  The integrated sensing and communication (ISAC) technique has the potential
to achieve coordination gain by exploiting the mutual assistance between
sensing and communication (S&C) functions. While the sensing-assisted
communications (SAC) technology has been extensively studied for high-mobility
scenarios, the communication-assisted sensing (CAS) counterpart remains widely
unexplored. This paper presents a waveform design framework for CAS in 6G
perceptive networks, aiming to attain an optimal sensing quality of service
(QoS) at the user after the target's parameters successively ``pass-through''
the S$\&$C channels. In particular, a pair of transmission schemes, namely,
separated S&C and dual-functional waveform designs, are proposed to optimize
the sensing QoS under the constraints of the rate-distortion and power budget.
The first scheme reveals a power allocation trade-off, while the latter
presents a water-filling trade-off. Numerical results demonstrate the
effectiveness of the proposed algorithms, where the dual-functional scheme
exhibits approximately 25% performance gain compared to its separated waveform
design counterpart.
