arXiv subjects

Mark F. Flanagan

Publications and source records attributed to Mark F. Flanagan.

At least 19 recordsLinked to original sources

arXiv2026

Pinching Antenna Systems (PASS) for Air-Ground Integrated Sensing and Communications (ISAC)

This paper proposes a multi-user single-target air-ground integrated sensing and communication (ISAC) system, in which a ground pinching-antenna system (PASS) transmits communication and sensing signals and an unmanned aerial vehicle (UAV) receives the target echo. The communication and sensing beamforming vectors, pinching antenna positions, and UAV position are jointly optimized to minimize the weighted Cramér-Rao bound (CRB). A penalty dual decomposition (PDD) algorithm is developed for the resulting nonconvex problem. The auxiliary variable and beamforming subproblems admit globally optimal solutions in semi-closed form, while the positions of the pinching antennas and the UAV are updated by successive convex approximation (SCA). Under mild conditions, every accumulation point of the sequence generated by our proposed algorithm is a Karush-Kuhn-Tucker (KKT) point of the reformulated problem. Numerical results show that the proposed PASS-UAV design achieves a lower weighted CRB than conventional multiple-input multiple-output (MIMO) and PASS benchmarks.

eess.SP↗

arXiv2026

Belief Propagation-Informed Tesseract Decoding of Quantum LDPC Codes

In quantum error correction, the decoder must estimate the errors from the measurement data accurately and fast enough to avoid a backlog. Search-based decoders such as Tesseract achieve high accuracy by finding a minimum-weight set of error events with an A* search, but their search effort grows rapidly with the physical error rate and the code size. To reduce this search effort while maintaining a logical error rate close to that of Tesseract, we propose a belief propagation (BP)-guided modification of the Tesseract decoder for quantum low-density parity-check (LDPC) codes. Damped sum-product BP is applied to the triggered detectors before the Tesseract search. If the detectors triggered by the BP hard decision match the triggered detectors, this hard decision is accepted directly, and the Tesseract search is skipped. Otherwise, the BP marginal probability estimates, weighted by a trust coefficient, modify the search priority, while the path cost keeps the channel-prior weights so that the decoding objective is unchanged. The framework also supports BP-based initialization, in which selected BP decisions form the starting point of a residual search whose signed-cost objective allows incorrect initial decisions to be removed. A node-exploration limit bounds this residual search; if the limit is reached, the decoder falls back to the BP-guided search on the original triggered detector set. The method is evaluated on the [[72,12,6]], [[90,8,10]], and [[144,12,12]] bivariate bicycle codes under circuit-level noise. With BP-based initialization and a global trust coefficient, the number of Tesseract search nodes expanded is reduced by up to more than two orders of magnitude at low physical error rates, while the logical error rate remains close to that of Tesseract.

quant-ph↗

arXiv2026

Array Rotation in Near-Field ISAC: Communication, Detection, and Estimation Under Low-Complexity Transmit Architectures

This paper studies near-field integrated sensing and communication (ISAC) with a rotatable base-station array under an exact spherical-wave propagation model. For a general transmit covariance, we derive a Cramér--Rao bound (CRB) decomposition that separates target-side geometry from transmit-covariance-dependent terms. We then specialize the framework to two representative low-complexity architectures: a single-stream superposition architecture, and a dual-waveform split architecture design. We characterize the communication-, detection-, and estimation-optimal rotations for each architecture using the exact finite-array model. In addition, we show how a second-order approximation yields compact approximate expressions for the optimal rotation angles. We also study uncertainty in the target state used for beamforming design through a correlated Gaussian perturbation model, and derive approximations of the optimal robust rotation angles for target sensing and detection. The analysis shows that the two architectures behave differently under rotation: the single-stream design can align communication and detection tasks, whereas the dual-waveform design exhibits a direct communication--detection tradeoff and can incur a larger communication loss when sensing is prioritized. Numerical results show clear gains over a fixed array orientation, especially for range estimation, and close agreement between the approximate and exact rotation choices. Detection is less affected by target-state errors than estimation in the considered cases.

