This is the accepted version of the journal article: Paredes, Ferran; Herrojo, Cristian; Escudé Pujol, Roger; [et al.]. «High data density near-field chipless-RFID tags with synchronous reading». IEEE journal of radio frequency identification, Vol. 4, Issue 4 (December 2020), p. 517-524. DOI 10.1109/JRFID.2020.2996586 This version is available at https://ddd.uab.cat/record/258240 under the terms of the license > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 1 High Data Density Near-Field Chipless-RFID Tags with Synchronous Reading Ferran Paredes, Member, IEEE, Cristian Herrojo, Member, IEEE, Roger Escudé, Eloi Ramon, and Ferran Martín, Fellow, IEEE 1Abstract— Near-field chipless-RFID tags with high data RFID systems, the tags contain resonant elements tuned to density and synchronous reading capability are presented and different frequencies, and the presence or absence of experimentally validated in this paper. The tags consist of a chain functional resonant elements determine the logic state (‘1’ or of rectangular patches etched or printed at predefined positions ‘0’) of the corresponding resonator. It has been demonstrated on a dielectric substrate, including rigid or flexible (i.e., plastic or that the data density and capacity of frequency-domain tags even paper) substrates. Patch dimensions determine the binary state, where the larger and smaller patches are associated with (or spectral signature barcodes, the usual designation of such the logic ‘1’ and ‘0’ states, respectively, or vice versa. For tags) can be substantially enhanced by combining the sequentially and synchronously reading the bits, a sensitive frequency domain with other domains, e.g., phase, amplitude, element able to determine the presence of the patches and their polarization, etc. [20]-[33]. Nevertheless, the data capacity of size by proximity (through near field using microwaves) is these hybrid chipless-RFID systems is still far from the data considered. Such element is a microstrip line loaded with a pair storage capacity of chipped-tags (typically 96 bits according to of rectangular complementary split ring resonators (CSRRs), one the EPC UHF Gen2 standard). etched inside the other in the ground plane. When the tag chain is Time-domain chipless-RFID systems based on tags displaced at short distance over the CSRRs, the larger patches emulating the behavior of surface acoustic wave (SAW) based modify the resonance frequency of both sensing CSRRs, whereas tags [34] have been proposed. The working principle of such the lower patches do only alter the resonance frequency of the smaller CSRR. Consequently, the ID code is contained in the time-domain based tags is time-domain reflectometry (TDR), patch dimensions, and the presence of a patch (regardless of its where the ID code is inferred from the echoes generated by a size) determines the reading times (clock signal), necessary for delay line loaded with reflectors in response to a pulsed synchronous reading. Tag reading in this system proceeds by interrogation signal. Several examples of TDR-based tags feeding the CSRR-loaded line (reader) with a pair of harmonic have been reported [35]-[44], but the data density and capacity signals tuned to the resonance frequencies of the bare CSRRs. of these tags is very limited as compared to the one of chip- Both signals are amplitude modulated (AM) at the output port as based tags or SAW tags (the latter very competitive in terms consequence of tag motion, and the respective envelope functions of data capacity, but expensive, as far as they need acoustic contain both the clock signal and the tag ID code. The ID codes transducers). of several 16-bit tags, implemented on different substrates Recently, a novel time-domain approach for chipless-RFID (microwave substrate, plastic and paper) and exhibiting a per unit length density of 1.67 bit/cm, have been inferred with the systems, based on near-field coupling and sequential bit dedicated reader for validation purposes. reading, has been reported by some the authors [45]-[56]. In Index Terms — Chipless-RFID, synchronous reading, these systems, the tags are implemented by etching or printing microwave encoders, microstrip technology. a chain of metallic inclusions (typically, although not exclusively, resonant elements) on a dielectric substrate (rigid I. INTRODUCTION or flexible, including plastic or paper). Encoding is based on the presence/absence of a functional inclusion at its predefined C HIPLESS-RFID emerged more than one decade ago as an alternative to chip-based RFID, in order to reduce the cost of the tags (mainly dictated by the presence of the chip [1]). In position in the tag chain, where each inclusion corresponds to a bit of information. For tag reading, the tag (i.e., the most chipless-RFID systems, the identification (ID) code of inclusions chain) is displaced at short distance over the the tags is contained in a metallic pattern, etched