  Orthogonal Time Frequency Space (OTFS) is a novel modulation scheme designed
in the Doppler-delay domain to fully exploit time and frequency diversity of
general time-varying channels. In this paper, we present a novel discrete-time
analysis of OFDM-based OTFS transceiver with a concise and vectorized
input-output relationship that clearly characterizes the contribution of each
underlying signal processing block in such systems. When adopting cyclic prefix
in the time domain, our analysis reveals that the proposed MIMO OTFS and OFDM
systems have the same ergodic capacity despite the well-known fact that the
former has great advantages in low-complexity receiver design for high Doppler
channels. The proposed discrete-time vectorized formulation is applicable to
general fast fading channels with arbitrary window functions. It also enables
practical low-complexity receiver design for which such a concise formulation
of the input-output relationship is of great benefits.
