Principles and Implementation of Secure Free-Space Optical Wireless Communications

Hiroyuki Endo, Mikio Fujiwara, Mitsuo Kitamura +6 more

Hiroyuki Endo, Mikio Fujiwara, Mitsuo Kitamura +4 more

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.

Zixian Wei, Rui Deng, Binbin Zhu +3 more

Rajeev Ranjan, Pratham Vinayak Sharma

Abstract With increasing demands for secure high-speed wireless communications, integration of quantum key distribution (QKD) into light fidelity (LiFi) systems is a promising method for boosting data confidentiality in optical wireless networks. The BB84 protocol and Mach–Zehnder modulator (MZM) are utilized with a pump-assisted continuous wave laser at 1,550 nm and polarization control components to encode quantum states onto photons. They are transmitted on free space optical (FSO) links with a length ranging from 2 km to 8 km and received using Avalanche photodiodes (APDs) and optical filters. Performance analysis conducted using bit error rate (BER) and Q-factor analysis. BER maintained at zero across all tested configuration and transmission distance up to 8 km indicating excellent signal integrity and security. A highest Q-factor achieved a maximum of 3,055.9, which ensures signal integrity and low interference noise. The system operates at a simulated data rate of 10 Gbps, suitable for high speed optical wireless communication. Consistently high eye height (∼172,210 units) and stable decision thresholds confirm minimal inter-symbol interference and accurate symbol recognition. And the Poincaré sphere showed even polarization stability for different optical power levels. This work presents a simulation-based implementation of a polarization encoded QKD system over a LiFi-enabled FSO communication channel, aiming to enhance secure optical wireless communication. Calculated based on quantum bit error rate (QBER) and included in the theoretical analysis to demonstrate practical secure throughput.

Baris I. Erkmen, Bruce E. Moision, Kevin M. Birnbaum

Mingze Cai, Zhenyu Wan, Yuzhuo Cao +4 more

Free-space optical (FSO) communication offers high capacity, license-free spectrum, and immunity to electromagnetic interference, making it a promising solution for future wireless links. However, practical deployment of FSO systems remains challenged by issues of link security and reliable transmission under atmospheric turbulence. In this work, we investigate the use of nonseparability encoding based on vectorial structured light as a turbulence-resilient and secure transmission approach. Through simulations and experiments, we compare multi-level nonseparability encoding with conventional amplitude modulation under various turbulence strengths and eavesdropping scenarios. The results demonstrate that vectorial structured light exhibits inherent security advantages, imposing higher optical signal-to-noise ratio penalties on eavesdroppers and making information recovery from partial interception significantly more difficult. These findings confirm the potential of nonseparability-based encoding for enhancing both security and robustness in FSO communication systems.