Performance Improvement for Mixed RF–FSO Communication System by Adopting Hybrid Subcarrier Intensity Modulation

Round 1

Reviewer 1 Report

This paper studies a hybrid RF-FSO system with PPM-BPSK-SIM modulation. The paper relies on a mathematical background but there are many equations which might have typos or they are not presented adequately.

 

1) Equation 1: 3,0 and 1,3?

2) Equation 2: the author should check that the integral is correct.

3) In line 147, can they correct the equation of the capacity?

4) Can the authors specify what is E in line 147?

5) In Eq. 4, what do the authors mean with 7,0 and 2,7?

Whereas the remaining of the paper looks of much interest to the scientific community, at this stage I can only suggest the readers to improve the presentation of the mathematical framework in order to make it readable to a general audience.

The following minor issues should also be addressed by the authors:

 

6) The authors should provide a citation for the BER equation of a PPM signal.

7) The authors should clarify that L is the number of time slots of the PPM symbol.

8) The authors should explain what they mean by conditional BER.

9) The authors should specify whether they are considering a coherent optical receiver or direct detection?

10) The authors should discuss why the FSO SNR does depend on the electro-optical conversion efficiency and not in the optic-electro conversion efficiency.

11) In line 198, it should read "average BER".

 

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

It is a very interesting work concerning  the performance of mixed RF/FSO systems. This work could be published as long as the authors make the following changes:

a) The Rayleigh distribution is used for  the RF link. Why this distribution was chosen along many other distributions and which phenomenon is modeled according to this distribution?

b) More references are required for equation 1

c) In section 4 "Results and discussion", in table 1, why these values for a and b were chosen? References should be provided.

d) In figure 2, parameter "ζ" is mentioned in the figure caption, but such parameter is not mention anywhere else

e) In figure 3, why the single FSO has higher BER that the serial hybrid system? Is this possible and how can this be justified?

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

This paper studies a hybrid RF-FSO system with PPM-BPSK-SIM modulation. The paper presents a sound theoretical background with simulation results to show the suitability of the proposed system. I recommended the paper to be accepted for publication. The following minor comments should however be taken into account by authors:

 

The authors should clarify that the Meijer’s function G has associated the subscript and superscript number. This has led to confusion in the previous review. The authors should be aware of two recent experimental papers on free-space optics regarding (both published at one of the major conferences of optical communications):

[a] MIMO operation using orbital angular momentum.

[b] Increasing the spectral efficiency and performance of FSO links without increasing the receiver complexity using Kramers-Kronig receiver.

The authors are recommended to cite those papers as experimental demonstration of state-of-art free space optical communications.

The equation for the capacity does show a square and no the symbol equal or defined as. I recommended the authors to correct this typo.

[a] H. Song et al. "Experimental Mitigation of Atmospheric Turbulence Effect using Pre-Channel Combining Phase Patterns for Uni-and Bidirectional Free-Space Optical Links with Two 100-Gbit/s OAM-Multiplexed Channels." In Optical Fiber Communication Conference. Optical Society of America, 2019.

[b] A. Lorences-Riesgo, F. P. Guiomar, A. N. Sousa, A. L. Teixeira, N. J. Muga, and P. P. Monteiro, “200 Gbit/s free-space optics transmission using a Kramers-Kronig receiver,” in 2019 Optical Fiber Communications Conference and Exhibition (OFC), March 2019, paper W4A.3.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf