The CTIA and the Big 3 US mobile carriers have warned that the use of certain spectrum for direct-to-device (D2D) could hamper their core 5G networks.

As reported first by Light Reading, CTIA and executives from AT&T, Verizon, and T-Mobile met with the Federal Communications Commission (FCC) last week to argue that 2.5GHz and 3.45GHz should be removed from the rules being drafted for Supplemental Coverage from Space (SCS) and D2D connectivity.

Those two bands have been heavily used for 5G connectivity and are part of the Time-Division Duplex (TDD) bands that the CTIA and US carriers want removed from the draft.

The main gripes put forward by the operators and the CTIA center around the incompatibility between the use of TDD technology in those 5G bands and the use of paired Frequency-Division Duplex (FDD) for D2D.

It comes as the FCC gets ready to vote this week on a proposed item to make an additional 482MHz of spectrum available for SCS, while looking at measures to harmonize the SCS and Mobile Satellite Service (MSS) rules.

FDD separates uplink and downlink traffic into separate, independent channels, while TDD utilizes uplink and downlink traffic to share the same frequency.

"Today, more than 300 million users rely on these bands for wireless services, and they should not have their service compromised by technically unworkable changes to the SCS framework," said the CTIA in its filing.

"Supplemental Coverage from Space is not technically feasible in TDD bands, and the Commission should remove the 2.5GHz and 3.45GHz bands from its proposal for inclusion in the SCS framework."

The Big 3 recently formed a D2D satellite-focused spectrum-pooling joint venture (JV), while SpaceX subsidiary Starlink sealed approval from the FCC to deploy 15,000 high-capacity direct-to-device (D2D) satellites.

While all three carriers have clear ambitions to complement their respective terrestrial coverage, the operators have put forward concerns around how spectrum usage could hurt their 5G coverage.

In a separate filing, T-Mobile outlined some of its own fears around this, noting that D2D services can't share the 2.5GHz band, as it could seriously impact T-Mobile's 5G network.

The carrier said any satellite signal that uses the 2.5GHz could land on top of a tower and collide with terrestrial signals, "leading to dropped and incomplete calls (including calls to emergency 911 services), delayed uplink transmissions, and reduced indoor coverage."

"TDD works because every tower keeps the same rhythm. A satellite can't keep that rhythm, so it cannot share the band. And the existing license structure makes SCS unworkable," said T-Mobile.

In an effort to quantify how much of an impact this could have on its network, T-Mobile said up to 96 percent of a cell site's uplink coverage area could be lost where satellite coverage is present, while customers would see uplink speeds slow down by 88 percent to 99 percent.

"Coexistence in unsynchronized operations requires restrictive emission limits, custom filters to mitigate receiver blocking, and a spectral guard band of approximately 20-25 megahertz," noted T-Mobile in its filing.

"T-Mobile estimates that implementing these measures would ultimately result in a substantial reduction in the utility of the 2.5GHz band and approximately 20-25 percent of the spectrum lying fallow in a 100-megahertz block. That outcome would not serve the public interest, let alone achieve the Commission’s goal to “maximize use of spectrum resources."