This is the accepted version of the journal article: Havlíček, Jaroslav; Herrojo, Cristian; Paredes, Ferran; [et al.]. «Enhancing the Per-Unit-Length Data Density in Near-Field Chipless-RFID Systems With Sequential Bit Reading». IEEE antennas and wireless propagation letters, Vol. 18, Issue 1 (January 2019), p. 89-92. DOI 10.1109/LAWP.2018.2881356 This version is available at https://ddd.uab.cat/record/258464 under the terms of the license > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 1 Enhancing the Per-Unit-Length Data Density in Near-Field Chipless-RFID Systems with Sequential Bit Reading Jaroslav Havlicek, Cristian Herrojo, Member, IEEE, Ferran Paredes, Member, IEEE, Javier Mata- Contreras, and Ferran Martín, Fellow, IEEE compared to time-domain reflectometry (TDR) based tags Abstract— In this paper, a novel near-field chipless-RFID [4],[5], spectral signature barcodes [6]-[9], or tags exploiting system with sequential bit reading is reported. The main novelty hybrid coding techniques [10]-[14], the proposed time-domain concerns the tags, implemented by means of straight half- based tags are implemented by printing all-identical resonators wavelength resonators separated a small distance and forming a at equidistant positions forming a linear chain [2]. Coding linear chain in the direction orthogonal to their axis. By this (programming) is achieved by detuning (cutting) certain means, the number of resonant elements (or bits) per unit length of the tag is significantly increased, as compared to previous resonant elements and it has been demonstrated that tag implementations, with the result of very competitive tags in terms erasing is also possible by simply short-circuiting those of data density. The active part of the reader is a microstrip line previously detuned resonant elements [2]. loaded with a shunt stub. Through a proper design, it is possible The interrogation signal is a harmonic signal tuned to the to achieve a significant excursion of the transmission coefficient frequency of the resonant elements of the tag, f0 (or close to when the stub is either loaded or unloaded with a functional it), and tag reading proceeds by displacing the tag over a resonant element of the tag. Consequently, by tag motion over the dedicated transmission line fed by the interrogation signal. reader (stub), it is possible to infer the ID code by simply Through tag motion, the amplitude of the feeding signal is injecting a harmonic signal to the input port of the stub-loaded modulated by the coupling between the line and the functional line (the ID code is contained in the amplitude modulated, AM, tag resonators, and the ID code is contained in the envelope signal at the output port). This system has been validated by considering 100-bit tags exhibiting a surface data density of 4.9 function present at the output port of the line (which can be bits/cm2. However, the relevant parameter is the data density per inferred by means of an envelope detector). The system works unit length, as high as 16.7 bit/cm, i.e., much larger than previous similar to an amplitude modulator (AM), where the carrier values reported in the literature in similar chipless-RFID signal is the interrogation signal and modulation is achieved systems. by tag motion (Fig. 1). Similar to spectral signature barcodes, Index Terms — Chipless-RFID, microstrip, half-wavelength the envelope function exhibits dips, or peaks, providing the ID resonator, secure paper. code, but in time domain. For this reason, we can designate these chipless-RFID tags as time-domain signature barcodes. I. INTRODUCTION Since the spectral bandwidth is virtually null, it follows that A novel chipless-RFID concept where tags are read by the number of bits is only limited by tag size, proportional to proximity (through near-field) and sequentially was recently the number of bits. reported [1]-[3]. The main advantage of this time-domain approach, of special interest for secure paper applications, is the data storage capability, only limited by tag size. As This work was supported in part by MINECO-Spain under Project TEC2016-75650-R, in part by Generalitat de Catalunya (project 2017SGR- 1159), in part by the Institució Catalana de Recerca i Estudis Avançats (who awarded F. Martín), and in part by FEDER funds. The work of C. Herrojo was supported by MINECO through the FPI under Grant BES-2014-068164. The work of J. Havlicek was supported by Czech Science Foundation and Czech Technical University in Prague through projects 17-02760S and Fig. 1. Illustration of the working principle of the proposed chipless-RFID SGS16/226/OHK3/3T/13, respectively. system. In a reading operation, the tag is displaced at short distance over the J. Havlicek is with the Department of Electromagnetic Field, Czech reader through a mechanical guiding system providing tag/reader alignment. Technical University in Prague, Technicka 2, 16627 Prague, Czech Republic. J. Havlicek C. Herrojo, F. Paredes, and F. Martín are with Although a figure of merit of chipless-RFID tags is usually GEMMA/CIMITEC (Departament d’Enginyeria Electrònica), Universitat Autònoma de Barcelona. 