  Integrated sensing and communications (ISAC) is potentially capable of
circumventing the limitations of existing frequency-division sensing and
communications (FDSAC) techniques. Hence, it has recently attracted significant
attention. This article aims to propose a unified analytical framework for ISAC
from a mutual information (MI) perspective. Based on the proposed framework,
the sensing performance and the communication performance are evaluated by the
sensing MI and the communication MI, respectively. The unity of this framework
is originated from the fact that the sensing and communication (S\&C)
performance metrics, i.e., the S\&C MI, have the similar physical and
mathematical properties as well as the same unit of measurement. Based on this
framework, the S\&C performance of downlink and uplink ISAC systems is
investigated and compared with that of FDSAC systems. Along each considered
system settings, numerical results are provided to demonstrate the superiority
of ISAC over conventional FDSAC designs. Finally, promising open research
directions are provided in the context of MI-based ISAC.
