Clearwire released preliminary fourth-quarter results today indicating that attempts to reinvigorate its business showed some results last quarter.

The struggling WiMAX operator managed to double its sales, which soared to an estimated $362 million from $181 million during the same period the previous year.

The number beat estimates from analysts polled by Thomson Reuters, who expected Clearwire to post sales of $355.3 million for the three months ended Dec. 31. Retail sales came in at $198 million, outpacing $164 million in wholesale revenue. Clearwire has been working to bolster its wholesale business, yesterday announcing a deal with Simplexity to make it easier for outside companies to resell its service under their own brand.

Despite the upbeat revenue figures, the number of new wholesale customers using Clearwire's network was down slightly to 900,000, from 1.42 million net new wholesale customers during the fourth quarter of 2010.

The vast majority of wholesale customers comprise Sprint smartphone subscribers, who access Clearwire's WiMAX network from Sprint's 4G devices. Data traffic from its wholesale customers contributed to a 165 percent year-over-year increase in network usage during 2011.

Clearwire estimates it ended the year with 10.4 million customers, including 1.3 million retail subscribers. Wholesale churn is pegged at 2.9 percent, with retail churn at 3.9 percent for the fourth quarter.

The growth was good for Clearwire's bottom line. The company expects to post positive adjusted EBITDA (earnings before income taxes, depreciation and amortization) for the quarter, compared to a loss of adjusted EBITDA during the previous quarter.

The increase in sales also helped strengthen Clearwire's cash reserves. It ended the year with $1.11 billion in cash, cash equivalents and investments, from $711 million at the end of September. Clearwire sold off $716 million in stock to Sprint and public investors last quarter to keep its business afloat and pay a $237 million interest payment on its debt.

Sprint also agreed to invest more money in Clearwire, forking over $926 million for unlimited access to its WiMAX network for the next two years and shelling out extra cash for the construction of Clearwire's planned TD-LTE network.

The company said today it will sell off $300 million worth of additional senior secured notes.

Clearwire has yet to set a date for the full release of its financial results from last year but says complete financial information will be made available "in the coming weeks."

Investors appeared to be pleased by the news, raising Clearwire's stock nearly 4 percent in morning Nasdaq trading.