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Christian Deppe

Publications and source records attributed to Christian Deppe.

At least 19 recordsLinked to original sources
arXiv2026

Communication with Adversarial Users: Byzantine Models in Multi-User Information Theory

We consider communication systems operating in environments where some participating users cannot be trusted. This motivates the study of Byzantine communication models, in which users may behave adversarially. In this survey, we provide a unified information-theoretic perspective on communication with adversarial users in multi-user settings. We review recent advances in Byzantine multiple access channels, including reliable communication, adversary identification, and distributed function computation. Furthermore, we discuss extensions to classical-quantum channels and highlight fundamental differences between classical and quantum models. Finally, we outline key open problems and future research directions at the intersection of communication, computation, and security.

cs.IT↗
arXiv2026

Two-Dimensional Search for a Moving Target on Rectangular and Cylindrical Grids

We study how precisely a moving target can be localized on finite rectangular grids and their cylindrical counterparts. Each query asks whether the target lies in a chosen subset of vertices and receives a noiseless answer. Queries may depend on previous answers. Between successive queries, the target may stay in place or move along one horizontal or vertical edge. Accuracy is measured by the number of positions still possible immediately after the final answer, with no subsequent movement. A strategy achieves accuracy $s$ if it always terminates with at most $s$ possible positions, whatever admissible trajectory the target follows. Thus $s=1$ means exact localization. We denote by $s^*(M,N)$ the smallest value of $s$ achievable by a finite adaptive strategy on a finite grid. Our main result shows that every finite rectangular grid admits a strategy that localizes the target to at most eight positions, regardless of the grid dimensions: $s^*(M,N)\leq8$. The construction uses nine local query patterns and their vertical reflections. Its termination follows from the progress these patterns force toward the finite grid boundaries. We also prove the dimension-dependent bound $s^*(M,N)\leq\min\{M+1,N+1\}$. For finite cylindrical grids $P_M\square C_N$, where the columns are periodic but the first and last rows remain boundaries, we introduce the corresponding accuracy parameter and state the proposed dimension-independent bound $s^*_{\mathrm{cyl}}(M,N)\leq9$.

cs.DM↗
arXiv2026

Secure Deterministic Identification with Feedback and Sensing over State-Dependent Wiretap Channels

Identification differs from classical transmission in that the receiver determines whether a particular identity was sent rather than recovering a message. We study secure deterministic identification over state-dependent discrete memoryless wiretap channels with private noiseless feedback and a sensing distortion constraint. The encoder has no local randomness and uses only the identity and past feedback. A symbol-wise input restriction guarantees the sensing constraint. We show that the secure deterministic identification capacity equals its non-secure feedback counterpart when the admissible main channel has positive ordinary transmission capacity and H(Y|Z,X=x)>0 for some fixed admissible input x. This equality does not require positive classical wiretap secrecy capacity. For achievability, noiseless feedback generates common randomness for identification and enables secret-key extraction by privacy amplification. The extracted key one-time-pad encrypts a short identification tag, whose reliable transmission requires only a sublinear number of additional channel uses. We also present an alternative construction in which feedback-generated randomness replaces the local randomization of a stochastic wiretap encoder. We further prove that secure identification capacity vanishes whenever non-secure identification capacity is zero. We also identify a class of channels with input-independent wiretapper observations that achieves full non-secure identification capacity even when the private conditional feedback entropy is zero, illustrated by a binary state-dependent wiretap channel. Thus, feedback can support both identification and secrecy without local encoder randomization while satisfying sensing constraints.

cs.IT↗
arXiv2026

Identification for Inverse Gaussian Channels

We derive lower and upper bounds on the identification capacity of inverse Gaussian (IG) channels, a fundamental model for molecular communications in fluid environments. The analysis considers deterministic encoding schemes under a peak time constraint and characterizes the asymptotic growth of optimal codebook sizes. In the converse, by leveraging the Borel-Cantelli lemma and the super-exponential near-zero tail behavior of the IG distribution, we establish an almost-sure lower bound on the first-passage time of drifting diffusing molecules. This leads to the conclusion that the identification capacity exhibits super-exponential growth with the codeword length $n$, i.e., $\sim 2^{(n \log n)R_n}$, where $R_n$ is the coding rate.

cs.IT↗
arXiv2026

Gaussian Identification in MIMO Channels

We study the identification capacity of discrete-time Gaussian multiple-input multiple-output (MIMO) channels under per-antenna, per-symbol peak-amplitude constraints. We establish upper and lower bounds on the identification capacity that scale linearly with the channel rank $r.$ The number of reliably identifiable messages can grow super-exponentially with blocklength $n,$ following $M_n(R)=2^{(n\log n)R},$ where $R$ is an achievable rate. The channel rank provides a spatial-multiplexing advantage, enabling higher achievable rates and, consequently, stronger super-exponential codebook growth. These results characterize the rank-dependent gain in achievable identification performance.

