Security issues of free-space optical (FSO) communications are discussed. Based on a subcarrier intensity-modulated air-to-ground FSO system model, we first analyze the coherence time of the air-to-ground FSO link and show that under practical assumptions, scintillation reciprocity holds in the FSO communication system. A private secret-key-based cryptosystem with key management is introduced to enhance FSO security, and a key agreement approach is proposed based on statistics of the random atmospheric-turbulence-induced fading channel measurements. The secret key rate of the key agreement scheme is investigated for the gamma–gamma turbulence model. Practical key agreement protocols based on bidirectional channel identification are designed for different FSO communication scenarios. Our numerical results reveal that the key rate is not sensitive to the strength of the atmospheric turbulence; the per-symbol signal-to-noise ratio and the training sequence length are the dominating factors.
Security issues of free-space optical (FSO) communications are discussed. Based on a subcarrier intensity-modulated air-to-ground FSO system model, we first analyze the coherence time of the air-to-ground FSO link and show that under practical assumptions, scintillation reciprocity holds in the FSO communication system. A private secret-key-based cryptosystem with key management is introduced to enhance FSO security, and a key agreement approach is proposed based on statistics of the random atmospheric-turbulence-induced fading channel measurements. The secret key rate of the key agreement scheme is investigated for the gamma–gamma turbulence model. Practical key agreement protocols based on bidirectional channel identification are designed for different FSO communication scenarios. Our numerical results reveal that the key rate is not sensitive to the strength of the atmospheric turbulence; the per-symbol signal-to-noise ratio and the training sequence length are the dominating factors.
Security issues of terrestrial free-space optical (FSO) communications over strong atmospheric turbulence are investigated. Based on a subcarrier intensity modulated FSO system model, we first study the scintillation reciprocity of the system from a coherence time perspective. A private secret key-based cryptosystem with key management is introduced to enhance FSO security, and a key agreement approach is proposed based on statistics of the random atmospheric turbulence-induced fading channel measurements. Secret key rate of the key agreement scheme is studied for the strong turbulence channel characterized by the K-distribution. Practical key agreement protocol is designed based on channel identification.
Free-space optical (FSO) communications are becoming promising schemes for high-capacity wireless links due to their plentiful characteristics originated from higher carrier frequency. These characteristics also yield a greater security advantage over radio frequency counterparts: the physical ability of a wiretapper is reasonably restricted due to the high directionality of communication beam and the line-of-sight configuration of the link. Secret key agreement over FSO links (FSO-SKA) employs this security advantage as well as the post-processing over an authenticated public channel to establish an information-theoretic secure key which cannot be broken even with unbounded computer resources. In the previous works, the authors demonstrated the full-field implementations of FSO-SKA with a 7.8-km FSO link testbed including a probing station to estimate the possible wiretapping risks from the sidelobe of the communication beam. In the demonstration, however, there is still room to improve the secret key rate by exploiting the optical fading which contains additional information about random states of the FSO links. We here propose a novel protocol for FSO-SKA employing such channel state information. In the protocol, the legitimate receiver decides whether to discard the received symbols or not according to the received optical power at the time. Based on the experimental data from the FSO link testbed, we demonstrate that the proposed protocol improves the secret key rate compared with our previous result. To our best knowledge, this is the first demonstration that exploits the effect of atmospheric turbulences to improve the security performance of communication systems. We anticipate that this idea will be applicable on the broader areas of FSO communications and opens a way toward practical wireless network spanned by FSO links.
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.
This paper proposes a novel two-way free-space quantum key distribution (QKD) protocol, which can be implemented using standard free-space optical (FSO) systems with subcarrier intensity-modulation (SIM) binary phase-shift-keying (BPSK) and direct-detection (DD) receiver. Different eavesdropping threats, including unauthorized receiver, beam-splitting, and intercept-resend attacks, are considered in the security analysis of the proposed protocol. Under the constraints on security requirements, we analytically investigate the design criteria for transmitter and receiver, in particular, the intensity modulation depth at the transmitter and the dual-threshold setting of the receiver in two-way free-space QKD systems. Quantum bit error rate (QBER), the ergodic secret-key rate, and the probability of detecting eavesdropper of the proposed system are analytically derived in closed-form expressions, considering the atmospheric loss, turbulence, and receiver noises.
This paper considers secure communication over free-space optical (FSO) links suffering from turbulence-induced fading. In particular, we study the average secrecy capacity of applying physical layer security to achieve security in an FSO communication system with binary-level on-off keying intensity modulation and threshold detection, a widely used system in FSO communication. We first define the instantaneous secrecy capacity for our system and then provide a lower bound of it, which is achieved based on a simple design principle for practical applications and yields a simpler equation. We next derive a general expression of the average secrecy capacity and the corresponding lower bound based on bit error probabilities. Finally, numerical analysis is conducted for the special case of the correlated log-normal fading model to show the impacts of fading and channel correlation on the average secrecy capacity.
In our recent paper, we proposed the secret-key generation (SKG) scheme leveraging both atmospheric turbulence and background radiation as sources of randomness, which provides high secret-key rates and is compatible with state-of-the-art optical communications. Unfortunately, its application is limited to the symmetric line-of-sight free-space optical (FSO) links. In this paper, we propose two SKG schemes suitable for asymmetric and non-line-of-sight (NLOS) communications over FSO links. The proposed schemes employ turbulence, solar background radiation, and post-quantum cryptography-based randomly generated Alice and Bob's PSK sequences. To reduce the system cost, an unmodulated carrier is transmitted alongside the modulated PSK signal. On the receiver side, the coherent detection is performed by beating the unmodulated carrier with the received modulated signal on the PIN-photodetector (or the avalanche photodiode), followed by the DC blocker to remove undesired DC components. The proposed SKG schemes are applicable to both physical-layer security (PLS) cryptosystems and protocols similar to the quantum key distribution (QKD). The proposed SKG schemes show excellent tolerance to the atmospheric turbulence effects compared to the traditional PLS schemes. Experimental results indicate that the secret-key capacity of the NLOS SKG scheme is significantly higher than the corresponding traditional PLS counterpart.