(Reuters) - Clearwire Corp (CLWR.O) said its 2012 revenue may fall, or at best remain at 2011 levels, as it faces increased competition at its biggest customer, Sprint Nextel (S.N), sending its shares down nearly 9 percent in after hours trade.

The wireless service provider, which is majority-owned by Sprint, also forecast higher capital spending this year as it kicks off an upgrade to its network.

It forecast 2012 revenue of $1.15 billion to $1.25 billion compared with its 2011 revenue of $1.25 billion partly due to the timing of revenue recognition from a recent agreement with Sprint.

Clearwire is expected to lose out because of Sprint's own plans to build a high-speed wireless service and its launch of Apple Inc's (AAPL.O) iPhone, which doesn't work on the Clearwire network.

The company also warned that it would revert to an adjusted loss before interest, taxes, depreciation and amortization of $250 million to $350 million for 2011.

Since the company posted earnings before interest, taxes, depreciation and amortization for the fourth quarter, maybe there's some disappointment they're guiding to an EBITDA loss after a positive Q4, Pacific Crest analyst Steve Clement said.

Clearwire Chief Executive Erik Prusch said that the company's numbers would improve once it completes its network upgrade, which starts this year.

This is a build year for us, Prusch told Reuters. He also noted that Clearwire is working on gaining new wholesale customers beyond Sprint, on which it is almost entirely dependent for wholesale customers.

The company does not expect to have devices to support its high-speed service upgrade until the end of 2012 or early 2013.

It forecast 2012 capital spending of $450 million to $550 million, with most spending coming in the second half of the year.

For the fourth quarter, Clearwire posted a wider quarterly net loss as costs increased and it wrote down the value of certain assets and took a charge related to a change of its preferred network technology.

Clearwire posted a loss of $236.85 million, or 81 cents per share, compared with a loss of $128.01 million, or 79 cents per share, in the year ago quarter. Revenue rose to $361.9 million from $175.2 million.

Clearwire shares fell to $2.15 in after-market trade after closing at $2.36 on the Nasdaq.