To address one of key problems for optical communication links, physical layer-security (PLS), Bessel modes are employed in proposed-PLS-scheme, significantly outperforming conventional schemes in secrecy capacity. Simulations indicate that atmospheric turbulence effects can help in improving security under beam-splitting attack.
When an eavesdropper performs an optical beam-splitting attack in a free-space optical communications channel, it is referred to as an optical wiretap channel, which is an extension of Wyner's wiretap channel model. Even though physical-layer security can be compromised, it is possible to exploit the noisy and degraded channel conditions experienced by the eavesdropper to obtain positive secrecy capacity even when a shared secret key is not used. In our previous work, we found that employing Bessel-Gaussian beams can help to improve physical-layer security and provide higher secrecy capacity over that of Laguerre-Gaussian beams in a turbulent free-space optical communications channel. In this companion paper, we conducted an experiment exclusively with Bessel-Gaussian beams onto which we encoded a pseudorandom binary sequence to emulate data transmission over this optical wiretap channel. Bit-error rate curves for the intended receiver and the eavesdropper were calculated from which estimates of secrecy capacity were derived. We found that the bit-error rate curves for the eavesdropper were consistently worse than those of the intended receiver under several turbulence conditions and that further evidence of an error floor even when the eavesdropper uses an optical amplifier is promising for secure communications.
The security issue is investigated from a physical layer perspective in the context of free-space optical (FSO) communications. More precisely, the performance of secure FSO link with intensity modulation/direct detection over Málaga (M) atmospheric turbulence channels that accounts for pointing errors is addressed in this letter. The closed-form expressions for the average secrecy capacity, the secrecy outage probability and the strictly positive secrecy capacity are derived by considering the specific nature of FSO channels modeled by the general M distribution. Monte-Carlo simulations are provided to verify the accuracy of our derivations.
Physical layer security of a free-space optical (FSO) communication link is investigated for certain eavesdropping scenarios in which the eavesdropper exploits the divergence of the transmitted laser beam, which is subject to atmospheric turbulence, to compromise the security of the transmission link. A solution for physical layer security in which the legitimate receiver is positioned to receive only the coherent component of the transmitted laser beam is proposed. The irradiance fluctuations are modeled with separate distributions for the coherent and non-coherent components of the transmitted beam, corresponding to the main channel and the eavesdropper’s channel, respectively. Complete secrecy analysis of this setup is presented, and closed form expressions of the probability of strictly positive secrecy capacity, average secrecy capacity, and secrecy outage probability are derived. The analytical results are verified by Monte-Carlo simulations. Secrecy performance for worst-case scenarios corresponding to an adversary with powerful resources to capture all of the leaked information is explored. Results are analyzed for different turbulence regimes, and it was found that secure communication is possible even in the region of strong turbulence.
Free-space optical (FSO) communication provides a promising alternative solution to the traditional radio frequency (RF) communications. Compared to the RF technology, there are many advantages of FSO communications, such as ultra-high bandwidth, license-free spectrum, low inter-channel interference, and energy-efficient transmission. The potential applications for FSO communication include the last-one-mile access link from the fiber-based backbone network, low earth orbit (LEO) satellite communications, high-altitude base station platforms or vehicles, deep-space communications, and in-building automation connections. As the FSO technology becomes mature and increasingly involved, the security and reliability requirements of such applications also become crucial. Regarding the security issues, although the FSO communication is inherently more secure than the RF counterpart due to high directivity of its line-of-sight (LOS) link, it can still suffer from optical wire-tapping attack. There are many different application schemes that an eavesdropper can still probe the FSO link. Traditionally, to protect the communication, many works have been done on encryption layer and upper network layer, such as computationally intensive cryptographic algorithms and message exchanging protocols. In recent years, physical-layer security (PLS) has been getting more attentions as it provides an extra layer of security against eavesdropping. The PLS schemes, in general can be categorized into two main categories: classical PLS and quantum key distribution (QKD) protocols. The classical PLS is based on the information theoretic security analysis, while the QKD security is guaranteed by the quantum mechanics laws. Both techniques have been studied for FSO channels. In the first half of this dissertation, we study the PLS in a LOS FSO channel using orbital angular momentum (OAM) multiplexing and show that higher secrecy capacity can be achieved using OAM multiplexing technology in the presence of atmospheric turbulence effects. Then, we proposed an adaptation scheme for QKD system over FSO channel with different QKD protocols, i.e. BB84 and decoy state protocols. By adapting the source brightness based on the channel condition, we optimized the secret key rate (SKR) of the QKD system. Furthermore, we proposed a multiple spatial modes-based QKD system with backpropagation method to increase the total SKR through parallel channels. Regarding the reliability of an optical communication system as well as information reconciliation for both PLS and QKD schemes, the forward error correction (FEC) becomes an essential technique to enable high-speed transmission. The FEC with low-density parity-check (LDPC) codes has been studied for decades from LDPC block codes to the most recent spatially-coupled (SC) LDPC codes. For FSO communications, due to the time-varying nature of FSO channels, the capability of rate adaptation is important to achieve a consistent reliable transmission. In the second half of this dissertation, we proposed unified FPGA-based architecture to implement both quasi-cyclic (QC) LDPC codes and SC-LDPC codes with rate adaptation capability. The performance is verified through FPGA emulation to show good error floor properties. To further validate the rate adaptation capability, we have built a spatial light modulator (SLM)-based FSO channel emulator to test the adaptive coding scheme. Furthermore, we have conducted a fiber-based experiment with our rate adaptative coding scheme to show that the proposed adaptive coding scheme can be also used in dynamic fiber-based networks.
Optical wireless communication (OWC) has emerged as one promising candidate to mitigate the frequency spectrum crisis. Due to the broadcast nature of OWC channel, physical layer security (PLS) techniques have been considered and explored to improve the transmission confidentiality of OWC links. However, almost all current schemes focus on scenarios including multiple transmitters and fail to match the scenarios with limited transmitters, even single transmitter. For addressing this issue, in this work, the dynamic optical beam based PLS enhancement scheme is proposed. Unlike conventional Lambertian beam based technique paradigm, the above scheme tentatively utilize the commercially available non-Lambertian beam to configure the secure OWC links. Numerical results show that, compared with about 1.10 bps/Hz average secrecy capacity (SC) of the conventional configuration, up to 1.93 bps/Hz average SC gain could be provided by the proposed dynamic beam scheme. Moreover, this potential gain will be increased to about 2.39 bps/Hz when the diversity combing is available for the legitimate user receiver.
To overcome various problems of QKD, postquantum cryptography (PQC), and physical-layer security (PLS) encryption protocols, in our recent paper we have proposed to employ the atmospheric turbulent channel as the source of randomness in a source type PLS cryptosystem. However, a long coherence time of atmospheric turbulence channels limits the secret-key rates (SKRs). In this invited paper, we describe an encryption scheme that overcomes the problem of the long coherence time of the turbulent channel. The proposed scheme has security better than PQC and is comparable to QKD but enables significantly higher SKRs.