Channel-Coherence-Adaptive Two-Stage Fully Digital Combining for mmWave MIMO Systems

TL;DR

This paper introduces a two-stage fully digital combining scheme for mmWave MIMO systems that reduces computational complexity by leveraging channel geometry and updates at different times, improving spectral efficiency.

Contribution

It proposes a novel two-stage digital combining method that adapts to channel coherence times, along with a pilot-based channel estimation framework for mmWave MIMO systems.

Findings

Outperforms hybrid beamforming in spectral efficiency.

Reduces baseband processing complexity.

Effective channel estimation in both uplink and downlink.

Abstract

This paper considers a millimeter-wave wideband point-to-point MIMO system with fully digital transceivers at the base station and the user equipment (UE), focusing on mobile UE scenarios. A main challenge when building a digital UE combining is the large volume of baseband samples to handle. To mitigate computational and hardware complexity, we propose a novel two-stage digital combining scheme at the UE. The first stage reduces the $N_{\text{r}}$ received signals to $N_{\text{c}}$ streams before baseband processing, leveraging channel geometry for dimension reduction and updating at the beam coherence time, which is longer than the channel coherence time of the small-scale fading. By contrast, the second-stage combining is updated per fading realization. We develop a pilot-based channel estimation framework for this hardware setup based on maximum likelihoodestimation in both uplink and downlink. Digital precoding and combining designs are proposed, and a spectral efficiency expression that incorporates imperfect channel knowledge is derived. The numerical results demonstrate that the proposed approach outperforms hybrid beamforming, showcasing the attractiveness of using two-stage fully digital transceivers in future systems.