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Backscatter communication is an emerging paradigm for pervasive connectivity of low-power communication devices. Wirelessly powered backscattering wireless sensor networks (WSNs) become particularly important to meet the upcoming era of the Internet of Things (IoT), which requires the massive deployment of self-sustainable and maintenance-free low-cost sensing and communication devices. This article will introduce the state-of-the-art antenna design and radio frequency (RF) system integration for wirelessly powered backscatter communications, covering both the node and the base unit. We capture the latest development in ultralow-power RF front ends and coding schemes for μW-level backscatter modulators, as well as the latest progress in wireless power transfer (WPT) and energy harvesting (EH) techniques. Newly emerged rectenna system, waveform design, and channel optimization are reviewed in light of the opportunities for adaptively optimizing the WPT/EH efficiency for low-power signals with varying conditions. In addition, advanced device packaging and integration technologies in, e.g., additively manufactured RF components and modules for microwave and millimeter-wave ubiquitous sensing and backscattering energy-autonomous RF structures are reported. Inkjet printing for the sustainable and ultralow-cost fabrication of flexible RF devices and sensors will be reviewed to provide a prospective insight into the future packaging of backscatter communications from the chip-level design to complete system integration. Finally, this article will also address the challenges in fully wireless powered backscatter radio networks and discuss the future directions of backscatter communication in terms of ``Green IoT'' and ``Low Carbon'' smart home, smart city, smart skin, and machine-to-machine (M2M) applications.

Powering the increasing number of sensor nodes used in the Internet of Things creates a technological challenge. The economic and sustainability issues of battery-powered devices mean that wirelessly powered operation—combined with environmentally friendly circuit technologies—will be needed. Large-area electronics—which can be based on organic semiconductors, amorphous metal oxide semiconductors, semiconducting carbon nanotubes and two-dimensional semiconductors—could provide a solution. Here we examine the potential of large-area electronics technology in the development of sustainable, wirelessly powered Internet of Things sensor nodes. We provide a system-level analysis of wirelessly powered sensor nodes, identifying the constraints faced by such devices and highlighting promising architectures and design approaches. We then explore the use of large-area electronics technology in wirelessly powered Internet of Things sensor nodes, with a focus on low-power transistor circuits for digital processing and signal amplification, as well as high-speed diodes and printed antennas for data communication and radiofrequency energy harvesting.

Radio technologies are appealing for unobtrusive and remote monitoring of human activities. Radar-based human activity recognition proves to be a success, for example, Project Soli developed by Google. However, it is expensive to scale up for multiuser environments. In this article, we propose a solution—the HoloTag system—which circumvents the multichannel-radar scaling problem through the use of a quasivirtual ultralow-cost UHF RFID array over which a holographic projection of its environment is measured and used to both localize and monitor the health of several targets. The method is first described in detail, before the image reconstruction process, employing known beamforming algorithms—Delay & Sum, and Capon—is shown and its scaling properties simulated. Then, the idiosyncrasies of the implementation of HoloTag using low-cost off-the-shelf hardware are explained, before its ability to simultaneously measure the breathing rates and positions of multiple real and synthetic targets with accuracies of better than 0.8 bpm and 20 cm is demonstrated.

The pandemic caused by the SARS-COv2 virus demanded actions in universities to ensure safety public health of their community. Several fronts are working in this direction, from physical distancing and sanitation protocols, to the use of indoor and outdoor environments, which includes the circulation of pedestrians and cyclists through the campuses. The University of Campinas is carrying out a tactical urbanism project, consisting of a set of actions aimed at low-cost changes in the built environment to test possible improvements through a living laboratory that involves the entire local community. The living laboratory is an innovative systemic approach that enables the integration of research and innovation in a territorial context through co-creation that involves the local community. The objective of the living laboratory of tactical urbanism at Unicamp is to encourage and guide the circulation of pedestrians and cyclists on the campuses in conditions that ensure physical distance. For this, workshops, questionnaires, decisions and collaborative actions are carrying out, so the community is involved in all stages of the project. After the situational survey and planning, actions should involve surface design with patterns and artistic paintings along the streets in order to promote shared use between modes of transport with special consideration for pedestrians and cyclists. As a result, it is expected that Unicamp will be able to restart face-to-face activities during the pandemic with public safety, mobility and low investment.Keyword: Sustainable Urban Mobility, Pedestrian, Cyclist, Sustainable Campus

