This paper presents a comprehensive review/tutorial on multi-functional metasurfaces integrated with M-type ferrite materials for millimeter-wave (mmWave) absorption and beam control. As wireless communication systems transition toward beyond-5G architectures, including non-terrestrial networks (NTNs), the demand for adaptive, low-profile electromagnetic surfaces that can manage interference while enabling beam reconfiguration becomes increasingly critical. Conventional metasurfaces often struggle to simultaneously achieve high absorption and beamforming over wide frequency ranges due to intrinsic material and structural limitations. This paper reviews the state-of-the-art in metasurface design for dual-functionality— particularly those combining frequency-selective magnetic materials with periodic surface lattices—to enable passive, compact, and reconfigurable reflectors and absorbers. Special emphasis is placed on the role of M-type ferrites in enhancing absorption via ferromagnetic resonance (FMR), and on the use of surface-wave trapping mechanisms to achieve narrowband and broadband functionality. A case study of a ferrite-based hybrid “reflectsorber” (reflectrarray + absorber) is presented to demonstrate key design concepts, analytical models, and application scenarios relevant to satellite, UAV, and NTN ground station deployments. Future directions for low-loss, tunable, and scalable metasurfaces in next-generation wireless infrastructures are also discussed.

This paper presents a novel approach to address the challenges in backscatter communication for the Internet of Things. The traditional use of I/Q load modulator based on parametric transistor models often suffers from performance degradation due to discrepancies with actual transistor models caused by thermal and environmental noise sources. To overcome this issue, the paper proposes an active circuit modeling technique based on artificial neural network (ANN)-based transfer learning, which utilizes actual measurement data to model the I/Q load modulator accurately. Furthermore, an optimization algorithm is applied to achieve an optimal high-order modulation scheme, leading to improved energy efficiency by 40%. By leveraging machine learning-based modeled I/Q modulators, the proposed approach enables high-speed wireless data communication in a dual-channel configuration. The paper also conducts theoretical analysis to define the required performance of a dual-polarized Vivaldi antenna for implementing polarization diversity in backscatter communication. This analysis provides guidelines for achieving optimal performance in terms of spectral efficiency and error vector magnitude (EVM). The experimental results demonstrate that the proposed approach achieves a spectral efficiency of 2.0 bps/Hz based on 4-QAM modulation within a 150 MHz bandwidth. The measured EVM is 9.35%, indicating the effectiveness of the proposed technique in achieving reliable and efficient wireless data communication in backscatter systems. This paper presents a comprehensive approach combining accurate circuit modeling, optimization algorithms, and theoretical analysis to enable high-speed, ultra-low-power wireless data communication in backscatter communication systems.

This article describes sensors and radio frequency (RF) components manufactured by applying selective-metallization processes (metal foil tape, liquid metal filling, electro and electroless plating, and surface embossing) to 3-D-printed and flexible dielectric substrates. All these technologies can be implemented at room temperature, thus enabling the adoption of unconventional materials and biopolymers with low glass transition temperatures. In this article, we also describe how the above technologies are used to manufacture wireless transponders for tracking and sensing applications. Several examples of RF components are reported, including antennas, beamforming networks, Doppler radars, and wireless passive transponders based on backscatter radio. Innovative circuit design approaches (such as the via-less approach) are presented and their impact on circuit manufacturing and recyclability is discussed.

Emerging additive manufacturing (AM) technologies, specifically additively manufactured electronics (AME), 4-D printing, and origami, are reshaping the design capabilities and functionalities of contemporary electronic devices. Cutting-edge 3-D/4-D printing technologies facilitate the prototyping and realization of complex electronic functions that are challenging to conventional methods. This article provides a comprehensive overview of the evolving techniques in AME, 4-D printing, and origami, employing multimaterials (conductive and dielectric materials) and shape-memory materials (SMMs) to fabricate functional electronic components and devices. Additionally, the overview delves into the state-of-the-art AME and 4-D-printed electronic components across diverse fields, including biomedical electronics, space engineering, and the advancements in the next-generation wireless communications and sensing.

