Low-Profile and Closely Spaced Four-Element MIMO Antenna for Wireless Body Area Networks
, Amjad Iqbal 2,3,*, Chemseddine Zebiri 4, Abdul Basir 5, Jonathan Rodriguez 1,6, Maryam Sajedin 1, Artur de Oliveira Pereira 1, Widad Mshwat 7, Raed Abd-Alhameed 7,8 and Sadiq Ullah 5Round 1
Reviewer 1 Report
1. Please provide simulation and measurement results on a single-element (instead of the four-element antenna, which is thoroughly investigated in the manuscript) and justify the array gain of the four-element design.
2. Although the purpose of the majority of MIMO antennas at 2.4GHz band is to provide diversity gain under fading channels, the far field beam steering capability of array antennas provides additional benefits to MIMO systems (e.g., Arno Thielens et al., "A Comparative Study of On-Body Radio-Frequency Links in the 420 MHz–2.4 GHz Range," Sensors, Dec. 2018). I would like to suggest the authors discussing the beam steering capability of the proposed antenna. If available, please provide simulation/measurements results on far field beam steering.
3. If the antenna is integrated into a system, there can be a large ground plane near the antenna. Please discuss the impact of a nearby ground plane on the antenna performance.
Author Response
Reviewer 1:
Please provide simulation and measurement results on a single-element (instead of the four-element antenna, which is thoroughly investigated in the manuscript) and justify the array gain of the four-element design.
Reply: We updated the manuscript by adding simulation and measurement results as Fig. 2. The discussion on the single-element antenna is provided in the 1st paragraph of the section. 2. The concept of the MIMO antenna and Array antenna in terms of gain is different. The improvement in gain by designing an array system does not apply to the MIMO system because in a MIMO system, only one element transmits/receives the signal at one instance. Hence, the gain of single-element and MIMO system will
Although the purpose of the majority of MIMO antennas at 2.4GHz band is to provide diversity gain under fading channels, the far field beam steering capability of array antennas provides additional benefits to MIMO systems (e.g., Arno Thielens et al., "A Comparative Study of On-Body Radio-Frequency Links in the 420 MHz–2.4 GHz Range," Sensors, Dec. 2018). I would like to suggest the authors discussing the beam steering capability of the proposed antenna. If available, please provide simulation/measurements results on far field beam steering.
Reply: The suggested reference is cited as Ref. [54] in the main text. We agree that the beam steering capability of a system highly reduce the interference and thus increase the throughput. However, our proposed work focus on the design of a compact MIMO system for On-body communication. We, of course, will consider the beam steering capability of the MIMO system in our future work.
If the antenna is integrated into a system, there can be a large ground plane near the antenna. Please discuss the impact of a nearby ground plane on the antenna performance.
Reply: Of course there will be some impact of the large ground plane on the performance of the antenna, however, the impact is highly dependent on the position and gap between the large ground plane and the antenna. Normally, a large ground plane below the antenna acts as a reflector and reduces the backward radiation and thus enhance the gain. Moreover, the cavity effect generated by the large ground plane enhances the bandwidth of the system. But still, all of the improvement in the antenna parameters is dependent on the position and gap between the antenna and ground plane [1].
[1] Dash, Sounik Kiran Kumar, et al. "Wideband cylindrical dielectric resonator antenna operating in HEM11δ mode with improved gain: a study of superstrate and reflector plane." International Journal of Antennas and Propagation 2017 (2017).
Author Response File: 
Author Response.pdf
Reviewer 2 Report
This paper describes a novel design for 2x2 MIMO antenna that is intended for operation at the 2.4 GHz band. The study can be considered thorough, as several operation parameters are estimated and assessed. This work is interesting, however the authors need to clarify certain points, before the paper can be suitable for publication. These points are analyzed in the following comments:
1) The phrase "The ground plane is printed on the lower side of the substrate" should be modifies, as no ground plane appears in the corresponding figure. In fact, it is later mentioned that, instead of a classic ground plane, four conducting rectangular parts are printed on the lower side of the substrate.
2) The phrase " there is only fraction of mismatch between both results which validate our circuit model" is suggested to be modified, as the differences appear to be more pronounced,.
3) Regarding Fig. 3, why aren't the S11, S22, etc parameters shown?
4) In several figures, results are plotted over extended frequency bands. Why are we interested in such broad frequency bands? Is the proposed antenna supposed to operate at frequencies away from 2.4 GHz? It is required that the authors comment on this issue.
5) Due to the geometric symmetry in Fig. 7, the three figures are redundant (the current distribution remains identical) and they should be omitted. The corresponding paragraph of the paper should be modified accordingly.
6) I do not completely agree with the statement "that skin of the human body behaves as an extension of the ground plane". The body skin absorbs some of the radiated power and, at the same time, reflects a fraction of it. Consequently, I am not convinced that it operates as a ground plane.
