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International Conference on Space Optics (ICSO)

Modelling concurrent IM/DD key distribution and data transmission over an optical LEO-to-ground link

Michał Jachura, Mikołaj Lasota, Piotr Leszek Kolenderski, Konrad Banaszek

Published Jul 12, 2023• 6 citations
Publisher

Abstract

We propose and analyze numerically concurrent intensity modulation/direct detection (IM/DD) optical key distribution (OKD) combined with conventional data transmission over a LEO-to-ground optical link. The idea is validated by numerical simulations which indicate that in a realistic scenario it is possible to generate simultaneously up to 200 Mbits of a secret key and downlink transmit up to 140 Gbits of data during a single nearly-zenithal pass of a LEO satellite. Such lengths of the secret key are three orders of magnitude higher in comparison to typical quantum key distribution systems, although they are generated under different security assumptions. The simplicity and robustness of IM/DD OKD protocols could make them an attractive and cost-effective option to provide security in the physical layer of space communication systems.

DOI
10.1117/12.2689278

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We propose and analyze numerically concurrent intensity modulation/direct detection (IM/DD) optical key distribution (OKD) combined with conventional data transmission over a LEO-to-ground optical link. The idea is validated by numerical simulations which indicate that in a realistic scenario it is possible to generate simultaneously up to 200 Mbits of a secret key and downlink transmit up to 140 Gbits of data during a single nearly-zenithal pass of a LEO satellite. Such lengths of the secret key are three orders of magnitude higher in comparison to typical quantum key distribution systems, although they are generated under different security assumptions. The simplicity and robustness of IM/DD OKD protocols could make them an attractive and cost-effective option to provide security in the physical layer of space communication systems.
In this study, we analyze the secret key capacity of intensity modulation/direct detection optical key distribution (IM/DD OKD) for a free-space optical (FSO) link between a low-Earth orbit satellite and an optical ground station. Focusing on downlink communication, we account for atmospheric turbulence, which causes random variations in the transmittance of the FSO channel. We implement an atmospheric channel model that accounts for absorption and scattering, geometric losses, pointing errors, and intensity fluctuations. The secret key capacity is quantified under different noise scenarios and reconciliation code efficiencies, assuming a hard decoding scheme. The performance of the IM/DD OKD protocol is compared under direct and reverse reconciliation regimes. Additionally, we examine the impact of weak and strong wind on the strength of atmospheric turbulence, leading to different results of the secret key capacity. Furthermore, we analyze the characteristics of error distributions that arise from protocol optimization. Our results provide insights into optimizing IM/DD OKD protocols for varying atmospheric conditions.
Optical key distribution (OKD) protects the physical layer of communication links by taking advantage of the inherent noise present in the photodetection process. It allows for efficient generation of a shared random key between two distant users that is secure against passive eavesdropping and can be subsequently used for cryptographic purposes. Moreover, it can be straightforwardly implemented over standard intensity modulation and direct detection links, making it an attractive alternative to quantum key distribution. Here we present a comprehensive security analysis against more powerful eavesdroppers possessing the ability to perform either coherent detection, or even quantum-optimal measurements on the intercepted transmission.
Australia and New Zealand are currently developing an optical communications ground station network to test technologies for optical and quantum communications that will enable faster data transfers and security during information exchange. A goal of the network is to implement Gaussian modulated continuous variable quantum key distribution (GM-CVQKD) between LEO satellites and optical ground stations. This protocol offers quantum cryptography, providing information-theoretic security through quantum mechanics and is compatible with hardware needed for coherent optical communications that satellites are starting to use to combat the crowded spectrum from increased traffic in radio-frequency communications. GM-CVQKD avoids the need for the specialised and complex equipment required by discrete variable QKD to achieve secret key exchange, as it can use cost-effective commercially available off-the-shelf hardware. We present a study to determine the feasibility of a proposed implementation of GM-CVQKD. As a first step, a model of the quantum signal downlink propagation from a LEO satellite to an optical ground station (OGS) has been created. This was used to explore adverse effects from varying atmospheric conditions, the Doppler shift owing to the relative motion between a LEO satellite and an OGS, and the presence of an attack/hack from an eavesdropper. The results show that suitable atmospheric conditions (negligible turbulence and higher visibility) produce a larger positive secret key rate (SKR) while less favourable conditions (high turbulence and lower visibility) significantly decrease the achievable SKRs. The effects of Doppler shift are negligible and still permit a positive SKR. The protocol allows the presence of an eavesdropper to be detected so that communications can be terminated. The results indicate that GM-CVQKD is a viable protocol for space-based QKD and is a stepping-stone towards a global quantum network.
We report the experimental distribution of a cryptographic key based on binary intensity-modulation/direct-detection (IM/DD) transmission of an optical signal. The implemented secret key agreement protocol ensures security against passive wiretap eavesdropping, notably in the regime when the eavesdropper has an advantage in the intercepted signal strength over the legitimate recipient. Operating at a 1 GBaud symbol rate, the experimental fiber optic testbed yielded secret key rates between 43.5 Mbps at 61.1 km and 640 kbps at 81.6 km of the effective link length, with a 9% wiretap installed at the transmitter. The protocol performance was analyzed using a theoretical model incorporating transmitter noise and non-ideal key reconciliation efficiency. The presented results validate IM/DD optical key distribution as an efficient low-complexity solution for protecting optical communication systems against the predominant physical-layer threat of wiretap eavesdropping.
Abstract Establishing secure communication links at a global scale is a major potential application of quantum information science but also extremely challenging for the underlying technology. Although milestone experiments using satellite-to-ground links and exploiting singe-photon encoding for implementing quantum key distribution have shown recently that this goal is achievable, it is still necessary to further investigate practical solutions compatible with classical optical communication systems. Here, we examine the feasibility of establishing secret keys in a satellite-to-ground downlink configuration using continuous-variable encoding, which can be implemented using standard telecommunication components certified for space environment and able to operate at high symbol rates. Considering a realistic channel model and state-of-the-art technology, and exploiting an orbit subdivision technique for mitigating fluctuations in the transmission efficiency, we find positive secret key rates for a low-Earth-orbit scenario, whereas finite-size effects can be a limiting factor for higher orbits. Our analysis determines regions of values for important experimental parameters where secret key exchange is possible and can be used as a guideline for experimental efforts in this direction.
Intensity modulation/direct detection (IM/DD) optical key distribution (OKD) is a method to generate a secret key whose security against passive eavesdropping is guaranteed by the shot noise inherent to the photodetection process. Here the effects of intensity-dependent symbol shape distortion on the IM/DD OKD security are investigated assuming that the eavesdropper can implement temporal mode demultiplexing using e.g. the quantum pulse gating technique. The quantitative analysis includes key generation based on either hard- or soft-decoding of the detected signal as well as the impact of excess detection noise. A simple, rule-of-thumb relation between the severity of the symbol shape distortion and the signal strength required to ensure key security is presented.