The Wireless Future podcast is back with a new season, and we have released the 46th episode. It has the following abstract:
One of the first topics covered in this podcast was reconfigurable intelligent surfaces (RIS). Five years later, Erik G. Larsson and Emil Björnson return to this topic to reflect on what has happened since then. The conversation covers how these surfaces can improve wave propagation between transmitters and receivers, and identifies the most convincing practical use cases. Core challenges overcome in recent years are discussed, and Emil describes the RIS used in his lab and the lessons learned from his measurements. They also go through new forms of RIS, known as Beyond-Diagonal RIS, STAR-RIS, and Stacked Intelligent Metasurfaces. To learn more, you can read the paper “Reconfigurable Intelligent Surfaces in Upper Mid-Band 6G Networks: Gain or Pain?”.
You can watch the video podcast on YouTube:
You can listen to the audio-only podcast at the following places:
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.
Most of the populated parts of the world have cellular network coverage. You have likely seen base station antennas at both rooftops and in towers, but have you reflected on what the different boxes are?
In the following short video, I take you on a tour of a 4G cell site in Stockholm, where there are antennas and radios for the 700-900 MHz, 1.8+2.1 GHz, and 2.6 GHz bands.
We have now released the 44th episode of the podcast Wireless Future. It has the following abstract:
Coverage holes exist in cellular networks despite decades of wireless technology evolution, but new potential solutions are on the horizon. In this episode, Emil Björnson and Erik G. Larsson discuss network-controlled repeaters, reconfigurable intelligent surfaces, and half-duplex relays. Network-controlled repeaters have attracted particular attention from 3GPP in recent years; the conversation focuses on how these can create strong propagation paths through signal amplification. Implementation challenges related to synchronization, band selectivity, and stability are also covered. A detailed overview is provided in “Achieving Distributed MIMO Performance with Repeater-Assisted Cellular Massive MIMO”. Technical details can be found in: https://arxiv.org/pdf/2405.01074 and https://arxiv.org/pdf/2403.17908
You can watch the video podcast on YouTube:
You can listen to the audio-only podcast at the following places:
We have now released the 43rd episode of the podcast Wireless Future. It has the following abstract:
There are many textbooks to choose between when learning the basics of wireless communications. In this episode, Erik G. Larsson and Emil Björnson discuss the recent book “Introduction to Multiple Antenna Communications and Reconfigurable Surfaces” that Emil has written together with Özlem Tugfe Demir. The conversation focuses on ten subtopics that are covered by the book and differentiates it from many previous textbooks. These are related to the dimensionality of physical constants, the choice of performance metrics, and the motivation behind OFDM signaling. Various system modeling characteristics are discussed, including how the antenna array geometry impacts the channel, dual-polarized signals, carrier frequency dependencies, and the connection between models for small-scale fading and radar cross-sections. The role of non-orthogonal multiple access, hybrid beamforming, and reconfigurable intelligent surfaces are also covered. The textbook is meant for teaching an introductory course on the topic and can be freely downloaded from https://www.nowpublishers.com/NowOpen
You can watch the video podcast on YouTube:
You can listen to the audio-only podcast at the following places:
We have now released the 42nd episode of the podcast Wireless Future. It has the following abstract:
Even if the 6G standardization is just beginning, the last five years of intensive research have illuminated the contours of the next-generation technology. In this episode, Emil Björnson and Erik G. Larsson discuss the recent paper “6G takes shape” written by leading researchers at UT Austin and Qualcomm. The conversation covers lessons learned from 5G, the potential role of new frequency bands and waveforms, and new coding schemes and forms of MIMO. The roles of machine learning and generative AI, as well as satellite integration and Open RAN, are also discussed. The original paper by Jeffrey G. Andrews, Todd E. Humphreys, and Tingfang Ji will appear in the IEEE BITS magazine, and the preprint is available on arXiv.
You can watch the video podcast on YouTube:
You can listen to the audio-only podcast at the following places:
We have now released the 41st episode of the podcast Wireless Future. It has the following abstract:
New cellular network deployments are often associated with new frequency bands. 6G will likely use the upper mid-band from 7-24 GHz. It is called the “golden band” since it provides more spectrum than in current 5G networks and features decent propagation conditions. In this episode, Erik G. Larsson and Emil Björnson discuss the coexistence issues that must be overcome when operating in this band and how much spectrum we can expect to utilize. The future role of multi-antenna technology and its associated challenges are detailed, including the emerging “Gigantic MIMO” term. The prospects of exploiting near-field propagation effects in 6G and the road towards distributed cell-free MIMO are also covered. You can read Emil’s paper about Gigantic MIMO and Nokia’s white paper about coverage evaluation.
You can watch the video podcast on YouTube:
You can listen to the audio-only podcast at the following places:
