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arXiv · 2602.11901

Cramer-Rao Bounds for Activity Detection in Conventional and Fluid Antenna Systems

Zhentian Zhang·Kai-Kit Wong·Hao Jiang·Christos Masouros·Chan-Byoung Chae
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Abstract

In this letter, we develop a unified Cram\'{e}r-Rao bound (CRB) framework to characterize the fundamental performance limits of transmission activity detection in fluid antenna systems (FASs) and conventional multiple fixed-position antenna (FPA) systems. To facilitate CRB analysis applicable to activity indicators, we relax the binary activity states to continuous parameters, thereby aligning the bound-based evaluation with practical threshold-based detection decisions. Closed-form CRB expressions are derived for two representative detection formulations, namely covariance-oriented and coherent models. Moreover, for single-antenna FASs, we obtain a closed-form coherent CRB by leveraging random matrix theory. The results demonstrate that CRB-based analysis provides a tractable and informative benchmark for evaluating activity detection across architectures and detection schemes, and further reveal that FASs can deliver strong spatial-diversity gains with significantly reduced complexity.

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Zhentian Zhang, Kai-Kit Wong, Hao Jiang, Christos Masouros, Chan-Byoung Chae. 2026-02-12. Cramer-Rao Bounds for Activity Detection in Conventional and Fluid Antenna Systems. https://doi.org/10.1109/lwc.2026.3691385

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