Frequency-spectra-based high coding capacity chipless RFID using an UWB-IR approach

sensors Article Frequency-Spectra-Based High Coding Capacity Chipless RFID Using an UWB-IR Approach Kawther Mekki 1, Omrane Necibi 2, Hugo Dinis 3, Paulo Mendes 3,* and Ali Gharsallah 1   Citation: Mekki, K.; Necibi, O.; Dinis, H.; Mendes, P.; Gharsallah, A. Frequency-Spectra-Based High Coding Capacity Chipless RFID Using an UWB-IR Approach. Sensors 2021,21, 2525. https://doi.org/ 10.3390/s21072525 Academic Editor: Jemal H. Abawajy Received: 15 February 2021 Accepted: 29 March 2021 Published: 4 April 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1Laboratory for Research on Microwave Electronics, Physics Department, Faculty of Science, University of Tunis El Manar, 2092 El Manar, Tunisia; kawther[email protected] (K.M.); [email protected] (A.G.) 2Computer Science Department, College of Arts and Sciences at Tabarjal, Jouf University, 72388 Jouf, Saudi Arabia; [email protected] 3CMEMS-Uminho, University of Minho, 4710-057 Braga, Portugal; [email protected] *Correspondence: [email protected] Abstract: A novel methodology is proposed to reliably predict the resonant characteristics of a multipatch backscatter-based radio frequency identification (RFID) chipless tag. An ultra-wideband impulsion radio (UWB-IR)-based reader interrogates the chipless tag with a UWB pulse, and analyzes the obtained backscatter in the time domain. The RFID system consists of a radar cross-section (RCS)-based chipless tag containing a square microstrip patch antenna array in which the chipless tag is interrogated with a UWB pulse by an UWB-IR-based reader. The main components of the backscattered signal, the structural mode, and the antenna mode were identified and their spectral quality was evaluated. The study revealed that the antenna-mode backscatter includes signal carrying information, while the structural mode backscatter does not include any tag information. The simulation findings were confirmed by experimental measurements obtained in an anechoic chamber environment using a 6-bit multipatch chipless RFID tag. Finally, the novel technique does not use calibration tags and can freely orient tags with respect to the reader. Keywords: RFID; antenna; chipless tag; reader; UWB-IR; backscatter; amplitude 1. Introduction Radio frequency identification (RFID) is a technique that has been applied to transmit and receive wireless data. It is intensively used in various applications such as automatic identification, asset tracking, and security surveillance [ 1 , 2 ]. To overcome the weaknesses of barcodes and other identification means such as the limitations of asset tracking, RFID technology has been employed in security access control, logistics, asset tracking, and supply chain management [ 3 , 4 ]. Traditional passive RFID systems use tags with no internal power source. These tags are powered by the electromagnetic energy transmitted from a RFID reader. However, the cost of RFID reduces its potential to replace trillions of barcodes [ 1 , 2 ]. To solve this problem, a chipless RFID tag is considered a good alternative [ 1 ]. The chipless RFID tag has no electronic circuitry and thus no intelligent signal processing capability. This option makes it less expensive and easier to mass produce at a unit cost comparable to optical barcodes [4–8]. RFID technology contains two major components: readers and tags [ 1 , 9 , 10 ]. Recently, the application of chipless tags has been extensively studied by the research community to develop high-performance RFID tags [ 3 , 11 – 19 ]. Several topologies relating to those used based on resonators have been suggested for chipless tags. Such topologies include circular loop [ 20 – 22 ], square loop [ 23 – 25 ], U-shaped [ 26 – 28 ], C-shaped [ 29 , 30 ], L-shaped [ 31 – 34 ], slotted [ 34 – 36 ], rhombic [ 37 , 38 ], octagonal [ 39 ], and microstrip-line [ 18 , 40 , 41 ]. It is also worth noting that the authors have suggested textile wearable application technology devices in the literature [ 42 , 43 ], which can be used as chipless RFID sensor tags in identification and tracking applications. There is also the three-dimensional chipless RFID tag Sensors 2021,21, 2525. https://doi.org/10.3390/s21072525 https://www.mdpi.com/journal/sensors Sensors 2021,21, 2525 2 of 17 suggested in [ 44 ]. Since the commercialization