CSI Reconstruction in Fluid Antenna Systems Without Spatial Covariance Priors
2026-08-10 • Information Theory
Information Theory
AI summaryⓘ
The authors study fluid antenna systems, which have many potential antenna ports but can only observe signals from a few at a time due to hardware limits. They show that the signal information actually lies in a smaller, simpler space defined by the environment, not by the total number of ports. This means that if the number of observed ports reaches this smaller space's size, the full signal can be recovered accurately without prior knowledge. They also identify a clear cutoff point below which recovery isn't possible, and explain how different factors like power and training affect performance.
fluid antenna systemschannel state information (CSI)spatial diversityClarke isotropic scattering modelmodal subspaceRF chainspilot overheadsignal recoveryspatial modal dimensionsignal-to-noise ratio (SNR)
Authors
Zhentian Zhang, Kaitao Meng, Tuo Wu, Kai-Kit Wong, Hao Xu, Liang Liu, Pei Xiao, Chao Wang, Kin-Fai Tong
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.