arXiv subjects

Christos Masouros

Publications and source records attributed to Christos Masouros.

At least 19 recordsLinked to original sources

arXiv2026

Localization MSE Analysis for Cooperative OFDM-ISAC

This paper develops a receiver-specific mean-squared-error (MSE) framework for cooperative integrated sensing and communication (ISAC) localization using random orthogonal frequency-division multiplexing (OFDM) payloads. It captures local peak perturbations and incorrect ambiguity-peak acquisition for two classical receiver processing schemes: matched filtering (MF) and reciprocal filtering (RF). For scatterers well separated in delay, we quantify how frequency spread and interference induced by symbol-energy fluctuations affect local delay errors under MF. RF removes symbol-dependent weighting but amplifies noise for low-energy symbols. Payload-conditioned joint Gaussian delay profiles yield peak-selection probabilities, which combine with nonlinear multistatic fusion errors to approximate position MSE while accounting for network geometry. Numerical results illustrate receiver-dependent estimation errors and the benefits of frequency-dependent constellation allocation.

cs.IT↗

arXiv2026

Movable-Antenna-Empowered ISAC: 3D Ambiguity Function Analysis, Waveform Design and Antenna Position Optimization

Multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) provides abundant spatial and time-frequency degrees of freedom for integrated sensing and communication (ISAC), while movable-antennas (MAs) further introduce reconfigurable spatial freedom through array geometry adjustment. However, how the array geometry and the MIMO-OFDM waveform design jointly influence the sensing performance, i.e., the three dimensional (3D) ambiguity function property, remains insufficiently understood. Against this background, we investigate MA-empowered ISAC design under the MIMO-OFDM architecture, where the radio-frequency (RF) antenna positions and the baseband waveform are co-designed with symbol-level precoding (SLP) philosophy. The discrete periodic angle-range-Doppler ambiguity function is firstly derived for the MA-enabled ISAC framework, revealing its interesting features with respect to the waveform matrix rank. Specifically, it is proved that a rank-one waveform yields a 3D ambiguity function that is separable into angular and range-Doppler components. On the other hand, a higher-rank waveform combines multiple spatial components with angle-dependent weights, allowing the range-Doppler ambiguity function to vary across candidate angles and enabling the MA positions to further shape the 3D ambiguity function. Based on the aforementioned analysis, a 3D integrated sidelobe level ratio (ISLR) is defined, a joint optimization problem of the MA positions and the SLP waveform matrix is formulated, and a penalty dual decomposition based alternating optimization algorithm is developed. For the radar-only case, the existence of a rank-one globally optimal waveform is established, which reduces the original high-dimensional waveform optimization to a low-dimensional joint design of the spatial beam and MA positions.

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arXiv2026

Flexible Beamforming via Fluid Antenna Array: From Discrete to Continuous Array

Fluid antenna arrays (FAAs) reconfigure their effective aperture by activating radiation ports from a dense set of candidate positions within a finite physical region, thereby offering geometry diversity beyond conventional fixed arrays. This paper studies arbitrary-direction far-field beam synthesis with a planar FAA. Since a fixed array adapts its beamforming weights on a predetermined spatial sampling structure, its ability to match desired beam patterns is limited when both the aperture and the number of active elements are constrained. To exploit the geometry diversity of the FAA, we propose a coarse-to-fine scheme that couples discrete port selection with continuous joint refinement. In the coarse stage, a fast Fourier transform (FFT) based phase retrieval step converts the amplitude-only beam specification into an aperture-compatible complex target, after which an FFT-tailored greedy method selects the active ports and least squares initializes their complex weights. In the fine stage, a projected adaptive moment estimation procedure jointly refines the continuous port positions and beamforming weights while enforcing the minimum interelement spacing. For the evaluated two-beam configuration, the proposed scheme increases the mainlobe energy fraction from 35.73% to 48.92% and reduces the peak sidelobe level from -2.30 dB to -3.83 dB relative to a fixed array with the same aperture and active-port count.

