Performance analysis of 10 Gbps LDPC coded 4-QAM CO-OFDM–based FSO communication system under real-time atmospheric conditions across five Indian cities

Abstract Free Space Optical (FSO) communication is rapidly gaining attention as a high-capacity, license-free wireless technology. However, its real-world deployment across geographically diverse regions remains challenging due to the significant influence of localized atmospheric conditions on link performance. This paper presents a detailed simulation-based study of a Low-Density Parity-Check (LDPC) coded 4-Quadrature Amplitude Modulation (QAM) modulated Coherent Orthogonal Frequency Division Multiplexing (CO-OFDM) FSO system evaluated across five Indian cities: Bangalore, New Delhi, Mumbai, Kochi, and Guwahati. Real-time atmospheric data from the year 2025 was used to derive city-specific attenuation coefficients using the Kim visibility model, while the Hufnagel–Valley (H–V) model characterized optical turbulence through refractive index structure parameters ( C n 2 ). The complete system was designed and simulated using OptiSystem software. Results indicate that LDPC coding significantly enhances transmission range and bit error rate (BER) performance across all geographical locations, with peak communication distances reaching up to 25.00 km under clear weather conditions in Bangalore. Even under severe weather, LDPC extended the FSO transmission range by up to 40 m. Where, uncoded systems failed entirely. These findings establish the practical viability of FSO as a robust next-generation communication solution for the Indian subcontinent when paired with robust error-correction strategies.