internet of Things (IoT) is expected to be the new technology to drive the semiconductor industry, significant research efforts have been made to develop new circuit and system techniques for autonomous/very low-power operation of wireless sensor nodes. Very low-power consumption of sensors is key to increase battery lifetime or allow for battery-less (autonomous) operation of sensors, which contributes to preventing or reducing the high maintenance costs of battery supplied sensors and reduce the amount of discarded batteries. This thesis, entitled Radio Frequency Energy Harvesting and Low Power Data Transmission for Autonomous Wireless Sensor Nodes, presents very low-power consumption circuit and system techniques combined with energy harvesting that allow the creation of autonomous wireless sensor nodes. This work focuses on three main challenges: 1) how to improve energy harvesting efficiency, 2) how to minimize power consumption of data transmission and 3) how to combine low-power techniques and energy harvesting in a system. These challenges are addressed in this thesis with on-PCB and Integrated Circuits (IC) solutions. The efficiency of radio frequency (RF) energy harvesting is improved by proposing a new topology of a charge-pump rectifier. The proposed topology uses a voltage boosting network to compensate for the voltage drop in the transistors. The new topology is presented and analyzed. Simulation results are compared to the analytical analysis and measurement results of the circuit that has been fabricated in a 0.18um CMOS technology and operates at 13.53 MHz. Although the efficiency of RF energy harvesting is improved using the above technique, at the same time, low power techniques in data transmission should be developed to save energy. Pulse width modulation and impulse transmission techniques to minimize power consumption have been developed and are presented in this thesis. The developed pulse modulation circuitry has been fabricated in 0.18um CMOS technology as part of a System on Chip (SoC). The new impulse transmitter topology for low-voltage low-power operation has been fabricated on PCB with micro-wave discrete components. Theoretical analysis, simulations and measurements results are shown to prove the impulse transmitter concept. The circuits developed are integrated in a SoC with energy harvesting to prove the concept of autonomous wireless sensor nodes. Two sensor nodes have been designed and measured: one for autonomous temperature monitoring and the second for autonomous ECG monitoring. Both designs operate from wireless power without the use of batteries. Finally, the work developed in this thesis is summarized and future research possibilities are discussed.

Introduction: The energy supply challenge in wireless charging applications is currently a significant research problem. To address this issue, this study introduces a novel small-scale long-distance radio frequency (RF) energy harvesting system that utilizes a hybrid model incorporating CNN, LSTM, and reinforcement learning. This research aims to improve RF energy harvesting and wireless charging efficiency.Method: The methodology of this study involves data collection, data processing, model training and evaluation, and integration of reinforcement learning algorithms. Firstly, RF signal data at different distances are collected and rigorously processed to create training and testing datasets. Next, the CNN-LSTM model is trained using the prepared data, and model performance is enhanced by adjusting hyperparameters. During the evaluation phase, specialized test data is used to assess the accuracy of the model in predicting RF energy harvesting and wireless charging efficiency. Finally, reinforcement learning algorithms are integrated, and a reward function is defined to incentivize efficient wireless charging and maximize energy harvesting, allowing the system to dynamically adjust its strategy in real time.Results: Experimental validation demonstrates that the optimized CNN-LSTM model exhibits high accuracy in predicting RF energy harvesting and wireless charging efficiency. Through the integration of reinforcement learning algorithms, the system can dynamically adjust its strategy in real time, maximizing energy harvesting efficiency and charging effectiveness. These results indicate significant progress in long-distance RF energy harvesting and wireless charging with this system.Discussion: The results of this study validate the outstanding performance of the small-scale long-distance RF energy harvesting system. This system is not only applicable to current wireless charging applications but also demonstrates potential in other wireless charging domains. Particularly, it holds significant prospects in providing energy support for wearable devices, Internet of Things (IoT), and mobile devices.