or printed on sensitive part of the reader, an element able to discriminate a dielectric substrate (rigid or flexible, including plastic and between the functional and inoperative inclusions. The paper substrates). There are different encoding techniques in sensitive part of the reader is typically a transmission line, chipless-RFID, but the most extended one exploits the eventually loaded with a resonant element, fed by a harmonic frequency domain [1]-[19]. In frequency-domain chipless- signal conveniently tuned. In contrast to frequency-domain or hybrid chipless-RFID This work was supported by MINECO-Spain (project TEC2016-75650-R), systems, where the interrogation signal is a wideband signal, MICIU (project RTI2018-102070-B-C21), by Generalitat de Catalunya in these novel time-domain systems, a single-tone (harmonic) (project 2017SGR-1159), by Institució Catalana de Recerca i Estudis signal suffices for tag reading (two harmonic signals are Avançats (who awarded Ferran Martín), and by FEDER funds. needed for synchronous reading, as it will be shown later). F. Paredes, C. Herrojo, and F. Martín are with GEMMA/CIMITEC, Departament d’Enginyeria Electrònica, Universitat Autònoma de Barcelona, Moreover, since the bits are read sequentially, the number of 08193 Bellaterra, Spain. E-mail: Ferran.Martin@uab.es. bits of the tags is theoretically unlimited (in practice the R. Escudé, and E. Ramon are with Institut de Microelectronica de Barcelona IMB-CNM (CSIC), 08193 Bellaterra, Spain. > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 2 limitation is dictated by tag size). The sketch of the working II. THE PROPOSED READER AND TAGS principle (including synchronous reading) is depicted in Fig. The proposed near-field chipless-RFID system with 1. Note that the ID code is contained in the envelope function synchronous reading capability is based on the approach first at the output port, with peaks or dips corresponding to the presented in [57]. The tags are implemented by means of a logic state ‘1’ or ‘0’, or vice versa. linear chain of rectangular patches, where each patch The difference between these time-domain near-field corresponds to a bit and encoding is achieved by patch size. chipless-RFID systems and the frequency-domain, or hybrid, Namely, patch dimensions determine the logic state, ‘1’ or ‘0’, systems mentioned above concerns the different type of ID of the corresponding bit. With this encoding system, a data multiplexing. In frequency-domain and hybrid systems sensitive element able to determine, through near field the data are multiplexed in frequency (the resonators are tuned coupling, the presence of a patch on top of it and patch to different frequencies), whereas in near-field chipless-RFID dimensions, suffices to obtain both the ID code of the tag and systems with sequential bit reading, the ID code is read bit by the clock signal necessary for synchronous reading. Such bit at different times, in a time-division multiplexing scheme. sensitive element can be implemented by means of a By analogy to the spectral signature barcodes (the usual microstrip transmission line loaded with a pair of designation of frequency-domain based tags), the tags in this complementary split ring resonators (CSRRs). It is well novel time-domain and near-field approach of chipless-RFID known that CSRR-loaded lines exhibit a transmission zero have been called time-domain signature barcodes [52],[56]. [58], which is intimately related to CSRR dimensions (and, For reading these time-domain signature barcodes, a obviously, to the characteristics of the considered substrate, mechanical guiding system able to displace at short distance, mainly thickness and dielectric constant). However, the and with proper alignment, the linear inclusions’ chain of the presence of a metal patch in close proximity to it also modifies tag over the reader, is needed. Moreover, a constant relative the position of the transmission zero frequency, provided the velocity between the tag and the reader is required in order to patch boundaries extend beyond the perimeter of the CSRR. avoid false readings. If such velocity is not known, but it is According to the previous works, the presence of a patch in constant, the tags can be equipped with header bits in order to the tag chain and its size can be inferred by considering two determine such velocity, and consequently the instants of time CSRR, one inside the other one, rather than one. The specific for bit reading [50]-[52]. Nevertheless, in order to provide topology of the reader CSRR-loaded line is depicted in Fig. major levels of robustness to the system, synchronous reading 2(a), whereas Fig. 2(b) shows the layout of the tag, with the is very convenient (Fig. 1). presence of both binary states (larger and smaller patches). The main advantage of a synchronous system (a preliminary Note that the CSRRs are single loop slot resonators with the version was reported