The auctioning of the C-band for 5G use has raised concerns in North America that signals could interfere with aircraft altimeter operations. 5GTW asked a filter engineer and a test expert if there’s a problem and how to fix it.
When the FCC auctioned off spectrum in the C-band — 3.7 GHz to 3.98 GHz – winners including Verizon and AT&T spent over $81 billion to acquire portions of that spectrum. The use of the C-band for 5G raised concerns from the aviation industry regarding possible interference with aircraft altimeters.
The issue has caused delays in C-band implementation, now scheduled to begin on January 5, 2022 in the U.S. Furthermore, Canada has restricted the use of the C-band around 26 airports. In response, Verizon and AT&T have reluctantly agreed to reduce transmit power in the C-band, which the aviation industry calls “inadequate.” In another action, the FAA has restricted some flight operations in response to the issue.
While these actions may address the potential problem, they’re hardly a long-term solution, according to Mike Eddy VP, corporate business development & product marketing at Resonant, a manufacturer of RF filters. “Energy from 5G bands can enter the altimeter’s receiver,” said Eddy in an interview with 5G Technology World. “Any transmit system produces spurious noise, which can enter the band of a receiver.”
A 231 page report issued by aeronautics technical group RTCA in November 2020 explains the issue.
The fundamental emissions may lead to blocking interference in the radar altimeter receiver, wherein a strong signal outside of the normal receive bandwidth cannot be sufficiently filtered in the receiver to prevent front-end overload or other effects. The spurious emissions, on the other hand, fall within the normal receive bandwidth of the radar altimeter, and may produce undesirable effects such as desensitization due to reduced signal-to-interference-plus-noise ratio (SINR), or false altitude determination due to the erroneous detection of the interference signal as a radar return.
Figure 1, adapted from the report, provides a visual representation of how signals from a base station may occur at the same time and in proximity to altimeter signals.
Figure 1. Signals from 5G towers that use the C-band will likely occur at the same time as those transmitted from an aircraft altimeter, though at different frequencies. Image adapted from RTCA report “Assessment of C-Band Mobile Telecommunications Interference Impact on Low Range Radar Altimeter Operations, RTCA Paper No. 274-20/PMC-2073.”
“Altimeters have poor filters,” said Eddy. “Under that condition, spurious noise can enter band of receiver. That noise can come from both base stations and handsets. Eddy claims that poor filtering at the receivers raises the potential problem, though he noted that these interference problems are nonexistent in other parts of the world. As you might expect, the telcom industry, through CTIA, contends that claim saying “5G networks using a set of radio waves called ‘C-band spectrum’ operate safely and without causing harmful interference to aviation equipment.”
Figure 2. According to the RTCA report, altimeter receiver filters are inadequate to prevent C-band transmission energy from interfering with the altimeter’s receiver despite a 200-MHz guard band. Image: RTCA report “Assessment of C-Band Mobile Telecommunications Interference Impact on Low Range Radar Altimeter Operations, RTCA Paper No. 274-20/PMC-2073.”
Figure 2, from the report, shows the reason for concern in the aviation industry. Spurious emissions such as harmonics from 5G transmissions could leak into the altimeter’s receiver, potentially preventing it from receiving signals reflected from the ground. The lack of a sharp cutoff in an altimeter’s receiver filter — even with a 200 MHz guard band — is the primary source of concern.
Is my transmitter a good neighbor?” asks Roger Nichols, Keysight’s 5G/6G program manager. “Measurement of adjacent-channel leakage ratio (ACLR) and adjacent-channel power ratio (ACPR) tells if a transmitter (Tx) leaks into adjacent radio channels. There are related measurements called ‘spurious emissions,’ which are not necessarily in the adjacent radio channels, but they can cause interfere in adjacent channels in convoluted ways given a phenomenon called passive intermodulation distortion (PIM).”
Nichols continued by asking “Can my receiver handle a bad neighbor? Receiver measurements include adjacent channel rejection, the measurement of how well a radio receiver (Rx) blocks interfering energy from an unwanted transmit signal before the receiver can no longer provide adequate sensitivity.”
