Japanese smartphone users are increasingly adopting high-speed data connections, as telecoms operator NTT Docomo announced today that the number of subscribers to its Xi LTE network has exceeded 15 million — making up about one-quarter of its more than 61 million mobile customers.

The company said that subscriptions to its LTE network have grown rapidly since its launch in December 2010, but noted that the number has increased by 50 percent in a mere five months, after reaching 10 million subscribers in February this year.

The recent growth spurt has largely been due to promotions introduced by smartphone manufacturers for their new summer lineup, including discounts introduced in May for the LTE-compatible Sony Xperia A SO-04E and Samsung Galaxy S4 SC-04E.

Last week, Docomo — the largest mobile network operator in Japan — announced it would be starting prelaunch operations for an upgrade to its Xi LTE network, offering downlink speeds of up to 150 Mbps — similar to speeds being offered by South Korean operators SK Telecom and LG Uplus for their LTE-Advanced services.

Korean consumers have been quick on the uptake – more than 150,000 subscribers signed up for SK Telecom’s LTE-A service just 14 days after its launch, making for a take-up rate of more than 10,000 new subscribers a day.

This comes even as LTE is still taking off in other parts of the world. It was just confirmed yesterday that UK operator O2 will launch its 4G service on August 29, finally providing a challenger to EE’s sole offering in the market. In April this year, reports said that China Mobile was aiming to launch its LTE network for public usage — the first 4G service in China — by August, though that is yet to come.

Image Credit: Toshifumi Kitamura via AFP/Getty Images