Limitations on US midband 5G spectrum

Beyond RF challenges, US midband spectrum has limitations built into service rules

As demand for various wireless services has grown, however, it has gotten harder and harder for federal regulators to come up with bands that can be (relatively) easily repurposed—either the cost or the logistics of relocating incumbents (or both) has become a higher hurdle, particularly when it comes to mission-critical systems on which the Department of Defense, public safety users or established satellite service providers with large customer bases rely.

When it came time for the FCC to consider spectrum for 5G, it first turned to millimeter-wave—in part because of the large amount of spectrum available, but also because the cost of mmWave systems and equipment meant that there simply weren’t many existing incumbent systems that were using those airwaves. The midband proved to be a tougher nut to crack, which is evident from the licensing and service rules that the FCC has imposed.

-The Citizens Broadband Radio Service (CBRS) presents perhaps the most radical departure from previous spectrum regimes. In order to accommodate incumbents including ship-mounted U.S. Navy  radar systems and fixed satellite providers, the FCC adopted a three-tiered spectrum-sharing framework for the 150-megahertz slice of spectrum: Incumbent users had the highest priority to the full band as needed, Priority Access Licenses were auctioned for 70 megahertz of the band, and General Authorized Access to the spectrum was granted under essentially unlicensed use coordinated by a Spectrum Access System in concert with a network of receiver sensors (the Environmental Sensing Capability, or ESC) in coastal Dynamic Protection Zones, where the ESC is used to detect the presence of incumbents and notify the SAS that PAL and GAA users have to clear out of the band.

In addition to the sharing regime, CBRS is also constrained by significantly lower transmit power limitations than cellular network operators usually work with; plus the fact that licenses are county-level and that PAL holders are limited to aggregation of 40 megahertz of PAL spectrum.

“The biggest clash we are seeing at the moment around CBRS is the [Effective Isotropically Radiated Power, or EIRP] requirement, which is determined … by the output and the antenna gain,” explains Baha Badran, global head of engineering for antenna OEM Taoglas. “The EIRP is quite low.” This means, he continues, that “one of the biggest challenges we have from the antenna design perspective is, the higher [in spectrum] you go, the gain tends to also increase – so how can we limit the gain? The other big challenge is, how can we have an antenna that works for this band, CBRS, and continues to serve other bands?

“It might be possible to have a solution work only for CBRS, but … customers are looking for antennas to cover all the bands and meet FCC requirements; that’s the challenge we are seeing. It’s introducing a tricky situation, and we are seeing some companies trying to scratch their heads to solve this problem.” Generally, he added, Taoglas is not seeing high demand for antennas above 5 GHz yet, and most of the demand is for antennas serving the 3-5 GHz range—the most commonly used midband range around the world.

-The FCC has also auctioned 100 megahertz of 3.45-3.55 GHz is immediately adjacent to the CBRS band. While commercial network operators will generally have use of the entire band in the contiguous United States, under the FCC’s rules there are still some locations where federal systems will continue to operate. The Department of Defense has been using the 3.45 GHz band for high- and low-powered radar systems, including fixed, mobile, shipborne and airborne systems, as well as testing and training related to those systems; the coordination areas are primarily around military training facilities, test sites, Navy home ports and shipyards.

The places with federal incumbent systems have been dubbed Cooperative Planning Areas (CPAs, of which there are 33) or Periodic Use Area (PUAs; 23 of them overlap with CPAs), where coordination with federal incumbents will be required and operators who win 3.45 GHz licenses are being encouraged to enter into operator-to-operator agreements to figure out the details on efficiently sharing the band in those locations. In all but two of them, winners of 3.45 GHz licenses have to coordinate operations across all 100 megahertz of the band.

According to a July 2022 DoD presentation from a workshop on 3.45 GHz, of the 4,041 licenses that the FCC granted as a result of the auction, 1,172 of them (held by 15 entities) have requirements to coordinate with DoD.

For national wireless network operators specifically, 29% of the 1,624 licenses that AT&T bought have that requirement, as do 61% of the 199 licenses that T-Mobile US purchased; 34% of the 380 licenses for US Cellular and 25% of the 1,232 licenses purchased by Dish Network. For two small providers, 100% of the 3.45 GHz licenses they purchased require DoD coordination.

