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IEEE World Forum on Internet of Things (WF-IoT)

Quantum Leaps in the Sky: Composable XOR–Relay QKD over HAPs for Next-Gen 6G Networks

Hasan Abbas Al-Mohammed, Elias E. Yaacoub

Published Oct 27, 2025• 1 citation
Publisher

Abstract

Quantum key distribution (QKD) via free-space optical (FSO) links suffers from severe turbulence losses and limited range in satellite-to-ground scenarios. We propose a two-hop XOR-relay architecture employing a high-altitude platform (HAP) to segment the link, reducing both geometric and atmospheric attenuation. Our simulation framework incorporates beam wandering, finite-key constraints, and realistic link geometry. Simulation results show up to a 55 times transmittance gain, 18% energy-per-bit savings, and a 64.2 kb/s secret key rate with under 3 ms latency. We formulate a constrained min–max optimization to minimize worst-case delay while meeting key rate, quantum bit error rate (QBER), and protocol overhead bounds. The XOR relay ensures composable secrecy with negligible information leakage, confirming the feasibility of stratospheric relays for low-latency, quantum-secure backhaul in future 6G non-terrestrial networks.

DOI
10.1109/wf-iot64238.2025.11270581

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International Conference on Advanced Technologies for Communications•Oct 8, 2020•0 citations
Quantum key distribution (QKD) via free-space optical (FSO) links suffers from severe turbulence losses and limited range in satellite-to-ground scenarios. We propose a two-hop XOR-relay architecture employing a high-altitude platform (HAP) to segment the link, reducing both geometric and atmospheric attenuation. Our simulation framework incorporates beam wandering, finite-key constraints, and realistic link geometry. Simulation results show up to a 55 times transmittance gain, 18% energy-per-bit savings, and a 64.2 kb/s secret key rate with under 3 ms latency. We formulate a constrained min–max optimization to minimize worst-case delay while meeting key rate, quantum bit error rate (QBER), and protocol overhead bounds. The XOR relay ensures composable secrecy with negligible information leakage, confirming the feasibility of stratospheric relays for low-latency, quantum-secure backhaul in future 6G non-terrestrial networks.
The emergence of massive, latency-sensitive Internet of Things (IoT) and high-mobility services in 6G networks amplifies long-standing limits of radio systems (bandwidth, interference, Doppler fragility) and exposes classical cryptography to quantum threats. This dissertation presents a framework that integrates Quantum Key Distribution (QKD) with Free-Space Optics (FSO) to provide quantum-resilient connectivity, while exploring applications to terrestrial Internet of Things (IoT), high-speed trains (HST), ultra-high-speed trains (UHST), and non-terrestrial networks (NTN) with high-altitude platforms (HAPs). On the physical layer, we build weather- and turbulence-aware FSO models (attenuation, beam propagation, pointing/beam-wander, scintillation) and derive fundamental link budgets. A core innovation for mobility is an optimization framework that maximizes the distance between base stations along a rail corridor subject to stringent Gbps-class data rate and availability constraints. This front-end optimization directly informs the subsequent design of robust FSO architectures and handover mechanisms for HST/UHST. The resulting system-achieved through divergence control, advanced acquisition-tracking-pointing (ATP), and geometry-aware handover zones-significantly reduces infrastructure density while sustaining high-capacity links. For IoT security, we propose two practical QKD delivery modes (rail-side and onboard) and quantify energy per secret bit and realistic key provisioning. For extended reach, we introduce HAP-assisted QKD via a standards-aligned trusted relay and a composable finite-key decoy-state pipeline using the Entropy Accumulation Theorem. A key contribution is a symmetry-aware altitude optimizer that selects the HAP height to bound the transmittance gap, complete with concavity proofs and implementable solvers. We show that-under realistic HAP turbulence-advanced measurement-device-independent (MDI-), twin-field (TF-), and Gaussian-modulated coherent-state continuous-variable (GM-CV-) QKD protocols all collapse, whereas the simpler per-hop BB84 with decoys remains feasible under finite-key constraints. Results include rate/feasibility maps, robust operating rules, and integrated "security dashboards," complemented by ML-based attacker detection. The framework adheres to tight size-weight-power (SWaP) constraints by prioritizing geometric control and efficient solvers over hardware escalation. Collectively, this dissertation bridges physics, security, and control into a deployable blueprint for quantum-secure communications, advancing FSO for HSTs/UHSTs through optimized base-station placement and highvelocity handovers to minimize infrastructure costs.
This paper studies the design and performance analysis of relay-assisted satellite free-space optics (FSO) quantum key distribution (QKD) systems for secure vehicular networks. High-altitude platforms (HAPs) equipped with optical amplify-and-forward nodes are used as relay stations. Secrecy performances in terms of quantum bit error rate and ergodic secret-key rate are analytically investigated under the effects of transceiver misalignment, receiver's velocity variation, receiver noises, and atmospheric turbulence conditions. Based on the analyzed results, the design criteria for the legitimate user are determined so that the security of the considered system could be guaranteed.
