Hybrid Precoding Applied to Multi-Beam Transmitting Reconfigurable Intelligent Surfaces (T-RIS)

Round 1

Reviewer 1 Report

The authors have job research and presented the paper in a well mannered.

Just minor remarks

I found many mistakes in sentence constructions, grammatical errors are there

make conclusion more precise and to-the -point

include more recent references from last 3 years.

make a performance comparison table with other reported works

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

 

The manuscript entitled “Hybrid Precoding applied to Multi-Beam Transmitting Reconfigurable Intelligent Surfaces (T-RIS)”, authored by D. Demmer, F. F. Manzillo, S. Gharbieh, M. Smierchalski, R. D’Errico, J-B Doré and A. Clemente has been reviewed.

This paper describes the use of Transmitting Reconfigurable Intelligent Surfaces (T-RIS) for practical multi-user massive MIMO access. The authors propose a multi-beam T-RIS propagation model with strong phase quantification constraints as well as limited beam codebooks. The quad-beam 7 T-RIS structure is evaluated by tuning the focal distance between the lens and the focal sources according to the per-user antenna gain and average rate. In both cases, the system performance was evaluated by assuming imperfect channel state information.

A ray-tracing model has been derived including the wave propagation through the T-RIS and the unit-cells with finite-resolution phase shifters, RF and baseband precoding schemes. It is  very interesting to note that the theoretical performance of a K-band T-RIS is in agreement with the proposed model and simulation results. The presented results are based on the performance evaluation for a 28.0 GHz T-RIS with low complex hybrid precoding schemes, which is valid for all mmWave bands. Also, the proposed model is scalable and can be easily adapted to numerous applications as well.

Although the manuscript is very well written, there are few corrections required to be completed before publishing it. The scientific content of the paper is novel and adequate for publication after making the following corrections.

Q1. P2, L 49-51, “The fully-connected approach provides more degrees of freedom, because of the higher number of phase shifters, however such network becomes unrealistic with large antenna arrays”.

Please correct the sentence structure.

Q2. P3, L109-110, “Excluding the varactor-based design which provides a quasi-continuous phase resolution at the cost of a limited bandwidth”.

Please correct the spelling.

Q3. P3, L110-112, “It is important to consider strong phase quantification constraints”.

Briefly explain the quantification constraints you need to take care of while designing the reconfigurable elements.

Q4. P4, L156-157, “the distance from the origin, the elevation angle θ ∈ [0, π/2[ corresponds to the angle with respect to the polar axis O~ z and the azimuth angle φ ∈ [0, 2π[ is the angle between the azimuth axis O~ x and the orthogonal projection of the point P over the reference plane (Oxy).

Please check the statement.

Q5. P5, L175-176, “We assume no signal attenuation induced by the phase shifters”

Why there is no attenuation in the signal? Explain briefly. How do you address the attenuation caused by temperature and humidity?

Q6. P5, L-177, “Assuming far-field propagation, i.e. ∀m rk,m = rk , θk,m = θk and φk,m = φk , it is is possible to derive the propagation channel expression

Please correct the sentence

Q7. P9, L-77-78, “we investigate and compare two methods to determine the optimal focal distance df: via

(1)   Gain optimization: maximizing the antenna array gain

(2)   Capacity optimization: another proposed method is to directly optimize the T-RIS structure T so as to maximize the per-user capacity

What are the results of your investigation based on Gain optimization and capacity optimization? How do you compare your results and what is your conclusion? Explain briefly.

Q8. P11, L280-285, “A very good agreement is observed between simulated and experimental results. Nonetheless, there is an inevitable difference between these results and the scattering parameters calculated with periodic boundary conditions and normal incidence. In particular, we can observe a 500-MHz shift towards lower frequencies. It is mainly due to the oblique incidence of the waves in the wave guides simulators”.

How do you justify the difference between your experimental and simulated results? Explain briefly: How do you nullify or reduce this “difference” by changing the periodic boundary conditions/normal incidence/oblique incidence of the wave guides?

Q9. P11, L 289-290, “The prototype is a 20 × 20 transmitarray comprising the 1-bit unit cells previously described”

Please correct the sentence.

Q10. P13, L315-316, “The objective is emphasize the impact of user density and antenna aperture limit”.

Please correct the sentence.

Q11. P13, L317-319, “From now on and for the rest of the work, we consider imperfect channel knowledge 317 and assume a noisy channel estimation corrupted by thermal noise (temperature of 318 25 °C)”.

Please correct the sentence structure.

Q12. P16, L397-398, “We compared the theoretical performance of our model with measurements with a K-band T-RIS which justifies that the proposed model and radiation pattern is valid.

 

Please correct the English Grammar.

 

 

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

Comments for author File: Comments.pdf

Author Response

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Author Response File: Author Response.pdf