Photonic-Assisted E-Band Millimeter-Wave 1 × 2 MIMO Near-Sea-Surface Long-Distance Communication

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Comments on paper titled “Photonic-Assisted E-Band Millimeter-Wave 1×2 MIMO Near- 2 Sea-Surface Long-Distance Communication”:

This paper uses photon-assisted millimeter wave generation and MRC technology at the receiver to improve performance, achieving 5Gbaud QPSK 26km transmission. The results were verified through simulation and experiments. The results of the article are instructive. The following issues require improvement and supplementation.

• None of the experimental parameters, including those for the devices, modules, and equipment, were provided. In particular, parameters such as the output and bandwidth of the transmitter power amplifier, the transmitter antenna gain, the receiver antenna, and the low-noise amplifier gain.
• The receiver uses photonics for downconversion and coherent detection. This principle needs to be explained.
• Some of these are clearly excessive and should be deleted: 4. Discussion Authors should discuss the results and how they can be interpreted from the per- spective of previous studies and of the working hypotheses. The findings and their impli- cations should be discussed in the broadest context possible. Future research directions may also be highlighted.
• The constellations in Fig. 7 look good, how much is the BER?
• Authors claim that MRC achieves nearly 5 dB improvement. It can not find in the figures. Authors should add one figure show SNR improvement.
• Some related references should be added:

About MRC, these references should be cited:

Zhang Q T, Yu J J, Zhang J, et al. Exploiting polarization isolation for high diversity gain in a THz MIMO system. Sci China Inf Sci, 2025, 68(12): 220310, https://doi.org/10.1007/s11432-025-4605-2

Zhang, Q., Yu, J., Zhao, X., Li, W., Wei, Y., Yang, X., ... & Yu, J. (2024). 4.6-km D-band photonic-assisted terahertz wireless communication employing SIMO and MRC technology. IEEE Transactions on Microwave Theory and Techniques, 72(11), 6657-6668.

About long distance mm-wave or THz transmission, these references should be added:

Zhang, Q., Yu, J., Zhao, X. et al. 18 Gbps D-band wireless signal transmission over 30.2 km land and sea surface based on photonics aided technology. Sci. China Technol. Sci. 68, 2280901 (2025). https://doi.org/10.1007/s11431-025-3059-x

Yu, Jianjun, et al. "Broadband photon-assisted terahertz communication and sensing." Journal of Lightwave Technology 41.11 (2023): 3332-3349.

Li, W., Yu, J., Zhu, B., Wang, F., Ding, J., Zhang, J., ... & Yu, J. (2023). Photonic terahertz wireless communication: Towards the goal of high-speed kilometer-level transmission. Journal of Lightwave Technology, 42(3), 1159-1172.

About mm-wave generation based on photonics, this paper should be cited:

Li, L., Long, T., Yang, X. et al. Modified uni-travelling-carrier photodiodes with 206 GHz bandwidth and 0.81 A W−1 external responsivity. Nat. Photon. 19, 1301–1308 (2025). https://doi.org/10.1038/s41566-025-01784-0

About mm-wave generation by a phase or intensity modulator, this paper should be cited:

Yu, Jianjun, et al. "Optical Millimeter-Wave Generation or Up-Conversion Using External Modulators." IEEE PHOTONICS TECHNOLOGY LETTERS 18.1 (2006): 265.

 

Comments on the Quality of English Language

NA

Author Response

Response to Reviewer 1 Comments

1. Summary

We are extremely grateful for your insightful feedback and the effort taken to review our work. We have carefully addressed all the issues raised and revised the manuscript accordingly to improve the quality of our work. A specific point-by-point response to each comment is provided below, with all corresponding revisions have been highlighted in the re-submitted version.

2. Point-by-point response to Comments and Suggestions for Authors

Comments 1: None of the experimental parameters, including those for the devices, modules, and equipment, were provided. In particular, parameters such as the output and bandwidth of the transmitter power amplifier, the transmitter antenna gain, the receiver antenna, and the low-noise amplifier gain.

Response 1: Thank you for pointing this out. The output and bandwidth of the transmitter power amplifier is 26 dBm@P1dB and 15 GHz(from 71 GHz to 86 GHz) respectively, the transmitter antenna gain is 48 dBi, the receiver antenna gain is 45 dBi, and the low-noise amplifier gain is 23 dBi. We have added the above experimental parameters into the Section 3.1 and 3.2 in the revised manuscript.

