We’ve heard about delays in C-band commercialization, as several pilots’ organizations have popped up with concerns about interference. Let me explain why this question was inevitable, and why it’s so difficult to resolve completely.
First of all, here are the basics: The FCC has chosen to clear the 3.7-4.0 GHz band of satellite users, to replace directional satellite links with high-power mobile links transmitted in all directions. This is a real change in the nature of the energy in the 3.7 to 4.0 GHz band.
The concern is that a change in RF emissions would impact radar altimeters that use an FMCW radar at 4.2 GHz. This radar system is critical to pilots because this is their primary way to determine their height above the ground.
The FCC, as a government agency, regulates the use of spectrum, and it’s clearly their responsibility to license spectrum in a way that ensures no interference between these systems. They try to do that by regulating the transmitters. In short, they only try to control the emissions, and they assume that receivers will only receive the signals in their desired band. If it was this simple, a 200 MHz guard band would be enough.
Unfortunately, in the real world the receivers are not ideal. The filters on the front end of the receiver are typically designed for the actual field environment, not for any regulatory standard. That means that radar altimeters have been manufactured for 30 years using wideband filters on the receiver to improve performance and save cost. Thousands of aircraft are flying around in the USA with wide filters on their receivers, because they never had problems with the satcom links in the 3.7-4.0 GHz band.
A very similar situation happened with Lightsquared in the GPS band. The FCC re-allocated L-band spectrum from satellite use to terrestrial use, and did not realize that a new terrestrial system would cause massive problems for GPS receivers in the adjacent band at 1.575 GHz. Everyone built GPS receivers to be very small, and that required wideband receiver filters that were open to a new, unplanned source of interference. So, even though Lightsquared had a legal license to use the band, their venture was strangled before it could get started.
In the case of C-band, several OEMs have analyzed the interference issue and in general the mobile industry has reached a consensus that the radar altimeters should be safe. I agree, and I have not reduced my forecasts.
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However, a typical pilot is not convinced. Of course not! After all, it is their butt in the seat and they are responsible for the lives of their passengers. So the industry needs to support a rigorous and comprehensive review of all FMCW radar altimeters built over the past 30-40 years and the mobile industry should make allowances for any aircraft that will be impacted. (Sorry, I shouldn’t say that aircraft will be impacted. Let’s say “affected”).
It won’t do any good to tighten the specifications for spurious emissions. This is not an issue of base stations emitting too much energy at 4.2 GHz. The issue is that legal high-power signals at 4.0 GHz will be coming into the altimeter, and the altimeter will be confused. So, instead of re-testing the 5G base station the FCC needs to kick off a process of testing the radar systems to fully understand the vulnerability of every model ever built.
Joe Madden is principal analyst at Mobile Experts, a network of market and technology experts that analyze wireless markets.
"Industry Voices" are opinion columns written by outside contributors—often industry experts or analysts—who are invited to the conversation by FierceWireless staff. They do not represent the opinions of FierceWireless.