High-Q Slot Resonator Used in Chipless Tag Design

Round 1

Reviewer 1 Report

The paper presents a good idea - the chipless tag with a retransmission based on the U shaped slot resonators. Unfortunately the presented results are are only of very low quality. Comparing to the results presented by authors of ref. [15], they are really not sufficient. Presented tag has only 6 bits in the wide frequency band of about 2.3 GHz. Fig. 9 presents the detuning of the particular resonances of about 200 MHz probably due to the mutual coupling. The minumum resonance distance is about 300 MHz. Authors of ref [15] presentend in their particular works tags with 20 bits at the frequency band tipically of about 1.8 GHz with the minimum distance between resonances 50-100 MHz. Those tags assure the perfect readability for particular combinations of storred data due to minimum mutual coupling between resonators.

So the results presented in this paper are not sufficient fro practical applications.

Author Response

The paper presents a good idea - the chipless tag with a retransmission based on the U shaped slot resonators. Unfortunately the presented results are only of very low quality. Comparing to the results presented by authors of ref. [15], they are really not sufficient. Presented tag has only 6 bits in the wide frequency band of about 2.3 GHz. Fig. 9 presents the detuning of the particular resonances of about 200 MHz probably due to the mutual coupling. The minumum resonance distance is about 300 MHz. Authors of ref [15] presented in their particular works tags with 20 bits at the frequency band typically of about 1.8 GHz with the minimum distance between resonances 50-100 MHz. Those tags assure the perfect readability for particular combinations of stored data due to minimum mutual coupling between resonators.

So the results presented in this paper are not sufficient for practical applications.

Author response: With respect to the reviewer’s comment- the response is that the design of this manuscript is different from the ref. [15]. The chipless tag of the ref. [15] is based on the encoding of RCS spectral features, and our design is based on the encoding of S21 spectral features with a retransmission structure. Although the structure of ref. [15] is more compact, the design based on RCS is more susceptible to the influence of the external environment in practical applications. The design of 6-bit encoding is much smaller in spectral capacity than ref. [15]. The spectrum utilization of our design does not seem to be high, but 6-bit coded tag is just to illustrate the feasibility of the design. According to the principle of frequency position coding, the coding capacity of this structure chipless tag can reach 31 bits. Furthermore, our design has high encoding density than Ref. [15]. The Ref. [15] is only 0.7 bit/cm². However, our design is 1.03 bits/cm². The Ref [15] uses RCS features for encoding, which will be strongly affected by the surrounding environment when it is applied the actual environment. The purpose of studying chipless is to replace the currently barcode, but that Ref [15] of manufacturing process is complex. We also found that the chipless tag of the Ref. [15] only rotated horizontal direction, but the normal direction of the chipless tag is limited.

15. Svanda M.; Havlicek J.; Machac J.; Polivka M. Polarisation independent chipless RFID tag based on circular arrangement of dual-spiral capacitively-loaded dipoles with robust RCS response. IET Microwaves Antennas and Propagation, 2018; 12(14): 2167-2171.

Author Response File: Author Response.pdf

Reviewer 2 Report

Point 1: The third section must add more information of the numerical simulations using ANSYS HFSS. Which were the main conditions and constraints used in the numerical models?

Point 2: Figures 6 and 9 should be modified not to have such thick lines, which cannot be read easily.

Point 3: Experimental Results should be section 4 instead of 3.

Point 4: At the beginning of the section dedicated to experimental results, a paragraph explaining the experimental setup should be presented instead of pictures. Also, a discussion regarding the limitations or challenges of the proposed research would be helpful.

Point 5: In Table 2, a space between the word and the reference is needed.

 Point 6: Give more information on how the Q-value, capacity and spectral capacity of the U-shaped slot proposed has been measured/deduced.

Point 7: This manuscript should include more recent references between 2016 and 2020, regarding the research done in this direction.

Author Response

Point 1: The third section must add more information of the numerical simulations using ANSYS HFSS. Which were the main conditions and constraints used in the numerical models?

Author response: With respect to the reviewer’s comment- the response is that we would like to thank you for your comments that helped us to improve the quality of our paper. Related content has been added to the text.

(In page 4, line 153)

The U-shaped slot has been widely used in patch antenna design [38,39]. The structural parameters of the U-shaped slot are shown in Figure 3, where L_t is the width of the microstrip line, W_t is the length of the microstrip line, and L_u is the length of the U-shaped slot. w_m is the width of the U-shaped slot, w_u is the length of the bottom side of the U-shaped slot, H_t is the distance from the U-shaped slot in the microstrip line, and h is the thickness of the substrate.