in [57]) is the fact that the displacement slits etched at the same position. This is important, in order to velocity of the tag does not need to be constant (as it may avoid inter-slot coupling [59]. Therefore, with this pattern in occur in a real scenario). This paper, an extension of the work the ground plane, the two CSRRs behave as individual, presented in [57], reports a synchronous near-field chipless- roughly uncoupled, resonators. A slot ring surrounds the outer RFID system where the inclusion’s size in the direction of the CSRR in order to tailor its quality factor. chain axis are small, thereby reducing substantially the period of the inclusions. The result is a tag with high data density per d1 unit length, the main figure of merit of this type of chipless- l3 RFID tags. In [57], the reported prototype tags were l2 implemented on a rigid microwave substrate. In this paper, the CSRRclk l1 functionality of the system is demonstrated by reading the new designed tags by considering its implementation not only on w2 w3 w1 microwave substrates, but also on plastic and paper substrates. s CSRRID g1 ID Code w Z0 CSRRID CSRRclock 1010101 g2 V1 @ fc,ID Z0 E.D. 1111111 d2 V2 @ fc,clock Combiner Splitter E.D. Clock (a) (b) Signal Sensitive Fig. 2. Topology of the proposed reader (a) and tag (b). The considered Element substrate for the reader is the Rogers RO4003C with dielectric constant ID r = 3.38, thickness h = 0.81 mm and loss factor tan = 0.0022. Relevant Tag reader dimensions are: l1 = 10.3 mm; l2 = 12.3 mm, l3 = 13.5 mm, Clock w1 = 1.9 mm, w2 = 3.1 mm, w3 = 4.3 mm, g1 = 1.0 mm, g2 = 7.2 mm. CSRR slots width are c1 = 0.5 mm and c2 = 0.3 mm for the inner and the outer CSRR, Fig. 1. Sketch of the time-domain near-field chipless-RFID systems with respectively, and the outer slot ring width is c = 0.3 mm. The considered sequential and synchronous bit reading, showing the working principle. microwave substrate for one of the fabricated tags is the Rogers RO4003C Through tag motion over the reader, electromagnetic coupling between the with dielectric constant r = 3.38, thickness h = 0.2 mm and loss factor transmission line and the inclusions of the tag arise, and the output signals (for tan = 0.0022. Tag dimensions are: d1 = 9.3 mm; d2 = 14.5 mm, s = 3 mm and the ID code and clock) are amplitude modulated. w = 3 mm. > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 3 The functionality of the system proceeds as follows. patch on top of the CSRR, whereas for the ID curve, the peaks Regardless of its size, a patch on top of both resonators is able correlate with the presence of a larger patch above the position to modify the resonance frequency of the smaller CSRR. of the CSRRs (logic state ‘1’). However, the external CSRR is only altered by the larger patches. Consequently, the larger CSRR is used to III. EXPERIMENTAL VALIDATION OF TAG READING discriminate the binary state, whereas the inner CSRR is The photograph of the fabricated CSRR-loaded reader line is devoted to generate the synchronization clock signal. For that depicted in Fig. 5 (the considered substrate is indicated in the purpose, two harmonic signals tuned to the resonance caption of Fig. 2). Concerning the tag, we have considered as frequencies of the bare resonators (designated as fc,ID and first option the fabrication on a microwave substrate, fc,clock) must be injected to the input port of the CSRR-loaded particularly, the Rogers RO4003C with dielectric constant line. It is expected that the presence of any patch modifies the r = 3.38, thickness h = 0.2 mm and loss factor tan = 0.0022. resonance of the inner CSRR, generating a large excursion of Nevertheless, since the tag reading scheme is based on the the transmission coefficient at fc,clock, and thereby producing a variation of the transmission coefficient generated by the significant variation (increment) of the amplitude of the metallic patches of the tags, it is expected that the influence of harmonic signal tuned to that frequency. Thus, the clock signal the tag substrate on system functionality is not critical. is merely the envelope function of the amplitude modulated Consequently, we have also fabricated tags on flexible (AM) signal with carrier frequency fc,clock, with as many peaks substrate, including plastic substrate (Polyethylene as patches in the tag chain. Conversely, for the envelope naphthalate, or also known as PEN, with a thickness function of the output AM signal with carrier frequency tuned h = 125 μm, from Dupont) and paper (Powercoat XD with a to fc,ID, it is expected that peaks appear only at those clock thickness h = 200 μm, from Arjowinngs), in both cases by instants where the patches on top of the CSRRs are those with means of inkjet printing (details of the inkjet printing larger dimensions. Figure 3 depicts the frequency response of the CSRR- loaded reader line without patch loading, as well as the responses with larger