Peer review of: Metasurface-assisted massive backscatter wireless communication with commodity Wi-Fi signals (2020)

Reviewer #1 (Remarks to the Author): 
 
 
In this paper, the authors propose a method to achieve passive wireless communication with a metamaterial 
panel by using opportunistic wi-fi signals. It is one new application of the smart walls or engineered tunable 
metamaterial panels that can be tuned to focus wireless signals onto a wireless device. The authors propose 
an original communication scheme based on the use of such panels. The idea could be of interest and could 
deserve publication, but I have a few fundamental questions to be addressed by the authors before I can 
make a recommendation. 
 
1) 
The minimal distance required between the two antennas (master receiver and slave receiver) is a critical 
problem. It should be large enough to allow for selective focusing on the master receiver. This problem is 
briefly addressed experimentally in Supplementary Note 6, in which the distance is more than one meter. 
From the practical point of view, this distance seems very large to be useful for typical objects and typical 
indoor applications. 
From the theoretical point of view, it seems to be much larger than the focal spot radius given in the paper. 
Moreover: the formula of the focal spot radius lambda R/D is a transverse radius; the longitudinal radius is 
larger, of the order of lambda R^2/D^2. 
 
2) 
The authors state (in several different ways): 
"Our experiments demonstrate wireless communication without any active radio components". 
However, the method requires to know with precision the positions of the receiver antennas. Therefore, the 
receiver should find a way to give its relative position with respect to the metamaterial panel with relative 
precision (at the scale of the focal spot). How is it supposed to do it without any active radio component ? 
 
3) 
The role of the type of the source is not sufficiently addressed in my point of view. 
An analysis is given in the case of one point-like source in free space; a complementary analysis is given 
with a set of uniformly distributed random sources. 
However, a typical indoor situation will have one or a few point-like sources, with multipathing. In this case 
some of the statements such as "an important conclusion that, wherever the non-cooperative wireless 
source is, its energy can be well focused toward the master receiver at r_m by choosing a suitable control 
coding pattern of the metasurface" are not clear at all. 
 
4) 
A few statements are given without justification: 
- page 8: "The proposed PWC scheme is expected to virtually not affect the background AWC." 
- supplementary page 8 : "For this case, the machine learning (ML) technique can be applied to find a ML 
agent for the Green's function." 
 
5) 
typo (S13): r_A->r_m, r_B->r_S 
typo page S14: metasurace 
I think that some early papers on the use of tunable metamaterial panels should be cited, such as: 
N. Kaina, M. Dupré, G. Lerosey, M. Fink, 
Shaping complex microwave fields in reverberating media with binary tunable metasurfaces, 
Scientific Reports, volume 4, 6693 (2015). 
 
 
Reviewer #2 (Remarks to the Author): 
 
The manuscript entitled “Metasurface-Assisted Passive Wireless Communication with Commodity Wi-Fi 
Signals” introduce a “passive wireless communication” (PWC) architecture, which is totally different from the 
traditional active wireless communication (AWC), by a coding metasurface. The concept presented in the 
manuscript really attracts me and I strongly believe such a SIMO (e.g. single-input multi-output) wireless 
communication technique will bring many enticing applications and might change our daily life in the era of 
IOT, 5G or even 6G communication in the future. Therefore, I believe the conceptual innovation of this paper 
meets the criteria of Nature Communications. 
However, I still have following questions and doubt after I read the manuscript and supplementary 
materials. My questions are as following. 
 
1. Through the manuscript, the key innovation should be the PWC architecture. As shown in Fig. 1c, the 
ambient stray wireless signals were utilized as the input power flow. However, the coding metasurface is still 
active indeed. I mean, in contrast to the modulator and LO used in the AWC, a FPGA-based encoder is used 
in the PWC. To control the coding process in the FPGA and switch “on”-“off” state of diode in the 
metasurface, power supply is still required in this so-called PWC architecture. Therefore, it seems the 
present architecture is still “active”. I can understand that authors use the word “passive” because the input 
power of signal is from environment rather than their metasurface, but the “passive wireless 
communication” may introduce misunderstanding to some extent. 
 
2. The statement “(ii) directly modulating the ambient stray electromagnetic waves with high signal-to-noise 
ratio (SNR)” is lack of evidence. Authors had better to explain it in detail. For example, the SNR might be 
influenced by following points: A) the gain of transmitting antenna at Alice side and B) the transmission 
power in the wireless channel. If we talk about point A, it is not the advantage of PWC. It is about the 
performance of RF hardware. If we change the dipole in Fig. 1b for AWC architecture to a beam-scanning 
antenna with large apertures, just like the metasurface used for PWC architecture, maybe the AWC will have 
a very high SNR as well. Regarding to point B, if the radiating power of wifi router in the PWC case is 
identical to that for the AWC case and they work in the same environment (e.g., they may share the same 
wireless channels and the similar noise), I am not sure whether the power of coded signal is high or low in 
the case of PWC compared to the traditional AWC case This is because that the coding metamaterial might 
receive only a few energy from the stray wireless signals. Is it possible to provide some additional result to 
prove this conclusion? 
 
