Clearwire Corp plans to sell more than $1.1 billion in debt after months of negotiations to stave off a looming funding gap as it spends billions on building a new wireless network.

But its shares fell 6.6 percent after the news as some investors worried about the cost of the debt, while others were disappointed the wireless service provider, majority owned by Sprint Nextel Corp , had not been able to raise more.

It's only $1 billion, and in view of the other options this is likely one of the higher-cost of capital options, Mizuho Securities analyst Michael Nelson said.

Sprint shares rose 5 percent as investors saw the offering as a sign that the No. 3 U.S. mobile provider would not need to raise its investment in Clearwire -- at least for now.

The news was more mixed for Clearwire investors, some of whom were hoping for a lower-cost solution, like an equity investment.

Clearwire had also considered raising funding from other sources including another investment from Sprint or from T-Mobile USA, a unit of Deutsche Telekom AG , which would in turn rent network space at a favorable rate.

It also looked at the option of selling wireless spectrum. The company did not say in its statement on Thursday if those other options were still in the cards.

Wells Fargo analyst Jennifer Fritzsche said in a research note that she did not believe this was the ideal path (Clearwire) wanted to go down.

Some analysts also estimate that Clearwire still needs another $3 billion or so to build its network to compete with rivals such as Verizon Wireless, a venture of Verizon Communications and Vodafone Group Plc .

But others saw the move as good news for the company, which had set the year's end as a deadline for its financing plans.

A billion is nice. It gives them breathing room in the near term, said BTIG analyst Walter Piecyk. It obviously increases their negotiating position because they're not as desperate for cash.

Early last month, after failing to come up with a financing agreement despite months of negotiations, Clearwire had warned that there was doubts it could continue as a going concern, causing investors to view it as a much higher-risk option.

Part of the debt offering -- $500 million exchangeable notes due 2040 -- is exchangeable for cash or common shares in the company.

While the debt offer also appears to take immediate pressure off Sprint to make another investment in Clearwire, Piecyk said it may end up buying into the exchangeable debt.

It's possible it forces them to write a check in order to ensure their equity stake doesn't drop below 50 percent, the analyst said. Sprint declined to comment on the matter.

Clearwire said it is giving certain shareholders with about 85 percent of voting rights the option, for 30 days from the offer date, to buy notes in proportion to their stake.

Aside from Sprint, other Clearwire investors also include Comcast Corp , Time Warner Cable , Google Inc and Intel Corp .

Clearwire's funding negotiations took place against a backdrop of tensions in its relationship with Sprint, which uses Clearwire's network to sell high-speed wireless services, but objects to Clearwire pursuing its own retail sales effort.

The fact they made a decision on raising capital through debt probably tells you there's still a disagreement between Sprint and Clearwire regarding Clearwire's retail efforts, Mizuho's Nelson said.

Clearwire also announced three new director nominees for its board on Thursday. The nominees were chosen by Sprint, who had to remove three of its executives from the Clearwire board for technical reasons.

Clearwire said the new debt would come in the form of an offer of $175 million in first-priority, senior secured notes due 2015, $500.0 million of second-priority secured notes due 2017 as well as the exchangeable notes due 2040.

It said it would also grant the initial purchasers of the exchangeable notes an option to purchase up to an additional $100.0 million of exchangeable notes.

Clearwire shares fell 6.6 percent, or 45 cents, to $6.37 on Nasdaq, while Sprint shares rose nearly 5 percent, or 19 cents, to $3.97 on the New York Stock Exchange.

(Reporting by Sinead Carew; editing by Gerald E. McCormick and Maureen Bavdek)