T-Mobile said Friday that it has signed an agreement to buy 10MHz of Advanced Wireless Services (AWS) wireless spectrum from U.S. Cellular for $308 million in cash.

The spectrum is meant to improve and expand T-Mobile's existing 4G LTE network coverage in the Mississippi Valley region, the company said in a news release.

The spectrum covers 32 million people in 29 markets—notably in St. Louis, Nashville, Kansas City, Memphis, Lexington, Little Rock-North Little Rock, Birmingham, New Orleans and Louisville, T-Mobile said.

The deal expands the network to allow for a broader roll-out of 4G LTE, which gives customers a fast, reliable 4G connection, and spurs competition, T-Mobile said.

Securing AWS spectrum is a rare opportunity that will be put to use by both T-Mobile and MetroPCS users, T-Mobile said. The agreement with U.S. Cellular still has to be approved by the U.S. Federal Communications Commission and the Department of Justice, it added.

The transaction is expected to close in the fourth quarter of this year, T-Mobile said.

T-Mobile completed the merger with MetroPCS in May and said earlier in June that the migration of MetroPCS customers onto its network for 4G HSPA+ and LTE is ahead of schedule. The migration frees up spectrum for deployment of the new network, which provides faster data speeds for customers with HSPA+ and LTE compatible devices, T-Mobile said at the time.