Research Note

November 2021 Hey Mr. Altimeter, I (C-band) mean no harm

November 2021

Abstract

This is what I call the “America exception.” The American 5G story started with the U.S. spectrum at odds with the rest of the world: no communications service provider (CSP) had the full quiver composed of low, mid and high band. In short, the sweet 3.5GHz spot, known as C-band (e.g., 3.7GHz – 4.2GHz) was missing. AT&T and Verizon had millimeter wave spectrum (e.g., 28GHz and 39GHz), T-Mobile US had its 600MHz and Sprint (now part of T-Mobile US) had its 2.5GHz. Put them all together and you have a formidable 5G contender!

Fortunately, the FCC acted swiftly to reorganize the C-band spectrum used by satellites and auction it such that the U.S. could join the 5G mid-band club and be on par with the rest of the world. In addition, it’s worth noting that the C-band addresses capacity issues and closes the gap between AT&T and Verizon, and T-Mobile US.

However, as C-band rollouts were just about to start this fall, the Federal Aviation Administration (FAA) issued a warning on potential safety risks from C-band, specifically interferences with radio altimeters used in the aeronautical radionavigation service. Given that:
• theoretically, interferences between the cellular radio and the radio altimeter waves should not occur because their respective center frequency are different, and the 2 radio waves do not start in phase,
• the ITU addressed this potential risk 8 years ago and published a comprehensive set of recommendations,
• there is a guard band large enough between the 5G C-band frequency band and that of the radio altimeter,
• 3.5GHz is widely used in 4G LTE TDD networks; by at least 230 CSPs in 101 countries/territories that have launched TDD LTE or TDD 5G networks in the 3.3-3.8GHz spectrum band,
• and early 5G adopter South Korea, now joined by more than 100 countries using 3.5GHz, has not reported a single incident,
this “Made in America” story sounds like a hoax.

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