Finite Blocklength Covert Communication over Quasi-Static Multiple-Antenna Fading Channels

2026-03-31

AI Summary

This study addresses the performance limits of finite-blocklength covert communication over quasi-static multi-antenna fading channels under the stringent ultra-low latency and high-security requirements set by the ITU. Under a Kullback–Leibler (KL) divergence-based covertness constraint, the authors systematically analyze the covert capacity between legitimate transceivers under four distinct channel state information (CSI) availability scenarios. By leveraging finite-blocklength information theory, modeling covert outage probability, and carefully controlling higher-order terms, they demonstrate that multi-antenna systems yield substantial spatial diversity gains and prove that CSI availability does not affect covert performance at finite blocklengths. Key contributions include extending the quasi-η neighborhood framework to fading channels, establishing that the first-order covert rate follows a square-root law Θ(n⁻¹/²), the second-order rate vanishes, and its leading coefficient is uniquely determined by the traces of the legitimate user’s and warden’s channel matrices.

Abstract

The white book released by the International Telecommunications Union (ITU) calls for extremely high-security and low-latency communication over fading channels. Under the low-latency requirement, the corresponding fading model is quasi-static fading while high-security can be achieved via covert communication. In response to the call of ITU, we study the finite blocklength performance of optimal codes for covert communication over quasi-static multi-antenna fading channels, under the covertness metric of Kullback-Leibler (KL) divergence. In particular, we study all four cases regarding the availability of channel state information (CSI) for legitimate transmitter and receiver, and assume that the warden knows perfect CSI for the channel from the legitimate transmitter to itself. Specifically, we show that, when the blocklength is $n$, the first-order covert rate satisfies the square root law, scaling as $Θ(n^{-\frac{1}{2}})$ with the coefficient determined by the traces of the channel matrices of the legitimate users and the warden, and the second-order rate vanishes. In contrast to the non-covert result of Yang et al. (TIT, 2014), we show that CSI availability at the legitimate users does not affect the finite blocklength performance for covert communication. Furthermore, we reveal the significant spatial diversity gain provided by multiple-antenna systems for covert communication. For the covertness analysis, we extend the quasi-$η$-neighborhood framework to fading channels and address challenges arising from the random channel matrices. For the reliability analysis, due to the vanishing power imposed by the covertness constraint, we refine the non-covert analysis by Yang et al. (TIT, 2014), by carefully controlling higher-order terms and exploiting the properties of covert outage probability.

Problem

• covert communication
• finite blocklength
• quasi-static fading channels
• multiple-antenna systems
• channel state information

Innovation

• covert communication
• finite blocklength
• quasi-static fading
• multiple-antenna
• channel state information