Japanese ADSL provider Acca Networks Co Ltd signed up NTT DoCoMo Inc to its WiMAX joint venture, tapping the mobile phone giant's resources to launch ultra high-speed wireless Internet access.

Acca, which is fighting to win new subscribers to its broadband Internet access service, will invest about 30 billion yen in the 72 billion yen ($620 million) venture and hold an over 40 percent stake. DoCoMo, Japan's biggest mobile phone operator, would take a 26 percent stake at 19 billion yen, Acca said on Friday.

WiMAX, or Worldwide Interoperability for Microwave Access, allows high-speed and cheap Internet access and file downloads from laptops, phones or other mobile devices over greater distances than previous technologies.

But in Japan's saturated market, where many users have already switched to advanced 3.5-generation phones and where broadband use is high, WiMAX may be a hard sell, analysts have said.

Acca is in talks with other firms, including financial companies, its President Masaharu Kimura said earlier this month. Other Japanese companies interested in WiMAX technology include Softbank Corp (9984.T: Quote, Profile, Research), Japan's smallest mobile phone operator, which is looking to form a joint venture with Internet access provider eAccess Ltd.

Shares in Acca rose to close up 4.7 percent at 245,000 yen while DoCoMo rose 1.1 percent ahead of the announcement, compared with the Nikkei average, which rose 2.57 percent.