08193 BELLATERRA (Barcelona), Spain. E-mail: the data density per area unit, in these chipless-RFID tags Ferran.Martin@uab.es. based on linear chains of resonant elements, the relevant J. Mata-Contreras is with the Departamento de Ingeniería de parameter is the number of bits per unit length. The reason is Comunicaciones, Escuela Técnica Superior de Ingeniería de that the shape factor of the tag is very extreme, and the Telecomunicación, Universidad de Málaga, 29071 Málaga, Spain. number of bits is typically limited by tag length, rather than > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 2 width. In [1]-[3], by using split ring resonators (SRRs), a per- After an exhaustive analysis, we have concluded that the unit-length data density of 3 bits/cm was achieved. In this preferred option to implement the chipless-RFID system is to paper, a novel tag (and reader) with much higher density of tune the half-wavelength resonators in the vicinity of the information per unit length (i.e., 16.7 bit/cm) is reported and second resonance of the stub. The reason is that the effects of validated. The tags are based on straight half-wavelength multiple couplings between the stub and the tag resonators are resonators, and the reader is a 50  transmission line loaded less severe by tuning the system at this frequency, as with an open shunt stub. compared to working in the vicinity of the fundamental frequency (where glitches that appear in the response, not II. PROPOSED CHIPLESS-RFID TAGS AND READER shown, may prevent from a correct and robust tag reading, The active part of the reader is a stub-loaded microstrip line, unless the tag period is substantially increased). implemented on the Rogers RO3010 substrate with thickness h For tag and reader design, we have tuned the dimensions in = 0.635 mm and dielectric constant r = 10.2 [Fig. 2(a)]. This order to achieve the response shown in Fig. 3 corresponding to structure exhibits a stop band response, with transmission resonator tuning at the second resonance. Maximum zeros at the frequency where the electrical length of the open transmission at the second resonance, f0, of the unloaded line stub is 90º and at its odd harmonics (Fig. 3). By etching a can be appreciated when a half-wavelength resonator is straight half-wavelength resonator in a different substrate, and perfectly aligned with the stub. By this means, a significant locating it on top of the stub, as Fig. 2(b) illustrates, the excursion of the transmission coefficient is expected by tuning response of the structure is modified as depicted in Fig. 3, the interrogation signal to that frequency (f0 = 3.47 GHz) or where two transmission zeros in the vicinity of the close to it (a convenient situation in order to obtain a high fundamental (first) resonance of the stub-loaded line appear modulation index and hence clearly discern between the two (frequency splitting). Note that the half wavelength resonator logic states). can be tuned to operate at different frequencies, not necessarily at the fundamental frequency of the stub-loaded line. The response for the half-wavelength resonator tuned to the second resonance is also included in Fig. 3. It can be seen that it is possible to achieve frequency splitting in the vicinity of any of the stub resonances. (a) (b) Fig. 2. Layout of the active part of the reader (a) and 3D view of the loaded reader with one of the half-wavelength resonators of the tag aligned with the reader stub (b). The length and width of the stub are 26 mm and 0.2 mm, respectively. The width of the transmission line is 0.6 mm (50 Ω line). The separation between adjacent resonators in the tag chain is 0.2 mm and resonator dimensions are 34 mm  0.4 mm (so that the tag period is 0.6 mm). The tag chain has been implemented on the Rogers RO4003C substrate with thickness h = 0.2 mm and dielectric constant r = 3.55. Fig. 4. Variation of the transmission coefficient at the interrogation signal frequency that results from a relative displacement between the tag and the reader for the indicated codes. Several air gaps are considered. Fig. 4 depicts the variation of the transmission coefficient at the frequency of the interrogation signal, set to f0, as a 7-bit tag is displaced above the reader. Two different bit f0 combinations, indicated in the figure, have been considered, in order to validate, at simulation level, the proposed system. An important aspect is the robustness against air gap variations. Fig. 3. Magnitude of the