 (In page 6, line 198)

According to the literature [40], for very thin conductors (that is, thickness→0), the expression of the characteristic impedance Z₀ is as follows:

Where ɛ_re is the effective dielectric constant, Ω is the wave impedance in free space, u = L₂ / h, and

The accuracy of this expression applies to ε_r ≤ 128 and 0.01≤ u ≤10.

From formula (11), the width L₂ of the Z₀ = 50Ω trapezoidal microstrip can be obtained. Due to the limitation of processing accuracy, the size adjustment step length of numerical simulation cannot be lower than the process requirements of chipless tag manufacturing. According to the relevant theoretical formula of the U-shaped slot [37, 38], after the HFSS software simulation and design, the structural parameters of the 6 bits U-shaped slot resonator are finally determined in Table 1.

(In page 15, line 456)

40. Hammerstad E.O.; Jensen O. Accurate models for microstrip computer-aided design. International Microwave Symposium Digest IEEE 1980. p. 407−

Point 2: Figures 6 and 9 should be modified not to have such thick lines, which cannot be read easily.

Author response: With respect to the reviewer’s comment- This is a very important suggestion. Thank you very much. The Figures 6 and 9 are updated in the revised manuscript.

(In page 7, line 223)

Figure 7. Simulate the resonance curves of the 6 bits U-shaped slot resonator chipless tags with a short-circuit logic state of "0". (a) ID111111 and ID101010; (b) ID111111 and ID010101;(c) ID111111 and ID111000.

Figure 8. The simulation remove the resonance curves of the 6 bits U-shaped slot resonator chipless tags with a logic state of "0". (a) ID111111 and ID101010; (b) ID111111 and ID010101;(c) ID111111 and ID111000.

(In page 10, line 278)

Figure 12. Simulation and measurement results of U-shaped slot resonator chipless tags. (a) ID111111; (b) ID101010; (c) ID010101; (d) ID111000.

Point 3: Experimental Results should be section 4 instead of 3.

Author response: With respect to the reviewer’s comment- the response is that the Experimental Results have been revised to Section 4. Really sorry for the mistake.

Point 4: At the beginning of the section dedicated to experimental results, a paragraph explaining the experimental setup should be presented instead of pictures. Also, a discussion regarding the limitations or challenges of the proposed research would be helpful.

Author response: With respect to the reviewer’s comment- This is a very important suggestion. A paragraph has been already added in section 4.

(In page 8, line 229)

Fig. 9 is the test system of a retransmission chipless tag based on multi-state resonators. The experimental setup architecture is composed of a UWB reader, Tx/Rx antennas and a chipless tag. Ceyear vector network analyzer 3672D is used as an alternative to the UWB reader. The two ports of the network analyzer are respectively connected to the L-shaped slot-loaded stepped-impedance UWB antennas [41], which are orthogonal to each other to improve the transceiver isolation of the reader. The tag is connected to two orthogonal L-shaped slot-loaded stepped-impedance UWB antennas through two microwave adapters. In order to prevent the received signal and the transmitted signal of the tag from interfering with each other, the two-sided UWB antennas of the tag are also orthogonal to each other. Both the reader antennas and the tag antennas are fixed on the foam, 10 cm apart. The gain of the L-shaped slot-loaded stepped-impedance UWB antenna is 4 dBi to 8.54 dBi in the range of 2.39 GHz to 13.78 GHz.

Figure 9: Test system.

(In page 15, line 458)

41. Ma Z. H.; Jiang Y. F. L-shaped Slot-loaded stepped-impedance microstrip structure UWB antenna. Micromachines 2020; 11(9):828.

Point 5: In Table 2, a space between the word and the reference is needed.

Author response: With respect to the reviewer’s comment- the response is that the Table 2 is updated. Sorry for the mistakes.

 Point 6: Give more information on how the Q-value, capacity and spectral capacity of the U-shaped slot proposed has been measured/deduced.

Author response: With respect to the reviewer’s comment- This is a very important suggestion. The related measured/deduced content is added to the revised manuscript.

(In page 4, line 149)

The Q-value can be calculated by equation (4).

where B is defined as the -10 dB impedance bandwidth of the U-shaped slot resonator. The f is the center frequency of U-shaped slot resonator.

(In page 5, line 182)

 When the side length of the single U-shaped slot resonator is 7 mm, the fundamental frequency of the resonator is 8.28 GHz, and the notch depth of the spectrum characteristic achieves −22 dB. Moreover, the impedance bandwidth B is 0.04 GHz. Q-value is about 207.

