The name “Massive MIMO” has been debated since its inception. Tom Marzetta introduced it ten years ago as one of several potential names for his envisioned MIMO technology with a very large number of antennas. Different researchers used different terminologies in their papers during the first years of research on the topic, but the community eventually converged to calling it Massive MIMO.
The apparent issue with that terminology is that the adjective “massive” can have different meanings. The first definition in the Merriam-Webster dictionary is “consisting of a large mass”, in the sense of being “bulky” and “heavy”. The second definition is “large in scope or degree”, in the sense of being “large in comparison to what is typical”.
It is probably the second definition that Marzetta had in mind when introducing the name “Massive MIMO”; that is, a MIMO technology with a number of antennas that is large in comparison to what was typically considered in the 4G era. Yet, there has been a perception in the industry that one cannot build a base station with many antennas without it also being bulky and heavy (i.e., the first definition).
Massive MIMO products are not heavy anymore
Ericsson and Huawei have recently proved that this perception is wrong. The Ericsson AIR 6419 that was announced in February (to be released later this year) contains 64 antenna-integrated radios in a box that is roughly 1 x 0.5 m, with a weight of only 20 kg. This can be compared with Ericsson’s first Massive MIMO product from 2018, which weighed 60 kg. The product is designed for the 3.5 GHz band, supports 200 MHz of bandwidth, and 320 W of output power. The box contains an application-specific integrated circuit (ASIC) that handles parts of the baseband processing. Huawei introduced a similar product in February that weighs 19 kg and supports 400 MHz of spectrum, but there are fewer details available regarding it.
These products seem very much in line with what Massive MIMO researchers like me have been imagining when writing scientific papers. It is impressive to see how quickly this vision has turned into reality, and how 5G has become synonymous with Massive MIMO deployments in sub-6 GHz bands, despite all the fuss about small cells with mmWave spectrum. While both technologies can be used to support higher traffic loads, it is clear that spatial multiplexing has now become the primary solution adopted by network operators in the 5G era.
Open RAN enabled Massive MIMO
While the new Ericsson and Huawei products demonstrate how a tight integration of antennas, radios, and baseband processing enables compact, low-weight Massive MIMO implementation, there is also an opposite trend. Mavenir and Xilinx have teamed up to build a Massive MIMO solution that builds on the Open RAN principle of decoupling hardware and software (so that the operator can buy these from different vendors). They claim that their first 64-antenna product, which combines Xilinx’s radio hardware with Mavenir’s cloud-computing platform, will be available by the end of this year. The drawback with the hardware-software decoupling is the higher energy consumption caused by increased fronthaul signaling (when all processing is done “in the cloud”) and the use of field-programmable gate arrays (FPGAs) instead of ASICs (since a higher level of flexibility is needed in the processing units when these are not co-designed with the radios).
Since the 5G technology is still in its infancy, it will be exciting to see how it evolves over the coming years. I believe we will see even larger antenna numbers in the 3.5 GHz band, new array form factors, products that integrate many frequency bands in the same box, digital beamforming in mmWave bands, and new types of distributed antenna deployments. The impact of Massive MIMO will be massive, even if the weight isn’t massive.
Mobile networks are divided into semi-autonomous cells. It is essentially a divide-and-conquer approach to network operation, where each cell becomes simple to operate and the reuse of radio resources over the cells can be planned in advance. This network structure was proposed already in the 1950s and has been vital for the wide-spread adoption of mobile network technology. However, the weaknesses of the cellular architecture have become increasingly apparent as mobile data has replaced voice calls as the main type of traffic. While the peak data rates are high in contemporary networks, the user-guaranteed rates are very modest, due to the largest pathloss variations and inter-cell interference that is inherent in the cellular architecture.
A promising solution to these issues is to leave the cellular paradigm behind and create a new network architecture that is free from cells. This vision is called Cell-free Massive MIMO.
This is a technology that essentially combines three main components that have been previously considered separately: 1) the efficient physical-layer operation with many antennas that enabled wide-spread adoption of Massive MIMO in cellular networks; 2) the vision of deploying many access points close to the users, to create a reality where users are surrounded by access points instead of the opposite; 3) the joint transmission and reception from distributed access points, that have been analyzed under many names over the last two decades, including coordinated multipoint (CoMP).
This blog post is about the first book on the topic: “Foundations of User-Centric Cell-Free Massive MIMO” by Özlem Tuğfe Demir, Emil Björnson, and Luca Sanguinetti. We provide the historical background, theoretical foundations, and state-of-the-art signal processing algorithms. The book is 300 pages long and is accompanied by a GitHub repository with all the simulation code. We hope that this book will serve as the starting point for much further research. The last section of the book outlines many future research directions.
NOW publishers is offering a free PDF until April 2, 2021. To obtain it, go to the book’s website, create a free account, and then click on download. For the same period, they are offering printed copies for the special prize of $40 (including non-trackable shipping). To purchase the printed version, go to the secure Order Form and use the Promotion Code 584793.
Since 5G is designed to be future-proof and enable decoupling of the control signaling and data transmissions, I believe that the 5G networks will become increasingly cell-free during this decade, while beyond 5G networks will embrace the cell-free architecture from the outset.
