Search

Due to their high price, RFID tags are not yet widely used in applications for monitoring, tracing, and information inscribing, such as manufacturing lines, supply chains, and inventory management. The currently utilized barcode-like tracing techniques lack security, experience data limitation, are semi-automated, and are prone to external damage. Chipless RFID technology is a potential replacement, able to overcome shortcomings. However, due to the uncertainty created by the absence of a communication antenna and microchip, the detection of chipless RFID tags has a 2–5% detection error rate. The recent advancement in spatial chipless RFID technology has opened up a plethora of opportunities to use deep learning, which is becoming increasingly popular, to detect chipless RFID tags from a 2D virtual image created from the backscattered signal. In this study, we presented a comprehensive and exhaustive investigation of several deep learning techniques to improve the detection capability of deep learning-based chipless RFID tag detection. Our proposed fine-tuning methods yielded better mathematical metrics on the state-of-the-art on its original setup. A confusion matrix used to measure error rates on unseen data shows that the improved deep learning architecture we introduced works at roughly 99.99% accuracy. [ABSTRACT FROM AUTHOR]

Due to their high price, RFID tags are not yet widely used in applications for monitoring, tracing, and information inscribing, such as manufacturing lines, supply chains, and inventory management. The currently utilized barcode-like tracing techniques lack security, experience data limitation, are semi-automated, and are prone to external damage. Chipless RFID technology is a potential replacement, able to overcome shortcomings. However, due to the uncertainty created by the absence of a communication antenna and microchip, the detection of chipless RFID tags has a 2–5% detection error rate. The recent advancement in spatial chipless RFID technology has opened up a plethora of opportunities to use deep learning, which is becoming increasingly popular, to detect chipless RFID tags from a 2D virtual image created from the backscattered signal. In this study, we presented a comprehensive and exhaustive investigation of several deep learning techniques to improve the detection capability of deep learning-based chipless RFID tag detection. Our proposed fine-tuning methods yielded better mathematical metrics on the state-of-the-art on its original setup. A confusion matrix used to measure error rates on unseen data shows that the improved deep learning architecture we introduced works at roughly 99.99% accuracy.

This paper presents a wireless chipless resonator-based sensor for measuring the absolute value of an external time-varying electric field. The sensor is developed using contactless air-filled substrate-integrated waveguide (CLAF-SIW) technology. The sensor employs a low-impedance electromagnetic band gap structure to confine the electric field within the sensor's air cavity. The air cavity is loaded with varactor diodes whose reverse bias voltage is modified by the to-be-measured external electric field. Variation in the external electric field results in a variation of the sensor's resonant frequency. The CLAF-SIW sensor offers a high unloaded quality factor, which is required for a long-distance ringback-based interrogation system. A prototype of the proposed sensor is fabricated and tested. It can measure a time-varying external electric field up to 6.9 kV/m, has a sensitivity of 1.86 (kHz)/(V/m), and can be interrogated from a distance of 80 cm. The feasible maximum bandwidth of the external electric field is 25 kHz. The proposed sensor offers a compact planar multilayer structure that can easily be incorporated with a planar antenna and its size can be reduced by selecting a higher operating frequency without an increase in dielectric loss. [ABSTRACT FROM AUTHOR]

Frequency encoding chipless Radio Frequency Identification (RFID) tags have been frequently using the radar cross section (RCS) parameter to determine the resonant frequencies corresponding to the encoded information. Recent advancements in chipless RFID design have focused on the generation of multiple frequencies without considering the frequency position and signal amplitude. This article proposes a novel method for chipless RFID tag design, in which the RCS response can be located at an exact position, corresponding to the desired encoding signal spectrum. To achieve this, the empirical Taguchi method (TM), in combination with particle swarm optimization (PSO), is used to automatically search for optimal design parameters for chipless RFID tags with a fast response time, to comply with the frequency encoding requirements in the presence of the mutual coupling effect. The proposed design method is validated using I-slotted chipless tag structures that are fabricated and measured with different sets of resonant frequencies. [ABSTRACT FROM AUTHOR]

Chipless RFID tags have attractive low-cost advantages. However, traditional RFID anti-collision algorithms cannot be applied due to a lack of computing and processing capabilities. Problems with multitag detection must be solved to commercialize chipless RFID tags. In this paper, an innovative method for frequency-domain chipless RFID tag detection is proposed. The tags' scattered signals are processed via wavelet analysis, and a time–frequency plot that can read the code is obtained. When the distance between tags is too close to distinguish in the time–frequency plot, independent component analysis is used to separate individual scattered signals from mixed echo signals; then, the code is read by means of wavelet analysis. To validate the proposed method, C-shaped frequency-domain chipless RFID tag models and a multitag detection simulation scenario were constructed in selected software. The short-time matrix pencil method (STMPM), short-time Fourier transform (STFT), and the proposed method were compared. When the tag spacing is 0.05 m, the code can be read successfully. Compared with the STMPM, the proposed method greatly reduces the computational quantity and shortens the reading time. Furthermore, adjustment of the window width and search step parameters is avoided. [ABSTRACT FROM AUTHOR]

