  Orthogonal frequency-division multiplexing (OFDM) is widely adopted for
providing reliable and high data rate communication in high-speed train
systems. However, with the increasing train mobility, the resulting large
Doppler shift introduces intercarrier interference (ICI) in OFDM systems and
greatly degrades the channel estimation accuracy. Therefore, it is necessary
and important to investigate reliable channel estimation and ICI mitigation
methods in high-mobility environments. In this paper, we consider a typical HST
communication system and show that the ICI caused by the large Doppler shift
can be mitigated by exploiting the train position information as well as the
sparsity of the conventional basis expansion model (BEM) based channel model.
Then, we show that for the complex-exponential BEM (CE-BEM) based channel
model, the ICI can be completely eliminated to get the ICI-free pilots at each
receive antenna. After that, we propose a new pilot pattern design algorithm to
reduce the system coherence and hence can improve the compressed sensing (CS)
based channel estimation accuracy. The proposed optimal pilot pattern is
independent of the number of receive antennas, the Doppler shifts, the train
position, or the train speed. Simulation results confirms the performance
merits of the proposed scheme in high-mobility environments. In addition, it is
also shown that the proposed scheme is robust to the respect of high mobility.
