TOKYO (Reuters) - NTT Docomo Inc said on Wednesday it plans to apply for a license to upgrade its base stations to tap the 700 megahertz spectrum for high-speed mobile services, as it races with smaller rivals to attract smartphone users.

The cost of migration to support a network for 3.9-generation phones could cost as much as 150 billion yen ($1.9 billion), it said.

Japan's Communications Ministry allocated the coveted 900 MHz spectrum in February to smaller rival Softbank Corp, which supplies Apple Inc's iPhone in Japan along with KDDI Corp. ($1 = 79.9100 Japanese yen)

(Reporting by Mayumi Negishi; Editing by Michael Watson)