cs.IT↗

arXiv2026

Multi-Functional Chirp Signalling for Next-Generation Multi-Carrier Wireless Networks: Communications, Sensing and ISAC Perspectives

To meet the increasingly demanding quality-of-service requirements of the next-generation multi-carrier mobile networks, it is essential to design multi-functional signalling schemes facilitating efficient, flexible, and reliable communication and sensing in complex wireless environments. As a compelling candidate, we advocate chirp signalling, beneficially amalgamating sequences (e.g., Zadoff-Chu sequences) with waveforms (e.g., chirp spread spectrum and frequency-modulated continuous wave (FMCW) radar), given their resilience against doubly selective channels. Besides chirp sequences, a wide range of chirp waveforms is considered, ranging from FMCW to affine frequency-division multiplexing (AFDM), to create a promising chirp multicarrier waveform. This study also highlights the advantages of such waveforms in supporting reliable high-mobility communications, plus integrated sensing and communications (ISAC). Finally, we outline several emerging research directions for chirp signalling designs.

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arXiv2026

Joint Subcarrier and Power Allocation for Localization-Oriented Multi-Carrier Underwater ISAC Systems

Multi-carrier underwater integrated sensing and communication (ISAC) realizes concurrent acoustic communication and localization, yet the frequency selectivity of the acoustic channel creates uneven positioning capabilities across subcarriers. This paper proposes a localization-oriented subcarrier and power allocation scheme for multi-anchor underwater ISAC systems. We build a unified orthogonal subcarrier multiplexing framework and derive closed-form Cramer-Rao bounds (CRBs) under frequency-selective channels with colored noise. We derive a subcarrier-level localization information contribution from the time of arrival Fisher information, which enables localization-oriented resource allocation. We derive several theoretical properties regarding the distance and frequency dependence of the localization information metric, and establish a distance-frequency matching principle. These results provide a theoretical foundation for localization-oriented resource allocation and guide the design of a low-complexity greedy algorithm that improves positioning accuracy. Specifically, the algorithm iteratively allocates subcarriers to minimize the residual positioning-error objective, thereby improving positioning accuracy while satisfying communication quality-of-service (QoS) constraints. Simulations verify that our method yields much lower positioning error bound than uniform, random and high-frequency-first subcarrier allocation in symmetric or asymmetric topologies while guaranteeing communication QoS, achieving superior communication-localization tradeoffs.

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arXiv2026

Optical ISAC with Single-Photon Avalanche Diode (SPAD) Arrays

Single-photon avalanche diode (SPAD) arrays offer photon-level sensitivity for sensing in optical integrated sensing and communication (O-ISAC). However, after each detection, a SPAD is blind for a dead time, which saturates the array at high received power. Existing O-ISAC analyses assume linear receivers and do not capture this effect. In this paper, we study ranging with a SPAD array in a monostatic O-ISAC system based on direct-current-biased optical orthogonal frequency-division multiplexing (DCO-OFDM). Sampling the array once per dead time, we model the number of pixels that detect a photon in each sampling interval as a binomial random variable whose success probability accounts for the intensity variation of the echo within a dead time. Based on this model, we derive the Cramér-Rao bound (CRB) for range estimation. We show that the SPAD count model of optical wireless communication, which assumes a constant photon rate over a dead time, causes a range bias that depends on the received power. Moreover, unlike for a linear photodiode, saturation degrades the ranging accuracy beyond a sensing-optimal transmit power. The rate-constrained operating point is then the larger of this power and the power needed for the target data rate. Numerical results validate the analysis against a continuous-time photon-counting simulation and show that the proposed model is needed for ranging.

