arXiv subjects

Hüseyin Arslan

Publications and source records attributed to Hüseyin Arslan.

At least 19 recordsLinked to original sources

arXiv2026

Timing Misalignment and Its Impact on OTFS and Other Delay-Based Waveforms

The shift towards 6G wireless networks calls for advanced waveforms to support emerging applications such as ultra-reliable and massive machine-type communications. Among the proposed candidates, OTFS, ODDM, OTSM, and OCDM share the common feature of exploiting the delay domain for data transmission. While these waveforms offer significant advantages in high-mobility and sensing scenarios, their integration into multi-user uplink systems reveals a critical challenge: timing misalignment (TM). Unlike OFDM, where residual timing offsets translate into phase rotations, delay-based waveforms experience actual shifts of resources, whose impact we investigate in this work. We show that the aforementioned waveforms can be unified under a time-invariant setup through a common input--output relation, which enables a unified analysis of TM in uplink resource allocation. The results reveal a fundamental trade-off between interference mitigation and guard overhead in the presence of TM. We further benchmark the observed behavior against OFDM under Doppler spread as a reference impairment and highlight the analogy between TM in delay-based allocation and Doppler spread in frequency-based allocation. The results show that delay-based waveforms can maintain a net performance advantage over OFDM in the considered setup. Beyond this unified Doppler-free analysis, we evaluate the joint impact of Doppler spread and TM on each waveform individually. We also investigate BER performance and show how coding can mitigate the resulting interference. This study therefore establishes a problem framework that motivates the development of future compensation and scheduling strategies for practical multi-user operation of delay-based waveforms.

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arXiv2026

A Novel Coexistence Scheme for OFDM and OTFS Waveforms for Uplink in 6G Networks

The upcoming sixth generation (6G) wireless networks are designed to support advanced applications that the current air interface based on orthogonal frequency division multiplexing (OFDM) cannot accommodate, necessitating innovations in waveform design. Orthogonal time frequency space (OTFS) has emerged as a promising alternative, offering enhanced performance particularly in high-mobility channel environments. However, deploying fundamentally different waveforms over the same frequencies fails to exploit the mutual relationships between them. In this paper, a forward-compatible yet backward-compatible coexistence framework is proposed, where various OFDM waveform options are incorporated while seamlessly integrating OTFS within the same network. The proposed coexistence paradigm involves utilizing a specially designed spreading matrix in the delay-Doppler (DD) domain to process OTFS data. The matrix is tailored to establish orthogonality between OTFS and OFDM waveforms, effectively reducing cross-interference when both coexist within the same time-frequency (TF) resources. Our results demonstrate up to a 6 dB improvement in bit error rate (BER) performance for the OTFS waveform compared to other coexistence schemes. The proposed approach also shows greater interference resilience and approaches the upper bounds of achievable rates. By utilizing the full bandwidth and time duration, it enhances range and velocity resolutions, leading to improved sensing performance. These findings are supported by mathematical analysis and extensive simulations for uplink scenarios.

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arXiv2026

Backward-Compatible Tag-Based PHY Authentication for Secure WLAN Sensing

Driven by ongoing standardization efforts, Wi-Fi sensing has recently emerged as a promising functionality within wireless local area networks (WLANs). However, due to the use of standard null-data packets (NDPs) to estimate the channel, conventional Wi-Fi sensing systems are susceptible to spoofing attacks. To counter this threat, we propose a lightweight tag-embedding scheme that enables physical layer authentication (PLA) of sensing signals. Specifically, a secret tag is superimposed onto the orthogonal frequency division multiplexing (OFDM) channel sounding signal, allowing legitimate WLAN receivers to verify signal authenticity and detect spoofed transmissions. A unique tag is generated for each transmission to avoid replay attacks, and the subcarrier occupancy of each tag is randomized to minimize the mean channel estimation error at each subcarrier. Simulation results demonstrate reliable authentication performance with a 95% correct authentication rate at a signal-to-noise ratio (SNR) of 10dB, while introducing an additional channel estimation error of only 0.52dB at legacy receivers that are unaware of the embedded tag. Thus, our proposed scheme provides an effective and backward-compatible countermeasure against spoofing attacks, enhancing the trustworthiness of future Wi-Fi sensing systems.

