Secrecy Rate Bounds in Spatial Modulation-Based Visible Light Communications under Signal-Dependent Noise Conditions
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThis reviewer has following comments:
1. In this paper, the spatial modulation (SM) concept is not well introduced. Note that the recent SM related literature should be well introduced, including the review of SM [1-2], etc., which could largely improve the quality of this paper.
[1] A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications, IEEE JSAC, 2019.
[2] Index Modulation Multiple Access for 6G Communications: Principles, Applications, and Challenges, IEEE Network, 2023.
2. If there are more than one Bob or Eve, is the proposed method still available?
3. The authors should emphasize the difference between the conventional wireless channel and the VLC channel.
4. What if Eve is co clear (or close to Bob) that could acquire the channel state information between Alice and Bob? In this case, Eve might be able to intercept the information of Bob.
5. Please draw all curves smoother. In addition, please improve the resolution of all figures.
6. Please correct some typo errors.
Comments on the Quality of English LanguagePlease correct some typo errors.
Author Response
Comment 1: In this paper, the spatial modulation (SM) concept is not well introduced. Note that the recent SM related literature should be well introduced, including the review of SM [1-2], etc., which could largely improve the quality of this paper.
[1] A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications, IEEE JSAC, 2019.
[2] Index Modulation Multiple Access for 6G Communications: Principles, Applications, and Challenges, IEEE Network, 2023.
Response 1: Thank you for your valuable feedback. I have revised the introduction to provide a more comprehensive explanation of the SM concept. Additionally, I have incorporated the recent SM-related literature you mentioned, including the detailed review on SM progress and applications from IEEE Journal on Selected Areas in Communications, as well as the exploration of index modulation multiple access for 6G communications from IEEE Network. These references have been introduced to enhance the context and relevance of the presented work. The reviewer mar refer to the highlighted texts in the introduction section (Section I) on pages 1 and 2, lines 42 to 49 as follows:
SM has gained increasing attention for its ability to balance spectral efficiency and energy efficiency, while maintaining a simple design through the use of a single RF chain. A comprehensive survey of recent developments in SM, including its applications across various domains such as time, frequency, and code, as well as its integration with emerging wireless technologies, is provided in [5]. Furthermore, the potential of SM in 6G networks is highlighted in [6], where the concept of index modulation multiple access is explored to enhance system performance and enable massive connectivity, further extending the applicability of SM in future communication systems.
Comment 2: If there are more than one Bob or Eve, is the proposed method still available?
Response 2: Thank you for your question. Yes, the proposed method applies to scenarios with multiple Bobs and Eves. As detailed in Section VII, on page 15, the analysis extends to these cases, assuming non-colluding adversaries. The secrecy rate and capacity bounds, outlined in equations (6) and (25), remain valid under these conditions. The reviewer may refer to Section VII as follows:
VII. Scenarios With Multiple Receivers
In this section, we analyze scenarios involving multiple legitimate receivers and illegitimate receivers (adversary). For generality, we assume the presence of legitimate receivers and adversaries. The signals received by each legitimate receiver and adversary are expressed as follows:
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(50) |
where represent the signal-independent noise at the u-th legitimate receiver and v-th adversary, respectively. Similarly, denote the signal-dependent noise at the u-th legitimate receiver and v-th adversary, respectively.
We assume that the adversaries do not collude, meaning each processes their received signals independently. The adversary with the highest mutual information is considered the worst-case adversary. The secrecy rate for the -th legitimate receiver, as defined in (6) and (25), becomes:
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where
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(51) |
Thus, the secrecy capacity is formulated according to problems (6) and (25). This confirms that the secrecy capacity bounds obtained in Sections III to V remain valid in scenarios with multiple receivers, assuming non-colluding adversaries.
Comment 3: The authors should emphasize the difference between the conventional wireless channel and the VLC channel.
