Signal Processing
Holographic Beamforming for Range-Doppler Sidelobe Suppression in OFDM-ISAC
Amirhossein Azarbahram and Onel L. A. López
This paper investigates range–Doppler (RD) sidelobe suppression in an integrated sensing and communications system with a reconfigurable holographic surface (RHS). We jointly design the digital feed precoders and RHS amplitudes to minimize the integrated RD sidelobe level subject to transmit-power, target-illumination, and communication constraints. For this, we develop an alternating successive convex approximation method updating both variable blocks through quadratic subproblems. Numerical results reveal diminishing returns from additional feeds, while increasing the aperture remains more effective and allows the RHS design to reduce sidelobe level.
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Signal Processing
Robust Near-Field Beam Focusing Under Imperfect Localization
Nima Mozaffarikhosravi, Amirhossein Azarbahram, Prathapasinghe Dharmawansa et al. (+1 more)
The transition to 6G-and-beyond wireless systems with large-scale
antenna arrays and high-frequency deployments significantly extends the near-field region, where channels exhibit a strong dependence on user location. While this enables location-based beam focusing as a low-overhead alternative to conventional channel estimation, its performance is highly sensitive to localization errors. In this paper, we study robust near-field beam focusing under imperfect user localization. We explicitly characterize the impact of localization errors on the line-of-sight-dominated channel by deriving a tractable uncertainty model via a first-order Taylor approximation, which captures the coupled effects of distance and angle in near-field propagation. Building on this model, we formulate a max-min signal-to-interference-plus-noise ratio optimization problem that guarantees performance under worst-case channel realizations induced by bounded localization errors. The resulting problem is reformulated into a feasibility problem using semidefinite relaxation. Numerical results demonstrate that the proposed robust design significantly improves the worst-user rate compared to non-robust beam focusing, particularly under high total transmit power levels and large localization error ranges.
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All authors Nima Mozaffarikhosravi, Amirhossein Azarbahram, Prathapasinghe Dharmawansa, Italo Atzeni
Signal Processing
COBALT: Column-swapping Optimized Bit-serial Accelerator for LSTM Tasks
Mohd Tasleem Khan
Long Short-Term
Memory (LSTM) networks continue to be widely deployed for real-time sequential tasks on edge devices, yet their computational demands challenge deployment on resource-constrained hardware. This work introduces COBALT, a bit-serial compressed LSTM accelerator built on a
matrix (input)-vector (weight) reformulation of the standard circulant
matrix-vector multiplication (MVM) with offset-binary coding. A novel column-swapping scheme operates on partial products at the bit level when generated in pairs, systematically exposing redundancy across output rows to reduce the number of PP generators and selectors. This redundancy is further exploited using a lightweight correction unit that derives a row's output directly from its paired row. Additionally, for block-circulant MVMs, relocating the shift-accumulate and correction units of each sub-MVM further reduces resource usage. The compressed network achieves weight compression of up to 93.6% while maintaining accuracy on the TIMIT and LibriSpeech-100h
benchmarks. On a field-programmable gate array, COBALT achieves superior overall efficiency relative to state-of-the-art LSTM accelerators.
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Signal Processing
Untangling Dense Crowds with mmWave Radar: From Crowd Semantics to Individual Spatial Behaviors
Aaditya Prakash Kattekola, Anurag Pallaprolu and Yasamin Mostofi
In this paper, we present a novel framework for recovering individual footprints and pedestrian spatial behaviors in dense crowds. This is a highly challenging problem, as pedestrians can merge or block one another often and for prolonged periods in crowded areas, leading to strongly entangled observations. Our approach jointly leverages learned crowd semantics and principled
reasoning over persistent radar observability loss to systematically infer individual spatial footprints. At the macroscopic level, we learn spatial usage patterns of the crowd from radar point clouds, while at the microscopic level we develop a physics-informed model of radar observability loss caused by sustained merging and occlusion. Building on this macro-micro structure, we introduce a semantic-guided multi-hypothesis
reasoning foundation that evaluates competing hypotheses via principled
reasoning over many-to-one and one-to-none observation mappings, enabling untangling of individual spatial footprints. We evaluate the framework through 18 real-world experiments using an off-the-shelf mmWave radar, involving crowds of up to (and including) 10 individuals across six categories of diverse pedestrian behaviors, and four environments. Our methodology robustly recovers individual spatial footprints, even during prolonged and persistent observability loss, demonstrating strong structural consistency with the ground-truth. The recovered footprints further enable accurate zonal analysis of pedestrian space usage. Finally, we present an ablation study and further compare against state-of-the-art. The proposed foundation substantially outperforms the state-of-the-art across all experiments, achieving an average median DTW of 30.5 cm compared to 95.94 cm, highlighting the framework's robustness to frequent and prolonged merging and occlusion.
