Even though it only recently introduced third-generation (3G) wireless service, Japan's NTT DoCoMo said Tuesday that it had started developing an experimental fourth-generation system.

The 4G service is aimed at providing data access speeds of 100 Mbps for downloads and 20 Mbps for uploads, the company said in a statement. It said it has started constructing the infrastructure for system and would probably start what it called "practical evaluations of key technologies" this summer.

The 4G system will provide high-quality video that DoCoMo claims will equal that of high-definition television, which is more than double the resolution of old-technology analog television.

The company said that the 4G system will use a bandwidth of about 100 MHz, which is far broader than 3G bandwidth. It said that it expected the primary technical impediment to be multipath, which is the occurrence of secondary signals reflected off objects such as buildings. It said it will implement several technologies, including variable spreading factor and orthogonal frequency code division multiplexing to minimize that problem.

DoCoMo became the first wireless operator to launch widespread 3G service last October when it made its FOMA service widely available in its service area. That service has attracted 60,000 subscribers, which some industry analysts say is disappointing.

DoCoMo said it wasn't in a hurry to launch 4G service, saying it estimated it would not be commercialized until 2010.

Separately, an executive for the Japanese wireless carrier told an audience at the CTIA Wireless 2002 show this week that he expected U.S. carrier AT&T Wireless to launch i-mode services later this year.

I-mode is wildly popular with DoCoMo's Japanese customers with more than 30 million subscribers. DoCoMo took a 16 percent stake in AT&T Wireless last year. It also owns stakes in a variety of other European and Asian wireless operators, most of which either already have launched i-mode service or are expected to do so in the future.