Fluid reconfigurable intelligent surfaces (FRIS) enable joint position and phase reconfigurability by integrating fluid antennas (FA) with conventional reconfigurable intelligent surfaces (RIS). In this paper, we propose a novel FRIS-based index modulation (IM) framework that exploits the additional spatial degrees of freedom introduced by FRIS element-position reconfiguration. Based on this framework, two transmission schemes are developed, namely FRIS-assisted receiver spatial modulation (FRIS-RSM) and receiver spatial shift keying (FRIS-RSSK), where information bits are conveyed through receiver-antenna index selection. The proposed framework supports both continuous and finite-bit phase control while accounting for FRIS-side spatial correlation. To balance detection complexity and bit error rate (BER) performance, a two-stage reduced-complexity list detector is proposed. For performance analysis under double-Rayleigh cascaded fading with strongest-link selection, tractable post-selection statistics are developed for both continuous-phase and quantized-phase FRIS and incorporated into a moment-generating-function (MGF)-based framework to derive unconditional pairwise error probability (UPEP) and union-bound BER expressions. Simulation results demonstrate significant BER gains over conventional RIS-assisted schemes and verify the accuracy of the analysis.

翻译：流体可重构智能表面通过将流体天线与传统可重构智能表面相结合，实现了位置与相位的联合可重构性。本文提出了一种新颖的基于FRIS的索引调制框架，该框架利用FRIS单元位置重构引入的额外空间自由度。基于此框架，我们开发了两种传输方案：FRIS辅助的接收机空间调制与接收机空间移位键控，其中信息比特通过接收天线索引选择进行传递。所提框架同时支持连续相位与有限比特相位控制，并考虑了FRIS侧的空间相关性。为平衡检测复杂度与误码率性能，提出了一种两阶段低复杂度列表检测器。针对采用最强链路选择的双瑞利级联衰落信道下的性能分析，本文推导了连续相位与量化相位FRIS在链路选择后的易处理统计量，并将其纳入基于矩生成函数的分析框架，从而导出无条件成对错误概率与联合界误码率闭合表达式。仿真结果表明，所提方案较传统RIS辅助方案获得显著的误码率性能增益，并验证了理论分析的准确性。