<p>With the expansion of Internet of things (IoT) scale and the extension of its application fields, node energy acquisition has become one of the constraints on the development and application of IoT technology. Wireless energy acquisition is faced with the challenge of how to enhance power conversion efficiency under low input power in the broadband range. This study aimed to improve the capability of energy collection and transmission in wireless communication, an efficient dual-frequency energy harvesting system was proposed, moreover a novel dual-frequency planar inverted-f antenna (PIFA) and its energy conversion circuit for Radio frequency (RF) energy harvesting were designed by studying the characteristics of ambient RF energy and antenna. The developed antenna is designed and manufactured for GSM 900mhz or DCS 1.8 GHz to harvest the ambient energy provided by nearby devices. It is proved that the new dual-frequency antenna still has high efficiency when the input power is less than -20 dbm, and can maximize the receiving energy. The system works with an energy conversion and storage module to convert weak signals into required voltages, the simulation and test results show that the maximum conversion efficiency of the prototype is about 65% at 920MHZ and 1.8GHz at -20 dBm input power.</p> <p> </p>

This paper presents the design and implementation of an RF energy harvester system at 5.8[Formula: see text]GHz for low-power wireless transmission applications. The potential application of the proposed system is to wirelessly power sensor nodes. First, a design methodology of the rectifier based on a theoretical approach is presented. The simulation results show an excellent correlation with the theoretical ones, proving the accuracy of the proposed design methodology. A prototype is fabricated and the simulation results are validated by the measurements. Then, the rectenna is combined to a commercial power management circuit and a load which emulates the behavior of a sensor. The power management circuit boosts and regulates the output DC voltage as well as stores the collected energy into a capacitor. Finally, the complete system is experimentally tested and excellent performances are demonstrated. The efficiency of the RF energy harvester is 24% at [Formula: see text]10[Formula: see text]dBm input power and 47% at [Formula: see text]5[Formula: see text]dBm input power which are the highest reported measured efficiencies at this frequency and at those power levels. The complete rectenna system is able to harvest 4.62[Formula: see text]mJ in 40 s and 192[Formula: see text]s for [Formula: see text]6[Formula: see text]dBm and [Formula: see text]10[Formula: see text]dBm input power, respectively allowing us to power wirelessly low-power electronic devices.

This paper proposes a hybrid microwave power receiving (MPR) metasurface array with efficient dual matching of surface impedance and phase gradient. The hybrid array comprises three components: a reflective phase gradient metasurface (R-PGM) array, a surface wave focusing array, and an energy harvesting port. The R-PGMs efficiently convert incident electromagnetic waves into surface waves. The surface wave focusing array then concentrates the energy onto the integrated harvesting port through dual matching of surface impedance and phase gradient. Connecting a rectifier circuit enables efficient microwave energy reception and RF-DC conversion, avoiding the need for multiple rectifiers or complex feeding networks in traditional MPR designs. Numerical analysis and experimental tests verify the superior performance of this hybrid array design in efficient microwave energy harvesting and RF-DC conversion, achieving 90.84% plane wave-to-surface wave conversion efficiency, 76.67% surface wave energy harvesting efficiency, and 49.28% overall RF-DC conversion efficiency. Across the wideband range of 5.6–6.0 GHz, the energy harvesting efficiency remains consistently high, demonstrating the superior characteristics and promising potential of this design in MPR device development.

With the advancement of wireless communication, Radio Frequency (RF) energy harvesting has gained significant attention over the past decade. RF energy harvesting is emerging as a sustainable alternative to conventional batteries, enabling self-powered operation in wireless sensor networks and Internet-of-Things (IoT) devices. This work presents a single-band cylindrical dielectric resonator antenna (DRA) operating at the 2.4 GHz Wi-Fi band for efficient RF energy harvesting. The proposed antenna (120 × 120 × 1.6 mm3) achieves a gain of 6.5 dBi at 2.4 GHz. A single-diode shunt rectifier converts RF signals into usable DC power. Simulation and experimental results exhibit strong agreement. At 5 dBm input power, the rectifier achieves a peak power conversion efficiency of 66%, while the overall rectenna reaches 64.4% efficiency with a maximum output voltage of 1.34 V. The proposed rectenna is suitable for low-power applications such as sensors, smartwatches, and energy storage devices like batteries and supercapacitors.