(In page 13, line 340)

Point 7: This manuscript should include more recent references between 2016 and 2020, regarding the research done in this direction.

Author response: With respect to the reviewer’s comment- the response is that the References have been updated to those of recent years.

(In page 14, line 388)

7.Herrojo C.; Mata-Contreras J.; Paredes F.; Núñez A.; Ramon E.; and Martín F. Near-field chipless-RFID system with erasable/programmable 40-bit tags inkjet printed on paper substrates. IEEE Microwave and Wireless Components Letters, 2018; 28(3):272-274.

8. Havlicek J.; Herrojo C.; Paredes F.; 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, 2019, 18(1): 89-92.

9.Paredes F.; Herrojo C.; Escudé R.; Ramon E.; Martín F. High Data Density Near-Field Chipless-RFID Tags with Synchronous Reading, IEEE Journal of Radio Frequency Identification, 2020,4(4): 517-524.

18.Babaeian F.; Karmakar N. A Cross-Polar Orientation Insensitive Chipless RFID Tag, 2019 IEEE International Conference on RFID Technology and Applications, IEEE, Pisa, Italy, 2019: 116-119.

20.Tariq N.; Riaz M. A.; Shahid H.; et al. Orientation Independent Chipless RFID Tag Using Novel Trefoil Resonators, IEEE Access, 2019, 7(99): 122398-122407.

30.Rance O.; Siragusa R.; Lemaitre-Auger P.; and Perret E. "Toward RCS magnitude level coding for chipless RFID. IEEE Transactions on Microwave Theory and Techniques 2016; 64(7): 2315-2325.

31.Marindra A. M. J.; and Tian G. Y. Chipless RFID sensor tag for metal crack detection and characterization. IEEE Transactions on Microwave Theory and Techniques 2018; 66(5): 2452-2462.

32.Bibile M. A.; and Karmakar N. C. Moving Chipless RFID Tag Detection Using Adaptive Wavelet-Based Detection Algorithm. IEEE Transactions on Antennas and Propagation 2018; 66(6): 2752-2760.

33.Zhang Y. J.; Gao R. X.; HeY.; and Tong M. S. Effective Design of Microstrip-Line Chipless RFID Tags Based on Filter Theory. IEEE Transactions on Antennas and Propagation 2018; 67(3): 1428-1436.

35.Abdulkawi W. M; and Sheta A. A. Design of Chipless RFID Tag Based on Stepped Impedance Resonator for IoT Applications. In: International Conference on Innovation and Intelligence for Informatics, Computing, and Technologies (3ICT); 2018. p. 1-4.

36.Abdulkawi W. M.; and ShetaA.-F. A. Four-State Coupled Line Resonator for Chipless RFID Tags Application. Electronics 2019, 8(5): Art no. 581.

41.Ma Z. H.; Jiang Y. F. L-shaped Slot-loaded stepped-impedance microstrip structure UWB antenna. Micromachines 2020; 11(9):828.

Author Response File: Author Response.pdf

Reviewer 3 Report

The technology shown in the paper is well known in the scientific community, similar and better results related to multi-resonator chipless RFID have been already reported 10 years ago. For instance, Preradovic et al in “Chipless RFID: The bar code of future” (IEEE Microwave magazine, 2010) have presented a 35 bits chipless RFID with a capacity of 0.61 bits/cm2. This work is not mentioned in this manuscript. We kindly recommend the author improve the reference list by adding more relevant papers in this field, focusing the references to multi-resonator chipless RFID technology.  

The introduction section is too naïve and not well focused on the main subject developed in the manuscript which is a multi-resonator-type chipless RFID. One decade ago, this technology received a lot of attention due to its huge potential as a replacement for a barcode. Nevertheless, it decayed slowly and become considered impractical for a real scenario due to the following reasons. On the one hand, the barcode is very difficult to beat since it has cost “cero” of production and the chipless RFID has a residual cost. On the other hand, the multi-resonator chipless RFID which uses orthogonal cross-polarized antennas to retransmit the information to the reader suffers from a huge disadvantage related to the alignment of the polarized fields necessary to make the system work properly. In a real scenario, this perfect alignment of reader antennas and chipless RFID tag is not guaranteed (but in the laboratory). This limitation pushed the development of e.g. chipless RFID tags insensitive to polarization but at cost of reduction of tag capacity. These factors jeopardize the introduction of chipless RFID in the real market. Please consider rewriting the introduction section by adding the real disadvantages of your technology as above described.  

The statement in line 12 is not clear, please rephrase it.  

Please remove the last statement in the abstract (line 20) or move this affirmation to the introduction section with respective references.  

