With 55 million customers, the 800-pound gorilla in the world’s most advanced mobile society, Japan, is NTT Docomo. And over the weekend, the company made clear it wants to retain its position in an LTE future: Docomo is ready to invest $3.4 billion in the next three years to build base stations and will start an LTE service in December this year (as the first of Japan’s three major carriers).

Under the plan, Docomo subscribers can initially expect a maximum downlink speed of 37.5 Mbps. The service will be made available for PCs first, while compatible cell phones will be rolled out sometime in 2011. Tokyo, Nagoya and Osaka are the cities that will be covered first, with around 50% of the nation to follow by 2014.

The logo you can see above shows the brand name for the LTE service, “Xi”, which is read “crossy”. Docomo plans to make Xi available to MVNOs from the get-go, too.