This communication proposes the reconfiguration of a coplanar-fed single arm bowtie antenna structure by relying on the integration of a digital tunable capacitor (DTC). The coplanar-fed single arm bowtie antenna is presented in two configurations. The first configuration is a 2-D topology with a modified ground plane arrangement. The second configuration employs vertical folding for the ground plane as well as the radiating arm. In addition, vertical copper inserts are embedded within the bowtie single arm to further enhance the compactness of the structure and transform it into a 3-D topology. Both antennas are initially designed to operate at 868 MHz for Internet of Things (IoT) applications, with a digitally reconfigurable frequency operation for wireless communication. The 2-D antenna structure exhibits a 77% size reduction at the IoT operational frequency of 868 MHz in comparison with a similar rectangular patch antenna operating at the same frequency. On the other hand, the 3-D antenna structure reaches 84% of size reduction at the same frequency. A figure of merit (FOM) is derived to prove the superior performance of the presented antenna structure.

The efficacy of the deployment of backscatter networks in sensor networks and the internet of things has been severely limited by the maximum read range of backscatter radio frequency identification (RFID) tags. The work presented here demonstrates a low-power two-way repeater architecture capable of increasing the read range and extending the coverage of tags to non-line-of-sight (NLOS) scenarios. This is achieved through the use of a two-way frequency divided tunnel diode-based reflection amplifier optimized for use in backscattering channels. The system reports a more than 2X increase in the read range of a semi-passive RFID tag at 17 dBm output power. The work presented here shows a system capable of giving a comparable performance with the state of the art while consuming up to 1000X less power in a simple single-component front end.

The advent of wearable and Internet of Things (IoT)‐connected technologies has enabled and brought to light the potential for a cornucopia of new health‐applied wearable devices. Given a dense implementation of RFID devices for medical and biomonitoring applications owing to the advent of the IoT, a proper way to simultaneously interrogate and gather useful information from an array of sensors is required. An attractive means to achieve this is through the employment of RADAR principles, specifically frequency‐modulated continuous wave. The use of additive manufacturing technologies–such as the commonly encountered inkjet‐printing technology or the popular forms of 3D printing approaches–enables, through their versatility, a surprising wealth of solutions to the limitations of the state‐of‐the‐art health wearables. This chapter presents new strategies enabling the powering of printed wearable systems using both legacy radio‐frequency wireless networks and upcoming 5G millimeter‐wave implementations.

This paper addresses backscatter communications as a new paradigm for Internet of Things (IoT) sensor wireless solutions. The paper clearly presents the history background of the technology and then makes a summary of the more recent developments form several groups around the world, with emphasis in US and Europe groups. The analysis done in this paper will address circuit and physical layer approaches for such a solution and its real implementations, having in mind IoT sensors for long range high speed communications.

With the revolutionary developments in the fields of millimeter-wave (mm-wave) and Internet of Things (IoT) technologies and the billion devices promised to be implemented by the end of the decade, the realization of inexpensive, low-power, and intelligent systems is highly desirable. Additive manufacturing (AM) is a technology seeing widespread adoption due to its ability to enable rapid prototyping for iterative design; its reduced setup costs, facilitating economic small-batch production; and its ability to significantly reduce waste by-products, resulting in both environmental benefits as well as lower manufacturing costs. On the other hand, current lithography-based manufacturing technologies-a huge contributor to the growing RF and 5G wireless electronics industry-require extensive design verification, have longer turnaround times, and produce harmful byproducts. Although AM can offer time and cost benefits under the correct conditions, among the more impactful demonstrations of the manufacturing technology are the novel topologies enabled by design rules that allow feature sets, including nonorthogonal planes, conformal surfaces, multimaterial deposition, simultaneous thick- and thin-film deposition [1], complex 3D structures, integrated voids, and gradient index materials.