In paragraph (lines 36 to 45) the reasons to ground the necessity of a chipless RFID are introduced but the idea is difficult to be followed. Please consider rephrasing this paragraph to a better understanding of a normal reader.  

In line 37 and 43, please quantify the capacity of a chipped RFID and a SAW chipless RFID  

In line 58, three bits are considered high capacity, and in line 71, 4 bits are considered low capacity. Please clarify this and please unify criteria on what is considered high/low capacity  

In line 122, change Fiirs by Friis  

Please provide comments on the effect of the structural RCS of your tag and its effects on the recovering of the retransmitted signal from the tag.  

In line 136, please consider rephrasing the definition of “m”  

In line 159, how Q is evaluated?  

In line 168, please provide additional info on estimated dimensions to have a 31 bit chipless RFID tag using your technology.  

Figure 6 is not clear, please consider splitting the info plotted in several figures, e.g. comparing the ref tag with one coding at a time.  

Please provide more details on the measurement setup and how the alignment of the antennas is achieved and verified. Please provide more pictures of the fabricated tag, which includes both cross-polarized antennas.  

In table 2, please consider adding and compare the work presented by Preradovic et al with a multi-resonator chipless RFID tag with 35 bits and a capacity of 0.61bits/cm2. The comparison shown in this table is not fair, you are comparing your potential chipless RFID (with 31 bits) and comparing it against real proof-of-concepts fabricated and measured. A fair comparison must include the values obtained by your proof of concept, i.e, the chipless tag with 6 bits, or let clear that the values shown in the table correspond to simulations.  

In line 286, please consider replacing the term “intelligently” with another adjective better suited for your technology.   

Author Response

1. The technology shown in the paper is well known in the scientific community, similar and better results related to multi-resonator chipless RFID have been already reported 10 years ago. For instance, Preradovic et al in “Chipless RFID: The bar code of future” (IEEE Microwave magazine, 2010) have presented a 35 bits chipless RFID with a capacity of 0.61 bits/cm². This work is not mentioned in this manuscript. We kindly recommend the author improve the reference list by adding more relevant papers in this field, focusing the references to multi-resonator chipless RFID technology.  

Author response: With respect to the reviewer’s comment- This is a very important suggestion. The related content is added to the revised manuscript.

(In page 2, line 90)

Literature [21, 22] first proposed a retransmission chipless tag composed of 6-bit spiral resonators. In order to increase the encoding capacity of the tag, the spiral resonator was increased to 35, the encoding capacity was increased to 35 bits, and the tag size was 88×65 mm². Although in order to reduce the size of the tag, the microstrip line is bent and the spiral resonator is placed on both sides of the microstrip line, the tag size is correspondingly increased, and there is a small amount of coupling between the resonators. Retransmissible tags require orthogonal polarization antennas to retransmit information to the orthogonal polarization antennas of the reader. The polarization fields of the reader and the tag must be aligned, otherwise the system sensitivity will be reduced and the error code rate will rises.

(In page 15, line 417)

21.Preradovic S.; Balbin I.; Karmakar N. C.; Swiegers G. F. Multiresonator-Based Chipless RFID System for Low-Cost Item Tracking. IEEE Transactions on Microwave Theory and Techniques 2009;57(5):1411-1419.

22.Preradovic S.; Karmakar N. C. Design of fully printable planar chipless RFID transponder with 35-bit data capacity. 2009 European Microwave Conference (EuMC); 2009. p. 13-16.

2. The introduction section is too naïve and not well focused on the main subject developed in the manuscript which is a multi-resonator-type chipless RFID. One decade ago, this technology received a lot of attention due to its huge potential as a replacement for a barcode. Nevertheless, it decayed slowly and become considered impractical for a real scenario due to the following reasons. On the one hand, the barcode is very difficult to beat since it has cost “cero” of production and the chipless RFID has a residual cost. On the other hand, the multi-resonator chipless RFID which uses orthogonal cross-polarized antennas to retransmit the information to the reader suffers from a huge disadvantage related to the alignment of the polarized fields necessary to make the system work properly. In a real scenario, this perfect alignment of reader antennas and chipless RFID tag is not guaranteed (but in the laboratory). This limitation pushed the development of e.g. chipless RFID tags insensitive to polarization but at cost of reduction of tag capacity. These factors jeopardize the introduction of chipless RFID in the real market. Please consider rewriting the introduction section by adding the real disadvantages of your technology as above described.  

Author response: With respect to the reviewer’s comment- the response is that the introduction section is updated in the revised manuscript. Thank you for pointing it out.

