  Although the combination of the orthogonal time frequency space (OTFS)
modulation and the massive multiple-input multiple-output (MIMO) technology can
make communication systems perform better in high-mobility scenarios, there are
still many challenges in downlink channel estimation owing to inaccurate
modeling and high pilot overhead in practical systems. In this paper, we
propose a channel state information (CSI) acquisition scheme for downlink
massive MIMO-OTFS in presence of the fractional Doppler, including
deterministic pilot design and channel estimation algorithm. First, we analyze
the input-output relationship of the single-input single-output (SISO) OTFS
based on the orthogonal frequency division multiplexing (OFDM) modem and extend
it to massive MIMO-OTFS. Moreover, we formulate an accurate model for the
practical system in which the fractional Doppler is considered and the
influence of subpaths is revealed. A deterministic pilot design is then
proposed based on the model and the structure of the pilot matrix to reduce
pilot overhead and save memory consumption. Since channel geometry changes very
slowly relative to the communication timescale, we put forward a modified
sensing matrix based channel estimation (MSMCE) algorithm to acquire the
downlink CSI. Simulation results demonstrate that the proposed downlink CSI
acquisition scheme has significant advantages over traditional algorithms.