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arXiv2026

Joint Probabilistic Shaping and Symbol-Level Precoding for Covert LEO Satellite Downlinks

The wide footprint of low-Earth-orbit (LEO) satellite downlinks exposes the mere occurrence of a transmission to any receiver within it. This paper studies covert multiuser LEO downlinks, where a passive warden at an unknown but fixed ground location accumulates observations as the satellite moves. We jointly optimize the precoding and the occurrence probabilities of the multiuser symbol combinations to provide covertness against such wardens. Each phase-shift keying (PSK) symbol vector is decomposed into a relative pattern and a common rotation, so that shaping the pattern probabilities leaves the symbols of every legitimate user uniformly distributed. We develop probabilistically shaped symbol-level precoding and probabilistically shaped block-level precoding. We derive the exact chi-square divergence of the warden's Gaussian-mixture observation over the complete session and, by bounding the leakage over a continuous location uncertainty region, obtain certificates that guarantee covertness throughout that region. The certified covertness level is minimized under robust constructive-interference, power, and entropy constraints through an alternating optimization whose updates are equivalent to convex conic problems, and the optimized probabilities follow a Maxwell--Boltzmann-type law. Numerical results show that shaping substantially reduces the certified covertness level of both precoding structures, particularly under large location uncertainty, low elevation, and scarce power.

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arXiv2026

Information-Theoretic Limits of Fluid Antenna-Aided ISAC with Representation Information Constraints

This paper characterizes the fundamental limits of integrated sensing and communication (ISAC) when the transmitter is subject to a representation-constrained encoder modeled via a mutual information (MI) bottleneck and the receiver employs a fluid antenna system (FAS). Specifically, the message is first encoded into an ideal Gaussian waveform and then mapped by a stochastic encoder into a finite capacity representation that serves as the channel input in the adopted analytical abstraction, while the FAS receiver selects the port with the most favorable channel conditions. From an information-theoretic perspective, we show that the representation bottleneck is equivalent to an additive representation noise, which reduces both the communication and sensing signal-to-noise ratios (SNRs) at the selected port. We then derive the resulting ISAC capacity-distortion region and establish tight converse and achievability bounds under general fading models, including Jakes correlated channels. Leveraging the spatial degrees of freedom (DoF) characterization of the Jakes' model, we prove that the port selection gain is fundamentally constrained by the physical length of the FAS region: the effective diversity order equals the numerical rank of the Jakes' correlation matrix and increases only with the FAS length. Consequently, enlarging the FAS length allows the selected communication SNR to approach the representation-limited ceiling, driving the achievable rate toward its fundamental bound. The sensing mean square error (MSE) similarly approaches its bound when the selected sensing gain also increases with the FAS aperture. Numerical results corroborate the analysis and scaling laws.

cs.IT↗

arXiv2026

Communication and Sensing Coverage of UAV Corridor under Beam Misalignment Error

In this paper, a unmanned aerial vehicle (UAV) corridor-assisted network is proposed to study the joint sensing and communication (JCAS) coverage probability (CP) of a terrestrial (BS) equipped with a realistic three-dimensional (3D) 3GPP antenna pattern. By employing the 3D binomial point process (BPP) to model the spatial locations of the UAV-UEs, a fixed number of UAV-UEs are deployed in an aerial corridor modeled as a finite cylinder. The BS exploits radar sensing capabilities to track a known UAV-UE under a 3D beam misalignment error, which follows a multivariate normal distribution. Subsequently, under clutter and uplink communication interference, exact- form analytical expressions for JCAS CP are derived and used to analyze the performance at the terrestrial BS. The results highlight corridor-length saturation at high UAV heights and the beamwidth trade-off between interference suppression and misalignment sensitivity.

cs.IT↗

arXiv2026

Index Modulation Aided Phase-Rotated AFDM for Efficient Integrated Sensing and Communication: Framework, Optimization and Performance Analysis