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arXiv2026

FM-OFDM: A Constant-Envelope Sensing Waveform with Phase-Differencing Receiver Processing

Cyclic-prefix orthogonal frequency-division multiplexing (CP-OFDM) is widely adopted as a reference waveform for integrated sensing and communication (ISAC). However, its high peak-to-average power ratio (PAPR) requires power amplifier back-off, thereby reducing the available sensing link budget. Frequency-modulated OFDM (FM-OFDM) provides a constant-envelope signal with 0 dB PAPR and has demonstrated reliable communication performance in high-mobility scenarios, but its sensing characteristics remain largely unexplored. This paper characterizes FM-OFDM as a sensing waveform based on its bandwidth, ambiguity function, sidelobe behavior, and Doppler estimation capability. The analysis shows that its data dependent sidelobe floor is incoherent and decreases through across frame integration, whereas the corresponding CP-OFDM floor remains unchanged. Consequently, under equal occupied bandwidth and transmit power, frame-level integration reverses the single-symbol performance ordering and provides FM-OFDM with approximately 20 dB of additional dynamic range for weak-target detection. Moreover, the zero-delay cut of the FMOFDM ambiguity function is shown to be deterministic and independent of the transmitted data realization, in contrast to linearly modulated waveforms, while a closed-form expression is derived for the sidelobe floor away from the zero-delay cut. Since the nonlinear mapping between the data symbols and time-domain samples prevents the direct application of the conventional CP-OFDM range-Doppler processing chain, a weighted phase-increment Doppler estimator is developed. The proposed estimator enables closed-form prediction of the sensing floor and its crossover point with a minimum computational complexity.

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arXiv2026

CP-Aware OFDM-Based OOK Signaling

This letter addresses the challenge of cyclic prefix (CP) problem in orthogonal frequency division multiplexing (OFDM)-based on-off keying (OOK) generation for low-power Internet of Things (IoT) systems. We propose a CP-aware waveform design that generates the OOK signal over the entire CP-OFDM symbol duration, potentially breaking subcarrier orthogonality after passing through the channel if the CP is not matched. To enable such a mechanism, OOK has to be generated independently and a modification to the existing OFDM-based OOK generation block is needed. Additionally, a block-wise time shift can be applied to minimize CP mismatch. Analytical discussions and simulations confirm that the resulting interference on legacy subcarriers remains controlled, while OOK detection performance significantly improves, albeit with trade-offs that should be carefully considered in practical deployments.

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arXiv2026

HAP-Centric Flying Ad-Hoc Networks With Cell-Free Non-Terrestrial Connectivity

High-altitude platforms (HAPs) are key enablers of next-generation non-terrestrial networks (NTNs), offering wide coverage, long endurance, and rapid deployment. Despite these advantages, current NTN designs remain satellite-centric and rely on terrestrial cellular assumptions, limiting flexibility and scalability. To overcome these limitations, this article proposes a multi-layer HAP-centric flying ad-hoc network (FANET). In this framework, HAPs are integrated with distributed uncrewed aerial vehicles (UAVs) to form a standalone, cell-free (CF) non-terrestrial system capable of autonomous operation. The layered architecture consists of an inter-HAP ad-hoc layer, a HAP-to-UAV cooperative layer, and a UAV-to-ground access layer, collectively enabling aerial connectivity, adaptive coverage, and interference-aware user access. Unique challenges for each layer are analyzed, including inter-HAP connectivity, FANET co-existence with terrestrial networks (TNs), and user access under heterogeneous conditions. Moreover, the article introduces enabling strategies such as fast beam alignment for high data rate connectivity, uncoordinated FANET/TN co-existence, and user localization and environment classification. Validated by three case studies, the discussion also outlines standardization pathways. The results highlight HAP-centric FANETs as a foundation for resilient, scalable, and application-oriented 6G NTN deployments.

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arXiv2026

Scalable Multi-BD Access for OFDM-Based Symbiotic Backscatter Communications

We propose an interference-free multi-backscatter device (BD) access scheme for orthogonal frequency division multiplexing (OFDM)-based symbiotic backscatter communication (SBC). Specifically, we introduce an orthogonal frequency-code spread (OFC) scheme in which the transmitter reserves empty subcarriers for BDs to incorporate their information through frequency shifting, which effectively suppresses direct-link interference (DLI). Since enabling simultaneous access over shared subcarriers introduces inter-backscatter device interference (IBDI), a blockwise orthogonal code-spread is applied to assign each BD to a unique codeword, thereby mitigating IBDI with orthogonal transmissions of all BD signals in the code domain. We develop a non-coherent energy detector whose performance is analyzed in terms of average probability of missed detection (PMD) and sum-rate under Rayleigh fading. The simulation results validate the OFC scheme and demonstrate low PMD values and high spectral efficiency compared to conventional code-assisted multiple-BD access schemes.

