Wireless, Battery-Free, High-Sensitivity 5G RF Energy Harvesters for Next Generation IoT Sensor Tags

Abstract

The Internet of Things (IoT) is revolutionizing various industries, enabling a new wave of smart applications such as automated asset tracking in warehouses, substation monitoring in smart grids, and precision agriculture. However, as IoT devices proliferate, powering these devices in a sustainable and maintenance-free manner has become a critical challenge. Traditional IoT systems rely on batteries, which present issues of limited lifespan, environmental impact, and maintenance costs, especially in large-scale deployments. As a result, the development of battery-free IoT devices powered by ambient energy harvesting has gained significant attention. Among various energy-harvesting technologies, radio frequency (RF) energy harvesting has emerged as a promising solution for powering IoT devices. By harvesting energy from ambient RF signals in licensed frequency bands, RF energy-harvesting systems eliminate the need for batteries and allow for continuous, maintenance-free operation. This is especially crucial in environments where battery replacement is impractical or impossible, such as in large industrial warehouses, remote infrastructure, and hazardous environments. However, achieving high sensitivity and reliable operation in RF energy-harvesting systems poses several challenges. High-sensitivity rectifiers are required to capture and convert weak RF signals into usable energy, but integrating these rectifiers with ultra-low power baseband data processing circuits remains a significant hurdle. Moreover, antenna-rectifier matching calibration must be compatible with the duty-cycled operation of these tags, where brief communication periods are followed by long charging intervals. Additionally, the antenna system must be robust to detuning when placed on various objects, ensuring that the system can operate effectively in diverse environments. This thesis presents two integrated circuits to work towards these goals. The first chip is designed with the goal of minimizing the charging time as much as possible, which is critical in scenarios such as inventory management in warehouses, and tamper detection. The goal was to achieve < 1-minute charging time while maintaining sensitivity competitive with the state-of-the-art. Unlike previous harvesters that either focused solely on sensitivity without integrating baseband processing and communication, or included those features but considered continuous communication at low sensitivity, the IC developed in this work achieves a sensitivity of −31 dBm and is capable of backscattering data approximately 18 seconds after a cold start. It also provides a detailed description of the difficulty of achieving higher sensitivities at higher 5G frequencies. The second chip in this thesis builds upon the first one and integrates an analog front-end to convert sensor data for environmental monitoring. We implemented an antenna-rectifier calibration method that is maintained as long as there if RF power, even though the tag goes into long charging periods. Even though the charging time, or the data readout interval, for these tags is more relaxed compared to the inventory management applications, we have also developed a design methodology to minimize the energy required to generate a data packet for backscattering, through which we were able to keep the charging time at 4 minutes while having additional functionalities and backscattering at a higher data rate compared to the first chip. Finally, a simple shielding method was implemented to enable the tags to be placed on any objects without resonance frequency detuning. All of these were achieved while still obtaining a sensitivity of −30 dBm, competitive with the state of the art. In addition, the third project investigates the use of heterogeneously integrated “beyondCMOS” devices to enhance overall rectifier performance. These emerging devices, fabricated by Palacios Group at MIT, show promise in overcoming sensitivity limitations commonly found in rectifiers, thereby extending the range and coverage of energy-harvesting IoT systems. We conduct a detailed characterization of these devices, highlighting their unique physical behaviors not present in standard CMOS technology, and provide system-level design guidelines for building improved rectifiers. Preliminary simulation results show that rectifiers using negative-capacitance field-effect transistors (NCFETs) can harvest up to four times as much power than their CMOS-based counterparts, while maintaining the same sensitivity. This thesis outlines the design, implementation, and evaluation of all three systems. The two aforementioned ICs are tested both in simulation and in real-world scenarios such as a typical office environment. Meanwhile, the novel device technologies are explored through simulation, demonstrating their significant potential for next-generation rectifier design.