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Receiver impairments play a significant role in determining system performance, particularly in adversarial environments subject to intentional jamming that seek to impair operation by collapsing the dynamic range of the receiver. Clock jitter is one of the key impairments that limits analog-to-digital (A/D) converters, and it causes significant challenges for channel decoding as it distorts the received signal’s timing, making compensation difficult. Motivated by this challenge, we consider performance and design for the two-look channel, which utilizes two receivers with independent clock jitter. Using a statistical characterization of the jitter, we derive the degree to which mutual information is increased in the two-look scenario versus the case when a single receiver is employed. This analysis motivates the consideration of optimal decoder design, but this is complicated by the nonlinear timing jitter model. Hence, we turn to a machine learning (ML)-based decoding framework. Numerical results demonstrate the improvement in mutual information from having two looks, and simulation results demonstrate the improved channel decoding performance. 

The onset of quantum computing calls for secrecy schemes that can provide everlasting secrecy resistant to increased computational power of an adversary. One novel physical layer scheme proposes that an intended receiver capable of performing analog cancellation of a known key-based interference would hold a significant advantage in recovering small underlying messages versus an eavesdropper performing cancellation after analog-to-digital conversion. This advantage holds even if an eavesdropper later obtains the key and employs it in their digital cancellation. Inspired by this scheme, a flexible software-defined radio receiver design capable of maintaining analog cancellation ratios over 40 dB, reaching up to and over 50 dB, is implemented. Using analog cancellation levels from the hardware testbed, practical everlasting secrecy rates up to 2.0 bits/symbol are shown to be gained by receivers performing interference cancellation in analog rather than on a digital signal processor. 

Covert communication is achieved when a transmitter Alice can successfully transmit a message to a receiver Bob without being detected by an attentive and capable adversary Willie. Early results demonstrated the difficulty of the covert communications problem: with AWGN discrete-time channels between all parties, only O(sqrt(n)) bits can be sent in n channel uses. But it was soon recognized that uncertainty about the environment at Willie, for example, uncertainty in his own noise statistics, could allow for a positive rate: O(n) bits can be sent covertly in n channel uses. However, most covert communication results, including this promising positive rate result, have been obtained for a discrete-time communications channel. Here, we demonstrate that the assumption of a discrete-time channel is problematic when trying to exploit Willie's noise uncertainty. In particular, we demonstrate that if Alice transmits ω(sqrt(T)) bits in a length T interval to Bob on a continuous-time channel, then there exists a detector at Willie that can detect her transmission, as the probability of false alarm and missed detection PMD+PFA→0 as T→∞. In other words, the communication is not covert, unlike the case of a discrete-time channel. 

Today's information society relies on cryptography to achieve security goals such as confidentiality, integrity, authentication, and non-repudiation for digital communications. Here, public-key cryptosystems play a pivotal role to share encryption keys and create digital signatures. However, quantum computers threaten the security of traditional public-key cryptosystems as they can tame computational problems underlying the schemes, i.e., discrete logarithm and integer factorization. The prospective arrival of capable-enough quantum computers already threatens today's secret communication in terms of their long-term secrecy when stored to be later decrypted. Therefore, researchers strive to develop and deploy alternative schemes.In this work, we evaluate a key exchange protocol based on combining public-key schemes with physical-layer security, anticipating the prospect of quantum attacks: If a powerful quantum attacker cannot immediately obtain a private key, legitimate parties have a window of short-term secrecy to perform a physical-layer jamming key exchange (JKE) to establish a long-term shared secret. Thereby, the protocol constraints the computation time available to the attacker to break the employed public-key cryptography. In this paper, we outline the protocol, discuss its security, and point out challenges to be resolved.