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arXiv2026

Towards Standardizing Affine Frequency Division Multiplexing (AFDM) for Future Wireless Networks

Affine frequency division multiplexing~(AFDM) has emerged as a compelling waveform candidate for future wireless networks, owing to its strong resilience to doubly selective channels and its ability to enable the seamless integration of communication and sensing functionalities. Against this context, this article provides a systematic study of AFDM from a standardization perspective. We first introduce the principles of AFDM and discuss the major considerations involved in waveform standardization. We then examine the backwards compatibility of AFDM with 4G/5G multi-numerology frameworks and their anticipated evolution, frequency-modulated continuous-wave (FMCW) radar waveforms, and long-range (LoRa) modulation, demonstrating that AFDM can be incorporated into legacy processing chains with limited modification. Key standardization-critical capabilities are further discussed, including multiple-antenna and multi-user support, and peak-to-average power ratio (PAPR). Finally, we investigate the potential of AFDM in several emerging scenarios, including non-terrestrial networks~(NTN), integrated sensing and communications (ISAC), vehicle-to-everything (V2X), and underwater acoustic (UWA) communications, whereby severe delay-Doppler dispersion places stringent demands on waveform robustness. Through these explorations, it is shown that that AFDM represents a timely and compelling technology for future wireless networks.

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arXiv2026

Health-Conditioned Vision-Language-Action Models for Malfunction-Aware Robot Control

Research on Vision Language Action (VLA) models has been increasing rapidly in recent years. Although some of them focus on detecting, preventing, and recovering from task failures, they usually don't deal with adapting to robot's physical failures. In real-life scenarios, most robots face physical degradations in various ways such as joint degradation, actuator failure, or weak gripper. We introduce malfunction-aware (health-conditioned) VLA that takes a health vector as an input that gives information about robots' joints' operation angle and torque capability, and adapts its predictions to complete the tasks with the degraded joints. To achieve this, we inject a Health Projector module to the VLA-Adapter architecture and train it on malfunction robot data we collected on the LIBERO environment [1]. We collect 128 teleoperated episodes on Libero-Spatial tasks. Our results show that, with a very lightweight addition, the model can learn to operate successfully with different configurations of degraded joints which the default pretrained VLA-Adapter's Libero-Spatial-Pro model cannot. The code and dataset will be available soon at https://github.com/h-arslan/health-aware-vla

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arXiv2026

Phase-Shifted Pilot Design for NOMA-Empowered Uplink ISAC Systems

The deployment of multiple transmitters (TXs) in integrated sensing and communication (ISAC) networks necessitates efficient resource sharing to overcome the limitations of orthogonal allocation. While conventional interleaved (CI) pilots combined with non-orthogonal multiple access (NOMA) improve spectral efficiency (SE), they inherently compromise sensing resolution due to spectral sparsity, rendering the CI nulling (CIN) extension a strictly limited remedy. This paper proposes a phase-shifted (PS) pilot design and its novel PS nulling (PSN) variant to integrate a communication TX (CTX) over the PS-ISAC framework. The PSN variant strategically punctures sensing signals at CTX pilot locations to preserve initial channel estimates, enabling a dense data overlay. To resolve the resulting multi-TX interference, joint iterative interference cancellation (IIC) is adapted for non-nulling configurations and sequential IIC is adapted for nulling variants, optimizing for both detection robustness and convergence speed. Simulation results across varying STX densities and modulation orders demonstrate that the phase-shifted frameworks maintain sensing integrity while explicitly reducing receiver-side computational complexities by $18.8\%$ and $21.0\%$ against their respective interleaved baselines.

