Review Reports - High-Precision Localization of Passive Intermodulation Source in Radio Frequency Transmission Lines Based on Dual-Frequency Signals

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Zhang et al demonstrated a passive intermodulation localization scheme through dual-frequency signals. The principles are analyzed and the method is verified by experiment. It is a well written paper with clear theoretical derivations, and claims well supported by measurements. I recommend acceptance after addressing my following concerns.

The authors have shown that the positioning accuracy for their dual frequency method will decrease drastically when the distance of the PIM source along the transmission line is longer than the corresponding wavelength of the high-frequency signal. For practical applications, the distance between the PIM source and the detection port is unknown. Could the authors propose an operation procedure to locate the position of a PIM source under this scenario? Shall one tune the frequencies for both the low-frequency and high-frequency signals, and look for the measurement with the smallest error?

The experiment demonstrates good accuracy in single-source PIM detection. How does its performance translate to multi-source PIM scenarios? How does it compare to established methods like TDR in double or triple source?

Author Response

Thank you for your comments and suggestions.

If the distance of the PIM source along the transmission line exceeds the half-wavelength of the low-frequency signal, the position of the PIM source obtained by this method is invalid, mainly due to phase blurring. To ensure that the measurement results are valid, it is necessary to ensure that the half-wavelength of the low-frequency signal is greater than the total length of the entire transmission line.

In addition, due to the excessive ranging error of the low-frequency signal, when the error is larger than the half-wavelength of the high-frequency signal, it will decrease the accuracy of the ranging results. The solution given in this paper is to determine the frequency of the high-frequency signal based on both the ranging results of the low-frequency signal and the size of the ranging error of the low-frequency signal.

However, this method cannot localize and measure multiple PIM sources. Based on the literature search conducted, there is currently no evidence of the TDR technique being used for PIM localization. Although TDR is used in some companies for this purpose, its exact performance is unknown. It is safe to assume that the hardware requirements of the method used in this article are much lower than those of TDR.

Reviewer 2 Report

Comments and Suggestions for Authors

This paper discusses the growing issue of passive intermodulation interference in communication systems due to advancements in communication technology. The proposed solution involves a dual-frequency signal localization method based on phase ranging principles to accurately identify and locate passive intermodulation sources in RF cables. The method switches the test signal frequency to analyze the phase of passive intermodulation signals at different frequencies, enabling precise localization. The paper outlines the principles, establishes an experimental localization system, and suggests optimization strategies based on error analysis. Experimental verification demonstrates the method's stability in localizing passive intermodulation errors in RF transmission lines, particularly in communication scenarios like satellite and base stations. The proposed approach offers a new technical solution for addressing passive intermodulation interference sources in 5G and 6G communication systems.

The paper is interesting. However, some clarifications are necessary

1. In this work the authors focused solely on the summation component. Why did you neglect the difference component?

2. Line 129: signals instead of signal

3. Both Dj and Dj' are considered as deterministic quantities. Why deterministic when both variables are created under random processes?

4. Table 2 and Table 3: I suggest to add a new column for the relative error.

5. References must be updated and enriched.

Author Response

Thank you for your comments and suggestions.

In this paper, we use the second-order PIM difference frequency signal as a feature signal to localize the PIM source, which has the following main advantages:

1) The f1-f2 frequency is smaller than f1+f2, which has low requirements for signal sampling devices.

2) Low differential frequency signal can be measured over a longer range.

Thank you for carefully reviewing this article. I have changed the signal to signals in line 129.

For question 3, I'm sorry I didn't find the variable you mentioned in the article. The only variables that can be directly determined when the system is working are the propagation speed of the signal and the order of the intermodulation signal. The phase difference of the signal has to be measured with a phase detection module, and the signal's frequency has to be adjusted according to the length of the cable to be measured and the phase detection error.

Relative error terms have been added to Tables 2 and 3. Thank you for your suggestions.

There is very little research on PIM source localization techniques in RF transmission lines in published academic sources, and the references cited herein have largely encompassed the current literature on PIM source localization in RF transmission lines.

Reviewer 3 Report

Comments and Suggestions for Authors

This paper proposes a dual-frequency signal localization method based on the principle of phase ranging to locate the passive intermodulation sources in RF cables.

1. The novelty of the paper is extremely limited.

2. Please rewrite the paper abstract to be more directive and to contain work results.

3. The paper lacks clarity in explaining how exactly the proposed method is applied to localize passive intermodulation sources in RF cables.

4. The study lacks a discussion on the limitations and potential challenges of the proposed method.

5. In line 261, the authors wrote “this system can only localize a single PIM source” and this a big limitation for the proposed method.

6. There is a lack of discussion on the potential trade-offs between localization accuracy and computational complexity.

7. In table 2, for the 4.5 m case, how the location results to be 4.513 m?

8. The comparison with existing methods lacks depth and specificity. Merely stating average error without providing insights into the underlying factors or limitations of the compared methods undermines the credibility of the evaluation.

9. The paper does not seem to offer a comprehensive comparison with existing baselines which is crucial to establish the proposed scheme's superiority or uniqueness.

