NTT DoCoMo will begin offering cellular data transmission at up to 10 times the speed of 3G from late December when it switches on the biggest upgrade to its cellular data network in years, it said Monday.

Japan's largest cellular carrier will start offering LTE (long term evolution) data service under the "Xi" brand name (read "Crossy") from December 24.

LTE is a new packet data transmission technology that's based on IP, the same basic protocol as the Internet. For users it promises faster data transmission speeds, and for carriers it helps make more efficient use of available spectrum.

It will offer download speeds as fast as 37.5Mbps and uploads at up to 12.5Mbps in general use, but these are increased to 75Mbps downloads and 37.5Mbps uploads inside buildings that are fitted with LTE antennas and equipment. During a demonstration of the service in central Tokyo, a PC equipped with an LTE modem was measured streaming data at around 56Mbps.

At launch the new service will cover Tokyo, parts of Osaka, Nagoya and a handful of other areas near these cities. NTT DoCoMo built the launch network at a cost of ¥35 billion ($430 million) and plans to spend an additional ¥270 billion over the following two years to roll out service to other parts of Japan.

Most users will pay only slightly more for LTE than they already do for 3G data. A monthly charge of ¥6,510 (£50) will cover up to 5GB of data. After that, ¥2,625 will cover each additional 2GB of data.

DoCoMo currently charges ¥5,985 for unlimited 3G use. The 5GB limit covers more than 99 percent of all current users, so for most customers the faster speeds will cost around 9 percent more than current 3G charges, DoCoMo said. However, with far faster data speeds users may find many more ways to use the wireless service and their mobile bills could rise. For very heavy users the additional fees could make LTE significantly more expensive.

At launch the carrier will offer a USB modem from LG Electronics. An Express Card modem from Fujitsu will be available from April. Both the LG and Fujitsu products also include 3G modems for use outside the LTE service area.

While initially restricted to data, voice calls over LTE will begin to be offered sometime during the fiscal year that begins in April 2011.