-The 2.5 GHz auction raised the smallest total of the four midband spectrum auctions, for several reasons. T-Mobile US was the juggernaut holding large amounts of the band, including lease arrangements, that it acquired at 2.5 GHz when it bought Sprint. As a result, this auction was expected to be dominated by T-Mobile US cleaning up licenses that it didn’t already hold—and that’s very much how it played out. Of the 8,017 licenses available, 98% of them sold and the vast majority of those (7,156, to be exact) were won by T-Mobile US at a cost of about $304 million.

The low-grossing total for this auction reflected both T-Mo’s pre-existing position in the band as well as the fact that many of the spectrum licenses on offer were encumbered, particularly in urban areas that typically drive big bids by the national carriers. When the FCC set the rules for auctioning this band, they not only had to take existing licenses and leases into account, they also gave rural Native American tribes the opportunity to obtain licenses prior to the auction in order to help bridge the connectivity gap that Tribal areas often face.

While this auction on its face offered up a large number of county-level licenses and 117.5 megahertz of spectrum in three channel blocks—the “single largest contiguous portion of available mid-band spectrum below 3 GHz,” the FCC proclaimed—ultimately, the agency also warned auction participants that there would be “a substantial number of licenses … where the amount of unassigned area or frequency that is unassigned is very small”—as little as 0.333 megahertz in some cases, and a “substantial number” of licenses where there was less than a square mile geographically in which a license holder could operate. The winners of these “overlay licenses” can operate within their license areas but still have to protect the operations of existing licensees.

However, T-Mobile US’ existing spectrum position at 2.5 GHz band means that it can rapidly capitalize on the new spectrum—if the FCC gives permission. The carrier has already submitted an application to the FCC for special temporary authority to expand its existing 5G services in 45 states with the new spectrum, while the FCC goes through the final review process prior to officially granting licenses. “The 2.5 GHz spectrum T-Mobile won in Auction 108 is interspersed with 2.5 GHz spectrum T-Mobile has already deployed for 5G mobile broadband. The intermixture of newly won and operational spectrum provides the Commission with a unique opportunity to significantly increase 5G mobile broadband capacity for consumers by allowing T-Mobile to simply expand the channel bandwidths that its previously deployed 5G equipment already supports,” the carrier said in its filing with the FCC.

-The C-Band spectrum at 3.7-4.0 GHz was supposed to be the easiest of the lot for operators to deploy—but the trade-off was the cost, with the auction raising more than $80 billion, both in spectrum costs and incentive payments to satellite operators to move out of the band in a timely fashion, with the first tranche of spectrum slated for December 2021. But as turn-up approached and Verizon and AT&T prepared to begin operations, the Federal Aviation Administration raised red flags about the potential impact on on-board aviation altimeters, with altimeter receivers that had not been designed to operate in an environment with terrestrial cellular operations, even with 240 megahertz of separation between the bands at issue. The operators agreed twice to hold off temporarily as federal agencies parlayed, but as a new January 2021 deployment date inched closer, the FAA issued pilot warnings that altimeters—and travel by plane—could be disrupted. Ultimately, as part of an agreement that enabled AT&T and Verizon to go ahead with C-Band deployment in most areas as quickly as possible,the FAA rapidly tested and cleared the performance of came up with a list of 50 airports around the country where 5G buffers were put in place — meaning that the sites operate at lower power levels and with antennas tilted downwards, in order to head off potential interference with the aviation systems.

Since those buffers were put in place and the operators went ahead with the bulk of their C-Band deployment, the FAA has said that the phased retrofitting of regional aircraft with new RF filters is expected to be done by the end of 2022; meanwhile, conversations with wireless operators continued as the FAA worked with them to “identify airports around which their service can be enhanced with the least risk of disrupting flight schedules.” AT&T and Verizon had voluntarily agreed to keep mitigations in place until July 5th of this year; in June, the FAA said that the carriers had offered to “continue with some level of voluntary mitigations for another year.”

“Filters and replacement units for the mainline commercial fleet should be available on a schedule that would permit the work to be largely completed by July 2023. After that time, the wireless companies expect to operate their networks in urban areas with minimal restrictions,” the FAA said. However, an airline industry group has recently asked the FCC to codify existing down-tilts and reduced power levels rather than allow them to expire; it remains to be seen if the FCC would entertain that action.

Looking for more information on spectrum issues in midband 5G airwaves? Check out RCR Wireless News’ webinar on this topic, featuring insights from testing companies LitePoint and Anritsu.

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