of chipless RFID requires a communication specification, it is important to consider the appropriate bandwidth for a frequency spectrum system. Therefore, the operating frequency spectra have been selected as 2–4 GHz [ 13 , 26 , 27 ], 2–5 GHz [ 14 , 29 ], 2–8 GHz [ 24 , 25 ], 3–6 GHz [ 32 , 34 , 38 ], 3–8 GHz [ 19 , 21 , 33 ], 5–8 GHz [ 16 ], 4–8 GHz [ 17 ], and 3.1–10.6 GHz [ 22 , 37 ]. The objective of this study was to enhance the performance of RFID tags in major design parameters such as reading range, tag and reader separation distance, bandwidth, bit encoding ability, and bit states/resonators. Since a communication protocol is needed for the commercialization of chipless RFID, it is necessary to establish an acceptable frequency range for high-capacity devices. Chipless RFID spectral-signature-based tags using a radar cross-section (RCS) have some apparent merits such as small size and they are easily readable. We can distinguish between time domain (TD)-based tags and frequency domain (FD)-based tags. In the former, the interrogation of the tag by the RFID reader is carried out with a sequence of pulses. Subsequently, the tag code is determined based on the echoes reflected by a set of reflectors applied on the tag, which is linked to an object. More precisely, the existence or absence of these echoes and their time positions can be considered to specify the tag code [ 8 , 11 , 45 – 47 ]. On the other hand, in FD-based tags, an RFID reader uses a radio frequency (RF) signal to interrogate the tag. Afterward, the frequency signature is retransmitted by the tag to the reader. Each data bit shows the resonant frequency presence or absence in the operating spectrum [8,26,46,48–50]. Ultra-wideband (UWB) wireless communication technologies have recently attracted the attention of researchers due to their numerous benefits. Indeed, it enables lower power consumption, lower structural complexity, lower fabrication cost, size reduction, and a higher date rate. We would argue that (UWB) wireless communication technologies contain the majority of what researchers want to do in today’s technologies. For that, ultra-wideband (UWB) chipless tags for a RFID system may be a good solution for low-cost item tagging, as proven in [16–18]. The coding capacity of using UWB-impulse-based interrogation to remotely approximate the resonant features of a backscatter-based multipatch chipless RFID tag was enhanced in this study [ 51 ]. The use of ultra-wideband impulse radio (UWB-IR) based interrogation for the chipless RFID system has not been given much attention. However, a bit of research has been devoted to this field [52]. This paper presents an exploration of backscattering and its application to analyze the backscatter response created by an RCS-based RFID chipless tag for time and frequency domain analysis. The signal requests the tag when the response of a UWB signal is analyzed in the time and frequency domains in order to obtain the resonance information. The RCS and the advanced level signal processing of the backscattered signal play important roles in enhancing the operational range of chipless tags with enhanced tag design. To mitigate these challenges, the backscattered signals are recorded by a single antenna reader system using a standard frequency-domain backscattered tag with 6-bit data capability. A deeper insight into the mechanism of frequency-signature-based chipless RFID tags has been achieved through the time domain analysis of the tag backscatter. By time domain analysis, the valuable information-carrying component of the general backscattered signal was separated, and the frequency domain was then evaluated to estimate the tag’s frequency signature. In the CST Microwave studio suite, a condensed unit containing the reader antenna and the tag was simulated to gather the necessary simulated data. In both the time and frequency domains, different signal components were collected and assessed using the generated simulation data. The simulation results were also validated based on measurement findings provided by experiments carried out in an anechoic chamber environment. This multi-patch tag, which encodes 6-bit, has a large degree of freedom in the orientation angles with respect to the RFID reader antenna. Moreover, the maximum angular ranges can be detected from the measurement viewpoint. Therefore, the