  The 5G system has finally begun commercialization, and now is the time to
start discussing the road map for the 6G system. While the 5G system was
designed with a focus on discovering new service types for high speed,
low-latency, and massive connective services, the evolution of the network
interface for 6G should be considered with an eye toward supporting these
complicated communication environments. As machine-driven data traffic
continues to increase exponentially, 6G must be able to support a series of
connection methods that did not previously exist. In departure from
base-station-oriented cell densification, network diversification is necessary
if we are to satisfy the comprehensive requirements of end terminals for
diverse applications. In this article, we predict what will drive 6G and look
at what key requirements should be considered in 6G. We then diversify four
types of network architectures according to link characteristics, communication
ranges, and target services. The four types of networks play complementary
roles while at the same time collaborating across the entire 6G network.
Lastly, we call attention to key technologies and challenges in the air,
network, and assistive technologies that will have to be addressed when
designing the 6G system.