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Signal Processing
Distributed Target Tracking using Radar Networks
Shao-Hsuan Hung and Raj Thilak Rajan
Distributed target tracking is essential for scalable and robust sensing systems, as it enables multiple radar nodes to cooperatively estimate a target state without relying on a centralized fusion center. In this paper, we present a fully distributed framework for single-target tracking in frequency-modulated continuous-wave (FMCW) radar networks. Each monostatic radar node observes local range and Doppler measurements and exchanges information only with neighboring nodes. Two consensus optimization-based
estimators are developed. First, a Distributed Maximum A Posteriori (D-MAP)
estimator is formulated for batch-based tracking, where prior state information is incorporated and the resulting optimization problem is solved using consensus-based alternating direction method of multipliers (ADMM). Second, a Distributed Extended Kalman Filter (D-EKF) is proposed for recursive tracking, where each node performs local prediction and correction followed by consensus ADMM-based information exchange. We derive the posterior Cramér-Rao lower bound (PCRLB) as a theoretical performance
benchmark. Our simulation results show that D-MAP improves the accuracy of the estimation over the distributed maximum-likelihood baseline. These results demonstrate that the proposed framework provides a scalable and robust alternative to centralized radar tracking, given only local inter-node communication.
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Signal Processing · Cross-listed from cs.LG
Multi-Agent Off-Policy Deep Reinforcement Learning for Smart Campus Coverage
Omar Rady, Mohamed Ayman, Ali Arafa et al. (+1 more)
Deep reinforcement learning (DRL) has recently gained a great
attention due to its real-time adaptation and effectiveness in complex optimization problems. This paper investigates the optimal deployment of millimeter-wave (mmWave) base stations (BSs) in a realistic, non-convex campus
topology. The optimization problem is NP-hard, due to the non-convex, non-smooth nature of the max-min
fairness objective. To overcome these constraints, we formulate the BS placement as a Markov Decision Process (MDP) and systematically
benchmark four DRL schemes: a discrete single-
agent Deep Q-Network (DQN), a spatially partitioned Multi-
Agent DQN, a continuous single-
agent Deep Deterministic Policy Gradient (DDPG), and a geographically partitioned multi-
agent DDPG framework. Numerical evaluations reveal that the multi-
agent DDPG approach substantially outperforms single-
agent in dense scenarios. Additionally full coverage is achieved, and a
fairness Jain's index of 0.94 is obtained. Finally, the multi-
agent demonstrates highly efficient computational convergence of dense scenarios with $400$ users.
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All authors Omar Rady, Mohamed Ayman, Ali Arafa, Mohamed Shalma
Signal Processing
Robust and Efficient Feature Extraction for Spike Sorting via the Walsh-Hadamard Transform
Emily Yang, Liyuan Guo, Seyed Mohammad Ali Zeinolabedin et al. (+5 more)
Implantable neural interfaces require low-power real-time signal processing to remain within strict thermal and bandwidth constraints, motivating lightweight feature extraction methods for on-chip spike sorting. This work presents the Walsh-Hadamard Transform (WHT) as a hardware-efficient feature extraction method for neural spike classification. WHT can be implemented using only adders, subtractors, and registers without coefficient
memory. WHT performance is compared against the Compressed Hadamard Transform (CHT) and Principal Component Analysis (PCA), improving mean F1-scores from 55-60% to 70-75% on difficult high-noise
datasets and from 90-95% to 95-99% on all other simulated
datasets. In addition to improved classification performance, WHT demonstrates greater robustness to noise, downsampling, reduced training size, and distance metric selection, maintaining standard deviations typically below 5%, while CHT and PCA reach up to 10% under high-noise conditions.
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All authors Emily Yang, Liyuan Guo, Seyed Mohammad Ali Zeinolabedin, Meng Zhang, Ke Yang, Matthieu Couriol, Christian Mayr, Pierre-Emmanuel Gaillardon
Signal Processing
Towards Lawful ISAC in Cellular Networks
Stefano Tomasin, Annalisa Volpato and Marco Centenaro
Integrated sensing and communication (
ISAC) enables the tracking and detection of passive objects, including the human body, by leveraging standard wireless communication signals. While already standardized in IEEE 802.11bf for Wi-Fi,
ISAC is currently being defined by 3GPP for the upcoming generations of cellular networks. This transition scales the
ISAC technology from localized, uncoordinated Wi-Fi deployments into a pervasive, centralized network deployment managed by mobile network operators. However, such nationwide coverage introduces critical user
privacy challenges that must comply with stringent data protection legislative frameworks, most notably the General Data Protection Regulation in the European Union. This paper provides a comprehensive analysis of applicable norms to cellular
ISAC, highlights the open technical challenges in its implementation, and explores potential mitigation strategies at both the architectural and signal processing levels, establishing tiered data access levels for various stakeholder categories and detailing how these protocols can be lawfully integrated into cellular network architectures.
