  Amid the demand for densely integrated elements in techniques such as
holographic reconfigurable intelligent surfaces (RISs), the mutual coupling
effect has gained prominence. By performing a misspecified Cramer-Rao bound
analysis within an electromagnetics-compliant communication model, this letter
offers a quantitative evaluation of the impact of mutual coupling on
RIS-assisted channel estimation. Our analysis provides insights into situations
where mutual coupling can be disregarded safely. The analyses and numerical
results reveal that within practical scenarios, closer integration of RIS
elements or the enlargement of RIS size accentuates the impact of neglecting
mutual coupling. In addition, even with mutual coupling-aware setups,
excessively tight RIS element spacing can lead to substantial degradation in
the channel estimation performance.
