Reviewer: 1

Comments to the Author
- The paper investigates the noise whitening problem in AF relay protocol. In general the paper is well written and easy to follow. The main contribution in this paper is the application of the widely known whitening technique to AF system.  As example, some papers [1,2] studied this problem but with different  angles and formulation. The authors need to explain the difference and highlight his contribution clearly.   
[1] Fan-Shuo Tseng; Min-Yao Chang; Wen-Rong Wu, "Joint Tomlinson¨CHarashima Source and Linear Relay Precoder Design in Amplify-and-Forward MIMO Relay Systems via MMSE Criterion," Vehicular Technology, IEEE Transactions on , vol.60, no.4, pp.1687,1698, May 2011
[2] Jiayi Zhang; Lie-Liang Yang; Hanzo, L., "Power-Efficient Opportunistic Amplify-and-Forward Single-Relay Aided Multi-User SC-FDMA Uplink," Vehicular Technology Conference (VTC 2010-Spring), 2010 IEEE 71st , vol., no., pp.1,5, 16-19 May 2010
- In the other hand the author must also cite and discuss the non-coherent modulations AF in his paper where such system can work without NW and without any channel estimator at the receiver side [3, 4]
[3 ]Farhadi, G.; Beaulieu, N.C., "A Low Complexity Receiver for Noncoherent Amplify-and-Forward Cooperative Systems," Communications, IEEE Transactions on , vol.58, no.9, pp.2499,2504, September 2010 doi: 10.1109/TCOMM.2010.072710.090036
[4] Kaddoum, G.; Parzysz, F.; Shokraneh, F., "Low-complexity amplify-and-forward relaying protocol for non-coherent chaos-based communication system," Communications, IET , vol.8, no.13, pp.2281,2289, September 5 2014
- Finally,  by looking carefully to Fig 4 we can see that the improvement is marginal, can the author explain this results by taking into account the additional complexity added to the system compared to the on in reference  [5] of the paper
- Minor comments:
Some typos must be checked
After your equations you need to put comma or point
Your reference list must be cheeked to follow the IEEE format.
In my opinion the paper can be considered after revision



Reviewer: 2

Comments to the Author
The authors have tried to introduce an interesting noise whitening scheme applicable to AF relay systems. Having said so, the reviewer would like to share the following.

1. There are good number of grammatical errors and few typos. For instance, missing a ',' prior to 'and', burthen must be burden, page 2/9, lines 45-47 must be rephrased, if a 'which' is not preceded by a ',' then it must be a 'that', words starting with a vowel must be preceded with a 'an' and not 'a', and all the equations must end either with a ',' or '.'.
2. Isnt the assumption on perfect synchronization a very strong assumption? Is it really possible to this perfection?
3. The figures must have the Grid ON.
4. The power delay profiles are exponentially decaying with exp(-0.001l). Please cite some proper reference for this OR explain further.
5. The references must follow IEEE format.
6. The authors must share exactly where does the novelty stand in this work to be acceptable in IEEE Comms Letters. Is the presented work enough quantification to qualify publication in IEEE Comms Letters?

Based on the above, the reviewer will be happy to review the revised version.

Reviewer: 3

Comments to the Author
In this paper, the authors have proposed a noise whitening scheme in order to improve the performance of receivers in the amplify-and-forward relay systems. The authors have shown that the receiver with the noise whitening processing results in better estimation and detection performance than the original one at medium to high SNR.

However, the authors should make following changes in the paper in order to improve the quality of the paper. Specifically, the authors should explain that why the noise whitening receiver performance is worse than the original receiver. Moreover, the authors should provide more references about previous work and compare their proposed scheme with existing work.

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