  Delay alignment modulation (DAM) is an emerging technique for achieving
inter-symbol interference (ISI)-free wideband communications using
spatial-delay processing, without relying on channel equalization or
multi-carrier transmission. However, existing works on DAM only consider
multiple-input single-output (MISO) communication systems and assume
time-invariant channels. In this paper, by extending DAM to time-variant
frequency-selective multiple-input multiple-output (MIMO) channels, we propose
a novel technique termed \emph{delay-Doppler alignment modulation} (DDAM).
Specifically, by leveraging \emph{delay-Doppler compensation} and
\emph{path-based beamforming}, the Doppler effect of each multi-path can be
eliminated and all multi-path signal components may reach the receiver
concurrently and constructively. We first show that by applying path-based
zero-forcing (ZF) precoding and receive combining, DDAM can transform the
original time-variant frequency-selective channels into time-invariant ISI-free
channels. The necessary and/or sufficient conditions to achieve such a
transformation are derived. Then an asymptotic analysis is provided by showing
that when the number of base station (BS) antennas is much larger than that of
channel paths, DDAM enables time-invariant ISI-free channels with the simple
delay-Doppler compensation and path-based maximal-ratio transmission (MRT)
beamforming. Furthermore, for the general DDAM design with some tolerable ISI,
the path-based transmit precoding and receive combining matrices are optimized
to maximize the spectral efficiency. Numerical results are provided to compare
the proposed DDAM technique with various benchmarking schemes, including
MIMO-orthogonal time frequency space (OTFS), MIMO-orthogonal frequency-division
multiplexing (OFDM) without or with carrier frequency offset (CFO)
compensation, and beam alignment along the dominant path.