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$\mu \text{W}$-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.

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.

Internet of Things (IoT) is an emerging technology in the modern era of big data. It concerns a variety of applications ranging from smart homes, connected vehicles to smart factories, and more. IoT infrastructure typically comprises millions of connected objects and devices that store and exchange sensitive and confidential information. Theft and fraud scenarios, such as hacking and identity forgery, are serious threats to such IoT devices. Embedded hardware security techniques could be a potential solution to preserve the highest level of security within this infrastructure. Physically unclonable functions (PUFs) are among the potential solution to data security and counterfeiting problems. Many more intrinsic hardware security techniques are underway for a highly secure IoT infrastructure as strongly demanded by the IoT community. The focus of this Special Issue is to provide readers with the latest advances in securing IoT infrastructure from the physical layer point-of-view.

In this review, the particular importance and associated opportunities of additively manufactured radiofrequency (RF) components and modules for Internet of Things (IoT) and millimeter-wave ubiquitous sensing applications is thoroughly discussed. First, the current advances and capabilities of additive manufacturing (AM) tools are presented. Then, completely printed chipless radio-frequency identification (RFID) systems, and their current capabilities and limitations are reported. The focus is then shifted toward more complex backscattering energy autonomous RF structures. For each of the essential components of these structures, that encompass energy harvesting and storage, backscattering front ends, passive components, interconnects, packaging, shape-chaging (4-D printed) topologies and sensing elements, current trends are described and representative stateof- the-art examples reported. Finally, the results of this analysis are used to argue for the unique appeal of AM RF components and systems toward empowering a technological revolution of costefficient dense and ubiquitous IoT implementations.

In this paper, we present a novel wideband circularly polarized (CP) composite, called cavity-backed crossed dipole antenna for 2.45 GHz industrial, scientific, and medical (ISM) band wireless communication. To excite the CP radiation effectively, a curved-delay line providing an orthogonal phase difference among the cross-dipole elements is attached at corners of the sequentially rotated elements. By choosing a proper radius of the curved-delay line, a wide input impedance of the antenna can be realized. Unlike conventional cross-dipole antennas, the proposed cross-dipole antenna is designed with an open stub added to the radiating arms of the dipole so that both impedance and axial ratio bandwidths are enhanced. The antenna is center-fed by a 50-Ω coaxial cable and is placed above a cavity-backed reflector to obtain a directional CP radiation pattern. With the advantage of being center-fed, a symmetric CP radiation pattern can be achieved across the entire operating bandwidth. To further improve the directivity and the radiation pattern, a rectangular cavity-backed reflector is used. Simulated and measured results confirm that the proposed antenna has good CP characteristics. The proposed antenna obtains a broad 3-dB axial ratio bandwidth of 49% (1.20 GHz, 1.96–3.16 GHz) and an impedance bandwidth of 67.7% (1.66 GHz, 1.69–3.35 GHz) for reflection coefficient (S11) ≦ −10 dB. It also yields an average CP gain of 9.2 dBic across the operating bandwidth and a peak CP gain of 10 dBic. Copyright © 2016 John Wiley & Sons, Ltd.

Inkjet printing on flexible paper and additive manufacturing technologies (AMT) are introduced for the sustainable ultra-low-cost fabrication of flexible radio frequency (RF)/microwave electronics and sensors. This paper covers examples of state-of-the-art integrated wireless sensor modules on paper or flexible polymers and shows numerous inkjet-printed passives, sensors, origami, and microfluidics topologies. It also demonstrates additively manufactured antennas that could potentially set the foundation for the truly convergent wireless sensor ad-hoc networks of the future with enhanced cognitive intelligence and “zero-power” operability through ambient energy harvesting and wireless power transfer. The paper also discusses the major challenges for the realization of inkjet-printed/3-D printed high-complexity flexible modules as well as future directions in the area of environmentally-friendly “Green”) RF electronics and “Smart-House” conformal sensors.

