Sprint-Nextel Corp. (NYSE: S), the No. 3 U.S. telecommunications company, has acquired a majority interest in high-speed specialist Clearwire Inc. (Nasdaq: CLWR), which bolsters its wireless spectrum intended to speed communications for its 56 million wireless customers.

The $100 million deal came only three days after Overland Park, Kan.-based Sprint agreed to sell a 70 percent stake to Japan’s SoftBank Corp. (Tokyo: 9984) in deal valued around $20.1 billion. Adding control of Clearwire boosts Sprint's current operations and could make it even more valuable.

Sprint upped its interest in Clearwire to 50.8 percent from 47 percent, buying shares from founder Craig McCaw, the legendary U.S. wireless pioneer whose cellular network was later sold to AT&T Inc. (NYSE: T), the No. 1 telecommunications carrier.

The transaction was reported in a filing with the U.S. Securities and Exchange Commission.

Clearwire, of Bellevue, Wash., holds spectrum rights that are valuable to any mobile carrier. They can be used to help Sprint to further develop its long-term evolution network that’s required for newer smart phones such as the iPhone 5 from Apple (Nasdaq: AAPL), the world's most valuable technology company.

The status of Clearwire was left out of the official announcement of the SoftBank-Sprint acquisition, which isn’t expected to be completed until mid-2013 at the earliest.

But SoftBank CEO Masayoshi Son said in an investor call that he wanted to bring “this experience of high speed” enjoyed by Japanese mobile users to the U.S. SoftBank in Japan controls the No. 3 mobile network.

Shares of Clearwire fell 15 cents to $2.12 in early Thursday activity. Shares of Sprint rose a nickel to $5.78.