Design and Analysis of Enhanced IM/DD System with Nonorthogonal Code Shift Keying and Parallel Transmission

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The article provides an intensity modulation and direct detection (IM/DD) system with high data transmission efficiency by combining non-orthogonal code shift keying (CSK) and parallel transmission technology. The article has certain innovation and feasibility, but there are still the following problems before acceptance:

1. The background introduction of optical wireless communication is relatively brief, and the application scenarios and existing challenges of IM/DD systems should be described in detail.

2. The shortcomings of existing CSK systems should be pointed out more clearly, especially the potential problems and challenges of non-orthogonal CSK in IM/DD systems.

3. The impact of different noise sources on system performance should be further discussed. The analysis of APD noise and background noise under different transmission conditions should be added.

4. Necessary symbol explanations are missing in formulas (6) and (7).

5. The simulation part is mainly based on the performance analysis under ideal conditions. Although the author mentioned that the influence of background noise was considered, the simulation results did not fully demonstrate the performance of the system in actual noise environment.

6. Non-orthogonal codes may introduce inter-symbol interference (ISI) in a multi-user environment, but the article did not conduct an in-depth analysis of this.

7. The conclusion part does not adequately describe the innovation of the article.

8. Some formulas and symbols are not used in a standardized manner and need to be revised in detail.

Comments on the Quality of English Language

English level needs to be further improved.

Author Response

[Comment 1] The background introduction of optical wireless communication is relatively brief, and the application scenarios and existing challenges of IM/DD systems should be described in detail.

[Reply 1] Thank you for your valuable feedback. We acknowledge that the background explanation of IM/DD systems was too concise. To address this, we have added more details on application examples and existing research.

 

[Comment 2] The shortcomings of existing CSK systems should be pointed out more clearly, especially the potential problems and challenges of non-orthogonal CSK in IM/DD systems.

[Reply 2] We have strengthened the explanation of the challenges in conventional CSK and non-orthogonal CSK systems to provide a clearer understanding of their limitations.

 

[Comment 3] The impact of different noise sources on system performance should be further discussed. The analysis of APD noise and background noise under different transmission conditions should be added.

[Reply 3] We have included new numerical analysis results to illustrate the impact of avalanche photodiode (APD) noise and discussion on shot noise on APD and background noises on system performance.

 

[Comment 4] Necessary symbol explanations are missing in formulas (6) and (7).

[Reply 4] As suggested, we have supplemented the explanations for the symbols used in these equations to enhance clarity.

 

[Comment5] The simulation part is mainly based on the performance analysis under ideal conditions. Although the author mentioned that the influence of background noise was considered, the simulation results did not fully demonstrate the performance of the system in actual noise environment.

[Reply 5] Previously, we only considered the impact of background noise. However, to better reflect real-world conditions, we have now included additional numerical results that account for variations in APD shot noise intensity.

 

[Comment 6] Non-orthogonal codes may introduce inter-symbol interference (ISI) in a multi-user environment, but the article did not conduct an in-depth analysis of this.

[Reply 6] We have conducted numerical analysis to demonstrate the impact of inter-symbol interference (ISI) when using non-orthogonal codes in a single-user environment. Furthermore, we have added a discussion in the conclusion section on the potential extension to a multi-user environment.

 

[Comment 7] The conclusion part does not adequately describe the innovation of the article.

[Reply 7] To better highlight the novelty of this study, we have restructured the conclusion to emphasize the key features of the proposed method, namely parallel transmission and the systematic construction of non-orthogonal codes.

 

[Comment 8] Some formulas and symbols are not used in a standardized manner and need to be revised in detail.

[Reply 8] We have corrected inconsistencies in mathematical symbols and standardized notations throughout the manuscript.

Reviewer 2 Report

Comments and Suggestions for Authors

This paper proposes an enhanced Intensity Modulation and Direct Detection system for Optical Wireless Communication by combining nonorthogonal Code Shift Keying and parallel transmission to improve data transmission efficiency. The study constructs nonorthogonal codes using concatenated pseudo-orthogonal M-sequences and evaluates the system’s performance through analytical derivations and Monte Carlo simulations, in addition to background noise. 

