  The critical distinction between the emerging intelligent transparent surface
(ITS) and intelligent reflection surface (IRS) is that the incident signals can
penetrate the ITS instead of being reflected, which enables the ITS to combat
the severe signal penetration loss for high-speed railway (HSR) wireless
communications. This paper thus investigates the channel estimation problem for
an ITS-assisted HSR network where the ITS is embedded into the carriage window.
We first formulate the channels as functions of physical parameters, and thus
transform the channel estimation into a parameter recovery problem. Next, we
design the first two pilot blocks within each frame and develop a serial
low-complexity channel estimation algorithm. Specifically, the channel
estimates are initially obtained, and each estimate is further expressed as the
sum of its perfectly known value and the estimation error. By leveraging the
relationship between channels for the two pilot blocks, we recover the Doppler
shifts from the channel estimates, based on which we can further acquire other
channel parameters. Moreover, the Cramer-Rao lower bound (CRLB) for each
parameter is derived as a performance benchmark. Finally, we provide numerical
results to establish the effectiveness of our proposed estimators.