This paper presents a dual-band sub-1 GHz rectenna for near and far-field powering of Internet of Things (IoT) nodes. The rectifier is based on a coplanar waveguide (CPW) voltage doubler with inductive matching, achieving over 80% power conversion efficiency (PCE) at 10 dBm and 915 MHz. The rectifier is designed to directly charge a 6.8 mF supercapacitor with no DC power management circuitry or load tracking, achieving the highest reported average charging efficiency of 47.2% and 33.3%, at 433 and 915 MHz respectively. From 6 dBm input power, a 1.5-2.8 mA dummy load can be RF-powered with over 30% duty-cycle at 915 MHz. A dual-band single-layer antenna is designed and fabricated using dispenser printing on a flexible polyimide substrate. The antenna maintains over 2.1 dBi gain at 915 MHz with omnidirectional patterns, while occupying under 10 × 10 cm. The integrated rectenna is evaluated in two real-world applications: energy harvesting from a 915 MHz transmitter showing a 49% wireless charging efficiency for a $7.4 μW/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> incident power density; and ambient energy harvesting generating 47 mJ in 5 seconds at 3 to 5 cm from a handheld two-way radio.

This paper presents a novel circularly polarized rectenna designed for efficient electromagnetic energy harvesting at the 2.45 GHz ISM band. A compact antenna structure is designed to achieve high performance in terms of radiation efficiency, axial ratio, directivity, effective area, and harmonic rejection over the entire bandwidth of the ISM frequency band. The optimized rectifier circuit enhances the RF harvested energy efficiency, with an AC-to-DC conversion efficiency ranging from 36% to 70% for low-level input power ranging from −10 dBm to 0 dBm. The stable output of DC power confirms the suitability of this design for various practical applications, including wireless sensor networks, energy harvesting power supplies, medical implants, and environmental monitoring systems. Experimental validation, which includes both the reflection coefficient and radiation patterns of the designed antenna, confirms the accuracy of the simulation. The study found that the proposed energy harvesting system has a high total efficiency ranging from 53% to 63% and is well-suited for low-power energy harvesting (0 dBm) from ambient electromagnetic radiation. The proposed circularly polarized rectenna is a competitive option for efficient electromagnetic energy harvesting, both as a standalone unit and in an array, due to its high performance, feasibility, and versatility in meeting various energy harvesting requirements. This makes it a promising and cost-effective solution for various wireless communication applications, offering great potential for efficient energy harvesting from ambient electromagnetic radiation.

Energy harvesting in wireless sensor networks: A comprehensive review

With the rapid development of Internet of Things (IoT) and the popularity of wireless sensors, using internal permanent or rechargeable batteries as a power source will face a higher maintenance workload. Therefore, self-powered wireless sensors through environmental energy harvesting are becoming an important development trend. Among the many studies of energy harvesting, the research on rotational energy harvesting still has many shortcomings, such as rarely working effectively under low-frequency rotational motion or working in a narrow frequency band. In this article, a rotational magnetic couple piezoelectric energy harvester is proposed. Under the low-frequency excitation (<10 Hz) condition, the harvester can convert low-frequency rotational into high-frequency vibrational of the piezoelectric beam by frequency up-conversion, effectively increasing the working bandwidth (0.5–16 Hz) and improving the efficiency of low-speed rotational energy harvesting. In addition, when the excitation frequency is too high (>16 Hz), it can solve the condition that the piezoelectric beam cannot respond in time by frequency down-conversion. Therefore, the energy harvester still has a certain degree of energy harvesting ability (18–22 Hz and 29–31 Hz) under high-frequency conditions. Meanwhile, corresponding theoretical analyses and experimental verifications were carried out to investigate the dynamic characteristics of the harvester with different excitation and installation directions. The experimental results illustrate that the proposed energy harvester has a wider working bandwidth benefiting from the frequency up-conversion mechanism and frequency down-conversion mechanism. In addition, the forward beam will have a wider bandwidth than the inverse beam due to the softening effect. In addition, the maximum powers of the forward and inverse beams at 310 rpm (15.5 Hz) are 93.8 μW and 58.5 μW, respectively. The maximum powers of the two beams at 420 rpm (21 Hz) reached 177 μW and 85.2 μW, respectively. The self-powered requirement of micromechanical systems can be achieved. Furthermore, this study provides the theoretical and experimental basis for rotational energy harvesting.

