Reconfigurable Intelligent Surfaces: The Resurrection of Relaying

When I started my research career in 2007, relaying was a very popular topic. It was part of the broader area of cooperative communications, where the communication between the transmitter and the receiver is aided by other nodes that are located in between. This could be anything between a transparent relay that retransmits the signal that reaches it after amplification in the analog domain (so-called amplify-and-forward), to a regenerative relay to processes and optimizes the signal in the digital baseband before retransmission.

There is a large number of different relaying protocols and information theory that underpins the technology. Relaying is supported by many wireless standards but has not become a major commercial success, possibly because the deployment of “pico-cells” is more attractive to network operators looking for improved local-area coverage.

Is relaying is a technology whose time has come?

A resurrection of the relaying topic can be observed in the beyond 5G era. Many researchers are considering a particular kind of relays being called a reconfigurable intelligent surface (RIS), intelligent reflecting surface, or software-controlled metasurface. Despite the different names and repeated claims of RIS being something fundamentally new, it is clearly a relaying technology. An RIS is a node that receives the signal from the transmitter and then re-radiates it with controllable time-delays. An RIS consists of many small elements that can be assigned different time-delays and thereby synthesize the scattering behavior of an arbitrarily-shaped object of the same size. This feature can, for instance, be used to beamform the signal towards the receiver as shown in the figure below.

Using the conventional terminology, an RIS is a full-duplex transparent relay since the signals are processed in the analog domain and the surface can receive and re-transmit waves simultaneously. The protocol resembles classical amplify-and-forward, except that the signals are not amplified. The main idea is instead to have a very large surface area so it can then capture an unusually large fraction of the signal power and use the large aperture to re-radiate narrow beams.

Conventional full-duplex relays suffer from loop-back interference, where the amplified signals leak into the yet-to-be-amplified signals in the relay. This issue is avoided in the RIS technology but is replaced by several other fundamental research challenges. In our new paper “Reconfigurable Intelligent Surfaces: Three Myths and Two Critical Questions“, we are pointing out the two most burning research questions that must be answered. We are also debunking three myths surrounding the RIS, whereof one is related to relaying.

I have also recorded a YouTube video explaining the fundamentals:

15 thoughts on “Reconfigurable Intelligent Surfaces: The Resurrection of Relaying”

  1. Hello, Dr. Bjornson.
    Can you explain, for what reasons are the RIS’s used in beyond 5G? what are the main advantages of them?
    Do you know good references which fundamentally explain the basics of RIS’s?
    Thank you.

  2. I’m confused about reflectarray based RIS and metasurface based RIS as I read about in recent research.
    Can you tell me your point of view in this matter?

    1. I’m not sure what your question is. But, yes, there are different ways to implement RIS, based on reflectarray or metasurface technology.

      1. Hello Prof. Björnson,

        Thank you so much for your reply! I meant earlier about the differences between each design: reflectarray based and metasurfaces based RIS. It is mentioned here https://arxiv.org/pdf/2005.00938v1.pdf

        Metasurfaces based IRS gives better performance especially regarding its size and reconfigurable flexibility. I thought based on videos and blogs that reflectarray is the initial idea behind the concept of RIS. And RIS is the advanced form of reflectarray. However in this journal paper, RIS is implemented using reflectarray which got me confused.

        1. RIS is a theoretical concept and there are different potential ways to implement it. Reflectarrays and metasurfaces are two candidates, whereof reflectarrays has a longer history, while metasurfaces seem to perform better.

          However, I wouldn’t take everything that is said in that paper as being correct. For example, the section “Large-scale Path Loss” is making claims of the kind that we call Myth 3 in the paper https://arxiv.org/pdf/2006.03377.pdf

          1. Yes exactly! I have read this article https://arxiv.org/pdf/1912.06759v1.pdf about the pass loss model and how the size of RIS and distance between TX -and RX -to RIS Affect it which contradicts with this paper..

            I know this may sound a trivial Question; in this paper https://arxiv.org/pdf/1912.06759v1.pdf

            Sometimes, the author refer to path loss as path gain for example LRIS noted with -1
            (Equation 13)

            Im confused when shall I call it path loss or path gain. Is the pass gain the reciprocal of path loss?

            Thanks again!

          2. Yes, whenever someone is using both “path gain” and “path loss” in a paper, then they are reciprocal.

            However, it is common that people are only using the name “path loss” and then it can refer to any of these things, which makes it confusing for the reader…

    1. No, not that I’m aware of. If the industry becomes sufficiently interested in the technology, there might eventually be a work item on the topic in 3GPP. It remains to be seen.

  3. Hi, Professor Bjornson.
    If we suppose a scenario in which we use an IRS for both amplify the desired signal and also suppress the interference, for example, for a cell-edge user. How should the meta-atoms amplitude and phase should be selected? Based on desired signal or based on interferer signal or based on both?
    If they are selected based on both, how this should be done in practice knowing that each meta-atom have only one amplitude and phase?

    1. One cannot simultaneously maximize the desired signal power and minimize the interference, since these are partially conflicting metrics. The structured approach to define a single metric that combine these things, such as an SINR. Then you maximize that metric with the states of the meta-atoms as the design variables. I think there are papers that have already attempted to do this.

  4. Dear Emil,
    Thanks for your inspiring posts : )

    Sorry, I just got confused about the channel gain from the TX antenna to RIS.
    There should be also a $\lambda^2$ in the denominator or is it something normalized?

    And a more fundamental question, why is the gain is like
    1/((d_g)^2 * (d_h)^2) and not 1/((d_g+d_h)^2).
    I thought for the specular reflection, the incoming wave just changes its direction. So I didn’t expect to have a multiplication of distances squared and I expected the square of the summation of the distances.

    Thanks,
    Ashkan

    1. Thank you!

      The channel gain between the TX antenna and RIS is proportional to the area A of the RIS element. This area is typically proportional to lambda^2, so there is the behavior that you are looking for. The element can for instance be a square of size lambda/5 x lambda/5.

      No, the “sum-of-distance” formula will generally not appear, particularly not on an per-element-basis. There are basically two operating regimes of the RIS as a whole: 1) The RIS is too small to approximate the size of a mirror, and then it provides a worse channel gain. 2) The RIS is large enough to approximate a mirror, but it is suboptimal to operate it like that since one can get a much better pathloss by approximate the behavior of a curved reflector.

      This is something that we elaborate on in this paper (see Myth 3):
      https://arxiv.org/pdf/2006.03377.pdf

      You can find technical explanations in the following papers:
      https://arxiv.org/pdf/1911.03359
      https://arxiv.org/pdf/2002.04960

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