As the long term evolution (LTE) of the fourth generation mobile communication system (4G) is succe ssfully commercialized globally and mobile data are increase d explosively, the research and development on the fifth generation of mobile communication system (5G) become the focus of both industry and acad e mia. Here, the two drivers to develop 5G are firstly analyzed. Then th e international telecommunication union(ITU) timeline for global standardization of 5G is presented and the most recent updates are described. Subsequently, widely recognized key technologies for 5G a re pres e nted. Specifically, key wireless network technologies, including the ultra dense cel lular network (UDN) and the centralized radio access network, are described in d etai l. Finally, conclusions are drawn and open problems are discussed.