Fluid reconfigurable intelligent surfaces (FRIS) extend conventional reconfigurable intelligent surfaces (RIS) by adding spatial reconfigurability through switchable apertures, pattern-reconfigurable units, fluidic conductive materials, or movable surface elements. This article studies how FRIS can support index modulation (IM), where information bits select a surface configuration and the receiver detects the index from the induced receiver-side response. A key challenge is that many feasible FRIS layouts do not necessarily lead to many reliable spatial indices. After propagation, mutual coupling, hardware distortion, and receiver observation, different layouts may produce similar receiver-side responses and cause index-detection errors. To address this issue, we present a response-aware design view, in which FRIS spatial codebooks are selected according to response-domain separability rather than layout diversity alone. We also discuss actuation granularity as a practical design knob that balances spatial diversity, pilot overhead, coupling robustness, and hardware feasibility. The resulting workflow helps select compact, trainable, and controllable spatial-index codebooks from dense FRIS layouts, providing design guidance for future programmable wireless environments.

翻译：流体可重构智能表面（FRIS）通过可切换孔径、模式可重构单元、流体导电材料或可移动表面元件实现空间可重构性，从而扩展了传统可重构智能表面（RIS）的功能。本文研究FRIS如何支持索引调制（IM），其中信息比特选择表面配置，接收机通过感应接收端响应来检测索引。关键挑战在于，许多可行的FRIS布局不一定能产生足够多的可靠空间索引。经过传播、互耦合、硬件失真和接收机观测后，不同布局可能产生相似的接收端响应，导致索引检测错误。为解决此问题，我们提出一种响应感知的设计视角，其中FRIS空间码本基于响应域的可分离性而非单纯布局多样性进行选择。我们还讨论了执行粒度作为实际设计参量，它平衡了空间多样性、导频开销、耦合鲁棒性和硬件可行性。由此产生的工作流程有助于从密集的FRIS布局中选择紧凑、可训练且可控的空间索引码本，为未来可编程无线环境的设计提供指导。