Radio frequency (RF) energy harvesting and wireless power transmission (WPT) technologies —both near-field and far-field—have attracted significant interest for wireless applications and RFID systems. We already utilize near-field WPT products in our life and it is expected that RF EH and far-field WPT systems can drive the future low-power wireless systems. In this article, we initially present a brief historical overview of these technologies. The main technical challenges of rectennas and WPT transmitters are discussed. Furthermore, this paper presents the recent advances on the development of these technologies, including the possibility of powering RFID systems through the millimeter wave power from 5G networks, the trends in flexible rectennas design and the technological developments on the simultaneous wireless information and power transfer (SWIPT).

A novel diplexer-based fully passive transponder is presented in this paper, which targets sub-6-GHz 5G-compatible internet-of-things applications. To alleviate the antenna design restrictions of traditional transponder with two separate antennas, a new architecture has been proposed with the introduction of a diplexer, which allows transponder to simply employ a dual-band antenna. In this paper, a dual-band circularly polarized omnidirectional spiral slot antenna, with enhanced bandwidth and gain performance, is designed as the transponder Tx/Rx antenna. Besides the new architecture, a diode selection criterion is proposed as well. Analytical models are derived, showing relationships between the diode's SPICE parameters and the conversion efficiency or conversion loss (CL) of such diode-based transponders. With help of the analysis, transponder designers can easily identify diodes to implement transponders with better performance. Under the guidance of the criterion, low-barrier diode SMS7630 is chosen for verification. Measured CL results of the transponder circuitry part show a noticeable improvement over the state-of-the-art works. The complete prototype was tested with radar's transmitting power of 25 dBm, and it presents a maximum read-out distance up to 7 m when the operating fundamental frequency is 3.5 GHz.

Continuous advances in ultralow-power electronics are a fundamental driving force of developing far-field wireless power harvesting techniques, which generally harnesses low/ambient energy in free space or dedicated wireless power from a power base station. This article reviews recent developments and technology trends in far-field (radiative) wireless power harvesting, including modeling of rectification process inside nonlinear devices, insights into rectenna system design, and demonstrations of emerging applications. The rectification process inside nonlinear devices will be explored, upon which a device selection guideline is provided to help quickly identify suitable devices. Then, the optimization of rectenna design is discussed to realize efficiency enhancement at a limited input power level. Since far-field wireless power harvesting has become a critical enabling technology for battery-free Internet of Things, a series of promising applications is demonstrated in this article to highlight the challenges and opportunities in the field.

This work summarizes an outdoor dynamic measurement of ambient radiofrequency (RF) power density in the core areas of Montreal city, which can eventually be exploited for possible battery-free Internet of Things (IoT) sensor network planning. The measurement covers a frequency range of 400-2700 MHz. The power density along major streets, roads, and highways is captured and analyzed, which differs from previous stationary measurements. Results indicate that cellular communication bands (GSM/LTE850 and LTE700) are the best choice for RF energy recycling in downtown (populated) areas. The largest average power levels of GSM/LTE850 and LTE700 bands are -35.50 and -36.95 dBm, respectively. In contrast, in suburban areas where cellular communication signals are generally weak, the digital television band with a peak level of -47.68 dBm presents the largest among all frequency bands. Moreover, one separate measurement at a fixed location is conducted on a typical workday to study the effects of foot traffic on ambient RF power density. Results illustrate that no obvious fluctuations are detected during this daytime measurement. For example, both average power levels of GSM/LTE850 and LTE700 remain at a relatively stable level of -25 dBm throughout the measurement. With the advance of far-field wireless power transfer and backscattering techniques, ambient RF power-enabled battery-free IoT sensors are feasible and will contribute to the planning and implementation of a smarter and greener city.

An integrated cooperative harvester capable of collecting both electromagnetic [(principally radio frequency (RF)] and kinetic energy simultaneously is proposed and studied in this paper. One single substrate board is demonstrated to accommodate and integrate an electromagnetic kinetic generator, an F-shaped antenna, and a rectifying circuit, having a compact credit-card size. Cooperative harvesting mode enables a significant enhancement of the diode's power conversion efficiency (PCE) and also greatly strengthening the resilience of harvester at the same time. Measured results suggest that SMS7630 and HSMS-2850-based cooperative harvesters can increase dc output power by 50% to around 100% when both injecting power sources exhibit below -37 dBm with reference to the total added amount of dc power harvested directly from separate RF and kinetic excitation modes. A closed-form method is proposed and used to predict the enhanced efficiency performance of the cooperative harvester within a low power range of interest (from -50 to -30 dBm). Our analysis highlights the roles of the diode's nonlinear junction resistance and capacitance. Good agreements are observed when comparing the analysis results with harmonic balance simulations by ADS. Furthermore, this method is carefully verified by measured results of two different diode-based harvesters. Measured dc output power rectified from cooperative harvesting mode is reported to be 1.16 and 1.97 times larger with SMS7630 and HSMS-2850, respectively, when both power sources are -40 dBm, compared to the conventional single RF harvesting mode. The proposed cooperative harvester with its strong resilience is highly suitable for ambient power harvesting in an ever-changing environment.

In this paper, an ambient power harvester with a single diode device is proposed for simultaneously scavenging both radiofrequency (RF) and thermal energy in a mixed and cooperative manner. This cooperative harvesting process is theoretically examined through a proposed model of diode and then validated by simulation and measurement. In the proposed cooperative power harvester, the harvested dc voltage from thermal source is used to bias the diode for improving the diode's RF-to-dc power conversion efficiency (PCE). An accurate analytical model of Schottky diode is developed for specifying the constraining parameters of RF-to-dc PCE and accurately predicting diode's performances in a low RF power range ( $\leq \minus$ 25 dBm), respectively. The calculated results are found to be in a good agreement with the simulated ones obtained by the harmonic balance simulator in Advanced Design System (ADS). For demonstration and validation, the proposed mixed cooperative power harvester is designed and prototyped on the basis of diode SMS7630. A total measured output dc power around 0.8 μW is obtained with an RF-to-dc PCE around 33.4%, when the two injecting power sources at the diode are both $\minus$ 30 dBm. In addition, rectennas with and without a matching network are both fabricated and tested. By eliminating the L matching network, the rectenna is found to offer a higher dc output power. The proposed mixed cooperative power harvester is hoped to find potential real-world applications in ambient atmosphere with RF coverage and temperature gradient. It not only helps to produce a higher power but also provides a reliable way of improving the resilience of dc power production.