and smaller patch on top of the CSRRs. The considered air gap (distance between the patch and CSRRs) is 0.5 mm. As it can be appreciated, the notch frequencies vary in agreement to the predictions of the precedent paragraph. Moreover, a significant excursion of the transmission coefficient at both frequencies, fc,clock and fc,ID, when the patch is able to shift the corresponding frequency, can be seen. This is important in order to obtain a high modulation index of both AM signals at the output port, a key aspect in terms of system robustness, to clearly discriminate between the two binary states. To gain further insight on the functionality of the proposed Fig. 4. Simulated transmission coefficient at fc,clock and fc,ID, as the tag with the system, Fig. 4 depicts the variation of the transmission indicated code is displaced over the CSRR of the reader. The simulation has coefficient at fc,clock and fc,ID, as the tag with the indicated code been carried out by means of the Keysight Momentum software. is displaced over the CSRR of the reader. The peaks at periodic positions of the clock curve indicate the presence of a Fig. 3. Simulated frequency response (transmission coefficient) of the bare reader and reader loaded with smaller and larger patch. The simulation has been carried out by means of the Keysight Momentum software. Fig. 5. Photograph of the sensitive part of the reader. (a) Top; (b) bottom. > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 4 process are given in [50]-[52]) and by using the Orgacon terms of space and cost, is depicted in Fig. 8. Nevertheless, Nanosilver Inkjet Ink from the AGFA. The fabricated system validation in this paper is done by independently prototypes are shown in Fig. 6. injecting the harmonic signals to the input port of the reader For validation purposes, we have generated the interrogation line and inferring the corresponding envelope function. The harmonic signals (tuned to fc,clock = 5.31 GHz and results for the different considered tags are shown next. fc,ID = 4.63 GHz) by means of the function generator Agilent The envelope functions of the different measured tags (see E4438C. In order to obtain the envelope function, an AM Fig. 6), including the clock signals and the ID code signals, detector based on the Avago HSMS-2860 Schottky diode and are depicted in Fig. 9. It can be seen in this figure that the the Agilent N2795A active probe (with resistance and peaks in the ID signals perfectly correlate with the logic state capacitance R = 1 MΩ and C = 1 pF, respectively) has been ‘1’ of the corresponding ID code (larger patches), as expected, implemented. Moreover, the ATM PNR ATc4-8 circulator, and as many peaks as number of bits (or patches) in the clock configured as an isolator, has been cascaded between the signal are present. Therefore, these results validate the output port of the reader line and the AM detector in order to proposed near-field chipless-RFID approach with synchronous prevent from unwanted reflections from the diode. The reading. It is important to highlight that the system is envelope signals corresponding to the clock signal and ID functional even by considering the implementation of inkjet- code have been visualized on an oscilloscope (model Agilent printed tags on paper, representing an excellent solution in MSO-X 3104A). Finally, tag displacement over the reader line terms of tag cost. has been carried out by means of a linear stepper motor Concerning tag dimensions, an important aspect to (model THORLABS LTS300/M). The photograph of the emphasize is the fact that for synchronous reading, an complete experimental setup is depicted in Fig. 7. additional chain of inclusions is not needed (this is obviously It should be mentioned that in a real scenario, either a the most trivial solution, but it requires not only an additional combiner/diplexer (see Fig. 1) or a switching scheme is chain, but also a more complex sensing element for the reader, needed in order to separately obtain the envelope functions of with at least two separately allocated resonant elements). With the clock signal and ID code. The schematic of the system by the proposed reader, based on a pair of rectangular-shaped considering the switching scheme, the most convenient in Microcontroller VCO 1010101 Vin Vout Vts V2 1111111 E.D. V1 fc,ID fc,clock Switch E.D. Vin Vout CSRRclock V2 CSRRID V1 ID ts 2ts t ts 2ts t Clock Fig. 8. Sketch of the near-field chipless-RFID system with synchronous reading capability based on a switching scheme. (a) (b) (c) Fig. 6. Photograph of the fabricated tags on microwave substrate (a), plastic substrate (b) and paper (c). Signal Generator Circulator Osciloscope Probe Stepper motor Shottky Diode Fig. 9. Clock and ID signals inferred by reading the fabricated tags in Fig. 7. Experimental setup used for tag reading. microwave substrate, in plastic substrate and in ordinary paper, respectively, with the experimental