A group of regional wireless carriers is calling AT&T's bluff when it comes to claimed interference issues in the lower spectrum bands of the 700 MHz frequency.

On Tuesday, Cavalier Wireless, C Spire Wireless, Continuum 700, King Street Wireless, MetroPCS Communications, U.S. Cellular, and Vulcan Wireless filed a report to the Federal Communications Commission detailing results from a test conducted that shows there are no interference issues between devices operating in other parts of the 700 MHz spectrum frequency bands and the broadcast TV channel 51, which is right next to the lower A block portion of the 700 MHz frequency band.

AT&T has created a separate band class for its 4G LTE devices that would not be interoperable with services deployed in other parts of the 700 MHz spectrum frequency band that was bought by these smaller carriers in the FCC's 2008 auction. AT&T has said it created a new band class due to potential interference issues with adjacent broadcast TV spectrum.

But the regional wireless carriers claim that this study shows no such interference exists.

In a letter to the agency, the wireless operators said that "transmitters and channel 51 TV stations do not pose an interference threat to Lower B and C block device reception." Therefore, the operators conclude that there is no technical reason for AT&T to have its own separate "band class," which excludes smaller players also using similar spectrum in the 700 MHz frequency band.

The filing was made in response to a proceeding that the FCC is considering that looks at whether there are legitimate interference concerns in the lower portion of the 700 MHz frequency band of wireless spectrum.

The history

The FCC auctioned off several slivers of the 700 MHz spectrum in a 2008 auction. The spectrum, which was once used for transmitting analog TV, was carved up into licenses for wireless broadband services. Because it's relatively low frequency spectrum, it propagates over longer distances and can penetrate obstacles. This makes it great for launching new services like 4G LTE. And as a result, the 700 MHz spectrum was likened to "beach-front property" for wireless carriers. Everyone wanted a piece.

Several wireless carriers, including the two largest -- AT&T And Verizon Wireless -- as well as, the smaller regional carriers that filed the report with the FCC, bought spectrum in the 700 MHz auction. But because of how the spectrum licenses were divided and how the spectrum was cleared at different points through its history, no single "band class" exists throughout the whole 700 MHz hunk of spectrum. (This is different from other spectrum bands, such as PCS and AWS, which have one "band class" across all the spectrum.)

The downside of not having a single band class across a band of spectrum is that devices using different band classes are not interoperable. When the 700 MHz spectrum was auctioned in 2008, there was an assumption that there would be two major band classes for commercial use. Companies buying spectrum in the lower A, B, and C blocks would use Band Class 12. And Verizon Wireless, which bought most of the Upper C block, would use Band Class 13. The two band classes wouldn't be interoperable. But because AT&T bought spectrum in the Lower B and C block and because Verizon bought some Lower A and B block licenses, smaller players were confident that a viable ecosystem would be build around Band Class 12, which is for the lower 700 MHz spectrum.

But after the spectrum was auctioned off, AT&T, which only ended up with licenses in the Lower B and C blocks and none in the Lower A block, worked with the 3GPP standards body to develop Band Class 17. This new band class only includes support for Lower B and C block spectrum and it excludes A block spectrum, which are the licenses that the smaller regional players filing the letter to the FCC bought.

Meanwhile, Verizon has deployed its 4G LTE network exclusively using its nationwide Upper C block 700 MHz spectrum. But it has never deployed its A and B block 700 MHz spectrum and is now trying to sell that spectrum to other players in exchange for buying AWS spectrum from cable operators.

AT&T asserts that the reason it asked for a new band class for its Lower B and C block spectrum was because the lower A block is right next to channel 51, which is still used for TV broadcast. The company claims that high-power transmission for broadcast TV at channel 51 interferes with its network equipment and 4G LTE devices operating in the Lower B and C blocks.

But the regional carriers contend that these interference issues don't exist. In their own testing, they say that they have found no interference between devices operating on the Lower B and C Block spectrum and channel 51 broadcast transmission.

Where they acknowledge there is interference is for Lower A block and channel 51 transmitters. Specifically, cell towers transmitting over A block spectrum may experience interference from nearby channel 51 transmitters. But they say this is only an issue for carriers deploying service using Lower A block spectrum. In other words, it's a problem they face in their network deployment and it's not a problem that AT&T would face as a carrier that is not using A block spectrum. And the A block spectrum holders say they've already found ways to mitigate the interference. What they've done is move TV broadcasters to different channels in regions where cell towers and channel 51 transmitters interfere.

Since their study indicates there are no interference issues for AT&T, these regional carriers claim that AT&T has used interference as merely an excuse. They say that by excluding the A block from its band class, AT&T has effectively created its own proprietary band class, which excludes smaller players. They claim that suppliers, such as Qualcomm and Motorola, which have gone along with AT&T's plans for the separate band class, have been slow to develop devices for their own Band Class 12. And because AT&T now has its own band class, devices that are made for carriers that use Lower A block spectrum won't be able to interoperate with AT&T for nationwide roaming.

The companies claim that AT&T's purpose for asking for this separate band class was to exclude them from the 4G LTE market.

But AT&T says this is hogwash. AT&T said the new study that was filed this week is the same study that these companies referenced in a filing late last year when the FCC was considering conditions for AT&T's purchase of 700 MHz spectrum from Qualcomm.

"The filing is little more than a dressed-up version of a study previously filed and refuted," an AT&T spokesman said in an email. " In short, the study proves little more than 'garbage in' will produce 'garbage out.'"

In a previous response to the FCC on a similar interoperability study, AT&T poked holes in how the study was conducted. And it refuted assumptions that were made in the study that suggested that there would be no interference issues to devices operating in the Lower B and C block 700 MHz spectrum in areas where channel 51 is broadcasting.

"The study submitted by Vulcan, which it touts as a "real world" study, does not replicate real world experiences, is limited in scope, and suffers from numerous defects, all of which undermine its credibility," AT&T said in a letter to the FCC dated December 7, 2011.

Comments for the open proceeding at the FCC on the lower 700 MHz interoperability issue close on Friday.