  Future railway is expected to accommodate both train operation services and
passenger broadband services. The millimeter wave (mmWave) communication is a
promising technology in providing multi-gigabit data rates to onboard users.
However, mmWave communications suffer from severe propagation attenuation and
vulnerability to blockage, which can be very challenging in high-speed railway
(HSR) scenarios. In this paper, we investigate efficient hybrid beamforming
(HBF) design for train-to-ground communications. First, we develop a two-stage
HBF algorithm in blockage-free scenarios. In the first stage, the minimum mean
square error method is adopted for optimal hybrid beamformer design with low
complexity and fast convergence; in the second stage, the orthogonal matching
pursuit method is utilized to approximately recover the analog and digital
beamformers. Second, in blocked scenarios, we design an anti-blockage scheme by
adaptively invoking the proposed HBF algorithm, which can efficiently deal with
random blockages. Extensive simulation results are presented to show the sum
rate performance of the proposed algorithms under various configurations,
including transmission power, velocity of the train, blockage probability, etc.
It is demonstrated that the proposed anti-blockage algorithm can improve the
effective rate by 20% in severely-blocked scenarios while maintaining low
outage probability.
