{"ID":2885653,"CreatedAt":"2026-06-01T04:54:23.091178241Z","UpdatedAt":"2026-06-01T04:54:23.091178241Z","DeletedAt":null,"paper_url":"https://arxiv.org/abs/2508.04214","arxiv_id":"2508.04214","title":"Channel-Coherence-Adaptive Two-Stage Fully Digital Combining for mmWave MIMO Systems","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.","short_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...","url_abs":"https://arxiv.org/abs/2508.04214","url_pdf":"https://arxiv.org/pdf/2508.04214v1","authors":"[\"Yasaman Khorsandmanesh\",\"Emil Björnson\",\"Joakim Jaldén\",\"Bengt Lindoff\"]","published":"2025-08-06T08:48:39Z","proceeding":"eess.SP","tasks":"[\"eess.SP\",\"cs.AR\"]","methods":"[]","has_code":false}