The future of spectrum is at a crossroads
PITTSBURGH, Penn.—Good wireless spectrum is hard to find, and it’s getting harder and more complex. There are pressures all the way from the local level to the international one, for leadership in defining new bands that will shape advanced 5G and future 6G systems, as well as make more efficient use of the available airwaves.
During a panel discussion at the Competitive Carrier Association’s Mobile Carrier Show this week in Pittsburgh, the inflection point became clear. The Federal Communications Commission’s spectrum auction authority has been allowed to lapse for the first time in 30 years. There is an ongoing technical and policy scramble to come up with a spectrum pipeline of bands that could be shared or repurposed, with the goal of finding 1,500 megahertz—and that is shaping up to be far from an easy or fast. Meanwhile, the next World Radio Conference, where international discussions to shape 6G bands will take place, is looming, with the possibility that if the U.S. doesn’t have a clear leadership stance on spectrum, then it will cede a 6G advantage to China.
Meanwhile, on a very practical level, the lapse of the FCC’s authority has meant that the Commission is not issuing licenses won (and paid for) in the 2.5 GHz auction to companies like T-Mobile US, which dominated that auction in pursuit of bolstering its already substantial 2.5 GHz holdings. John Hunter, senior director of technology and engineering policy for T-Mo, said that the carrier’s position is that the FCC has the authority to issue those licenses because the auction itself has concluded and the monies paid. The agency could also issue special temporary authority (STAs) that would enable the spectrum to be used, he added.
But the agency’s auction authority remains in limbo as part of the broader spectrum discussions happening in Washington. Rebecca Murphy Thompson, VP of government affairs for US Cellular, said that the FCC is pushing hard on Congress and that political leaders she has spoken with seem to understand the importance of the related issues and the reauthorization. “I’m pretty confident that we’ll get there,” she said. In the meantime, there is spectrum-related work that continues to be done, she pointed out, including NTIA’s development of a long-term spectrum pipeline and the FCC’s new item on 12.7 GHz spectrum.
One of the potential bands that could be commercialized is 3.1-3.45 GHz, which is of interest because it is midband spectrum adjacent to bands already in use (3.45-3.55 GHz) and is already being looked at for sharing. But it’s a complex situation, with Department of Defense sea-, land- and air-based radar systems as incumbents in the bajd.
“There’s been a lot of good developments to make that work,” Hunter said, adding that there will be a report coming soon with more tangible details. His concern about the process—which was prompted by Congress directing DoD to study the band as part of the federal infrastructure bill—is that it focuses on studying interference into DoD systems, not interference into commercial systems, and there is a significant differential in the relative power of DoD’s megawatt systems compared to cellular networks using kilowatts of power. “It’s going to do us little good if we can show somehow through technical arrangements within our networks that we can protect DoD, only to find as we turn up our networks, that we have so much interference that it renders our networks useless, in a sense,” Hunter said. “I think we have to look at both sides of the coin. I think you need to do the study not in a vacuum, looking at both sides of interference. I think that’s the only way … you can get sharing to work in this band, particularly something this complex.”
The power sticking point is a broader fundamental issue, pointed out Murphy Thompson: While spectrum is crucial to mobile operators’ businesses, so are the details of power constraints, which directly affect the utility of spectrum. US Cellular operates in parts of 21 states, with 41% of its footprint in rural areas. “We don’t have a lot of clusters, which means we need higher power to reach those on the outer edge of the cell,” she said. This issue has come to the fore in CBRS, which mandates significantly lower transmit power in the shared band than is typical in cellular operators—to the tune of a roughly 1.5-kilometer radius for CBRS propagation versus 9 kilometers for C-Band, despite both bands being at 3 GHz, Murphy Thompson said. While US Cellular has CBRS Priority Access Licenses, she added, “we have found it to be very operationally challenging, especially in rural areas, because of this power issue.”
David Zylka, CTO of Carolina West Wireless, confirmed this. The carrier, which operates in 11 western North Carolina counties, has CBRS spectrum and wants to use it like other bands, he said. “Our infrastructure in rural America was designed for low-band” in terms of cell-site spacing, he explained. “By having CBRS at a lower power level, it requires us to go out and build more sites and invest more money into an economically challenged business model.” He gave the example of a site that sits on top of a mountain with good line-of-sight, but when the carrier has activated CBRS, “it doesn’t really propagate down to where the people are. By the time it gets to a [population] center or smaller town, the signal strength is diminished greatly.” There are techniques that can be employed to boost that signal with additional antennas, he said, but “it just makes it more complicated than it should.”
