7 research outputs found
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
Delay-Intolerant Covert Communications with Either Fixed or Random Transmit Power
In this paper, we study delay-intolerant covert communications in additive white Gaussian noise (AWGN) channels with a finite block length, i.e., a finite number of channel uses. Considering the maximum allowable number of channel uses to be N, it is not immediately clear whether the actual number of channel uses, denoted by n, should be as large as N or smaller for covert communications. This is because a smaller n reduces a warden’s chance to detect the communications due to fewer observations, but also reduces the chance to transmit information. We show that n=N is indeed optimal to maximize the amount of information bits that can be transmitted, subject to any covert communication constraint in terms of the warden’s detection error probability. To better make use of the warden’s uncertainty due to the finite block length, we also propose to use uniformly distributed random transmit power to enhance covert communications. Our examination shows that the amount of information that can be covertly transmitted logarithmically increases with the number of random power levels, which indicates that most of the benefit of using random transmit power is achieved with just a few different power levels.This work was supported by the Australian Research Council’s Discovery Projects under Grant DP180104062
Finite Blocklength Analysis of Gaussian Random Coding in AWGN Channels under Covert Constraint
This paper considers the achievability and converse bounds on the maximal
channel coding rate at a given blocklength and error probability over AWGN
channels. The problem stems from covert communication with Gaussian codewords.
By re-visiting [18], we first present new and more general achievability bounds
for random coding schemes under maximal or average probability of error
requirements. Such general bounds are then applied to covert communication in
AWGN channels where codewords are generated from Gaussian distribution while
meeting the maximal power constraint. Further comparison is made between the
new achievability bounds and existing one with deterministic codebooks.Comment: 18 page
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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