Frequency-Modulated Antipodal Chaos Shift Keying Chaotic Communication on Field Program Gate Array: Prototype Design and Performance Insights
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThe manuscript reports their new progress on prototype design and performance of FM-ACSK chaotic communication on FPGA. The analyzation of the performance on the critical parameters is comprehensive. The manuscript can be accepted directly after minor revisions.
1. The “Introduction” part is too long and does not quickly get to the point. Please simplify the content for better understanding.
2. In the Transmitter, the “R_out” depends on the Transmitted data bit value, which like a switcher. How fast is the switching speed? Is there a tiny delay on the switching and could influence the system performance after the system running for a long time?
Author Response
Thank You for taking Your time to review our manuscript and provide a valuable feedback.
All revisions have been highlighted using “Track Changes” feature.
Regarding your comments and suggestions:
Comment 1: The “Introduction” part is too long and does not quickly get to the point. Please simplify the content for better understanding.
Response 1: The introduction section was revised and slightly simplified and shortened, where we saw it possible. Some new text in the introduction was necessary to be added as well, to address comments from other reviewers and improve the overall quality of the paper.
Comment 2: In the Transmitter, the “R_out” depends on the Transmitted data bit value, which like a switcher. How fast is the switching speed? Is there a tiny delay on the switching and could influence the system performance after the system running for a long time?
Response 2: To be more exact, it is not the “R_out” signal (which is the output of chaos generator), but ACSK modulator output signal that depends on the transmitted data bit value. A switch unit consists of a 14-bit 2x1 multiplexer, and one 14-bit register after it, which defines the overall switching delay to 1 FPGA clock cycle (20ns at 50MHz). This cannot change over a long time, nor can it influence system performance. The signal processing sequence is defined by the initial structure of FPGA design, and it stays intact as long as system operates. We added small comments to the system’s general description to clarify the structure of the switch.
Reviewer 2 Report
Comments and Suggestions for AuthorsI would like to thank the authors for their efforts in producing this work. The paper reads well and brings something new to the field of research. The paper successfully highlights the need for secure communication systems leveraging chaotic signals. However, it would be beneficial to clarify how the proposed FM-ACSK system specifically addresses gaps in existing chaotic communication systems beyond. However, I have some comments and questions:
- The FPGA implementation and performance evaluation are detailed, but the irregularities in the BER curve require more thorough discussion, and comparaison with other chaotic and non-chaotic secure communication systems in terms of security, complexity, and robustness.
- The mathematical representation and FPGA implementation details are sufficiently described. However, the trade-offs in system design, such as resource usage versus precision, could be further analyzed.
- What motivated the use of the modified Chua circuit as the chaotic generator? Were other chaotic systems considered, and how does this choice affect system performance?
- How would this system handle real-world security challenges such as deliberate jamming
- Please discuss the limitations of the FM-ACSK system
- The choice of an intermediate frequency of 10.7 MHz seems appropriate for testing, but limits generalizability. How could you explain this?
Thank you for this high quality of your paper.
Author Response
Thank You for taking Your time to review our manuscript and provide a valuable feedback.
All revisions have been highlighted using “Track Changes” feature.
Regarding your comments and suggestions:
Comment 1: However, it would be beneficial to clarify how the proposed FM-ACSK system specifically addresses gaps in existing chaotic communication systems beyond.
Response 1: How the proposed FM-ACSK system specifically addresses gaps in existing chaotic communication systems is a difficult question. Some points that can be mentioned include: usage of FPGA in general, which addresses limitations of the analog circuit approach; an essentially novel approach in the design with the use of FPGA in chaotic communication (which is explained in details), that advances this study area; and the use of FM, which adds robustness against frequency offsets and ensures constant power envelope, addressing problems that are present when using pure chaotic signals as a carriers. Also, we demonstrate a novel approach in the design and FPGA implementation of the CSK-based chaotic coherent communication system, which is further explained in the revised Introduction section. This question is also addressed in the new Discussion section.
Comment 2: The FPGA implementation and performance evaluation are detailed, but the irregularities in the BER curve require more thorough discussion, and comparaison with other chaotic and non-chaotic secure communication systems in terms of security, complexity, and robustness.
Response 2: Irregularities in the BER curve have been addressed more in subsection 3.2. Brief discussion about design, performance and security aspects is added to the in the new Discussion section.
Comment 3: The mathematical representation and FPGA implementation details are sufficiently described. However, the trade-offs in system design, such as resource usage versus precision, could be further analyzed.
Response 3: In the end of subsection 3.1 we discussed some more about resource usage and related trade-offs.
Comment 4: What motivated the use of the modified Chua circuit as the chaotic generator? Were other chaotic systems considered, and how does this choice affect system performance?