The role of the orientation of the radio frequency identification (RFID) reader is vital in the RFIDbased communication system. This study presents the design and analysis of the impact of the orientation of the RFID reader (angle of incidence of the plane wave) on the detection and sensitivity characteristics of fractal chipless RFID tags. Four fractal (irregular) shaped tags are developed using four iterations of the Hilbert curve filling algorithm. The full-wave EM analysis of the designed tags in Matlab is performed by exporting them in Computer Simulation Technologies Micro-Wave Studio (CST MWS) in the frequency range of 2 to 20 GHz. Firstly, the performance of the tags is analyzed by observing the radar cross-section (RCS) of the tag for the fixed orientation of the incident plane wave in three different polarizations (horizontal, vertical, and oblique). Later, the variations in the EM spectrum (RCS results) are analyzed for oblique polarization by varying the incidence plane wave in both elevations (for two cases of 0 ° and 90°) and azimuth planes (sweeping from 0° to 180° with a step size of 10°). The analysis of the proposed aggregated RCS response for all cases in oblique polarization produces higher coding capacity (169 bits), coding spatial capacity (16.504 bits/cm2), coding spectral capacity (9.826 bits/GHz), and coding density (0.960 bits/GHz/cm2) for the realized highly irregular tag using fourth-iteration (4T) of Hilbert curve filling algorithm. The proposed procedure of detection based on aggregated response makes the developed RFID communication system more secure and reliable. [ABSTRACT FROM AUTHOR]

Packaging solutions have recently evolved to become smart and intelligent thanks to technologies such as RFID tracking and communication systems, but the integration of sensing functionality in these systems is still under active development. In this paper, chipless RFID humidity sensors suitable for smart packaging are proposed together with a novel strategy to tune their performances and their operating range. The sensors are flexible, fast, low-cost and easy to fabricate and can be read wirelessly. The sensitivity and the humidity range where they can be used are adjustable by changing one of the sensor's structural parameters. Moreover, these sensors are proposed as double parameter sensors, using both the frequency shift and the intensity variation of the resonance peak for the measure of the relative humidity. The results show that the sensitivity can vary remarkably among the sensors proposed, together with the operative range. The sensor suitability in two specific smart packaging applications is discussed. In the first case, a threshold sensor in the low-humidity range for package integrity verification is analyzed, and in the second case, a more complex measurement of humidity in non-hermetic packages is investigated. The discussion shows that the sensor configuration can easily be adapted to the different application needs. [ABSTRACT FROM AUTHOR]

The paper proposes a low-profile chipless RFID tag with an enhanced read range. To improve the read range, an artificial magnetic conductor (AMC) is used as the backplane of the resonator. The proposed technique is presented at the operating frequency of 5.07 GHz, which could further be shifted to other frequencies in the UWB band by optimizing the geometric parameters of the design. The octagonal-shaped resonator of the tag and the AMC backplane is designed on a 0.15-mm and 1.6-mm-thick FR-4 substrate, respectively. The AMC as backplane leads to seven times enhancement of the read range. The simulated results of the tag are experimentally validated using the bistatic measurement method. The overall dimension of the proposed tag is 28 × 28 mm2 and is suitable for tracking and identification of small-sized objects. The proposed technique of read range enhancement could further be matured for multi-resonance chipless RFID tags. [ABSTRACT FROM AUTHOR]

In the textile industry, passive radio frequency identification (RFID) tag integration is increasing from warehouse tracking to production follow-up and healthcare applications. The proposed work introduces the in-silico design of a 30x30 mm² textile-based chipless RFID (CRFID) tag with circular transmission lines for item identification using the CST Microwave Studio application. The tag antenna was designed with stainless steel/polyester twisted yarn integrated into the polypropylene tag fabric using the embroidery technique. The antenna was tested for its readability of the embedded code in the simulations with a range of 0-20 GHz frequency band, and its Radar Cross Section (RCS) responses, return losses (S1, 1), E-field, H-field, and Farfield performances were obtained. As a result, 9-bit code capacity and 1-bit/cm² code density were achieved. The UHF antenna design simulation results demonstrated the feasibility of using textile-based materials to create a CRFID tag that was lightweight, durable, and cost-effective. The CRFID tag can store and transmit data without a chip, making it ideal for healthcare applications. [ABSTRACT FROM AUTHOR]

This paper presents a wireless chipless resonator-based sensor for measuring the absolute value of an external time-varying electric field. The sensor is developed using contactless air-filled substrate-integrated waveguide (CLAF-SIW) technology. The sensor employs a low-impedance electromagnetic band gap structure to confine the electric field within the sensor’s air cavity. The air cavity is loaded with varactor diodes whose reverse bias voltage is modified by the to-be-measured external electric field. Variation in the external electric field results in a variation of the sensor’s resonant frequency. The CLAF-SIW sensor offers a high unloaded quality factor, which is required for a long-distance ringback-based interrogation system. A prototype of the proposed sensor is fabricated and tested. It can measure a time-varying external electric field up to 6.9 kV/m, has a sensitivity of 1.86 (kHz)/(V/m), and can be interrogated from a distance of 80 cm. The feasible maximum bandwidth of the external electric field is 25 kHz. The proposed sensor offers a compact planar multilayer structure that can easily be incorporated with a planar antenna and its size can be reduced by selecting a higher operating frequency without an increase in dielectric loss.

Frequency encoding chipless Radio Frequency Identification (RFID) tags have been frequently using the radar cross section (RCS) parameter to determine the resonant frequencies corresponding to the encoded information. Recent advancements in chipless RFID design have focused on the generation of multiple frequencies without considering the frequency position and signal amplitude. This article proposes a novel method for chipless RFID tag design, in which the RCS response can be located at an exact position, corresponding to the desired encoding signal spectrum. To achieve this, the empirical Taguchi method (TM), in combination with particle swarm optimization (PSO), is used to automatically search for optimal design parameters for chipless RFID tags with a fast response time, to comply with the frequency encoding requirements in the presence of the mutual coupling effect. The proposed design method is validated using I-slotted chipless tag structures that are fabricated and measured with different sets of resonant frequencies.

