Understanding the Role of Phase and Position Design in Fluid Reconfigurable Intelligent Surfaces

Abstract: Fluid Reconfigurable Intelligent Surfaces (FRISs) are gaining momentum as an improved alternative over classical RIS. However, it remains unclear whether their performance gains can be entirely attributed to spatial flexibility, or instead to differences in equivalent aperture or phase design. In this work, we shed light onto this problem by benchmarking FRIS vs. RIS performances in two practical scenarios: conventional RIS (same number of active elements and same overall aperture) and compact RIS (same number of active elements, and smaller aperture with sub-λ inter-element spacing). Statistical analysis demonstrates that: (i) spatial position optimization in FRIS provides noticeable gains over conventional RIS in the absence of phase-shift design; (ii) such benefits vanish when FRIS and conventional RIS employ optimal beamforming (BF) and phase shift (PS) design, making position optimization irrelevant; (iii) FRIS consistently outperforms compact RIS with optimized BF and PS design, owing to spatial correlation and smaller aperture.