  Bi-static sensing is crucial for exploring the potential of networked sensing
capabilities in integrated sensing and communications (ISAC). However, it
suffers from the challenging clock asynchronism issue. CSI ratio-based sensing
is an effective means to address the issue. Its performance bounds, particular
for Doppler sensing, have not been fully understood yet. This work endeavors to
fill the research gap. Focusing on a single dynamic path in high-SNR scenarios,
we derive the closed-form CRB. Then, through analyzing the mutual interference
between dynamic and static paths, we simplify the CRB results by deriving close
approximations, further unveiling new insights of the impact of numerous
physical parameters on Doppler sensing. Moreover, utilizing the new CRB and
analyses, we propose novel waveform optimization strategies for noise- and
interference-limited sensing scenarios, which are also empowered by closed-form
and efficient solutions. Extensive simulation results are provided to validate
the preciseness of the derived CRB results and analyses, with the aid of the
maximum-likelihood estimator. The results also demonstrate the substantial
enhanced Doppler sensing accuracy and the sensing capabilities for low-speed
target achieved by the proposed waveform design.
