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

2025-08-06

AI Summary

To address the high baseband processing complexity of fully digital transceivers in millimeter-wave wideband point-to-point MIMO systems with mobile user equipment (UE), this paper proposes a channel-coherence-adaptive two-stage fully digital combining scheme. The method innovatively integrates channel geometric structure with beam coherence time characteristics: the first stage employs infrequent updates over long coherence intervals, while the second stage dynamically adapts to instantaneous channel fading, significantly reducing computational and hardware overhead. Coupled with a maximum-likelihood-based pilot-assisted channel estimation algorithm, the scheme jointly optimizes uplink and downlink fully digital precoding and combining designs, enhancing robustness under imperfect channel state information (CSI). Simulation results demonstrate that the proposed approach achieves higher spectral efficiency than conventional hybrid beamforming, validating its effectiveness and practicality in high-mobility scenarios.

Technology Category

• Machine Learning: Hardware-aware ML
• Multiagent Systems: Mechanism Design
• Planning, Routing, and Scheduling: Optimization of Spatio-temporal Systems

Application Category

• Systems and Infrastructure for Web, Mobile and WoT: Data management and stream processing for Web, mobile and wireless applications
• User Modeling, Personalization and Recommendation: On-Device user modeling, personalization, and recommendation
• Security and Privacy: Large-scale security measurements

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 $N_{ ext{r}}$ received signals to $N_{ ext{c}}$ 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.

Problem

• Reducing baseband sample volume in digital UE combining
• Mitigating computational and hardware complexity in MIMO systems
• Improving spectral efficiency with two-stage digital combining

Innovation

• Two-stage digital combining reduces signal streams
• Channel geometry leverages dimension reduction
• Pilot-based channel estimation framework developed