For the last five years, most of the research into wireless communications has been motivated by its potential role in 6G. As standardization efforts begin in 3GPP this year, we will discover which of the so-called “6G enablers” has attracted the industry’s attention. Probably, only a few new technology components will be introduced in the first 6G networks in 2029, and a few more will be gradually introduced over the next decade. In this post, I will provide some brief predictions on what to expect.

6G Frequency Band

In December 2023, the World Radiocommunication Conference identified three potential 6G frequency bands, which will be analyzed until the next conference in 2027 (see the image below). Hence, the first 6G networks will operate in one of these bands if nothing unforeseen happens. The most interesting new band is around 7.8 GHz, where 650-1275 MHz of spectrum might become available, depending on the country.

This band belongs to the upper mid-band, the previously overlooked range between the sub-7 GHz and the mmWave bands considered in 5G. It has recently been called the golden band since it offers more spectrum than current 5G networks in the 3.5 GHz band and much better propagation conditions than at mmWave frequencies. But honestly, the new spectrum availability is quite underwhelming: It is around twice the amount that 5G networks will already be using in 2029. Hence, we cannot expect any large capacity boost just from introducing these new bands.

Gigantic MIMO arrays

However, the new frequency bands will enable us to deploy many more antenna elements in the same form factor as current antenna arrays. Since the arrays are two-dimensional, the number grows quadratically with the carrier frequency. Hence, we can expect five times as many antennas in the 7.8 GHz band as at 3.5 GHz, which enables spatial multiplexing of five times more data. When combined with twice the amount of spectrum, we can reach an order of magnitude (10×) higher capacity in the first 6G networks than in 5G.

Since the 5G antenna technology is called “Massive MIMO” and 6G will utilize much larger antenna numbers, we need to find a new adjective. I think “Gigantic MIMO“, abbreviated as gMIMO, is a suitable term. The image below illustrates a 0.5 × 0.5 m array with 25 × 25 antenna elements in the 7.8 GHz band. Since practical base stations often have antenna numbers being a power of two, it is likely we will see at least 512 antenna elements in 6G.

During the last few months, my postdocs and I have looked into what the gMIMO technology could realistically achieve in 6G. We have written a magazine-style paper to discuss the upper mid-band in detail, describe how to reach the envisioned 6G performance targets, and explain what deployment practices are needed to utilize the near-field beamfocusing phenomenon for precise communication, localization, and sensing. We also identify five open research challenges, which we recommend you look into if you want to impact the actual 6G standardization and development.

You can download the paper here: Emil Björnson, Ferdi Kara, Nikolaos Kolomvakis, Alva Kosasih, Parisa Ramezani, and Murat Babek Salman, “Enabling 6G Performance in the Upper Mid-Band Through Gigantic MIMO,” arXiv:2407.05630.

I started analyzing reconfigurable intelligent surfaces (RISs) in 2019, just when the hype began in the communication community. I was mostly interested in understanding the fundamentals of this technology because some claims that I found in papers were too good to be true. In 2020, we summarized our thoughts in the paper “Reconfigurable Intelligent Surfaces: Three Myths and Two Critical Questions,” which corrected three misconceptions that flourished then.

More importantly, we identified two open research problems that we found particularly important to solve if the RIS technology should become practically and commercially viable.

The first question was: What is a Convincing Use Case for RIS? The literature was already full of potential use cases; take any communication scenario and add an RIS, and you will improve the system performance by a little. However, the proposed use cases were either too niche-oriented to motivate technology development or aimed at “solving” problems that existing technology (e.g., relays or small cells) could already manage to some extent. A groundbreaking use case of clear commercial value was missing, and I think we have yet to find it.

The second question was: How can we estimate channels and control a RIS in real time? I was skeptical that it would be possible to solve this problem until I saw a demo video from the University of Surrey in late 2020 and participated in a measurement campaign at the Huazhong University of Science and Technology. I then decided to look for a solution together with my research group at KTH.

Real-time reconfigurability is challenging because a typical RIS has hundreds of elements; thus, there are hundreds of channel parameters to estimate every time the channel changes. A plausible solution is to narrow the scope to use cases where the channels can be parametrized efficiently. If the propagation geometry is such that the channel is determined by the same few parameters, regardless of how many RIS elements there are, we can focus on estimating those parameters instead. We realized that array signal processing contains the necessary tools for developing such geometry-based models and algorithms.

We now have a solid solution framework. In the following video, I explain our approach, from the first principles to the solution. The slides were originally prepared for a keynote speech at IEEE CAMSAP 2023 and then fine-tuned for my recent keynote at IEEE SITB 2024.

The technical details are found in the paper “Parametric Channel Estimation with Short Pilots in RIS-Assisted Near- and Far-Field Communications,” which will appear in the IEEE Transactions on Wireless Communications in 2024.