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arXiv2026

Near-Field Integrated Sensing and Communication with a Uniform Circular Array

Near-field (NF) integrated sensing and communication (ISAC) is studied for a uniform circular array (UCA) and monostatic sensing. A continuous-time channel model is derived for the spherical wavefront and orthogonal frequency-division multiplexing. A multi-slot Cramèr-Rao lower bound (CRLB) for joint range-angle-Doppler estimation is derived under a slow-motion approximation, showing how the signal delay and wavefront curvature affect the range bound. A transmit beamformer that minimizes the position CRLB is designed via gradient descent, and a maximum-likelihood estimator is obtained by using the Levenberg-Marquardt algorithm initialized with a grid search. Simulations show that increasing the UCA radius improves angle and motion estimation at all distances, but improves the range bound only at distances far inside the Rayleigh distance, and lowers the reliable communication rate.

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arXiv2026

A Reduced Greedy Minimum Distance Phase Shift Design for Quantized RIS-Assisted Spatial Modulation

Reconfigurable intelligent surfaces (RISs) have emerged as a key enabler for next-generation wireless networks, offering control over the propagation environment through nearly passive reflecting elements. When combined with spatial modulation (SM), an RIS shapes a finite set of noise-free signal points at the receiver, and the error rate performance under maximum-likelihood (ML) detection is governed by the smallest pairwise Euclidean distance among these points. In this paper, we propose a low-complexity RIS phase shift design algorithm for RIS-assisted SM under practical discrete phase shift control. The proposed method targets the minimum Euclidean distance directly through a reduced greedy search over the discrete set of RIS phase shifts, in contrast to existing approaches that align RIS phase shifts to maximize the effective channel gain and therefore do not directly address the dominant error events. The search effort is controlled by a small set of integer parameters, providing a tunable trade-off between bit error rate (BER) performance and computational cost. We evaluate the proposed method against continuous and quantized phase-alignment baselines, including a complexity-matched comparison against the most directly comparable benchmark. Numerical results show that the proposed method achieves a significant SNR gain over quantized phase alignment under the same discrete hardware model, and this advantage persists under imperfect channel state information.

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arXiv2026

Accelerating A*-Based Algorithms for Decoding Quantum Low-Density Parity-Check Codes

Quantum low-density parity-check (QLDPC) codes represent a promising approach for error correction in quantum computing. The recently proposed Tesseract decoder uses the A* search algorithm that guarantees finding the most likely error pattern. However, practical implementations of Tesseract often involve an extremely large graph, and the inherently sequential nature of the search results in high computational overhead and long runtime. To improve decoding efficiency, we propose a two-stage decoding framework. First, a belief propagation (BP) decoder efficiently processes the syndrome. This step generates hard decisions (a binary error vector) and soft information (per-qubit confidence levels). In non-convergent BP cases, a gating mechanism examines the BP decoder's output to identify and filter out qubits with oscillating confidence values. In these cases, the refined output serves as input to Tesseract, which then attempts to identify and correct any residual errors. This hybrid approach leverages the high speed of BP decoding and delegates the more challenging decoding instances to Tesseract. Numerical results demonstrate a substantial reduction in overall decoding complexity while maintaining the logical error rate (LER) of the stand-alone Tesseract. Across all tested physical error rates, the proposed method achieves at least 5x reduction in the number of expanded nodes in the Tesseract, with a peak reduction of nearly 15x at a physical error rate of p = 0.05 for the [[126, 12, d < 11]] T1 code, and approximately 8.8x at p = 0.05 for the [[72, 12, 6]] bicycle bivariate (BB) code.

quant-ph↗

arXiv2026

Fundamental Limits of Random Downlink Integrated Sensing and Communication over Rician Channels