The sidelobe level of a base station antenna is one of the important parameters to describe the performance of an antenna array. Given a required value of the sidelobe level, we can obtain a set of initial phases, and then further get a set of cable lengths. However, a tolerance (or error range) associated with manufacturing techniques will introduce an error in each cable length, thereby influencing the sidelobe level. This paper uses the knowledge of probability and mathematical statistics to make a statistical analysis for the reliability of the first sidelobe of the antenna array based on Monte Carlo simulations. We also obtain a distribution curve of reliabilities of the first sidelobe versus phase tolerances, which can bring great convenience for practical applications.

In this paper, various ambient energy-harvesting technologies (solar, thermal, wireless, and piezoelectric) are reviewed in detail and their applicability in the development of self-sustaining wireless platforms is discussed. Specifically, far-field low-power-density energy-harvesting technology is thoroughly investigated and a benchmarking prototype of an embedded microcontroller-enabled sensor platform has been successfully powered by an ambient ultrahigh-frequency (UHF) digital TV signal (512-566 MHz) where a broadcasting antenna is 6.3 km away from the proposed wireless energy-harvesting device. A high-efficiency dual-band ambient energy harvester at 915 MHz and 2.45 GHz and an energy harvester for on-body application at 460 MHz are also presented to verify the capabilities of ambient UHF/RF energy harvesting as an enabling technology for Internet of Things and smart skins applications.

This paper presents a review of existing works and solutions in the field of solar/electromagnetic energy harvesting and wireless power transmission. More specifically, the paper covers: solar/electromagnetic harvesters where solar antenna structures are used to obtain a compact implementation, direct current (dc) combining circuits necessary to combine the outputs of the solar and the electromagnetic harvesters, and efficient solar-to-electromagnetic (EM) converters that can be used to synthesize autonomous wireless power transmission radio-frequency (RF) signal generators. Finally, novel topologies to minimize the sensitivity of rectifier circuits to variations in the received RF power levels are presented.

The articles in this special issue cover recent advances in energy-harvesting and energy-scavenging systems with a focus on numerous "renewable" transducer technologies as well as emerging applications. energy-harvesting technologies are fundamental in enabling the realization of "zero-power" wireless sensors and implementing the Internet-of-Things (IoT) and machine-to-machine (M2M) communication. Their increasing utilization in low-power and power-efficient sensors and electronics could potentially find application in numerous critical areas ranging from health, agricultural, structural health monitoring to logistics, localization, and security. Energy-harvesting devices, including solar panels, piezoelectric devices, thermocouples, and RF energy scavengers, can dramatically extend the operating lifetime of nodes in wireless sensor networks (WSNs). Furthermore, this technology enables a completely battery-less operation and reduces the operation cost of WSNs, which is mainly due to battery replacement, thus making it very important for a sustainable "near-perpetual" WSN operability.

In this paper, we considered the possibility of using electricity harvested from the microwave field leaked from commercial microwave ovens. Our experimental results showed that the leakage received by a dipole antenna was about 0 dBm (1 mW) at a point 5 cm in front of the door. A rectenna consisting of a dipole antenna and charge pump can convert the leaked microwave energy into a DC current. When a microwave oven is operated for 2 min, 9.98 mJ of energy was harvested. We demonstrated that this energy is sufficient for powering a digital cooking timer to count down for 3 min and beep for 2.5 s. The operation of other kitchen devices was also demonstrated.

The use of passive ultra high-frequency (UHF) radio frequency identification (RFID) integrated into plywood boards is proposed to enable the identification and tracking of individual plywood boards and end products of plywood. For the first time, tags are embeddable inside plywood by direct inkjet-printing tag antennas on pure birch veneer. The use of passive UHF RFID technology in the applications of plywood industry is discussed, two tag antenna designs for plywood are presented and the tag fabrication procedures are described. Furthermore, results from tag performance measurements performed in the authentic application environment as well as in anechoic conditions are presented and discussed. Measurements show that tags printed on veneer and embedded inside 2 mm thick plywood board exhibited theoretical read ranges from 7.9 to 10.1 meters. The read ranges obtained meet the demands of the plywood industry and offer reliable identification even in challenging environments.