Response 3: Thank you for your feedback. We have revised the introduction to highlight key differences between RF-based wireless channels and VLC channels, emphasizing VLC's larger spectrum, reduced interference, and enhanced security, along with its susceptibility to eavesdropping. These distinctions justify the specialized security approaches explored in our study. The reviewer may refer to the introduction section, page 2, lines 50 to 61, as follows:
Conventional wireless communication systems, such as RF-based systems, operate in a spectrum that is highly susceptible to interference, fading, and congestion due to the shared nature of radio frequency bands. In contrast, Visible Light Communication (VLC) operates in a much larger, unregulated spectrum (400–800 THz), offering several advantages over traditional RF channels. VLC signals, transmitted through optical light, provide higher directivity and have a limited propagation range, reducing interference risk and enhancing spatial security. Moreover, VLC is highly resistant to electromagnetic interference, which is common in RF systems. However, due to its broadcast nature and line-of-sight requirements, VLC channels are also vulnerable to eavesdropping attacks. This makes physical-layer security techniques especially critical for VLC systems. These differences between VLC and conventional wireless channels necessitate specialized approaches to ensure data confidentiality, as explored in this study.
Comment 4: What if Eve is co clear (or close to Bob) that could acquire the channel state information between Alice and Bob? In this case, Eve might be able to intercept the information of Bob.
Response 4: Thank you for your insightful comment. In the case where Eve is close enough to Bob to potentially acquire the channel state information (CSI) between Alice and Bob, we acknowledge that the risk of eavesdropping increases. However, our proposed secrecy rate analysis and transmitter selection mechanisms, such as Channel Adaptive Selection (CAS) and Greedy Selection (GS), are designed to account for this challenge. These methods dynamically select transmitters to minimize the information leakage to any eavesdropper, including those with access to partial or full CSI.
Moreover, the physical-layer security techniques applied in our study leverage signal-dependent noise and the distinct characteristics of VLC channels, making it more difficult for Eve to reliably intercept Bob’s information, even with close proximity. Additionally, to further enhance the secrecy rate, we propose the use of optical helper jamming. This technique introduces controlled interference, specifically aimed at confusing potential eavesdroppers like Eve, without significantly affecting the legitimate receiver. By combining these strategies, we strengthen the system’s ability to maintain confidentiality, even in scenarios where Eve is close to Bob.
Comment 5: Please draw all curves smoother. In addition, please improve the resolution of all figures.
Response 5: Thank you for your feedback. We have improved the resolution and the presentation of the graphs, ensuring that all images can be identified easily despite being presented in Black & White print. We prefer not to use interpolation techniques to smooth the curves, as this can distort the results and cause deviations from the actual values.
Comment 6: Please correct some typo errors.
Response 6: Thank you for pointing this out. We have corrected the typographical errors in the manuscript.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsThe authors in this paper address a critical and important issue in the field of VLC, focusing on the physical-layer security using SM. Analyzing the secrecy rates under signal-dependent noise conditions is really relevant because it reflects such practical noise consideration in VLC systems. Also, the closed-form expressions derivation under different constraints (non-negativity, average optical intensity, and peak optical intensity) is thorough and mathematically rigorous. The reviewer has the following comments to be addressed:
1- The introduction should include more related works from the literature, explaining the main issues addressed by each work. This would provide a clearer background and highlight the significance of the current study within the broader research landscape.
2- The paper exclusively focuses on the spatial modulation (SM) approach. Considering more sophisticated approaches with enhanced spectral efficiency, such as Generalized Spatial Modulation (GSM) or Quadrature Spatial Modulation (QSM), could potentially offer greater insights and benefits. The authors might discuss why SM was chosen over these alternatives.
3- The discussion on how this work extends beyond the existing literature could be more comprehensive. Specifically, a detailed comparison with previous works that do not consider signal-dependent noise would help in highlighting the novelty and practical significance of this study.
4- I noticed several typographical and grammatical errors throughout the paper. A thorough proofreading is needed to enhance the readability and professionalism of the document.
5- Some references listed in the paper are incomplete. It is crucial to ensure that all references are fully detailed and formatted consistently, adhering to the journal’s guidelines. This will enhance the credibility and scholarly utility of the paper.