(In page 1, line 25)

The radio frequency identification (RFID), a wireless communication technology for the noncontact automatic identification and tracking, identifies remote tags by using radio frequency (RF) wave and extracts the encoded data from the backscatter wave. The system is composed of a reader, RFID tags, and a data processing system. The working principle of the RFID system is shown in Fig. 1. A continuous wave interrogation signal is transmitted to the tag through the reader antenna. The antenna of the chipped tag receives the interrogation signal and generates an induced current to obtain energy. In this way, the tag is awakened to work. The modulated coded information of the chipped tag is transmitted to the reader with backscatter wave, which decodes the modulated information of the tag. Finally, the encoded data of the tag is obtained by the system (1, 2)[1-2].

RFID system does not require manual intervention and have the advantages of non-line-of-sight (NLOS) reading and high data capacity. Therefore, chipless tags have the potential to replace barcodes [3,4]. However, the high cost of traditional RFID systems hinders its application in the market for low-value commodities (such as stamps, tickets, and envelopes). The cost of barcode is very low. About 0.005 dollars, while the cost of RFID tags is about 0.3 dollars [5]. The price of RFID tags is very high than barcodes, so barcodes still have a greater price advantage. The cost of the entire RFID system depends on the cost of the tag, because the reader is a one-time cost, and there is no need to replace it after it is put into use. The tag needs to be attached to the item and the quantity is huge. The traditional chip tag needs to be used Silicon chips, due to indispensable materials and manufacturing processes, cannot further reduce the cost of silicon chips [6]. Therefore, a chipless tag is proposed. The cost of the chipless tag is mainly determined by the cost of the conductive material constituting its resonant circuit. In the chipless tag, the cost has been greatly reduced due to the removal of the silicon chip of the tag. Moreover, it also has the advantages of the traditional chip tag. However, the chipless tags have shortcomings in terms of data capacity and tag size [7].

Figure 1. Working principle of the RFID system.

In the past few decades, many researchers have proposed various types of chipless tags, which are divided into two categories: time-domain (TD) [8-13], and frequency-domain (FD) chipless tags [15-29].

The ref. [8] proposed a near-field chipless tag system with sequential reading in the time domain. It is realized by multiple linear half-wavelength microstrip resonators which excited in the direction of a vertical straight line. The encoding capacity can reach 100 bits. The density per surface is 4.9 bit/cm². A series rectangular patches etching on a dielectric substrate is consisted of the chipless tag. A pair of rectangular complementary resonators (CSRR) is loaded on a microstrip line to move on the chipless tag printed on the substrate with rectangular patch of different sizes to generate time-domain codes [9]. The density per surface is 1.15 bit/cm². However, the moving speed of the chipless tag proposed in the literature [8-9] must be constant. The reader and the tag must be aligned, which is difficult to meet the requirements in practical applications.

Chipless tags based on TD also include SAW [10] and transmission delay line chipless tags [11-13]. Hartmann et al. [10] have proposed a new modulation method of time overlapped pulse position with simultaneous phase offset modulation. This method has an encoding capacity of up to 256 bits, reaching the encoding capacity of traditional chips tag, beacause the traditional tags equipped chips has 64, 96, 128 and 256-bit RFID standards [14]. However, the SAW tag’s nonprintable and high-cost characteristic limits its application in the market. Chipless tags based on the transmission delay line are printable TD tags. The literature [11] have proposed a transmission delay line-based identification (ID) generation circuit, which only realizes 4 bits of encoding capacity. If the number of encoding bits is increased, the corresponding dimension increases rapidly.

Chipless tags based on the FD are divided into the backscattered [15-20] and the retransmitted chipless tags [21-29]. The backscattering chipless tag depends on the self-resonance of its multi- resonator to generate spectral characteristics for encoding. The tag does not need a Tx/RX antenna, which advantages of small size and long reading distance. However, a complex algorithm is required to separate the radar cross-section (RCS) signal of the tag before it can be decoded in the actual environment [30-32]. The retransmission chipless tag consists of two cross-polarized antennas and multi-resonator that stores data. The two orthogonally polarized antennas can be remarkably decreased the interference between the transmitting and the receiving signals [33]. The system has a low reading error rate and a long reading distance. The tag encoding capacity is increased by adding more resonators. The distance between resonators can be adjusted to reduce the coupling effect [34-36]. Literature [21, 22] first proposed a retransmission chipless tag composed of 6-bit spiral resonators. In order to increase the encoding capacity of the tag, the spiral resonator was increased to 35, the encoding capacity was increased to 35 bits, and the tag size was 88×65 mm². Although in order to reduce the size of the tag, the microstrip line is bent and the spiral resonator is placed on both sides of the microstrip line, the tag size is correspondingly increased, and there is a small amount of coupling between the resonators. Retransmissible tags require orthogonal polarization antennas to retransmit information to the orthogonal polarization antennas of the reader. The polarization fields of the reader and the tag must be aligned, otherwise the system sensitivity will be reduced and the error code rate will rises.

