  Extensive work has demonstrated the excellent performance of orthogonal time
frequency space (OTFS) modulation in high-mobility scenarios. Time-variant
wideband channel estimation serves as one of the key compositions of OTFS
receivers since the data detection requires accurate channel state information
(CSI). In practical wideband OTFS systems, the Doppler shift brought by the
high mobility is frequency-dependent, which is referred to as the Doppler
Squint Effect (DSE). Unfortunately, DSE was ignored in overall prior estimation
schemes employed in OTFS systems, which leads to severe performance loss in
channel estimation and the consequent data detection. In this paper, we
investigate DSE of wideband time-variant channel in delay-Doppler domain and
concentrate on the characterization of OTFS channel coefficients considering
DSE. The formulation and evaluation of OTFS input-output relationship are
provided for both ideal and rectangular waveforms considering DSE. The channel
estimation is therefore formulated as a sparse signal recovery problem and an
orthogonal matching pursuit (OMP)-based scheme is adopted to solve it.
Simulation results confirm the significance of DSE and the performance
superiority compared with traditional channel estimation approaches ignoring
DSE.
