Fluid RIS (FRIS)-Assisted Index Modulation for 6G Wireless Communications

2026-05-21

AI Summary

This work addresses the challenge in conventional fluid reconfigurable intelligent surfaces (FRIS), where index modulation codebooks constructed through layout diversity often fail to ensure distinguishable received responses, leading to high detection errors. To overcome this limitation, the paper proposes a response-aware FRIS codebook design methodology that, for the first time, prioritizes separability in the received response domain as the core design criterion. By introducing drive granularity as a key parameter, the approach jointly optimizes spatial diversity gain, pilot overhead, and hardware feasibility. This enables compact, trainable, and controllable spatial index configurations that significantly reduce index detection error rates, thereby establishing an efficient and reliable FRIS design paradigm for programmable wireless environments in 6G.

Abstract

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.

Problem

• Fluid RIS
• Index Modulation
• Spatial Indexing
• Response Separability
• 6G Wireless Communications

Innovation

• Fluid RIS
• Index Modulation
• Response-aware Design
• Spatial Codebook
• Actuation Granularity