7) The authors use two symbols for the capacity loss: C_loss and CCL. Perhaps only one of them should be selected.
8) At the end of Section 6, a parenthesis is opened ("(ideally, CCL should..."), but it is never closed.
9) Regarding the antenna efficiency, how is it computed and how does it compare with other existing designs?
10) The paper's language needs to undergo a thorough revision, as certain errors need to be corrected. Some examples are: "an additional antenna characteristics", "the other three ports remains", "a certain irregularities", "this absorption of EM waves is exceed", etc.
Author Response
Reviewer 2:
This paper describes a novel design for 2x2 MIMO antenna that is intended for operation at the 2.4 GHz band. The study can be considered thorough, as several operation parameters are estimated and assessed. This work is interesting, however the authors need to clarify certain points, before the paper can be suitable for publication. These points are analyzed in the following comments:
1) The phrase "The ground plane is printed on the lower side of the substrate" should be modifies, as no ground plane appears in the corresponding figure. In fact, it is later mentioned that, instead of a classic ground plane, four conducting rectangular parts are printed on the lower side of the substrate.
Reply: The phrase is updated as: "The very small ground plane is printed on the lower side of the substrate, where this appellation is used in [R1] [R2] [R3] ".
[R1] Zhang, Z., Jiao, Y. C., Song, Y., Zhang, T. L., Ning, S. M., & Zhang, F. S. (2010). A modified CPW‐fed monopole antenna with very small ground for multiband WLAN applications. Microwave and Optical Technology Letters, 52(2), 463-466.
[R2] Suman P. Wadkar, B. G. Hogade, S. M. Rathod, Hemant Kumar & Girish Kumar (2018): Normal Mode Helical Antenna on Small Circular Ground Plane, IETE Journal of Research, DOI: 10.1080/03772063.2018.1510748.
[R3] M. Sanad, “Microstrip antenna on the very small ground plane for portable Communication systems”, A& P Society Symposium, 1994, Vol 12, pp 810-813.
2) The phrase " there is only fraction of mismatch between both results which validate our circuit model" is suggested to be modified, as the differences appear to be more pronounced,.
Reply: The phrase is updated as: "We can see that the plotted curves are almost identical, this validates our circuit model.".
3) Regarding Fig. 3, why aren't the S11, S22, etc parameters shown?
Reply: We updated the manuscript by adding the S11 plot as Fig. 4a.
4) In several figures, results are plotted over extended frequency bands. Why are we interested in such broad frequency bands? Is the proposed antenna supposed to operate at frequencies away from 2.4 GHz? It is required that the authors comment on this issue.
Reply: All plots in this work are plotted from 1 GHz to 4 GHz because of the reason that the proposed antenna has wide operating 10-dB bandwidth of around 1.71 GHz (1.64-3.35 GHz). If we plot the figures in limited frequency, then the reader will not be able to judge the operating band of the antenna.
5) Due to the geometric symmetry in Fig. 7, the three figures are redundant (the current distribution remains identical) and they should be omitted. The corresponding paragraph of the paper should be modified accordingly.
Reply: Thank you for the suggestion. The main aim of plotting Fig. 7 is to show the behavior of each antenna in the operating cycle and the coupling impact of the nearby antennas. The figure shows that if one antenna is excited then how much current is coupled with the nearby antennas. That’s why the current distribution on each antenna is plotted.
6) I do not completely agree with the statement "that skin of the human body behaves as an extension of the ground plane". The body skin absorbs some of the radiated power and, at the same time, reflects a fraction of it. Consequently, I am not convinced that it operates as a ground plane.
Reply: Thank you very much for your insightful comment. We agree with the reviewer that due to the conductive nature of human skin it reflects a fraction of the radiated power and absorbs the remaining. However, the conductivities of the human tissues depending on the operating frequencies. In our case, the portion of the reflected radiated power from the skin is greater than the absorbed one, and luckily it enhances the performance of our antenna, unlike the other cases which sometimes degrades the antenna performance.
7) The authors use two symbols for the capacity loss: C_loss and CCL. Perhaps only one of them should be selected.
Reply: We replaced C_Loss with CCL. Thank you.
8) At the end of Section 6, a parenthesis is opened ("(ideally, CCL should..."), but it is never closed.
Reply: closed parenthesis is added.
9) Regarding the antenna efficiency, how is it computed and how does it compare with other existing designs?
Reply: The efficiencies of the MIMO antenna is computed using EM simulator, HFSS. The efficiencies of existing designs with the proposed one are compared in a comparison Table (Table. 1).
10) The paper's language needs to undergo a thorough revision, as certain errors need to be corrected. Some examples are: "an additional antenna characteristics", "the other three ports remains", "a certain irregularities", "this absorption of EM waves is exceed", etc.
