  The integrated sensing and communication (ISAC), in which the sensing and
communication share the same frequency band and hardware, has emerged as a key
technology in future wireless systems. Early works on ISAC have been focused on
the design, analysis and optimization of practical ISAC technologies for
various ISAC systems. While this line of works are necessary, it is equally
important to study the fundamental limits of ISAC in order to understand the
gap between the current state-of-the-art technologies and the performance
limits, and provide useful insights and guidance for the development of better
ISAC technologies that can approach the performance limits. In this paper, we
aim to provide a comprehensive survey for the current research progress on the
fundamental limits of ISAC. Particularly, we first propose a systematic
classification method for both traditional radio sensing (such as radar sensing
and wireless localization) and ISAC so that they can be naturally incorporated
into a unified framework. Then we summarize the major performance metrics and
bounds used in sensing, communications and ISAC, respectively. After that, we
present the current research progresses on fundamental limits of each class of
the traditional sensing and ISAC systems. Finally, the open problems and future
research directions are discussed.
