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Link range optimisation for detection methods under scintillation effects on FSO system

Sowmyaa Vathsan M. S., Prakash P., Kasthuri P., Sasithradevi A.

Opto-Electronics Review

2025

Vol. 33, No. 2

art. no. e154745

Free-space optical (FSO) communication is a promising technology that aims to solve the problem of limited bandwidth in modern cellular communication systems. This work investigates the FSO communication systems under varying atmospheric conditions using direct, Mach-Zehnder interferometer (MZI), and coherent detection methods. A detailed analysis evaluates their trade-offs in cost, complexity, and performance. Results indicate that coherent detection achieves the highest link range with an optimal bit error rate (BER) across all turbulence regimes, with maximum feasible ranges of 7.4 km, 6.6 km, and 6.5 km under weak, moderate, and strong scintillation, respectively. Further, optimising local oscillator (LO) power significantly improves system reliability under strong turbulence, enabling robust communication. This study highlights the advantages of coherent detection for FSO systems, particularly in strong scintillation regimes which provide better receiver sensitivity and link range coverage.

Continuous-variable quantum key distribution with extended Kalman filter assisted by recurrent neural networks for phase estimation

Kasthuri P., Prakash P., Vathsan M.S. Sowmyaa, Sasithradevi A.

Optica Applicata

2024

Vol. 54, nr 4

483--495

Continuous-variable quantum key distribution (CV-QKD) holds promise for enhancing security in communication networks. However, obtaining a higher secure key rate poses challenges, particularly in reliable phase estimation. So, it is very necessary for CV-QKD implementations with independent local oscillator (LO) to employ carrier recovery along with precise phase estimation. Our methodology combines extended Kalman filters (EKF) with recurrent neural networks (RNNs) to enhance the accuracy of phase recovery for locally generated LO signals. Using numerical simulations, we evaluate the achievable secret key rates for different transmission distances and line widths. The proposed method achieves a phase error of approximately 1×10– 4, leading to positive secure key rates for distances up to 40 km. This method of phase tracking solves the problem and is effective in real-time deployment of CV-QKD in communication networks.