Japanese market-leader NTT Docomo has announced that subscribers to its Xi-branded LTE network surpassed the 2 million mark last weekend, adding the second million in just three months.

The operator launched Xi (pronounced “crossy”) on 24 December 2010 and announced the 1 million milestone a year later (25 December 2011). It said that the acceleration in subscriber numbers since then had been due to “the introduction of the first Xi-compatible smartphones in November 2011.” The firm has previously set a target of 30 million LTE connections by March 2016.

“Going forward, subscribers are expected to grow as Docomo continues to introduce additional Xi-compatible smartphones, tablets and data-communication devices and mobile Wi-Fi routers, as well as beneficial services, such as Xi Talk 24, which enables free, unlimited voice calls between Docomo users of Xi-compatible smartphones,” said the firm in a statement.

The first LTE-enabled tablet offered by the operator was the Samsung Galaxy Tab 10.1, launched in October 2011, while the first compatible smartphone (the Samsung Galaxy S II LTE) launched a month later.

While Docomo was an early pioneer of LTE, its Japanese rivals have since launched competing networks. EMOBILE launched a LTE-based data network this week, while SoftBank has recently started offering high-speed services using an evolution of Japan’s PHS cordless network, called AXGP.