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.

翻译：本文研究了一种在基站和用户设备（UE）端均采用全数字收发器的宽带毫米波MIMO系统，重点关注移动场景。为降低UE端的基带处理负担，我们提出了一种两阶段数字合并架构，将接收信号在基带处理前从$K$根天线压缩至维度$N_{\mathrm c}$。第一阶段合并矩阵利用信道几何特性，并在波束相干时间尺度（该时间尺度长于信道相干时间）上更新，而第二阶段则按每个信道相干时间进行更新。我们开发了一种适配所提出的两阶段数字合并架构的基于导频的信道估计框架，利用最大似然估计方法。此外，我们提出了一种时域方法，利用有限时延扩展从较少的导频子载波中重构完整信道。相应地设计了预编码与合并方案，并推导了具有非完美信道状态信息时的频谱效率表达式。数值结果表明，所提出的时域方法在降低导频开销的同时优于混合波束成形。我们进一步证明，该框架可扩展至多用户MIMO场景并保持其性能优势。这些结果凸显了全数字两阶段收发器在未来宽带系统中的潜力。