This letter investigates the challenge of channel estimation in a multiuser millimeter-wave (mmWave) time-division duplexing (TDD) system. In this system, the base station (BS) employs a multi-antenna uniform linear array (ULA), while each mobile user is equipped with a fluid antenna system (FAS). Accurate channel state information (CSI) plays a crucial role in the precise placement of antennas in FAS. Traditional channel estimation methods designed for fixed-antenna systems are inadequate due to the high dimensionality of FAS. To address this issue, we propose a low-sample-size sparse channel reconstruction (L3SCR) method, capitalizing on the sparse propagation paths characteristic of mmWave channels. In this approach, each fluid antenna only needs to switch and measure the channel at a few specific locations. By observing this reduced-dimensional data, we can effectively extract angular and gain information related to the sparse channel, enabling us to reconstruct the full CSI. Simulation results demonstrate that our proposed method allows us to obtain precise CSI with minimal hardware switching and pilot overhead. As a result, the system sum-rate approaches the upper bound achievable with perfect CSI.

翻译：本文研究多用户毫米波（mmWave）时分双工（TDD）系统中的信道估计难题。在该系统中，基站（BS）采用多天线均匀线性阵列（ULA），而每个移动用户配备流体天线系统（FAS）。精确的信道状态信息（CSI）对于FAS中天线的精准定位至关重要。由于FAS的高维度特性，传统针对固定天线系统设计的信道估计方法已不适用。为解决该问题，我们提出一种低样本量稀疏信道重构（L3SCR）方法，利用毫米波信道稀疏传播路径的特性。在该方法中，每个流体天线仅需在少数特定位置切换并测量信道。通过观测这些降维数据，我们能有效提取与稀疏信道相关的角度和增益信息，从而重构完整CSI。仿真结果表明，所提方法能以极低的硬件切换和导频开销获取精确CSI，系统总速率接近完美CSI条件下的性能上限。