We study energy harvesting in a binary phononic crystal (PC) beam at the defect mode. Specifically, we consider the placement of a mismatched unit cell related to the excitation point. The mismatched unit cell contains a perfect segment and a geometrically mismatched one with a lower flexural rigidity which serves as a point defect. We show that the strain in the defect PC beam is much larger than those in homogeneous beams with a defect segment. We suggest that the defect segment should be arranged in the first unit cell, but not directly connected to the excitation source, to achieve efficient less-attenuated localized energy harvesting. To harvest the energy, a polyvinylidene fluoride (PVDF) film is attached on top of the mismatched segment. Our numerical and experimental results indicate that the placement of the mismatched segment, which has not been addressed for PC beams under mechanical excitation, plays an important role in efficient energy harvesting based on the defect mode.

With the popularization of Internet-of-things (IoT) and wireless communication systems, a diverse set of applications in smart cities are emerging to improve the city-life. These applications usually require a large coverage area and minimal operation and maintenance cost. To this end, the recently emerging backscatter communication (BC) is gaining interest in both industry and academia as a new communication paradigm that provides high energy efficient communications that may even work in a battery-less mode and, thus, it is well suited for smart city applications. However, the coverage of BC in urban area deployments is not available, and the feasibility of its utilization for smart city applications is not known. In this article, we present a comprehensive coverage study of a practical cellular carrier-based BC system for indoor and outdoor scenarios in a downtown area of a Helsinki city. In particular, we evaluate the coverage outage performance of different low-power and wide area technologies, i.e., long range (LoRa) backscatter, arrow band-Internet of Things (NB-IoT), and Bluetooth low energy (BLE) based BC at different frequencies of operation. To do so, we carry out a comprehensive campaign of simulations while using a sophisticated three-dimensional (3D) ray tracing (RT) tool, ITU outdoor model, and 3rd generation partnership project (3GPP) indoor hotspot model. This study also covers the energy harvesting aspects of backscatter device, and it highlights the importance of future backscatter devices with high energy harvesting efficiency. The simulation results and discussion provided in this article will be helpful in understanding the coverage aspects of practical backscatter communication system in a smart city environment.

Hybrid Energy-Harvesting Systems Based on Triboelectric Nanogenerators

As a new technology for high-entropy energy harvesting, a triboelectric nanogenerator (TENG) has broad applications in sensor networks and internet of things as a power source, but its average power density is limited by the fixed low-frequency output. Here, a frequency-multiplication TENG based on intrinsic high frequency of tuning fork is proposed which enables converting low-frequency mechanical energy into high-frequency electric energy. A tuning-fork TENG is used to systematically study the effects of intrinsic frequency, dielectric's thickness, and gap distance on its electric performance, and a total transferred charges of 4.3µC and an average power density of 9.42mWm-2 are realized at the triggering frequency of 0.2Hz, which are 71 times and 5.7 times than that of the single-cycle output of conventional contact-separation TENG, respectively. Moreover, the crest factor also decreases from 3.5 to around 1.5. Then, a homemade tuning fork-like TENG is reasonably designed for harvesting ambient wind energy, achieving an average power density of 20.02mWm-2 at a wind speed of 7ms-1 . Specially, its impedance resistance is independent of the mechanical triggering frequency, simplifying the back-end power management circuit design. Therefore, the frequency-multiplication TENG shows a great potential for efficient distributed energy harvesting.