Review Reports

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript investigates an interesting RIS-enabled joint symbiotic backscatter NOMA system and develops continuous and on-off mechanisms to balance the primary and backscatter channel gains. The symbiotic-map formulation and induction-based treatment of the on-off mechanism are potentially useful contributions. The paper also provides comparisons with standalone and benchmark decoding approaches and validates several analytical expressions through simulations. However, several aspects should be addressed to strengthen the technical rigor and clarity of the manuscript:

1. The manuscript should provide more details on the practical architecture of the proposed RIS, particularly how the primary-signal reflecting elements and backscatter elements are physically implemented and independently controlled.
   
   
2. The analysis appears to rely on ideal/perfect SIC. The authors should clearly state this assumption and discuss how residual SIC errors would affect the COP, especially since eliminating the error floor is one of the main claims.
   
   
3. The continuous mechanism assumes an ideal level of control over the RIS reflection coefficients. The authors should clarify its practical feasibility and clearly distinguish this ideal benchmark from the proposed on-off implementation.
   
   
4. The assumptions regarding channel gains, phase alignment, coherent combining, and statistical independence/correlation among RIS elements should be explicitly stated. These assumptions are important for validating the derived COP expressions.
   
   
5. The induction-based derivation for large numbers of RIS elements is interesting, but several intermediate steps are difficult to reproduce. More details should be provided, particularly regarding the transition between successive activation states and the derivation of the compensation probability.
   
   
6. The manuscript should clearly distinguish exact analytical expressions, upper bounds, numerical integrations, asymptotic expressions, and approximations. In particular, the role of the compensation probability $C(N_1, N_2)C(N_1,N_2)$ should be clarified so that the analytical results are not interpreted as exact when they are bounds or approximations.
   
   
7. More extensive Monte Carlo validation is needed for the proposed on-off mechanism. The analytical/numerical results should be compared over a wider range of SNRs and RIS-element numbers to demonstrate the accuracy of the derived expressions.
   
   
8. The comparison with standalone and benchmark systems should be described more systematically. The authors should ensure that all schemes use consistent channel, power-allocation, RIS-element, and threshold parameters, and explain precisely what distinguishes each benchmark from the proposed scheme.
   
   
9. The discussion of batched activation is useful, but the corresponding derivation and performance-complexity tradeoff should be presented more clearly. The claim that the proposed approach reduces decoding/control time should preferably be supported by a quantitative complexity, latency, or number-of-RIS-control-operations comparison.
   
   
10. Several notation definitions and terminology should be made more consistent throughout the manuscript. The authors should also moderate broad claims regarding superiority and interference suppression unless they are directly supported by the presented results. A careful English-language revision and improvement of figure captions/legends would further improve readability.
11. The outage probability analysis of NOMA systems is commonly reported in the literature. Also, the outage probability analysis under imperfect SIC is also well studied. It would be better to provide a literature on them as well. Also better to justify why imperfect SIC is not considered in your analysis. Followings are some related papers to refer.

a). Y. Liu and H. Cheng, "Outage Performance Analysis and Optimization of Two-User Downlink NOMA Systems With Imperfect SIC and Improper Signaling," in IEEE Transactions on Wireless Communications, vol. 23, no. 9, pp. 11649-11661, Sept. 2024.

b). K. W. S. Palitharathna, H. A. Suraweera, R. I. Godaliyadda, V. R. Herath and Z. Ding, "Impact of Receiver Orientation on Full-Duplex Relay Aided NOMA Underwater Optical Wireless Systems," ICC 2020 - 2020 IEEE International Conference on Communications (ICC), Dublin, Ireland, 2020, pp. 1-7.

   12. There are typos in the paper. Please check carefully and correct. Also, the reference               format should be improved. Use abbreviations for journals, use "In Proc. xx" for                     conferences, and if more than 6 authors use "et al.".

 

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Comments to the Authors:

This paper studies an RIS-assisted backscatter NOMA system and proposes continuous reflection control and on-off element activation to reduce interference between the primary and backscatter transmissions. The authors analyze coexistence outage probability and introduce a symbiotic map to describe the activation process. The study addresses a relevant problem in reliable IoT communications. However, the manuscript still needs further revision and improvement. My comments are as follows.

 

1.Section 2 assumes accurate CSI and perfect SIC, but the effect of estimation and cancellation errors on the results is left unclear. The authors should discuss the limits of these assumptions and state which conclusions apply only under ideal conditions.

 

2.Section 3 does not specify how the next RIS element is chosen. Is activation based on a fixed order or on the current channel conditions? Please state the selection rule and set out the activation steps in pseudocode.

 

3.In Section 4, Fig. 5 is crowded, and several labels are too small to read at normal size. Please split the figure into smaller panels and increase the font size.