Comments 2: The receiver uses photonics for downconversion and coherent detection. This principle needs to be explained.

Response 2: We sincerely thank you for pointing this out. We acknowledge that the original principle was not sufficiently detailed. We have added a new subsection, Section 2.1: The principle of photonic down-conversion and coherent detection, within “Section 2 Principle” of the revised manuscript. This new subsection provides a rigorous theoretical derivation for photonics-assisted down-conversion and coherent detection.

“2.1. The principle of photonic down-conversion and coherent detection

We take one path of the 1x2 MIMO system as an example, as the principles of photonic down-conversion for the two paths at the receiver are identical. We assume that the received millimeter-wave signal be expressed as:

     (1)

where represents the transmitted baseband complex signal, and denotes the carrier frequency of the millimeter-wave signal. At the receiver, a phase modulator (PM) is employed to modulate the millimeter-wave signal onto the optical carrier. The output of the PM can be given as:

(2)

where are the amplitude and frequency of the optical carrier at the receiver respectively, and is the half-wave voltage of the PM. Focusing on the -1st order sideband of the PM output, and according to the Jacobi-Anger expansion, it can be written as:

(3)

where . The LO light is given by:

(4)

where denote the amplitude and frequency of the LO at the receiver respectively. When is satisfied, homodyne coherent detection can be performed to recover the baseband signal.”

Comments 3: Some of these are clearly excessive and should be deleted: 4. Discussion Authors should discuss the results and how they can be interpreted from the perspective of previous studies and of the working hypotheses. The findings and their implications should be discussed in the broadest context possible. Future research directions may also be highlighted.

Response 3: Thank you for pointing this out. We apologize for the oversight. We have carefully reviewed the Discussion section and deleted the excessive content in the revised manuscript.

Comments 4: The constellations in Fig. 7 look good, how much is the BER?

Response 4: Thank you for the comment. We added the BER of channel1, channel2 and that after MRC into Section 4 in the revised manuscript. The descriptions are shown as follows.

The constellation diagrams of the two channels are shown in Figure 7 with 7 dB optical power attenuation after EDFA5, the BER of channel1 and channel2 is 2.1x10-2 and 1.9x10-2 respectively. After applying MRC, the constellation points become significantly more concentrated and the BER is reduced to 2.6x10-3, indicating improved SNR.

Comments 5: Authors claim that MRC achieves nearly 5 dB improvement. It can not find in the figures. Authors should add one figure show SNR improvement.

Response 5: Thank you for the comment. There is a mistake above the reference of BER to claim the improvement of MRC. It should be 1×10⁻². Therefore, at the target BER of 1×10⁻², MRC achieves about 4 dB improvement. We added the checked description about MRC improvement and an updated Figure 8 into the revised manuscript.

Comments 6: Some related references should be added:

About MRC, these references should be cited:

Zhang Q T, Yu J J, Zhang J, et al. Exploiting polarization isolation for high diversity gain in a THz MIMO system. Sci China Inf Sci, 2025, 68(12): 220310, https://doi.org/10.1007/s11432-025-4605-2

Zhang, Q., Yu, J., Zhao, X., Li, W., Wei, Y., Yang, X., ...&Yu, J. (2024). 4.6-km D-band photonic-assisted terahertz wireless communication employing SIMO and MRC technology. IEEE Transactions on Microwave Theory and Techniques, 72(11), 6657-6668.

About long distance mm-wave or THz transmission, these references should be added:

Zhang, Q., Yu, J., Zhao, X. et al. 18 Gbps D-band wireless signal transmission over 30.2 km land and sea surface based on photonics aided technology. Sci. China Technol. Sci. 68, 2280901 (2025). https://doi.org/10.1007/s11431-025-3059-x

Yu, Jianjun, et al. "Broadband photon-assisted terahertz communication and sensing." Journal of Lightwave Technology 41.11 (2023): 3332-3349.

Li, W., Yu, J., Zhu, B., Wang, F., Ding, J., Zhang, J., ...&Yu, J. (2023). Photonic terahertz wireless communication: Towards the goal of high-speed kilometer-level transmission. Journal of Lightwave Technology, 42(3), 1159-1172.