This paper studies the stochastic performance of a downlink multiple-input multiple-output integrated sensing and communication (ISAC) system over Rician fading channels. Rician fading is important in line-of-sight (LoS)-dominated deployments, where a deterministic propagation component can strongly affect sensing and communication reliability. The base station (BS) simultaneously serves a user and senses a target. The BS-user channel contains LoS and non-line-of-sight components. The user LoS angle may be fixed or random, and the target angle may follow an arbitrary distribution potentially correlated with the user angle. Compared with Rayleigh fading, the deterministic LoS component introduces angle-dependent terms and leads to generally independent but non-identically distributed random vectors, requiring new analysis. We analyze two beamforming strategies: subspace joint beamforming (SJB), optimal for the shared waveform structure, and linear beamforming (LB), a practical alternative using separate sensing and communication beamformers. For both schemes, we derive communication outage probability (OP) and sensing OP based on the Cramer--Rao bound (CRB). We also identify special cases with simpler expressions. For LB, we derive upper and lower bounds on sensing OP and a tractable approximation. We characterize large-system and high-power scaling laws. LB without dirty paper coding (DPC) is interference-limited at high power due to radar self-interference. Results show the Rician K-factor affects communication more strongly than sensing, with non-monotonic behavior across regimes. LB with DPC achieves the best overall performance in strong LoS environments and is the only scheme achieving ultra-high communication reliability in Rayleigh fading, while SJB provides a robust lower-complexity alternative across operating conditions.

cs.IT↗

arXiv2026

Weighted Sum Rate Optimization for Movable Antenna Enabled Near-Field ISAC

Integrated sensing and communication (ISAC) has been recognized as one of the key technologies capable of simultaneously improving communication and sensing services in future wireless networks. Moreover, the introduction of recently developed movable antennas (MAs) has the potential to further increase the performance gains of ISAC systems. Achieving these gains can pose a significant challenge for MA-enabled ISAC systems operating in the near-field due to the corresponding spherical wave propagation. Motivated by this, in this paper we maximize the weighted sum rate (WSR) for communication users while maintaining a minimal sensing requirement in an MA-enabled near-field ISAC system. To achieve this goal, we propose an algorithm that optimizes the sensing receive combiner, the communication precoding matrices, the sensing transmit beamformer and the positions of the users' MAs in an alternating manner. Simulation results show that using MAs in near-field ISAC systems provides a substantial performance advantage compared to near-field ISAC systems with only fixed antennas. Additionally, we demonstrate that the highest WSR is obtained when larger weights are allocated to the users placed closer to the BS, and that the sensing performance is significantly more affected by the minimum sensing signal-to-interference-plus-noise ratio (SINR) threshold compared to the communication performance.

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arXiv2026

Beyond the Limits of Rigid Arrays: Flexible Intelligent Metasurfaces for Next-Generation Wireless Networks

Following recent advances in flexible electronics and programmable metasurfaces, flexible intelligent metasurfaces (FIMs) have emerged as a promising enabling technology for next-generation wireless networks. A FIM is a morphable electromagnetic surface capable of dynamically adjusting its physical geometry to influence the radiation and propagation of electromagnetic waves. Unlike conventional rigid arrays, FIMs introduce an additional spatial degree of design freedom enabled by mechanical flexibility, which can enhance beamforming, spatial focusing, and adaptation to dynamic wireless environments. This added capability enables wireless systems to shape the propagation environment not only through electromagnetic tuning but also through controllable geometric reconfiguration. This article explores the potential of FIMs for next-generation wireless networks. We first introduce the main hardware architectures of FIMs and explain how they can be integrated into wireless communication systems. We then present representative application scenarios, highlighting the advantages of FIMs for future wireless networks and comparing them with other emerging flexible wireless technologies. To illustrate their potential impact, we present case studies comparing FIM-enabled architectures with conventional rigid-array systems, demonstrating the performance gains enabled by surface flexibility for both communication and sensing applications. Finally, we discuss key opportunities, practical challenges, and open research directions that must be addressed to fully realize the potential of FIM technology in future wireless communication systems.

