Delay-Doppler Alignment Modulation for Spatially Sparse Massive MIMO Communication

TL;DR

This paper introduces delay-Doppler alignment modulation (DDAM), a novel technique for transforming time-variant MIMO channels into ISI-free channels using delay-Doppler compensation and path-based beamforming, enhancing wideband communication performance.

Contribution

The paper extends delay alignment modulation to time-variant MIMO channels, proposing DDAM with path-based zero-forcing precoding and beamforming, and provides conditions and asymptotic analysis for its effectiveness.

Findings

DDAM can convert time-variant channels into ISI-free channels.

When BS antennas are much larger than channel paths, DDAM achieves time-invariant channels.

Numerical results show DDAM outperforms benchmarks like OTFS and OFDM.

Abstract

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.