COVERT COMMUNICATIONS IN CONTINUOUS-TIME SYSTEMS

This dissertation studies covert wireless communications where a transmitter (Alice) intends to transmit messages to a legitimate receiver (Bob) such that the presence of the message is hidden from an attentive warden (Willie). Here we consider pertinent aspects of covert communications that focus on moving such systems closer to implementation. For example, previous studies use the standard discrete-time communication model when analyzing covert communications, since this is commonly assumed without loss of generality in standard communication theory. However, it is not clear that such a model captures the salient aspects of the continuous-time covert communications problem. A power detector that is optimal for the warden in a discrete-time covert communications scenario may not be optimal on a continuous- time model. Thus, it is of interest to consider this more realistic model for physical channels. After analyzing a power optimization problem using the standard discrete-time model, we move to the key part of system implementation: the instantiation in true continuous-time systems of the discrete-time models studied to this point in the literature. A key goal is to examine Willie’s detection capability on a continuous-time model and study how the limits of covert communications change from the discrete-time case. In particular, we show that detectors for Willie can benefit from the continuous-time setting and outperform detectors based on the discrete-time model; not surprisingly, this has a significant impact on the true covert throughput of the system. Nevertheless, we establish constructions such that efficient covert communications can still be achieved in a continuous-time model, and prove the fundamental limit on the covert communication rate. After considering the continuous-time problem in detail, we then turn to addressing another limitation of previous work - the requirement for an intentional jammer to facilitate efficient covert communication. Instead, we consider how to exploit a pre-existing interference source – a radar - to achieve covert communication. We establish a covert communication scheme in such an environment, and analyze the corresponding covert rate. Finally, we consider the use of a detection technique similar to that in the covert communications problem, in the area of quantized signal detection

Achieving Covert Communication With A Probabilistic Jamming Strategy

In this work, we consider a covert communication scenario, where a transmitter Alice communicates to a receiver Bob with the aid of a probabilistic and uninformed jammer against an adversary warden's detection. The transmission status and power of the jammer are random and follow some priori probabilities. We first analyze the warden's detection performance as a function of the jammer's transmission probability, transmit power distribution, and Alice's transmit power. We then maximize the covert throughput from Alice to Bob subject to a covertness constraint, by designing the covert communication strategies from three different perspectives: Alice's perspective, the jammer's perspective, and the global perspective. Our analysis reveals that the minimum jamming power should not always be zero in the probabilistic jamming strategy, which is different from that in the continuous jamming strategy presented in the literature. In addition, we prove that the minimum jamming power should be the same as Alice's covert transmit power, depending on the covertness and average jamming power constraints. Furthermore, our results show that the probabilistic jamming can outperform the continuous jamming in terms of achieving a higher covert throughput under the same covertness and average jamming power constraints

Jamming Signal Cancellation by Channel Inversion Power Control for Preserving Covert Communications, Journal of Telecommunications and Information Technology, 2023, nr 2

Uninformed jammers are used to facilitate covert communications between a transmitter and an intended receiver under the surveillance of a warden. In reality, the signals the uniformed jammer emits to make the warden’s decision uncertain have inadvertently interfered with the detection of the intended receiver. In this paper, we apply truncated channel inversion power control (TCIPC) to both the transmitter and the uninformed jammer. The TCIPC scheme used on the uninformed jammer may help the intended receiver remove jamming signals using the successive interference cancellation (SIC) technique. Under the assumption that the warden knows the channel coefficient between two intended transceivers and achieves the optimal detection power threshold, we form the optimization problem to maximize the effective transmission rate (ETR) under covertness and decoding constraints. With the aim of enhancing covertness-related performance, we achieve the optimal power control parameters and determine system parameter-related constraints required for the existence of these solutions. According to the simulations, the use of the TCIPC scheme on the uninformed jammer significantly improves covertness-related performance in comparison to that of random power control (RPC) and constant power control (CPC) schemes. In addition, simulation results show that, for the TCIPC scheme: 1) the maximum ETR tends to converge as the transmitter’s or the uninformed jammer’s maximum transmit power increases, and 2) there exists an optimal value of the transmitter’s predetermined transmission rate to achieve the optimal performance

Finite Blocklength Analysis of Gaussian Random coding in AWGN Channels under Covert constraints II: A Viewpoint of Total Variation Distance

Covert communication over an additive white Gaussian noise (AWGN) channel with finite block length is investigated in this paper. The attention is on the covert criterion, which has not been considered in finite block length circumstance. As an accurate quantity metric of discrimination, the variation distance with given finite block length n and signal-noise ratio (snr) is obtained. We give both its analytic solution and expansions which can be easily evaluated. It is shown that K-L distance, which is frequently adopted as the metric of discrimination at the adversary in asymptotic regime, is not convincing in finite block length regime compared with the total variation distance. Moreover, the convergence rate of the total variation with different snr is analyzed when the block length tends to infinity. The results will be very helpful for understanding the behavior of the total variation distance and practical covert communication

Perfectly Covert Communication with a Reflective Panel

This work considers the problem of \emph{perfect} covert communication in wireless networks. Specifically, harnessing an Intelligent Reflecting Surface (IRS), we turn our attention to schemes that allow the transmitter to completely hide the communication, with \emph{zero energy} at the unwanted listener (Willie) and hence zero probability of detection. Applications of such schemes go beyond simple covertness, as we prevent detectability or decoding even when the codebook, timings, and channel characteristics are known to Willie. We define perfect covertness, give a necessary and sufficient condition for it in IRS-assisted communication, and define the optimization problem. For two IRS elements, we analyze the probability of finding a solution and derive its closed form. We then investigate the problem of more than two IRS elements by analyzing the probability of such a zero-detection solution. We prove that this probability converges to $1$ as the number of elements tends to infinity. We provide an iterative algorithm to find a perfectly covert solution and prove its convergence. The results are also supported by simulations, showing that a small amount of IRS elements allows for a positive rate at the legitimate user yet with zero probability of detection at an unwanted listener.Comment: 30 pages, 5 figure

Covert Wireless Communications in a Dynamic Environment

This dissertation investigates covert communication in dynamic wireless communication environments. A key goal is to provide insight about the capabilities of a transmitter desiring to remain covert and analogously, the capabilities of the party attempting to detect covert communications. The first chapter provides background on covert communications prior to this work. The second chapter studies the theoretical limits of covert communication and proves that positive rate is achievable when a jammer is added to the classical Alice/Bob/Warden Willie model. The third chapter expands on the second chapter by considering more generally the impact of the dynamics of the environment on the Alice/Bob/Warden Willie model. The dynamics of the environment generate uncertainty at Willie even if the jammer does not vary his/her power or even if Willie employs an antenna array to mitigate the jamming. The fourth and fifth chapters investigate the impact of considering the exact continuous-time model rather than a discrete-time model approximation. In particular, detectors at Willie which leverage information in the continuous-time domain outperform detectors based on the discrete-time model approximation. The fourth and fifth chapters consider the continuous-time model of the Alice/Bob/Willie scenario and the Alice/Bob/Willie/Jammer scenarios respectively. The fourth and fifth chapters may appear to question the results of Chapter 2, Chapter 3 and prior wireless covert communication related research. However, these final chapters provide insight about different detectors available to Willie and the importance of Alice implementing communication schemes which do not contain features that significantly differ from Willie\u27s observation under the null hypothesis. Our work has demonstrated how the covert throughput critically depends on Willie\u27s knowledge of the environment and how the covert transmitter, allies in the area, or the dynamics of the environment itself might impact that knowledge. Future work will continue to move covert communications closer to practice by integrating further aspects of practical communication system design