It is unrealistic to fully align the orthogonal antennas in practical applications. This situation has promoted the research of chipless tags that are unsensitive to polarization [18-20]. Current research is based on backscatter chipless tags.

The inherent low isolation between the transmitter and receiver in a chipless tag reader greatly reduces the dynamic range and sensitivity of the reader. The strong excitation signal leaks to the receiver will reduce the sensitivity of the reader to detect weak backscattered signals and reduce the reading range.

This paper has designed a retransmission chipless tag with multiple U-shaped slot bandstop resonators. The U-shaped slot bandstop resonator unit used in the tag has a high Q-value, low coupling effect, high spectrum efficiency, and high encoding capacity. The high isolation of the receiving and the transmitting signals of the tag are due to the orthogonal transceiver antenna. Furthermore, the tag uses transmitting and receiving antennas, resulting in prolonged reading distance.

3.The statement in line 12 is not clear, please rephrase it.  

Author response: With respect to the reviewer’s comment- the response is that the line 12 is updated.

(In page 1, line 12)

The U-shaped slot resonator has high Q value and narrow impedance bandwidth.

4. Please remove the last statement in the abstract (line 20) or move this affirmation to the introduction section with respective references.  

Author response: With respect to the reviewer’s comment- the response is that the line 20 is removed.

Deleted the sentence “ The chipless tag proposed in this paper has large coding capacity, low cost, and high sensitivity, thereby indicating its potential to replace barcodes. The chipless tag is used widely in logistics, supermarkets, and other fields.”

5. In paragraph (lines 36 to 45) the reasons to ground the necessity of a chipless RFID are introduced but the idea is difficult to be followed. Please consider rephrasing this paragraph to abetter understanding of a normal reader.  

Author response: With respect to the reviewer’s comment- the response is that the line 36 to 45 have been updated.

(In page 1, line 36)

RFID system does not require manual intervention and have the advantages of non-line-of-sight (NLOS) reading and high data capacity. Therefore, chipless tags have the potential to replace barcodes [3,4]. However, the high cost of traditional RFID systems hinders its application in the market for low-value commodities (such as stamps, tickets, and envelopes). The cost of barcode is very low. About 0.005 dollars, while the cost of RFID tags is about 0.3 dollars [5]. The price of RFID tags is very high than barcodes, so barcodes still have a greater price advantage. The cost of the entire RFID system depends on the cost of the tag, because the reader is a one-time cost, and there is no need to replace it after it is put into use. The tag needs to be attached to the item and the quantity is huge. The traditional chip tag needs to be used Silicon chips, due to indispensable materials and manufacturing processes, cannot further reduce the cost of silicon chips [6]. Therefore, a chipless tag is proposed. The cost of the chipless tag is mainly determined by the cost of the conductive material constituting its resonant circuit. In the chipless tag, the cost has been greatly reduced due to the removal of the silicon chip of the tag. Moreover, it also has the advantages of the traditional chip tag. However, the chipless tags have shortcomings in terms of data capacity and tag size [7].

6.In line 37 and 43, please quantify the capacity of a chipped RFID and a SAW chipless RFID 

Author response: With respect to the reviewer’s comment- the response is that the line 37 to 43 have been updated.

(In page 2, line 69)

Hartmann et al. [10] have proposed a new modulation method of time overlapped pulse position with simultaneous phase offset modulation. This method has an encoding capacity of up to 256 bits, reaching the encoding capacity of traditional chips tag, beacause the traditional tags equipped chips has 64, 96, 128 and 256-bit RFID standards [14].

(In page 15, line 402)

14. "EPCglobal Tag Data Standards Version 1.5," ed, Available: http://www.gs1.org/ gsmp/ kc/epcglobal/tds/tds15-standard-20100818. pdf, August 18, 2010 (accessed on July 26, 2011).
15. In line 58, three bits are considered high capacity, and in line 71, 4 bits are considered low capacity. Please clarify this and please unify criteria on what is considered high/low capacity  

Author response: With respect to the reviewer’s comment- Line 58 describes that each resonator can achieve a 3 bits encoding capacity. The chipless tag includes multi-resonator, so it has high encoding capacity. In line 71, the design of the tag only realizes the encoding capacity of 4 bits [13], so the encoding capacity is low.