setup of Fig. 7. > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 5 CSRRs one inside the other, the period of the inclusions is of method for tag programming, or reprogramming, as discussed 6 mm. This represents an important improvement in terms of in [50]. However, the effects of a metallic patch on top of a data density per unit length (DPL), as compared to the first CSRR (resonance shift) do not vanish if small-unexpected cuts implementation of the approach, reported in [57]. For the tags or cracks (that may appear as consequence of mechanical of Fig. 6, such density is DPL = 1.67 bit/cm, whereas in [57] wearing) are present. Obviously, this is not true if large cracks the result is DPL = 0.67 bit/cm. Despite the fact the main or a significant damage in the metal layer of the patch (e.g., figure of merit of these tags based on linear chains of caused by extreme friction) occur. To demonstrate this inclusions is the DPL (it must be as much as small as possible robustness of the proposed tags against wearing/friction, we in order to avoid excessively lengthy tags), the density per have deliberately generated cuts in the fabricated tags. To do surface (DPS) is very reasonable. By considering the that, we have simply generated transverse cuts, as depicted in minimum rectangle that contains the chain patches, the density Fig. 10. The responses of these cracked tags are depicted in per surface is found to be DPS = 1.15 bit/cm2. These values of Fig. 11. According to the obtained responses, the periodic DPL and DPS are not as good as those, e.g., reported in [55], peaks in the clock signals are perfectly visible, and the ID where chains based on closely spaced linear strip were code signal for each cracked tag perfectly provides the ID considered. However, in [55], the tags do not exhibit code. These results confirm the capability of the system to synchronous reading capability. maintain the ID code despite the presence of cracks in the Another important aspect of the proposed tags is their patches. robustness against mechanical wearing. In the reported near- field chipless-RFID systems reported so far [45]-[55], where IV. DISCUSSION the metallic tag inclusions are either resonant elements or One important aspect of this type of chipless-RFID systems linear strips, an unexpected cut or crack in those tag inclusions is the tolerance against lateral misalignments and air gap may generate a bit error, as far as the inclusion losses its variations between the tag and the reader, eventually caused functionality. Indeed, deliberate resonator’s cutting is a by vibration, or by any other mechanical issue. An estimation of such tolerances can be inferred by repeating the simulations Zoom of Fig. 3, considering small lateral misalignments (Fig. 12) and variations in the air gap distance (Fig. 13), for the larger and smaller patches. These results demonstrate that, in the (a) case of larger patches, lateral misalignments higher than 1 mm deteriorate the dynamic range of the lower frequency fc,ID (4.63 GHz), but they have no effect in the higher frequency fc,clock (5.31 GHz). On the other hand, the variation (excursion) (b) between the curves for the bare reader and reader loaded with the smaller patches at higher frequencies is not very significant. However, at low frequencies, for misalignments higher than 1 mm, the smaller patch has influence on the external CSRR, and consequently, the dynamic range is (c) (d) considerably affected. This effect could be improved by Fig. 10. Photograph of the cracked tags on microwave substrate (a), plastic shortening the smaller patches. Similarly, Fig. 13 indicates substrate (b), paper (c), and zoom view of cracked patches of the microwave substrate (d). that the air gap can be expanded from 0.25 up to 0.75 mm (outside this range, it cannot be guaranteed that the patches are correctly detected). Concerning the data density of the reported tags, it is better than the one of the tags of [57]. However, it is not as good as the one of previous tags based on a similar principle (but not including synchronous reading capability). Particularly, the tag density in papers [54] and [55] is very good (e.g., a DPL = 17.7 bit/cm and a DPS = 26.04 bit/cm2 were obtained in [55], by virtue of the considered tag inclusions, i.e., narrow strips). The worst tag density of the proposed tags is compensated by synchronous reading, an advantageous aspect not reported in [54],[55], and necessary in certain applications where a constant and well known velocity between the tag and the reader cannot be guaranteed. Moreover, the tags of this paper are robust against potential effects caused by tag friction Fig. 11. Clock and ID signals inferred by reading a microwave substrate, (eventually generating cracks in the tags), as demonstrated in plastic and paper-based tag with cracks in all patches. Figs. 10 and 11. > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 6 for