 

4.Section 5.2 introduces batch activation without explaining how to choose the batch size. Larger batches reduce the number of updates but can skip a state in which both transmissions succeed. What should determine the batch size in practice? Please give a recommendation based on the results and explain when a smaller batch is preferable.

 

5.The caption of Fig. 6 says User 1, while the text describes the on-off results as those of User 3. Please check which user was actually simulated and correct the caption or the text.

 

6.The computational cost of the activation procedure is not clearly discussed in Section 6. Please clarify how it scales with the number of RIS elements, and compare sequential and batched activation in terms of activation steps or execution time.

 

7.Please add a few more closely related papers from the past three years, particularly on RIS-assisted backscatter NOMA and practical RIS control.

 

8.The manuscript contains grammatical errors, including incorrect use of singular and plural forms. Please check the language throughout the paper and correct these errors.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Authors have well addressed the comments from my side. I recommend to have a careful checking though the paper to correct minor typos.

Author Response

We sincerely thank the reviewer for the positive assessment of our revisions and for recognizing our efforts. Following the reviewer’s suggestion, we have carefully checked the entire manuscript, including the main text, equations, figure captions, tables, and references, and corrected the remaining minor typos, grammatical errors, and formatting inconsistencies. All changes have been incorporated into the revised manuscript and highlighted for ease of reference.

Reviewer 2 Report

Comments and Suggestions for Authors

Comments to the Authors:

I have reviewed the revised manuscript and the authors’ responses. A few minor issues remain in the presentation of the results and simulation parameters, as well as in the wording. I recommend minor revision, with the specific comments listed below.

1.The abstract claims better performance but does not give a specific numerical result. Please include one representative comparison from the existing simulations, stating the comparison scheme and the conditions under which the result was obtained.

2.Please organize the simulation parameters listed in Section 6 into a concise table, including their names, values, and units. Clearly indicate the settings that differ among the three users.

3.In the discussion of Fig. 8, the phrase “the only viable method” is too strong for the results presented. Please limit this statement to the conditions considered in the simulations.

4.Please check the remaining spelling and sentence errors.

Author Response

Comments 1: The abstract claims better performance but does not give a specific numerical result. Please include one representative comparison from the existing simulations, stating the comparison scheme and the conditions under which the result was obtained.

Response 1: We sincerely thank the reviewer for this valuable suggestion. We agree that the original abstract described the performance improvement only qualitatively and did not provide a representative numerical comparison. Accordingly, we have revised the abstract based on the existing simulation results. Specifically from the simulation results in Fig.8, at an SNR of 30 dB, with 12 primary elements and 12 backscatter elements, the proposed joint symbiotic mechanism reduces the outage probability from approximately 6e-4 for the conventional symbiotic scheme to approximately 5e-6. This corresponds to a COP reduction of approximately two orders of magnitude under on-off control and shows that the proposed mechanism eliminates the high-SNR error floor observed with the continuous scheme. The corresponding revision has been made in the Abstract section.

We hope you find our revisions satisfactory. Thank you.

Comments 2: Please organize the simulation parameters listed in Section 6 into a concise table, including their names, values, and units. Clearly indicate the settings that differ among the three users.

Response 2: We thank the reviewer for this helpful suggestion. We have reorganized the simulation parameters described in Section 6 into a concise table that lists each parameter, its value, and its unit. To make the user-specific settings clear, the table presents the parameters that differ among the three users in separate columns for User 1, User 2, and User 3. The corresponding descriptions in Section 6 have also been revised to refer to the table. Please see Table 1 in the revised manuscript.

We hope you find our revisions satisfactory. Thank you.

Comments 3: In the discussion of Fig. 8, the phrase “the only viable method” is too strong for the results presented. Please limit this statement to the conditions considered in the simulations.

Response 3: We thank the reviewer for pointing this out. We agree that the phrase “the only viable method” was too strong, since Fig. 8 considers only a limited set of simulation configurations and does not rule out other possible approaches. We have revised the discussion to describe the observed trend more cautiously and explicitly limit the conclusion to the configurations considered in the simulations. Specifically, the revised text states that, for the configurations considered in Fig. 8, increasing the number of RIS elements allocated to User 1 reduces the COP of the on-off mechanism. We have also revised the description of its high-SNR behavior to state that the COP approaches an error floor.

We hope you find our revisions satisfactory. Thank you.

Comments 4: Please check the remaining spelling and sentence errors.

Response 4: We thank the reviewer for this helpful comment. We have carefully proofread the entire manuscript and corrected the remaining spelling, grammatical, punctuation, and sentence-structure errors. We have also revised several expressions to improve clarity and readability, and checked the consistent use of technical terminology, abbreviations, notation, capitalization, and figure/table references throughout the manuscript. Since these corrections were made at multiple locations, the changes are highlighted in the revised manuscript. Moreover, the proof of Lemma 1 has been renewed for more explicit expression.

We hope you find our revisions satisfactory. Thank you.