cs.IT↗
arXiv2026

Joint Identification, Transmission, and Sensing over State-Dependent Channels with Noiseless Feedback

We consider a discrete memoryless state-dependent channel with noiseless feedback, where the transmitter uses randomized encoding to jointly perform message transmission, identification, and state sensing. In this setting, the transmitter sends a conventional transmission message and an identification message while estimating the unknown channel state sequence under an average distortion constraint. We establish a single-letter characterization of the randomized identification-transmission-sensing capacity-distortion region. The result reveals a single-letter separation structure: the communication rates are determined by the averaged channel, subject to the sensing-induced constraint on admissible input distributions. Specifically, the transmission rate is characterized by mutual information, while the identification rate is characterized by output entropy under the same distortion constraint. The achievability proof combines constant-composition transmission coding, output partitioning, and combinatorial identification coding, while the converse follows from an averaged-channel reduction, distortion lower bounds, and feedback-identification converse arguments.

cs.IT↗
arXiv2026

TetraCultures of Neurons, Astrocytes, Oligodendrocytes, and Microglia: A Murine In Vitro Model to Dissect Region-Specific CNS Immune Dynamics

Neuroinflammation and glial scar formation in the central nervous system (CNS) emerge from coordinated signaling between neurons, astrocytes, oligodendrocytes and microglia. Conventional embryonic neuron-astrocyte-oligodendrocyte (NAO) TriCultures lack microglia and cannot model innate immune responses. Here, we introduce postnatal neuron-astrocyte-oligodendrocyte-microglia (NAOM) TetraCultures, in which resident microglia are retained from the original brain tissue and co-develop with other CNS cell populations from the onset of culture. Hippocampal and cortical cultures were maintained for 14 days in vitro and challenged with LPS, poly I or scratch injury. Only NAOM TetraCultures mounted robust, region- and stimulus-specific inflammatory responses. LPS induced marked TNF-αand IL-1βsecretion and increased IBA1 in hippocampal microglia, whereas poly I elicited IFN-βrelease without increasing IBA1. CD68 regulation differed between hippocampus and cortex, and microglial morphology showed pronounced amoeboid transitions after LPS. Astrocytes in NAOM TetraCultures expanded their territorial coverage after LPS, an effect absent in NAO TriCultures. Following scratch injury, IL-1βincreased, microglia infiltrated lesion zones and astrocytes formed lesion borders. Thus, NAOM TetraCultures recapitulate key features of CNS immunity, glial scar formation and region-specific glial responses while providing a physiologically relevant microglia-containing model.

q-bio.CB↗
arXiv2026

Commitment over Gaussian channels with deterministic identification codes

We establish a previously unnoticed connection between commitment (BC), a fundamental cryptographic primitive, and deterministic identification (DI), a post-Shannon communication setting. Specifically, for additive white Gaussian noise channels $\mathcal{G}$ we show that a reliable commitment protocol can be obtained from deterministic identification codes in both the honest but curious and fully dishonest security settings. This viewpoint allows BC schemes to inherit the asymptotic performance of DI codes. Indeed, we show that commitment is naturally achievable in the linearithmic regime, i.e., with message sets of size $N_n=\exp [Θ(n\log n)]$, and we establish a lower bound on the linearithmic-scale BC capacity of $\dot C_{\text{hBC}}(\mathcal G)\geq\frac12$ in the honest but curious case and $\dot C_{\text{BC}}(\mathcal G)\geq\frac14$ in the fully dishonest picture. This refines the best previously known characterisation of the commitment capacity over $\mathcal{G}$, which established only an infinite linear-scale capacity.

cs.IT↗
arXiv2026

Identification Codes for Ad Hoc Networks: Use Cases and Challenges

In this study, we focus on large-scale ad hoc networks characterized by a high node density. To achieve energy efficiency, the devices must minimize their data transmission, thereby reducing the total number of bits sent and the associated energy consumption. We consider using ID codes for the address field in the MAC layer and reduce the header size. The work explores a range of use cases and challenges that demonstrate the benefits of incorporating these ID codes into packet routing mechanisms for ad hoc networks, thereby providing a foundation for understanding their applicability and the necessary design adaptations.

