Bounds on Covert Capacity with Sub-Exponential Random Slot Selection

TL;DR

This paper establishes bounds on the covert capacity for communication with random slot selection over noisy channels, revealing that the capacity is within a factor of er of the bounds and unaffected by the covertness metric choice.

Contribution

It provides the first bounds on covert capacity with sub-exponential random slot selection, bridging a gap between known capacities for different slot selection regimes.

Findings

Bounds on covert capacity within a factor of er, independent of channel.

Random slot selection affects the covertness constraint analysis.

Covertness metric choice does not influence capacity with random slot selection.

Abstract

We consider the problem of covert communication with random slot selection over binary-input Discrete Memoryless Channels and Additive White Gaussian Noise channels, in which a transmitter attempts to reliably communicate with a legitimate receiver while simultaneously maintaining covertness with respect to an eavesdropper. Covertness refers to the inability of the eavesdropper to distinguish the transmission of a message from the absence of communication, modeled by the transmission of a fixed channel input. Random slot selection refers to the transmitter's ability to send a codeword in a time slot with known boundaries selected uniformly at random among a predetermined number of slots. Our main contribution is to develop bounds for the information-theoretic limit of communication in this model, called the covert capacity, when the number of time slots scales sub-exponentially with the codeword length. Our upper and lower bounds for the covert capacity are within a multiplicative factor of $\sqrt{2}$ independent of the channel. This result partially fills a characterization gap between the covert capacity without random slot selection and the covert capacity with random selection among an exponential number of slots in the codeword length. Our key technical contributions consist of i) a tight upper bound for the relative entropy characterizing the effect of random slot selection on the covertness constraint in our achievability proof; ii) a careful converse analysis to characterize the maximum allowable weight or power of codewords to meet the covertness constraint. Our results suggest that, unlike the case without random slot selection, the choice of covertness metric does not change the covert capacity in the presence of random slot selection.