Polarization control improves hidden wireless communication rates
Polarforming for MIMO Covert Communications
Information Theory
Summary
The paper investigates how to hide wireless signals better by using antennas that can change their polarization, which is like adjusting the antenna’s angle to send and receive signals differently. The researchers show how to optimize this polarization alongside other settings to keep a message secret while maximizing how fast it can be sent. They also figure out how hard it is for a watcher to detect the hidden signal over different time spans. Their method improves data rates significantly compared to older ways of hiding wireless signals. The work helps wireless devices communicate more secretly even with limited antenna space.
What this means in practice
- •For wireless communication engineers: Design MIMO systems that use polarization control to enhance covertness and data rates under power limits while avoiding detection.
- •For military communication teams: Implement covert radio links that jointly optimize transmit and receive antenna polarization to reduce the chance of interception in hostile environments.
Authors
Xin Xie, Jinpeng Xu, Yihang Huang, Li Zhou, Zhaolong Ning
Abstract
Covert communication conceals wireless transmission activity, but conventional multi-antenna designs rely mainly on spatial beamforming and can be constrained by limited spatial degrees of freedom. This paper investigates polarization-reconfigurable antenna (PRA)-aided multiple-input multiple-output (MIMO) covert communication, where Alice and Bob perform transmit and receive polarforming, respectively, and Willie uses a vertically polarized antenna for radiometric detection. We jointly optimize the transmit covariance matrix and the transmit and receive phase shift vectors to maximize Bob's achievable rate under the covertness and transmit power constraints. Under Gaussian signaling, perfect polarized channel state information, and known noise power at Willie, we derive his optimal radiometric threshold and exact minimum detection error probability (DEP) for a finite observation length, and convert the DEP requirement into a deterministic constraint on his received signal power. We further prove that, for a fixed transmission design, positive signal leakage becomes detectable as the observation length tends to infinity. To solve the design problem for a finite observation length, we develop an alternating optimization (AO) algorithm with a generalized water-filling covariance update and closed-form phase updates. Numerical results validate the exact DEP analysis and the rapid convergence of the proposed algorithm. Compared with the sufficient divergence-based condition, the exact covertness constraint permits a $0.99$--$1.12$~dB higher received signal-to-noise ratio (SNR) at Willie, while joint transmit and receive polarforming provides a rate gain of up to $53.8\%$ over the considered benchmark schemes.