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Signal Processing · Cross-listed from cs.IT
Integrated Sensing and Communications over Hierarchical Cellular and Cell-Free MIMO Systems
Wei Jiang and Hans D. Schotten
This paper studies integrated sensing and communications (
ISAC) over a hybrid system that seamlessly combines legacy cellular base stations with distributed
cell-free (CF) access points (APs). We propose a hierarchical
ISAC architecture where a central base station (CBS) serves its near users and simultaneously operates as a monostatic radar for aerial target detection, while distributed APs—many idle under user-centric clustering—act as cost-free bistatic receivers. The CBS jointly handles communication processing and multi-static sensing fusion, reducing fronthaul overhead compared to conventional
cell-free
ISAC. To achieve this, a five-phase time-division duplexing workflow with precise
ISAC role assignment is specified. Closed-form expressions for spectral efficiency and multi-static sensing signal-to-noise ratio analytically characterize the communications–sensing Pareto frontier. Numerical results confirm that the proposed hierarchical design simultaneously achieves higher sum throughput and superior sensing accuracy than conventional
cell-free
ISAC.
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Signal Processing
Exploiting Pilot Contamination to Improve UL Sum Rate under Maximum Ratio Combining
Emanuel Wennemo, Ki Won Sung and Emil Björnson
In this work, we show that pilot contamination can be exploited to jointly increase the uplink sum rate and reduce pilot overhead. By leveraging the effect of pilot contamination on minimum mean-square-error (MMSE) channel estimates, we demonstrate that allowing users to share or use correlated pilots can, in certain regimes, yield better sum rate than with mutually orthogonal pilots or perfect channel state information (CSI) under maximum-ratio (MR) combining. To establish this result, we derive the use-and-then-forget (UATF) capacity bound for an arbitrary pilot design under correlated Rayleigh fading with MR combining. We show that, in the presence of pilot contamination, the key mechanism is the directional suppression of the channel estimate towards nearby interfering angular regions, enabling interference reduction. Numerical results confirm this behavior, demonstrating the sum-rate gain and showing that it grows with the number of
antennas.
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Signal Processing
Comparing GNSS Derived Sea Ice Drift in the Arctic and Antarctic using Rotary Spectra and Principal Component Analysis
James H. Hepworth and Amit Kumar Mishra
Sea ice drift underpins air-sea-ice coupling and model evaluation, but the Southern Ocean remains chronically undersampled relative to the Arctic. This work presents a cross-polar comparison of GNSS-tracked ice-drift time series using rotary spectral analysis, principal component analysis, inter-buoy coherence, complementary shape- and amplitude-sensitive spectral differences. Arctic data are aggregated into Beaufort Gyre and Transpolar Drift seasonal composites (2017–2024); Antarctic data comprise eight heterogeneous campaigns (2000–2022) with small buoy counts and short durations. After common quality control, the clearest cross-polar similarity is spectral organisation: both regions show strong low-frequency variance and a hemisphere-appropriate rotary enhancement near the Coriolis frequency, interpreted cautiously as a combined inertial–semidiurnal response. Coherence is highest at synoptic scales and declines toward higher frequencies. Southern Ocean campaigns occupy a higher-energy envelope, so similar band structure does not imply similar drift amplitude. Principal components capture much array-scale motion, but high cumulative variance does not substitute for dense sampling of smaller-scale processes. The results provide transferable frequency-resolved
benchmarks (dominant variance below 0.5 cpd and in the near-inertial band, an expanded Arctic reference, and explicit separation of spectral shape from amplitude) to inform future observing-system design without assuming identical dynamical regimes.
