  With the development of high speed trains (HST) in many countries, providing
broadband wireless services in HSTs is becoming crucial. Orthogonal
frequency-division multiplexing (OFDM) has been widely adopted for broadband
wireless communications due to its high spectral efficiency. However, OFDM is
sensitive to the time selectivity caused by high-mobility channels, which costs
large spectrum or time resources to obtain the accurate channel state
information (CSI). Therefore, the channel estimation in high-mobility OFDM
systems has been a long-standing challenge. In this paper, we first propose a
new position-based high-mobility channel model,in which the HST's position
information and Doppler shift are utilized to determine the positions of the
dominant channel coefficients. %In this way, we can reduce the estimation
complexity and to design the transmitted pilot.Then, we propose a joint pilot
placement and pilot symbol design algorithm for compressed channel estimation.
It aims to reduce the coherence between the pilot signal and the proposed
channel model, and hence can improve the channel estimation accuracy.
Simulation results demonstrate that the proposed method achieves better
performances than existing channel estimation methods over high-mobility
channels. Furthermore, we give an example of the designed pilot codebook to
show the practical applicability of the proposed scheme.
