  The rapid developments of high-speed trains (HSTs) introduce new challenges
to HST wireless communication systems. Realistic HST channel models play a
critical role in designing and evaluating HST communication systems. Due to the
length limitation, bounding of tunnel itself, and waveguide effect, channel
characteristics in tunnel scenarios are very different from those in other HST
scenarios. Therefore, accurate tunnel channel models considering both
large-scale and small-scale fading characteristics are essential for HST
communication systems. Moreover, certain characteristics of tunnel channels
have not been investigated sufficiently. This article provides a comprehensive
review of the measurement campaigns in tunnels and presents some tunnel channel
models using various modeling methods. Finally, future directions in HST tunnel
channel measurements and modeling are discussed.
