  Orthogonal time frequency space (OTFS) modulation is a recently introduced
multiplexing technique designed in the 2-dimensional (2D) delay-Doppler domain
suited for high-Doppler fading channels. OTFS converts a doubly-dispersive
channel into an almost non-fading channel in the delay-Doppler domain through a
series of 2D transformations. In this paper, we focus on MIMO-OTFS which brings
in the high spectral and energy efficiency benefits of MIMO and the robustness
of OTFS in high-Doppler fading channels. The OTFS channel-symbol coupling and
the sparse delay-Doppler channel impulse response enable efficient MIMO channel
estimation in high Doppler environments. We present an iterative algorithm for
signal detection based on message passing and a channel estimation scheme in
the delay-Doppler domain suited for MIMO-OTFS. The proposed channel estimation
scheme uses impulses in the delay-Doppler domain as pilots for estimation. We
also compare the performance of MIMO-OTFS with that of MIMO-OFDM under high
Doppler scenarios.
