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), it reported in a filing with the U.S. Securities and Exchange Commission Thursday.

The deal came only three days after the 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.

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.

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 (LTE) 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 14 cents to $2.12 in early Thursday activity. Shares of Sprint rose a penny to $5.74.

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