NTT DoCoMo, Japan's biggest cellular carrier, will soon launch improvements to its network that promise data transfers up to 15 times faster than are currently possible - and it plans even greater changes next year.

The network improvements will begin in June and will increase the data upload speed from a user's cell phone or PC to the network to as fast as 5.7Mbps from the current 384kbps.

The service, which is based on a technology called HSUPA (High-Speed Uplink Packet Access), will initially be available in Tokyo with coverage spreading from there.

NTT DoCoMo already offers downloads at speeds of up to 7.2Mbps through a complementary technology called HSDPA (High-Speed Downlink Packet Access). A nationwide roll-out was completed in the financial year just ended.

The download speed has steadily increased since 3G (third generation) mobile service began in October 2001 and will be increased again this year to a planned 14Mbps, the company said Tuesday.

The upgrades are scheduled to be the last before DoCoMo moves to an even more advanced technology called LTE (Long Term Evolution).

Promising download speeds as fast as 300Mbps and upload speeds of 75Mbps the technology will be rolled out in DoCoMo's next fiscal year (April 2010 to March 2011), according to the roadmap unveiled on Tuesday.

DoCoMo said the launch will make it one of the first carriers in the world to deploy LTE. In February, Verizon Communications said it will deploy LTE in two U.S. cities late this year, extending it to 25 to 30 others in 2010 before launching the service.

Shortly after DoCoMo's new HSUPA service launches, the company will also cut its rates for PC data users.

From July customers accessing the Internet through a PC will be able to subscribe to a data plan that begins at ¥1,000 (US$10) and tops out at ¥5,985 no matter how much data is used.

For most of the decade Japanese wireless carriers have concentrated on data services delivered via the handset but in recent years have begun to look more at business users who want to access the Internet from their PCs.