The fundamental technologies that have made 5G possible are unequivocally massive MIMO (multiple-input multiple-output) and millimeter wave (mmWave) technologies. Without these two technologies there would be no 5G network as we now know it.
The two men, who were the key architects behind these fundamental technologies for 5G, have been leading one of the premier research institutes in mobile telephony since 2012: NYU Wireless, a part of NYU's Tandon School of Engineering.
Ted RappaportNYU Wireless
Ted Rappaport is the founding director of NYU Wireless, and one of the key researchers in the development of mmWave technology. Rappaport also served as the key thought leader for 5G by planting the flag in the ground nearly a decade ago that argued mmWave would be a key enabling technology for the next generation of wireless. His earlier work at two other wireless centers that he founded at Virginia Tech and The University of Austin laid the early groundwork that helped NYU Wireless catapult into one of the premier wireless institutions in the world.
Thomas Marzetta, who now serves as the director of NYU Wireless, was the scientist who led the development of massive MIMO while he was at Bell Labs and has championed its use in 5G to where it has become a key enabling technology for it.These two researchers, who were so instrumental in developing the technologies that have enabled 5G, are now turning their attention to the next generation of mobile communications, and, according to them both, we are facing some pretty steep technical challenges to realizing a next generation of mobile communications.
"Ten years ago, Ted was already pushing mobile mmWave, and I at Bell Labs was pushing massive MIMO," said Marzetta. "So we had two very promising concepts ready for 5G. The research concepts that the wireless community is working on for 6G are not as mature at this time, making our focus on 6G even more important."
This sense of urgency is reflected by both men, who are pushing against any sense of complacency that may exist in starting the development of 6G technologies as soon as possible. With this aim in mind, Rappaport, just as he did 10 years ago, has planted a new flag in the world of mobile communications with his publication last year of an article with the IEEE, entitled "Wireless Communications and Applications Above 100 GHz: Opportunities and Challenges for 6G and Beyond"
"In this paper, we said for the first time that 6G is going to be in the sub-terahertz frequencies," said Rappaport. "We also suggested the idea of wireless cognition where human thought and brain computation could be sent over wireless in real time. It's a very visionary look at something. Our phones, which right now are flashlights, emails, TV browsers, calendars, are going to be become much more."
Tom MarzettaPhoto: NYU Wireless
While Rappaport feels confident that they have the right vision for 6G, he is worried about the lack of awareness of how critical it is for the US Government funding agencies and companies to develop the enabling technologies for its realization. In particular, both Rappaport and Marzetta are concerned about the economic competitiveness of the US and the funding challenges that will persist if it is not properly recognized as a priority.
“These issues of funding and awareness are critical for research centers, like NYU Wireless," said Rappaport. “The US needs to get behind NYU Wireless to foster these ideas and create these cutting-edge technologies."
With this funding support, Rappaport argues, teaching research institutes like NYU Wireless can create the engineers that end up going to companies and making technologies like 6G become a reality. “There are very few schools in the world that are even thinking this far ahead in wireless; we have the foundations to make it happen," he added.
Both Rappaport and Marzetta also believe that making national centers of excellence in wireless could help to create an environment in which students could be exposed constantly to a culture and knowledge base for realizing the visionary ideas for the next generation of wireless.
“The Federal government in the US needs to pick a few winners for university centers of excellence to be melting pots, to be places where things are brought together," said Rappaport. “The Federal government has to get together and put money into these centers to allow them to hire talent, attract more faculty, and become comparable to what we see in other countries where huge amounts of funding is going in to pick winners."
While research centers, like NYU Wireless, get support from industry to conduct their research, Rappaport and Marzetta see that a bump in Federal funding could serve as both amplification and a leverage effect for the contribution of industrial affiliates. NYU Wireless currently has 15 industrial affiliates with a large number coming from outside the US, according to Rappaport.
“Government funding could get more companies involved by incentivizing them through a financial multiplier," added Rappaport
Of course, 6G is not simply about setting out a vision and attracting funding, but also tackling some pretty big technical challenges.
Both men believe that we will need to see the development of new forms of MIMO, such as holographic MIMO, to enable more efficient use of the sub 6 GHz spectrum. Also, solutions will need to be developed to overcome the blockage problems that occur with mmWave and higher frequencies.
Fundamental to these technology challenges is accessing new frequency spectrums so that a 6G network operating in the sub-terahertz frequencies can be achieved. Both Rappaport and Marzetta are confident that technology will enable us to access even more challenging frequencies.
“There's nothing technologically stopping us right now from 30, and 40, and 50 gigahertz millimeter wave, even up to 700 gigahertz," said Rappaport. “I see the fundamentals of physics and devices allowing us to take us easily over the next 20 years up to 700 or 800 gigahertz."
Marzetta added that there is much more that can be done in the scarce and valuable sub-6GHz spectrum. While massive MIMO is the most spectrally efficient wireless scheme ever devised, it is based on extremely simplified models of how antennas create electromagnetic signals that propagate to another location, according to Marzetta, adding, “No existing wireless system or scheme is operating close at all to limits imposed by nature."
Tom Marzetta with an arrayPhoto: NYU Wireless
While expanding the spectrum of frequencies and making even better use of the sub-6GHz spectrum are the foundation for the realization of future networks, Rappaport and Marzetta also expect that we will see increased leveraging of AI and machine learning. This will enable the creation of intelligent networks that can manage themselves with much greater efficiency than today's mobile networks.
“Future wireless networks are going to evolve with greater intelligence," said Rappaport. An example of this intelligence, according to Rappaport, is the new way in which the Citizens Broadband Radio Service (CBRS) spectrum is going to be used in a spectrum access server (SAS) for the first time ever.
“It's going to be a nationwide mobile system that uses these spectrum access servers that mobile devices talk to in the 3.6 gigahertz band," said Rappaport. “This is going to allow enterprise networks to be a cross of old licensed cellular and old unlicensed Wi-Fi. It's going to be kind of somewhere in the middle. This serves as an early indication of how mobile communications will evolve over the next decade."
These intelligent networks will become increasingly important when 6G moves towards so-called cell-less ("cell-free") networks.
Currently, mobile network coverage is provided through hundreds of roughly circular cells spread out across an area. Now with 5G networks, each of these cells will be equipped with a massive MIMO array to serve the users within the cell. But with a cell-less 6G network the aim would be to have hundreds of thousands, or even millions, of access points, spread out more or less randomly, but with all the networks operating cooperatively together.
“With this system, there are no cell boundaries, so as a user moves across the city, there's no handover or handoff from cell to cell because the whole city essentially constitutes one cell," explained Marzetta. “All of the people receiving mobile services in a city get it through these access points, which in principle, every user is served by every access point all at once."
One of the obvious challenges of this cell-less architecture is just the economics of installing so many access points all over the city. You have to get all of the signals to and from each access point from or to one sort of central point that does all the computing and number crunching.
While this all sounds daunting when thought of in the terms of traditional mobile networks, it conceptually sounds far more approachable when you consider that the Internet of Things (IoT) will create this cell-less network.
“We're going to go from 10 or 20 devices today to hundreds of devices around us that we're communicating with, and that local connectivity is what will drive this cell-less world to evolve," said Rappaport. “This is how I think a lot of 5G and 6G use cases in this wide swath of spectrum are going to allow these low-power local devices to live and breathe."
To realize all of these technologies, including intelligent networks, cell-less networks, expanded radio frequencies, and wireless cognition, the key factor will be training future engineers.
To this issue, Marzetta noted: “Wireless communications is a growing and dynamic field that is a real opportunity for the next generation of young engineers."
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