degradation of detection performance is quantified. The proposed system does not Sensors 2021,21, 2525 3 of 17 require calibration tags, and with respect to the reader location, the new approach produces appropriate results under various tag orientations and positions [4,19,34,35,53,54]. The rest of this manuscript is organized as follows. Section 2presents the background on RCS-based chipless RFID tags; Section 3describes the analyzed system model, the chipless RFID system, and the backscatter formed by the chipless RFID tag in which the responses is derived to the structural mode and the antenna mode backscatter; Section 4 describes the electromagnetic simulation environment as well as the experimental setup, which were used to obtain the simulation and measurement results; and Section 5presents our concluding remarks. 2. Operating Principles of the Chipless Radio Frequency Identification (RFID) Reader System The wireless data capturing method, namely RFID, uses RF waves to systematically identify objects. This technique employs RF waves to transmit data from the data carrying device, namely the RFID tag, to the reader [1,55,56]. 2.1. Chipless Radio Frequency Identification (RFID) Reader System The chipless radio frequency identification (RFID) system consists of a multiresonator chipless tag and an UWB reader antenna. The former system is a completely passive microwave circuit that employs spectral signatures to encode data. It is made of a multiresonator to accommodate multiple bits and operates over the UWB frequency spectrum. In fact, UWB antennas are generally operated to allow the interrogation signal to be forwarded by the reader and transfer the signal back to the reader after modulation of the frequency spectra by the multiresonator. In cases where the chipless RFID tag is hit by a vertically polarized electromagnetic (EM) wave produced by the transmitter antenna, a unique frequency signature is coded and the receiver antenna receives the encoded backscattered signal. This unique frequency signature is then recorded and extracted by the RFID reader device, which enables the identification of the chipless RFID tag. The main function of the RFID reader is transmitting the interrogational signal toward the chipless tag, with magnitude and phase data. Subsequently, the interrogation signal is encoded into different frequency spectra according to the magnitude and phase and then retransmitted to the reader by the multiresonator chipless tag [ 7 , 21 , 49 , 57 , 58 ]. Figure 1shows a block diagram of the introduced chipless RFID system. 2.2. Design of Ultra-Wideband (UWB) Reader Antenna In our current work, we used circular monopole antennas, because, compared to other shapes with omnidirectional radiation characteristics and a simple structure, they have shown a much wider bandwidth. A radiating patch and ground plane on the same side of a dielectric substrate are the characterizing elements of an UWB monopole antenna. The transmission and the reception of the interrogation signals by the reader were performed by employing a single monopole antenna. Figure 2a shows the antenna utilized in the experiment. Circular UWB monopole antennas have a simple design and a very wide bandwidth. This antenna was a coplanar waveguide (CPW)-fed circular disk loaded monopole antenna [ 17 , 18 , 59 ]. The CPW feed line was separated from the ground by 0.25 mm. Due to its lower dielectric loss, decreased signal loss, and reduced thickness, RO4350B dielectric material is used to fabricate reader antennas at a lower cost. This allows us to miniaturize our proposed antennas. The Rogers RO4350B substrate had the following properties: εr = 3.48, δ= 0.0037, thickness of 0.76 mm, and copper cladding thickness of 175 µm. Figure 2a shows the dimensions of the UWB monopole antenna and Figure 2b shows the fabricated antenna. The reflection coefficients of the simulated and measured monopole antenna are shown in Figure 2c. The antenna performed from 2 to 7 GHz, and it had reflection coefficients of less than −10 dB. Sensors 2021,21, 2525 4 of 17 Sensors 2021, 21, x FOR PEER REVIEW 4 of 18 Figure 1. Operation principles of the chipless radio frequency identification (RFID) system. 