  In this paper, we introduce a novel communication-assisted sensing (CAS)
framework that explores the potential coordination gains offered by the
integrated sensing and communication technique. The CAS system endows users
with beyond-line-of-the-sight sensing capabilities, supported by a
dual-functional base station that enables simultaneous sensing and
communication. To delve into the system's fundamental limits, we characterize
the information-theoretic framework of the CAS system in terms of
rate-distortion theory. We reveal the achievable overall distortion between the
target's state and the reconstructions at the end-user, referred to as the
sensing quality of service, within a special case where the distortion metric
is separable for sensing and communication processes. As a case study, we
employ a typical application to demonstrate distortion minimization under the
ISAC signaling strategy, showcasing the potential of CAS in enhancing sensing
capabilities.
