Free-Space Optical (FSO) systems are emerging as a viable solution for highthroughput and are considered as a promising reference to future 6G in high-speed opticalwireless communication. The proposed framework considers DWDM-QKD (DenseWavelength Division Multiplexing – Quantum Key Distribution) enabled FSO system withExplainable Artificial Intelligence (XAI) driven intelligent control to explore the performanceunder varying atmospheric conditions such as Haze, Light Fog and Heavy fog (Turbulence).The purpose of the proposed work is to study the underlying performance of varyingatmospheric conditions for future 6G by comparing the results with classical DWDM to thatof DWDM-QKD with XAI SHAP (SHapley Additive exPlanations) analysis for secure Datatransmission. The performance metrics such as Signal-to-Noise Ratio (SNR) for classical linkquality, Secret Key Rate (SKR) for effective security throughput, SKR collapse, SHAPdependence plot and SHAP Interaction analysis for receiver diameter aperture has been foundfor safe and secure data transmission. The outcome of the study integrates to figure out thatthe data transmission is safe and the quantum security requirements has been met undervarying atmospheric conditions.
Free-Space Optical (FSO) systems are emerging as a viable solution for highthroughput and are considered as a promising reference to future 6G in high-speed opticalwireless communication. The proposed framework considers DWDM-QKD (DenseWavelength Division Multiplexing – Quantum Key Distribution) enabled FSO system withExplainable Artificial Intelligence (XAI) driven intelligent control to explore the performanceunder varying atmospheric conditions such as Haze, Light Fog and Heavy fog (Turbulence).The purpose of the proposed work is to study the underlying performance of varyingatmospheric conditions for future 6G by comparing the results with classical DWDM to thatof DWDM-QKD with XAI SHAP (SHapley Additive exPlanations) analysis for secure Datatransmission. The performance metrics such as Signal-to-Noise Ratio (SNR) for classical linkquality, Secret Key Rate (SKR) for effective security throughput, SKR collapse, SHAPdependence plot and SHAP Interaction analysis for receiver diameter aperture has been foundfor safe and secure data transmission. The outcome of the study integrates to figure out thatthe data transmission is safe and the quantum security requirements has been met undervarying atmospheric conditions.
Quantum key distribution (QKD) provides secure keys with information-theoretic security ensured by the principles of quantum mechanics. The flexibility and efficient deployment of free-space optical (FSO) quantum key distribution systems can make them highly advantageous. Despite the benefits of free-space transmission, there is still a chance of running into unfavorable weather conditions like fog. According to this study, free-space continuous variable (CV) QKD systems may be applicable when fog is present. Using attenuation based on the Kim and Kruse empirical visibility models, the performance of the models that explain the relationship between fog attenuation and air visibility is examined in order to evaluate its impact on the secure key rate (SKR) at wavelengths of 850 and 1550 nm. A comparative analysis between the two wavelengths is carried out to highlight their suitability under different weather conditions. Based on the results, 1550 nm performs better in foggy situations, reaching an SKR of 1.389379 bits/symbol, because of lower scattering losses, whereas 850 nm achieves a longer range of 12 km in clear air because of smaller diffraction divergence. The results demonstrate that wavelength selection plays a critical role in enhancing the robustness of CV-QKD systems that operate over FSO links. In addition, optimizing reconciliation efficiency (β) under thick-fog conditions greatly improved the SKR, indicating that 1550 nm operation has the ability to achieve reliable and secure quantum communication. Results show that varying β from 0.90 to 1.00 for both 850 nm and 1550 nm produces SKR enhancement of (0.63-0.80 bits/symbol) and (0.97-1.13 bits/symbol), respectively.