2.2. Design of Ultra-Wideband (UWB) Reader Antenna In our current work, we used circular monopole antennas, because, compared to other shapes with omnidirectional radiation characteristics and a simple structure, they have shown a much wider bandwidth. A radiating patch and ground plane on the same side of a dielectric substrate are the characterizing elements of an UWB monopole antenna. The transmission and the reception of the interrogation signals by the reader were performed by employing a single monopole antenna. Figure 2a shows the antenna utilized in the experiment. Circular UWB monopole antennas have a simple design and a very wide bandwidth. This antenna was a coplanar waveguide (CPW)-fed circular disk loaded monopole antenna [17,18,59]. The CPW feed line was separated from the ground by 0.25 mm. Due to its lower dielectric loss, decreased signal loss, and reduced thickness, RO4350B dielectric material is used to fabricate reader antennas at a lower cost. This allows us to miniaturize our proposed antennas. The Rogers RO4350B substrate had the following properties: ε r = 3.48, δ = 0.0037, thickness of 0.76 mm, and copper cladding thickness of 175 µm. Figure 2a shows the dimensions of the UWB monopole antenna and Figure 2b shows the fabricated antenna. The reflection coefficients of the simulated and measured monopole antenna are shown in Figure 2c. The antenna performed from 2 to 7 GHz, and it had reflection coefficients of less than −10 dB. Figure 1. Operation principles of the chipless radio frequency identification (RFID) system. 2.3. Chipless Tag Antenna Design and Operation The microstrip patch antennas used in this paper were designed to achieve narrow band operation, in contrast to the wide band antennas discussed earlier. This resonant behavior is desired in the design of frequency signature based chipless RFID tags. To encode tag-ID data as a frequency signature, we used sharp resonances, which is discussed later in Section 3. To demonstrate the functionality of the system, an array containing six square microstrip patch antennas was used in the 6-bit chipless RFID tag [51]. The design of the multipatch-based chipless RFID tag is shown in Figure 3a. This tag was modeled on a RO4350B substrate material with a dielectric constant of 3.48, a loss tangent of 0.0019, a thickness of 0.76 mm, and a copper thickness of 175 µ m. The lengths of the six patch antennas constituting the tag were 17.5, 16.5, 15.5, 14.5, 13.5, and 12 mm and they resonated at frequencies of 4.4, 4.7, 5.1, 5.4, 5.7, and 6.1 GHz. Each patch antenna creates backscatter only at its corresponding resonant frequency, producing a distinctive frequency signature in the total backscattered signal. This resonant behavior has been proven to be efficient in designing chipless RFID tags based on the magnitude of the RCS [ 51 ]. Indeed, a RCS tag shows the tag-ID data-encoding frequency signature. Six different ID tags were fabricated, and are illustrated in Figure 3b. The RCS was simulated and measured versus frequency and the peaks can be seen in Figure 3c. Sensors 2021,21, 2525 5 of 17 Sensors 2021, 21, x FOR PEER REVIEW 5 of 18 (a) (b) (c) Figure 2. Dimensions of (a) the reader antenna and (b) the fabricated reader antenna. (c) Simulated and measured reflection coefficients of the reader antenna. 2.3. Chipless Tag Antenna Design and Operation The microstrip patch antennas used in this paper were designed to achieve narrow band operation, in contrast to the wide band antennas discussed earlier. This resonant behavior is desired in the design of frequency signature based chipless RFID tags. To encode tag-ID data as a frequency signature, we used sharp resonances, which is discussed later in Section 3. To demonstrate the functionality of the system, an array containing six square microstrip patch antennas was used in the 6-bit chipless RFID tag [51]. The design of the multipatch-based chipless RFID tag is shown in Figure 3a. This tag was modeled on a RO4350B substrate material with a dielectric constant of 3.48, a loss Figure 2. Dimensions of ( a ) the reader antenna and ( b ) the fabricated reader antenna. ( c ) Simulated and measured reflection coefficients of the reader antenna. Sensors 2021,21, 2525 6 of 17 Sensors 2021, 21, x FOR PEER REVIEW 6 of 18 tangent of 0.0019, a thickness of 0.76 mm, and a copper thickness of 175 µm. The lengths of the six patch antennas constituting the tag were 17.5, 16.5, 15.5, 14.5, 13.5, and 12 mm and they resonated at frequencies of 4.4, 4.7, 