9. In line 122, change Fiirs by Friis 

Author response: With respect to the reviewer’s comment- the response is that the line 122 is updated. Really sorry for the mistake.

10. Please provide comments on the effect of the structural RCS of your tag and its effects on the recovering of the retransmitted signal from the tag.  

Author response: With respect to the reviewer’s comment- The related comments content is added to the revised manuscript.

(In page 2, line 80)

The backscattering chipless tag depends on the self-resonance of its multi- resonator to generate spectral characteristics for encoding. The tag does not need a Tx/RX antenna, which advantages of small size and long reading distance. However, a complex algorithm is required to separate the the radar cross-section (RCS) signal of the tag before it can be decoded in the actual environment [30-32]. The retransmission chipless tag consists of two cross-polarized antennas and a multiresonator that stores data. The two orthogonally polarized antennas can be remarkably decreased the interference between the transmitting and the receiving signals [33]. The system has a low reading error rate and a long reading distance. The tag coding capacity is increased by adding more resonators. The distance between resonators can be adjusted to reduce the coupling effect [34-36].

(In page 15, line 436)

30.Rance O.; Siragusa R.; Lemaitre-Auger P.; and Perret E. "Toward RCS magnitude level coding for chipless RFID. IEEE Transactions on Microwave Theory and Techniques 2016; 64(7): 2315-2325.

31.Marindra A. M. J.; and Tian G. Y. Chipless RFID sensor tag for metal crack detection and characterization. IEEE Transactions on Microwave Theory and Techniques 2018; 66(5): 2452-2462.

32.Bibile M. A.; and Karmakar N. C. Moving Chipless RFID Tag Detection Using Adaptive Wavelet-Based Detection Algorithm. IEEE Transactions on Antennas and Propagation 2018; 66(6): 2752-2760.

33.Zhang Y. J.; Gao R. X.; HeY.; and Tong M. S. Effective Design of Microstrip-Line Chipless RFID Tags Based on Filter Theory. IEEE Transactions on Antennas and Propagation 2018; 67(3): 1428-1436.

34.Islam M. A.; Karmakar N. C. A Novel Compact Printable Dual-Polarized Chipless RFID System. IEEE Transactions on Microwave Theory and Techniques 2012;60(7):2142-2151.

35.Abdulkawi W. M; and Sheta A. A. Design of Chipless RFID Tag Based on Stepped Impedance Resonator for IoT Applications. In: International Conference on Innovation and Intelligence for Informatics, Computing, and Technologies (3ICT); 2018. p. 1-4.

36.Abdulkawi W. M.; and ShetaA.-F. A. Four-State Coupled Line Resonator for Chipless RFID Tags Application. Electronics 2019, 8(5): Art no. 581.

11. In line 136, please consider rephrasing the definition of “m”  

Author response: With respect to the reviewer’s comment- the response is that the line 136 is updated.

(In page 5, line 164)

and m is the side length difference between adjacent U-shaped slots.

12. In line 159, how Q is evaluated?  

Author response: With respect to the reviewer’s comment- the response is that the line 159 is updated.

(In page 4, line 149)

The Q-value can be calculated by equation (4).

where B is defined as the 10 dB impedance bandwidth of the U-shaped slot resonator. The f is the center frequency of U-shaped slot resonator.

13. In line 168, please provide additional info on estimated dimensions to have a 31 bit chipless RFID tag using your technology. 

Author response: With respect to the reviewer’s comment- The related information of dimension has been added to the discussion.

(In page 12, line 325)

Figures 18 and Figure 19 show the resonance curves of the resonator with the longest side length and the resonator with the shortest side length as the number of resonators constituting the chipless tag is increased 31. Through HFSS software simulation, it was found that the side length of the U-shaped slot resonator with the shortest side length and the longest side length is 2.9 mm and 11 mm, respectively. The side length difference between adjacent resonators is 0.27 mm. At the same time, the width of all U-shaped slot resonator is the same as 2.9 mm. The dimension of the 31 bits chipless tag is 113.94 mm ´ 21.2 mm. The cascades of the U-shaped resonators are arranged side by side. The resonant frequency of the shortest resonator in the 31 bits chipless tag is 13.88 GHz. Meanwhile, the longest resonator corresponds to a resonant frequency of 4.7 GHz.

Figure 18. the maximum resonators of the 31-bit retransmitted chipless tag.

Figure 19.the minimum resonators of the 31-bit retransmitted chipless tag.

14. Figure 6 is not clear, please consider splitting the info plotted in several figures, e.g. comparing the ref tag with one coding at a time.  