The paper is good and I have the following comments to improve it:

 

1. Some modulations are missing in the introduction such as dual-mode time domain multiplexed chirp spread spectrum in [REF01]. Kindly include.
2. Why was nonorthogonal CSK specifically chosen over other modulation schemes like Orthogonal CSK or PPM for improving the IM/DD system?
3. Include a comparative analysis in terms of complexity and feasibility to explain to readers why nonorthogonal CSK is the optimal choice.
4. How does parallel transmission impact the system’s signal-to-noise ratio (SNR), and does it introduce additional interference between parallel signals?
5. How is the background noise modeled in the numerical analysis, and how does it correlate with real-world IM/DD optical communication noise sources? Clearly define the assumptions and limitations of the noise model.
6. Authors use Monte Carlo simulation, but how many independent runs were conducted for statistical reliability?
7. Can the proposed system be generalized towards  multi-user environments or is it limited to single-user transmission? Please discuss whether the framework can support multiple users while maintaining efficient data transmission.
8. There are some typos and weak statements like "Previous researches" which should be "Previous research”. Please revise.
9. The complementary error function (erfc) is used to model the probability of correct frame demodulation. Kindly explain the physical intuition of the threshold and mean correlation output.
10. Some figures, such as Figures 5 and 6, contain repeated captions. 
11. Can turbulence fading in addition to atmospheric turbulence and pointing errors impact simulations ? 

 

 

References 

[REF01] A. W. Azim, A. Bazzi, M. Fatima, R. Shubair and M. Chafii, “Dual-Mode Time Domain Multiplexed Chirp Spread Spectrum,” in IEEE Transactions on Vehicular Technology, vol. 72, no. 12, pp. 16086-16097, Dec. 2023, doi: 10.1109/TVT.2023.3295270

Comments on the Quality of English Language

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

Author Response

[Comment 1] Some modulations are missing in the introduction such as dual-mode time domain multiplexed chirp spread spectrum in [REF01]. Kindly include.

[Reply 1] We appreciate your valuable feedback. The background explanation of IM/DD systems was too concise, so we have added more details on application examples and existing research.

 

[Comment 2] Why was nonorthogonal CSK specifically chosen over other modulation schemes like Orthogonal CSK or PPM for improving the IM/DD system?

[Reply 2] We have revised the Introduction to provide a clearer explanation of conventional CSK, non-orthogonal CSK, and the motivation behind this study.

 

[Comment 3] Include a comparative analysis in terms of complexity and feasibility to explain to readers why nonorthogonal CSK is the optimal choice.

[Reply 3] Since the proposed method systematically constructs non-orthogonal codes, we believe that its complexity is comparable to that of conventional CSK. We have added the explanation of the complexity of the proposed method.

 

[Comment 4] How does parallel transmission impact the system’s signal-to-noise ratio (SNR), and does it introduce additional interference between parallel signals?

[Reply 4] We have included new numerical analysis results to illustrate the impact of APD noise. Additionally, we have discussed the relationship between parallel transmission, APD shot noise, and background light noise.

 

[Comment 5] How is the background noise modeled in the numerical analysis, and how does it correlate with real-world IM/DD optical communication noise sources? Clearly define the assumptions and limitations of the noise model.

[Reply 5] We have enhanced the explanation of noise characteristics and provided explanation on background noise modeling.

 

[Comment 6] Authors use Monte Carlo simulation, but how many independent runs were conducted for statistical reliability?

[Reply 6] We have conducted simulations with 10,000,000 trials.

 

[Comment 7] Can the proposed system be generalized towardsmulti-user environments or is it limited to single-user transmission? Please discuss whether the framework can support multiple users while maintaining efficient data transmission.

[Reply 7] We have added a new discussion on the applicability of the proposed method to multi-user environments and have examined the potential for multiple access.

 

[Comment 8] There are some typos and weak statements like "Previous researches" which should be "Previous research”. Please revise.

[Reply 8] We have corrected errors such as changing “Previous researches” to “Previous research” and made other necessary grammatical revisions.

 

[Comment 9] The complementary error function (erfc) is used to model the probability of correct frame demodulation. Kindly explain the physical intuition of the threshold and mean correlation output.

[Reply 9] We have included an explanation of the threshold setting in the erfc function.

 

[Comment 10] Some figures, such as Figures 5 and 6, contain repeated captions.

[Reply 10] We have corrected duplicated captions.

 

[Comment 11] Can turbulence fading in addition to atmospheric turbulence and pointing errors impact simulations ? 

[Reply 11] Thank you for your insightful comments. We recognize that further analysis is required to fully evaluate the effects of atmospheric turbulence and pointing errors. In the revised manuscript, we have clarified the scope of this study and mentioned that the impact of atmospheric turbulence, pointing errors, and turbulence fading will be addressed in future research. We greatly appreciate your valuable suggestions.

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

I have no further comments

Author Response

We appreciate the time and effort the reviewer has invested in evaluating our work.