Partial Pilot Allocation Scheme in Multi-Cell Massive MIMO Systems for Pilot Contamination Reduction

Round 1

Reviewer 1 Report

The topic of this paper is quite interesting. It proposes a pilot allocation scheme for cooperative multicell massive MIMO systems. The paper is relatively well written and organized. However the review have some concerns,

 

• In this paper it is assumed that the BSs are equipped with a massive number of antennas. In the reviewer opinion this assumption is realistic for millimeter wave bands, for current sub-6GHz is difficult. For sub-6GHz massive MIMO makes more sense if the antennas are distributed and not collocated in the same terminal. In massive MIMO context (collocated approach) is used to consider hybrid architectures to reduce the number of RF chains (see for example the reference below but much more can be found), but in this paper full digital architectures is considered. This should be clarified in the introduction. It should be stressed why a full digital architecture is followed instated a hybrid one.

 

L. Magueta, et all, “Hybrid Multi-User Equalizer for Massive MIMO Millimeter-Wave Dynamic Subconnected Architecture”, IEEE Access, vol. 7, pp. 79017- 79029, Junho 2019.

 

• In the introduction is mentioned “In multi-cell massive MIMO systems…” but no reference is added related with multi-cell systems. The reviewer suggest to add 1 or 2 references, e.g.

 

Yu, et al, “Multicell Coordination via Joint Scheduling, Beamforming, and Power Spectrum Adaptation,” IEEE Trans. Wireless Commun., vol. 12, no. 7, pp. 1–14, July 2013.

Niu, et all, “Downlink Scheduling with Transmission Strategy Selection for Multi-Cell MIMO Systems,” IEEE Trans. Wireless Commun., vol. 12, no. 2, pp. 736–747, February 2013.

 

• The reviewer would like to see the contributions of this paper after the related work and not before.

 

• It is assumed single antenna user terminals. In the context of future wireless systems this is not a realistic assumption. Is it straightforward to extend the proposed techniques to multi-antenna user terminals?

 

• Why are you considered ZF at the receiver side and not MMSE? It is well known that equalizers based on MMSE performs better.

 

• Regarding the simulation parameters, namely the channel model. Eq. (16) presents the large scale fading followed but the reviewer cannot find anything regarding small scale fading. What channel model have been considered? Was it assumed that the channel between antennas are uncorrelated? This is not realistic with tens of antennas. Please clary this point.

 

 

Author Response

Please see the attachment

Author Response File: Author Response.pdf

Reviewer 2 Report

This paper deals with the problem of pilot allocation in multicell MIMO system. A heuristic optimization algorithm is proposed to allocate pilots to users. The paper considers an interesting problem in massive MIMO systems. However, there are some issue to be clarified.  

1) The problem of pilot contamination is akin to the problem of pilot spoofing in multiantenna wireless systems with active eavesdroppers, see, e.g,  

- Design and performance analysis of channel estimators under pilot spoofing attacks in multiple-antenna systems, IEEE Transactions on Information Forensics and Security 15, 3255-3269, 2020  

- An Energy-Ratio-Based approach for detecting pilot spoofing attack in multipleantenna systems, IEEE Trans. Inf. Forensics Security, vol. 10, no. 5, pp. 932–940, May 2015.  

- Pilot spoofing attack detection and countermeasure, IEEE Trans. Commun., vol. 66, no. 5, pp. 2093–2106, May 2018.   It should be interesting to link the pilot contamination problem to the pilot spoofing one.  

2) My main concern pertains the formulation of the optimization problem. The Authors assume the knowledge of the squared channel gains of the users. The procedure to estimate such channel state information is unclear and additional details should be added.  

3) Moreover, what is the impact on pilot allocation of imprecise knowledge of the channel gains?  

Author Response

Please see the attachment

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The authors have satisfactory addresses most of the reviewer concerns. The English should be improved. 

Reviewer 2 Report

The Authors have accounted for my concerns.