A new EU-funded 6G initiative, the REINDEER project, joins forces from academia and industry to develop and build a new type of multi-antenna-based smart connectivity platform integral to future 6G systems. From Ericsson’s new site:
The project’s name is derived from REsilient INteractive applications through hyper Diversity in Energy-Efficient RadioWeaves technology, and the development of “RadioWeaves” technology will be a key deliverable of the project. This new wireless access infrastructure consists of a fabric of distributed radio, compute and storage. It will advance the ideas of large-scale intelligent surfaces and cell-free wireless access to offer capabilities far beyond future 5G networks. This is expected to offer capacity scalable to quasi-infinite, and perceived zero latency and interaction with a large number of embedded devices.
In my latest magazine paper, I identified real-time reconfigurability as the key technical research challenge: we need fast algorithms for over-the-air channel estimation that can handle large surfaces and complex propagation environments. In other words, we need hardware that can be reconfigured and algorithms to find the right configuration.
The literature contains several theoretical algorithms but it is a very different thing to demonstrate real-time reconfigurability in lab experiments. I was therefore impressed when finding the following video from the team of Dr. Mohsen Khalily at the University of Surrey:
The video shows how a metasurface is used to reflect a signal from a transmitter to a receiver. In the second half of the video, they move the receiver out of the reflected beam from the metasurface and then press a button to reconfigure the surface to change the direction of the beam.
I asked Dr. Khalily to tell me more about the setup:
“The metasurface consists of several conductive printed patches (scatterers), and the size of each scatterer is a small proportion of the wavelength of the operating frequency. The macroscopic effect of these scatterers defines a specific surface impedance and by controlling this surface impedance, the reflected wave from the metasurface sheet can be manipulated. Each individual scatterer or a cluster of them can be tuned in such a way that the whole surface can reconstruct radio waves with desired characteristics without emitting any additional waves.”
The surface shown in the video contains 2490 patches that are printed on a copper ground plane. The patches are made of a new micro-dispersed ceramic PTFE composite and designed to support a wide range of phase variations along with a low reflection loss for signals in the 3.5 GHz band. The design of the surface was the main challenge according to Dr. Khalily:
“Fabrication was very difficult due to the size of the surface, so we had to divide the surface into six tiles then attach them together.Our surface material has a higher dielectric constant than the traditional PTFE copper-clad laminates to meet the design and manufacturing of circuit miniaturization. This material also possesses high thermal conductivity, which gives an added advantage for heat dissipation of the apparatus.”
The transmitter and receiver were in the far-field of the metasurface in the considered experimental setup. Since there is an unobstructed line-of-sight path, it was sufficient to estimate the angular difference between the receiver and the main reflection angle, and then adjust the surface impedance to compensate for the difference. When this was properly done, the metasurface improved the signal-to-noise ratio (SNR) by almost 15 dB. I cannot judge how close this number is to the theoretical maximum. In the considered in-room setup with highly directional horn antennas at the transmitter and receiver, it might be enough that the reflected beam points in roughly the right direction to achieve a great SNR gain. I’m looking forward to learning more about this experiment when there is a technical paper that describes it.
This is not the first experiment of this kind, but I think it constitutes the state-of-the-art when it comes to bringing the concept of reconfigurable intelligent surfaces from theory to practice.
I am giving a tutorial on “Beyond Massive MIMO: User-Centric Cell-Free Massive MIMO” at Globecom 2020, together with my colleagues Luca Sanguinetti and Özlem Tuğfe Demir. It is a prerecorded 3-hour tutorial that can be viewed online at any time during the conference and there will be a live Q/A session on December 11 where we are available for questions.
The tutorial is based on our upcoming book on the topic: Foundations on User-Centric Cell-free Massive MIMO.
Until December 11 (the last day of the tutorial), we are offering a free preprint of the book, which can be downloaded by creating an account at the NOW publishers’ website. By doing so, I think you will also get notified when the final version of the book is available early next year, so you can gain access to the final PDF and an offer to buy printed copies.
If you download the book and have any feedback that we can take into account when preparing the final version, we will highly appreciate to receive it! Please email me your feedback by December 15. You find the address in the PDF.
The abstract of the tutorial is as follows:
Massive MIMO (multiple-input multiple-output) is no longer a promising concept for cellular networks-in 2019 5G it became a reality, with 64-antenna fully digital base stations being commercially deployed in many countries. However, this is not the final destination in a world where ubiquitous wireless access is in demand by an increasing population. It is, therefore, time for MIMO and mmWave communication researchers to consider new multi-antenna technologies that might lay the foundations for beyond 5G networks. In particular, we need to focus on improving the uniformity of the service quality.
Suppose all the base station antennas are distributed over the coverage area instead of co-located in arrays at a few elevated locations, so that the mobile terminals are surrounded by antennas instead of having a few base stations surrounded by mobile terminals. How can we operate such a network? The ideal solution is to let each mobile terminal be served by coherent joint transmission and reception from all the antennas that can make a non-negligible impact on their performance. That effectively leads to a user-centric post-cellular network architecture, called “User-Centric Cell-Free Massive MIMO”. Recent papers have developed innovative signal processing and radio resource allocation algorithms to make this new technology possible, and the industry has taken steps towards implementation. Substantial performance gains compared to small-cell networks (where each distributed antenna operates autonomously) and cellular Massive MIMO have been demonstrated in numerical studies, particularly, when it comes to the uniformity of the achievable data rates over the coverage area.
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.