In this paper we show a low cost and environmentally friendly fabrication for an agricultural sensing application. An antenna, a soil moisture sensor, and a leaf wetness sensor are inkjet-printed on paper substrate. A microprocessor attached to the paper substrate is capable of detecting the capacitance change on the surface of the sensor, and report the data over the wireless communication interface. This sensing system is useful to optimize irrigation systems.

This paper discusses the evolution towards the first integrated radio-frequency identification (RFID)-enabled wireless sensor network infrastructure using ultra-high frequency/radio frequency (UHF/RF) RFID-enabled sensor nodes and inkjet-printed electronics technologies on flexible and paper substrates for the first time ever. The first sections highlight the unique capabilities of inkjet printed electronics as well as the benefits of using paper as the ultra-low-cost, conformal and environmentally friendly substrate for the mass-scale ubiquitous implementation of the first RFID-enabled wireless sensing applications. Various inkjet-printed antenna configurations are presented for enhanced-range compact RFID-enabled sensing platforms in “rugged” environments up to 7 GHz, followed by the discussion of their 2-D integration with integrated circuit (IC) and sensors on paper. This integration is extended to a power-scavenging “smart-shoe” batteryless integrated RFID module on paper that could be used for autonomous wearable sensing applications with enhanced range. The paper concludes discussing the details for establishing for the first time an asynchronous wireless link between the aforementioned RFID-tags and a widely used commercial wireless sensor network (WSN) mote using a simplified protocol; a paramount step that could potentially create ubiquitous ultra-low-cost sensor networks and large-scale RFID implementations eliminating the need of expensive RFID reader infrastructure and linking RFIDs to the mature level of WSNs.

A conceptual design of a MIMO phased antenna array on liquid crystal polymer (LCP) is introduced. Because of the properties of the substrate, the overall design is comparably light weight and semi-flexible to previous MIMO designs. Each input output unit array is capable of steering the beam plusmn6deg in a 1-D space. This beam steer is enabled with the use of a 2-bit reconfigurable phase shifter using RF MEMS. The building blocks of a single input and output has been simulated to prove the concept. In addition, the array can be expanded to beam steer in a 2-D space for each individual unit. Overall, the entire system will be capable of having the beam steering function in multiple directions.

This paper presents a conformal, compact, omnidirectional, elliptical monopole antenna on liquid crystal polymer (LCP) which is studied mounted on a cylindrical surface and is proven to operate equally well compared to its planar rival. The presented return loss, gain and radiation pattern measurements demonstrate that the characteristics of the antenna are not compromised when the antenna is mounted on cylindrical surfaces of various radii when the direction of curvature is in the transverse direction. The presented study shows that the proposed antenna can be optimized and tested in planar shape and still be trusted to operate equally well under conditions like in wearable electronics, where their planar condition cannot be guaranteed.

In this paper, an overview of novel design and integration approaches for improved performance UHF radio frequency identification (RFID) tags with embedded power source and sensing capability is presented. Ultra-low-cost organic substrates, such as paper, with inkjet-printing capability are investigated for the UHF frequency band. The proposed technology could potentially revolutionize sensor nodes and RFID tags for various applications such as security, logistics, automotive and pharmaceutical.

In this paper, design requirements and advanced approaches for improved performance UHF radio frequency identification (RFID) tags are presented. Organic substrates are examined as highly potential candidates for RFID fabrications. An inkjet-printed tag on paper substrate is realized for ultra-low- cost mass production. RFID with sensor integration is also demonstrated for automotive sensing and vehicle security applications. Measurements are performed for RFID tags embedded in tires. These results show that proper designed RFID can serve for the purpose of tire pressure/temperature monitoring.