ADVANCING SECURE COMMUNICATIONS IN HIGH-MOBILITY ENVIRONMENTS: INTEGRATING QUANTUM KEY DISTRIBUTION, FREE SPACE OPTICS, AND HIGH-ALTITUDE PLATFORMS FOR ENHANCED IOT NETWORKS

Abstract

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.