Abstract The limited and inconvenient functionality of conventional light switches is out of pace with current advancements in wireless sensors. A chipless RFID light switch (CLS) that is passive, battery‐free and relocatable and maintains the convenience of having physical buttons for controlling lightbulbs or other electrical devices is introduced. These characteristics have been achieved by attaching single‐pole‐single‐through toggle switches to the edges of radio frequency spiral resonators. The status of the switches activates or deactivates the resonators, allowing the CLS tag to passively communicate the status of the switches. A CLS tag with two ID resonators (used for tag identifications), and two measurement resonators [MRs] (connected with switches and used to communicate the status of the switches) was designed and fabricated using a 1‐mm‐thick FR4 substrate. Measurement results showed resonant frequencies at 1115 and 1220 MHz corresponding to the ID resonators and frequencies at 848 and 971 MHz corresponding to the MRs. Turning the switches OFF and ON successfully activated and deactivated the MRs.

Chipless radio frequency identification (RFID) technology is expected to replace barcode technology due to its ability to read in non-line-of-sight (NLOS) situations, long reading range, and low cost. Currently, there is extensive research being conducted on frequency-coded (FC) co-polarized radar cross-section (RCS)-based tags, which are widely used. However, detecting co-polarized chipless RFID tags in cluttered environments is still a challenge, as confirmed by measuring two co-polarized tags in front of a perfect metal reflector ( 30.5 cm × 22.5 cm ). To address this challenge, a realistic mathematical model for a chipless RFID system has been developed that takes into account the characteristics of the reader and the tag, as well as reflections from cluttered objects. This extensive mathematical model developed for linear chipless RFID systems in clutter scenarios holds the potential to greatly assist researchers in their exploration of RCS-based tags. By relying solely on simulations, this model provides a tool to effectively analyze and understand RCS-based tags, ultimately simplifying the process of generating more authentic tag designs. This model has been simulated and verified with measurement results by placing a single flat metal reflector behind two co-polarized one-bit designs: a dipole array tag and a square patch tag. The results showed that the interfering signal completely overlaps the ID of the co-polarized tag, severely limiting its detectability. To solve this issue, the proposed solution involves reading the tag in cross-polarization mode by etching a diagonal slot in the square patch tag. This proposed tag provides high immunity to the environment and can be detected in front of both dielectric and metallic objects. [ABSTRACT FROM AUTHOR]

Packaging solutions have recently evolved to become smart and intelligent thanks to technologies such as RFID tracking and communication systems, but the integration of sensing functionality in these systems is still under active development. In this paper, chipless RFID humidity sensors suitable for smart packaging are proposed together with a novel strategy to tune their performances and their operating range. The sensors are flexible, fast, low-cost and easy to fabricate and can be read wirelessly. The sensitivity and the humidity range where they can be used are adjustable by changing one of the sensor’s structural parameters. Moreover, these sensors are proposed as double parameter sensors, using both the frequency shift and the intensity variation of the resonance peak for the measure of the relative humidity. The results show that the sensitivity can vary remarkably among the sensors proposed, together with the operative range. The sensor suitability in two specific smart packaging applications is discussed. In the first case, a threshold sensor in the low-humidity range for package integrity verification is analyzed, and in the second case, a more complex measurement of humidity in non-hermetic packages is investigated. The discussion shows that the sensor configuration can easily be adapted to the different application needs.

NO2 is an important environmental pollutant and is harmful to human health even at very low concentrations. In this paper, we propose a novel chipless RFID sensor able to work at room temperature and to detect sub-ppm concentration of NO2 in the environment. The sensor is made of a metallic resonator covered with NO2-sensitive tin oxide and works by monitoring both the frequency and the intensity of the output signal. The experimental measurements show a fast response (a few minutes) but a very slow recovery. The sensor could therefore be used for non-continuous threshold monitoring. However, we also demonstrated that the recovery can be strongly accelerated upon exposure to a UV source. This opens the way to the reuse of the sensor, which can be easily regenerated after prolonged exposure and recycled several times. [ABSTRACT FROM AUTHOR]

The limited and inconvenient functionality of conventional light switches is out of pace with current advancements in wireless sensors. A chipless RFID light switch (CLS) that is passive, battery‐free and relocatable and maintains the convenience of having physical buttons for controlling lightbulbs or other electrical devices is introduced. These characteristics have been achieved by attaching single‐pole‐single‐through toggle switches to the edges of radio frequency spiral resonators. The status of the switches activates or deactivates the resonators, allowing the CLS tag to passively communicate the status of the switches. A CLS tag with two ID resonators (used for tag identifications), and two measurement resonators [MRs] (connected with switches and used to communicate the status of the switches) was designed and fabricated using a 1‐mm‐thick FR4 substrate. Measurement results showed resonant frequencies at 1115 and 1220 MHz corresponding to the ID resonators and frequencies at 848 and 971 MHz corresponding to the MRs. Turning the switches OFF and ON successfully activated and deactivated the MRs. [ABSTRACT FROM AUTHOR]

