  In this paper, we propose a Doppler pre-compensation scheme for high-mobility
orthogonal frequency division multiplexing (OFDM) uplink, where a high-speed
terminal transmits signals to the base station (BS). Considering that the
time-varying multipath channel consists of multiple Doppler frequency offsets
(DFOs) with different angle of departures (AoDs), we propose to perform DFO
pre-compensation at the transmitter with a large-scale uniform linear array
(ULA). The transmitted signal passes through a beamforming network with
high-spatial resolution to produce multiple parallel branches. Each branch
transmits signal towards one direction thus it is affected by one dominant DFO
when passing over the time-varying channel. Therefore, we can compensate the
DFO for each branch at the transmitter previously. Theoretical analysis for the
Doppler spread of the equivalent uplink channel is also conducted. It is found
that when the number of transmit antennas is sufficiently large, the
time-variation of channel can be efficiently suppressed. Therefore, the
performance will not degrade significantly if applying the conventional
time-invariant channel estimation and equalization methods at the receiver.
Simulation results are provided to verify the proposed scheme.
