We would like to thank our podcast listeners for all the questions that they have asked on social media. We decided to categorize the questions and answer those related to Massive MIMO in the sixth episode of the podcast Wireless Future. There will be further Q/A episodes next year. The new episode has the following abstract:
In this New Year’s special, Erik G. Larsson and Emil Björnson answer questions from the listeners on the topic of Massive MIMO. Some examples are: How are the antennas calibrated? Will digital beamforming replace analog beamforming? What is channel hardening and how is it related to power control? Can Massive MIMO interact with drones? Practical issues such as the peak-to-average-power ratio (PAPR) and effective isotropic radiated power (EIRP) are also discussed.
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 fifth episode of the podcast Wireless Future, with the following abstract:
What happened to millimeter wave communications? It is often described as synonymous with 5G, but barely any of the brand new 5G networks make use of it. In this episode, Erik G. Larsson and Emil Björnson discuss the basic properties of millimeter waves, whether it is the long sought “holy grail” in wireless communications, and where the technology stands today. To learn more, they recommend the articles “Antenna Count for Massive MIMO: 1.9 GHz versus 60 GHz” and “Massive MIMO in Sub-6 GHz and mmWave: Physical, Practical, and Use-Case Differences”.
You can watch the video podcast on YouTube:
You can listen to the audio-only podcast at the following places:
I taught a course on complex networks this fall, and one component of the course is a hands-on session where students use the SNAP C++ and Python libraries for graph analysis, and Gephi for visualization. One available dataset is DBLP, a large publication database in computer science, that actually includes a lot of electrical engineering as well.
In a small experiment I filtered DBLP for papers with both “massive” and “MIMO” in the title, and analyzed the resulting co-author graph. There are 17200 papers and some 6000 authors. There is a large connected component, with over 400 additional much smaller connected components!
Then I looked more closely at authors who have written at least 20 papers. Each node is an author, its size is proportional to his/her number of “massive MIMO papers”, and its color represents identified communities. Edge thicknesses represent the number of co-authored papers. Some long-standing collaborators, former students, and other friends stand out. (Click on the figure to enlarge it.)
To remind readers of the obvious, prolificacy is not the same as impact, even though they are often correlated. Also, the study is not entirely rigorous. For one thing, it trusts that DBLP properly distinguishes authors with the same name (consider e.g., “Li Li”) and I do not know how well it really does that. Second, in a random inspection all papers I had filtered out dealt with “massive MIMO” as we know it. However, theoretically, the search criterion would also catch papers on, say, MIMO control theory for a massive power plant. Also, the filtering does miss some papers written before the “massive MIMO” term was established, perhaps most importantly Thomas Marzetta’s seminal paper on “unlimited antennas”. Third, the analysis is limited to publications covered by DBLP, which also means, conversely, that there is no specific quality threshold for the publication venues. Anyone interested in learning more, drop me a note.
We have now released the fourth episode of the podcast Wireless Future, with the following abstract:
We are creating a society that is increasingly reliant on access to wireless connectivity. In Sweden, you can barely pay for parking without a mobile phone. Will this wireless future have a negative impact on the security of our data and privacy? In this episode, Emil Björnson and Erik G. Larsson discuss security threats to wireless technology, including eavesdropping, jamming, and spoofing. What impact can these illegitimate techniques have on our lives and what do we need to be aware of?
You can watch the video podcast on YouTube:
You can listen to the audio-only podcast at the following places:
When T. Marzetta introduced the Massive MIMO concept in his seminal article from 2010, he concluded that “the phenomenon of pilot contamination impose[s] fundamental limitations on what can be achieved with a noncooperative cellular multiuser MIMO system.”
More precisely, he showed that the channel capacity under i.i.d. Rayleigh fading converges to a finite limit as the number of base stations goes to infinity. The value of this limit is determined by the interference level in the channel estimation phase. There are hundreds of papers on IEEEXplore that deals with the pilot contamination issue, trying to push the limit upwards or achieve higher performance for a given number of antennas. Various advanced mitigation methods have been developed to cure the symptoms of pilot contamination.
But was pilot contamination really a fundamental limitation to start with? In 2018, we published a paper called “Massive MIMO Has Unlimited Capacity” where we showed that there is an unexpectedly simple solution to the problem. You don’t need a sledgehammer to “crack the pilot contamination nut“, but the right combination of state-of-the-art tools will do. While I have written about this in previous blog posts and briefly mentioned it in videos, I have finally recorded a comprehensive lecture on the topic. It is 82 minutes long and was given online by invitation from Hacettepe University, Turkey. No previous knowledge on the topic is required. I hope you will enjoy it in small or big doses!
We have now released the third episode of the podcast Wireless Future, with the following abstract:
The research towards 6G has already been initiated. One of the most hyped concepts in the research community is “reconfigurable intelligent surfaces”, which can be utilized to create smart walls that capture wireless signals and reflect them towards the user device. In this episode, Erik G. Larsson and Emil Björnson discuss the prospects and limitations of this new technology. Is it the next big thing in wireless? To learn more, they recommend their new overview article “Reconfigurable Intelligent Surfaces: Three Myths and Two Critical Questions”, to appear in IEEE Communications Magazine, which can be downloaded at https://arxiv.org/pdf/2006.03377.
You can watch the video podcast on YouTube:
You can listen to the audio-only podcast at the following places:
5G used to be described as synonymous with millimeter-wave communications, but now when 5G networks are being rolled out all around the world, the focus is instead on Massive MIMO in the 3 GHz band. Moreover, millimeter-wave communications used to be synonymous with hybrid beamforming (e.g., using multiple analog phased arrays), often described as a necessary compromise between performance and hardware complexity. However, digital implementations are already on the way.
Last year, I wrote about experiments by NEC with a 24-antenna base station that carries out digital beamforming in the 28 GHz band. The same convergence towards digital solutions is happening for the chips that can be used in 5G terminals. The University of Michigan published experimental results at the 2020 IEEE Radio Frequency Integrated Circuits Symposium (RFIC) regarding a 16-element prototype for the 28 GHz band. The university calls it the “first digital single-chip millimeter-wave beamformer“. It is manufactured as a single chip using 40 nm CMOS technology and has a dimension of around 3 x 3 mm. The chip doesn’t include the 16 antenna elements (which are connected to it, see the image below and click on it to find larger images) but transceiver chains with low-noise amplifiers, phase-locked loops, analog-to-digital converters (ADCs), etc. While each antenna element has a separate ADC, groups of four adjacent ADCs are summing up their digital signals before they reach the baseband processor. Hence, from a MIMO perspective, this is essentially a digital four-antenna receiver.
One reason to call this a prototype rather than a full-fleshed solution is that the chip can only function as a receiver, but this doesn’t take away the fact that this is an important step forward. In an interview with the Michigan Engineering News Center, Professor Michael P. Flynn (who lead the research) is explaining that “With analog beamforming, you can only listen to one thing at a time” and “This chip represents more than seven years of work by multiple generations of graduate students”.
Needless to say, the first 5G base stations and cell phones that support millimeter-wave bands will make use of hybrid beamforming architectures. For example, the Ericsson Street Macro 6701 (that Verizon is utilizing in their network) contains multiple phased arrays, which can take 4 inputs and thereby produce up to 4 simultaneous beams. However, while the early adopters are making use of hybrid architectures, it becomes increasingly likely that fully digital architectures will be available when millimeter-wave technology becomes more widely adopted around the world.