This paper proposes a chipless radio frequency identification (RFID) multi-tag detection system. The one-dimensional convolutional neural network (1D CNN) was employed as an intelligent classifier of the proposed system, which was fed by complex natural resonances (CNRs). Experiments with contexts of a single chipless RFID tag were conducted to collect input datasets for training and validating the 1D CNN. The CNRs and natural frequencies only extracted from the individual tag’s responses by using the short-time matrix pencil method (STMPM) and then obtained after performing data augmentation were separately fed to the 1D CNN for training and validation in order to compare their performance. The accuracy obtained from the training and validation of the 1D CNN fed by the CNRs was significantly higher than that of the 1D CNN fed by the frequencies only. The performance matrices in terms of precision, recall, and F1-score also confirmed the superiority of the use of CNRs over that of frequencies only. In order to verify the performance of real-time multi-tag detection utilizing the proposed system, experiments with contexts of multiple tags were carried out, and the experimental results have shown that the system using the 1D CNN fed by the frequency only failed to detect multiple tags. In contrast, the proposed system was able to deliver 100% accurate multi-tag detection. However, as demonstrated by the experimental results, the proposed chipless RFID multi-tag detection system was restricted to a resolution of 3 cm.

As one of the important components of internet of things (IOT) systems, radio frequency identification (RFID) tags need to be improved in terms of power consumption, size reduction and coding capacity. In conventional methods, designers focus on optimizing resonators, but by focusing on the use of phase coding, a new method can be presented that results in a tag with smaller dimensions and more coding capacity. Hybrid encoding for RFID is based on the phase coding by changing the position of the resonator in the chipless RFID tag. In this paper a C-shaped resonator is proposed to design a chipless tag on the Rogers RT/duroid 5880 laminates. This method proposes different phase values in the resonance frequency as separate codes. The difference between the phases ( $\Phi$ ) of the scattering parameters S11 and S22 is measured to show the effect of position varying of the proposed C-shaped resonator. With the three different values of $\Phi $ at the resonance frequency, the proposed RFID tag creates codes 0, 1 and 2, in comparison with the conventional structures that create codes 0 and 1 only, based on the presence or absence of a transition zero at the resonance frequency. The proposed structure was designed, fabricated and measured and measurement results validate this method. Accordingly, an RFID tag with five C-shaped resonators is proposed, where the simulation results of its $3^{5}=243$ different states are presented in this paper.

In this study, a novel encoding approach for chipless radio frequency identification (RFID) tags is proposed. This tag is designed with $N$ resonant elements, each of which can be parameterized to resonate at $M$ individual desired frequencies. Therefore, the number of encoded data points is $\sum \nolimits _{k=0}^{N} C_{k}^{N\times M}$ . The introduced design concept is supported by a new mechanism to deal with the mutual coupling effect between elements, thus making the tag resonate at any desired frequency. The proposed mechanism is based on applying the particle swarm optimization (PSO) algorithm in the optimization of the tag structure parameters so that the resonant frequencies approach those determined by the user in advance. Simulation and experimental results show that the proposed approach not only overcomes one of the most troublesome pitfalls in designing chipless RFID tags, which include the limitation in data encoding capacity while keeping the tag small and uncomplex, but also changes the steps the tags are designed in such a way that the resonant frequencies can be determined before designing the tag structure.

In the realm of radio frequency identification (RFID) technology, the integration of reconfigurable intelligent surfaces (RISs) has opened up new possibilities for real-time remote data capturing and seamless connectivity. By manipulating the electromagnetic properties of the environment, RIS enables the control of electromagnetic wave propagation and allows for virtual line-of-sight (LOS) in cases where physical LOS is blocked. This has tremendous implications for the future of RFID applications, particularly with the emergence of chipless RFID technology. In this regard, this paper develops free-space path loss models for RIS-assisted RFID wireless communications. The proposed models in this study have taken into account several crucial physical factors, including tag radar cross-section (RCS), the physical properties of the RIS, and the radiative near-field/far-field effects of the RIS. To further validate the theoretical findings, we have conducted experimental measurements using a fabricated RIS. Numerical simulations were also utilized to validate the models and verify our findings. The channel measurements have demonstrated good agreement with the proposed path loss models, further bolstering the potential of RIS-assisted RFID wireless communications.

This paper addresses the new type of backscatter communication based on motion-modulated chipless Radio Frequency IDentification (RFID) tags. To clearly explain the concept, the different methods of backscatter communication are classified from a system point of view based on the two fundamental properties of linearity and variation in time. The principle of classical chipless RFID technology, as a non-modulated backscatter method, and the motion-modulated chipless RFID are described with general mathematical demonstrations, while the performance of the two approaches is compared in terms of read range. Motion-modulated chipless RFID is presented as an effective backscatter communication method for identification and sensing of moving objects at large distances. Three different types of motion-induced modulation as phase (Doppler) modulation, polarization modulation, and directional modulation are addressed based on three specially designed moving resonant scatterers. The modulation process in each case is theoretically described, and the performance of the motion-modulated tag is experimentally verified in terms of identification capability and large read range.