This paper proposes a high-altitude platform (HAP)-aided relaying satellite free-space optics (FSO) quantumkey distribution (QKD) system for the secure vehicular networks supporting unmanned vehicles (UAV) and regular vehicles. The subcarrier intensity modulation (SIM) binary phase shift keying (BPSK) and dual-threshold/direct detection (DT/DD) receiver are employed. The relaying HAP is equipped with optical amplifyand-forward (OAF) relaying node. We analytically derive the ergodic secret-key rate of the proposed system under the impact of atmospheric turbulence and other noises. Gaussian beam model is used to evaluate the impact of geometrical spreading on the signal received by the legitimate users and the probability of being eavesdropped. The numerical results confirm the feasibility of the proposed system.
To explore the pathways toward establishing a global quantum network, we investigate several link architectures for transatlantic quantum entanglement distribution over a 6,500 km ground distance. We define free-space link configurations involving satellites and stratospheric high altitude platforms (HAPs), using today's technology and without relying on quantum memories and repeaters. Considering link budgets, space radiation, orbital characteristics, and system complexity we find that a hybrid architecture consisting of an entangled photon source located on a low Earth orbit (LEO) satellite supported by two passive optical relays located on HAPs provides the overall highest entanglement distribution rate. In addition, the satellite HAP architecture offers practical advantages in payload design and launch requirements, and the ability to lower the weather-related link interruptions assuming some maneuverability of HAPs. Overall, this hybrid configuration yields on the order of 5X10^6 secure key bits per year using 30 cm aperture ground receivers, nearly two orders of magnitude higher than achievable with a single MEO satellite and 1 m aperture ground receivers. Our results highlight the major benefits of hybrid satellite HAP architectures by reducing system complexity while enabling scalable and more accessible long-range quantum communication networks.
Quantum key distribution (QKD) is one of the most mature quantum technologies and can provide quantum-safe security in future communication networks. Since QKD in fiber is limited to a range of few hundred kilometers, one approach to bridge continental scale distances may be the use of high altitude pseudo satellites (HAPS) as mobile trusted nodes in the stratosphere. In parallel, free-space laser communication for high rate data transmission has been a subject of research and development for several decades and its commercialization is progressing rapidly. Important synergies exist between classical free-space communication and QKD systems since the quantum states are often implemented using the same degrees of freedom such as polarization or field amplitude and phase. These synergies can be used to benefit from the progress in classical free-space laser communication in QKD applications. In this paper, the use case of QKD in a stratospheric environment is described wherein HAPS may serve as relay station of secret keys and encrypted data. The mission scenario and HAPS capabilities are analyzed to derive special requirements on the stratospheric laser terminal, the link geometry and the ground segment with respect to a feasibility demonstration. To obtain a flexible and compatible system, discrete variable and continuous variable QKD protocols are considered to be implemented side by side in the HAPS payload. Depending on the system parameters, it can be beneficial to use the one or the other kind of protocol. Thus, a direct comparison of both in one and the same system is of scientific interest. Each of the protocols has particular requirements on coupling efficiency and implementation. Link budget calculations are performed to analyze possible distances, key rates and data transmission rates for the different schemes. In case of the QKD system, the mean coupling efficiency is of main interest, i.e. signal fluctuations arising from atmospheric turbulence must be taken into account in the security proof, but the buffered key generation relaxes real-time requirements. This is different to classical communications, where the corresponding fading loss must be assessed. A system architecture is presented that comprises the optical aircraft terminal, the optical ground terminal and the most important subsystems that enable implementation of the considered QKD protocols. The aircraft terminal is interfaced with the dedicated quantum transmitter module (Alice) and the ground station with the dedicated quantum receiver module (Bob). The optical interfaces are SMF couplings which put high requirements on the receiving optics, in particular the need for wave-front correction with adaptive optics. The findings of the system study are reviewed and necessary next steps pointed out.
This paper proposes to use high-altitude platform (HAP)-based relaying and link-layer automatic repeat request (ARQ) techniques to improve the performance of the satellite quantum key distribution (QKD) over free-space optics (FSO) systems. We also develop analytical frameworks based on two 3D Markov chain models allowing us to comprehensively analyze the performance of the proposed system in terms of key loss rate and delay outage rate. Our performance analysis takes into account the physical layer impairments induced by FSO channel and receiver noise. Numerical results quantitatively demonstrate that the satellite FSO/QKD system using HAP-based relaying and link-layer ARQ techniques can offer considerable performance improvement over the conventional ones.