1.
Trinh, Phuc V.; Sugiura, Shinya; Xu, Chao; Hanzo, Lajos
Optical RISs Improve the Secret Key Rate of Free-Space QKD in HAP-to-UAV Scenarios Journal Article
In: IEEE Journal on Selected Areas in Communications, vol. 43, no. 8, pp. 2747–2764, 2025, ISSN: 1558-0008.
Abstract | Links | BibTeX | Tags: Atmospheric modeling, Drones, Fluctuations, Free-space optics (FSO), Global Positioning System, high-altitude platforms (HAPs), Laser beams, low-altitude platforms (LAPs), Optical beams, Optical reflection, Power distribution, quantum key distribution (QKD), reconfigurable intelligent surface (RIS), Reconfigurable Intelligent Surfaces, Satellites
@article{trinh_optical_2025,
title = {Optical RISs Improve the Secret Key Rate of Free-Space QKD in HAP-to-UAV Scenarios},
author = {Phuc V. Trinh and Shinya Sugiura and Chao Xu and Lajos Hanzo},
url = {https://ieeexplore.ieee.org/document/10993364},
doi = {10.1109/JSAC.2025.3568050},
issn = {1558-0008},
year = {2025},
date = {2025-08-01},
urldate = {2025-10-08},
journal = {IEEE Journal on Selected Areas in Communications},
volume = {43},
number = {8},
pages = {2747–2764},
abstract = {Large optical reconfigurable intelligent surfaces (ORISs) are proposed for employment on building rooftops to facilitate free-space quantum key distribution (QKD) between high-altitude 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.},
keywords = {Atmospheric modeling, Drones, Fluctuations, Free-space optics (FSO), Global Positioning System, high-altitude platforms (HAPs), Laser beams, low-altitude platforms (LAPs), Optical beams, Optical reflection, Power distribution, quantum key distribution (QKD), reconfigurable intelligent surface (RIS), Reconfigurable Intelligent Surfaces, Satellites},
pubstate = {published},
tppubtype = {article}
}
Large optical reconfigurable intelligent surfaces (ORISs) are proposed for employment on building rooftops to facilitate free-space quantum key distribution (QKD) between high-altitude 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.
2.
Safi, Hossein; Ihsan, Asim; Tavakkolnia, Iman; Haas, Harald
Optimizing Spatial Acquisition in Inter-Satellite FSO Links Under Beam jitter Miscellaneous
2024.
Abstract | Links | BibTeX | Tags: Free-space optics (FSO), LRDC
@misc{safi_optimizing_2024,
title = {Optimizing Spatial Acquisition in Inter-Satellite FSO Links Under Beam jitter},
author = {Hossein Safi and Asim Ihsan and Iman Tavakkolnia and Harald Haas},
url = {https://www.repository.cam.ac.uk/handle/1810/374012},
doi = {10.17863/CAM.112232},
year = {2024},
date = {2024-12-01},
urldate = {2024-10-30},
publisher = {Apollo - University of Cambridge Repository},
abstract = {The acquisition system is a key element in intersatellite free-space optics (FSO) links. Because satellite vibrations cause the scanning beam to jitter, making the acquisition process both challenging and time-consuming, minimizing acquisition time is crucial for the rapid establishment of an FSO link. In this paper, we present an optimization framework aimed at minimizing acquisition time while maintaining a predefined failure probability threshold. The optimization problem considers the total available power, the amount of overlap between the scanning beams (overlapping factor), and the beamwidth of the scanning beam. Additionally, we incorporate a weighting parameter to account for the varying power costs associated with adjusting the beamwidth and the overlapping factor. To solve this non-convex and NP-hard problem, we utilize an iterative successive convex approximation-based algorithm. When the cost of increasing laser power is high, the results show that as jitter variance increases, the optimal overlapping factor must also increase to maintain the acquisition success probability. Simultaneously, the beamwidth slightly decreases to adhere to power constraints. On the other hand, when the cost for increasing laser power is low, the optimal strategy involves maximizing the beamwidth and increasing the overlapping factor to mitigate jitter effects.},
keywords = {Free-space optics (FSO), LRDC},
pubstate = {published},
tppubtype = {misc}
}
The acquisition system is a key element in intersatellite free-space optics (FSO) links. Because satellite vibrations cause the scanning beam to jitter, making the acquisition process both challenging and time-consuming, minimizing acquisition time is crucial for the rapid establishment of an FSO link. In this paper, we present an optimization framework aimed at minimizing acquisition time while maintaining a predefined failure probability threshold. The optimization problem considers the total available power, the amount of overlap between the scanning beams (overlapping factor), and the beamwidth of the scanning beam. Additionally, we incorporate a weighting parameter to account for the varying power costs associated with adjusting the beamwidth and the overlapping factor. To solve this non-convex and NP-hard problem, we utilize an iterative successive convex approximation-based algorithm. When the cost of increasing laser power is high, the results show that as jitter variance increases, the optimal overlapping factor must also increase to maintain the acquisition success probability. Simultaneously, the beamwidth slightly decreases to adhere to power constraints. On the other hand, when the cost for increasing laser power is low, the optimal strategy involves maximizing the beamwidth and increasing the overlapping factor to mitigate jitter effects.