Review Reports

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The authors present a long-range distributed antenna method based on microwave-photonics for frequency synchronization. In the manuscript it is not evident what is the novelty that authors propose. Distributed antennas, microwave-photonic frequency dissemination, and distributed radar are all known. Review of existing research is thin.

Authors need to make a strong point on the novelty of their work and its contribution to the field. An experiment alone is not enough.

Further smaller comments:
1) Figures appear before referenced in text. This leads to the confusion. Figures should appear after being referenced in text.
2) In the Introduction coherent and full-coherent systems are mentioned. However, it is not specified what is the difference.
3) DCAR is mentioned without explanation in line 44.
4) The device structures in Figure 1 contain errors. The laser is missing? Is PFD connected to OCXO output or mixer output? 
5) In line 102 it is mentioned that the transmitting end generate dual-frequency signals? Nowhere else is it mentioned. How does it impact on the operation? Which frequencies?
6) Multiple time it mentions microwave-photonic PLL. However, it seems like only simple analog optic links are used for signal transfer and PLL is implemented in electronics only. Provide additional explanation if photonics are used more than just signal transfer over fiber.
7) From equations (5) and (6) it follows that the gain changes with the beam angle? How does this impact the beamstearing?
8) The point of distributed antenna is their spatial distribution. In the first experiment these are only 43.5cm apart and 40km of fiber are senseless. The anechoic chamber experiment is useful but not explained and lacks the motivation for it. This further degrades the value of results in Figure 5.
9) Degrees on x-axis of Figure 5abcd are ambiguous. They represent the beam deflection angle, and the parameter is the signal phase? The caption for Figure 5d is missing.
10) In line 282 says two frequency points, than in line 283 it gives three frequency points. Which are the ones used?
11) In the paragraph following equation (7) the phase shifts and beam deflection angles have the opposite signs as the corresponding Figure 5.
12) In line 325, it says "jitter is controlled within 1 dB". Jitter is measured in units of time.

The authors tackled the important and relevant research topic, however, the implementation and contribution of the work to the field is not obvious. This has to be improved to consider publication.

Author Response

Please see the attachment

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The paper presents a test of a coherent microwave beamforming system with two antennas synchronized via a fiber optic link. Two sets of experiments were conducted: one in anechoic chamber and the second in the field with 150 m distance between the antennas.

Coherent gain enhancement of 5.9 dB is reported.

Reported frequency stability of the fiber-optic synchronization link is 8.0 × 10−15 at 1 s and 2.0 × 10−16 at 1000 s.

 

However, the paper lacks vital information for evaluating the system design and performance.

 

1) The authors claim that the frequency stability of the fiber synchronization sub-system is a key factor in the whole setup. However, very little information is given about the system and verification of its performance.

The schemes presented in Fig. 1 are too general to represent the details of the experimental setup.

What types of critical components are used there: an optical source, an optical modulator, EDFA, etc.?

It is not even clear whether the fiber link is composed of two separate fibers (forward and backward) or a single bidirectional one.

 

2) The area of frequency distribution over fiber links has been developing for almost two decades.

What are specific new features of the developed system?

For example, frequency distribution with better stability of 5 × 10−15 at one second measurement time and 2 × 10−19 at 30 000 s in much longer (540 km) real fiber link was reported long before [O. Lopez, A. Haboucha, B. Chanteau et al. , "Ultra-stable long distance optical frequency distribution using the Internet fiber network," Opt. Express 20, 23518-23526 (2012)].

 

It is not clear how the performance of the system was tested. Apparently, all 40 km of fiber are on the spool (see photo in Fig. 4), which does not represent the noise and vibration environment of real fiber links between the antenna sites in large-scale coherent RF systems. 

 

In the field test only 150 meters of fiber are actually deployed in the filed. This provides some confirmation of the principle, but not true test of the system performance.

 

Overall, the paper can not be accepted for publication in its current form.

Author Response

Please see the attachment

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Authors addressed all the comments of my review with a clear answers. 

Only thing minor editorial revisions are required. For example Figure 5c includes typo MZN instead of MZM, same for Figure 5d where it also says PDRP instead of PDRO. In Figure 6b has µs written in italic, etc.

Content-wise I believe that the manuscript is suitable for publication.

Author Response

Dear Reviewer,

We are very grateful for your positive assessment of our revised manuscript (entropy-4500620), and for recommending it for publication pending minor editorial corrections. Your careful reading has caught several typos that we had missed.

We have now corrected all the editorial issues you identified, specifically:

Figure 5(c): the mislabelled "MZN" has been corrected to "MZM" (Mach–Zehnder modulator);
Figure 5(d): "PDRP" has been corrected to "PDRO" (phase-locked dielectric resonator oscillator);
Figure 6(b): the unit "µs" is now set upright (roman) rather than italic, consistent with SI unit formatting.
In addition, we have gone through all the remaining figure labels and axis annotations in the manuscript once more to check for similar inconsistencies.

We sincerely appreciate your time and the constructive guidance throughout the review process, which have made the manuscript considerably stronger.

Yours sincerely,

Bo Liu (On behalf of all the authors).

Reviewer 2 Report

Comments and Suggestions for Authors

The revised version can be accepted for publication. 

Author Response

Dear Reviewer,

We would like to express our sincere gratitude for your time and careful evaluation of our manuscript entitled "A Long-Range Distributed Antenna Method based on Microwave-Photonics Frequency Synchronization Technique" (entropy-4500620). 

We especially appreciate your detailed and constructive comments in the previous round, which guided us to strengthen the novelty statement, refine the experimental descriptions, and improve the clarity of the presentation. Your careful review has substantially improved the quality of this manuscript.

Thank you again for your professional guidance and for recommending our work for publication.

Yours sincerely,

Bo Liu ( On behalf of all the authors).