Affine frequency division multiplexing (AFDM) has emerged as a competitive waveform for integrated sensing and communication (ISAC) in high-mobility scenarios, but its data-dependent high peak-to-average power ratio (PAPR) and ambiguity function (AF) sidelobes may degrade the power-amplifier efficiency as well as the sensing performance. To mitigate the issues, one promising candidate is the phase-rotation optimization philosophy, which necessitates auxiliary side information (SI) to guarantee successful data recovery at the receiver. However, using additional signalling overhead to deliver the SI degrades the spectral efficiency. Against the background, this paper proposes a power level index modulation (PLIM)-aided phase-rotated AFDM (PLIM-PR-AFDM) scheme, which embeds the phase-rotation SI within the transmitted block. At the transmitter, the proposed scheme jointly optimizes the waveform by coordinating the PLIM power allocation, phase rotation, and embedding of SI. Specifically, subcarriers are flexibly allocated with different transmit power levels. High-power subcarriers are phase-rotated to suppress the PAPR and fast-slow-time AF sidelobes. By decomposing the fast-slow-time AF sidelobe energy into per-block periodic autocorrelation function (PACF) terms, the sensing objective can be formulated as a PACF-weighted integrated sidelobe level (WISL) minimization problem, while selected low-power subcarriers embed the phase bits as SI through constellation labels. Furthermore, the PLIM power patterns of each transmitted block can convey additional index bits, which compensates for the spectral efficiency loss caused by SI transmission. Moreover, the error performance is analyzed by deriving an upper bound of the average bit error probability (ABEP) and identifying the condition for full diversity.

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arXiv2026

Secure Near-Field ISAC with Pinching-Antenna Systems: Exploiting Constructive-Destructive Interference

This paper investigates secure near-field integrated sensing and communication (ISAC) with pinching-antenna systems (PASS), where a base station serves legitimate users while localizing a target that may also eavesdrop on the transmitted data. We jointly design symbol-level precoding (SLP) and the transmit and receive pinching-antenna positions to minimize the Cartesian position error bound (PEB) averaged over the data symbols. Constructive interference ensures reliable reception at the legitimate users, whereas destructive interference drives the target's observation toward incorrect symbol regions without sacrificing the illumination required for sensing. We derive the PEB for unknown complex target reflectivity and establish an exact factorization into transmit-illumination and receive-geometry terms. This decomposition reveals the distinct sensing roles of the two apertures, enables separate receive-placement and joint transmit-placement/SLP optimization, and provides analytical insights into local position identifiability, the range-information limitations of compact arrays, sensing-oriented receive placement, and in-waveguide attenuation. Building on these results, we develop an efficient algorithm by combining geometry-informed receive initialization, one-dimensional transmit-position searches, and convex per-symbol precoding. Numerical results demonstrate sub-millimeter localization, erroneous symbol decisions at the target, an expanded feasible operating region, and substantial gains over fixed-placement PASS and fully digital extremely large-scale MIMO (XL-MIMO) benchmarks.

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arXiv2026

An Operational Leakage Metric for Eavesdropping in Joint Sensing and Communications Systems

In upcoming integrated sensing and communications (ISAC) systems, security depends on whether an eavesdropper (Eve) can recover data symbols or infer sensing information from a common observation with a legitimate receiver. However, existing information and estimation measures do not quantify this dual risk under finite observation records. This letter introduces the novel metric of operational leakage, which signifies Eve's optimal probability, in the Bayesian sense, of reconstructing communication symbols or estimating a sensing parameter within prescribed distortions. By showing that prior adversarial success probability separates the effect of side information from that of the observation, we propose a conditional Laplace approximation for efficiently evaluating the proposed metric and jointly quantifying the eavesdropping risk to the sensing and communication functionalities. Numerical results showcase that the risk enabled by adversarial observation increases with Eve's signal-to-noise ratio (SNR), conventional metrics do not uniquely determine finite-record adversarial success probability, and that the proposed conditional Gaussian approximation closely follows numerical Bayes evaluation.

