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Delay alignment modulation (DAM) is an emerging technique for achievinginter-symbol interference (ISI)-free wideband communications usingspatial-delay processing, without relying on channel equalization ormulti-carrier transmission. However, existing works on DAM only considermultiple-input single-output (MISO) communication systems and assumetime-invariant channels. In this paper, by extending DAM to time-variantfrequency-selective multiple-input multiple-output (MIMO) channels, we proposea 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 beeliminated and all multi-path signal components may reach the receiverconcurrently and constructively. We first show that by applying path-basedzero-forcing (ZF) precoding and receive combining, DDAM can transform theoriginal time-variant frequency-selective channels into time-invariant ISI-freechannels. The necessary and/or sufficient conditions to achieve such atransformation are derived. Then an asymptotic analysis is provided by showingthat when the number of base station (BS) antennas is much larger than that ofchannel paths, DDAM enables time-invariant ISI-free channels with the simpledelay-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 optimizedto maximize the spectral efficiency. Numerical results are provided to comparethe proposed DDAM technique with various benchmarking schemes, includingMIMO-orthogonal time frequency space (OTFS), MIMO-orthogonal frequency-divisionmultiplexing (OFDM) without or with carrier frequency offset (CFO)compensation, and beam alignment along the dominant path.

Delay-Doppler Alignment Modulation for Spatially Sparse Massive MIMO Communication