Here’s What 6G Will Be, According to the Creator of Massive MIMO

Tom Marzetta is the director of NYU Wireless, New York University's research center for cutting-edge wireless technologies. Prior to joining NYU Wireless, Marzetta was at Nokia Bell Labs, where he developed massive MIMO. Massive MIMO (short for multiple-input multiple-output") allows engineers to pack dozens of small antennas into a single array. The high number of antennas means more signals can be sent and received at once, dramatically boosting a single cell tower's efficiency.

Massive MIMO is becoming an integral part of 5G, as is an independent development that came out of NYU Wireless by the center's founding director Ted Rappaport: Millimeter waves. And now the professors and students at NYU Wireless are already looking ahead to 6G and beyond.

Marzetta spoke with IEEE Spectrum about the work happening at NYU Wireless, as well as what we all might expect from 6G when it arrives in the next decade. The conversation below has been edited for clarity and length.

For several years, NYU Wireless has hosted the Brooklyn 5G Summit. This year, for the first time, it's the Brooklyn 6G Summit instead, which raises the question: When did you start doing 6G research?

I joined NYU Wireless four years ago this September, and I never considered myself to be doing anything at that point but 6G research. I had come from Nokia Bell Labs, where I originated massive MIMO. In 2016, I finished a book, Fundamentals of Massive MIMO, with four other authors, and at that point, I was finished with massive MIMO. And with 5G, as far as I was concerned.

I wanted to try to do research to do something ten times better than massive MIMO. So far, massive MIMO is the most spectrally efficient wireless scheme yet devised. And, and to some extent, it uses familiar principles. But it stretches those principles about as far as they can be stretched. The real question is, are we at the end of the line? Are there going to be no great new innovations in wireless? Or can we do ten or one hundred times better? And so that was why I considered I was working on 6G.

While developing the next, better version of massive MIMO is a clear research path for you, how do others at NYU Wireless pick their projects?

It's a fact of life that professors have to pursue research funds where research funds can be obtained. We're fortunate to have two sources of funding: Our professors get funding from traditional government agencies-the National Science Foundation, DARPA, the Air Force Office of Scientific Research, the Army Research Lab and so on. But we also have approximately 15 affiliate members who give donations. And this is the very best sort of research money because there are absolutely no strings attached, and our professors can use it for any advanced research that they wish to do.

How many competent engineers are there out in industry? I mean, Nokia, for instance, must have on the order of 30,000 very good engineers, and so does its competitors. So it would be a waste if we just added, we'll say, a one percent increment to the very competent development that's already going on in industry. Now that said, with 5G, there are lingering problems that need the attention of NYU Wireless.

What sorts of lingering problems?

One problem is the blockage problem with millimeter waves. Foliage, glass, the human body can all essentially completely attenuate a millimeter wave signal. If my hand or my body can block the signal, and I'm moving around, sometimes I'm going to block the signal. People aren't going to want to use millimeter wave if that happens. One remedy is a larger number of base stations. And now that itself is a problem because there's a handover problem. If you lose connection with one base station, it takes a while to connect with another one. That's an unacceptable delay.

So your next thought is, let's connect to two or more base stations at once. In one sense, that's a waste of resources. But on the other hand, we're operating at 10 times the cellular frequencies, so there's 10 times as much spectrum available. You can afford to be a little bit profligate with spectrum then. There are then a lot of questions about how many base stations do you have in the area, how many people wanting service, and so on, which is not at all a trivial problem.

There's also the applications. Some of our people are looking at using 5G equipment and protocol for remote control of robots and drones. When you want to do closed loop control of something, the very worst thing is if there's a delay. When you're applying a control input, you want to have some feedback as to what that's doing to the drone or to the robot. And if there's a delay, disaster can occur.

NYU WIRELESS NYU students studying measurements for 142 GHz signals on NYU Tandon's campus.

That covers 5G research, but what about 6G research? What's happening there?

Well, we're doing both experimental and theoretical 6G research. Let's talk about the experimental first. Ted Rappaport and Sundeep Rangan are looking at the terahertz band now. Ted, of course, became world-famous for his advocacy of millimeter waves, which go roughly from 28 gigahertz to 98 gigahertz. He wants to go up by another factor of five or so in frequency. And so they're doing both laboratory and outdoor experiments in these bands of frequencies to investigate the propagation. That to be done experimentally-you can theorize about it, but in the end, you've got to do experiments. Otherwise, you don't even know what to simulate. They're doing experiments up to, I believe, 280 gigahertz and getting positive results so far.

Ted and Sundeep have in mind other potential applications beyond cellular communication. The wavelength is one-tenth what millimeter wave is, so, in principle, you can locate a cell phone with 10 times greater precision. You can also start to do very interesting things such as terahertz imaging, for example. These frequencies just bounce off of skin. For instance, you can actually detect heartbeats, so there are medical sensing applications as well.

And what about the theoretical side of 6G?

I want to invent something that's 10 times better than massive MIMO. And my focus, where I think this is going to be applied is not at millimeter wave or terahertz bands, but in the sub-6 GHz bands. In my opinion, these bands will always be, hertz per hertz, the most valuable spectrum. That is where some of these advanced concepts-if they can be made to work-would pay off, economically. It's worth noting the results of FCC spectrum auctions over the last year. They had a very large spectrum auction for millimeter wave spectrum. I don't have the numbers at the moment, but millimeter wave spectrum sold at about US $2 or $3 per hertz. In January, the FCC sold some a block of spectrum in the 3.7 to 3.8 gigahertz band-prime stuff-and that sold for about $290 per hertz. Sub-6 bands, that's always going to be the most valuable spectrum because it can penetrate buildings.

Insomuch as anyone talks about what 6G will be," it oftentimes treated as synonymous with terahertz waves. But even just given your own work, it's clear 6G is more than just making terahertz waves work. So what will 6G be?

What we're really after is, what is the next level of human-to-human communication going to be? In one sense, 5G didn't deliver any new level of human-to-human communication, because 4G, and then the early parts of 5G, enabled ubiquitous streaming video. When I started to work at NYU, I started to commute from New Jersey into Brooklyn, every day. And everybody on the subway was streaming video. It's now commonplace, and I happen to know that 15 or 20 years ago, this was absolutely rejected even at Bell Labs.

But what have we had since then? In other words, 5G comes along, it says, we'll give you this sort of service faster and more reliably. And we'll do things like Internet of Things, and this, that and the other." But much of this has not, and will not, directly affect the average consumer.

What will the next level of human-to-human communication look like?

I, and many people, think the next level of human-to-human communication is going to be ubiquitous augmented reality. How could most of us have done our jobs over the last year without high-quality telecommunications and so on? But people are sick of the zoom experience. The next level is obviously augmented reality, and making it good enough so that the other person is effectively in the same room as you. That would be a transformative thing. From a wireless communications perspective, this imposes simply staggering requirements. People talk about sustained AR requiring throughputs per user of 2 gigabits per second, how are you going to do that? Suppose you have 50,000 people in Time Square all wanting AR at the same time? How are you going to give 2 Gbps each to 50,000 people crammed into a quarter square mile? We don't know how to do that yet.