Rakuten Inc. said Thursday it is considering an investment which would make it one of Japan's major mobile carriers and put it into direct competition with the country's three established telecommunications giants.The e-commerce firm said it will file for an allocation of wireless spectrum with the Ministry of Internal Affairs and Communications in January at the earliest. It is considering raising up to 600 billion yen ($5.3 billion) through 2025 to build its wireless network.The market is currently dominated by NTT Docomo Inc., KDDI Corp. and SoftBank Group Corp., with the three companies…READ FULL ARTICLE