synchronous bit reading. Additionally, the larger CSRR is only sensitive to the larger patches, this being the resonator devoted to determine the ID code of the tag. The system has been validated by means of a dedicated setup, which uses a step motor to displace the tag over the reader. Moreover, tag reading is based on an AM detector, necessary to provide the envelope function of the AM modulated clock and ID signals. System functionality has been demonstrated by considering tags implemented on microwave and flexible substrates, including paper, and it has been found that the tags are tolerant to the presence of cuts or cracks in the patches, which may be caused by wearing or friction in a real scenario. The key figures of merit in these near-field chipless-RFID tags based on linear chains of inclusions, the information density per unit length and the information density per surface, have been found to be very reasonable, i.e., DPL = 1.67 bit/cm and DPS Fig. 12. Simulated frequency response (transmission coefficient) of the reader loaded with smaller and larger patch, by considering lateral misalignments. = 1.15 bit/cm2. This later value is high taking into account the The simulations have been carried out by means of the Keysight Momentum synchronous reading capability of the system, and has been software. achieved by avoiding an extra chain for the generation of the clock signal. As compared to optical barcodes, the proposed tags are not as cheap and easy to read. However, the optical barcodes can be easily reproduced (by simply photocopying them), and the copy has exactly the same properties as the original. By contrast, with the proposed system, a copied tag cannot be read with the designed and fabricated ad hoc reader, unless a printing process (using conductive inks) makes the copy. This is the main advantage over optical barcodes, i.e., the proposed system provides certain level of security against plagiarism of coded documents, for instance, or other premium products. REFERENCES [1] S. Preradovic and N. C. Karmakar, "Chipless RFID: bar code of the future," IEEE Microwave Magazine, vol. 11, pp. 87-97, 2010. [2] S. Preradovic and N. C. Karmakar, Multiresonator-Based Chipless RFID: Barcode of the Future, Springer, 2011. Fig. 13. Simulated frequency response (transmission coefficient) of the reader [3] I. Jalaly and I. D. Robertson, “RF barcodes using multiple frequency loaded with smaller and larger patch, by considering air gap variations. 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Martín, “High- Proc. of IEEE Ultrasonics Symposium, October 2002, vol. 1, pp. 65–69. density microwave encoders for motion control and near-field chipless- [35] A. Chamarti and K. Varahramyan, "Transmission delay line based ID RFID”, IEEE Sensors J., vol. 19, pp. 3673-3682, May 2019. generation circuit for RFID applications," IEEE Microw. Wireless [55] C. Herrojo, F. Paredes, and F. Martín, “Double-stub loaded microstrip Compon. Lett., vol. 16, pp. 588-590, 2006. line reader for very high data density microwave encoders”, IEEE [36] M. Schüßler, C. Damm, and R. Jakoby, “Periodically LC loaded lines Trans. Microw. Theory Techn., vol.67, pp. 3527-3536, Sep. 2019. for RFID backscatter applications,” in Proc. of Metamaterials 2007, Rome, Italy, October 2007, pp. 103-106. > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 8 [56] F. Martín, C. Herrojo, F. Paredes, and J. Mata-Contreras, Time Domain Signature Barcodes for Chipless-RFID and Sensing Applications, FERRAN MARTIN (M’04-SM’08-F’12) was born in Springer, 2020. Barakaldo (Vizcaya), Spain in 1965. He received the [57] F. Paredes, C. Herrojo, F. Martin, “An approach for Synchronous B.S. Degree in Physics from the Universitat Autònoma Reading of Near-Field Chipless-RFID Tags”, 10th IEEE International de Barcelona (UAB) in 1988 and the PhD degree in Conference on RFID Technology and Applications (IEEE RFID-TA 1992. From 1994 up to 2006 he was Associate 2019), Pisa, Italy, 25-27 Sep. 2019. Professor in Electronics at the Departament [58] F. Falcone, T. Lopetegi, J.D. Baena, R. Marqués, F. Martín and M. d’Enginyeria Electrònica (Universitat Autònoma de Sorolla, "Effective negative- stop-band microstrip lines based on Barcelona), and since 2007 he is Full Professor of complementary split ring resonators", IEEE Microw. Wireless Compon. Electronics. In recent years, he has been involved in Lett., vol. 14, pp. 280-282, Jun. 2004. different research activities including modelling and simulation of electron [59] F. Martín, Artificial Transmission Lines for RF and Microwave devices for high frequency applications, millimeter wave and THz generation Applications, Wiley, Hoboken, NJ, 2015. systems, and the application of electromagnetic bandgaps to microwave and millimeter wave circuits. He is now very active in the field of metamaterials and their application to the miniaturization and optimization of microwave circuits and antennas. Other