Emerging intelligent reflecting surface (IRS) technology, also known under the names “reconfigurable intelligent surface” and “software-controlled metasurface”, is sometimes marketed as an enabling technology for 6G. How do they work, what are their use cases and how much will they improve wireless access performance at large?
The physical principle of an IRS is that the surface is composed of 
atoms, each of which acts as an “intelligent” scatterer: a small antenna that receives and re-radiates without amplification, but with a controllable phase-shift. Typically, an atom is implemented as a small patch antenna terminated with an adjustable impedance. Assuming the phase shifts are properly adjusted, the scattered wavefronts can be made to add up constructively at the receiver. If coupling between the atoms is neglected, the analysis of an IRS essentially entails (i) finding the Green’s function of the source (a sum of spherical waves if close, or a plane wave if far away), (ii) computing the impinging field at each atom, (iii) integrating this field over the surface of each atom to find a current density, (iv) computing the radiated field from each atom using physical-optics approximation, and (v) applying the superposition principle to find the field at the receiver. If the atoms are electrically small, one can approximate the re-radiated field by pretending the atoms are point sources and then the received “signal” is basically a superposition of phase-shifted (as 
), amplitude-scaled (as 
) source signals.
A point worth re-iterating is that an atom is a scatterer, not a “mirror”. A more subtle point is that the entire IRS as such, consisting of a collection of scatterers, is itself also a scatterer, not a mirror. “Mirrors” exist only in textbooks, when a plane wave is impinging onto an infinitely large conducting plate (none of which exist in practice). Irrespective of how the IRS is constructed, if it is viewed from far enough away, its radiated field will have a beamwidth that is inversely proportional to its size measured in wavelengths.
Two different operating regimes of IRSs can be distinguished:
1. Both transmitter and receiver are in the far-field of the surface. Then the waves seen at the surface can be approximated as planar; the phase differential from the surface center to its edge is less than a few degrees, say. In this case the phase shifts applied to each atom should be linear in the surface coordinate. The foreseen use case would be to improve coverage, or provide an extra path to improve the rank of a point-to-point MIMO channel. Unfortunately in this case the transmitter-IRS-path loss scales very unfavorably, as 
where is the number of meta-atoms in the surface, and the reason is that again, the IRS itself acts as a (large) scatterer, not a “mirror”. Therefore the IRS has to be quite large before it becomes competitive with a standard single-antenna decode-and-forward relay, a simple, well understood technology that can be implemented using already widely available components, at small power consumption and with a small form factor. (In addition, full-duplex technology is emerging and may eventually be combined with relaying, or even massive MIMO relaying.)
2. At least one of the transmitter and the receiver is in the surface near-field. Here the plane-wave approximation is no longer valid. The IRS can then either be sub-optimally configured to act as a “mirror”, in which case the phase shifts vary linearly as function of the surface coordinate. Alternatively, it can be configured to act as a “lens”, with optimized phase-shifts, which is typically better. As shown for example in this paper, in the near-field case the path loss scales more favorably than in the far-field case. The use cases for the near-field case are less obvious, but one can think of perhaps indoor environments with users close to the walls and every wall covered by an IRS. Another potential use case that I learned about recently is to use the IRS as a MIMO transmitter: a single-antenna transmitter near an IRS can be jointly configured to act as a MIMO beamforming array.
So how useful will IRS technology be in 6G? The question seems open. Indoor coverage in niche scenarios, but isn’t this an already solved problem? Outdoor coverage improvement, but then (cell-free) massive MIMO seems to be a much better option? Building MIMO transmitters from a single-antenna seems exciting, but is it better than using conventional RF? Perhaps it is for the Terahertz bands, where implementation of coherent MIMO may prove truly challenging, that IRS technology will be most beneficial.
A final point is that nothing requires the atoms in an IRS to be located adjacently to one another, or even to form a surface! But they are probably easier to coordinate if they are in more or less the same place.
Very nice summary. I also think the IRS will be more useful in higher frequency bands where the blockage often happens and the RF devices are quite costly and energy-consuming.
Assuming no coupling between atoms close together, close to half a wavelength, is unrealistic and will lead to poor models.
The coupling as a function of frequency is important, maybe the most important, parameter.
For very wideband, octave, the coupling is essential for performance.
That’s a good point. The analysis I have seen in the open literature neglects coupling, which in turn requires significant separation between the atoms. It appears to me that accurate calculations considering coupling would require full-blown EM simulations which may give accurate numerical results, but perhaps less intuitive insights (hence, disfavored by academics who aim at “closed-form” results). Yet this should be done, as the next step in bringing physical realism to the analysis of IRS technology for wireless communications.
When researchers say ‘reconfigurable intelligent surface’, doesn’t it contradict itself. if something is made reconfigurable it automatically implies that it is an intelligent surface.
What do you think?
I agree with you. I like the term “Intelligent reflecting surface” better since every word contributes to describing the concept, while that is not the case with “reconfigurable intelligent surfaces”. However, it seems that the latter term is gaining more traction from the community.
Yes. I agree with you that “intelligent” also implies that both the software (signal processing) and hardware (surface and reconfigurable element) are intelligent.
Hi, Dr. Bjornson
I have been confused by the assumption that a separate reliable wireless control link could be used for the control of the RIS, which is typically adopted in the current studies. However, the so-called passive features would be incorrect, because the smart controler installed at the RIS require identify the phase-shift instructions to adjust the RIS elements. This means we should equip the RIS controler with an active RF antenna to perform the instruction signal reception.
Hope for your help!
Yes, a wireless or wired control link is needed. Some people imagine that the RIS is capable of estimating channels itself and then it will need multiple RF chains (to capture spatial properties). But that is not enough since the base station also must inform the RIS of what is preferable operation.
If the RIS is not sensing anything itself, it can have a single RF chain and the control link can potentially be implemented in a different frequency band.
But as pointed out in our paper on myths and critical questions (https://arxiv.org/abs/2006.03377), the design of the control interface is still under development. Maybe you can figure out a better solution!
Thank you very much for your kind comments!
From (https://arxiv.org/abs/2006.03377), it is shown that ” There is a hope that the RIS technology will be energy-efficient since the array is passive [8] but this remains to be demonstrated quantitatively. The RIS will require a power source for reconfigurability and wireless control channels. It is likely that the control interface will consume most of the power at the RIS, so one cannot predict the total power consumption of the technology before the channel estimation and reconfigurability have been solved and validated.”
This indicates that the current studies of the RIS are under the perfect conceptions!
Yes, this is exactly the point. Unfortunately, the research community is prone to hearsay just as the rest of the society. If someone publishes a paper with a simple anecdotal example how of RIS is more energy efficient than a few other technologies, then other papers might cite this result by claiming “it is known that RIS an energy-efficient solution”. The more time this statement is repeated, there more it seems to be true, even if the support is just a single set of simulations…
These are the tendencies that we try to challenge with that text.