Bounds on Covert Capacity in the Sub-Exponential Slotted Asynchronous Regime

Abstract: We develop tight bounds for the covert capacity of slotted asynchronous binary-input Discrete Memoryless Channels (DMCs) and Additive White Gaussian Noise (AWGN) channels, in which a codeword is transmitted in one of several slots with known boundaries, where the number of slots is sub-exponential in the codeword length. Our upper and lower bounds are within a multiplicative factor of $\sqrt{2}$ independent of the channel. This result partially fills a characterization gap between the covert capacity without asynchronism and the covert capacity with exponential asynchronism. Our key technical contributions consist of i) a tight upper bound for the relative entropy characterizing the effect of asynchronism on the covertness constraint in our achievability proof; ii) a careful converse analysis to characterize the maximum allowable weight or power of codewords to meet the covertness constraint. Our results suggest that, unlike the case without asynchronism, the choice of covertness metric does not change the covert capacity in the presence of asynchronism.