By Paul Carsten BEIJING (Reuters) – Apple Inc is finally launching its iPhone on China Mobile Ltds vast network on Friday, opening the door to the worlds largest carriers 763 million subscribers and giving its China sales a short-term jolt. The deal could trigger a limited turnaround for Apple, whose fortunes have wavered in China in the face of stiff competition from market-leader Samsung Electronics Co Ltd and up-and-coming local rival Xiaomi Tech. Samsung had a 21 percent share of Chinas smartphone market in the third quarter of 2013, with Apple trailing in fifth place with just 6 percent, according to research firm Canalys. But the arrival of the iPhone could be a double-edged sword for China Mobile, with some analysts predicting a costly subsidy war as rival carriers compete to lure customers.