This paper presents an in-depth overview of the ten key enabling technologies which will be landmarks in 5G systems. Specifically, the requirements of the next generation cellular network are first explained and how these ten technologies would aid the realization of these requirements is examined. Furthermore, the strengths and limitations faced by these technologies are investigated in detail, in which the open problems that need to be explored and resolved before the commercialization of these solutions are fully summarized.. Finally, the current research activities being undertaken around the world are discussed. Although some network operators start to promote their “5G services”which are actually based on LTE-Advanced technologies, we predict that the 5G cellular standardization will be completed by the early years of the next decade and commercial services would be launched by mid-late 2020s. Although drastic paradigm shifts have never been easy and require massive upgrading of network infrastructures, the need for better services, which motivated such transitions in the past, will continue to occur in the future.