5.1, 5.4, 5.7, and 6.1 GHz. Each patch antenna creates backscatter only at its corresponding resonant frequency, producing a distinctive frequency signature in the total backscattered signal. This resonant behavior has been proven to be efficient in designing chipless RFID tags based on the magnitude of the RCS [51]. Indeed, a RCS tag shows the tag-ID data-encoding frequency signature. Six different ID tags were fabricated, and are illustrated in Figure 3b. The RCS was simulated and measured versus frequency and the peaks can be seen in Figure 3c. (a) (b) (c) Figure 3. Dimensions of (a) the chipless tags loaded with resonators and (b) the fabricated chipless tags. (c) The simulated and measured radar cross section (RCS) magnitude versus frequency. Figure 3. Dimensions of ( a ) the chipless tags loaded with resonators and ( b ) the fabricated chipless tags. ( c ) The simulated and measured radar cross section (RCS) magnitude versus frequency. 3. Model of Chipless RFID System for Backscatter In this section, we present the chipless RFID system model. The backscattering process is also described, and the major components of the backscattered signal are illustrated. In addition, the tag-ID data representation in the chipless RFID tag is shown, and the detection of the tag-ID data bits is investigated. 3.1. Chipless Tag Interrogation and Backscatter Response The analysis of the backscatter signal from a multipatch-based chipless RFID tag was carried out in the time domain [ 51 , 56 , 60 ] and the overall system was described. Then, the system functioning, the chipless RFID tag design, and the nature of the backscatter were clarified based on the CST Microwave Studio simulation results. The applied chipless RFID system model is illustrated in Figure 4. Sensors 2021,21, 2525 7 of 17 Sensors 2021, 21, x FOR PEER REVIEW 7 of 18 3. Model of Chipless RFID System for Backscatter In this section, we present the chipless RFID system model. The backscattering process is also described, and the major components of the backscattered signal are illustrated. In addition, the tag-ID data representation in the chipless RFID tag is shown, and the detection of the tag-ID data bits is investigated. 3.1. Chipless Tag Interrogation and Backscatter Response The analysis of the backscatter signal from a multipatch-based chipless RFID tag was carried out in the time domain [51,56,60] and the overall system was described. Then, the system functioning, the chipless RFID tag design, and the nature of the backscatter were clarified based on the CST Microwave Studio simulation results. The applied chipless RFID system model is illustrated in Figure 4. The circular monopole antenna utilized as the RFID reader represents an ultra-wide bandwidth system. In fact, the reader was employed to transmit the interrogation pulse and to receive the backscatter from the tag. The tag was located 30 cm in front of the reader antenna. Figure 4. System model for the multipatch-based chipless RFID reader. To interrogate the chipless tags, it was required to use a broadband pulse. The signal x(t) denotes the UWB pulse utilized to interrogate the chipless RFID tag. It represents a modulated Gaussian pulse with a 20 dB bandwidth of 6 GHz, whose frequency content varies between 2 and 8 GHz. This signal was applied to interrogate the chipless tags. The transmitted interrogation pulse, x(t), was obtained as follows [60]: X(t) = A0 cos (2π fc t) exp 󰇡  󰇛–󰇜  󰇢 (1) The mean and the standard deviation parameters specifying the form of the Gaussian pulse were μ = 0.6 ns and σ = 0.114 ns, respectively. The sinusoidal signal carrier amplitude and the frequency were A 0 = 1 and f c = 5 GHz, respectively. Figure 5a shows the time domain of the UWB interrogation pulse, while Figure 5b shows the frequency spectrum of the interrogation pulse. Figure 4. System model for the multipatch-based chipless RFID reader. The circular monopole antenna utilized as the RFID reader represents an ultra-wide bandwidth system. In fact, the reader was employed to transmit the interrogation pulse and to receive the backscatter from the tag. The tag was located 30 cm in front of the reader antenna. To interrogate the chipless tags, it was required to use a broadband pulse. The signal x(t) denotes the UWB pulse utilized to interrogate the chipless RFID tag. It represents a modulated Gaussian pulse with