SpaceX has agreed to acquire Grain Management's nationwide 800 MHz spectrum portfolio.

The Wall Street Journal reported that Elon Musk's company will pay $8 billion for the spectrum. The transaction is subject to Federal Communications Commission (FCC) approval and other customary closing conditions.

SpaceX plans to use the spectrum as part of its satellite-based Starlink Mobile direct-to-device (D2D) service.

The acquisition comes a couple of days after SpaceX secured the go-ahead to deploy 15,000 high-capacity D2D satellites as the company looks to compete with AT&T, Verizon, and T-Mobile US. Those three carriers notably formed a D2D satellite-focused spectrum-pooling joint venture (JV) last week.

SpaceX's Starlink Mobile has existing access to spectrum in the 2 GHz band, which it's using to deliver service in the U.S. The newly acquired spectrum from Grain will enable its signal to pass through obstacles such as walls and extend service to devices indoors.

"For nearly 20 years, we have built Grain to see opportunity where others see complexity," Grain Management CEO David Grain noted in a statement. "Our spectrum expertise allows us to connect the strategic value of these assets with the technologies and operators that can realize their potential. This agreement with SpaceX brings that capability to bear at extraordinary scale, with the potential to change where and how Americans connect."

Grain acquired the 800 MHz airwaves back in August as part of a $2.9-billion deal with T-Mobile US, with the latter scooping up 600 MHz frequencies from Grain as part of the exchange.

The 800 MHz spectrum was initially set to be picked up by EchoStar-owned Dish Network as part of an agreement with T-Mobile US when Dish was set to operate as the country's fourth mobile carrier. However, that deal fell through when Dish said it was unable to afford the spectrum.

Shares of AT&T, Verizon, and T-Mobile US fell by around 6% following news of SpaceX's latest spectrum purchase. in extended trading.