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arXiv2026

Secure ISAC with Sensing Privacy under Eavesdropper Uncertainty

This paper investigates the joint protection of confidential data and legitimate-user directional information in integrated sensing and communication (ISAC) networks. We consider a multiuser downlink in which passive multi-antenna eavesdroppers (Eves) attempt to decode confidential signals while exploiting legitimate-user reflections for unauthorized angular sensing. To address both threats, we develop a robust covariance-design framework that jointly limits Eve decoding, maintains the transmitter's accuracy in estimating the Eves' directions and shifts the dominant passive-sensing response toward prescribed deceptive directions. A Bayesian angular prior and the corresponding Bayesian Cramer-Rao bound (BCRB) characterize the transmitter's eavesdropper-angle estimation accuracy. Angular uncertainty is represented through geometry-consistent samples, such that each candidate Eve direction jointly determines the corresponding transmitter-Eve channel, user-Eve bearing, and deceptive direction. The resulting design balances worst-user secrecy, sensing accuracy, sensing privacy, and deception power. Robust Eve-decoding constraints are handled through finite sufficient conditions with intersample margins, while continuous ghost dominance is enforced using interval sum-of-squares (SOS) constraints. The resulting nonconvex problem is addressed through successive convex approximation (SCA) and semidefinite relaxation. Numerical results show that the proposed design effectively preserves secrecy and sensing privacy under Eve-angle uncertainty, provides controlled angular deception in single- and multiple-Eve scenarios, and outperforms benchmarks.

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arXiv2026

Physical-Layer Sensing Privacy via Constellation Shaping for OFDM-ISAC Systems: Theory, Design, and Experiments

The integration of sensing into communication networks introduces a new privacy risk, as a passive eavesdropper (Eve) may exploit ISAC data signals as signals of opportunity to perform unauthorized sensing of targets. In this paper, we develop a sensing-privacy-enhancing geometric constellation shaping (GCS) framework for OFDM-ISAC systems. The key observation is that constellation-dependent ranging performance is receiver-specific. For matched filtering at Eve, the ranging MSE is governed by the constellation kurtosis $\kurt$, whereas reciprocal filtering at the legitimate receiver (Alice) is governed by the inverse second-order moment $\ism$. Based on closed-form MSE expressions, we define sensing privacy as the ranging MSE gap between Eve and Alice and characterize its dependence on these two moments. The analysis shows that positive skewness of the symbol-power distribution is necessary for a positive intrinsic moment gap, namely $\kurt-\ism$. We further derive an exact skewness-based decomposition of the intrinsic moment gap and a canonical two-ring characterization, providing analytical guidelines for privacy-enhancing constellation geometries. We then formulate Eve-aware and Eve-agnostic GCS designs that balance sensing privacy and communication reliability through the minimum Euclidean distance (MED), with the Eve-agnostic design depending only on the intrinsic moment gap. Numerical results demonstrate scalable privacy--communication trade-offs, while over-the-air experiments show that the proposed constellation shaping substantially increases the ranging error gap between Eve and Alice with only a small communication throughput loss.

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arXiv2026

Full-Frame OFDM-ISAC: Joint Constellation, Pilot, and Receive-Filter Design

Orthogonal frequency-division multiplexing (OFDM) integrated sensing and communication (ISAC) reuses the entire time-frequency frame, including pilots and data payloads, for radar sensing. The random data payload, however, reduces the sensing dynamic range and degrades the target detection and parameter estimation performance. Existing studies typically optimize the modulation constellation, the pilot pattern, the power allocation, and the receive filter in isolation, overlooking their coupled effects on the sensing ambiguity structure. To address this gap, we propose a joint design of the constellation, the pilot placement and power split, and the mismatched receive filter for full-frame OFDM-ISAC systems, which minimizes the residual interference inside declared regions of interest (ROIs) under a communication rate constraint. For a given transmit signaling scheme, the receive filter admits a closed-form solution. With this filter in place, we derive closed-form expressions for the receiver-specific interference floor and noise enhancement over the 2-D range-Doppler domain, which show that the floor is governed by a single effective statistic of the full-frame signaling. This law reduces the joint design to two steps. A balanced staggered pilot placement removes the deterministic grating lobes of pilot-power boosting, and the shaped constellation and the power split are then selected jointly on the boundary of the rate constraint under the channel estimation error, which attains the joint optimum over the constructed shaping levels to first order in the declared ROI size. Under a quality-of-service (QoS) constraint of $98\%$ of the rate of a conventional 16-QAM frame, the proposed design significantly lowers the interference floor, improves the target detection probability, and achieves better ranging accuracy compared to the conventional receive filter-aware design.

