Covert Multi-Access Communication with a Non-Covert User

TL;DR

This paper investigates the fundamental limits of covert communication in a three-user system, revealing how a non-covert user can enhance covert user capacities and proposing a multiplexing-based coding scheme to achieve these limits.

Contribution

It characterizes the capacity tradeoffs in a multi-user covert communication system with a non-covert user and introduces a novel multiplexing coding scheme to attain these limits.

Findings

Presence of a non-covert user can boost covert user capacities.

A multiplexing coding scheme achieves the fundamental limits.

Tradeoff between rates of covert and non-covert users is characterized.

Abstract

In this paper, we characterize the fundamental limits of a communication system with three users (i.e., three transmitters) and a single receiver where communication from two covert users must remain undetectable to an external warden. Our results show a tradeoff between the highest rates that are simultaneously achievable for the three users. They further show that the presence of a non-covert user in the system can enhance the capacities of the covert users under stringent secret-key constraints. To derive our fundamental limits, we provide an information-theoretic converse proof and present a coding scheme that achieves the performance of our converse result. Our coding scheme is based on multiplexing different code phases, which seems to be essential to exhaust the entire tradeoff region between the rates at the covert and the two non-covert users. This property is reminiscent of the setup with multiple non-covert users, where multiplexing is also required to exhaust the entire rate-region.