About mm-wave generation based on photonics, this paper should be cited:

Li, L., Long, T., Yang, X. et al. Modified uni-travelling-carrier photodiodes with 206 GHz bandwidth and 0.81 A W−1 external responsivity. Nat. Photon. 19, 1301–1308 (2025). https://doi.org/10.1038/s41566-025-01784-0

About mm-wave generation by a phase or intensity modulator, this paper should be cited:

Yu, Jianjun, et al. "Optical Millimeter-Wave Generation or Up-Conversion Using External Modulators." IEEE PHOTONICS TECHNOLOGY LETTERS 18.1 (2006): 265.

Response 6: We sincerely thank you for this valuable suggestion. We have added the suggested references into the revised manuscript. We believe these additions significantly enhance the completeness and richness of our paper.

“Thus, millimeter waves hold significant potential for long-range terrestrial and maritime communication [8–15].

Although commercial E/W-band electronic components have matured, limitations remain in modulation and frequency conversion, including local oscillator leakage and harmonic distortion. Microwave photonic up/down-conversion techniques can directly convert baseband signals to millimeter-wave frequencies or down-convert millimeter-wave signals to baseband, leveraging the wide bandwidth and low loss of optical devices [16-17]. This reduces the complexity of high-frequency mixing and filtering and improves overall system performance [18-21]. In recent years, increasing attention has been given to experimental demonstrations of E/W-band millimeter-wave communication systems. For example, Ref. [22] reported a 4.6 km wireless transmission at a center frequency of 88.5 GHz using photonic-assisted generation, although the receiver still relied on a conventional electronic down-conversion architecture. The use of photonic techniques can effectively avoid the noise accumulation associated with electronic frequency multiplication when generating high-frequency millimeter-wave signals, while also overcoming the bandwidth limitations of electronic components, making photonic approaches more suitable for high-frequency and wideband systems. In addition, MRC technology can significantly enhance system performance [23]. In Ref. [24], the authors employed MRC-based diversity reception to achieve approximately 4.6 km of ground-level wireless transmission. However, compared with terrestrial environments, near-sea-surface propagation is subject to much more severe impairments, including high humidity, wave-induced fluctuations, and strong multipath effects, making long-distance communication far more challenging [25-27].”

 

 

 

 

 

 

 

 

 

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript presents a photonics-assisted E-band 1×2 MIMO system for long-distance, near-sea-surface communication. The 26-km field trial is commendable and the reported ~5 dB gain from spatial diversity with MRC is a valuable result. However, the manuscript requires significant revisions to address technical ambiguities, incomplete descriptions, and insufficient analysis before it can be considered for publication.

1. In Figs. 1-2, the BER performance is simulated. However, the expression of BER is not included in the principle part. It should be given and while the ITU-R model is cited, the parameters of the actual temperature, pressure, and crucially, the water vapor density measured or estimated for the Lianyungang test site on the experiment day should also be given.
2. The linewidths of the transmitter and LO ECLs are not provided. Phase noise is a key limitation for coherent QPSK and higher-order modulation. An analysis of its impact on the system BER is recommended.
3. The receiver antenna spacing is vaguely described as ">80 cm". The exact distance should be provided. Also, the difference between the two branches need to be analyze and also the influence on the practical MRC gain.
4. Using an attenuator (EDFA5) to emulate increased distance simulates path loss but does not emulate the changing multipath delay spread and fading statistics that would occur over actual varying distances. Therefore, the conclusion that the 5 dB gain "translates to more than 50% increase in transmission distance" is an oversimplification. A more cautious interpretation is advised.
5. The authors attribute the difference between Channel 1 and 2 spectra to both hardware response and multipath. To strengthen the claim, could the authors quantify the relative contribution of each factor? A back-to-back test in an anechoic chamber could isolate and measure the intrinsic hardware difference between the two channels. This would more powerfully demonstrate that distinct multipath fading is the primary reason for employing diversity.
6. In the experiment, the ~5 dB improvement is obtained by MRC combination. Does it agrees with the previous simulation? The authors should comment on it.
7. The statement that this experiment is "the first of its kind in China" is difficult for a reviewer to verify. Please provide a reference to support this claim.
8. There is a formatting error on P6 where a standalone "4. Discussion" heading appears without content, followed by "5. Experimental Results and Discussion". This should be corrected.

