  It has been 20 years since the concept of cognitive radio (CR) was proposed,
which is an efficient approach to provide more access opportunities to connect
massive wireless devices. To improve the spectrum efficiency, CR enables
unlicensed usage of licensed spectrum resources. It has been regarded as the
key enabler for intelligent communications. In this article, we will provide an
overview on the intelligent communication in the past two decades to illustrate
the revolution of its capability from cognition to artificial intelligence
(AI). Particularly, this article starts from a comprehensive review of typical
spectrum sensing and sharing, followed by the recent achievements on the
AI-enabled intelligent radio. Moreover, research challenges in the future
intelligent communications will be discussed to show a path to the real
deployment of intelligent radio. After witnessing the glorious developments of
CR in the past 20 years, we try to provide readers a clear picture on how
intelligent radio could be further developed to smartly utilize the limited
spectrum resources as well as to optimally configure wireless devices in the
future communication systems.
