A Survey on MIMO-OFDM Systems: Review of Recent Trends

Round 1

Reviewer 1 Report

In this survey paper, the authors review the state of art research studies and recent trends related to MIMO-OFDM systems. However, there are many missing important studies and recent research papers related to MIMO and OFDM systems. The current paper is not discussing, reviewing, or even citing some of the most recently published articles in the domain of MIMO-OFDM. Examples of such papers that should be reviewed and discussed are given below.
1) ---, (2021) Interference Signal Superposition-aided MIMO with Antenna Number Modulation and Adaptive Antenna Selection for Achieving Perfect Secrecy. RS Open Journal on Innovative Communication Technologies, 2(6). https://doi.org/10.46470/03d8ffbd.b23dc12e 

2) ---, (2021) Physical Layer Security Analysis of Hybrid MIMO Technology. RS Open Journal on Innovative Communication Technologies, 2(5). https://doi.org/10.46470/03d8ffbd.bd54c70c    

3) -----, (2021). Multi-User Auxiliary Signal Superposition Transmission (MU-AS-ST) for Secure and Low-Complexity Multiple Access Communications. RS Open Journal on Innovative Communication Technologies, 2(4). https://doi.org/10.46470/03d8ffbd.92a40b85  

Finally, careful proofreading of the paper is highly recommended to remove some of the remaining writing and grammatical mistakes.

Author Response

• In this survey paper, the authors review the state of art research studies and recent trends related to MIMO-OFDM systems. However, there are many missing important studies and recent research papers related to MIMO and OFDM systems. The current paper is not discussing, reviewing, or even citing some of the most recently published articles in the domain of MIMO-OFDM. Examples of such papers that should be reviewed and discussed are given below.

1) ---, (2021) Interference Signal Superposition-aided MIMO with Antenna Number Modulation and Adaptive Antenna Selection for Achieving Perfect Secrecy. RS Open Journal on Innovative Communication Technologies, 2(6). https://doi.org/10.46470/03d8ffbd.b23dc12e 

2) ---, (2021) Physical Layer Security Analysis of Hybrid MIMO Technology. RS Open Journal on Innovative Communication Technologies, 2(5). https://doi.org/10.46470/03d8ffbd.bd54c70c    

3) -----, (2021). Multi-User Auxiliary Signal Superposition Transmission (MU-AS-ST) for Secure and Low-Complexity Multiple Access Communications. RS Open Journal on Innovative Communication Technologies, 2(4). https://doi.org/10.46470/03d8ffbd.92a40b85  

Thank you for your valuable comment. The adopted references were filtered and restricted to some defined sources, mainly IEEE and Elsevier databases. We agree on the fact that we had missed some recently published papers in the field. This issue was corrected by adding some defined references:

• KIRIK, M.; HAMAMREH, J.M. Interference Signal Superposition-aided MIMO with Antenna Number Modulation and Adaptive Antenna Selection for Achieving Perfect Secrecy. 2021, doi:10.46470/03d8ffbd.b23dc12e
• Yang, S.; Kobayashi, M.; Piantanida, P.; Shamai, S. Secrecy degrees of freedom of MIMO broadcast channels with delayed CSIT. IEEE Transactions on Information Theory 2013, 59, 5244-5256.

Please check the corresponding added text in the revised version:

‘More recently, the need to provide full secrecy in sensitive data applications has spurred research in physical layer secure communications. The usage of MIMO by in-creasing the number of parallel and independent channels in a rich scattering environment, has been recognized to be a tool to increase the uncertainty about the channel and therefore increase the equivocation rate at an illegitimate receiver (e.g. [143], [144] and references therein).’

• Chen, S.; Zhang, J.; Zhang, J.; Björnson, E.; Ai, B. A survey on user-centric cell-free massive MIMO systems. Digital Communications and Networks 2021, doi:https://doi.org/10.1016/j.dcan.2021.12.005.

Please check the added line in table 1 and the added text line:

Besides some surveys for channel estimation and signal processing over massive MIMO systems [15].

• Belgiovine, M.; Sankhe, K.; Bocanegra, C.; Roy, D.; Chowdhury, K.R. Deep learning at the edge for channel estimation in beyond-5G massive MIMO. IEEE Wireless Communications 2021, 28, 19-25.

The reference was added in the sentence of lines 847-849:

‘Despite the traditional LS and LMMSE based estimator that have relatively limited performance with the existence of strong interferences, novel innovative approaches based on machine learning are introduced [123,124,142].’

• Finally, careful proofreading of the paper is highly recommended to remove some of the remaining writing and grammatical mistakes.

Thank you. Several remaining grammatical issues are addressed in the revised version.

