Measurement Techniques for Gray-Coded WDM-FSO System with LDPC Channel-Conditioned Modulation under Atmospheric Turbulence
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Free-space optical (FSO) communication has emerged as a compelling solution for high-speed data transmission due to its high bandwidth availability, security, and cost-efficient deployment. However, existing FSO systems have focused on single-channel configurations and static modulation or coding schemes, which limit the performance under dynamic atmospheric conditions. This paper presents a measurement based wavelength-division multiplexing free-space optical (WDMFSO) communication system designed to maintain reliability under atmospheric turbulence. The proposed architecture integrates Gray-coded M-ary quadrature amplitude modulation (MQAM), low-density parity-check (LDPC) coding, and a channelconditioned modulation formats driven by carrier quality indicators (CQI) and signal-to-noise ratio (SNR) thresholds. To enhance transmission across multiple wavelengths, we develop a joint physical-layer model that incorporates Gamma-Gamma turbulence, rain-induced attenuation, and geometric beam divergence based on ITU-R standards. A modulation controller adjusts the modulation order per channel according to SNR estimates, ensuring optimal spectral efficiency (SE) and bit error rate (BER) performance. Simulation results across 4 to 32 WDM channels demonstrate BER below 10−6 under atmospheric turbulence, like fog, rain and Gmma-Gamma and 10–25 mm/hr rainfall, confirming improved measuremetn accuracy and spectral efficiency compared to fixed-modulation and uncoded schemes.