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arXiv2026

Exploiting Structural Flexibility in SIM-Enabled Communications: From Adaptive Inter-Layer Spacing to Fully Morphable Layers

Stacked intelligent metasurfaces (SIMs) have recently emerged as a promising metasurface-based physical-layer paradigm for wireless communications, enabling wave-domain signal processing through multiple cascaded metasurface layers. However, conventional SIM designs rely on rigid planar layers with fixed interlayer spacing, which constrain the propagation geometry and can lead to performance saturation as the number of layers increases. This paper investigates the potential of introducing structural flexibility into SIM-enabled communication systems. Specifically, we consider two flexible SIM architectures: distance-adaptive SIM (DSIM), where interlayer distances are optimized, and stacked flexible intelligent metasurface (SFIM), where each metasurface layer is fully morphable. We jointly design the meta-atom positions and responses together with the transmit beamformer to maximize the system sum rate under per-user rate, quantization, morphing, and interlayer distance constraints. An alternating optimization framework combining gradient projection, penalty-based method, and successive convex approximation is developed to address the resulting non-convex problems. Perturbation analysis reveals that the flexibility gains of both DSIM and SFIM scale approximately linearly with the morphing range, with SFIM exhibiting a faster growth rate. Simulation results demonstrate that flexible SIM designs mitigate performance saturation with increasing layers and achieve significant transmit power savings compared to rigid SIMs.

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arXiv2025

Optimal Beamforming Design for Multi-user MIMO Near-Field ISAC Systems with Movable Antennas

Integrated sensing and communication (ISAC) has been recognized as one of the key technologies capable of simultaneously improving communication and sensing services in future wireless networks. Moreover, the introduction of recently developed movable antennas (MAs) has the potential to further increase the performance gains of ISAC systems. Although the gains of MA-enabled ISAC systems are relatively well studied in the far field, they remain almost unexplored in near-field scenarios. Motivated by this, in this paper we maximize the weighted sum rate (WSR) for communication users while maintaining a minimum sensing requirement in an MA-enabled near-field ISAC system. To achieve this goal, we propose algorithms that optimize the communication precoding matrices, the sensing transmit beamformer, the sensing receive combiner, the positions of the users' MAs and the positions of the base station (BS) transmit MAs in an alternating manner for the considered ISAC system, for the cases where linear procoding and zero-forcing (ZF) precoding are employed at the BS. Simulation results show that using MAs in near-field ISAC systems provides a substantial performance advantage compared to near-field ISAC systems equipped with fixed antennas only. We show that the scheme with linear precoding achieves larger WSR for unequal users' weight rates, while the scheme with ZF precoding maintains an approximately constant WSR for all users' weight rates. Additionally, we demonstrate that the WSRs of the proposed schemes are highly dependent on the inter-antenna interference between different user's MAs, and that the sensing performance is significantly more affected by the minimum sensing signal-to-interference-plus-noise ratio (SINR) threshold compared to the communication performance.

cs.IT↗

arXiv2025

RIS-Assisted Generalized Receive Quadrature Spatial Modulation with Extension to Multicast Communications

