  Recent advances in wireless communication and solid-state circuits together
with the enormous demands of sensing ability have given rise to a new enabling
technology, integrated sensing and communications (ISAC). The ISAC captures two
main advantages over dedicated sensing and communication functionalities: 1)
Integration gain to efficiently utilize congested resources, and even, 2)
Coordination gain to balance dual-functional performance or/and perform mutual
assistance. Meanwhile, triggered by ISAC, we are also witnessing a paradigm
shift in the ubiquitous IoT architecture, in which the sensing and
communication layers are tending to converge into a new layer, namely, the
signaling layer.
  In this paper, we first attempt to introduce a definition of ISAC, analyze
the various influencing forces, and present several novel use cases. Then, we
complement the understanding of the signaling layer by presenting several key
benefits in the IoT era. We classify existing dominant ISAC solutions based on
the layers in which integration is applied. Finally, several challenges and
opportunities are discussed. We hope that this overview article will serve as a
primary starting point for new researchers and offer a bird's-eye view of the
existing ISAC-related advances from academia and industry, ranging from
solid-state circuitry, signal processing, and wireless communication to mobile
computing.

Signal Processing