Beam-Coherence-Aware Two-Stage Digital Combining for mmWave MU-MIMO Systems

TL;DR

This paper introduces a two-stage digital combining architecture for wideband mmWave MIMO systems that reduces processing complexity and pilot overhead while maintaining high spectral efficiency, especially in multi-user scenarios.

Contribution

It proposes a novel two-stage digital combining method exploiting channel geometry and delay spread, with tailored channel estimation and reconstruction techniques for improved performance.

Findings

Time-domain channel reconstruction outperforms hybrid beamforming.

Proposed architecture reduces pilot overhead significantly.

Framework extends effectively to multi-user MIMO.

Abstract

This paper considers a wideband millimeter-wave MIMO system with fully digital transceivers at both the base station and the user equipment (UE), focusing on mobile scenarios. To reduce the baseband processing burden at the UE, we propose a two-stage digital combining architecture, where the received signals are compressed from $K$ antennas to dimension $N_{\mathrm c}$ before baseband processing. The first-stage combining matrix exploits channel geometry and is updated on the beam-coherence timescale, which is longer than the channel coherence time, while the second stage is updated per channel coherence time. We develop a pilot-based channel estimation framework tailored to the proposed two-stage digital combining architecture, leveraging maximum likelihood estimation. Furthermore, we propose a time-domain method that exploits the finite delay spread to reconstruct the full channel from a reduced number of pilot subcarriers. Precoding and combining schemes are designed accordingly, and spectral efficiency expressions with imperfect channel state information are derived. Numerical results show that the proposed time-domain approach outperforms hybrid beamforming while reducing pilot overhead. We further demonstrate that the framework extends to multi-user MIMO and retains its performance advantages. These results highlight the potential of two-stage fully digital transceivers for future wideband systems.