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arXiv2026

Chirp-Based Multi-Device Ambient Backscatter Communication and Sensing Enabled by OFDM-AFDM Symbiotic Radio

This paper presents a novel symbiotic radio system for integrated sensing and backscatter communication (ISABC) technique that enables signal-domain interference-free coexistence of the primary communication signal and the backscatter communication (BC) signal within the same spectrum. The proposed system design allows simultaneous backscatter devices (BDs) sensing and data transmission without mutual interference by exploiting waveform-domain orthogonality between orthogonal frequency division multiplexing (OFDM) and affine frequency domain multiplexing (AFDM) signals. Specifically, a chirp-based AFDM waveform is adopted due to its inherent processing gain, which enhances the detectability and reliability of the weak backscatter signal while simultaneously supporting high-resolution sensing. Unlike conventional methods that attempt to suppress direct-link interference (DLI), this approach embeds the backscatter transmission within the affine domain while maintaining reliable OFDM-based primary communication. Furthermore, by assigning distinct affine-domain shifts to each backscatter device, the proposed framework inherently suppresses inter-backscatter device interference (IBDI). Comprehensive simulation results demonstrate that the proposed coexistence scheme effectively mitigates interference without affecting the error rate of the primary link and improves the miss-detection probability performance of the BC, making it a promising candidate for future low-power and interferenceresilient systems.

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arXiv2026

A Unified Multicarrier Waveform Framework for Next-generation Wireless Networks: Principles, Performance, and Challenges

Next-generation wireless networks require enhanced flexibility, efficiency, and reliability in physical layer waveform design to address the challenges posed by heterogeneous channel conditions and stringent quality-of-service demands. To this end, this paper proposes a unified multicarrier waveform framework that provides a systematic characterization and practical implementation guidelines to facilitate waveform selection for the sixth-generation (6G) mobile networks and beyond. We commence by examining the design principles of the state-of-the-art waveforms, which are categorized into one-dimensional modulation waveforms (e.g., orthogonal frequency division multiplexing (OFDM) and affine frequency division multiplexing (AFDM)) and two-dimensional modulation waveforms (e.g., orthogonal time frequency space (OTFS)). Their inherent resilience against various channel-induced interference is further studied, revealing their distinct suitability in diverse channel conditions. Furthermore, an in-depth performance analysis is presented by comparing their key performance indicators (KPIs), followed by an extensive exploration of these advanced waveforms in various applications. Consequently, this work aims to serve as a pivotal reference for waveform adoption in future 6G standardization and network deployment.

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arXiv2025

Adaptive Phase-Shifted Pilot Design for Uplink Multiple Access in ISAC Systems

In uplink integrated sensing and communication (ISAC) systems, pilot signal design is crucial for enabling accurate channel estimation and reliable radar sensing. In orthogonal frequency-division multiple access (OFDMA)-based frameworks, conventional pilot allocation schemes face a trade-off between spectral efficiency (SE) and sensing performance. Interleaved pilots improve user equipment (UE) multiplexing through sparse allocation but reduce the maximum unambiguous range. Conversely, orthogonal block-based pilots reduce range ambiguity but degrade sensing resolution due to limited delay granularity. To address this trade-off, the phase-shifted ISAC (PS-ISAC) scheme was recently proposed for uplink multiple access in ISAC systems. However, PS-ISAC suffers from spectral inefficiency due to the fixed cyclic prefix (CP) constraints. To overcome these limitations, we propose adaptive phase-shifted-ISAC (APS-ISAC), an enhanced pilot scheme that employs an overlapped block-pilot structure with UE-specific phase shifts determined by maximum excess delay of each UE. This design enables UEs to share the same time-frequency resources while preserving separable and contiguous channel impulse responses (CIRs) at the base station (BS). Simulation results show that APS-ISAC significantly outperforms conventional pilot allocation methods in terms of SE, approximately doubling the number of multiplexed UEs. It also achieves lower mean square error (MSE) under power constraints with reduced complexity. Furthermore, it yields maximum range resolution and unambiguous sensing performance. These results establish APS-ISAC as a scalable, spectrally efficient, ambiguity-resilient, and low-complexity pilot design paradigm for future uplink ISAC systems.

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arXiv2025

Efficient handover based on Near-field and Far-field RIS for seamless connectivity

Reconfigurable Intelligent Surfaces (RIS) is becoming a transformative technology for the upcoming 6G communication networks, providing a way for smartly maneuvering the electromagnetic waves to enhance coverage and connectivity. This paper presents an efficient handover (HO) management scheme leveraging RIS in the Fresnel region i.e., in both the near-field (NF) and far-field (FF) regions to reduce signaling overhead and optimize mobility management. For this, we analyzed the signal strength variations in the considered RIS-aided networks, considering the radiative NF and FF regions, and derive the probability density function (PDF) of the RIS-UE distance in the NF region to quantify RIS reflection gains along the user equipment (UE) trajectory. We propose a new HO algorithm incorporating several HO categories like hard handover (HHO), soft handover (SHO), RIS-aided cell breathing (RIS-CB), and RIS-aided ping-pong avoidance (RIS-PP) strategies. The proposed algorithm uses bit error rate (BER) as a key parameter to predict the minimization of unnecessary HOs by using RIS-aided pathways to retain connectivity with the serving base station (BS), which minimizes the requirement for frequent target BS searching and ultimately optimizes the HO. By restricting measurement reports and HO requests, the suggested method improves spectrum efficiency (SE) and energy efficiency (EE), especially in crowded cellular networks. Numerical results highlight significant reductions in HO rates and signaling load, ensuring seamless connectivity and improved quality of service (QoS) in 6G systems.

