The hype around reconfigurable intelligent surfaces (RIS) has escalated over the past five years. It began with communication theoretic studies based on “guesstimated” models, but gradually became more grounded through experimental validations in sub-6 GHz bands and the formation of an ETSI Industry Specification Group, which has laid the foundation for future standardization of RIS technology.
The outcomes of these efforts are mixed. On the one hand, it is clear that one can build and operate RIS roughly as envisioned. Several universities and companies have built functional RIS prototypes, and there are efficient channel estimation and beamforming algorithms for scenarios where the goal is to create a virtual line-of-sight (LOS) path around a block object (as illustrated in the figure).
On the other hand, it has become clear that practical hardware design is associated with signal losses that are often unaccounted for in theoretical studies, but limit the practical usefulness of RIS. Many actors in the telecom industry remain convinced that alternative solutions (e.g., network-controlled repeaters and small-cell base stations) are more attractive to deploy than RIS. This is aligned with my five-year-old assessment: “RIS is a hammer looking for a nail“. Since we have not yet found a practical problem that RIS can solve significantly better than other technologies, despite intensive research and pre-standardization work, I believe the RIS technology has passed the Peak of Inflated Expectations on the Gartner hype cycle curve.
I currently see the largest potential for RIS deployments in mmWave bands. This assessment is based on three observations:
- The propagation losses through objects grow with the carrier frequency, making it more attractive to send signals around objects than through them at mmWave frequencies compared to sub-6 GHz bands. The coverage of mmWave base stations is typically limited to LOS, e.g., a room or a street segment. Hence, a RIS could extend that coverage to a neighboring room or crossing street by creating a virtual LOS path.
- A RIS of a given physical size provides a larger gain at higher frequencies. The RIS collects incident signal energy proportionally to its size, so that part is frequency-independent. However, the directivity of the reflected beam (beamforming gain) increases with the carrier frequency, as more elements can be fitted into the aperture.
- The deployment of small-cell base stations is less attractive at higher frequencies, as coverage is limited. One RIS can replace one base station in a mmWave band, whereas multiple RISs are needed to replace one small cell at lower frequencies, as the coverage area is larger from such a base station (e.g., it could cover multiple rooms or streets).
The deployment of 5G mmWave networks is currently very limited, but once we identify the right use cases for these bands, we should also consider deploying networks using a combination of base stations and RIS.
The following video explains the fundamentals of RIS and demonstrates how effectively it can improve coverage at mmWave frequencies. The experiments were made using the XRifle Dynamic RIS and Developer Kit from TMYTEK.
Hello Prof. Björnson,
After reading your text, I was left wondering if in the future companies like TP-LINK would consider commercially viable to sell kits with one router and multiple RIS instead of pushing for selling the maximum number of routers. This considering, for example, buyers from large indoor spaces like big homes, university/private companies facilities and so forth. It seems to me that in these kind of spaces the only solution available is to spread as many routers as necessary to provide satisfactory signal coverage. However, the routers used for coverage extension are basically of the same model that the one connected to the ISP (I think). Maybe the reduced cost of the RIS, in comparison to routers/repeaters, could drastically decrease the cost for WLAN implementation in large indoor spaces?
Thank you for your time.
Yes, this would indeed be a possible future scenario. The only thing to keep in mind is WLAN technology for mmWave bands have existed for 15 years (IEEE 802.1ad), but is not widely deployed yet. In the WLAN bands that are mostly used today, deploying a network of routers might be a simpler and still rather affordable solution.
Sir, your blog emphasizes evaluating RIS against alternative deployment options rather than in isolation. Looking toward 6G, do you think future network planning should optimize jointly over base stations, repeaters, RIS, and distributed antenna arrays, selecting the most suitable technology for each location instead of assuming a single architecture?
Yes, future networks should be planned using a heterogenity of nodes. Current networks use a variety of base stations with different spectrum, EIRP, MIMO capability, etc., and repeaters are used to extend coverage into tunnels. RIS and distributed/cell-free MIMO could be added to that mix in the future.
However, as emphasized in this post, I think that RIS is mostly attractively in mmWave bands. I’ve heard people in the telecom industry say the same thing. So looking towards 6G networks, which likely will operate in bands up to 8 GHz, I think that RIS won’t be utilized.
Maybe we will see a renaissance for mmWave technology for indoor usage cases and then RIS might become a part of the mix.
Professor, if future 6G networks primarily operate below 8 GHz, do you think distributed/cell-free Massive MIMO will provide most of the coverage and reliability benefits that many researchers currently expect from RIS?
For PhD students interested in wireless communications, would you recommend focusing on heterogeneous network architectures rather than individual technologies such as RIS or Massive MIMO, since future systems are likely to integrate multiple solutions?
Distributed/cell-free MIMO is not primarily about enhancing coverage, but boost the capacity in the areas where the users are covered by multiple base stations. Instead of letting these base stations interfere with one another, they collaborate to avoid interference and boost desired signal powers.
Coverage improvements can be achieved by integrating mobile networks with LEO satellites, or to develop technology that would be easier/cheaper for telecom operators to deploy. But at the end of the day, coverage is more of a question of economics than technology. The operators could fill coverage gaps by densifying the networks, but they will only do it if they can make more money from it (or to retain customers who would otherwise abandon them).
The primary goal of PhD studies is to learn how to perform research, not to make a direct impact on 6G technology. Hence, if I were you, I would look for some more future-looking technology component that is currently underexplored and that you find interesting. You can then contribute to the knowledge base on that topic, and if you are lucky, then the outcomes could eventually impact future technologies.
Thank you Sir..