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Wireless Networks

Quantum key distribution over FSO channel using error reconciliation protocol

Bandana Mallick, Priyadarsan Parida, Chittaranjan Nayak, Pritam Keshari Sahoo, Gopinath Palai

Published Mar 8, 2023• 14 citations
Publisher
DOI
10.1007/s11276-023-03289-6

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Satellite-based free-space optics (FSO) quantum key distribution (QKD) systems have recently attracted significant research interest due to their potential to enable globally secured applications. However, the inherent uncertainty of FSO channels, caused by weather conditions and satellite mobility, induces severe fluctuations in quantum bit-error rate (QBER) between legitimate users. This makes designing an efficient key reconciliation, an essential step in the QKD post-processing, particularly challenging. In this work, we propose a key reconciliation scheme that combines protograph rate-compatible (RC) low-density parity-check (LDPC) codes with a syndrome-based error estimation method. The proposed error estimation method reduces the number of communication rounds without requiring additional information disclosure. Furthermore, to our best knowledge, an analytical framework is first developed to evaluate end-to-end secret-key throughput (SKT), accounting for the impact of imperfect error estimation and dynamic FSO channel conditions. Numerical results demonstrate that the proposed scheme consistently outperforms conventional blind reconciliation under diverse FSO channel conditions and provide practical guidelines for system parameter selection. Finally, we validate the proposed framework through a case study that incorporates data from a Starlink low-Earth orbit (LEO) satellite and moving ground vehicles.
A significant breakthrough in space-based quantum key distribution (QKD) inaugurated by the Micius satellite has brought us closer to the era of global QKD networks. An essential step in the post-processing QKD is key reconciliation (KR), which eliminates the mismatch in the raw keys between two legitimate users. However, the fluctuation in the quantum bit-error rate (QBER) caused by the free-space optical (FSO) turbulence channels poses various challenges for the KR design. This paper addresses the design of blind KR schemes for FSO-based low Earth orbit (LEO) satellite QKD systems, which allow operating without a prior QBER estimation. Specifically, we present a novel blind KR scheme with a protograph low-density parity check (LDPC) code extension. The proposed LDPC structure is constructed by gradually extending and exhaustively searching among all possible solutions. The proposed design is evaluated in terms of the final key rate performance over the QKD systems using a dual-threshold/direct detection (DT/DD) scheme. Simulation results confirm the effectiveness of our proposed design compared to the state-of-the-art design in different turbulence channel conditions.
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.