Covert Communication over a K-User Multiple Access Channel

TL;DR

This paper characterizes the limits of covert communication over a K-user multiple access channel, showing each transmitter can reliably send on the order of √n bits without detection, and identifies conditions for secret key use.

Contribution

It provides a complete characterization of the covert capacity region for the K-user MAC, including optimal scaling constants and conditions for secret key necessity.

Findings

Each transmitter can reliably transmit on the order of √n bits.

Sum-rate constraints are inactive in the covert setting.

Conditions for operation without secret keys are identified.

Abstract

We consider a scenario in which $K$ transmitters attempt to communicate covert messages reliably to a legitimate receiver over a discrete memoryless MAC while simultaneously escaping detection from an adversary who observes their communication through another discrete memoryless MAC. We assume that each transmitter may use a secret key that is shared only between itself and the legitimate receiver. We show that each of the $K$ transmitters can transmit on the order of $\sqrt{n}$ reliable and covert bits per $n$ channel uses, exceeding which, the warden will be able to detect the communication. We identify the optimal pre-constants of the scaling, which leads to a complete characterization of the covert capacity region of the $K$-user binary-input MAC. We show that, asymptotically, all sum-rate constraints are inactive unlike the traditional MAC capacity region. We also characterize the channel conditions that have to be satisfied for the transmitters to operate without a secret key.