topics of interest include microwave sensors and Ferran Paredes was born in Badalona (Barcelona), RFID systems, with special emphasis on the development of high data Spain in 1983. He received the Telecommunications capacity chipless-RFID tags. He is the head of the Microwave Engineering, Engineering Diploma (specializing in Electronics) and Metamaterials and Antennas Group (GEMMA Group) at UAB, and director the Telecommunications Engineering degree from the of CIMITEC, a research Center on Metamaterials supported by TECNIO Universitat Autònoma de Barcelona in 2004 and 2006, (Generalitat de Catalunya). He has organized several international events respectively and the PhD degree in Electronics related to metamaterials and related topics, including Workshops at the IEEE Engineering from the same university in 2012. He was International Microwave Symposium (years 2005 and 2007) and European Assistant Professor from 2006 to 2008 at the Microwave Conference (2009, 2015 and 2017), and the Fifth International Universitat Autònoma de Barcelona, where he is Congress on Advanced Electromagnetic Materials in Microwaves and Optics currently working as a Research Assistant. His (Metamaterials 2011), where he acted as Chair of the Local Organizing research interests include metamaterial concepts, Committee. He has acted as Guest Editor for six Special Issues on passive microwaves devices, antennas and RFID. metamaterials and sensors in five International Journals. He has authored and co-authored over 600 technical conference, letter, journal papers and book chapters, he is co-author of the book on Metamaterials entitled Metamaterials Cristian Herrojo was born in Barcelona, Spain, in with Negative Parameters: Theory, Design and Microwave Applications (John 1983. He received the Telecommunications Technical Wiley & Sons Inc.), author of the book Artificial Transmission Lines for RF Engineering degree in electronic systems and and Microwave Applications (John Wiley & Sons Inc.), co-editor of the Telecommunications Engineering degree from the book Balanced Microwave Filters (Wiley/IEEE Press) and co-author of the Universitat Autònoma de Barcelona in 2010 and 2012, book Time-Domain Signature Barcodes for Chipless-RFID and Sensing respectively and the PhD degree in Electronics Applications (Springer). Ferran Martín has generated 19 PhDs, has filed Engineering from the same university in 2018. His several patents on metamaterials and has headed several Development research interests include RF/microwave devices, Contracts. Chipless-RFID and RFID technology, and Prof. Martín is a member of the IEEE Microwave Theory and Techniques Metamaterials. Society (IEEE MTT-S). He is reviewer of the IEEE Transactions on Microwave Theory and Techniques and IEEE Microwave and Wireless Components Letters, among many other journals, and he serves as member of Roger Escudé was born in Barcelona, Spain. He the Editorial Board of IET Microwaves, Antennas and Propagation, received the M.S. degree in Industrial Electronics and International Journal of RF and Microwave Computer-Aided Engineering, and Automation Engineering from the Polytechnic Sensors. He is also a member of the Technical Committees of the European University of Catalonia, Spain, in 2004. He is working Microwave Conference (EuMC) and International Congress on Advanced at IMB-CNM from 2004, and from 2016 he is in Electromagnetic Materials in Microwaves and Optics (Metamaterials). Among charge of printed electronics laboratory. He has co- his distinctions, Ferran Martín has received the 2006 Duran Farell Prize for authored seven articles and holds one patent. His main Technological Research, he holds the Parc de Recerca UAB – interest is the research on the field of electronic Santander Technology Transfer Chair, and he has been the recipient of three hybridization mixing printed electronic technologies ICREA ACADEMIA Awards (calls 2008, 2013 and 2018). He is Fellow of with others. the IEEE and Fellow of the IET. Eloi Ramon graduated in Telecom Engineering from the Polytechnic University of Catalonia (UPC), holds a Master in Micro- and Nanoelectronics Engineering and a PhD on inkjet printed devices and circuits from the Autonomous University of Barcelona (UAB). Since 1999, he is an Associate Professor at the Electronic Dept. (UAB) where he is teaching Telecom and CS BsC and MA. In 2014 he joined IMB-CNM as Printed Microelectronics researcher & group leader of Printed Electronics R&D line. His work is focused in the area of printed & organic devices for electronic systems, radiofrequency and biomedical applications, with large focus on the application of inkjet printing technologies for functional devices manufacturing. He has participated in more than 55 industrial and research projects, being co-author of more than 40 papers and 50 conference presentations and serving as a reviewer and scientific expert for different journals and public funding agencies.