T-Mobile US (NYSE:TMUS) is making it crystal clear: It does not like the idea of a Spectrum Access System (SAS) for coordinating terrestrial licensees and satellite operators in millimeter wave bands.

While a SAS is fine for the 3.5 GHz band, where the FCC designated three tiers -- Incumbent Access, Priority Access (PAL) and General Authorized Access (GAA) – it might not even be technically feasible at the higher millimeter bands, according to John Hunter, director of spectrum policy at T-Mobile, who was one of the panelists at the FCC's Spectrum Frontiers workshop last week.

One of the proposals in the FCC's Spectrum Frontiers Notice of Proposed Rulemaking (NPRM) suggested importing the SAS model from the 3.5 GHz band. Under the SAS option in the NPRM, the FCC proposed the idea of requiring terrestrial licensees to provide satellite operators with information, like geographic coordinates, that the satellite operators would need in order to avoid causing interference to terrestrial operations.

"I think we need to take a step back and really think about what the SAS was intended for," which involved military radar operations, Hunter said. In 3.5 GHz, "those are known locations and those will just be added to the database, so there's really no need for a SAS in millimeter wave bands to offer up the type of protection we're going to try to afford for the incumbents," he said, adding that the 3.5 GHz is a very different scenario than what's been purported in the hybrid solution for 37 GHz.

In 3.5 GHz, a portion of the spectrum will be set aside for GAA use and a portion will be set aside for PAL use, so "you're not going to have interference … you've got the sensing mechanism, but when you're talking about trying to implement a system like 3.5 in millimeter wave bands," where the sensing is going to be probably much more difficult to do at those frequencies than it is at 3.5, "I'm just not sure … if it's even technically viable at this time," Hunter said.

John Marsh, vice president of regulatory affairs at AT&T (NYSE: T), agreed. "You look at a band like 37 to 39, there's a lot of territory there, there's a lot of real estate, and we think there is opportunity for all types of licenses there," she said. "Our proposal is look at that band as one contiguous band, build some very broad bands there, and I think the record supports 200 megahertz minimum, maybe larger."

AT&T has told the commission that rather than experimenting with a hybrid licensing regime, the FCC should combine the 37 GHz band with the 39 GHz band and license the spectrum on a flexible-use basis.

"I certainly agree, the hybrid licensing scheme with the indoor building proposal … we think that's not a really good idea at all," Marsh said. If that kind of experimentation is desired, she suggested the FCC could pick a single block in the band and pursue it there.

Both T-Mobile and AT&T have registered their opposition to the SAS model in the millimeter wave bands in separate comments filed with the commission.

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