  As of today, the fifth generation (5G) mobile communication system has been
rolled out in many countries and the number of 5G subscribers already reaches a
very large scale. It is time for academia and industry to shift their attention
towards the next generation. At this crossroad, an overview of the current
state of the art and a vision of future communications are definitely of
interest. This article thus aims to provide a comprehensive survey to draw a
picture of the sixth generation (6G) system in terms of drivers, use cases,
usage scenarios, requirements, key performance indicators (KPIs), architecture,
and enabling technologies. First, we attempt to answer the question of "Is
there any need for 6G?" by shedding light on its key driving factors, in which
we predict the explosive growth of mobile traffic until 2030, and envision
potential use cases and usage scenarios. Second, the technical requirements of
6G are discussed and compared with those of 5G with respect to a set of KPIs in
a quantitative manner. Third, the state-of-the-art 6G research efforts and
activities from representative institutions and countries are summarized, and a
tentative roadmap of definition, specification, standardization, and regulation
is projected. Then, we identify a dozen of potential technologies and introduce
their principles, advantages, challenges, and open research issues. Finally,
the conclusions are drawn to paint a picture of "What 6G may look like?". This
survey is intended to serve as an enlightening guideline to spur interests and
further investigations for subsequent research and development of 6G
communications systems.