a 20 dB bandwidth of 6 GHz, whose frequency content varies between 2 and 8 GHz. This signal was applied to interrogate the chipless tags. The transmitted interrogation pulse, x(t), was obtained as follows [60]: X(t)=A0cos (2πfct)exp −(t–µ)2 2σ2(1) The mean and the standard deviation parameters specifying the form of the Gaussian pulse were µ = 0.6 ns and σ = 0.114 ns, respectively. The sinusoidal signal carrier amplitude and the frequency were A 0 = 1 and f c = 5 GHz, respectively. Figure 5a shows the time domain of the UWB interrogation pulse, while Figure 5b shows the frequency spectrum of the interrogation pulse. As shown in Figure 4, x(t) is the UWB pulse transmitted by the RFID reader and y(t) denotes the overall received signal at the reader. In cases of interaction of the transmitted UWB pulse, x(t), with the tag, a portion of the UWB pulse will be captured by the individual patch antennas forming the tag, while the remaining part will be instantly reflected. The total received signal, y(t), at the RFID reader is composed of the following three components: y(t) = yr(t) + ys(t) + ya(t) (2) The transmitted pulse rejection, y r (t), is considered as the largest and the first received component due to the antenna reflection coefficients. At this point, the reader antenna fully transmits x(t) and receives any backscatter sent by the tag. The structural mode backscatter is the second received component, y s (t), whereas the weakest and the last received component is the antenna mode of the backscatter, ya(t). Sensors 2021,21, 2525 8 of 17 Sensors 2021, 21, x FOR PEER REVIEW 8 of 18 (a) (b) Figure 5. (a) Time domain of the ultra-wideband (UWB) interrogation pulse and (b) frequency spectrum of the interrogation pulse. As shown in Figure 4, x(t) is the UWB pulse transmitted by the RFID reader and y(t) denotes the overall received signal at the reader. In cases of interaction of the transmitted UWB pulse, x(t), with the tag, a portion of the UWB pulse will be captured by the individual patch antennas forming the tag, while the remaining part will be instantly reflected. The total received signal, y(t), at the RFID reader is composed of the following three components: y(t) = y r (t) + y s (t) + y a (t) (2) The transmitted pulse rejection, y r (t), is considered as the largest and the first received component due to the antenna reflection coefficients. At this point, the reader antenna fully transmits x(t) and receives any backscatter sent by the tag. The structural mode backscatter is the second received component, y s (t), whereas the weakest and the last received component is the antenna mode of the backscatter, y a (t). 3.2. Analysis of Chipless Tag Backscatter Response Figure 6 illustrates the received signal of the full-time domain, y(t), provided by applying full-wave EM simulation in cases where the tag was located 30 cm away from the reader antenna. First, a part of the signal yr(t), which represents the first and largest component of y(t), decreased progressively and disappeared. A small echo, which was the backscatter produced by the chipless tag, was noticed at 2.55 ns. Compared to the rejected signal, the backscatter was smaller and cannot be visibly observed in Figure 6. The structural mode backscatter and the antenna mode backscatter were clearly separated. Figure 6 also depicts the ys(t) and ya(t) components. Figure 5. ( a ) Time domain of the ultra-wideband (UWB) interrogation pulse and ( b ) frequency spectrum of the interrogation pulse. 3.2. Analysis of Chipless Tag Backscatter Response Figure 6illustrates the received signal of the full-time domain, y(t), provided by applying full-wave EM simulation in cases where the tag was located 30 cm away from the reader antenna. Sensors 2021, 21, x FOR PEER REVIEW 9 of 18 Figure 6. Total received signal, y(t) at the antenna. Here, the various components that formed the total signal arriving at the reader can be distinguished and separated. The backscatter contained a large but short-duration initial component as well as a small but longer-duration component. The shape of the former resembles the transmitted Gaussian pulse represented in Figure 5b. We hypothesize that the structural mode backscatter, ys(t), was the larger component and that the antenna mode, ya(t), was the smaller component. Nevertheless, as no transmission line presenting a controlled