The row between the wireless and aviation industries on potential interference between 5G and aircraft radio altimeters is sounding like a storm in a tea cup! The genesis of this dispute is a report in October 2020 by the Radio Technical Commission for Aeronautics (RTCA) citing potential interference to altimeters operating in the 4.2 – 4.4 GHz band from 5G base stations operating in mid-band spectrum, particularly 3.7 – 3.98 GHz [see here]. This alerted telecom regulators to a possible interference problem that may threaten aircraft safety leading some regulators to impose restrictions on 5G radio emissions and deployments.
Background on Radio Altimeters
Most civil and military aircraft use radio altimeters to measure an aircraft’s altitude above ground and its clearance over terrain. Altimeters feed information to other aircraft systems such as landing and collision avoidance systems.
Altimeters RF performance is designed to comply with ITU-R M.2059 specification. There are classes of altimeters featuring different RF characteristics which makes some more susceptible to interference than others. The aviation industry claims that emissions from 5G could lead to altimeter receiver overload. How much overload will depend on a number of factors, which in addition to the type of altimeter include the operating parameters and geometry of the aircraft and the cell site antennas.
National transportation administrators regulate and approve the use of altimeters (e.g. FAA in the US) which must comply with minimum operational performance standards (MOPS). One problem is that the current standards date back to 1980, if not older. The RTCA and EUROCAE (European Organization for Civil Aviation Equipment), which sets the MOPS, plan to release new standards for radio altimeters later this year to address potential interference concerns. Nevertheless, there is already a large install base; and it would take a few years and a fair amount of money to retrofit fleets with new altimeters.
Telecom Regulatory Approaches
38 countries have launched 5G services in midband spectrum, including: Japan, South Korea, Taiwan, China, Hong Kong, Philippines, Australia, New Zealand, Singapore, Saudi Arabia, Bahrain, Oman, United Arab Emirates, Qatar, Kuwait, Greece, Spain, Italy, Austria, Switzerland, France, Luxembourg, Germany, Ireland, United Kingdom, Czech Republic, Denmark, Norway, Sweden, Finland, Latvia, Slovenia, Slovakia, Croatia, Hungary, Romania, Bulgaria, and Peru, according to CTIA submission to the FCC, without any reports of interference [see here].
Two national regulators acted to restrict 5G deployments. The French (ANFR) and Canadian (ISED) regulators are noteworthy, because they are the only ones imposing such restrictions as of the date of this article [see below for the case of Japan].
France: In March 2021 France mandated exclusion and protection zones affecting 17 airports [see here]. The rules apply to deployments in 3.4 – 3.8 GHz, or 400 MHz below the altimeter operating range between 4.2 – 4.4 GHz.
- Exclusion zones: The exclusion zones extend 910 m on either side of the runway edge and 2100 metres from the runway thresholds.
- Protection zones: Rectangular areas that extend from the edge of the exclusion zones at either end for 6100 m. A protection zone is 800 m wide.
Additionally, France will not allow base station antenna vertical uptilt. This scales-back an earlier requirement issued in November 2020 mandating downtilts on all antennas.
Canada: In November 2021, ISED mandated exclusion and protection zones affecting 26 airports, that are almost identical to those in France [see here]. Restrictions apply to the 3450 – 3650 MHz band – or 550 MHz below the altimeter operating frequency band. The plan is to follow similar restrictions for the 3650 – 3980 MHz band which ISED intends to auction early next year.
- Exclusion zones: The exclusion zones extend 910 m on either side of the runway edge and 2100 metres from the runway thresholds. Base stations cannot be deployed outdoors in exclusion zones.
- Protection zones: Rectangular areas that extend from the edge of the exclusion zones at either end for 6100 m. A protection zone is 1000 m wide. Base stations are allowed in protection zones as long as they meet the power flux density (pfd) limit of -38.80 dBW/m2 in 1 MHz at 91.44 m (300 feet) from the ground.