In this letter, the authors demonstrate, for the first time, the implementation of a fully-flexible, energy autonomous Rotman lens-based backscattering RFID tag operating in the 28 GHz 5G band. The Rotman lens front end was designed, simulated, fabricated on a thin flexible substrate and its gain and differential radar cross section (RCS) tested. The structure displays a simultaneous high gain and wide angular coverage, resulting in a remarkable RCS, measured in both planar and severe bending conditions. A low power oscillator was then added to the retrodirective array to test its detectability as a backscatterer at an unprecedented range of up to 64 m while displaying a power consumption as low as 2.64 $\mu$ W, effortlessly supplied by its flexible solar cell in normal office indoor conditions. Enabled by the implementation of a high degree of focalizing retrodirectivity, this unique Rotman lens-based system could enable the birth of RFIDs with practical reading ranges exceeding 1.8 km.

In this letter, waking-up wireless sensors is achieved by solely relying on existing ambient radio frequency signals from commercial Wi-Fi routers. The wake-up is accomplished based on the captured Wi-Fi signal that is received by an antenna and then fed into a rectifier circuit. A novel packet injection algorithm is developed to enable the rectifier to efficiently convert the captured Wi-Fi signal into a dc power. This algorithm can be implemented on any commercial router with no modifications to the router's hardware or protocol stack and without compromising the performance of the Wi-Fi network. The design is experimentally validated under controlled conditions.

This article presents a new compact lightweight radio frequency (RF) energy harvesting system. The system relies on a dual-tapered transmission line-based matching network that stretches the rectification capability of an integrated Schottky diode. This topology is demonstrated on a 2.4 GHz rigid harvesting system with a resulting power conversion efficiency that reaches up to 58% over 0 dBm input RF power. Its performance is compared with a reference rectifier that relies on a typical open circuit shunt-stub matching network. The rectifier along with a miniaturized monopole antenna is then tailored for a flexible substrate with a resulting efficiency around 50% at 0 dBm input power. The rectifier exhibits an almost flat efficiency over the 2.3–2.5 GHz frequency span despite wide load variations. The system is characterized in multiple bent configurations featuring high and stable performance. It is demonstrated that for different bent states, the proposed flexible harvester does not display large variations in harvested power. Thus, the presented rectenna demonstrates the remarkable combination of compactness, flexibility, and stability. Equipped with these features, such rectifier can be plugged into a variety of sensors, even on wearable surfaces, which makes it ideal for Internet of Things (IoT) applications.

Wireless sensors have become commonplace in our daily lives for such tasks as tracking temperature, measuring humidity, and sensing light and gas. Many technologies capable of relaying sensor information are continually being refined, but the search for a robust and customizable technology to satisfy the exponentially increasing demand for largescale and low-cost deployment of these wireless sensor nodes while minimizing power consumption has led to backscatter radio as a potential solution. Backscatter radio, first proposed in the 1940s as "communication by means of reflected power" [1], is a fairly mature technology that has been used in a variety of modern applications, especially those employing passive RFID technology in the ultrahigh-frequency band. The tags used in these applications are usually made up of just a transistor or switch and an antenna. These tags can be activated using only the RF power from an interrogating signal and, as a result, can be operated without any external energy source. Communication ranges of up to a few meters have been reported using this technology [2], [3].

Unlike conventional energy harvesters that are tuned to individual frequencies and are directional, the proposed wideband (23% fractional bandwidth), omnidirectional system harvests from all FM towers in vicinity. Hence, it does not require each sensor node to be carefully aligned to any one source and makes itself suited for mass-deployment applications such as precision agriculture and structural health monitoring. It harvests as much as 923 $\mu \text{W}$ on a rooftop at 1.54 km from an FM tower while retaining omnidirectionality partly thanks to the proposed antenna, which has a wider bandwidth and a higher measured gain (2.0 dBi) than the commercial reference wideband antenna. It achieves competitive efficiency (56%) and sensitivity (−17 dBm) while retaining wideband operation rather than being specifically optimized to individual frequencies. The system (fabricated on low-cost FR4) harvested outside ambient FM energy indoors at the same 1.54-km location to power a wireless sensor node without needing to shut down periodically.

This letter investigates the incorporation of support vector machines (SVMs) to predict the time and location of maximum Wi-Fi coverage for energy harvesting. Such incorporation of machine learning, along with radio frequency energy harvesting systems, improves the proposed rectenna efficiency especially when scavenging wireless routers and access points. Experiments showed that the proposed framework that uses SVM was able to predict the time and location of maximum Wi-Fi coverage. Such prediction constitutes the basis of a scheduling mechanism for directed harvesting of Wi-Fi signals.