An earlier version of this paper was presented at the 2021 15th European Conference on Antennas and Propagation (EuCAP) and was published in its Proceedings. This paper describes a design, fabrication, and characterization of a planar frequency-coded retroreflector for indoor localization. The retroreflector is fabricated in high-resistive silicon and consists of a Luneburg lens antenna and a photonic crystal high- Q resonator reflective layer, providing ranging and identification within the same tag and bandwidth. The Luneburg lens antenna presents a measured gain of 21.63 dBi at a design frequency of 240 GHz. The frequency-coded retroreflector allows for ranging in a continuous 130 degree angular range in azimuthal plane, with a discrete but repeatable two-resonance identification over multiples of 15 degree. Its maximum measured radar cross-section is −23.48 dBsqm at a frequency of 240 GHz. The retroreflective tag set in ideal line-of-sight situation is compared to a non-line-of-sight arrangement showing the influence of a metallic rod as an obstacle on the overall tag detection parameters. Finally, the successful read-out of the retroreflective tag is discussed in unknown environments, where no a-priori information is available. [ABSTRACT FROM AUTHOR]

This paper presents a unique geometry of a chipless radio frequency identification (RFID) tag for encoding a large number of bits in a very small form factor. The tag geometry consists of semi-octagonal copper strips, sequentially laid on a single side of an ultra-thin substrate. A unique and robust encoding mechanism for the tag identification (ID) is proposed. The operating frequency spectrum of the tag ranges from 3.1 to 10.5 GHz. The tag is compact, having an overall size of 14.5 × 28 mm2. The proposed tag exhibits very high code density of 9.85 bits/cm2 and spectral efficiency of 5.4 bits/GHz. The unique geometric configuration of the proposed tag allows it to encode up to 40 bits of data as an RCS signature. This chipless RFID tag seems to be a potential candidate for a wide range of modern RFID applications. [ABSTRACT FROM AUTHOR]

The analysis and experimental verification of the properties of four types of chipless RFID tags with an increased RCS response level designed and fabricated by conductive screen-printing using silver paste on foil and paper substrates was performed. The analytical formula for the quality factor of microstrip structures with a reduced conductivity of the metal layers was used to predict the changes and detectability of the backscattered RCS response. The analysis provides insight into the limitations and outlines the possibilities of chipless structures screen-printed on foil and paper substrates, which can be of significant benefit to further reducing the cost, and to speed up the production of these tags for identification and sensing purposes. [ABSTRACT FROM AUTHOR]

A chipless RFID tag was designed using two hexagonal resonators. By defining a relation for the phases of S‐parameters and rotating the resonators around their axes, a senary coding scheme was obtained at the resonant frequency. The designed resonator was able to generate six different codes using six different phase parameters at 3.03 GHz. While the conventional binary ("0" and "1") coding of ordinary tags makes minimal use of bandwidth capacity, the senary coding in the proposed design allowed the use of a larger bandwidth capacity. Requiring much fewer resonators for generating more codes, the proposed technique leads to significantly smaller tags. Using the proposed tag with two resonators, 62 = 36 different codes can be generated; more generally, an n‐resonator tag of the proposed type can create 6n different codes. [ABSTRACT FROM AUTHOR]

Detecting strain in structures is crucial for structural health monitoring (SHM) to prevent severe damage in buildings, power plants, and aircraft. Chipless radio frequency identification (RFID) has widened sensing applications, providing intelligent strain detection and risk assessment in SHM with reduced cost and complexity. Proper systematic analysis of chipless RFID strain sensors is necessary to improve their performance in terms of detectability, sensitivity, and resolution. This article presents a novel classification of chipless RFID strain sensors, categorized by their sensing mechanisms and materials such as metallic, carbon-type, and hybrid. It investigates the modern development of chipless RFID strain sensors and their significance in strain-sensing capability based on sensing materials, substrates, and sensor geometry. Additionally, the paper highlights the challenges and future directions that need to be addressed for reliable strain detection in the chipless RFID sensing approach. Finally, it analyses the performance of strain sensors using a novel graphene-based chipless sensor to prove the future direction of the chipless strain sensing. [Display omitted] [ABSTRACT FROM AUTHOR]

Chipless RFID systems can be considered as a special case of passive RFID systems, where the tag contains no power source and no electronics. Instead, the tag’s information is stored in its structure and accessed through its electromagnetic (EM) scattering response. However, robust response detection, which is primarily a function of the measurement method, measurement equipment, and processing method used, is still a major challenge in the chipless RFID field. The consequences of not properly capturing a tag response include, incorrectly assigning an ID or incorrectly reporting a sensing parameter. Due to the criticality of these challenges, this review seeks to provide an overview of the current measurement methods, equipment architectures, and processing methods as they relate to chipless RFID tag response detection and decoding. Since chipless RFID started gaining popularity around 2005, the developments in this area have been focused on three major categories: time-domain, frequency-domain, and spatial-domain systems. Frequency-domain systems have emerged as the most popular among these three categories, and thus, this review focuses on techniques used for these systems.

In this paper, a concept of a reconfigurable chipless radio frequency identification (RFID) sensor tag for detecting solvent vapors/gas in IoT applications was presented. The concept was based on the authors' previously published rectangular loop structure equipped with a U-folded dipole loaded with a glide-symmetrical interdigital capacitor coated with a thin layer of tetrasulfonated copper phthalocyanine deposited as a sensing layer to improve the sensing capability in the presence of acetone vapor. In order to further maximize the sensitivity of the designed structure to the desired solvent, a circuit for a central frequency adjustment using a radio frequency varactor diode biased with a wireless power transfer (WPT) was designed. By varying the DC bias of the diode, a continuous tunable range of approximately 200 MHz was achieved. The proposed reconfigurable wireless sensor tag was manufactured and the frequency shift was verified by measurement. The proposed external frequency control can be applied to a wide class of electrical resonators. [ABSTRACT FROM AUTHOR]