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arXiv2025

Unified Interference-Aware Water-Filling for QoS-Constrained Communication, Sensing, and JRC

Water-filling (WF) algorithms are pivotal in maximizing capacity and spectral efficiency in multiple-input and multiple-output (MIMO) systems. However, traditional WF approaches cater solely to communication requirements, neglecting the emerging heterogeneity of 6G, including sensing and joint radar-communication (JRC). As these diverse demands grow in importance and have different Quality of Service (QoS) constraints, traditional WF becomes inadequate. Therefore, in this paper, we propose a unified interference-aware and QoS-constrained WF algorithm for systems with communication, sensing, and JRC. The proposed algorithm enables power allocation for multi-user MIMO systems, effectively addressing interference and balancing the support for heterogeneous user requirements.

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arXiv2025

Scalable Association of Users in CF-mMIMO: A Synergy of Communication, Sensing, and JCAS

Cell-free massive multiple-input multiple-output (CF-mMIMO) is a key enabler for the sixth generation (6G) networks, offering unprecedented spectral efficiency and ubiquitous coverage. In CF-mMIMO systems, the association of user equipments (UEs) to access points (APs) is a critical challenge, as it directly impacts network scalability, interference management, and overall system performance. Conventional association methods primarily focus on optimizing communication performance. However, with the emergence of sensing and joint communication and sensing (JCAS) requirements, conventional approaches become insufficient. To address this challenge, we propose a scalable user association (SUA) scheme for CF-mMIMO networks, considering heterogeneous UE requirements. Designed to enhance the performance of both sensing and communication, the proposed SUA scheme aims to ensure network scalability. This is achieved by dynamically assigning APs to UEs based on their specific service requirements (communication, sensing, or JCAS), while considering link quality, interference mitigation, and network-related constraints. Specifically, the proposed SUA scheme employs AP masking, link prioritization, and an optimization-based association mechanism to select the most suitable APs for each UE. Simulations show that, compared to conventional CF-mMIMO methods, the proposed SUA scheme significantly reduces interference and computational runtime, while improving the symbol error rate for communication and the probability of detection for sensing.

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arXiv2025

A Novel Approach to Secure RSMA Networks

This letter introduces a novel data-dependent interleaving technique designed to enhance the security of rate-splitting multiple access (RSMA) networks by protecting the common stream from eavesdropping threats. Specifically, we exploit the RSMA structure by interleaving the common bits of each user based on a sequence derived from their private bits. By decoding its private stream, the legitimate receiver reconstructs the interleaving sequence set by the transmitter and successfully de-interleaves the common stream. Therefore, the common part is successfully prevented from being intercepted by an eavesdropper who is unable to deduce the dynamic changing interleaving permutations. To ensure dynamic interleaving sequences, a private bit selection approach that balances the trade-off between security and system efficiency is proposed. Simulation findings confirm the effectiveness of the suggested method, showing notable security improvements while maintaining robust overall system reliability.

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arXiv2025

A Novel Splitter Design for RSMA Networks

Rate splitting multiple access (RSMA) has firmly established itself as a powerful methodology for multiple access, interference management, and multi-user strategy for next-generation communication systems. In this paper, we propose a novel channel-dependent splitter design for multi-carrier RSMA systems, aimed at improving reliability performance. Specifically, the proposed splitter leverages channel state information and the inherent structure of RSMA to intelligently replicate segments of the private stream data that are likely to encounter deep-faded subchannels into the common stream. Thus, the reliability is enhanced within the same transmission slot, minimizing the need for frequent retransmissions and thereby reducing latency. To assess the effectiveness of our approach, we conduct comprehensive evaluations using key performance metrics, including achievable sum rate, average packet delay, and bit error rate (BER), under both perfect and imperfect channel estimation scenarios.

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