US: new 5G delay to study interference with planes


US authorities have asked telecom operators AT&T and Verizon to delay for up to two weeks their already postponed rollout of 5G networks amid uncertainty about interference with vital flight safety equipment.

The two companies said Saturday they are reviewing the request.

The US rollout of the high-speed mobile broadband technology had been set for December 5, but was delayed to January 5 after aerospace giants Airbus and Boeing raised concerns about potential interference with the devices planes use to measure altitude.

US Transportation Secretary Pete Buttigieg and the head of the Federal Aviation Administration, Steve Dickson, asked for the latest delay in a letter sent Friday to AT&T and Verizon, two of the country’s biggest telecom operators.

The US letter asked the companies to “continue to pause introducing commercial C-Band service” – the frequency range used for 5G—”for an additional short period of no more than two weeks beyond the currently scheduled deployment date of January 5.”

Asked by AFP for comment, Verizon spokesman Rich Young replied, “We’ve received the government’s letter after 6 pm on New Year’s Eve. We’re in the process of reviewing it.”

AT&T also said it was reviewing the government request.

In the letter, the US officials assure the companies that 5G service will be able to begin “as planned in January with certain exceptions around priority airports.”

The officials say their priority has been “to protect flight safety, while ensuring that 5G deployment and aviation operations can co-exist.”

Last February, Verizon and AT&T were authorized to start using 3.7-3.8 GHz frequency bands as of December 5, after obtaining licenses worth tens of billions of dollars.

But when Airbus and Boeing raised their concerns about possible interference with airplanes’ radio altimeters – which can operate at the same frequencies – the launch date was pushed back to January.

The FAA requested further information about the instruments, and it issued directives limiting the use of altimeters in certain situations, which sparked airline fears over the potential costs.

When Verizon and AT&T wrote to federal authorities in November to confirm their intention to start deploying 5G in January, they said they would take extra precautions beyond those required by US law until July 2022 while the FAA completes its investigation.

The conflict between 5G networks and aircraft equipment led French authorities to recommend switching off mobile phones with 5G on planes in February.

France’s civil aviation authority said interference from a signal on a nearby frequency to the radio altimeter could cause “critical” errors during landing.

RF Spectrum Surrounding Radar Altimeters

• Majority of current usage near the radar altimeter band is Fixed Service (FS) and Fixed-Satellite Service (FSS) links
• FS links may have relatively high EIRP, but are sparsely distributed and utilize highly directional antennas
• FSS is Space-to-Earth (downlink) only, resulting in very low interference power
• Mobile allocation above 4.4 GHz is used in the U.S. for Aeronautical Mobile Telemetry (AMT)
• Currently limited to Federal use—generally isolated to specific test ranges

From ITU Radio Regulations, Edition of 2020, Vol I.

Current High-Power Potential Interference Sources

• The immediate spectrum neighborhood around the radar altimeter band is relatively quiet
• High-power emissions sources (such as ground-based surveillance radars) are generally far from the band, e.g. below 3.5 GHz
• Airworthiness standards from the FAA and other aviation authorities include requirements to tolerate High-Intensity Radiated Fields (HIRF)
• Includes continuous wave (CW) and pulsed high-power interference signals
• However, an exclusion band is allowed ±10% around the band, within which the HIRF levels are very low—for radar altimeters, this spans 3.78–4.84 GHz
• Therefore, HIRF requirements are not sufficient to protect against interference from modern communications waveforms close to the radar altimeter band

The Push for “Mid-Band” Spectrum

• The radar altimeter band lies in the middle of a spectrum sweet spot, allowing for large bandwidths with favorable terrestrial propagation properties
• Spectrum regulators worldwide are working to make more of this “mid-band” spectrum available for 5G mobile wireless applications
• In the United States, the FCC has made mid-band spectrum a key priority in its Facilitate America’s Superiority in 5G Technology (5G FAST) Plan1
• Includes new allocations of 3.45–3.55 GHz and 3.7–3.98 GHz
• Mid-band spectrum is extremelyvaluable—70 MHz in the 3.45–3.55 GHz band was recently auctioned for $4.6B in the United States2
• 3.7–3.98 GHz band expected to be even more valuable—total auction proceeds likely to be in the tens of billions3


Anticipated Changes to RF Spectrum

New mobile allocations (primarily for 5G) being actively developed or considered:

3.7–3.98 GHz: Background

• In 2018, FCC announced their intent to establish a new spectrum allocation for 5G in the 3.7–4.2 GHz band
• Aviation industry representatives, led by the Aerospace Vehicle Systems Institute (AVSI) at Texas A&M University, conducted testing and analysis to evaluate the risk of harmful interference to radar altimeters and submitted results to the FCC
• In March 2020, the FCC released a Report and Order finalizing the spectrum allocation from 3.7–3.98 GHz, to be auctioned in December 2020
• FCC acknowledged further study was needed regarding coexistence issues with radar altimeters, but no operating limitations for the band were defined specifically to prevent interference
• In April 2020, RTCA established a 5G Task Force within the newly-formed Special Committee 239 to further investigate the issue and generate a technical report
• In October 2020, SC-239 report released publicly and provided to the FCC

• 5G usage in the 3.7–3.98 GHz band will lead to widely-deployed, high-power emissions sources in this band
• Two means of potential interference to radar altimeters:
• 5G fundamental emissions overcoming the filtering in the altimeter receivers (blocking)
• 5G spurious emissions landing within the 4.2–4.4 GHz band directly
• Radar altimeters are inherently wideband systems, and currently have no requirements for front-end rejection—may be more susceptible to blocking than other types of receivers

SC-239 Study Methodology
Overall guiding principle (based on ICAO guidance): worst-case analysis of currently fielded TSO-approved radar altimeters

SC-239 Study Results

Usage Category 1: Commercial airplanes used for passenger travel and cargo transport

Red dots mean that the safe interference limit is exceeded


The impact to radar altimeters is limited to a set of specific scenarios, with only some base station configurations producing interference above the safe limit, and only for certain combinations of aircraft altitude and lateral distance between the aircraft and base station.

Although the interference impacts for Usage Category 1 only arise in certain scenarios, the extent and safety consequences of those impacts are extreme:
• Unreliable radar altimeter signals delivered to the critical aircraft systems: Auto Pilot, Flight Controls, Terrain Avoidance, Cockpit Displays.
• Catastrophic impact with the ground, leading to multiple fatalities, is possible.

Usage Category 2: Business aviation, general aviation, and regional transport airplanes
Usage Category 3: Both transport and general aviation helicopters


Every base station configuration produces harmful interference both from 5G fundamental emissions in the 3.7–3.98 GHz band and 5G spurious emissions* in the 4.2–4.4 GHz band, across virtually all operational scenarios and relative geometries between the aircraft and base station.
5G user equipment (mobile phones) that may be operating onboard aircraft were also found to exceed the safe interference limits for Usage Categories 2 and 3.

*Note: 5G spurious emissions level is not compliant with Recommendation ITU-R M.2059 protection criteria


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