I have been watching the kerfuffle between the airlines and the wireless providers over the U.S. rollout of 5G cellular service with some consternation. (For a bit of background on the conflict, see this recent New York Times story.) Some international airlines have canceled or restricted flights to the United States, and U.S. carriers have also raised alarms. In turn, the Federal Aviation Administration issued an emergency order, temporarily delaying 5G activation near airports while it investigated.
Let’s start with a bit of background. Wireless spectrum is like beachfront property. You cannot make more and some is more valuable than others. Different spectrum regions have different propagation characteristics, making them more or less useful for different purposes. Hence, the electromagnetic spectrum is chopped up in fixed bands, allocated for specific usage. For example, some is for emergency responders and hospitals, some for radio and television, some for police and civil defense, some for military purposes, some for amateur radio, and some for commercial use. In recent years, there has been intense competition among the U.S. cellular carriers for spectrum, with the Federal Communications Commission (FCC) auctioning spectrum for 5G deployment.
In the U.S., the Verizon and AT&T 5G deployment operates in the C band, between 3.7 and 3.98 GHz, which is near the 4.2-4.4 GHz bands used by aircraft radar altimeters for ground proximity tracking during low visibility landings. The FAA has warned that the radar altimeters might be adversely affected due to possible interference with those radar altimeters. Under International Telecommunications Union (ITU) rules, radio altimeters are classified as safety services and must be protected from interference.
However, the FCC has required a 220 MHz interference buffer (a so called guard band) between the upper end of the U.S. 5G usage (3.98 GHz) and the lower end of the avionics usage at 4.2 GHz. It all comes down to how good the spectrum filters are on the radio receivers, where there is a good bit of variation, based on the age of the airplanes.
On January 20, 2022, the FAA issued new approvals that allow most commercial flights to perform low visibility landings where there are 5G deployments in the C band. The FAA notes, “We anticipate some altimeters will be too susceptible to 5G interference. To preserve safety, aircraft with those altimeters will be prohibited from performing low-visibility landings where 5G is deployed because the altimeter could provide inaccurate information.”
You might ask why this is an issue in the United States, given the long planned, global rollout of 5G. The technical answer is simple – the details of the deployments differ slightly, but the political answer is complex. First, Europe is deploying 5G in the 3.4-3.8 GHz range, lower than the 3.7-3.98 GHz used in the U.S. and further from the spectrum used for radar altimeters (i.e., there is a larger buffer). There are also differences in the heights of 5G antennas, the power levels used, and – for antennas near airports – how they are tilted relative to airplane flight paths.
Could all of this U.S. kerfuffle have been avoided? Of course, in many possible ways. It brought to mind the iconic line from Cool Hand Luke, when the prison warden, played by Strother Martin, tells Luke, played by Paul Newman, “What we've got here is failure to communicate.”
As someone who served on the FCC Technological Advisory Council (TAC) while at Microsoft and who worked regularly with the ITU, I have seen spectrum politics from a front row seat. Tom Wheeler, who chaired the TAC when I was a member, has written one perspective in these issues for the Brookings Institution. The FAA, as I noted above, has a somewhat different perspective, with the wireless providers and airlines battling in between. What is undeniable is that this could have been prevented by clearer and better coordinated policy leadership in Washington. Working together, the FCC and the FAA, along with the aircraft manufacturers, airlines, and wireless carriers could have quietly resolved these issues years ago.
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