Chipless radio frequency identification (RFID) technology is expected to replace barcode technology due to its ability to read in non-line-of-sight (NLOS) situations, long reading range, and low cost. Currently, there is extensive research being conducted on frequency-coded (FC) co-polarized radar cross-section (RCS)-based tags, which are widely used. However, detecting co-polarized chipless RFID tags in cluttered environments is still a challenge, as confirmed by measuring two co-polarized tags in front of a perfect metal reflector (30.5cm×22.5cm). To address this challenge, a realistic mathematical model for a chipless RFID system has been developed that takes into account the characteristics of the reader and the tag, as well as reflections from cluttered objects. This extensive mathematical model developed for linear chipless RFID systems in clutter scenarios holds the potential to greatly assist researchers in their exploration of RCS-based tags. By relying solely on simulations, this model provides a tool to effectively analyze and understand RCS-based tags, ultimately simplifying the process of generating more authentic tag designs. This model has been simulated and verified with measurement results by placing a single flat metal reflector behind two co-polarized one-bit designs: a dipole array tag and a square patch tag. The results showed that the interfering signal completely overlaps the ID of the co-polarized tag, severely limiting its detectability. To solve this issue, the proposed solution involves reading the tag in cross-polarization mode by etching a diagonal slot in the square patch tag. This proposed tag provides high immunity to the environment and can be detected in front of both dielectric and metallic objects.

NO2 is an important environmental pollutant and is harmful to human health even at very low concentrations. In this paper, we propose a novel chipless RFID sensor able to work at room temperature and to detect sub-ppm concentration of NO2 in the environment. The sensor is made of a metallic resonator covered with NO2-sensitive tin oxide and works by monitoring both the frequency and the intensity of the output signal. The experimental measurements show a fast response (a few minutes) but a very slow recovery. The sensor could therefore be used for non-continuous threshold monitoring. However, we also demonstrated that the recovery can be strongly accelerated upon exposure to a UV source. This opens the way to the reuse of the sensor, which can be easily regenerated after prolonged exposure and recycled several times.

Miniaturization, flexibility and high coding capacity design are very important while considering the development of chipless tags. In this paper, a flexible high-encoding capacity chipless radio frequency identification (RFID) tag with a compact structure is designed using Polyimide, a flexible material with high-temperature resistance as the substrate. The tag works in the band of 1.5∼10.5 GHz, the coding density of 4.348 bits/cm2 is obtained with four alternating nested improved U-shaped resonance structures. Frequency shift coding is realized by changing the structural parameters of the resonator. The production process is simple because only one conductive layer is needed. Because of its flexible and low-cost characteristics, it provides more possibilities for the promotion of RFID technology. A good agreement between simulation and measurement validates the feasibility of the design concept. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Test & Measurement Technology is the property of Publishing Center of North University of China and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The ground plane has never been considered as a degree of freedom to be used for chipless radio frequency (RF) identification (RFID) tag design purposes. For the first time, we show that the interaction with the ground plane can be exploited to achieve challenging designs, such as depolarizing tag insensitive to orientation (i.e., nonzero cross polar for any roll angle). The principle relies on the perturbation of the ground plane symmetry with merely one microstrip dipole placed near the edge of the substrate. Chipless RFID tags are realized using three different shapes of the substrate (square, octagonal, and circular) to achieve multiple edges. The measurements are performed in a semianechoic and office environment with tag attached to objects (cardboard box and metallic plate). The measured results of the square chipless RFID tags present magnitude variations (< 10 dB) but are detectable over a full 360° even in real environment yielding an “orientation-insensitive” detection system. Compared to previous designs, the frequency variability of the peaks is negligible. [ABSTRACT FROM AUTHOR]

In this paper, we report two different strategies to enhance the data density in electromagnetic encoders with synchronous reading. One approach uses a periodic chain of rectangular metallic patches (clock chain) that determines the encoder velocity, and dictates the instants of time for retrieving the bits of the identification (ID) code. However, contrary to previous electromagnetic encoders, the ID is inferred at both the rising and the falling edges of the clock signal generated by the clock chain. Moreover, the bits of information are not given by the presence or absence of metallic patches at their predefined positions in the so-called ID code chain. With this novel encoding system, a bit state corresponding to a certain instant of time is identical to the previous bit state, unless there is a change in the envelope function of the ID code signal, determined by the additional non-periodic ID code chain. The other encoding strategy utilizes a single chain of C-shaped resonators, and encoding is achieved by considering four different resonator dimensions, corresponding to four states and, hence, to two bits per resonator of the chain. Thus, with these two strategies, the data density is twice the one achievable in previously reported synchronous electromagnetic encoders. [ABSTRACT FROM AUTHOR]

In this paper, a compact and fully passive chipless radio frequency identification planar tag is proposed based on a modified rectangular resonator. The tag is having a simple design and is fabricated using a low‐cost FR4 substrate and acquires a physical footprint of 50 mm × 30 mm. The novelty of the proposed work describes a single passive tag having a reading distance of 84 cm with a 15‐bit coding capacity in a compact profile. A spectral bit capacity of 2.727 bits/GHz is achieved in a wide range of frequencies from 0.5 to 6 GHz. A reference wideband horn antenna is used as the interrogator to study the tag detection procedure having gains of 6.6 and 18.77 dBi at 0.5 and 5.8 GHz respectively. The backscattered tag response is well suited to the measurement characteristic of the reader. The measurements are performed in a bistatic mode and the results confirm the 15‐bit coding capacity of the chipless tag using frequency domain analysis with a maximum radar cross section of −15 dBsm. The multi‐bit coded tag can be suitable for a wide range of wireless applications, viz. tracking, retailing, identification, and healthcare utilities. [ABSTRACT FROM AUTHOR]