cs.NI↗
arXiv2026

Common Randomness: A Key Enabler of Trustworthy 6G Communication Systems

Common randomness (CR) is a valuable resource for enhancing the trustworthiness of 6G communication systems. This article highlights the role of CR in improving scalability, security, and resilience in distributed 6G architectures. It provides an overview of information-theoretic two-source models for CR generation and summarizes known capacity results, together with their implications for practical systems under different communication settings, including one-way, two-way, helper-assisted, and interactive scenarios. Furthermore, the article explores CR-assisted secure identification over Gaussian channels as an application and highlights selected open research problems in CR generation for next-generation wireless systems. The article targets researchers and practitioners interested in bridging information-theoretic tools with emerging 6G communication systems.

cs.IT↗
arXiv2026

Identification for ISI Gaussian Channels

We establish lower and upper bounds for the identification capacity of discrete-time Gaussian channels subject to inter-symbol interference (ISI), a canonical model in wireless communication. Our analysis accounts for deterministic encoders under peak power constraint. A principal finding is that, even when the number of ISI taps scales sub-linearly with the codeword length $n$ as $\sim n^κ$ with $κ\in [0,1/2),$ the number of messages that can be reliably identified grows super-exponentially in $n$ as $\sim 2^{(n \log n)R},$ where $R$ denotes the coding rate.

cs.IT↗
arXiv2026

The QTF-Backbone: Proposal for a Nationwide Optical Fibre Backbone in Germany for Quantum Technology and Time and Frequency Metrology

The recent breakthroughs in the distribution of quantum information and high-precision time and frequency (T&F) signals over long-haul optical fibre networks have transformative potential for physically secure communications, resilience of timing infrastructure (such as that supporting Global Navigation Satellite Systems (GNSS)) and fundamental physics. To date, these capabilities remain confined to isolated testbeds, with quantum and T&F signals accessible, for example in Germany, to only a few institutions. In this white paper we propose the QTF Backbone: a dedicated national fibre-optic infrastructure in Germany for the networked distribution of Quantum and T&F signals using dark fibres and specialised hardware. The QTF Backbone is planned as a four-phase deployment over ten years to ensure scalable, sustainable access for research institutions and industry. The concept builds on successful demonstrations of time and frequency distribution at high Technology Readiness Levels (TRLs) across Europe, including PTB-MPQ links in Germany, REFIMEVE in France, and the Italian LIFT network. The QTF Backbone will enable transformative Research and Development (R&D), support a nationwide QTF ecosystem, and ensure the transition from innovation to deployment. As a national and European hub, it will position Germany and Europe at the forefront of quantum networking, as well as T&F transfer.

physics.ins-det↗
arXiv2026

Network Oblivious Transfer via Noisy Broadcast Channels

This paper investigates information-theoretic oblivious transfer via a discrete memoryless broadcast channel with one sender and two receivers. We analyze both non-colluding and colluding honest-but-curious user models and establish general upper bounds on the achievable oblivious transfer capacity region for each case. Two explicit oblivious transfer protocols are proposed. The first ensures correctness and privacy for independent, non-colluding receivers by leveraging the structure of binary erasure broadcast channels. The second protocol, secure even under receiver collusion, introduces additional entropy-sharing and privacy amplification mechanisms to preserve secrecy despite information leakage between users. Our results show that for the non-colluding case, the upper and lower bounds on oblivious transfer capacity coincide, providing a complete characterization of the achievable region. The work provides a unified theoretical framework bridging network information theory and cryptographic security, highlighting the potential of noisy broadcast channels as powerful primitives for multi-user privacy-preserving communication.

cs.IT↗
arXiv2026

Identification Codes and Post-Shannon Communication: Theory, Architectures, and Emerging Applications

Identification (ID) coding, introduced by Ahlswede and Dueck, extends Shannon's classical communication paradigm by replacing message reconstruction with hypothesis testing. Instead of decoding the transmitted message, the receiver only decides whether a particular message was sent. A fundamental result of ID theory is the double-exponential growth in the number of identifiable messages with respect to (w.r.t.) the blocklength. This scaling behavior enables fundamentally new communication architectures for large-scale distributed systems and forms a key building block of post-Shannon communication. While ID cannot replace classical communication in general, it is particularly well-suited for scenarios in which full message reconstruction is unnecessary, such as monitoring, alarming, and control systems. In this survey, we review the theoretical foundations of ID coding and discuss emerging communication architectures and application domains based on this paradigm. Particular emphasis is placed on practical use cases, including monitoring systems, special-purpose data storage, joint identification and sensing (JIDAS), semantic communications, mobile-network control systems and networked consensus testing systems. We further highlight recent system concepts, industrial perspectives, and implementation examples that illustrate how ID-based principles can be realized in practical communication systems.

