A Review on Cell-Free Massive MIMO Systems
Round 1
Reviewer 1 Report
Comments to the Authors
In this paper, the authors reviewed the Cell-free massive multiple-input multiple-output techniques at higher frequencies for physical layer. On the basis of cell free techniques, it is shown that the system performance can be improved.
The work of this paper is interesting; especially the cell free massive MIMO techniques such as RIS, AI, etc., makes this paper have practical significance. To be honest, the expression, readability, and analysis of this manuscript still need to be revised, but in any case, I think it deserves a major revision before the next consideration.
1. This paper presents the cell free implementation using massive MIMO. The physical meaning of the proposed work needs to elaborate.
2. Although this paper lists many literature, it does not seem to clearly point out the shortcomings of existing research, which blurs the motivation and contribution of this paper.
3. Analysis lacks quantitative analysis and modeling. In addition, there is a lack of comparison with existing related work. The complexity of the proposed work should be presented.
4. Some discussion regarding channel modeling schemes, such as deterministic and stochastic channel modeling schemes can be appended for better understanding at higher frequencies.
5. More background related to massive MIMO channel modeling for indoor environment needs to be added and some highly related work in Introduction part can be cited, e.g., “Channel Propagation Characteristics for Massive Multiple-Input/Multiple-Output Systems in a Tunnel Environment [Measurements Corner]”, and “Statistical characteristics of 3D MIMO channel model for vehicle-to-vehicle communications”.
6. In Cell-Free Promising Technologies, the Reconfigurable Intelligent Surface (RIS) was described. Could you pls append several sentences to describe the intelligent transmission method? In my opinion, the intelligent transmission method can obtain the narrower beam.
7. Some discussion on potential THz communications in UAV will be good to have.
8. Some formatting problems, such as missing punctuation marks in formulas and unnecessary text indentation.
9. Writing can be improved, as I have seen many grammatical errors in the manuscript.
Comments for author File: 
Comments.pdf
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Reviewer 2 Report
It would be nice if the authors could include a section regarding:
1. The simulation of MIMO designs via several numerical schemes; a critical item prior to their fabrication.
2. The limitations of MIMO usage in 6G problems and their relatively slow assimilation in such configurations.
3. The inevitable EMI issues at large MIMO arrangements and means to overcome.
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
The reviewer queries have been answered very-well and the revised version of the paper is fulfilling all my concerns. So, i think there is nothing more to be changed and i accept it for publication in present form.
Reviewer 2 Report
I think that the authors have tried to improve their work in this revised version of the paper.
