T-Mobile is petitioning the FCC to let them test network equipment in the 3.5GHz band as they believe that it will be beneficial to the advancement of deploying 5G networks and because they see it as a means to enhancing LTE with more spectrum. In addition to seeking permission for testing in this particular band, T-Mobile also hopes that the FCC will take another look at the rules for the 3.5GHz band spectrum as they want the rules to "align" with the requirements for a 5G network.
T-Mobile's goal is to test the use and performance of pre-commercial Nokia equipment on the band and through the testing they're hoping to understand how use of the band might open up a path for new and innovative services, and it wants to complete these tests in Bellevue, Washington as well as Bothell, Washington, as Bellevue is their HQ and Bothell is a nearby location. The FCC voted to open the use of 3.5GHz spectrum bands way back in 2015, and in late August of last year the CBRS Alliance was launched in attempts to help manage details revolving around the band, which now includes T-Mobile along with Verizon Wireless, AT&T, and Sprint, all of which have their own plans for 5G networks in development.
Verizon, which also plans to make use of the 3.5GHz band spectrum has asked the FCC around October of last year to re-purpose the use of the spectrum faster so they could work towards testing it. Although T-Mobile is hoping to get the approval to begin testing the 3.5GHz band soon, it's reported that they're aiming to begin testing on April 15th which is just next Saturday, but it wants to be able to continue testing the band all the way through September 30th of next year, which means it's very possible that any actual usage of the band on T-Mobile's own network wouldn't be implemented for quite some time as their proposed testing period is for about a year and a half. T-Mobile and other carriers are already working towards deploying 5G networks with the use of other technologies and T-Mobile sees the 3.5GHz spectrum band as a potential way to improve things.