Comments on the Quality of English LanguageI noticed several typographical and grammatical errors throughout the paper. A thorough proofreading is needed to enhance the readability and professionalism of the document.
Author Response
- RESPONSES TO REVIEWER II
Comment 1: The introduction should include more related works from the literature, explaining the main issues addressed by each work. This would provide a clearer background and highlight the significance of the current study within the broader research landscape.
Response 1: Thank you for your insightful feedback. We revised the introduction to include additional relevant literature, summarizing the main issues addressed by each study. This enhances the background context and better highlight the significance of our current research within the broader landscape. The reviewer may refer to the highlighted texts in the introduction section on pages 1 and 2.
Comment 2: The paper exclusively focuses on the spatial modulation (SM) approach. Considering more sophisticated approaches with enhanced spectral efficiency, such as Generalized Spatial Modulation (GSM) or Quadrature Spatial Modulation (QSM), could potentially offer greater insights and benefits. The authors might discuss why SM was chosen over these alternatives.
Response 2: Thank the reviewer for the comment. We acknowledge the potential benefits of exploring more advanced techniques like Generalized Spatial Modulation (GSM) and Quadrature Spatial Modulation (QSM) for enhanced spectral efficiency. In our future work, we plan to investigate these sophisticated approaches to gain deeper insights into their advantages and benefits compared to SM. Additionally, we will provide a more comprehensive analysis of the selected modulation technique. We have added a paragraph at the end of the conclusion as follows:
In the future work, we plan to delve into the analysis of the secrecy rate within VLC systems, utilizing advanced SM techniques such as Generalized Spatial Modulation (GSM) and Quadrature Spatial Modulation (QSM). These methodologies will be scrutinized in terms of their efficacy in enhancing the secrecy rate, juxtaposed against the conventional SM approach.
Comment 3: The discussion on how this work extends beyond the existing literature could be more comprehensive. Specifically, a detailed comparison with previous works that do not consider signal-dependent noise would help in highlighting the novelty and practical significance of this study.
Response 3: We appreciate your valuable feedback. In response to your suggestion, we have added the discussion section to provide a more detailed comparison with previous works. Specifically, we now highlight how most prior studies ([11]-[43]) assumed signal-independent noise, which is less practical for VLC systems, while our work addresses the more realistic scenario of signal-dependent noise. We have also emphasized the novelty of our contributions by incorporating transmitter selection techniques (US, CAS, GS) and optical jamming under signal-dependent noise, which were not extensively explored in earlier research, particularly in the context of SM-based VLC systems. These additions underscore the practical significance of our study, especially in enhancing secrecy under challenging noise conditions. Also, we have added Table IV that provides a comparison of the recent works. The reviewer may refer to Section IX “Discussion” on page 20.
Comment 4: I noticed several typographical and grammatical errors throughout the paper. A thorough proofreading is needed to enhance the readability and professionalism of the document.
Response 4: Thank you for your valuable feedback. We thoroughly proofread the paper to improve its readability and professionalism. Your comments are much appreciated.
Comment 5: Some references listed in the paper are incomplete. It is crucial to ensure that all references are fully detailed and formatted consistently, adhering to the journal’s guidelines. This will enhance the credibility and scholarly utility of the paper.
Response 5: Thank you for pointing out the issues with the references. We have carefully reviewed and revised all references to ensure they are complete and formatted consistently according to the journal's guidelines. We appreciate your feedback, as it helps enhance the credibility and scholarly utility of our paper.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Comments and Suggestions for AuthorsThis paper is well revised. No further comments.
Author Response
Comment: This paper is well revised. No further comments.
Thank you for your positive feedback. I'm glad the revisions meet your expectations. I appreciate your time and consideration throughout the review process.
Reviewer 2 Report
Comments and Suggestions for AuthorsMy comments were addressed well, I am satisfied with the current version.
Author Response
Comment: My comments were addressed well, I am satisfied with the current version.
Response: Thank you for your thoughtful review and for acknowledging the revisions. I'm pleased that the current version meets your satisfaction.