Coating degradation is a critical issue when steel surfaces are subject to weathering. This paper presents a chipless, passive antenna tag, which can be applied onto organically coated steel. Simulations indicated that changes associated with organic coating degradation, such as the formation of defects and electrolyte uptake, produced changes in the backscattered radar cross section tag response. This may be used to determine the condition of the organic coating. Simulating multiple aging effects simultaneously produced a linear reduction in tag resonant frequency, suggesting coating monitoring and lifetime estimation may be possible via this method. For coatings thinner than calculations would suggest to be optimum, it was found that the simulated response could be improved by the use of a thin substrate between the coated sample and the antenna without vastly affecting results. Experimental results showed that changes to the dielectric properties of the coating through both the uptake of water and chemical degradation were detected through changes in the resonant frequency. [ABSTRACT FROM AUTHOR]

A 32-bit chipless RFID tag operating in the 4.5–10.9 GHz band is presented in this paper. The tag has a unique multiple-arc-type shape consisting of closely packed 0.2 mm wide arcs of different radii and lengths. The specific tag geometry provides multiple resonances in frequency domain of an RCS plot. A frequency domain coding technique has also been proposed to encode the tag's RCS signature into a 32-bit digital identification code. The tag has an overall dimension of 17.9 × 17.9 mm2, resulting in a high code density of 9.98 bits/cm2 and spectral efficiency of 5 bits/GHz. The proposed tag is built on a low loss substrate bearing a very small footprint, thereby making it extremely suitable for large-scale product identification purposes in future chipless RFID tag systems. [ABSTRACT FROM AUTHOR]

In this article, a chipless RFID reader antenna based on a modified rectangular‐resonator is presented. The reader antenna is fabricated and measured to validate multiple narrowband resonance behavior. The measurement results show that the reader antenna can detect 11‐bit multi‐frequency coded chipless tag and depict gains of 4.7 and 5.2 dBi at 0.58 and 2.4 GHz, respectively. The cubic interpolation approach as a signal processing technique is applied at the reader end to eliminate the unnecessary drops at the measurement. Further study of Chu and Mclean's fundamental limit demonstrates that the physical dimensions of the proposed antenna and the radiation characteristics fulfilling the limitations. The multiple narrow‐band responses of the reader antenna can be suitable to detect multi‐bit tags which are of high demand because a multibit tag can be used for multiple frequency applications for hassle‐free wireless environment, viz. tracking, retailing, identification, and healthcare utilities. [ABSTRACT FROM AUTHOR]

This paper proposes a detection technique for the chipless RFID system. In the proposed technique, the experiments for measuring the scattering responses of all possible tags were conducted first in a free-space anechoic chamber in order to avoid the effect of unwanted signals due to the environment. The responses of all possible tags were exploited in order to extract poles, including natural frequencies and damping factors, by using the short-time matrix pencil method. All extracted poles successively chosen from the late-time portions were exploited to create the decision boundary by using the k- nearest neighbor algorithm. In order to validate the robustness of the proposed detection technique, experiments with the chipless RFID system with tags attached to containers, i.e. parcel and plastic boxes, were conducted. Poles extracted from the response of the tag attached to a container were fed to the decision boundary for ID detection. The poles that had fallen into the region labeled as logic “1” were detected as logic “1,” and vice versa. The experiment results showed that the attachment of the tags to the containers caused a change in the poles. The conventional technique using only natural frequencies has exhibited poor performance regarding tag ID detection. On the other hand, the proposed detection technique achieved a 100% detection rate, although the extracted poles used to detect the bit logic were disturbed by the container. The experimental results confirm the superiority of the proposed system over conventional chipless RFID detection. Moreover, the proposed technique can be considered a robust detection technique.

The recent emergence of Radio Frequency Identification (RFID) applications have attracted the attention of all the stakeholders, namely developers, manufacturers, and end-users. In essence, RFID has permeated the broad spectrum of item tracking, identification, and sensing. Alternatively, it is safe to say that RFID has revolutionized item tracking, sensing, and monitoring mechanisms. However, RFID’s proliferation often faces a roadblock due to its associated cost due to silicon-based integrated circuits. This aspect can be addressed by the emerging design techniques and performance enhancement approach adopted for the realization of Chipless RFIDs. This has catapulted the Chipless RFIDs at the forefront in recent years. The chipless RFID tag uses electromagnetic properties to store the information and eliminate memory chips. It is anticipated that the usage of Chipless RFIDs will increase by leaps and bounds in the coming years. Therefore, this paper discusses some of the critical applications that can be directly served by Chipless RFIDs. This paper also elaborates on the associated constraints which limit the mass deployment of the Chipless RFIDs. Furthermore, this paper throws some light on some of the exciting research directions for future development.

RFID has permeated in wide-ranging applications that may eventually fall under the broad IoT umbrella. However, the adaption of RFID for commercial purposes of IoT-centric applications still faces numerous technical challenges due to its high cost. The Chipless RFID can become a boon considering its significantly reduced cost and tag complexity in this context. In this article, therefore, different aspects of the chipless RFID are discussed and analyzed, and a potential road map is explored for the effective seamless integration of chipless RFID in the amalgamated IoT. At first, a brief theoretical preliminary about the Chipless RFID is presented, and it is succeeded by the application’s specific roles of this emerging RFID in the wide variety of domains. Subsequently, the analysis of various physical parameters shows that chipless RFID technology can greatly broaden IoT’s horizon. It essentially shows that it has the potential to bring a paradigm shift in the way RFID can coexist with IoT. Finally, a section which briefs about the future directions is also included to throw some light on Chipless RFID’s direct fitment in the IoT.