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Signal Processing · Cross-listed from cs.CV
EgoHRV: Continuous Heart Rate Variability Estimation from Egocentric Systems for Autonomic Response and Skill Assessment
Berken Utku Demirel and Christian Holz
Egocentric
vision systems capture human behavior from visible cues, but overlook physiological indicators of autonomic states such as stress, engagement, and
attention. Heart rate variability (HRV) is a widely used noninvasive marker of autonomic regulation under stress. HRV reflects small timing differences between successive heartbeats and has so far been out of reach for egocentric platforms, where motion and noise in gaze
video mask exactly this fine-grained timing. We propose EgoHRV, a method that estimates HRV as well as heart rate (HR) from the gaze cameras that are already integrated into egocentric headsets. Our pipeline combines a 3D backbone with a novel low–high decomposition module that extracts the blood volume pulse (BVP) signal from gaze
video. Our cross-domain pretraining aligns the frequency-domain representations of contact-based and camera-derived signals. This
alignment gives EgoHRV the temporal precision to recover HRV from the subtle fluctuations in gaze
video. EgoHRV achieves state-of-the-art accuracy for HR and HRV estimation from egocentric
video, and its uncertainty-aware design improves downstream behavioral modeling. Integrating our HRV estimates and confidence measures into EgoExo4D's proficiency
estimator raises accuracy by 17.8%. Beyond skill, continuous HRV estimation also opens egocentric systems to stress- and arousal-aware estimation tasks. Code: https://github.com/eth-siplab/EgoHRV
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Signal Processing
Change Point–Aware Evaluation and Re-Calibration of PPG-Based Blood Pressure Estimation
Yunwon Tae, Minje Park, Gyunho Rho et al. (+2 more)
Non-invasive continuous blood pressure (BP) monitoring using photoplethysmography (PPG) is a promising alternative to cuff-based measurements. However, existing PPG-based BP estimation studies predominantly rely on aggregated performance metrics (e.g., mean absolute error) computed over entire evaluation intervals, which can obscure model failures during rapid BP fluctuations and limit clinical relevance. In this work, we propose a fluctuation-aware evaluation framework for PPG-based BP estimation based on time-series change point detection. Instead of heuristic BP thresholding (e.g., $\Delta\mathrm{BP} > 10\mathrm{mmHg}$), we identify BP change points by capturing abrupt distributional shifts in BP trajectories and evaluate estimation performance specifically during these fluctuation periods. Our analysis shows that several state-of-the-art models exhibit substantial performance degradation around BP change points, and that periodic test-time calibration is insufficient to handle such dynamic BP variations. To address this limitation, we introduce a targeted re-calibration framework triggered by detected BP change points, improving robustness without modifying model architectures. To the best of our knowledge, this is the first systematic evaluation of PPG-based BP estimation from a BP change point perspective, highlighting the importance of fluctuation-aware evaluation and calibration for real-world continuous BP monitoring.
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All authors Yunwon Tae, Minje Park, Gyunho Rho, Dongjoon Yoo, Sunghoon Joo
Signal Processing
Coordination of Ground-to-Space Reference Networks for High-Precision GNSS
Xue Xian Zheng, Xing Liu, José A. López-Salcedo et al. (+2 more)
High-precision Global Navigation Satellite System (GNSS) services rely on accurate orbit, clock, atmospheric, and hardware-bias corrections generated from reference observations. These products are traditionally derived from terrestrial reference networks, whose performance strongly depends on the density and geographic distribution of ground stations. Consequently, sparse or regionally concentrated networks can suffer from tracking gaps and limited global observability, reducing their ability to support globally consistent high-precision products. Low Earth Orbit (LEO) constellations equipped with onboard GNSS receivers and inter-satellite links (ISLs) can serve as a network of spaceborne reference stations, offering a promising way to extend terrestrial reference networks into space, thereby improving observability for ground networks and enabling future direct correction broadcast. However, most existing network-based GNSS correction-generation workflows assume that observations can be centrally collected and processed. This assumption fails in practical ground–to–space architectures, where dynamic satellite geometry and system constraints render communication links intermittent, asymmetric, capacity-limited, and lossy. To address these limitations, this paper proposes a decentralized processing architecture that coordinates the ground-to-space GNSS reference network. By modeling ground stations and LEO satellites as interacting subnetworks over a dynamic graph, our approach allows frequent intra-tier communication while restricting cross-tier exchanges to compact estimation summaries transmitted opportunistically under probabilistic link availability....
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All authors Xue Xian Zheng, Xing Liu, José A. López-Salcedo, Gonzalo Seco-Granados, Tareq Y. Al-Naffouri
Signal Processing
Score-based Fading-Aware Decision Fusion
Domenico Ciuonzo
Distributed detection of an unknown deterministic signal is studied in a wireless sensor network with low-cost nodes. Sensors apply one-bit quantization to noisy observations and transmit over Rayleigh fading to a Fusion Center (FC). Score tests, including variants using observed Fisher information, are proposed as low-complexity alternatives to the generalized likelihood ratio test (GLRT). The FC performs joint decoding and fusion, optimizing quantizers in a channel-aware manner to improve asymptotic detection performance. Simulations confirm the appeal of score-based tests under realistic wireless conditions.
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