CSI Reconstruction in Fluid Antenna Systems Without Spatial Covariance Priors

2026-08-10

AI Summary

This work addresses the challenge of reconstructing full-port channel state information (CSI) in fluid antenna systems, where hardware constraints permit observation of CSI from only a limited number of ports and prior spatial covariance statistics are unavailable. Leveraging the Clarke isotropic scattering model, the study reveals that the channel resides in a low-dimensional modal subspace dictated by the scattering environment. It establishes, for the first time, a theoretical framework for CSI recoverability without requiring prior statistical knowledge and derives an exact feasibility threshold. By integrating spatial modal modeling, error decomposition analysis, and low-rank recovery techniques, the proposed approach enables high-fidelity reconstruction of full-port CSI using only a few active ports, substantially reducing the number of required RF chains, pilot overhead, and training data.

Abstract

Fluid antenna systems (FASs) exploit many candidate ports for spatial diversity, but hardware constraints allow channel observations at only a few active ports. Whether full-port CSI can be recovered without pre-acquired channel statistics remains open. Under the Clarke isotropic scattering model, we show that the channel lies in a low-dimensional spatial modal subspace determined by the scattering environment rather than the total port count. Consequently, recovery becomes feasible when the number of observed ports reaches the modal dimension (i.e., $M\geq r$), even when $M\ll N$. We further establish a sharp feasibility threshold: reliable recovery is impossible below this dimension regardless of SNR, whereas accuracy improves with additional observations above it. By decomposing the recovery error into modal truncation, estimation, and learning components, we derive explicit tradeoffs among RF chains, pilot overhead, transmit power, and training data. These results enable scalable prior-free full-port CSI recovery with few active ports.

Problem

• CSI reconstruction
• fluid antenna systems
• spatial covariance priors
• channel state information
• modal subspace

Innovation

• Fluid Antenna Systems
• CSI Reconstruction
• Spatial Modal Subspace
• Prior-Free Recovery
• Low-Dimensional Channel Modeling