A 3-in-1 depolarizing circular scatterer for the polarimetric radar calibration is proposed in this article. The proposed scatterer is low cost, compact, planar circuit, and well suited for the radar operating in a compact range, for example, chipless RFID technology. By the virtue of its circular shape, the rear side of the proposed scatterer acts as a metallic disk with strong copolarization backscattered signals. The front side is composed of eight resonant dipoles that make it nondepolarizing and depolarizing scatterers at the inclination of 0° and 45°, respectively. The features of proposed circular scatterer are tested as reference calibration objects for single antenna-based polarimetric radar calibration techniques. Two test objects are utilized: a dihedral tilted at 45° and a depolarizing multiresonant planar structure commonly called chipless RFID tag. The performance of proposed circular scatterer is also compared with the standard reference calibration objects: a metallic disk and a dihedral tilted at 22.5°. The performance of proposed circular scatterer is comparable to the standard reference calibration objects for the calibration techniques (namely, Type 1 and Type 2). The proposed scatterer is potentially tolerant of displacement up to 1 cm and misalignment equals 2°. [ABSTRACT FROM AUTHOR]

This paper reports a novel, fully passive chipless RFID tag covering a minuscule profile of 16 × 15 mm 2 emanating data capacity of 15 bits. The ON–OFF keying phenomenon has been utilized for data encoding in the compact structure. Moreover, the tag renders 1:1 slot-to-bit correspondence while incorporating loop-based pentagonal-shaped geometry as a resonating structure. The proposed chipless RFID tag offers an appreciable bit density of 6.25 bits/cm 2 in a spectral range of 4.8 to 18.8 GHz. The electromagnetic performance of the design is scrutinized over the ungrounded laminates, i.e., Rogers RT/duroid® 5880 and Rogers RT/duroid® 5870. The multi-resonant signature exhibits fair agreement between measured and computed results. The reported work credits its potential and application in contactless tracking and identification. [ABSTRACT FROM AUTHOR]

In this paper, we realize a non-invasive, bio-compatible, reliable and compact sensor to measure relative humidity (RH) levels by depositing keratin bio-polymer on a dual-polarized ultra wide band (UWB) chipless radio frequency identification (RFID) tag. The bio-compatible and hydrophilic properties of keratin make the proposed sensor an appropriate solution for monitoring RH level in different food and pharmaceutical industries, where non-toxic materials are in high demand. The proposed sensor comprises two main parts: a three-armed Fermat spiral resonator and the keratin bio-polymer mounted on the surface of the resonator. Keratin is extracted through the alkaline hydrolysis process and analyzed using X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR) methods. The three-armed Fermat spiral resonator acts as a scatterer, generating both co- and cross-polarized resonant responses when interrogated by a signal in a UWB frequency range. Having a cross-polarized response increases the robustness of the proposed sensor in the presence of interference and helps enhance the reliability of the sensor by offering three additional parameters. Amplitude, quality factor, and resonant frequencies are three main parameters that have been extracted from the responses of the proposed RH sensor at both co- and cross-polarization by modelling its responses as a function of frequency. To examine the performance of the sensor, laboratory measurements are carried out in the Esky box. The results show that the proposed sensor can be used to monitor RH changes from 54% to 74% at 26° Celsius. [ABSTRACT FROM AUTHOR]

This paper introduces a chipless Radio Frequency Identification tag to be used for oil and gas pipelines monitoring applications. The proposed tag uses multiple E-shaped resonators to introduce multiple notches in the operation band, giving any required number of bits in the spectrum signature. Two cross-polarized semi-circular patches with partial grounds are used as the tag antennas. The proof of the concept is done using vector network analyzer measurements with two implemented Vivaldi antennas as transmitter (Tx) and receiver (Rx). The experimental data match will with the expected results. [ABSTRACT FROM AUTHOR]

This paper presents the design and analysis of a paper-based chipless RFID sensor for wetness content measurement in plant leaves. The sensor is designed using a passive microwave resonator to measure the ambient physical parameter. The resonator is designed with a rectangular loop inductively coupled with an interdigital capacitor. The experimental analysis of the designed sensor is illustrated here to prove its efficacy. The resonator provides adequate backscattered signal for wireless measurement in the microwave frequency band. When interrogated with a couple of UWB antennas and a VNA, it yields accurate leaf wetness response of the plant. The commercial competence of this sensing device is validated through its experimental analysis on three different types of leaf samples. This sensor is suitable for plant health monitoring in any small-scale gardening applications such as nurseries and greenhouses.

This article introduces retroreflective lenses for millimeter-wave radio-frequency indoor localization. A three-dimensional (3D) gradient-index Luneburg lens is employed to increase radar cross section (RCS) of photonic-crystal high-Q resonators and its performance is compared to conventional radar retroreflectors. A classic Luneburg lens with and without a reflective layer is realized with 25 mm diameter (6.7 λ0), showing a realized gain of 24.6 dBi and a maximum RCS of -9.22 dBm2 at 80 GHz. The proposed Luneburg lens with embedded high-Q resonators as frequency-coded particles in a photonic crystal structure, operating as a reflective layer, achieved a maximum RCS of -15.84 dBm2 at the resonant frequency of 76.5 GHz and showed a repeatable response each 18° over ±36° in two perpendicular planes. With this high RCS of the Luneburg lens, a maximum readout range of 1.3 m could be achieved compared to 0.15 m without the lens at 76.5 GHz for the same transmit power, receiver sensitivity, and gain of the reader antenna.