Samsung Electronics Co Ltd said October-December operating profit fell 18 percent to 8.3 trillion won ($7.8 billion) from the prior quarters record earnings, missing analyst estimates, due primarily to a one-off special bonus. Samsung is bracing itself for its weakest smartphone profit growth this year since it started making the devices in 2007, as arch rival Apple Inc challenges its domination in Chinas $80 billion market. Samsungs mobile devices business, which earns two thirds of the companys profit, will come under pressure when Apple makes its phones available from January 17 via China Mobile Ltd, through which Samsung has been selling smartphones for around seven years. Apple is also widely expected to sell smartphones with larger screens come autumn when it traditionally announces products, neutralizing a selling point that Samsung has enjoyed since introducing its Galaxy Note in late 2011.