  The sixth-generation (6G) network is envisioned to integrate communication
and sensing functions, so as to improve the spectrum efficiency (SE) and
support explosive novel applications. Although the similarities of wireless
communication and radio sensing lay the foundation for their combination, there
is still considerable incompatible interest between them. To simultaneously
guarantee the communication capacity and the sensing accuracy, the
multiple-input and multiple-output (MIMO) technique plays an important role due
to its unique capability of spatial beamforming and waveform shaping. However,
the configuration of MIMO also brings high hardware cost, high power
consumption, and high signal processing complexity. How to efficiently apply
MIMO to achieve balanced communication and sensing performance is still open.
In this survey, we discuss joint communication and sensing (JCAS) in the
context of MIMO. We first outline the roles of MIMO in the process of wireless
communication and radar sensing. Then, we present current advances in both
communication and sensing coexistence and integration in detail. Three novel
JCAS MIMO models are subsequently discussed by combining cutting-edge
technologies, i.e., cloud random access networks (C-RANs), unmanned aerial
vehicles (UAVs) and reconfigurable intelligent surfaces (RISs). Examined from
the practical perspective, the potential and challenges of MIMO in JCAS are
summarized, and promising solutions are provided. Motivated by the great
potential of the Internet of Things (IoT), we also specify JCAS in IoT
scenarios and discuss the uniqueness of applying JCAS to IoT. In the end, open
issues are outlined to envisage a ubiquitous, intelligent and secure JCAS
network in the near future.