3. It seems that authors use equation (3) to demodulate PWC signals from unknown wireless signals. Is this 
for general cases? Is it just for single path or LOS (line-of-sight) case? If multi-path influence and NLOS 
(non-line-of-sight) should be considered when the environment is very complicate, should the PWC 
operational principle change? Under an NLOS case, will the signal be hard to distinguished due to the 
multiple reflected path? 
 
4. Authors present very sufficient experimental results to support their PWC concept and show that the 
transmission rate of their PWC demo system is 500 Kbps. Is this transmission rate equivalent to the 
throughput (or similar concept) of a wireless communication system? According to my knowledge, 
throughput is one of the most important factors in the present wireless communication systems (base 
station and UEs). Therefore, is it possible to compare this transmission rate/throughput between AWC and 
PWC architectures with the use of the same wifi router? Of course, the wifi router in both cases should be 
arranged at the same position. Furthermore, what is the spectral width used in the experiment? How much 
is the spectrum efficiency in the experiment? Please compare it to the PWC case as well. If the throughput 
and spectrum efficiency could be higher than those in the AWC case, authors’ PWC concept could attract 
much more interest from industrial engineers. 
 
Reviewer #3 (Remarks to the Author): 
 
The main problem of this manuscript is that the claim of the authors that they "introduce the fundamentally 
different concept of passive wireless communication (PWC)" is really too much since the papers published on 
this topic are very many. It appears that there are even start-up companies that develop this technique. 
 
In fact, some earlier work was discussed in one of the papers in the list of references, ref. 27. Moreover, in 
the same reference it is mentioned that the principle is the same as spatial modulation (there is a lot of 
papers on that subject also) and backscattering. 
 
Here are some references on papers that presented and developed this principle of passive wireless 
communications (the first one is a review, with many relevant references in its list, and the second one uses 
WiFi, the same approach as in this manuscript): 
 
C. Xu, L. Yang, P. Zhang, Practical backscatter communication systems for battery-free Internet of Things: a 
tutorial and survey of recent research. IEEE Sig. Process. Mag. 35(5), 16–27 (2018) 
 
J. Zhao, W. Gong, J. Liu, Spatial stream backscatter using commodity WiFi. ACM MobiSys (2018). 
https://dl.acm.org/citation.cfm?id=3210240.3210329 
 
There are research works on the same principle using WiFi, TV signals, 4G, and under research 5G signals. 
Here is an example of an early system that utilizes TV signals: 
 
Vincent Liu, Aaron Parks, Vamsi Talla, Shyamnath Gollakota, David Wetherall, Joshua R. Smith, Ambient 
Backscatter: Wireless Communication Out of Thin Air, SIGCOMM’13, August 12–16, 2013, Hong Kong, China 
http://conferences.sigcomm.org/sigcomm/2013/papers/sigcomm/p39.pdf 
 
In my opinion, the authors need to be better position their research work. 
 
I would like to note also that this work is similar to an earlier publication from same team: W. Tang, J. Y. 
Dai, M. Z. Chen, K.-K. Wong, X. Li, X. Zhao,S. Jin, Q. Cheng, and T. J. Cui, "MIMO transmission through 
reconfigurable intelligent surface: System design, analysis, and implementation", IEEE J. Sel. Commun., 
2020. [Online]. Available: https://arxiv.org/pdf/1912.09955.pdf. 
 
There, the principle is similar, although the application is different. In that paper the authors use the 
surfaces as low complexity transmitters. In this submission, the surface is for backscattering applications. 
The function of the metasurface is similar, although not exactly the same. 
 
As to the hardware realization, this varactor-switchable metasurface design was presented earlier by the 
same team in many publications, including papers in this journal (ref. 30). 
 
On the positive note, this paper presents a working prototype whose construction is not an easy task, and 
the results may be of interest and, possibly, of some significance to the engineering community. However, in 
view of the lack of significant novelty, I would not recommend this manuscript for consideration by Nature 
Communications. 
 
As an advice for paper revisions, I note that this paper is very difficult to read. There are lots of notations 
and abbreviations that are not generally known and used except perhaps some very specialized 
communities. For example, let’s look at Fig. 1. What is “A-demodulator”? What is “P-demodulator”? What is 
“Antenna-S”? What is “PMS encoder”, etc. It is very difficult to get an idea of the used approach from this 
picture.