arXiv subjects

Shinya Sugiura

Publications and source records attributed to Shinya Sugiura.

At least 19 recordsLinked to original sources

arXiv2026

Technical Report OFDM-Assisted Simultaneous Quantum and Classical THz Communications

The feasibility of cost-effective simultaneous quantum and classical communication (SQCC) transmitting both the quantum key and classical information via a superimposed coherent state is investigated both in optical and Terahertz (THz) bands. Since the existing THz SQCC schemes assume single-carrier (SC) transmission over flat fading channels, we embark on investigating SQCC in realistic frequency-selective multipath THz fading channels. We then propose an orthogonal frequency division multiplexing (OFDM) based SQCC system for time-invariant frequency-selective THz scenarios, supported by low-density parity-check coded (LDPC) multidimensional QKD reconciliation schemes. Our simulation results demonstrate that the OFDM-based SQCC scheme is capable of achieving a practical secret key rate (SKR) over a wide range of power sharing scenarios between the classical and quantum signals. By contrast its single-carrier counterpart requires the classical signal to be at least 100 times stronger than the quantum signal in THz SQCC.

quant-ph↗

arXiv2026

Learning to Agree under Pseudo-Reciprocity

We characterize the communication networks that ensure consensus in rational social learning under the sure-thing principle. A network ensures consensus if and only if it is pseudo-reciprocal, a property under which reciprocity may fail between individuals but holds at a coarser group level. Prior work has assumed message exchange to be bidirectional, yet such pairwise reciprocity proves largely dispensable. We show that a single bidirectional link can suffice, however large the population.

econ.TH↗

arXiv2026

Precoded Non-Orthogonal Frequency Division Multiplexing with Subcarrier Index Modulation

In this paper, we propose a novel precoded non-orthogonal frequency division multiplexing (NOFDM) with subcarrier index modulation (SIM) to increase spectral efficiency for multicarrier systems. The proposed NOFDM-SIM scheme is constituted by a filterbank multicarrier system under reduced subcarrier spacing, where precoded index modulated symbols are mapped onto the subcarriers. The detrimental effects of inter-carrier interference (ICI) resulting from the non-orthogonality between subcarriers are efficiently mitigated with the aid of eigenvalue-decomposition-based precoding. Our simulation results demonstrate that the proposed precoded NOFDM-SIM system exhibits a similar peak-to-average power ratio to OFDM and OFDM-SIM counterparts while achieving the bit error ratio limit of zero-ICI OFDM.

eess.SP↗

arXiv2026

Power-Domain-Multiplexed Precoded Faster-Than-Nyquist Signaling for NOMA Downlink

In this paper, we propose a novel precoded faster-than-Nyquist (FTN) signaling scheme with power-domain-multiplexing in non-orthogonal multiple access (NOMA) downlink for tapping the joint benefits of high spectral efficiency and simultaneous multiuser connectivity. Non-orthogonality is introduced in both the symbol-interval (time) domain as well as in the multiple-access (power) domain to achieve a flexible resource allocation. Eigendecomposition of the FTN-specific intersymbol interference (ISI) matrix is used to achieve efficient cancellation of ISI, while successive interference cancellation is used to eliminate multiuser interference induced by NOMA. We derive the achievable analytical rate bound and demonstrate the numerical results of the bit error rate performance for the proposed scheme.

eess.SP↗

arXiv2026

SLIPT-Enabled Ground-to-UAV FSO Systems with Optical Reconfigurable Intelligent Surfaces

This paper proposes an optical reconfigurable intelligent surface (ORIS)-assisted ground-to-unmanned aerial vehicle (UAV) free-space optical (FSO) communication system empowered by simultaneous lightwave information and power transfer (SLIPT). To overcome the line-of-sight (LoS) limitation of FSO-based SLIPT systems, we introduce an ORIS that reflects the laser beam towards a non-LoS UAV receiver. We model and analyze the combined channel characteristics, incorporating atmospheric loss, turbulence-induced fading, pointing error, and angle-of-arrival fluctuations due to UAV hovering. We derive closed-form expressions for harvested energy, outage probability, and symbol error rate (SER). Numerical results show that integrating ORIS improves EH efficiency while maintaining manageable outage and SER performance.