Response 4: This specific modified Chua circuit chaos generator is used as a core element of the designed communication system prototype for a few reasons. Firstly, its properties and chaotic synchronization qualities are well studied in our previous works, and effectively adapted to the functionality of the designed communication system, which let us build new prototype of the FM-ACSK system on the foundation of older ACSK system, as a next advancement step in refining the technology. Secondly, such general characteristics of the output chaotic signal as relatively large bandwidth (about 215 kHz) satisfy basic requirements for spread-spectrum chaotic communications (which is spread further using FM), which makes this chaos generator suitable for this role. And thirdly, while it was desirable to use a relatively simple chaos generator in the initial prototype design, usage of well-known and widely studied chaos generators would be less interesting from the research novelty aspects, than usage of this specific modified Chua circuit chaos generator, which is rarely used in other studies and chaotic communication systems.
Without doubt, the choice of the chaos generator can significantly affect system’s performance. There might exist chaos generators that would allow the designed FM-ACSK chaotic communication system to perform better, but such generators are not yet identified within the scope of the prior studies. It is one of the goals set for future research steps.
We also added some comments to the introduction of chaos generator in paper, with a reference to our study about it.
Comment 5: How would this system handle real-world security challenges such as deliberate jamming
Response 5: If we consider such security challenge as deliberate jamming, then most efficient way to jam FM-ACSK signal would be by utilizing the capture effect of the frequency demodulator, which would require transmitting same carrier with more power. It’s a well-known quality of any FM-based communication. How much more power jamming carrier needs to have compared to the FM-ACSK power level, is something that we consider testing in future studies. Other types of jamming, including flat noise over the signal band, would be less efficient, considering the wide bandwidth. It should also be noted that jamming effectiveness would also depend on the design of the radio part of the communication system, which was not part of the designed prototype. Comment about this was added to the new Discussion section.
Comment 6: Please discuss the limitations of the FM-ACSK system
Response 6: Some notable limitations are mentioned in the new Discussion section.
Comment 7: The choice of an intermediate frequency of 10.7 MHz seems appropriate for testing, but limits generalizability. How could you explain this?
Response 7: 10.7 MHz is the most commonly used intermediate frequency for conventional FM broadcasting systems, therefore this approach could allow adaptation of some existing designs or hardware modules for further frequency scaling and radio transmission tests. Additionally, this frequency is in a range that is well suited for generation on FPGA with 50MHz clock, as it is close to the half of the Nyquist frequency. Generalizability could be more necessary at future research stages, where, for example, radio transmission within common ISM bands will take place. We added notes about frequency choice to the section that describes FM.
Reviewer 3 Report
Comments and Suggestions for AuthorsThe article is devoted to developing tools for chaotic communications channels.
Unfortunately, the article does not comprise a complete study: it only presents an architecture and one experiment measuring that the device can be used. That could have been acceptable if the article was the first in the field, but the review of related works shows that there are other implementations that can be compared to the authors' work.
The chief problems of the article that needs addressing are as follows:
1. While introduction describes in vague words that the goal of this study is having a more secure communication channel, this parameter is not quantified (that is, no method of evaluating the "security" of a communication channel was described), which makes it impossible to determine if the proposed architecture makes anything better for security or not. Please, formulate the key efficiency indicators - the ways of measure what you're trying to achieve - so that your results can be measured and compared with the results of state-of-the-art systems.
2. Similarly, when you review related work regarding implementation of chaotic communication channels, you should point to the disadvantages of existing approaches, which you want to make better, and to ways of measuring the relevant parameters. For now, it is clear that you propose a new architecture, but it isn't clear why you are doing it and what should become better compared to state-of-the-art systems.
3. The only measurement that was done was signal-to-noise ration. While necessary, it is not enough to make a valid research. To make it a full study, you need to experimentally compare your architecture with state-of-the-art implementations, to show which parameters became better and which became worse. This is the most significant problem with this study right now.
4. There is no Discussion section, comparing your findings with findings of other researchers.
5. The reference list contains only 18 sources, which is significantly below the average for Applied Sciences and other journal articles.
Author Response
Thank You for taking Your time to review our manuscript and provide a valuable feedback.
All revisions have been highlighted using “Track Changes” feature.
Regarding your comments and suggestions:
Comment 1: While introduction describes in vague words that the goal of this study is having a more secure communication channel, this parameter is not quantified (that is, no method of evaluating the "security" of a communication channel was described), which makes it impossible to determine if the proposed architecture makes anything better for security or not. Please, formulate the key efficiency indicators - the ways of measure what you're trying to achieve - so that your results can be measured and compared with the results of state-of-the-art systems.
