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IEEE International Workshop on Signal Processing Advances in Wireless Communications (SPAWC)

On Covert Communication Over Infinite-Bandwidth Gaussian Channels

Ligong Wang

Published Jun 1, 2018• 7 citations
Publisher

Abstract

We consider a problem of communication over a continuous-time additive Gaussian noise channel. A covertness constraint is imposed on the communication protocol, which says that the channel output must statistically resemble pure noise. When there is no bandwidth constraint on the input, we argue that the covert communication capacity of this channel is positive, in contrast to the band-limited case where covertness requires that the amount of transmission grow at most like the square root of total communication time. This claim is formally proven in the case where the Gaussian noise is white with respect to the bandwidth used by the transmitter.

DOI
10.1109/spawc.2018.8445976

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We consider a problem of communication over a continuous-time additive Gaussian noise channel. A covertness constraint is imposed on the communication protocol, which says that the channel output must statistically resemble pure noise. When there is no bandwidth constraint on the input, we argue that the covert communication capacity of this channel is positive, in contrast to the band-limited case where covertness requires that the amount of transmission grow at most like the square root of total communication time. This claim is formally proven in the case where the Gaussian noise is white with respect to the bandwidth used by the transmitter.
We consider the problem of communication over a continuous-time Poisson channel subject to a covertness constraint: The relative entropy between the output distributions when a message is transmitted and when no input is provided must be small. In the absence of both bandwidth and peak-power constraints, we show the covert communication capacity of this channel, in nats per second, to be infinity. When a peak-power constraint is imposed on the input, the covert communication capacity becomes zero, and the “square-root scaling law” applies. When a bandwidth constraint but no peak-power constraint is imposed, the covert communication capacity is again shown to be zero, but we have not determined whether the square-root law holds or not.
We consider a problem of communication over a physically-degraded additive white Gaussian noise wiretap channel. A covertness constraint is imposed, which says that the output at the wiretapper must statistically resemble pure noise. Previous works have shown that the total amount of information that can be transmitted over n uses of the channel is proportional to n, but the communication schemes in those works are designed to transmit only one message of predetermined length. We propose a feedback-aided scheme that, at the cost of less-than-optimal message length, is easily extensible to transmitting multiple messages and an unbounded amount of information.
The problem of covert communication over Multiple-Input Multiple-Output (MIMO) Additive White Gaussian Noise (AWGN) channels is investigated, in which a transmitter attempts to reliably communicate with a legitimate receiver while avoiding detection by a passive adversary. The covert capacity of the MIMO AWGN channel is characterized under a variational distance covertness constraint when the MIMO channel matrices are static and known. The characterization of the covert capacity is also extended to a class of channels in which the legitimate channel matrix is known but the adversary’s channel matrix is only known up to a rank and a spectral norm constraint.
We consider the problem of covert communication over Multiple-Input Multiple-Output (MIMO) Additive White Gaussian Noise (AWGN) channels, in which a transmitter attempts to reliably communicate with a legitimate receiver while ensuring a low probability of detection by an adversary. We exactly characterize the covert capacity under a variational distance covertness constraint when MIMO channel matrices are static and known to all parties. We also characterize the covert capacity for a class of channels in which the legitimate channel matrix is known but the adversary's channel matrix is only known to satisfy a rank and a spectral norm constraint.
Physical-layer security aims to exploit the randomness inherent in communication channels in order to guarantee confidentiality even against computationally unlimited attackers. In this thesis, we focus on covert communication, also known as “communication with low probability of detection”. This is a scenario where a transmitter and receiver try to prevent an eavesdropper from making a good guess on whether a communication is ongoing or not. We assume that the receiver and the eavesdropper share the same channel and therefore see the same outputs. In this scenario, the transmitter shares a key with the legitimate receiver to provide the necessary advantage to detect the communication and reliably decode the message. Asymptotically, the amount of information that can be sent reliably and covertly scales like the square root of the number of channel uses; this is known as the square root law of covert communication.We study the corresponding scaling constant of the square root law for general memoryless channels including non-Gaussian additive noise, as well as Gaussian channels with memory. In the latter case, we show that the scaling constant is the same as over the memoryless Gaussian channel. For continuous memoryless channels, under mild integrability conditions, we show that the scaling constant is upper bounded by a simple expression which only involves the variance of the logarithm of the probability density function of the noise. Moreover, we show that under some additional assumptions, this upper bound is tight. Furthermore, we provide upper bounds on the length of the secret key required to achieve covertness.The second objective of this work is to investigate the limits of covert communication over continuous memoryless channels in the finite blocklength regime. We provide bounds for the first and second-order asymptotics for covert communication over an AWGN channel under a maximal error probability criterion and for the first-order asymptotics under an average error probability criterion.