On Fundamental Limits of Transmission Activity Detection in Fluid Antenna Systems

2026-02-12

AI Summary

This study investigates the fundamental performance limits of transmission activity detection in fluid antenna systems. For the first time, Cramér–Rao bound (CRB) analysis is introduced to this context by relaxing the discrete activity state into a continuous parameter, thereby establishing a unified framework applicable to both covariance-based and coherent detection models. In the case of single-fluid-antenna systems, a closed-form expression for the coherent CRB is derived, and an analytically tractable performance benchmark is provided using random matrix theory. The results demonstrate that fluid antennas can achieve substantial spatial diversity gains while significantly reducing system complexity, offering rigorous theoretical support for the design of activity detection schemes.

Technology Category

• Machine Learning: Active Learning
• Planning, Routing, and Scheduling: Activity and Plan Recognition
• Constraint Satisfaction and Optimization: Mixed Discrete/Continuous Optimization

Application Category

• Systems and Infrastructure for Web, Mobile and WoT: Web performance, measurement, and characterization
• Graph Algorithms and Modeling for the Web: Algorithms and analysis for incomplete, noisy, or partially observed Web-related graphs
• Responsible Web: Measurement, analysis, and circumvention of Web censorship

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.

Problem

• transmission activity detection
• fluid antenna systems
• fundamental limits
• Cramér-Rao bound
• spatial diversity

Innovation

• Cramér-Rao bound
• fluid antenna systems
• activity detection
• spatial diversity
• random matrix theory