eess.SP↗

arXiv2026

Denoising-Autoencoder-Assisted Physical Layer Secret Key Generation

In this paper, we propose denoising autoencoder (DAE)-assisted secret key generation (SKG), where channel noise reciprocity imperfections induced due to wireless channel measurements are suppressed, hence significantly enhancing the reliability and efficiency. More specifically, the DAE is capable of capturing the intrinsic structure of input distributions, reconstructing the original data structure, and removing additive noise while preserving the essential structure of signals. In our analysis, it is demonstrated that the proposed SKG scheme exhibits higher performance than the conventional schemes in terms of key disagreement rate (KDR), secret key capacity (SKC), and randomness of the generated keys.

eess.SP↗

arXiv2026

Iterative Optimization of Reconfigurable Intelligent Surface Aided Single-Carrier Spatial Modulation

This paper proposes a novel cyclic-prefixed single-carrier transmission scheme that amalgamate a reconfigurable intelligent surface (RIS) with spatial modulation in the frequency-selective fading channel. The discrete-input continuous-output memoryless channel's~(DCMC) capacities of the proposed schemes are formulated, while their gradients with respect to the RIS phase shifts are derived in the closed form. Then, the gradients are used for iteratively and efficiently optimizing the proposed RIS-aided schemes with the aid of the gradient-ascent algorithm. Our performance results demonstrate that the proposed iterative algorithm enhances the DCMC capacity of the schemes, while outperforming the conventional RIS benchmarks. Moreover, the convergence behavior and sensitivity in the proposed iterative algorithm are analyzed.

eess.SP↗

arXiv2026

Efficient Numerical Modeling of Near-Field Diffraction in ORIS-Assisted Free-Space Optical Links

This paper investigates near-field propagation in optical reconfigurable intelligent surface (ORIS)-assisted free-space optical (FSO) communication systems. Unlike conventional far-field scenarios, near-field propagation involves complex diffraction effects that hinder tractable closed-form analysis. To address this issue, a numerical framework for evaluating the optical field distribution of ORIS-assisted FSO links is proposed. Specifically, two numerical approaches are considered: direct Riemann-sum evaluation and a fast Fourier transform (FFT)-based method. Although the Riemann sum approach provides accurate field estimation, it incurs extremely high computational complexity due to the fine spatial discretization of the ORIS surface required at optical wavelengths. To improve computational efficiency, the optical-field calculation is reformulated as a convolution in the spatial-frequency domain, enabling efficient FFT-based propagation analysis. Simulation results demonstrate that the proposed FFT-based method achieves accuracy comparable to that of the Riemann-sum approach while significantly reducing computational complexity.

eess.SY↗

arXiv2026

Reduced-Overhead Channel Estimation and Iterative Detection of FTN Signaling Based on Pilot Superimposition and Spectral Interference Alignment

This paper proposes low-overhead and low-complexity channel estimation (CE) of frequency-domain equalization aided faster-than-Nyquist (FTN) signaling. In the proposed CE scheme, the concept of pilot superimposition is employed, where the FTN block is designed to superimpose pilot symbols with information symbols, and thus, no dedicated time and frequency resources nor guard bands are required, resulting in a 50% reduction of the overhead. Furthermore, interference induced by the pilot superimposition is eliminated by invoking a novel scheme, referred to as spectral interference alignment, where a data-dependent sequence is subtracted from transmitted information symbols. The theoretical mean-square error (MSE) of the proposed CE is derived, which verifies that the MSE is no longer affected by interference due to the pilot superimposition.

eess.SP↗

arXiv2026

Exploiting Skyrmions in Free-Space Optical Communication

In this paper, we propose a novel free-space optical (FSO) communication system utilizing optical skyrmions. We introduce a scheme referred to as skyrmion number modulation (SkM), which employs index modulation by encoding information onto the skyrmion number, a topological invariant preserved during free-space propagation. This topological nature offers the potential for inherent robustness against atmospheric turbulence-induced wavefront distortions, which limit the performance of conventional FSO systems. More specifically, we demonstrate that the fluctuation of the received skyrmion number is mitigated by a proposed intensity-based masking technique. Finally, our performance analysis based on a discrete memoryless channel framework confirms that the proposed system exhibits near-ideal robustness under weak turbulence and supports high-order modulation in moderate regimes.