Quantum key distribution (QKD) has emerged as a pillar of secure communication based on the inherent principles of quantum mechanics. Widespread adoption of QKD, however, is hindered by major technical limitations in both fiber-optic and free-space channels. This work explores the integration of QKD with Dense wavelength division multiplexing (DWDM), free space optics (FSO), and radio frequency (RF) networks in a bid to address these challenges. It presents a thorough review of major theories, recent experimental breakthroughs, system architectures, and advantages of hybrid terrestrial and satellite-based systems. The discussion outlines current limitations, security issues, deployment methods, and advancements in standardization initiatives. Additionally, the chapter highlights future research trends, such as RF-assisted switching, trusted node architectures, and cross-technology synchronization. These developments are envisioned to form a crucial aspect of the global networked quantum internet.
In response to the increasing demands for reliable, fast, and secure communications beyond 5G scenarios, the high-capacity networks have become a focal point. Quantum communication is at the forefront of this research, offering unmatched throughput and security. A free space optics (FSO) communication system integrated with fiber-end is designed and investigated using the Bennett–Brassard 1984 quantum key distribution (BB84-QKD) protocol. Simulation results show that reliable transmission can be achieved over a 10–15 km fiber length with a signal power of −19.54 dBm and high optical-to-signal noise of 72.28–95.30 dB over a 550 m FSO range under clear air, haze, fog, and rain conditions at a data rate of 1 Gbps. Also, the system using rectilinearly and circularly polarized signals exhibits a Stokes parameter intensity of −4.69 to −35.65 dBm and −7.7 to −35.66 dBm Stokes parameter intensity, respectively, over 100–700 m FSO range under diverse weather conditions. Likewise, for the same scenario, an FSO range of 100 m incorporating 2.5–4 mrad beam divergence provides the Stokes power intensity of −6.03 to −11.1 dBm and −9.04 to −14.12 dBm for rectilinearly and circularly polarized signals, respectively. Moreover, compared to existing works, this work allows faithful and secure signal transmission in free space, considering FSO–fiber link losses.
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.
Satellite-based Quantum Key Distribution (QKD) has emerged as a promising solution for global secure communication beyond the limitations of terrestrial optical fiber networks. However, practical free-space optical (FSO) satellite channels are highly dynamic due to atmospheric turbulence, background radiation, weather variability, and finite transmission durations. Most existing models rely on static channel assumptions and asymptotic secret key rate (SKR) estimations, which do not accurately reflect real-time operational conditions. This paper proposes an AI-assisted real-time satellite Continuous Variable QKD (CV-QKD) simulator that integrates live weather data with machine learning- based hybrid noise prediction to model realistic channel behavior. The predicted noise parameters are incorporated into an adaptive QKD framework to dynamically compute both asymptotic and finite-size corrected SKR values. The system further performs day–night comparative analysis and 24-hour temporal simulation to study channel variations over time. All operational parameters, including predicted noise, adaptive SKR, and finite-size adjustments, are logged for reproducibility and performance evaluation. The proposed work implemented in the QKD simulator bridges the gap between theoretical QKD modeling and practical satellite deployment by introducing environmental awareness, AI-driven channel adaptation, and finite-size security considerations within an interactive visualization platform. This framework provides a scalable and realistic tool for analyzing next-generation satellite quantum communication systems.
Integrated millimeter-wave (mmWave) and free-space optical (FSO) communication systems offer a complementary solution for secure and high-speed data transmission. In this hybrid system, mmWave serves as the classical channel and FSO operates as a quantum channel for quantum key distribution (QKD), creating a multi-layered communication infrastructure. This dual channel faces significant challenges, including vulnerability to atmospheric attenuation, blockage, beam misalignment, and high sensitivity to environmental factors such as weather conditions, turbulence, and obstacles, which impact their reliability and performance. Taking into account these challenges, the integration of mmWave and FSO enables robust applications such as secure 6G networks, satellite-to-ground communications, and urban connectivity, where mmWave ensures reliability during FSO disturbances. By combining classical high-speed data transfer with the quantum security of QKD, this hybrid architecture opens the road for next-generation secure and high-capacity networks.