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.

翻译：延迟对齐调制（DAM）是一种新兴技术，通过空间延迟处理实现无符号间干扰（ISI）的宽带通信，而无需依赖信道均衡或多载波传输。然而，现有关于DAM的研究仅考虑多输入单输出（MISO）通信系统，并假设信道是时不变的。本文通过将DAM扩展到时变频率选择性多输入多输出（MIMO）信道，提出了一种称为“延迟-多普勒对齐调制”（DDAM）的新技术。具体而言，通过利用“延迟-多普勒补偿”和“基于路径的波束赋形”，可以消除每条多径的多普勒效应，并使所有多径信号分量同时且建设性地到达接收端。我们首先证明，通过应用基于路径的迫零（ZF）预编码和接收合并，DDAM可以将原始的时变频选信道转化为时变无ISI信道。推导了实现这种转化的必要和/或充分条件。随后，通过渐近分析表明，当基站（BS）天线数量远大于信道路径数时，DDAM通过简单的延迟-多普勒补偿和基于路径的最大比传输（MRT）波束赋形，即可实现时变无ISI信道。此外，对于存在一定可容忍ISI的通用DDAM设计，我们优化了基于路径的发射预编码和接收合并矩阵以最大化频谱效率。最后，通过数值结果将所提出的DDAM技术与多种基准方案进行比较，包括MIMO正交时频空间（OTFS）、无/有载波频率偏移（CFO）补偿的MIMO正交频分复用（OFDM），以及沿主导路径的波束对齐。