This paper proposes a novel reconfigurable intelligent surface (RIS)-assisted generalized receive quadrature spatial modulation (RIS-GRQSM) scheme to enhance the spectral efficiency (SE) of RIS-aided \textit{quadrature} spatial modulation (QSM) systems. By leveraging the principle of \textit{generalized} spatial modulation (GSM), multiple receive antennas are independently activated for \textit{both} the in-phase and quadrature components of spatial symbols. To fully exploit the potential of RIS, we formulate a max-min optimization problem to adjust the phase shifts of all RIS elements, thereby maximizing the effective signal-to-noise ratios (SNRs) at the activated antennas. Using Lagrange duality, the original high-dimensional non-convex problem is reduced to a tractable problem with a smaller number of real variables, and a closed-form suboptimal solution is also proposed, which achieves near-optimal performance with a sufficiently large RIS. At the receiver, a low-complexity non-coherent energy-based greedy detector (GD) is introduced for efficient symbol detection. We further extend the RIS-GRQSM framework to a multicast communication system, where all users receive identical information with equal SNR levels, and provide a detailed performance analysis of both systems. In particular, we derive the average bit error probability (ABEP) for the proposed RIS-GRQSM and multicast systems under optimal and suboptimal optimization strategies. Numerical results show that RIS-GRQSM significantly improves the SE and error rate performance compared with benchmark schemes, while the multicast extension achieves performance close to benchmark methods at substantially lower complexity.

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arXiv2025

On Beamforming for Transmitter Location Privacy in MIMO Systems

In this paper, we introduce a beamforming framework to ensure transmitter location privacy against sensing-capable MIMO receivers. We propose a novel privacy metric called the direction-of-arrival obfuscation power ratio (DoA-OPR) to design the transmit beamformer. The design approach reshapes the angular power distribution observed at the receiver so that a false direction appears dominant without nulling the line-of-sight (LoS) component. We derive closed-form bounds on the feasible range of DoA-OPR via generalized eigenvalue analysis and formulate an achievable rate-maximization problem under the DoA-OPR constraint. The resulting problem is non-convex but can be efficiently solved using semidefinite relaxation, eigenmode selection, and optimal power allocation. Numerical results demonstrate that the proposed DoA-OPR-based beamformer achieves a trade-off between location privacy and communication rate. The proposed design attains higher achievable rates than existing LoS-nulling approaches while maintaining comparable location privacy. A suboptimal design strategy is also proposed with reduced complexity. It achieves a near-optimal communication rate with a reduction of nearly 85%.

eess.SP↗

arXiv2025

Stacked Flexible Intelligent Metasurface Design for Multi-User Wireless Communications

Stacked intelligent metasurfaces (SIMs) have recently emerged as an effective solution for next-generation wireless networks. A SIM comprises multiple metasurface layers that enable signal processing directly in the wave domain. Moreover, recent advances in flexible metamaterials have highlighted the potential of flexible intelligent metasurfaces (FIMs), which can be physically morphed to enhance communication performance. In this paper, we propose a stacked flexible intelligent metasurface (SFIM)-based communication system for the first time, where each metasurface layer is deformable to improve the system's performance. We first present the system model, including the transmit and receive signal models as well as the channel model, and then formulate an optimization problem to maximize the system sum rate under constraints on the transmit power budget, morphing distance, and the unit-modulus condition of the meta-atom responses. To solve this problem, we develop an alternating optimization framework based on the gradient projection method. Simulation results demonstrate that the proposed SFIM-based system achieves significant performance gains compared to its rigid SIM counterpart.

eess.SP↗

arXiv2025

Sensing Rate Optimization for Multi-Band Cooperative ISAC Systems

Integrated sensing and communication (ISAC) has been recognized as one of the key technologies for future wireless networks, which potentially need to operate in multiple frequency bands to satisfy ever-increasing demands for both communication and sensing services. Motivated by this, we consider the sum sensing rate (SR) optimization for a cooperative ISAC system with linear precoding, where each base station (BS) works in a different frequency band. With this aim, we propose an optimization algorithm based on the semi-definite rank relaxation that introduces covariance matrices as optimization variables, and we apply the inner approximation (IA) method to deal with the nonconvexity of the resulting problem. Simulation results show that the proposed algorithm increases the SR by approximately 25 % and 40 % compared to the case of equal power distribution in a cooperative ISAC system with two and three BSs, respectively. Additionally, the algorithm converges in only a few iterations, while its most beneficial implementation scenario is in the low power regime