With the development of Additive Manufacturing Technologies, the realization of inkjet-3D/-4D printing of complex structures for flexible wearable wireless modules has become a reality. This paper provides a review of the current state-of-the-art in semi-passive/fully-passive conformal, sensors, and the wireless powering of wearable wireless devices. This paper presents the advantages of mm-Wave additively manufactured modules for both high-speed wireless communications and wireless sensing with mmID as well as pathogen detection through inkjet-printing of multi-material electrochemical sensors enabling highly scalable low-cost 5G powered IoT devices for biomonitoring and pathogen sensing.

In this article, the particular importance of nanotechnology-enabled materials associated with additive manufacturing techniques for the realization of radio-frequency (RF) components and modules for Internet of Things (IoT) and millimeter-wave (mm-wave) applications is discussed.

5G has been designed for blazing fast and low-latency communications. To do so, mm-wave frequencies were adopted and allowed unprecedently high radiated power densities by the FCC. Unknowingly, the architects of 5G have, thereby, created a wireless power grid capable of powering devices at ranges far exceeding the capabilities of any existing technologies. However, this potential could only be realized if a fundamental trade-off in wireless energy harvesting could be circumvented. Here, we propose a solution that breaks the usual paradigm, imprisoned in the trade-off between rectenna angular coverage and turn-on sensitivity. The concept relies on the implementation of a Rotman lens between the antennas and the rectifiers. The printed, flexible mm-wave lens allows robust and bending-resilient operation over more than 20 GHz of gain and angular bandwidths. Antenna sub-arrays, rectifiers and DC combiners are then added to the structure to demonstrate its combination of large angular coverage and turn-on sensitivity—in both planar and bent conditions—and a harvesting ability up to a distance of 2.83 m in its current configuration and exceeding 180 m using state-of-the-art rectifiers enabling the harvesting of several μW of DC power (around 6 μW at 180 m with 75 dBm EIRP).

Backscatter front-ends are generally praised for their sub-µW power consumptions. However, this power consumption ends up being dwarfed by that of its modulating baseband circuitry. Furthermore, they are plagued by short reading ranges. The work reported in this paper demonstrates, for the first time, the use of a combined single-element oscillator/reflection-amplifier architecture. This remarkable system combines two critical features for range extension-the highest reflection-amplification RFID gain of the literature (48 dB) and higher-than-MHz sub-carrier offset frequency-while displaying a power consumption lower than that of any comparable commercial (amplifier-less) oscillator: 20 µW. This is achieved by using a tunnel-diode, whose properties as a baseband-oscillator and a 5.8 GHz reflection-amplifier are analyzed, before both are combined. This highly voltage-sensitive system is synergistically associated with a first-of-a-kind self-regulating tunnel diode-based rectifier to propose a fully-tunnel-diodes-based passive RFID design which could enable the future of practical km-range RFIDs.

This paper presents a state-of-the-art fully 3D-printed mm-wave origami-inspired reflectarray with one-shot deployability. The prototype is fabricated with flexible photosensitive resin using stereolithography 3D printing technology. The proposed design demonstrates a tremendous size reduction in its retracted state which can be very useful for various outer-space and terrestrial applications such as CubeSats, 5G stations and long-range mm-wave backscattering applications.

In this paper, a digitally tuned flexible antenna is proposed for integration onto devices within the Internet of Things (IoT) paradigm. The antenna design caters for the various IoT communication standards by reconfiguring its operating frequency. Accordingly, the antenna reconfiguration allows operation at 868 MHz, 915 MHz, 1.8 GHz and 2.1 GHz. The reconfiguration is achieved digitally by resorting to a digitally tunable capacitor (DTC). The DTC changes its capacitance, and hence its frequency operation, by relying on 5-bit sequence commands through a microcontroller. The proposed antenna is coplanar waveguide fed with its ground plane surrounding the radiating structure. The gain of the antenna varies between 4 dB and 6 dB for different frequencies of operation. Its radiation efficiency varies between 78% and 92%. The antenna is fabricated and tested on a flexible substrate, where the resulting reconfigurable prototype maintains its performance characteristics for different bent states.