  As the fifth-generation (5G) mobile communication system is being
commercialized, extensive studies on the evolution of 5G and sixth-generation
mobile communication systems have been conducted. Future mobile communication
systems are evidently evolving towards a more intelligent and
software-reconfigurable functionality paradigm that can provide ubiquitous
communication and also sense, control, and optimize wireless environments.
Thus, integrating communication and localization by utilizing the highly
directional transmission characteristics of millimeter-wave (mmWave) is a
promising route. This approach not only expands the localization capabilities
of a communication system but also provides new concepts and opportunities to
enhance communication. In this paper, we explain the integrated communication
and localization in mmWave systems, in which these processes share the same set
of hardware architecture and algorithm. We also perform an overview of the key
enabling technologies and the basic knowledge on localization. Then, we provide
two promising directions for studies on localization with extremely large
antenna array and model-based neural networks. We also discuss a comprehensive
guidance for location-assisted mmWave communications in terms of channel
estimation, channel state information feedback, beam tracking, synchronization,
interference control, resource allocation, and user selection. Finally, this
paper outlines the future trends on the mutual assistance and enhancement of
communication and localization in integrated systems.
