Radio Front-End for Frequency Agile Microwave Photonic Radars
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThe manuscript presents a system design for RF frontend applications in microwave photonic links. Its basic design objectives are a more economical and effective solution, ensuring that NF, phase noise reduction, and IMD are kept as low as possible while maintaining the unconditional stability of the amplifier chain. While the experimental work demonstrates some practical applications, there are several major concerns that need to be addressed. Besides, the current version is not recommended for publication in journals.
1. The simulation approach using two lasers to emulate MZM intensity modulation in Figure 4 is problematic. This method requires phase-locking between the two laser outputs to generate reliable beat frequency. However, the manuscript fails to describe or justify how phase synchronization is achieved. Without proper phase-locking, the validity of this simulation method is questionable.
2. The authors claim their system is suitable for RF frontend in microwave photonic links. However, the demonstrated signal processing bandwidth of only several hundred MHz is insufficient for typical microwave photonic applications. This significant limitation is not adequately addressed in the manuscript. A clear explanation of this bandwidth constraint and its implications for practical applications should be provided.
3. The experimental section lacks critical performance metrics. Notably, the complete S21 response curve of the RF amplification link is missing, which is essential for evaluating system performance. While the antenna testing results are well-presented, they appear tangential to the main objectives of the paper.
4. The Doppler frequency measurement of simulated aircraft propeller blades in Section 4.4 validates basic system functionality, but this single application case is insufficient to demonstrate comprehensive system capabilities.
5. While the system may solve certain engineering challenges, the manuscript lacks substantial scientific novelty. The theoretical foundation and technical innovation need significant strengthening.
Author Response
Dear Reviewer,
We are grateful for your careful review and helpful feedback! Please refer to the attached PDF document for our point-by-point response to your comments.
Please see the attachment.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsDear Authors, Dear Editor,
the paper "Radio Frontends for Frequency Agile Microwave Photonic Radars" submitted by Aljaž Blatnik, Luka Zmrzlak, and Boštjan Batagelj covers a nice research about a combined dual frequency microwave/photonic radars. Due to the actual industrial interest and growing demands for robotics, drones, 6G, V2V, V2X communication systems, etc., and respective anti-collision systems, I find the topic of the research very important and actual. The paper presents system considerations and demonstrates experimental verification. The results are nicely presented, and evaluated. The strengths of the paper are the combination of microwave and PIC technology and the author's own work in circuit development and testing.
I have only a few minor comments for consideration by the authors.
There is one mistake in the manuscript, that should be corrected. Figure 9 plots S11 but figure caption "Comparison of simulated and measured antenna input impedance (S11 parameter)" refers to input impedance. Please note that input impedance is not S11. S11 is rather reflection coefficient or return loss (with proper sign).
"in the optical as well as radio-frequency domain" - consider domains in plural as optical and RF are together 2 domains.
Figure 1 a.) I recommend to add some arrows to show better the TX and RX directions. I attach a highlighted PDF to this review... hoping the arrows added are visible.
Line 116-117: automotive avoiding systems.... maybe word 'collision' is missing. We do not avoid automotive, we avoid their collisions by radars.
Line 153 and other places: LNA amplifier.... sounds 'LN amplifier amplifier'. Like after FET, we do not write FET transistor as T already means transistor. Similarly for LNA. Simply LNA (or low noise amplifier) recommended.
Figure 3: TX and RX direction arrows recommended in the figure. Also I have a question regarding the RX branch. Usually BPF is also required. See e.g., Fig.1 a.) where 2 BPFs are inserted in the RX chain. Is BPF missing or there was no BPF applied? I suppose RX BPF is needed to reduce unwanted interference from other frequencies.
Some acronyms are not resolved. E.g., UAV, ISM. I suppose they stand for unmanned aerial vehicle and industrial, scientific and medical. However, I still recommend to follow the journal style, and preferably explain all acronyms at the place of their first appearence.
Front-end is mostly written as 'front-end' in the paper, however 5 times it is written as frontend (also in the title). Uniform writing is recommended.
Paper mentions transimpedance amplifiers, that are used in optical recieivers. I recommend adding a reference here, as they are different from the usual microwave amplifiers, that have 50 Ohm impedance at both their ports, and that are discussed in the rest of the paper.
Somewhere, e.g., in the introduction, it would be useful to explain the start/stop frequencies of the S-band and the X-band.
Paper mentions antenna directivity multiple times. As the substrate (Duroid) is given, that definitively has some loss. Maybe using Gain would be more accurate or more proper term. Is the simulation tool calculating Gain or Directivity for the microstrip patch antenna?
All my other comments are minor, the paper is 'nearly publish as it is'... So I let the Authors and the Scientific Editor to consider and either accept my recommendations or not. I personally do not request a second review of the paper. I hope the highligts and pop-up notes are visible in the PDF attached.
Best Regards,
the reviewer, 2024-10-23.
Author Response
Dear Reviewer,
We are grateful for your careful review and helpful feedback! Please refer to the attached PDF document for our point-by-point response to your comments.
Please see the attachment.
Author Response File: Author Response.pdf
Reviewer 3 Report
Comments and Suggestions for AuthorsThe authors should consider the following suggestions:
1. Remove random citation references such as [3-5], [6-8], and [8, 16, 22, 23, 25, 32] to maintain clarity and coherence in the text.
2. Include a literature survey to provide context and background for the research presented.
3. The authors should explicitly compare their proposed work with existing research to highlight the novelty of their approach.
4. Clearly outline the main contributions of the work in a separate section.
5. Add a performance comparison table to illustrate how the proposed work compares with existing research.
6. Create a separate column for conclusions, future research directions, and discussions of the results.
7. What future research directions do the authors envision following this study?
8. Present the measured and simulated results with greater technical clarity and explanation.
9. How does the proposed photonics-based radar system compare with traditional RF radar systems in terms of bandwidth, phase noise, and operational frequency range? What are the advantages and disadvantages of each approach?
Comments on the Quality of English LanguageN/A
Author Response
Dear Reviewer,
We are grateful for your careful review and helpful feedback! Please refer to the attached PDF document for our point-by-point response to your comments.
Please see the attachment.
Author Response File: Author Response.pdf
Reviewer 4 Report
Comments and Suggestions for AuthorsI recommend this manuscript for publication after minor revisions.
Comments for author File: Comments.pdf
Author Response
Dear Reviewer,
We are grateful for your careful review and helpful feedback! Please refer to the attached PDF document for our point-by-point response to your comments.
Please see the attachment.
Author Response File: Author Response.pdf
Reviewer 5 Report
Comments and Suggestions for Authors1. There are no quantitative results reported in the abstract.
2. Figure 3, the input power -70 dBm is relatively high in the link budget. Why the sensitivity requirement is not high?
3. There is not mathematical derivation for the design, such the relationship of the noise in photonics devices versus conventional circuit.
4. There is also no comparison with existing work.
5. The illustration of the measurement setups are good references. There is also lacking mathematics and measurement theories supporting the relational of the results.
Author Response
Dear Reviewer,
We are grateful for your careful review and helpful feedback! Please refer to the attached PDF document for our point-by-point response to your comments.
Please see the attachment.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Comments and Suggestions for AuthorsAfter two rounds of revisions, the manuscript has reached publishable quality. The authors should revise the grammar and details in the manuscript. For example, the abbreviation "TX" has slightly different meanings in lines 190 and 241.
Reviewer 3 Report
Comments and Suggestions for AuthorsThe author has incorporated all the questions well in the revised manuscript.