transmission coefficient (S21) of the unloaded reader and reader loaded with a single half-wavelength resonator perfectly aligned. Therefore, we have repeated the simulations of the considered For the loaded reader, two different cases have been considered in regard to 7-bit tags for air gaps of 0.2 mm and 0.4 mm (the nominal air the half-wavelength resonator, i.e., either tuned at the first (fundamental) or gap being 0.3 mm). The corresponding responses are also second resonance of the stub. The considered air gap is 0.3 mm. These results have been inferred by electromagnetic simulation, using Keysight Momentum. included in Fig. 4, and it can be appreciated that differences > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 3 with the air gap variation arise. These differences are caused by frequency shift of the second resonance of the stub as a result of the air gap variation, and are mainly manifested by a different level of the transmission coefficient when a detuned resonator is on top of the reader stub. Nevertheless, for all bit combinations, the responses exhibit the expected behavior. Namely, for a logic state ‘1’, corresponding to the presence of a functional resonator on top of the stub, maximum transmission is achieved; conversely, when the half- wavelength resonator is detuned (i.e., not functional) by cutting it, corresponding to the logic state ‘0’, the transmission coefficient is severely reduced. Note, therefore, that, as compared to papers [1]-[3], we use negative logic in this paper. III. EXPERIMENTAL VALIDATION To experimentally validate the proposed system, we have fabricated a 100-bit tag by means of standard photo-etching, where all-functional resonators have been etched. Conversely, the reader has been fabricated by means of a LPKF HF-100 drilling machine (the photographs are depicted in Fig. 5). We have then read this tag by means of the set-up present in our laboratory, described in [1], consisting of a function generator, an envelope detector and an oscilloscope (to display the time domain response of the tag, i.e., the AM signal containing the ID code), plus the mechanical guiding system. The measured envelope is depicted in Fig. 6, where it can be seen that dips are not present, as long as all bits are set to ‘1’. Then we have detuned some resonant elements by cutting them (hence setting some bits to ‘0’), and the measured normalized envelope reveals the presence of such ‘0’ states (as dips in the time-domain responses). Thus, tag programming can be simply done by cutting the required resonant elements, similar to [1], [2]. Note that three codes have been programmed, as it is visible in Fig. 6, and in all the cases such codes have been correctly read. (a) Fig. 6. Measured normalized envelope for the 100-bit tags with the indicated codes. For each code, the repeating period is shown. The reason of the variation of the maximum voltage in the figures is related to the fact that absolute uniformity in the gap space between the tag and the reader cannot be guaranteed. IV. DISCUSSION The surface information density of the tag is 4.9 bit/cm2 (not (b) as good as the one reported in [1]-[3], i.e., 8.9 bit/cm2), but, most important, we have accommodated 100 bits in a length of 60 mm, corresponding to a per-unit-length density of 16.7 bit/cm, i.e., substantially larger than the one of previous similar works based on tags implemented with split ring resonators [1]-[3]. Such high per-unit-length information density has been achieved by driving the separation of the resonant elements of the tag to the limit of the available technology (0.2 mm). Further reducing this spacing may have Fig. 5. Photograph of the fabricated 100-bit tag with all bits set to the logic negative effect on tag reading due to possible couplings state ‘1’ (a), and reader (b). The space occupied by the tag is 60 mm  34 mm. Line bending in the feeding lines of the reader has been applied in order to between the line stub (reader) and neighbor resonators in the reduce the coupling between the tag resonators and the feeding lines. tag. However, this aspect has not been considered as long as > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 4 for the limiting inter-resonator spacing (0.2 mm), the one [5] M. Schüßler, C. Damm, M. Maasch, and R. Jakoby, “Performance evaluation of left-handed delay lines for RFID backscatter applications,” considered in this work, this effect has been found to be IEEE MTT-S Int. Microw. Symp. 2008, Atlanta, USA, pp. 177-180. negligible. It should be also pointed out that it is possible to [6] S. Preradovic, I. Balbin, N. C. Karmakar, and G. F. Swiegers, reduce the width of the tag (in order to increase the data "Multiresonator-based chipless RFID system for low-cost item tracking," density per surface) by increasing the frequency of