10. English proofreading is necessary.

11. The paper conclusion is not written in a good way. Please enhance the conclusion by incorporating a quantitative summary of the results for a more comprehensive understanding.

Comments on the Quality of English Language

English proofreading is necessary.

Author Response

Thank you for your criticisms and suggestions. We have revised the manuscript and responded to your questions as follows.

1. This paper proposes a ranging technique based on dual-frequency signals based on the phase method of ranging, which significantly improves the positioning accuracy of the PIM source. The positioning system based on this method has the advantages of low hardware cost, high efficiency, and high positioning accuracy.

2. The paper's abstract has been revised; thanks for your suggestions.

3、This paper introduces the basic principle of the dual-frequency signal ranging method and verifies the correctness of the process through experiments. The derivation and explanation of the specific method are shown in the article "Fundamentals and Methodology" and "Experimental."

4. In the "Conclusions" section of the article, the limitations of the method and the next step to be studied are given. It is improved on the original basis of the manuscript.

5. Yes, the limitation of this method is that only single PIM sources can be localized.

6. As discussed in the new manuscript, There is not much connection between localization accuracy and computational complexity.

7, In Table 2, when the distance of the PIM source from the input port of the transmission line is 4.5 m, the result measured by the localization system is 4.513 m, with an error of 0.013 m. We carefully checked the calculation process and confirmed that there was no error.

8. In this work, a large amount of data is obtained through experimental testing to ensure the stability and accuracy of the positioning system with high credibility.

9. In the "Conclusions" part of the article, the method is compared with the existing positioning techniques, and the advantages and disadvantages of each method are analyzed in detail.

10. The English presentation of the manuscript has been revised and checked, and we thank you again for your suggestions.

11. The paper's conclusion was rewritten; thank you for your suggestions.

Reviewer 4 Report

Comments and Suggestions for Authors

1. The authors propose a dual-frequency signal localization method based on the principle of phase ranging to locate the passive intermodulation sources in RF cables accurately. This method switches the test signal frequency to obtain the phase of passive intermodulation signals at different frequencies to localize the passive intermodulation source.

2. Please compare the contributions of the proposed method to related technologies, in detail.

3. In the figure 4, principle of low-frequency signal ranging should be demonstrated in detail.

4. In the figure 5, principle of high-frequency signal ranging should be demonstrated in detail.

5. Please thoroughly revise the language before your submission.

Comments on the Quality of English Language

Extensive editing of English language required

Author Response

Thank you for the reminder that the contribution of this method to related technologies is complemented in the conclusion section of the article. Regarding the description of Figures 4 and 5, which can be confusing, this article provides an additional description of Figures 4 and 5. In addition, there may have been some incorrect representations in the article, which have been corrected.

Reviewer 5 Report

Comments and Suggestions for Authors

A nice and well-written paper in my opinion - however the localization method proposed specifically applies to transmission lines or waveguides, not to other systems that may be the source of PIMs, like antennas. I think this should be reflected in the title itself, something like "High-Precision Localization of Passive Intermodulation Sources in Transmission lines and Waveguides Based on Dual-Frequency Signals" (just a suggestion). Also in the introduction it would be nice to have some more references to the main PIM culprits both in cables / waveguide and other parts (like antennas, both for the presence of parasitic rectifying contact or by material nonlinearity issues, like in carbon fibers).

Minor points:

- the sentence "However, the 82 above two localization methods can only be used to localize PIM sources in open spaces 83 and cannot localize devices in closed spaces [10]." is not clear to me, please specify what you mean by closed and open spaces...

- in the sentence "The wave speed of the cable used in this paper is 2.45E+08m/s, and the 185 wave speeds in all the experiments in Part III are 2.45E+08m/s" please use 10^ in the standard scientific notation

Author Response

Thank you for your comments and suggestions.

The method proposed in this paper focuses on localizing PIM sources present in RF transmission lines, but it can also be used to localize PIM sources present in waveguide devices. This idea is also expressed in the abstract and introduction section of the article, and in order to make the title of the article concise and easy to understand, the title of the article is changed to "High-Precision Localization of Passive Intermodulation Sources in Transmission Lines Based on Dual-Frequency Signals".

In addition, the reason for not introducing much about PIM in the introduction is that the research goal of this paper is not the generation mechanism of PIM but the localization method of PIM sources.

Open space refers to the problem of PIM interference caused by external space in wireless communication systems. Closed space refers to the PIM interference caused inside the RF transmission line or waveguide device. I apologize for not clarifying this; additional notes on this section have been added to the article. Thank you for the reminder.

The error about scientific counting has been corrected in the article; thank you for your valuable suggestion; it is important to us.

Round 2

Reviewer 3 Report

Comments and Suggestions for Authors

Several comments have been addressed.

References list not sufficient, please update the paper references, 2018 and above.

Comments on the Quality of English Language

Minor editing of English language required.

Author Response

Thank you for your suggestions and comments. We have supplemented our new manuscript with references from 2018, mostly related to the mechanism of PIIM generation. In addition, we have revised the language of the manuscript.