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arXiv2026

Multi-UE Networked Sensing: A New Paradigm for 6G Perceptive Mobile Networks

Networked sensing, which jointly exploits observations from multiple distributed nodes, is essential for unlocking the full sensing potential of integrated sensing and communications (ISAC). This article introduces multi-UE sensing, a new networked sensing paradigm for future perceptive mobile networks that exploits the correlated sensing observations naturally arising from distributed user equipment devices (UEs) interacting with common targets. Representative uplink, downlink, and hybrid sensing architectures are presented, together with a multi-view signal processing framework encompassing synchronization, correlation-aware parameter estimation, and sensing fusion. Key open challenges, including correlation modelling, target association, sensing information compression, and communication-sensing co-optimization, are also discussed.

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arXiv2026

Finite-blocklength Fluid Antenna Systems

This paper investigates fluid antenna systems (FASs) subject to finite-blocklength (FBL) constraints, motivated by the strict reliability-latency and ultra-massive connectivity requirements of future wireless networks. While FAS performance has been widely studied in the asymptotic regime, its behavior under FBL remains largely unexplored. Our objective is to develop a unified set of analytical tools for evaluating FASs under FBL that remains applicable across different spatial-correlation models. First, to establish accurate benchmarks for non-orthogonal finite-length user signature design, we characterize both the average and the worst-case correlation coefficients via extreme value theory (EVT) and derive closed-form predictions of the achievable correlation levels. Second, taking block error rate (BLER) as the fundamental FBL metric, we study joint detection and decoding in FAS-assisted links and derive a closed-form BLER expression that is universally applicable across channel models. Additionally, we revisit outage probability (OP) in the FBL regime and obtain tractable OP characterizations for both FASs and conventional multiple fixed-position antenna (FPA) systems. In order to reduce the computational burden for multi-fold integrals in correlated fading models, we further propose a Taylor-expansion-assisted mean value theorem for integrals (MVTI), thus enabling efficient performance evaluation with marginal accuracy loss. Numerical results validate the analysis and reveal that even single-antenna FASs can have superior spatial diversity relative to conventional multi-FPA systems. Moreover, under both FBL and interference-limited environments, FASs provide improved energy, spectral, and hardware efficiencies, hence highlighting FAS as a promising enabler for next-generation wireless networks.

cs.IT↗

arXiv2026

OFDM-ISAC over Data Payloads: MSE Analysis, Constellation Design, and Experimentation

Orthogonal frequency division multiplexing (OFDM) is a key waveform for integrated sensing and communication (ISAC) systems due to its high spectral efficiency and inherent compatibility with modern wireless standards. However, its fundamental estimation-theoretic sensing performance under random data modulation remains largely unexplored. This paper presents a unified and explicit performance analysis of OFDM-based ISAC systems for multi-target range estimation, focusing on the distinct impacts of the modulation constellation on the sensing performance. We develop a comprehensive estimation-theoretic framework to characterize the range estimation mean-square error (MSE) for both matched filtering (MF) and reciprocal filtering (RF) sensing receiver architectures. Our theoretical analysis reveals that in multi-target and clutter-rich environments, the sensing performance of the MF receiver is fundamentally limited by the fourth-order moment (kurtosis) of the constellation, which determines the data-dependent sidelobe interference level. In contrast, the RF receiver eliminates such interference at the cost of noise enhancement, with its performance governed by the inverse second-order moment of the constellation. Building on these closed-form MSE derivations, we propose a sensing-receiver specific geometric constellation shaping (GCS) framework. By jointly optimizing the constellation geometry based on the minimum Euclidean distance (MED) and receiver-dependent sensing metrics, we enable a flexible trade-off between communication reliability and sensing precision. Our results demonstrate that the proposed constellation shaping provides significant performance gains and facilitates a tailored sensing and communication trade-off across different receiver architectures in practical over-the-air implementations.

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