Additionally, operators are required to implement mechanical and/or electrical downtilt on all base stations, similar to the earlier French mandate. Indoor operation in the 3.5 GHz band will be allowed in exclusion and protection zones.
Japan completed studies with recommended restrictions, which I presume are being enforced [see here]. These are:
- Avoid deploying 5G base stations within a 200 m from the airplane approach area of an airport.
- A separation distance between a small-cell base station and a heliport should be more than 20 m. Similarly, a separation distance between a macro-cell and a heliport should be more than 50 m.
- Implement additional filtering at the 5G base station to reduce unwanted emission interference.
There are a few key observations to make:
- Most regulators have not acted on the RTCA report, nor were there reports of interference. Japan deployed 5G up to 4.1 GHz and over 4.5 GHz, at only 100 MHz separation from the altimeter band. Canada has the most restrictions given the current band stops at 3650 MHz, or 550 MHz below the altimeter band.
- The number of airports affected by restrictions is small, and the impact on surrounding coverage is also small. For instance, ISED estimates that the exclusion zones impact less than 1% of the population of the respective license area. In the United States, only 50 airports will have exclusion zones (see below).
- Aside from airports there is the issue of vertical uptilt which could impact helicopters and other aircraft everywhere. Operators rarely use antenna uptilt, except in some very dense urban centres – I have seen a few in Hong Kong, for instance. However, this could change with 5G in midband spectrum where massive MIMO antennas (e.g. 64T64R) could beamform in the vertical plane in addition to the horizontal plane to cover tall buildings.
- The nature of the studies and assumptions vary widely. There is clearly a need for a streamlined analysis that uses realistic scenarios for different parameters used on both 5G systems and altimeters.
The US Developments
The initial RTCA report clearly targets the US wireless operators and the FCC. The CTIA issued its own rebuttal citing deficiencies in the RTCA report [see here]. What followed has been a mini battle between the wireless industry and FCC on one side, and the aviation industry and FAA on the other side. Perhaps the climax came a week ago when AT&T and Verizon said that the AIA “wants to hold the C-Band hostage until the wireless industry agrees to cover the costs of upgrading any obsolete altimeters that, in the view of some aviation interests, do an abnormally poor job of filtering signals in bands far removed from the 4.2-4.4 GHz aeronautical altimetry band.” [see here].
Over the past few days, the conflict has deescalated. Verizon and AT&T agreed to delay the January 5th planned 5G launch by two weeks. In return, the FAA will prioritize a list of 50 airports for exclusion zones [see here].
Future Developments and Concluding Thoughts
The hoopla of presumed 5G interference on aircraft altimeters emerged suddenly in late 2020 at the initiative of the aviation industry. It caught many regulators by surprise. For instance, ISED had to adjust the technical requirements in the “dead of night” just as the 3.5 GHz auction was winding down last summer.
Migration of the C-band from satellite and other services to mobile services was planned years ago. This raises the question on what the FAA aviation industry was doing all this time? Perhaps more poignant is where was the NTIA on this issue? [See here for more on this side of the story]
In both the US and Canada, operators have paid a premium for their spectrum rights [see here and here]. Putting restrictions at such a late stage is a bitter pill to swallow. Fortunately, these restrictions are not materially impactful on a grand scale. In fact, if the motive of the aviation industry is to get some compensation for the altimeter upgrade, the wireless service providers can easily wait it out and fight it off, in my opinion.
This case presented one of the rare occasions where two regulators – the FCC and FAA – and their ecosystem cohort, butt heads and pulled in different direction. Both were quick to realize the potential damage and came to a seemingly quick amend.
The issue will not go away as quickly as it developed. However, I don’t expect it to be headline news as it has over the past few weeks. Engineers will take the intervening time to test and analyze, so we come up with a methodical approach to address this issue. This has been lacking to date as much ambiguity surrounds altimeters in particular.
Note: we have conducted interference analysis between different types of networks and systems including airborne systems in AWS band, weather radars in S and C-bands. Contact me if this topic is of interest.