On the Hot Seat—Google’s Preston Marshall looks to the future of the 3.5 GHz CBRS band

Google Preston Marshall with his CBRS book
Google's Preston Marshall recently wrote a book on the CBRS band.

Preston Marshall is the principal wireless architect for Google Access, and he has been one of the leaders in developing spectrum sharing rules for the FCC's Citizens Broadband Radio Service (CBRS) at 3.5 GHz. Marshall, through Google, has been working to create rules around the band, as well as positioning Google as a Spectrum Access System (SAS) vendor. SAS providers will operate a database of CBRS users to ensure the spectrum is shared appropriately.

Prior to his work at Google, Marshall was deputy director of the University of Southern California (USC) Information Sciences Institute (ISI), the director of the Computational Science and Technology Division, and a research professor in the Viterbi School of Engineering Electrical Engineering Department. Before working at USC, he was a program manager at the Defense Advances Research Projects Agency (DARPA) as a member of the U.S. Senior Executive Service.

FierceWireless spoke with Marshall at last weeks Mobile World Congress Americas trade show, where he was a main participant in the CBRS Alliances meeting. Below is a transcript of that interview, edited for clarity and brevity.

FierceWireless: What is the current status of the 3.5 GHz CBRS band? What needs to happen next?

Marshall: We have to get through the [FCCs] SAS certification. I think the device certification is something everyone knows how to do. The SAS certification is a much more complex problem. It has stakeholder interest by the Navy, protecting their ships. And so industry has worked, I think, very closely with the FCC and Department of Defense and NTIA to come up with our process. But it's a brand new process. It's a much more complex testing regime than just testing other band signals and such that we would typically do.

FierceWireless: How is the SAS business, and shared spectrum in general, different in the CBRS band when compared with TV White Spaces?

Marshall: Philosophically, the idea of having spectrum managed by a cloud service is similar. Beneath that, the details are very different. We manage coexistence. TV White Space just grew. TV White Space has the burden of showing it doesn't interfere with TV receivers. And with a three-tier [licensing system in CBRS], we protect the rights of incumbent users. We protect the rights of protected users, but we let people use the spectrum that wasn't used. This is a fundamental change. In the past, if you had a license, anything in that area, you had a license, it couldn't be used by someone else. So, you could think of a spectrum license in the past as carrying two rights. It had the right that you wouldn't be interfered with, but it also had the right to exclude.

So, the broadcasters [in TV White Spaces] could exclude anyone within their footprint. A carrier who bought spectrum has a right to exclude anyone within the area they bought it. In this band, there's no right to exclude. You can purchase the right to be protected, but if you don't use that piece of spectrum, it's open to anyone. So this third tier [in CBRS] has its own spectrum, but it also has the right to use any unused spectrum in the PALs tier.

So if you're out in the middle of nowhere, maybe it doesn't make sense to buy PALs at all.

A lot of spectrum in the United States goes unused. Carriers have to buy spectrum to meet their densest demands. Areas with less density get less deployment, thus spectrum is unused. So this SAS allows us to deal with that. It makes use of every bit of spectrum and still provides people the rights that they would have purchased for the FCC licensing process. And that is a very, very significant increase.

FierceWireless: Have you been surprised by any of the actions recently in the CBRS space?

Marshall: I was shocked to see how many people came today [to the CBRS Alliances meeting at the MWC Americas show]. I've been shocked to see how many people are joining CBRS Alliance, and the WinnForum. I think we thought there would be a massive success in this band. Maybe we thought it would be a little slower. We were the, I think, the leading advocate for the band in its conception. Now we're just one of many. So, I think that's a fundamental change. We've seen companies that took a closer look at it that might have first been concerned about it.

FierceWireless: Which of the three tiers of licensing in CBRS do you expect companies to migrate to? The unlicensed tier or the licensed tier?

Marshall: In the past if you wanted to do wireless, paying whatever it took was existential. There was no way you could be a wireless supplier if you don't go buy spectrum and pay whatever it took. It was existential. Now what we've made is spectrum trade specs.

So if I pay a million dollars for a base station and someone interferes with it by 5%, you might say it was worth $50,000 to buy spectrum. If I pay $400 for that same base station, how much money would I spend to protect 10% of its input from interference? Not much.

Now that the cost of spectrum is fungible to the cost of equipment, if spectrum cost is high, we put more equipment out there, build a little denser network with shorter range, and you get the same effect and probably have a better network.

It was a bidding war for spectrum before, because there was no way to trade spectrum for any other asset. Now we can start to see that spectrum, infrastructure, radio count and performance are all tradable. And that puts a ceiling of valuable spectrum. The more boxes people build, the lower the cost of the boxes. And presumably the more flexibility people will have to do things other than pay a lot for spectrum.

Money paid for spectrum doesn't build infrastructure. It doesn't service users. It doesn't lower costs. It doesn't create any value for the consumer or the company. So bringing a reasonable market for spectrum, it represents its value into the ecosystem, but it is not extortion. It is going to be a fundamental change in how spectrum is looked at.

FierceWireless: So you expect relatively low prices in CBRS spectrum auctions?

Marshall: The price for the spectrum will be appropriate to the value of protection versus non protection. And the alternative is available. Cellular has always asked for more and more spectrum. But in the end, infrastructure and spectrum are fungible. If you don't have a lot of spectrum, you have to build a denser network. If you have a lot of spectrum, you don't have to be as dense. It's always been fungible. You could build any amount of throughput with either strategy: densifying or more spectrum.

We've now really brought that down to a very, very finite level and presumably lower values. So whether the value is low—I would view low-value spectrum as a good thing because what that means is we've created something good.

When someone says a successful auction is one that raised a lot of money, well, what did that mean? That meant that the government had created an artificial scarcity that drove the value up. The government should be trying to provision spectrum to the economy. So it shouldn't cost a lot of money to buy spectrum, because that's all lost. That's all going into infrastructure. It's not serving anyone.

FierceWireless: And success also for the wireless user?

Marshall: For public policy, for consumers. There's a massive amount of wealth tied up in spectrum acquisition that could just as well have gone to infrastructure. It could have gone to any number of things. So I don't view the success of CBRS as people pay a lot of money for the spectrum. If we've done our job right with coexistence with GAA, then people won't feel compelled to spend a lot. Spend some money, yes, to get protection. But when it gets exorbitant like what we've seen in some of the auctions, then we have technical solutions to solve it.

We incentivize people. In the past, you had an incentive to build systems that were intolerant to interference, because then you could go to the FCC and push people out. Now, you have an incentive to tolerate interference. Defined technologies that can deal with a little interference, because now you don't have to go buy spectrum. This is virtuous. The other system had moral hazard in it. It actually made people benefit by building poor systems. Now they benefit by building good ones.

We've got things maybe back on the right track.

FierceWireless: This viewpoint represents a major change for established wireless players.

Marshall: It was a fight for coverage. Now we're in a fight for capacity. And it's a fundamentally different thing.

For capacity, beachfront spectrum is high-band. All of the ideas that have sort of driven our assumptions about spectrum were geared towards a completely different problem [because spectrum was primarily used for coverage not capacity]. Now we want capacity in cities, in dense areas. I don't want to build a very, very high-capacity light area. I want to build lots of small networks. It's not having spectrum that will drive the future, it's infrastructure.

Whoever has infrastructure that's close to people -- not the ugly tower they put on the edge of town, which was the old model -- now it's going to be who has fiber in the building that can get high-capacity to small cells and CBRS. Who has fiber along the street. These will be the things that create capacity for people, not spectrum. And CBRS provides everyone spectrum so that now they can exploit their other natural advantages in existing infrastructure.