Secure Transmission for RIS-Assisted Downlink Hybrid FSO/RF SAGIN: Sum Secrecy Rate Maximization
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThe manuscript addresses a critical topic in secure communications within space-air-ground integrated networks (SAGIN) by proposing a novel RIS-assisted hybrid FSO/RF framework. While the work presents a solid contribution, I have identified several aspects that could benefit further clarification, analysis, and enhancement, as mentioned below.
1. The paper proposes a novel RIS-assisted hybrid FSO/RF SAGIN architecture. To further highlight the novelty of their work, could the authors provide a comparative analysis with existing architectures?
2. The manuscript mentions adopting the Málaga fading and Nakagami-m models. Are these models widely accepted in similar scenarios? If not, why were they chosen?
3. The proposed methodology relies on simulations. Are there plans for experimental validation in real-world scenarios? If so, what are the potential challenges?
4. The manuscript employs DDPG, SA, and SDP optimization methods. Could the authors provide insights into these algorithms' computational complexity and scalability to larger systems?
5. The paper focuses on enhancing the sum secrecy rate (SSR). However, it lacks a detailed analysis of the proposed system's performance under different eavesdropping scenarios. Could the authors address this aspect?
6. The introduction section needs to be revised according to the timeline of technology development. For example, these references with recent papers such as doi: 10.1109/TIM.2024.3406807, doi: 10.1177/15501329221104330, doi: 10.1109/TWC.2024.3480353,
doi: 10.1109/TITS.2022.3198046.
7. The cooling rates in the SA algorithm significantly influence the results (Figure 5). What criteria were used to select the final cooling rate? Would the performance hold for other parameter settings?
8. Does the proposed framework generalize to other SAGIN configurations, such as multi-drone systems or higher altitudes for the HAP?
9. Some terms, like RSMA and SAGIN, are used frequently. A brief introduction in the abstract or early in the introduction could help readers unfamiliar with the terminology.
10. It would be useful to specify why some parameters were chosen, such as the satellite's transmit power and the drone's flight speed, in Table 1.
11. The related works section is comprehensive but could benefit from including a summary table contrasting the proposed work with recent advancements.
12. The conclusion briefly mentions the potential for system enhancement. Could the authors elaborate on specific future directions or extensions of this work?
Comments for author File: 
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Author Response
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Author Response File: Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsThe manuscript addresses an alternative optimization approach for improving the PLS of RIS-Assisted Downlink Hybrid FSO/RF SAGIN. The work is well written, however, I have the following concerns:
1- Regarding the proposed system model under consideration, I can not see a use case and justification for the complicated multi-hop system model. Can the HAP or the UAV alone serve the users?
2- The function k_{\alpha,i} is not defined in equation (4)
3- The difference between Rk and Ck is not clear on page 7
4-Justify the addition of Ck in eq(18), while it is not part of the secure rate difference in the second term
5- Justify constraint 19b, why the summation should b less than the min
6- The complexity of the proposed alternating optimization algorithm with the three internal steps seems very high and not practical. provide a complexity analysis, please
7- How do you compare the PLS of the three systems in Fig (4), how can this be a fair comparison? no information is provided
Author Response
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Author Response File: Author Response.pdf
Reviewer 3 Report
Comments and Suggestions for AuthorsThis paper proposed a novel RIS-assisted downlink hybrid FSO/RF SAGIN, where the HAP and drone equipped with RIS dynamically adjust the signal propagation path, effectively reducing signal attenuation and enhancing RF link transmission. In addition, the authors adopted the Málaga fading and Nakagami-m models to model the transmission characteristics of FSO and RF channels under various weather conditions. Then, an alternating iterative framework is proposed for a joint solution using the simulated annealing algorithm and the designed deep deterministic policy gradient (DDPG) algorithm. The simulation results show that the proposed scheme can enhance security performance.
The article is organized in a clear and well-structured manner. The objectives of the study are thoroughly explained but the benefits of this system, at the level of implementation in the civil and perhaps even military field, in particular, are not sufficiently addressed. I mean usage scenarios, types of missions (if it is about the military field). Please consider adding this aspect to the article. Additionally, the article is highly relevant to the specified topic.
The references cited from the last 5 years account for 85% (3 out of 20), which is a very good proportion of recent citations. No excessive citations from a single author were identified, which is a positive aspect.
The article presents interesting scientific insights, and the results are original. Also, the results presented in the manuscript can be reproduced based on the details provided in the methods section.
The figures, tables, images, and diagrams are clear, easy to interpret, and align with the content described in the article.
The conclusions drawn are consistent with the results, but they are brief. Expanding on them with more detail would be beneficial.
The ethics and CoI statements are included at the end of the article and are appropriately addressed.
Author Response
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Author Response File: Author Response.pdf
Reviewer 4 Report
Comments and Suggestions for AuthorsPlease, see the attached file.
Comments for author File: Comments.pdf
Author Response
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Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
Comments and Suggestions for AuthorsThe authors have addressed my concerns. I have no further comments
Reviewer 3 Report
Comments and Suggestions for AuthorsThis paper proposes a novel reconfigurable intelligent surface (RIS)-assisted downlink hybrid free-space optics (FSO)/ radio frequency (RF) space-air-ground integrated network (SA GIN) architecture, where the high altitude platform (HAP) converts the optical signal sent by the satellite into an electrical signal through optoelectronic conversion. The simulation results show that the proposed scheme can significantly enhance security performance of the proposed SSR maximization. The paper formulated a sum secrecy rate maximization problem involving three variables: power allocation, RIS phase shift, and drone trajectory.
As I specified in the previous report, I consider that the article is well structured, the objectives and benefits of the study are thoroughly explained and the content is highly relevant to the specified topic. Additionally, article presents interesting scientific insights, like the transmission
characteristics of FSO and RF channels under various weather conditions. Also, the results are original and can be reproduced based on the details provided in the methods section.
The references cited from the last 5 years account for 90% (26 out of 29), which represents a very good proportion of recent citations. No excessive citations from a single author were identified, which is a positive aspect.
The figures, tables, images, and diagrams are clear, easy to interpret, and align with the content described in the article.
The conclusions drawn are consistent with the results and the ethics and data availability statements are included at the end of the article and are appropriately addressed.