the IEEE Trans. Microw. Theory Techn., vol. 57, pp. 1411-1419, May 2009. interrogation signal (by this means, both the reader stub and [7] A. Vena, E. Perret, and S. Tedjini, “High-capacity chipless RFID tag the resonant elements of the tag can be shortened). insensitive to the polarization”, IEEE Trans. Ant. Propag., vol. 60, pp. 4509-4515, Oct. 2012. Finally, concerning the substrate of the tag and tag [8] R. Rezaiesarlak and M. Manteghi, "Complex-natural-resonance-based fabrication, chemical etching of a laminated microwave design of chipless RFID tag for high-density data," IEEE Trans. Ant. substrate (the one specified in the caption of Fig. 2) has been Propag., vol. 62, pp. 898-904, Feb. 2014. considered in this paper (mainly focused on demonstrating the [9] M. Polivka, J. Havlicek, M. Svanda, J. Machac, “Improvement in potential of this new approach to enhance the density of bits robustness and recognizability of RCS response of U-shaped strip-based per unit length). Nevertheless, it was demonstrated in [2],[3] chipless RFID tags”, IEEE Ant. Wirel. Propag. Lett., vol 15, pp. 2000- 2003, 2016. that by replacing such tag substrates and fabrication procedure [10] A. Vena, E. Perret, S. Tedjini, "Chipless RFID tag using hybrid coding with a flexible substrate, such as paper, and inkjet printing, the technique," IEEE Trans. Microw. Theory Techn., vol. 59, pp. 3356-3364, functionality of the tags is preserved. Rather than the quality Dec. 2011. factor of the resonant elements, the key aspect for system [11] M. A. Islam and N. C. Karmakar, "A novel compact printable dual- functionality is the effect of coupling between the sensitive polarized chipless RFID system," IEEE Trans. Microw. Theory Techn., part of the reader (the stub in our case) and the resonant vol. 60, pp. 2142-2151, Jul. 2012. [12] S. Genovesi, F. Costa, A. Monorchio, G. Manara, “Chipless RFID tag element of the tag. Such coupling does not depend critically exploiting multifrequency delta-phase quantization encoding”, IEEE Ant. on the substrate material of the tag, and for that reason the Wireless Propag. Lett., vol. 15, pp. 738-741, 2015. functionality is preserved in [2],[3]. It should be pointed out, [13] O. Rance, R. Siragusa, P. Lemaître-Auger, E. Perret, "Toward RCS however, that the quality factor of the resonant elements may magnitude level coding for chipless RFID," IEEE Trans. Microw. have influence on the effects of neighbor resonators, if it is too Theory Techn., vol. 64, pp. 2315-2325, Jul. 2016. low. Nevertheless, under these circumstances, the solution is [14] C. Herrojo, F. Paredes, J. Mata-Contreras, S. Zuffanelli and F. Martín, “Multi-state multi-resonator spectral signature barcodes implemented by to simply increase the distance between adjacent resonators. means of S-shaped Split Ring Resonators (S-SRR)”, IEEE Trans. According to the period of the reported tags (0.6 mm), there is Microw. Theory Techn., vol. 65, pp. 2341-2352, Jul. 2017. margin to proceed in that way (if necessary, depending on the considered substrate), yet keeping a significant density of bits per unit length. V. CONCLUSIONS In conclusion, the functionality of a time-domain near-field chipless-RFID system with tags based on a chain of linear (straight) half-wavelength resonators has been demonstrated. We have designed the tags and the reader, a stub-loaded microstrip line, and the system has been optimized for operation at f0 = 3.47 GHz (the frequency of the interrogation signal). Programmable 100-bit chipless-RFID tags have been reported. Moreover the achieved information density per area and length (the most important one in this type of tags based on a resonator chain) is 4.9 bit/cm2 and 16.7 bit/cm, respectively. By using the proposed system, the shape factor of the tags has been severely improved as compared to those of previous tags based on sequential bit reading. REFERENCES [1] C. Herrojo, J. Mata-Contreras, A. Núñez, F. Paredes, E. Ramón, and F. Martín, “Near-field chipless-RFID system with high data capacity for security and authentication applications”, IEEE Trans. Microw. Theory Techn., vol. 65, no. 12, pp. 5298-5308, Dec. 2017. [2] C. Herrojo, J. Mata-Contreras, F. Paredes, A. Núñez, E. Ramon, and F. Martín, “Near-field chipless-RFID system with erasable/programable 40- bit tags inkjet printed on paper substrates”, IEEE Microw. Wireless Compon. Lett., vol. 28, pp. 272-274, Mar. 2018. [3] C. Herrojo, M. Moras, F. Paredes, A. Núñez, E. Ramón, J. Mata- Contreras, F. Martín, “Very low-cost 80-bit chipless-RFID tags inkjet printed on ordinary paper”, Technologies, vol. 6, p. 52, 2018. [4] F.J. Herraiz-Martínez, F. Paredes, G. Zamora, F. Martín, J. Bonache, “Printed magnetoinductive-wave (MIW) delay lines for chipless RFID applications”, IEEE Trans. Ant. Propag., vol. 60, pp. 5075-5082, Nov. 2012.