You may have to wait a while to take advantage of C-band 5G’s potentially higher speeds. The Wall Street Journal has learned that AT&T and Verizon are delaying their C-band rollouts by at least a month to help the Federal Aviation Administration address aircraft interference concerns. They had originally planned to use the newly available frequencies on December 5th, but AT&T said it was pushing its launch to January 5th after the Transportation Department requested a delay.

Verizon hasn’t formally confirmed its delay, but it will reportedly move the C-band 5G deployment by “about a month” in step with its rival. We’ve asked the carrier for comment.

The FAA wants to issue requirements that bar pilots from using some automated poor-weather safety systems that might conflict with the new 5G technology, according to The Journal‘s sources. Air transport industry figures are worried this could play havoc with flights, but telecoms claim there’s no evidence to suggest the expanded 5G will create problems.

Both wireless networks have a strong incentive to keep any delays to a minimum. The C-band slice they’re using (between 3.7GHz and 3.98GHz) represents a sweet spot for 5G service with more bandwidth than low- or mid-band 5G, but without nearly as much interference as millimeter wave frequencies. It could alleviate congestion in other frequency bands, too. The expansion promises a more substantial improvement over LTE that could drive subscriptions, not to mention more upgrades to 5G phones.