While these measurements are important, they take place in the lab and what we have here is a real-world problem. As the report points out, there can be more than one base station around an airport. That makes emulating the real world difficult.
Then here’s possible interference from handsets in the aircraft. “Handsets in the plane could be a problem,” said Eddy. “They transmit at lower power than base stations but are closer to the altimeter.” We all know that people often don’t follow the rules by turning off their phones setting them to airplane mode, nor turning off the 5G capability. Indeed, many people don’t even know how to invoke airplane mode, never mind turn off 5G. Phones without 5G won’t interfere with altimeters at all, but people are replacing their LTE phones with 5G every day.
“Some would argue,” said Nichols, “that radar altimeters use signals that travel in vertical directions and there is a significant guard-band (200 MHz) between commercial C-band and the radar-altimeter band. Large C-band deployments are already in place in Japan, Korea, China, Europe, and the co-existence (or adjacent existence) appears to be a non-issue. With radar altimeters in commercial aircraft flying over heavily populated urban areas when close to relatively dense mobile networks, higher Tx energy levels represent higher stakes in terms of safety and security. That’s why understanding this issue is critical.”
Nichols continued, saying “The long-term solution, should it be needed, is to retrofit or replace altimeters and use receiver filters with sharper cutoffs. That could significantly reduce unwanted energy from reaching the receiver. That costs money, takes time, and places the burden on aircraft manufacturers and aircraft operators which will certainly catch the eyes of the aviation industry.”
As the news reports indicate, Verizon and AT&T lowering their base-station transmit power and Canada’s limiting C-band transmissions are at best short-term solutions. Let’s face it, there’s too much money at stake. The telecom industry will continue to push for using the prime spectrum that the C-band provides.
“There’s a potential problem, added Eddy. “Politics and players: CTIA and FCC look after the telecom industry while FAA looks after the aircraft industry. The reports are all over the place.”
Interesting article and goes some way to investigate the problem. The Aviation industry should not have to bear any costs associated with modifying existing equipment. The equipment meets the current regulatory requirements domestic and internationally and the allocation of bandwidth expansion is purely dominated by financial considerations by sale and expansion, manufacture of user PEDs. If the industries associated with this financial gain would like to fund the required modifications, fine, however I suspect this probability veers towards the improbable.
Watch this space.
Build new altimeter devices for airlines. Then maybe airlines can utilize the 5g benift.
Robert Ladley has a point: Should airlines be forced to pay for altimeters with better filtering now that 5G has come along?
Fierce Wireless calls this an “aging altimeter problem.”
“Not our fault, blame the airlines” say the telecom carriers and press.
This is an ongoing story. The telecom news outlets are ablaze today with AT&T and Verizon’s letter to Secretary of Transportation Pete Buttigieg claiming that 5G won’t interfere with altimeters. The letter follow this one dated December 31 where Buttigieg and FAA Administrator Steve Dickson said:
Failure to reach a solution by January 5 will force the U.S. aviation sector to take steps to protect the safety of the traveling public, particularly during periods of low visibility or inclement weather. These steps will result in widespread and unacceptable disruption as airplanes divert to other cities or flights are canceled, causing ripple effects throughout the U.S. air transportation system.
The two telecom carriers base their conclusion on data citing no such incidents with altimeters and 5G in France. Is that good enough or is it just a matter of time?
The two companies have agreed to delay launch of C-band operations until January 19, pushing the launch date back by two weeks.
Right now, the talk is about interference from cellular base stations, but what about passengers who disobey crew instructions and keep right on using their phones during landings?
I’m a retired RF electronics engineer. This type of challenge, co-channel interference is well understood, and appears in many applications. The challenge/problem was then foreseeable to anyone expert in the art. In the past, the mission of the FCC was to manage compatibility issues among the many services using the RF spectrum. Why has this controversy arisen at such a late stage?
The FCC has decided to cede management of compatibility to the “free market”. If a compatibility issue arises, then the telecom companies and airlines will resolve their dispute through the courts. Meanwhile the FCC has gotten away with the $81 billion. In part this is a result of long standing budget cuts at the FCC. There is a role for government regulation to avoid the burdens of relying on legal processes.
However… that’s a terrible front end for aviation nav, and it stinks of leaving off a better one to cut down cost. Cheap can be very expensive.