Author response: With respect to the reviewer’s comment- the response is that Figures 6 is splitted in 6 figures. The original Fig. 6(a) is splitted into 3 figures and the original Fig. 6(b) is splitted into 3 figures.

(In page 7, line 223)

Figure 7. Simulate the resonance curves of the 6 bits U-shaped slot resonator chipless tags with a short-circuit logic state of "0". (a) ID111111 and ID101010; (b) ID111111 and ID010101;(c) ID111111 and ID111000.

Figure 8. The simulation remove the resonance curves of the 6 bits U-shaped slot resonator chipless tags with a logic state of "0". (a) ID111111 and ID101010; (b) ID111111 and ID010101;(c) ID111111 and ID111000.

15. Please provide more details on the measurement setup and how the alignment of the antennas is achieved and verified. Please provide more pictures of the fabricated tag, which includes both cross-polarized antennas.  

Author response: With respect to the reviewer’s comment- the response is that a paragraph is added in ‘Experiment Results’. Thank you for pointing it out.

(In page 8, line 229)

Fig. 9 is the test system of a retransmission chipless tag based on multi-state resonators. The experimental setup architecture is composed of a UWB reader, Tx/Rx antennas and a chipless tag. Ceyear vector network analyzer 3672D is used as an alternative to the UWB reader. The two ports of the network analyzer are respectively connected to the L-shaped slot-loaded stepped-impedance UWB antennas [40], which are orthogonal to each other to improve the transceiver isolation of the reader. The tag is connected to two orthogonal L-shaped slot-loaded stepped-impedance UWB antennas through two microwave adapters. In order to prevent the received signal and the transmitted signal of the tag from interfering with each other, the two-sided UWB antennas of the tag are also orthogonal to each other. Both the reader antennas and the tag antennas are fixed on the foam, 10 cm apart. The gain of the L-shaped slot-loaded stepped-impedance UWB antenna is 4 dBi to 8.54 dBi in the range of 2.39 GHz to 13.78 GHz.

Figure 9: Test system.

The experimental setup architecture is composed of a UWB reader and a chipless tag. The two ports of the network analyzer are respectively connected to the UWB antennas, which are orthogonal to each other to improve the transceiver isolation of the reader. The chipless tag needs two orthogonal transceiver antennas, similarly, the chipless reader also needs two orthogonal transceiver antennas, which will not affect each other. The polarization characteristics of the transmitting antenna of the reader and the receiving antenna of the tag are the same. The polarization characteristics of the receiving antenna of the reader and the transmitting antenna of the tag are the same. The receiving antenna of the reader can receive the signal that transmitting from the tag However, the transmitting antenna of the reader will not receive any signal from the tag.

(In page 15, line 458)

41. Ma Z. H.; Jiang Y. F. L-shaped Slot-loaded stepped-impedance microstrip structure UWB antenna. Micromachines 2020; 11(9):828.
42. In table 2, please consider adding and compare the work presented by Preradovic et al with a multi-resonator chipless RFID tag with 35 bits and a capacity of 0.61bits/cm². The comparison shown in this table is not fair, you are comparing your potential chipless RFID (with 31 bits) and comparing it against real proof-of-concepts fabricated and measured. A fair comparison must include the values obtained by your proof of concept, i.e, the chipless tag with 6 bits, or let clear that the values shown in the table correspond to simulations.  

Author response: With respect to the reviewer’s comment- the response is that Table 2 is updated.

(In page 13, line 340)

Table 2. Comparison of different types of retransmitted chipless tags.

Table 2 lists the performance comparisons of various types of retransmission chipless tags. The U-shaped slot resonator chipless tags proposed in this paper are simple in design, with high Q-value and small size. The capacity is 1.03 bits/cm². The spectral capacity is 2.71 bits/GHz. Compared with other retransmission chipless tags, such as ref. [22], the designed 6-bit U-shaped slot resonator has insufficient spectral capacity. However, due to the high Q value of the U-shaped slot resonator, the spectrum utilization rate is high. The substrate size of the 6-bit tag is designed to be 30×19.5 mm². Adding a resonator can increase the encoding capacity of the tag. However, the tag size of the U-shaped slot resonator will not be too large. The tag performance designed in this paper can be expanded.

17. In line 286, please consider replacing the term “intelligently” with another adjective better suited for your technology.   

Author response: With respect to the reviewer’s comment- the response is that the line 286 is updated.

(In page 14, line 364)

this tag can only works in the 8.28 - 17.67 GHz frequency bands.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

no coments

Reviewer 3 Report

The author re-worked adequately the comments and corrections proposed by the Reviewer. Thanks