  Covert communication, also known as low probability of detection (LPD)
communication, prevents the adversary from knowing that a communication is
taking place. Recent work has demonstrated that, in a three-party scenario with
a transmitter (Alice), intended recipient (Bob), and adversary (Warden Willie),
the maximum number of bits that can be transmitted reliably from Alice to Bob
without detection by Willie, when additive white Gaussian noise (AWGN) channels
exist between all parties, is on the order of the square root of the number of
channel uses. In this paper, we begin consideration of network scenarios by
studying the case where there are additional "friendly" nodes present in the
environment that can produce artificial noise to aid in hiding the
communication. We establish achievability results by considering constructions
where the system node closest to the warden produces artificial noise and
demonstrate a significant improvement in the throughput achieved covertly,
without requiring close coordination between Alice and the noise-generating
node. Conversely, under mild restrictions on the communication strategy, we
demonstrate no higher covert throughput is possible. Extensions to the
consideration of the achievable covert throughput when multiple wardens
randomly located in the environment collaborate to attempt detection of the
transmitter are also considered.
