The fluid antenna (FA)-enabled multiple-input multiple-output (MIMO) system based on index modulation (IM), referred to as FA-IM, significantly enhances spectral efficiency (SE) compared to the conventional FA-assisted MIMO system. This paper proposes an innovative FA grouping-based IM (FAG-IM) system to improve performance in mitigating the high spatial correlation between multiple activated ports. A block grouping scheme is employed based on the spatial correlation model and the distribution structure of the ports. Then, a closed-form expression for the average bit error probability (ABEP) upper bound of the FAG-IM system is derived. In order to reduce the receiver complexity of the proposed system, the message passing mechanism is first incorporated into the FAG-IM system. Subsequently, within the approximate message passing (AMP) framework, an efficient structured AMP (S-AMP) detector is devised by leveraging the structural characteristics of the transmission signal vector. Simulation results confirm that the proposed FAG-IM system significantly outperforms the existing FA-IM system in the presence of spatial correlation. The derived ABEP curve aligns well with the numerical results, providing an efficient theoretical tool for evaluating the system performance. Additionally, simulation results demonstrate that the proposed low-complexity S-AMP detector not only reduces the time complexity to a linear scale but also substantially improves bit error rate (BER) performance compared to the minimum mean square error (MMSE) detector, thus facilitating the practical implementation of the FAG-IM system.

翻译：基于索引调制（IM）的流体天线（FA）多输入多输出（MIMO）系统（简称FA-IM）相较于传统FA辅助MIMO系统，显著提升了频谱效率（SE）。本文提出一种创新的基于流体天线分组的索引调制（FAG-IM）系统，以改善在缓解多个激活端口间高空间相关性方面的性能。该系统基于空间相关性模型与端口分布结构，采用了一种块分组方案。随后，推导了FAG-IM系统平均误码概率（ABEP）上界的闭式表达式。为降低所提系统的接收机复杂度，首次将消息传递机制引入FAG-IM系统。接着，在近似消息传递（AMP）框架内，通过利用传输信号向量的结构特征，设计了一种高效的结构化AMP（S-AMP）检测器。仿真结果证实，在存在空间相关性的情况下，所提出的FAG-IM系统性能显著优于现有的FA-IM系统。推导的ABEP曲线与数值结果高度吻合，为评估系统性能提供了有效的理论工具。此外，仿真结果表明，所提出的低复杂度S-AMP检测器不仅将时间复杂度降低至线性量级，而且相较于最小均方误差（MMSE）检测器，其误码率（BER）性能也得到大幅提升，从而促进了FAG-IM系统的实际应用。