eess.SP↗

arXiv2026

Delay-Doppler-Domain Channel Estimation and Reduced-Complexity Detection of Faster-than-Nyquist Signaling Aided OTFS

We conceive a novel channel estimation and data detection scheme for OTFS-modulated faster-than-Nyquist (FTN) transmission over doubly selective fading channels, aiming for enhancing the spectral efficiency and Doppler resilience. The delay-Doppler (DD) domain's input-output relationship of OTFS-FTN signaling is derived by employing a root-raised cosine (RRC) shaping filter. More specifically, we design our DD-domain channel estimator for FTN-based pilot transmission, where the pilot symbol interval is lower than that defined by the classic Nyquist criterion. Moreover, we propose a reduced-complexity linear minimum mean square error equalizer, supporting noise whitening, where the FTN-induced inter-symbol interference (ISI) matrix is approximated by a sparse one. Our performance results demonstrate that the proposed OTFS-FTN scheme is capable of enhancing the achievable information rate, while attaining a comparable BER performance to both that of its Nyquist-based OTFS counterpart and to other FTN transmission schemes, which employ the same RRC shaping filter.

eess.SP↗

arXiv2026

Faster-than-Nyquist Signaling for Next-Generation Wireless: Principles, Applications, and Challenges

Future wireless networks are expected to deliver ultra-high throughput for supporting emerging applications. In such scenarios, conventional Nyquist signaling may falter. As a remedy, faster-than-Nyquist (FTN) signaling facilitates the transmission of more symbols than Nyquist signaling without expanding the time-frequency resources. We provide an accessible and structured introduction to FTN signaling, covering its core principles, theoretical foundations, unique advantages, open facets, and its road map. Specifically, we present promising coded FTN results and highlight its compelling advantages in integrated sensing and communications (ISAC), an increasingly critical function in future networks. We conclude with a discussion of open research challenges and promising directions.

cs.IT↗

arXiv2025

Quantum Internet in the Sky: Vision, Challenges, Solutions, and Future Directions

This article envisions the concept of a ``Quantum Internet in the Sky", aiming to establish ubiquitous quantum communication links among distant nodes via free-space optical channels. Our key focus is on deploying quantum communication terminals on non-terrestrial platforms, specifically unmanned aerial vehicles and satellites, at various altitudes. By highlighting the unique characteristics of these platforms compared to terrestrial counterparts, we address inherent challenges and discuss potential solutions through meticulous system designs and analyses of typical non-terrestrial quantum communication scenarios. Finally, we illuminate the path forward by proposing essential future directions that underscore the integration of high-dimensional multipartite quantum communications with sensing, computing, and intelligence for multiple users en route to realizing a fully operational Quantum Internet.

quant-ph↗

arXiv2025

Simultaneous Classical and Quantum Communications: Recent Progress and Three Challenges

A critical aspect of next-generation wireless networks is the integration of quantum communications to guard against quantum computing threats to classical networks. Despite successful experimental demonstrations, integrating quantum communications into the classical infrastructure faces substantial challenges, including high costs, compatibility issues, and extra hardware deployment to accommodate both classical and quantum communication equipment. To mitigate these challenges, we explore novel protocols that enable simultaneous classical and quantum communications, relying on a single set of transceivers to jointly modulate and decode classical and quantum information onto the same signal. Additionally, we emphasize extending quantum communication capabilities beyond traditional optical bands into the terahertz, even possibly to millimeter-wave and microwave frequencies, thereby broadening the potential horizon of quantum-secure applications. Finally, we identify open problems that must be addressed to facilitate practical implementation.

quant-ph↗

arXiv2025

An Experimental Validation of Reconfigurable Intelligent Surfaces Achieving Pulse Width-Modulated Singular Reflection Angles Without External Power Sources