cs.IT↗
arXiv2026

Square-Root Law for Covert Communication with Warden-Favorable Side Information

Covert communication enables Alice to transmit to Bob while making the transmission difficult for Willie to detect. We study a scalar Gaussian covert-overlay model in which Alice's low-power covert signal is superimposed on an aggregate public component generated by Alice or other trackable sources. Willie is given all physically obtainable side information, including protocol details, timing, pilots, channel estimates, and calibration information, and subtracts his best estimate of the public component before testing. Covertness is imposed on the resulting residual through a relative-entropy constraint with budget $δ$ conditioned on Willie's side information. In the stationary case, the residual under no covert transmission has variance $σ_0^2=σ_W^2+σ_e^2$, where $σ_W^2$ is Willie's receiver-noise variance and $σ_e^2$ is the irreducible cancellation error. Over $n$ channel uses, the maximal reliably transmissible covert payload is $R_C^\star\sqrt{n}(1+o(1))$ bits, where $R_C^\star=\frac{σ_0^2}{σ_B^2\ln 2}\sqrtδ$, and $σ_B^2$ is Bob's receiver-noise variance. Thus, the square-root-law (SRL) constant is governed by the variance at Willie's actual detector input, not by receiver noise alone. Low-power Gaussian signaling achieves this constant, and a matching converse establishes first-order optimality within the conditioned additive Gaussian innovation model. For known time-varying conditioned residual variances, we also derive the first-order allocation, which assigns more covert power to larger residual variances. The results require a Gaussian post-cancellation null residual with known conditioned variance; non-Gaussian residuals and fixed non-vanishing variance uncertainty are outside the scope of this paper.

cs.IT↗
arXiv2026

Identification for Colored Gaussian Channels

We study the identification capacity of discrete-time Gaussian channels impaired by correlated noise and inter-symbol interference (ISI). Our analysis is formulated for deterministic encoding functions subject to a peak power constraint and colored noise whose covariance matrix features a polynomially bounded singular value spectrum, i.e., $\sim [n^{-μ} , n^{μ/2}]$ where $n$ is the codeword length and $μ\in [0,1/2)$ is the spectrum rate. A central result establishes that, even when the ISI memory length grows sub-linearly with $n,$ i.e., $\sim n^κ$ where $κ\in [0,1/2)$ and $κ+ μ\in [0,1/2),$ the codebook size continues to exhibit super-exponential growth in $n$, i.e., $\sim 2^{(n \log n)R},$ with $R$ representing the associated coding rate. Moreover, by employing the well-known Mahalanobis-distance decoder induced by colored Gaussian noise statistics, we characterize bounds on the identification capacity, with the resulting bounds parameterized by $κ$ and $μ.$

cs.IT↗
arXiv2026

Deterministic identification for Bernoulli channels and related channels with continuous input

For memoryless channels with continuous input alphabets, deterministic identification (DI) typically exhibits a linearithmic ($n\log n$) message growth. However, the exact DI capacity has long remained open due to a persistent gap between the best known achievability and converse bounds. This gap was recently closed for AWGN channels via a novel code construction optimising the "galaxy" codes. Here, we extend this approach to the Bernoulli channel and subsequently to any channel $W$ whose image contains a continuous curve of output probability distributions, and hence admits a reduction to the Bernoulli channel restricted to a subinterval of inputs. As a consequence, we prove that the converse bound is tight and establish $\dot{C}_{\text{DI}}(W) = \frac 12$ for this broad class of channels, thereby closing the long-standing capacity gap. A similar gap was also observed for the DI rate-reliability tradeoff. We analyse the tradeoff between rate and error of the proposed code and derive improved lower bounds on the reliability function, approaching the converse at leading order in the regime of small error exponents.

cs.IT↗
arXiv2026

Optimal Codes for Deterministic Identification over Gaussian Channels: Closing the Capacity Gap

Deterministic identification (DI) has emerged as a promising paradigm for large-scale and goal-oriented communication systems. Despite significant progress, a fundamental open problem has remained unresolved: a persistent gap between the best known lower and upper bounds on the DI capacity, as well as on the corresponding rate-reliability tradeoff bounds. In this paper, we finally close this gap for Gaussian channels $\mathcal{G}$ by constructing an optimised code that achieves the known upper bound. This allows us to establish that the linearithmic capacity for deterministic identification is $\dot{C}_{\text{DI}}(\mathcal{G})=\frac{1}{2}$. Furthermore, we analyse the rate-reliability tradeoff and show that the proposed scheme matches the known upper bounds to first order, thereby closing the existing gap in reliability performance for all admissible error decay regimes. Finally, we demonstrate the existence of an optimum universal code, which does not require knowledge of the channel parameters and yet achieves capacity.

cs.IT↗