Selected Publications






All Publications
Benchmarking Quantum Reinforcement Learning
To enable valid performance comparisons and to streamline current research in this area, we propose a novel benchmarking methodology, which is based on a statistical estimator for sample complexity and a definition of statistical outperformance. Furthermore, considering QRL, our methodology casts doubt on some previous claims regarding its superiority. We conducted experiments on a novel benchmarking environment with flexible levels of complexity. While we still identify possible advantages, our findings are more nuanced overall. We discuss the potential limitations of these results and explore their implications for empirical research on quantum advantage in QRL.
Combining Multi-Objective Bayesian Optimization with Reinforcement Learning for TinyML
We propose a novel NAS strategy for TinyML based on multi-objective Bayesian optimization (MOBOpt) and an ensemble of competing parametric policies trained using augmented random search (ARS) reinforcement learning (RL) agents. Our methodology aims at efficiently finding tradeoffs between a DNN’s predictive accuracy, memory requirements on a given target system, and computational complexity. Our experiments show that we consistently outperform existing MOBOpt approaches on different datasets and architectures such as ResNet-18 and MobileNetv3.
Don’t get me wrong: How to apply deep visual interpretations to time series
A framework of six orthogonal metrics for gradient- or perturbation-based post-hoc visual interpretation methods. Its designed for time series classification and segmentation tasks.
Evaluating ML Robustness in GNSS Interference Classification, Characterization & Localization
We introduce an extensive dataset comprising snapshots obtained from a low-frequency antenna that capture various generated interferences within a large-scale environment, including controlled multipath effects. Our objective is to assess the resilience of machine learning (ML) models against environmental changes, such as multipath effects, variations in interference attributes, such as interference class, bandwidth, and signal power, the accuracy of jamming device localization, and the constraints imposed by snapshot input lengths. Furthermore, we evaluate the performance of a diverse set of 129 distinct vision encoder models across all tasks.
Federated learning with MMD-based early stopping for adaptive GNSS interference classification
We propose an FL approach using few-shot learning and aggregation of the model weights on a global server. We introduce a dynamic early stopping method to balance out-of-distribution classes based on representation learning, specifically utilizing the maximum mean discrepancy of feature embeddings between local and global models. An exemplary application of FL is to orchestrate machine learning models along highways for interference classification based on snapshots from global navigation satellite system (GNSS) receivers.
Dataset/Code: https://gitlab.cc-asp.fraunhofer.de/darcy_gnss/federated_learning
Passive Channel Charting: Locating Passive Targets using a UWB Mesh
Fingerprint-based passive localization enables high localization accuracy using low-cost UWB IoT radio sensors. However, fingerprinting demands extensive effort for data acquisition. The concept of channel charting reduces this effort by modeling and projecting the manifold of channel state information (CSI) onto a 2D coordinate space. So far, researchers have only applied this concept to active radio localization, where a mobile device intentionally and actively emits a specific signal.In this paper, we apply channel charting to passive localization. We use a pedestrian dead reckoning (PDR) system to estimate a target’s velocity and derive a distance matrix from it. We then use this matrix to learn a distance-preserving embedding in 2D space, which serves as a fingerprinting model. In our experiments, we deploy six nodes in a fully connected ultra-wideband (UWB) mesh network to show that our method achieves high localization accuracy, with an average error of just 0.24 m, even when we train and test on different targets.
Resilient Channel Charting Under Varying Radio Link Availability
We show a channel charting architecture that can natively handle variable numbers of CSI measurements and arbitrary antenna subsets. Across two public real-world SISO and MIMO datasets, AdaPos maintains state-of-the-art localization accuracy under missing-antenna conditions, replaces roughly 57 configuration-specific models with a single unified model, and provides resilience to both individual antenna failures and full-array outages.
Rethinking AI/ML for Air Interface in Wireless Networks
We provide an overview of AI/ML-related discussions in 3GPP standardization, highlighting key use cases, architectural considerations, and technical requirements. We outline open research challenges and opportunities where academic and industrial communities can contribute to shaping the future of AI-enabled wireless systems.
Simplicity is Key: An Unsupervised Pretraining Approach for Sparse Radio Channels
We introduce Sparse pretrained Radio Transformer (SpaRTran), a hybrid method based on the concept of compressed sensing for wireless channels. SpaRTran builds around a wireless channel model that constrains the optimization procedure to physically meaningful solutions and induces a strong inductive bias, cutting positioning error by up to 28% and increasing top-1 codebook selection accuracy for beamforming by 26 percentage points.
Variational Quantum Error Correction
Quantum error correction is critical for protecting quantum computations from noise, yet standard protocols demand significant qubit overhead. To address this, we introduce a learning-based approach that allows tailoring encodings to specific noise channels. This approach is based on a novel loss function that maximizes the distinguishability of noisy quantum states. We thereby realize a variational quantum algorithm that discovers new resource-efficient codes, which empirically outperform standard quantum error correction schemes on various noise structures. We furthermore derive promising theoretical properties of these variational quantum error correction (VarQEC) codes and perform proof-of-concept experiments on quantum hardware.
BCQQ: Batch-Constraint Quantum Q-Learning with Cyclic Data Re-uploading
In this paper, we investigate this potential advantage by proposing a batch RL algorithm that utilizes variational quantum circuits (VQCs) as function approximators within the discrete batch-constraint deep Q-learning (BCQ) algorithm. Additionally, we introduce a novel data re-uploading scheme by cyclically shifting the order of input variables in the data encoding layers. We evaluate the efficiency of our algorithm on the OpenAI CartPole environment and compare its performance to the classical neural network-based discrete BCQ.
C-MCTS: Safe Planning with Monte Carlo Tree Search
The Constrained Markov Decision Process (CMDP) formulation allows to solve safety-critical decision making tasks that are subject to constraints. While CMDPs have been extensively studied in the Reinforcement Learning literature, little attention has been given to sampling-based planning algorithms such as MCTS for solving them. Previous approaches perform conservatively with respect to costs as they avoid constraint violations by using Monte Carlo cost estimates that suffer from high variance. We propose Constrained MCTS (C-MCTS), which estimates cost using a safety critic that is trained with Temporal Difference learning in an offline phase prior to agent deployment. The critic limits exploration by pruning unsafe trajectories within MCTS during deployment. C-MCTS satisfies cost constraints but operates closer to the constraint boundary, achieving higher rewards than previous work. As a nice byproduct, the planner is more efficient w.r.t. planning steps. Most importantly, under model mismatch between the planner and the real world, C-MCTS is less susceptible to cost violations than previous work.
Comprehensive Library of Variational LSE Solvers
Linear systems of equations can be found in various mathematical domains, as well as in the field of machine learning. By employing noisy intermediate-scale quantum devices, variational solvers promise to accelerate finding solutions for large systems. Although there is a wealth of theoretical research on these algorithms, only fragmentary implementations exist. To fill this gap, we have developed the variational-lse-solver framework, which realizes existing approaches in literature, and introduces several enhancements. The user-friendly interface is designed for researchers that work at the abstraction level of identifying and developing end-to-end applications.