Author Response

1. Summary
We are truly thankful for the time and effort you dedicated to reviewing my manuscript. We have carefully considered each of your comments and have made the necessary revisions to address them in our manuscript. In the following, we provide a point-by-point response to each of your comments as presented as follows and the corresponding revisions highlighted in the re-submitted files.
2. Point-by-point response to Comments and Suggestions for Authors
Comments 1: In Figs. 1-2, the BER performance is simulated. However, the expression of BER is not included in the principle part. It should be given and while the ITU-R model is cited, the parameters of the actual temperature, pressure, and crucially, the water vapor density measured or estimated for the Lianyungang test site on the experiment day should also be given. Response 1: Thank you for the comment. We added the expression of BER into the Section 2 in the revised manuscript as follows.   “We first determine the required SNR at the receiver to maintain the QPSK signal at this specific BER threshold by , where the  is the complementary error function and  represents bit SNR.”   Due to the lack of precise measuring instruments, based on the weather on the day of the experiment and the testing time, the temperature, air pressure, and water vapor concentration are 16°C, 102.9kpa, and 13.5 g/m³ respectively. We added the environment parameters into the Section 3.3 in the revised manuscript.
Comments 2: The linewidths of the transmitter and LO ECLs are not provided. Phase noise is a key limitation for coherent QPSK and higher-order modulation. An analysis of its impact on the system BER is recommended.
Response 2: We sincerely thank you for highlighting this critical issue. We fully agree that phase noise significantly impacts the performance of fiber-wireless systems, particularly for coherent QPSK and higher-order modulation formats.   In our experiment, the optical carrier and the LO at the receiver are generated using an electro-optical frequency comb via electro-optic modulation. Since both tones are derived from the same master laser source, the system benefits from the inherent phase correlation between the comb lines. As a result, the generated signals exhibit high-frequency stability and excellent phase noise performance. We have cited relevant reference [30] in the revised manuscript to further support this explanation. In Section 2.1, we have added a description and a relevant published literature as reference. In the meantime, “The ECL linewidth used for the multi-wavelength generation structure is 100kHz.” is added into Section 3 in the revised manuscript.   “Furthermore, at the receiver, both the optical carrier and the LO are generated using an electro-optical frequency comb via electro-optic modulation. Since both tones are derived from the same master laser source, the system exhibits high frequency stability and low phase noise [28].”   28. Nopchinda, D.; Zhou, Z.; Liu, Z.; Darwazeh, I. Experimental Demonstration of Multiband Comb-Enabled Mm-Wave Transmission. IEEE Microw. Wirel. Technol. Lett. 2023, 33, 919–922.
Comments 3: The receiver antenna spacing is vaguely described as ">80 cm". The exact distance should be provided. Also, the difference between the two branches need to be analyze and also the influence on the practical MRC gain. Response 3: The receiver antenna spacing is 85cm after testing. Both channels have the hardware configuration of the same model, including antennas, LNAs, PAs, and phase modulators. The difference between the two branches is the broadband frequency responses between the hardware with the same model. From the received electrical spectra of the two channels, some differences can be observed, however, the BER performance is found to be comparable from Figure.8. Therefore, when performing MRC processing, the weight differences between the two channels are not significant. We added the receiver antenna spacing and the analysis about the difference between the two branches into Section 4.
Comments 4: Using an attenuator (EDFA5) to emulate increased distance simulates path loss but does not emulate the changing multipath delay spread and fading statistics that would occur over actual varying distances. Therefore, the conclusion that the 5 dB gain "translates to more than 50% increase in transmission distance" is an oversimplification. A more cautious interpretation is advised. Response 4: Thank you for the comment. We deleted the description about the transmission distance improvement in the revised manuscript.
Comments 5: The authors attribute the difference between Channel 1 and 2 spectra to both hardware response and multipath. To strengthen the claim, could the authors quantify the relative contribution of each factor? A back-to-back test in an anechoic chamber could isolate and measure the intrinsic hardware difference between the two channels. This would more powerfully demonstrate that distinct multipath fading is the primary reason for employing diversity.
Response 5: We sincerely thank you for this constructive suggestion. We acknowledge that the current analysis regarding the spectral differences between Channel 1 and Channel 2 is primarily based on theoretical inferences. We have noted this as a key direction and will dedicate efforts to investigating this quantitative relationship in our future research by carrying out a back-to-back test to measure the intrinsic hardware difference between the two channels.   “Although both channels receive the same 5-Gbaud QPSK E-band signal, their spectral amplitudes exhibit observable differences across the 5-GHz bandwidth. These discrepancies may originate from two primary factors: the different broadband frequency responses of the antennas, LNAs, PAs, phase modulators, photodiodes, and other electronic and optical components in each channel, and the distinct sea-surface-induced multipath characteristics experienced by the two antennas. Detailed analysis and further investigation into the specific contributions of these factors will be conducted in our future research.”   Comments 6: In the experiment, the ~5 dB improvement is obtained by MRC combination. Does it agrees with the previous simulation? The authors should comment on it. Response 6: Thank you for the comment. There is a mistake above the reference of BER to claim the improvement of MRC. It should be 1×10⁻². Therefore, at the target BER of 1×10⁻², MRC achieves about 4 dB improvement. We added the checked description about MRC improvement and an updated Figure 8 into the revised manuscript.   Comments 7: The statement that this experiment is "the first of its kind in China" is difficult for a reviewer to verify. Please provide a reference to support this claim. Response 7: Thank you for pointing this out. Our previous statement was not sufficiently rigorous. In the revised manuscript, we have cited relevant reference to better present our contribution. Compared with Ref. [27], we introduced the 1x2 MIMO architecture for the first time to achieve such a long-distance near sea transmission.   “A near-sea-surface 26 km 1×2 MIMO transmission experiment, the first in China to employ this specific architecture over such a long distance, is conducted in Lianyungang [27].”   In this section, the reference we cited to support this claim is:   27. Liu, Y.; Tao, L.; Dong, B.; Ping, D.; Zhao, J.; He, S.; Lu, Q.; Cheng, T.; Li, Y.; Li, R.; et al. Long-Distance Field Trial of E/W-Band Signal Wireless Delivery Based on Full Photonic Up- and Down-Conversions. J. Lightwave Technol. 2024, 1–12, doi:10.1109/JLT.2024.3496894. Comments 8: There is a formatting error on P6 where a standalone "4. Discussion" heading appears without content, followed by "5. Experimental Results and Discussion". This should be corrected. Response 8: Thank you for pointing this out. We apologize for this formatting error. We have carefully reviewed the “4. Discussion” section and deleted the excessive content in the revised manuscript.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

