  The fifth-generation (5G) of cellular networks use revolutionary and
evolutionary technologies to notably improve wireless communication
performance. However, given the envisioned new use cases (e.g., holographic
communication) and the ever-increasing deployment of smart-physical end
devices, the volume of network traffic has considerably raised, and the current
mobile networks cannot wholly meet the ever-increasing demands. Hence, it is
envisioned that 6G networks will play a critical role to alleviate these
challenges by providing new communication services, network capacity, and
ultra-low latency communications. In this paper, first, the need of 6G networks
is discussed. Then, the potential 6G requirements and trends, as well as the
latest research activities related to 6G are introduced. Furthermore, the key
performance indicators, applications, new services, and the potential key
enabling technologies for 6G networks are presented. Finally, several potential
unresolved challenges for future 6G networks are presented.