cs.IT↗

arXiv2025

On the performance of downlink NOMA in underlay spectrum sharing

Non-orthogonal multiple access (NOMA) and spectrum sharing are two potential technologies for providing massive connectivity in beyond fifth-generation (B5G) networks. In this paper, we present the performance analysis of a multi-antenna-assisted two-user downlink NOMA system in an underlay spectrum sharing system. We derive closed-form expressions for the average achievable sum-rate and outage probability of the secondary network under a peak interference constraint and/or peak power constraint, depending on the availability of channel state information (CSI) of the interference link between secondary transmitter (ST) and primary receiver (PR). For the case where the ST has a fixed power budget, we show that performance can be divided into two specific regimes, where either the interference constraint or the power constraint primarily dictates the performance. Our results confirm that the NOMA-based underlay spectrum sharing system significantly outperforms its orthogonal multiple access (OMA) based counterpart, by achieving higher average sum-rate and lower outage probability. We also show the effect of information loss at the ST in terms of CSI of the link between the ST and PR on the system performance. Moreover, we also present closed-form expressions for the optimal power allocation coefficient that minimizes the outage probability of the NOMA system for the special case where the secondary users are each equipped with a single antenna. A close agreement between the simulation and analytical results confirms the correctness of the presented analysis.

cs.IT↗

arXiv2025

Delay Violation Probability and Effective Rate of Downlink NOMA over $α$-$μ$ Fading Channels

Non-orthogonal multiple access (NOMA) is a potential candidate to further enhance the spectrum utilization efficiency in beyond fifth-generation (B5G) standards. However, there has been little attention on the quantification of the delay-limited performance of downlink NOMA systems. In this paper, we analyze the performance of a two-user downlink NOMA system over generalized α-μ fading in terms of delay violation probability (DVP) and effective rate (ER). In particular, we derive an analytical expression for an upper bound on the DVP and we derive the exact sum ER of the downlink NOMA system. We also derive analytical expressions for high and low signal-to-noise ratio (SNR) approximations to the sum ER, as well as a fundamental upper bound on the sum ER which represents the ergodic sum-rate for the downlink NOMA system. We also analyze the sum ER of a corresponding time-division-multiplexed orthogonal multiple access (OMA) system. Our results show that while NOMA consistently outperforms OMA over the practical SNR range, the relative gain becomes smaller in more severe fading conditions, and is also smaller in the presence a more strict delay quality-of-service (QoS) constraint.

cs.IT↗

arXiv2025

Opportunistic User Scheduling for Secure RIS-aided Wireless Communications

In this paper, we provide expressions for the secrecy outage probability (SOP) for suboptimal and optimal opportunistic scheduling schemes in a reconfigurable intelligent surface (RIS) aided {single antenna} system with multiple eavesdroppers in approximate closed form. A suboptimal scheduling (SS) scheme is analyzed, which is used when the channel state information (CSI) of the eavesdropping links is unavailable, and the optimal scheduling (OS) scheme is also analyzed, which is used when the global CSI is available. For each scheme, we provide a simplified expression for the SOP in the high signal-to-noise ratio (SNR) regime to demonstrate its behavior as a function of the key system parameters. At high SNR, the SOP saturates to a constant level which decreases exponentially with the number of RIS elements in the SS scheme and with the product of the number of RIS elements and the number of users in the OS scheme. We also show that the derived SOP of the SS scheme can directly provide the SOP for the best antenna-user pair scheduling scheme in a multiple antenna system. We compare the performance of the opportunistic user scheduling schemes with that of a non-orthogonal multiple access (NOMA) based scheduling scheme which chooses a pair of users in each time slot for scheduling and we show that the opportunistic schemes outperform the NOMA-based scheme. We also derive a closed-form expression for the SOP of a decode-and-forward (DF) relay-aided scheduling scheme in order to compare it with that of the RIS-aided system. It is found that the RIS-aided system outperforms the relay-aided systems when the number of RIS elements is sufficiently large. An increased number of RIS elements is required to outperform the relay-aided system at higher operating frequencies.

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