In this study, we introduce a design concept that leverages pulse width variation to enable a reconfigurable intelligent surface (RIS) and to autonomously switch reflection properties between two angles without any active control system. Our RIS alters its beam pattern from a singular specular reflection to another unique singular anomalous reflection when the incoming waveform changes from a short pulse to a continuous wave, even at the same frequency. Unlike conventional RISs, our passive control mechanism eliminates the requirements of active components and precise symbol-level synchronization with the transmitting antennas, reducing the system complexity level while offering dynamic material adaptability. We numerically show that the proposed RIS design is capable of varying the received magnitude of an incident wave by a factor of ten, which is also experimentally validated for the first time. Employing binary phase-shift keying (BPSK) modulation, we further report that the communication characteristics can be varied by 7 dB or more, which indicates that the proposed design is not limited to a single frequency component as long as the bandwidth of the given signal is covered by that of the RIS design. These results may present new opportunities for exploring and deploying pulse width-dependent RISs in practical scenarios involving next-generation communication systems.

cond-mat.other↗

arXiv2025

Towards Quantum SAGINs Harnessing Optical RISs: Applications, Advances, and the Road Ahead

The space-air-ground integrated network (SAGIN) concept is vital for the development of seamless next-generation (NG) wireless coverage, integrating satellites, unmanned aerial vehicles, and manned aircraft along with the terrestrial infrastructure to provide resilient ubiquitous communications. By incorporating quantum communications using optical wireless signals, SAGIN is expected to support a synergistic global quantum Internet alongside classical networks. However, long-distance optical beam propagation requires line-of-sight (LoS) connections in the face of beam broadening and LoS blockages. To overcome blockages among SAGIN nodes, we propose deploying optical reconfigurable intelligent surfaces (ORISs) on building rooftops. They can also adaptively control optical beam diameters for reducing losses. This article first introduces the applications of ORISs in SAGINs, then examines their advances in quantum communications for typical SAGIN scenarios. Finally, the road ahead towards the practical realization of ORIS-aided NG quantum SAGINs is outlined.

eess.SP↗

arXiv2024

Optical RISs Improve the Secret Key Rate of Free-Space QKD in HAP-to-UAV Scenarios

Large optical reconfigurable intelligent surfaces (ORISs) are proposed for employment on building rooftops to facilitate free-space quantum key distribution (QKD) between highaltitude platforms (HAPs) and low-altitude platforms (LAPs). Due to practical constraints, the communication terminals can only be positioned beneath the LAPs, preventing direct upward links to HAPs. By deploying ORISs on rooftops to reflect the beam arriving from HAPs towards LAPs from below, reliable HAP-to-LAP links can be established. To accurately characterize the optical beam propagation, we develop an analytical channel model based on extended Huygens-Fresnel principles for representing both the atmospheric turbulence effects and the hovering fluctuations of LAPs. This model facilitates adaptive ORIS beam-width control through linear, quadratic, and focusing phase shifts, which are capable of effectively mitigating the detrimental effects of beam broadening and pointing errors (PE). Consequently, the information-theoretic bound of the secret key rate and the security performance of a decoy-state QKD protocol are analyzed. Our findings demonstrate that quadratic phase shifts enhance the SKR at high HAP-ORIS zenith angles or mild PE conditions by narrowing the beam to optimal sizes. By contrast, linear phase shifts are advantageous at low HAP-ORIS zenith angles or moderate-to-high PE by diverging the beam to mitigate LAP fluctuations.

eess.SP↗

arXiv2024

Precoded faster-than-Nyquist signaling using optimal power allocation for OTFS

A precoded orthogonal time frequency space (OTFS) modulation scheme relying on faster-than-Nyquist (FTN) transmission over doubly selective fading channels is {proposed}, which enhances the spectral efficiency and improves the Doppler resilience. We derive the input-output relationship of the FTN signaling in the delay-Doppler domain. Eigenvalue decomposition (EVD) is used for eliminating both the effects of inter-symbol interference and correlated additive noise encountered in the delay-Doppler domain to enable efficient symbol-by-symbol demodulation. Furthermore, the power allocation coefficients of individual frames are optimized for maximizing the mutual information under the constraint of the derived total transmit power. Our performance results demonstrate that the proposed FTN-based OTFS scheme can enhance the information rate while achieving a comparable BER performance to that of its conventional Nyquist-based OTFS counterpart that employs the same root-raised-cosine shaping filter.

eess.SP↗