Craig McCaw, one of the founders of Clearwire, has quit as the company's chairman, according to a Securities and Exchange Commission filing.

In the filing, Clearwire said McCaw's decision to resign is not due to any disagreements with the company. Nor has McCaw said what he will do next. 

McCaw has been on the board since 2008. Eagle River Holdings, a major investor, originally nominated him to the position. Eagle River can still nominate a board member.

The departure comes at a delicate time for Clearwire. The Kirkland, Wash.-based company has plans to build a national WiMax network. The company recently raised $1.3 billion in debt, and is 54 percent owned by Sprint Nextel. Sprint has the option to buy an additional $750 million in securities, and has committed to using Clearwire's network.

But while Sprint needs Clearwire's network to provide 4G services, the two companies have their differences. They are in the midst of an arbitration case in which Clearwire claims Sprint still owes fees for the wireless devices that are 4G capable, even if the devices aren't using Clearwire's network. Sprint says otherwise.

The fees from Sprint add up to millions -- in their quarterly conference call in November Clearwire's CFO Erik Prusch put it at about $17 million.

At the same time, Clearwire is rapidly expanding its network, reaching two major cities, San Francisco and Bridgeport, Conn., in just the last two weeks.

Before Clearwire, McCaw was the founder of McCaw Cellular, which he sold to AT&T in 1994 for $11.5 billion. Later, he teamed up with Bill Gates to form Teledesic, which was supposed to provide satellite-based Internet service. That venture eventually failed. He founded Clearwire in 2003.