High-speed spectrally efficient duobinary-precoded NRZ free-space optical links with adaptive optical gain control under extreme atmospheric attenuation

Abstract Free-space optical (FSO) communication systems offer a robust high-capacity backbone for wireless transmission; however, their reliability is frequently undermined by atmospheric conditions such as haze, fog, and rain, which significantly diminish signal integrity and elevate bit error rates (BER). This study presents an enhanced 10 Gbps FSO architecture that incorporates duobinary precoding, non-return-to-zero (NRZ) modulation, and adaptive optical amplification. The duobinary precoder also minimizes bandwidth needs and eliminates intersymbol interference ISI, and dynamic amplification makes the link always reliable. OptiSystem 21 fully tested the architecture on a rigorously dual-validation framework. To set theoretical performance limits, the system was stress-tested at a 60 dBm benchmark. It kept a Q-factor of 11.70 with a BER of 5.43e-32 over a 5,000 m link in hazy conditions, and it reached a Q-factor of 47.01 with no BER at 333 m in rainy or misty conditions. To make sure the system could be used in the real world, it was tested at a practical, eye-safe launch power of 20 dBm. The proposed NRZ–duobinary framework consistently and quantitatively outperformed traditional NRZ systems in terms of signal integrity and power efficiency under realistic conditions in standard atmospheric models, such as haze, 20, rain, 30 and extreme fog, 70 dB/km. These results confirm that the architecture is a strong, simple solution for the next generation of optical wireless backhaul links.