It can be accepted as is.

Comments on the Quality of English Language

NA

Author Response

Comments 1: The English could be improved to more clearly express the research.

Response 1: Thank you for pointing this out. We agree with this comment. Therefore, we have have taken the following actions: We have invited a native English-speaking colleague with expertise in our field to review and polish the entire manuscript.Based on the feedback received, we have revised the text to ensure that the research findings are presented in a clear, concise, and academic manner, with particular attention to the Abstract, Introduction,Principle, Experiment and Discussion sections.

Comments 2: Figures and tables can be improved

Response 2: Agree. In response, we have carefully revised all figures and tables to improve their clarity and presentation. Key improvements include increasing font sizes for better readability, enhancing contrast in graphical elements, and ensuring all legends are comprehensive. We believe the revised visuals now more accurately and clearly represent our data.

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have responsed most of the comments and improved their manuscript correspondingly. However, I think  the description about the transmission distance improvement is valuable and it should be further discussed instead of being deleted.

Author Response

Comments 1: The English could be improved to more clearly express the research.

Response 1: Thank you for pointing this out. We agree with this comment. Therefore, we have have taken the following actions: We have invited a native English-speaking colleague with expertise in our field to review and polish the entire manuscript.Based on the feedback received, we have revised the text to ensure that the research findings are presented in a clear, concise, and academic manner, with particular attention to the Abstract, Introduction,Principle, Experiment and Discussion sections.

Comments 2: Figures and tables can be improved

Response 2: Agree. In response, we have carefully revised all figures and tables to improve their clarity and presentation. Key improvements include increasing font sizes for better readability, enhancing contrast in graphical elements, and ensuring all legends are comprehensive. We believe the revised visuals now more accurately and clearly represent our data.