delay existed, as was the case in the chipless RFID tag depicted in Figure 6 before, we could not adequately determine the start of the antenna mode backscatter and the end of the structural mode backscatter. To prove this hypothesis, it was necessary to separate the two components and investigate their spectral content. The components of the received backscatter were isolated by employing a raised cosine window, as shown in Figure 7. Figure 6. Total received signal, y(t) at the antenna. First, a part of the signal yr(t), which represents the first and largest component of y(t), decreased progressively and disappeared. A small echo, which was the backscatter produced by the chipless tag, was noticed at 2.55 ns. Compared to the rejected signal, the backscatter was smaller and cannot be visibly observed in Figure 6. The structural mode Sensors 2021,21, 2525 9 of 17 backscatter and the antenna mode backscatter were clearly separated. Figure 6also depicts the ys(t) and ya(t) components. Here, the various components that formed the total signal arriving at the reader can be distinguished and separated. The backscatter contained a large but short-duration initial component as well as a small but longer-duration component. The shape of the former resembles the transmitted Gaussian pulse represented in Figure 5b. We hypothesize that the structural mode backscatter, ys(t), was the larger component and that the antenna mode, ya(t), was the smaller component. Nevertheless, as no transmission line presenting a controlled delay existed, as was the case in the chipless RFID tag depicted in Figure 6before, we could not adequately determine the start of the antenna mode backscatter and the end of the structural mode backscatter. To prove this hypothesis, it was necessary to separate the two components and investigate their spectral content. The components of the received backscatter were isolated by employing a raised cosine window, as shown in Figure 7. Sensors 2021, 21, x FOR PEER REVIEW 10 of 18 Figure 7. Magnified portion showing the structural mode ys(t) and antenna mode backscatter ya(t). Figure 8 shows the spectral content of the windowed structural mode and windowed antenna mode provided by applying the fast Fourier transform (FFT) algorithm. The larger and first portion of the backscatter, ys(t), is characterized by a Gaussian amplitude spectrum resembling the spectrum of the transmitted UWB pulse (Figure 5b). It does not include any data on the resonant frequencies of the patch antennas in the chipless tag. However, the amplitude spectrum of the secondary smaller component backscatter after the structural mode clearly displayed six spectral peaks, precisely at the resonant frequencies of the patch antennas in the chipless tag. The windowed ya(t) spectral content presents the resonant frequencies of the six individual patch antennas (f1 = 4.4, f2 = 4.7, f3 = 5.1 f4 = 5.4, f5 = 5.7, and f6 = 6.1 GHz). Thus, the transient ya(t) after the initial strong backscatter ys(t) clearly contains the necessary information to approximate the resonant frequencies of the patch antennas in the chipless RFID tag. The backscatter antenna mode is viewed as a filtered variant of the tag’s UWB pulse incident, where only the frequencies associated with the patches are transmitted in, while the others are filtered out. Therefore, the antenna mode backscatter spectral peaks showing the frequencies with higher energy (f1) in the incident UWB pulse were higher than those showing frequencies with lesser energy (f2) in the incident UWB pulse. Figure 7. Magnified portion showing the structural mode ys(t) and antenna mode backscatter ya(t). Figure 8shows the spectral content of the windowed structural mode and windowed antenna mode provided by applying the fast Fourier transform (FFT) algorithm. The larger and first portion of the backscatter, ys(t), is characterized by a Gaussian amplitude spectrum resembling the spectrum of the transmitted UWB pulse (Figure 5b). It does not include any data on the resonant frequencies of the patch antennas in the chipless tag. Sensors 2021,21, 2525 16 of 17 9. Mekki, K.; Necibi, O.; Boussetta, C.; Gharsallah, A. Miniaturization of circularly polarized patch antenna for RFID reader applications. Eng. Technol. Appl. Sci. Res. 2020,10, 5655–5659. [CrossRef] 10. 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