Tom Marzetta developed the principles of Massive MIMO in the summer of 2006, and at the time, the idea of having 100 active antennas, each with its own electronics, seemed like science fiction.
But it’s happened, and as the industry gears up for next week's Mobile World Congress 2021, albeit physically smaller than years past, the topic of Massive MIMO is likely to come up a lot. Massive MIMO, which stands for Multiple-Input Multiple-Output, is a key enabler for 5G.
Now the director of the research center at NYU Wireless, Marzetta spent many years at Bell Labs, where he was given the freedom to pursue the concepts that led to his advancements in Massive MIMO, which at the time was considered rather radical thinking.
Today, multiple vendors offer Massive MIMO arrays of 64 or 128 active antennas, and they’re compactly packaged. That compactness is worth noting. John Saw, former CTO of Sprint and now EVP of Advanced and Emerging Technologies at T-Mobile, once lamented how the term “Massive MIMO” doesn’t really do it justice because it suggests the equipment is massive, when in fact, that’s not the intent. (Sprint, with its 2.5 GHz spectrum, was an early player in Massive MIMO.)
When it was first envisioned and Marzetta worked on its development, along with many others, there was no specific band of spectrum in mind. “I wasn’t really thinking of anything except a great new principle for wireless systems,” he told Fierce in a recent interview.
Massive MIMO is a relatively easy way for operators to add capacity to networks without shelling out a lot for new spectrum or adding base station sites. It also offers better spectral efficiency, and because it directs power only where it’s needed, it’s a naturally green technology.
Of course, these days, after the record $80 billion spent in the FCC’s C-band auction, mid-band spectrum is very much on everyone’s mind. “C-band, being a shorter wavelength, means the form factor for your antenna array is physically smaller than at 2.5 GHz,” he said. “That’s an advantage.”
One sort of hidden advantage of these recent 5G FCC allocations of spectrum in the U.S. is they’re time division duplex (TDD), whereas a big swath of the lower frequencies is still frequency division duplex, or FDD.
TDD is particularly conducive for Massive MIMO for the task of channel state information (CSI) acquisition. In other words, the base station has to learn the frequency responses of all the channels between its antennas and user antennas in order to do the clever stuff that Massive MIMO does, he said.
“This is just a beautifully simple scheme for the most efficient known way for the base station to get the information it needs to do its work,” he said.
Is Massive MIMO going to see its full potential in 5G? “No,” he said. “I don’t think Massive MIMO is going to reach its full potential in 5G.”
That’s for two reasons. For one, huge segments of the spectrum in the U.S. are still FDD. Over the years, he has recommended that the FCC change policy and mandate the replacement of all FDD spectrum with TDD. However, “that’s a very contentious issue,” he added.
Another reason Massive MIMO is unlikely to reach its full potential in 5G: Unlike previous generations of wireless, 5G was designed with a considerable amount of backward compatibility. In previous generations of wireless, each one made the earlier generation obsolete. For example, with the old 4G user equipment, you can get Massive MIMO service under 5G, but it’s not as good as it could otherwise be. In fact, “we could do 10 times better if we jettisoned FDD” and if didn’t require these new measures to work with old 4G terminals, he said.
The good news, according to Marzetta, is the newer spectrum has been TDD. C-band is TDD, “which is very good.” But the older spectrum at lower frequencies remains FDD for the most part.
Looking to 6G
At NYU Wireless, significant 5G work is still occurring. It’s where a lot of research was done in support of millimeter wave (mmWave) frequencies for wireless communications, and that continues under the leadership of Sundeep Rangan, associate director, and Ted Rappaport, founding director of NYU Wireless, including in the area of terahertz (THz).
However, 6G is also a big priority, and for Marzetta, “I want to do 10 or 100 times better than Massive MIMO can do,” he said.
Theoretically, great improvements are possible. The research that he and his students are doing is very physics-based – bringing in much more wave propagation physics into communications theory than typically has been done, and in the long term, he thinks that has an excellent chance of paying off.
The wireless R&D community needs to take a two-pronged approach, he said. That means continuing with the mmWave research and extending that into terahertz. In parallel, “we ought to be doing even more fundamental research on trying to get breakthroughs for the mid-band,” or the spectrum below 6 GHz, especially considering what operators spent in the recent C-band auction.
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Much more work remains to be done, and like so many folks, Marzetta is looking forward to returning to the classroom, as there’s no substitute for face-to-face meetings. And while the pandemic sidelined the Brooklyn Summit this past spring and caused a lot of upheaval, one thing can be counted on: The summit will be back live in April 2022.