Here are a few links if you’d like to follow the news of this mess.
https://www.rcrwireless.com/20220104/5g/verizon-att-change-course-agree-to-another-c-band-delay
https://www.sdxcentral.com/articles/news/verizon-att-pause-mid-band-5g-again/2022/01/
https://telecoms.com/512781/verizon-and-att-forced-to-delay-5g-expansion-once-more/
https://www.fiercewireless.com/wireless/airlines-file-emergency-petition-stop-5g-c-band-deployment-near-airports
Figure 2 seems to show that altimeters have been allocated 200 MHz of spectrum (which already seems a lot, considering the amount of information involved) but are using quite a lot more, and up until now have been getting away with it.
The mobile industry seems to be in the position of someone who has been sold a building plot and the finds that someone from two doors down the road is using the space because, hey, it was empty so they just spread across it.
Figure 2 lacks a scale for the Y axis, but, from the look of it (-3dB/octave?), radar altimeters have practically no selectivity at all. I honestly doubt that any such lousy instrument would be designed. Especially because there may be very powerful C band radars in airports which would pose the same intermodulation and interference issues, if not worse, since radar antennas point upward, as 5G. Something doesn’t sound right in all this brouhaha.
I think it’s a case of that the altimeter filters were good enough when implemented, but perhaps not anymore.
FAA list of airports with a 5G buffer.
https://www.faa.gov/sites/faa.gov/files/2022-01/50%20Airports%20with%205G%20Buffer.pdf
In first term, we have to stay in clear that altimeter and radio altimeter, (RA) are different concepts.
While altimeter refers to the measurement with respect an altitude above the medium level sea, radio altimeter measurment is the height from the terrain to the aircraft between zero to 2,500 feet. This signal is the only clearance reference between the aircraft and the terrain below the plane.
The probable cause of interference in the use of C band in 5G technologies is related to aircrafts’ radio altimeters on board. This transceiver equipment transmit a rf signal between 4.2 to 4.4 GHz. While 5G technologies are uthorized to use the bandwidth between 3.7 GHz and 3.98 GHz.
In other hand, radio altimeter system on aircraft board is associated with other important systems, which gives to the fly crew information during takeoff, approach and landing phases.
There are three categories in radio altimeter systems related with Automatic Flight System, AFS; and Flight Guidance System, FGS.
Category 1, radio altimeter system supply infromation approach and high decision readouts to the crew.
Category 2 and category 3 radio altimeter supply height over terrain data to systems AFS and FGS capable to develop an auto land task.
Futhermore, radio altimeter signal is provided to other system called Ground Proximity Warning System, GPWS to advice the crew the terrain clearance or its proximity in the range between zero and 2500 feet.
If we review a simple signal analysis, appeal to Nyquist theorem we can able to understand that the channel capability of a 5G signal can be able to interfere with radio altimeter signal, in spurious harmonics if the transmitter power is greater.
We have to understand that there is a possiblity of a harmful interference in the use of 5G technologies, close to the range of 4.2 to 4.4 GHz, in particuoar for avionics systems.
Arturo Garcia Orozco
Telecommunication and Electronic Eng.
Engineering and Planning Mngr
TAR Airlines
The frequency range used by 5G until 2024 in the US is actually 3.7-3.8 GHz. And, as a telecom engineer, you well know that a device authorized to operate at any frequency range, 4.0-4.2 GHz in this case, has to be able to operate with emissions in other neighboring frequencies.
Ahem, 4.2 -4.4 GHz.
It could be a solution……
https://www.israeldefense.co.il/en/node/53315
@Moshe,
Maybe, but what’s described here is essentially having a “moderator” to give priority to one spectrum user over another. That’s similar to CBRS in U.S. Indeed, we posted an article about CBRS and spectrum sharing just today.
https://www.5gtechnologyworld.com/the-cbrs-band-depends-on-spectrum-management/
@Martin
Your response may be almost correct. The solution is also “moderator” In some cases, in cases where there is 100% damage or damage to one of the sides that does not allow proper work on the spectrum then in such a case a full simulation is required.
Do you know if a full simulation has been done?