Reviewer 2 Report

1. In this paper, authors only focus on MIMO waveforms, channel estimations, and MIMO hardware/software technology. Actually, the practical application of MIMO-OFDM is also an important issue for beyond 5G/6G systems.
2. The receiver detection issue should be considered in this paper. Besides, the complexity comparison between different detection methods should be provided for a survey paper.
3. Please draw all curves more smooth. In addition, please improve the resolution of all figures.
4. Please correct some typo errors.

Author Response

• In this paper, authors only focus on MIMO waveforms, channel estimations, and MIMO hardware/software technology. Actually, the practical application of MIMO-OFDM is also an important issue for beyond 5G/6G systems. The receiver detection issue should be considered in this paper. Besides, the complexity comparison between different detection methods should be provided for a survey paper.

Thank you for your remark. Exactly the review was limited to some points since it is impossible to cover the entire subject axes. We had already explained this point in the part of the introduction, highlighting the introduced reviewed axes, that had not been considered anywhere else:

“Several MIMO-OFDM reviews papers and surveys, resumed in table 1., had been published to give a comprehensive analysis of the different challenging issues for the scientific community. There were some papers proposing a simple overview of some important research axes, pointing to the last technology evolutions [2] concerning MIMO systems namely: MIMO 5G candidate waveforms [3,4], channel models [5] and coding schemes [6] relative to MIMO technology. Other papers, that propose deeper review concerning MIMO systems, in general, are classified in Table 1 as well. Only few papers [7] discuss the waveforms tested using MIMO systems. Nonetheless, several authors [8-13] had analyzed the MIMO channel estimation and modeling problem. But, to the best of our knowledge, there is no review paper until now discussing the recent works concerning channel estimation and equalization process for MIMO-OFDM systems exactly. There is the exception of some review papers [14] for MIMO-OFDM systems used especially for underwater acoustic communication concept where the transmission conditions are completely different than over-air transmission. Thus, there is a need for comprehensive review papers that analyze the current MIMO waveforms, channel estimation and equalizations techniques. Therefore, the current paper gives a detailed overview of MIMO systems recent advantages in general and classify the MIMO-OFDM waveforms, channel estimators, and equalizers.”

• Please draw all curves more smooth. In addition, please improve the resolution of all figures.

Thank you for your valuable comment. The quality and the resolution of several figures were improved in the revised version.

• Please correct some typo errors.

Thank you. Several typo errors had been corrected in the revised version.

Reviewer 3 Report

Please address these comments

there are many figures with citations, these figures need permission from the authors or publishers - see the journal instruction or discuss with the office

insert the contribution of the work and values in the abstract. the abstract is too short.

the conclusions need to be clear on the implications of the findings. also the technical limitations of this review should be specified - e.g. any missing papers due to the search method for identifying papers to be reviewed? how comprehensive are the search and review

the value of findings and reliability of the findings

Author Response

• there are many figures with citations, these figures need permission from the authors or publishers - see the journal instruction or discuss with the office

Thank you for your valuable remark. The citation within the figure refers mainly to the theoretical concept, the figures were not copied from the references. The figures were made following our understanding of the concept and our proper conception. I don’t believe that in this case, it is necessary to have copyright permissions. To avoid any confusion, we opted to remove the references from the figures and put them in the accompanied explanation corresponding to every figure in the text.

• insert the contribution of the work and values in the abstract. the abstract is too short.

The following sentences were added to demonstrate the contribution and values of the work:

Through literature, only few authors had discussed the MIMO channel estimation and modeling problems for a variety of MIMO systems. But, to the best of our knowledge, there is no review paper until now discussing the recent works concerning channel estimation and equalization process for MIMO-OFDM systems exactly. Hence the current work focuses on analyzing the recently used algorithms in the field, which could be a rich reference for researchers. Moreover, some research perspectives are identified.

• the conclusions need to be clear on the implications of the findings. also, the technical limitations of this review should be specified - e.g. any missing papers due to the search method for identifying papers to be reviewed? how comprehensive are the search and review the value of findings and reliability of the findings

 The following sentences were added, to highlight some technical limitations in the part of the conclusion:

‘Nonetheless, some recent papers could be missed since we had limited the resources to mainly three databases: ISI Web of Science, IEEE Xplore, and Google Scholar. Hence automatically some papers were excluded from the search, but the papers indexed on those databases have more impact and relevance to the addressed topic.’

The implications of the findings are highlighted in the points of challenges and opportunities in the part of conclusion. However, to clarify some global noticed points we had added the following text to the part of the conclusion:

‘From the above extensive search, some points could be noticed:

-   The part of implementation is more limited to research testbeds that apply traditional channel estimations algorithms.

-   The most deployed channel estimation algorithms are complex and have a lower level of mitigation of ICI.

-   The equalization algorithms are quite limited in terms of performance.

In the next, some challenges and research opportunities are spotted.

’

Round 2

Reviewer 2 Report

This paper is well written. Please adjust the sizes of all figures. Some figures are out of range.