Response 1: The goal of this study by no means is to have a more secure communication channel, but rather provide a conceptual design example of a communication system that is inherently more secure on the physical layer than conventional communication systems, due to the chaotic modulation employment. Therefore, the enhanced security aspect of proposed system is a desirable feature that can be potentially used, studied and adapted, but it is not a goal of present study. Comparison of the state-of-the-art systems for dynamical chaotic communication (when such systems don’t use chaos for data or image encryption directly but use chaotic modulation to transmit data) cannot be done using measurable indicators, as such information is almost never present in relevant studies. For such systems it is only possible to estimate general security aspects, judging from the system’s design structure and characteristics of chaotic signals used. While our system is designed to transmit digital information, and uses digital base for its implementation, the modulation and transmission are analog. There certainly may be methods to analyze system’s security in terms of vulnerabilities to eavesdropping, and to specific attacks, such as jamming, or man-in-the-middle, but that is a subject of a separate future study. These aspects were discussed in the Discussion section.
Comment 2: Similarly, when you review related work regarding implementation of chaotic communication channels, you should point to the disadvantages of existing approaches, which you want to make better, and to ways of measuring the relevant parameters. For now, it is clear that you propose a new architecture, but it isn't clear why you are doing it and what should become better compared to state-of-the-art systems.
Response 2: We pointed out in the introduction (and in the discussion section as well) some of the disadvantages in existing implementations and specified what we aim to achieve. We cannot point out a specific measurable parameter, as we are not, at this point, competing with other systems in terms of performance parameters, but rather introduce a new design that has features not present in other implementations, which may be useful in practical applications.
Comment 3: The only measurement that was done was signal-to-noise ration. While necessary, it is not enough to make a valid research. To make it a full study, you need to experimentally compare your architecture with state-of-the-art implementations, to show which parameters became better and which became worse. This is the most significant problem with this study right now.
Response 3: A brief comparison has been provided in the new Discussion section. It is not possible to fully compare communication system performance parameters with other state-of-the-art implementations at this research stage, as the proposed communication technology is still under development and research, and its unique design complicates comparison, even by such basic parameters as white noise performance, because it is not enough for understanding system’s quality. The practical quality would depend on how effectively a chaotic communication system can be integrated into a network solution to perform in a dedicated role. What we aim to show is a novel design approach and the communication system conceptual implementation, that can have various potential applications, if developed further. It is true that our study lacks throughout comparative analysis of performance, but it was not an aim of this study, and it is planned to be experimentally verified in more details in future studies.
Comment 4: There is no Discussion section, comparing your findings with findings of other researchers.
Response 4: The discussion section was added.
Comment 5: The reference list contains only 18 sources, which is significantly below the average for Applied Sciences and other journal articles.
Response 5: More of the relevant references were added.
Round 2
Reviewer 3 Report
Comments and Suggestions for AuthorsMany of the points raised in the previous review weren't addresed in the manuscript. In particular:
1. You write in response to my comment "we are not, at this point, competing with other systems in terms of performance parameters, but rather introduce a new design that has features not present in other implementations, which may be useful in practical applications." Then, at least, you can add a comparison table of features that will show features present in your implementation and other implementations and so let the readers compare them. That would greatly enhance the clarity of the article.
2. Even with those new features, you still need experiments demonstrating that your proposed system indeed has them. For now, a significant part of discussion is your speculations, not objective facts. Please, prove with data that your system has those new features after you summarize them in a table.
Author Response
Comment 1: You write in response to my comment "we are not, at this point, competing with other systems in terms of performance parameters, but rather introduce a new design that has features not present in other implementations, which may be useful in practical applications." Then, at least, you can add a comparison table of features that will show features present in your implementation and other implementations and so let the readers compare them. That would greatly enhance the clarity of the article.
Response 1: Thank you for the suggestion. A table with design feature comparison of the related systems was added to the introduction section.
Comment 2: Even with those new features, you still need experiments demonstrating that your proposed system indeed has them. For now, a significant part of discussion is your speculations, not objective facts. Please, prove with data that your system has those new features after you summarize them in a table.
Response 2: To clarify, by “features” in our comment we meant design features, which are already described in detail within the paper. We believe we have described the structure and working principles of the design features (such as FM, symbol timing recovery method, etc.) within the communication system prototype.
The discussion relies on some general facts about technologies involved, as well as on sources listed. However, part of discussion referred to other systems design features that were not explicitly listed in a comparative table, which, as you kindly suggested, was done in this revision, moving part of this comparison discussion to the introduction section for better consistency.
Round 3
Reviewer 3 Report
Comments and Suggestions for AuthorsThe article is as good as it can be at this stage of studying the proposed device. The further improvement would require making new experiments with the device.