Reply: Thank you for highlighting the issue. We thoroughly checked the English.
Author Response File: Author Response.pdf
Reviewer 3 Report
The work presented in this paper appears sound, though it is presented in a way that is not always clear. In general, I encourage the authors to improve on this. furthermore, language editing would definitely improve the readability of this paper.
Here are a few more specific points that I would like to see addressed:
Page 6:
"The isolation values vary from 18 to 23 dB in the impedance band of the proposed antenna":
I'm not quite sure I understand what the authors mean here (frequency band?)
Page 7, Figure 7:
The surface current distributions look identical to me, except for being rotated by 90 degrees
each (as I would expect due to symmetry). Showing just one and pointing to symmetry in the caption should be sufficient.
Section 6:
A few sentences explaining the concepts of envelope correlation coefficient, diversity gain, and channel capacity loss, and why they are of relevance is definitely needed.
Comments for author File: Comments.pdf
Author Response
Reviewer 3:
The work presented in this paper appears sound, though it is presented in a way that is not always clear. In general, I encourage the authors to improve on this. furthermore, language editing would definitely improve the readability of this paper.
Here are a few more specific points that I would like to see addressed:
Page 6:
"The isolation values vary from 18 to 23 dB in the impedance band of the proposed antenna":
I'm not quite sure I understand what the authors mean here (frequency band?)
Reply: X-axis of the plot shows the frequency in GHz while Y-axis shows the isolation in dB. The proposed antenna has wide 10-dB bandwidth of around 1.71 GHz (1.64-3.35 GHz). By the sentence stated above means that the isolation value varies between 18 to 23 dB in the operating band (1.64-3.35 GHz).
Page 7, Figure 7:
The surface current distributions look identical to me, except for being rotated by 90 degrees each (as I would expect due to symmetry). Showing just one and pointing to symmetry in the caption should be sufficient.
Reply: Thank you for the suggestion. The main aim of plotting Fig. 7 is to show the behavior of each antenna in the operating cycle and the coupling impact of the nearby antennas. The figure shows that if one antenna is excited then how much current is coupled with the nearby antennas. That’s why the current distribution on each antenna is plotted.
Section 6:
A few sentences explaining the concepts of envelope correlation coefficient, diversity gain, and channel capacity loss, and why they are of relevance is definitely needed.
Reply: Thank you for the suggestion. We updated the manuscript by adding the definitions of MIMO parameters.
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
It seems that this submission needs further work, before it can be considered for publication: Specifically:
- When the authors replied to one of my comments, the claimed that they used HFSS for performing the simulations, which is not mentioned in the paper. On the other hand, CST is mentioned in the manuscript as the implemented simulator. Apparently, there is some inconsistency here and it should be clarified.
- Perhaps reference [45] for the CST Studio needs to be corrected, as it begins with "STudio, C.M. ".
- The authors mention that "Fig. 14b reveals that the proposed MIMO antenna achieves acceptable CCL values". This is clearly wrong, as Fig. 14b pertains to the radiation efficiencies.
- The statement "If this absorption of EM waves is exceeded some standard limits, the temperature of the tissues will rise [50]-[51]" is deceiving, as the temperature will rise, even when the limits are not exceeded.
- One of my initial requests was that the paper should undergo an extended revision, regarding the language. The authors replied that the have complied with this suggestion, however the following correction prove that significant work remains to be done:
1) "are giving more attention by industry" --> "are given more attention by industry".
2) "the MIMO technology has been extensively considered and exploited in order to the performance of array antennas in terms of improving the channel capacity and signal quality" (this needs to be re-phrased, it does not make sense).
3) "The MIMO approach help in " --> "The MIMO approach helps in ".
4) "and since then become broadly" (needs to be re-phrased).
5) "These include different kind" --> "These include different kinds".
6) "While, most wearable antennas" --> "On the other hand, most wearable antennas".
7) "Especial considerations" --> "Special consideration".
8) "The dimensions of the antenna was optimized" --> "The dimensions of the antenna were optimized".
9) "We can see that the plotted curves are almost identical" --> "We can see that the plotted curves present satisfactory agreement".
10) "The measured s-parameters of the proposed MIMO antenna in Fig. 5 shows" --> "The measured s-parameters of the proposed MIMO antenna in Fig. 5 show".
11) "in which can be attributed to several factors" --> "which can be attributed to several factors".
12) "in which generated by the CST software" --> "which are generated by the CST software".
13) "is exceeded some standard limits" --> "exceeds some standard limits".
14) "ECC of the MIMO system portrays that how independent" --> "ECC of the MIMO system portrays how independent".
15) "In the case of 2×2 MIMO system, assuming that only the receiving antenna patterns are
correlated and assuming the worst case where high SNR is occurring" (needs to be re-phrased).
16) "The compact 4-element MIMO antenna" --> "A compact 4-element MIMO antenna".
