MSIT Director Explains Why 5G Almost Halted Air Travel

The beginning of 2022 brought on a disagreement between two of the world's largest cell phone carriers and almost a dozen US airlines that nearly brought thousands of commuter and cargo flights to a grinding halt.

The issue? 5G. 

The problem, which played out in the media and made it to the White House before a temporary solution was brokered, is a lesson students in Northwestern Engineering's Master of Science in Information Technology (MSIT) program should learn from, said MSIT director and professor Randall Berry. 

"It's a little bewildering that this blew up like it did," Berry said.

In 2021, the Federal Communications Commission (FCC) auctioned C-band spectrum licenses for 5G deployment. This spectrum is seen as one of the best bands for 5G, and it allows signals to propagate further than other bands already in use. Verizon and AT&T combined to bid nearly $70 billion for licenses in the auction, and the companies were expected to introduce their expanded service in early 2022. 

The spectrum the FCC auctioned is separated by 220 MHz from the spectrum used by radio altimeters in aircrafts. The altimeter determines the height of an aircraft and is relied on by other systems within an airplane, including one used for landing when there is poor visibility.  

“The underlying issue here is that radio signals cannot be perfectly confined to their assigned frequency band and will result in some energy being transmitted in neighboring bands," Berry said. "Also, radio receivers cannot perfectly filter out the signals transmitted in neighboring bands and so will receive some out-of-band interference. Both of these effects result in interference from a neighboring band."

Learn more about 5G from MSIT professor Mike Honig. 

US airlines, along with the Federal Aviation Administration, worried  the use of the new spectrum near certain airports would interfere with older airplane altimeters, and, in a worst-case scenario, could lead to a plane crash. Ultimately, Verizon and AT&T agreed to temporarily reduce the power of their networks within a two-mile radius of 50 airports across the US. The agreement is only temporary because permanently reducing the power would decrease the value of the licenses the companies purchased, Berry said.  

Berry's bewilderment stems from the fact that both the airlines and the telecommunications company knew of this potential obstacle for years. Berry said there was a simple solution: the airplanes with outdated altimeters could install a filter that would reduce any out-of-band interference. The problem with that scenario is determining who should be responsible to pay for the filters: the airlines or the telecommunications companies.  

How the situation will ultimately play out is still unclear. For now, airplanes are flying as they should and the telecommunications companies are propagating their new signals from the vast majority of their cell phone towers.  

When Berry talks about the ongoing issue with MSIT students, he reiterates its complexities and the fact the problem didn't have to be as big of a problem as it turned out to be. He tells students they are in the right place to recognize why the problem exists, and potentially to help solve what the ultimate solution should be.

"To understand what's going on with this, you need to know the technology, but you also have to think about these economic issues, as well as law and policy issues," Berry said. "MSIT is a program that gives our students the tools to see all of those different sides of a problem."