It should be understood that the aforementioned corrections are a representative part of the corrections that need to be made.
Author Response
Manuscript ID : electronics-678267
Title :
Low-Profile and Closely Spaced Four Elements MIMO Antenna for Wireless Body Area Networks
Journal : Electronics
Dear Editor
Cover letter
The authors would like to thank the editor and the reviewers for their precious time and valuable recommendations. We also greatly appreciate the reviewers’ constructive criticisms and appreciated comments and suggestions.
We have carefully addressed all the reviewers comments. A point-by-point response to reviewers’ comments and the corresponding changes and refinements made in the revised paper are denoted to below.
The changes are marked in red in the new version.
We hope that you find our responses satisfactory and that the manuscript is now acceptable for publication.
Sincerely,
Dr Issa Elfergani
Senior Research Engineer
Instituto de Telecomunicações
Campus Universitário de Santiago 3810-193 Aveiro
Portugal
Reviewer 2:
It seems that this submission needs further work, before it can be considered for publication: Specifically:
- When the authors replied to one of my comments, the claimed that they used HFSS for performing the simulations, which is not mentioned in the paper. On the other hand, CST is mentioned in the manuscript as the implemented simulator. Apparently, there is some inconsistency here and it should be clarified.
Reply: Basically we used two different software (CST Microwave Studio and HFSS) for design and optimization of the antenna. The reason behind simulating antenna in both software was to check its validity in time domain and frequency domain solver. That’s why we have only included CST in the paper.
- Perhaps reference [45] for the CST Studio needs to be corrected, as it begins with "STudio, C.M. ".
Reply: Reference [45] is corrected.
- The authors mention that "Fig. 14b reveals that the proposed MIMO antenna achieves acceptable CCL values". This is clearly wrong, as Fig. 14b pertains to the radiation efficiencies.
Reply: We updated manuscript by correcting Fig. 14b to Fig. 14a.
- The statement "If this absorption of EM waves is exceeded some standard limits, the temperature of the tissues will rise [50]-[51]" is deceiving, as the temperature will rise, even when the limits are not exceeded.
Reply: We agree the worthy reviewer that yes the temperature will still increase but it will bot harm the human body if it is under IEEE defined standards. However, if it exceeds the specified limit, then it will harm the human body.
- One of my initial requests was that the paper should undergo an extended revision, regarding the language. The authors replied that the have complied with this suggestion, however the following correction prove that significant work remains to be done:
1) "are giving more attention by industry" --> "are given more attention by industry".
Reply: We updated manuscript by coorecting the sentence.
2) "the MIMO technology has been extensively considered and exploited in order to the performance of array antennas in terms of improving the channel capacity and signal quality" (this needs to be re-phrased, it does not make sense).
Reply: We updated manuscript by rephrasing the sentence.
3) "The MIMO approach help in " --> "The MIMO approach helps in ".
Reply: Corrected accordingly.
4) "and since then become broadly" (needs to be re-phrased).
Reply: Corrected accordingly.
5) "These include different kind" --> "These include different kinds".
Reply: Corrected accordingly.
6) "While, most wearable antennas" --> "On the other hand, most wearable antennas".
Reply: Corrected accordingly.
7) "Especial considerations" --> "Special consideration".
Reply: Corrected accordingly.
8) "The dimensions of the antenna was optimized" --> "The dimensions of the antenna were optimized".
Reply: Corrected accordingly.
9) "We can see that the plotted curves are almost identical" --> "We can see that the plotted curves present satisfactory agreement".
Reply: Corrected accordingly.
10) "The measured s-parameters of the proposed MIMO antenna in Fig. 5 shows" --> "The measured s-parameters of the proposed MIMO antenna in Fig. 5 show".
Reply: Corrected accordingly.
11) "in which can be attributed to several factors" --> "which can be attributed to several factors".
Reply: Corrected accordingly.
12) "in which generated by the CST software" --> "which are generated by the CST software".
Reply: Corrected accordingly.
13) "is exceeded some standard limits" --> "exceeds some standard limits".
Reply: Corrected accordingly.
14) "ECC of the MIMO system portrays that how independent" --> "ECC of the MIMO system portrays how independent".
Reply: Corrected accordingly.
15) "In the case of 2×2 MIMO system, assuming that only the receiving antenna patterns are
correlated and assuming the worst case where high SNR is occurring" (needs to be re-phrased).
Reply: Corrected accordingly.
16) "The compact 4-element MIMO antenna" --> "A compact 4-element MIMO antenna".
It should be understood that the aforementioned corrections are a representative part of the corrections that need to be made.
Reply: Corrected accordingly. Thank you
Author Response File: Author Response.pdf
Round 3
Reviewer 2 Report
I believe that the authors have improved their work, after taking into account the review comments, and their paper has reached a level of maturity that renders it suitable for publication.
