## 4 Results and Discussion
A similar trend is observed at 194.65 THz: at 12 dB OSNR, the uncoded BER is about \(10^{-0.45}\), which improves to \(10^{-0.76}\) with LDPC, and at 32 dB OSNR, it moves from \(10^{-4.39}\) without coding to \(10^{-7.03}\) when LDPC is applied. These results underscore LDPC's effectiveness in markedly reducing error rates under different OSNR conditions. Figure 3 illustrates BER performance over varying free-space optical (FSO) link distances for both the original and LDPC-coded systems. In the absence of LDPC coding (dashed lines), the BER increases as the FSO range extends from 300 m to 2200 m, highlighting the effects of optical losses and atmospheric turbulence. At 300 m, the BER for the original system at 193.1 THz is approximately \(10^{-4}\), increasing to nearly \(10^{-1}\) at 2200 m. A similar trend is observed across other frequencies, where, for example, at 194.65 THz, the BER degrades from approximately \(10^{-4.5}\) at 300 m to nearly \(10^{-1}\) at the maximum range. Employing LDPC coding (solid lines) substantially boosts performance across all frequency channels. At a distance of 300 m, the LDPC-coded BER at 193.1 THz reaches approximately \(10^{-6}\), and even at 2200 m it remains under \(10^{-3}\), indicating a coding gain of nearly two orders of magnitude. The most striking improvement occurs at higher frequencies--such as 194.65 THz--where the LDPC-coded BER stays below \(10^{-6}\) for most of the transmission span, in stark contrast to the uncoded scenario, which degrades severely. Uncoded transmission experiences rapid BER degradation beyond 1 km due to the combined effects of beam divergence and turbulence-induced signal fading. However, LDPC coding significantly extends the operational range of the system by mitigating deep fades. Even at 2 km, where the uncoded system exhibits a BER exceeding \(10^{-2}\), LDPC reduces BER to below \(10^{-5}\), ensuring reliable transmission. These findings confirm that LDPC coding effectively counters performance loss, thereby extending the maximum transmission range and ensuring stable data delivery in FSO systems. Figure 3: BER versus Range plots. Taken together, these outcomes confirm that LDPC coding not only strengthens error correction but also extends the operational range, ensuring more reliable data transmissions in FSO systems under varying OSNR and distance scenarios. The plotted results clearly illustrate how LDPC counteracts performance deterioration, highlighting its significance in improving both the reliability and the coverage of FSO communication links. In addition to turbulence and noise, real-world environmental conditions can further influence FSO link performance. Adverse weather such as fog and heavy rain introduces additional attenuation and scattering, which in turn raises the required power for a given BER. For instance, recent measurements show that under clear weather, an FSO link might achieve 26 km range, but in dense fog, this range can shrink to about 1 km due to extreme optical attenuation [48]. Rain can also reduce the link span (e.g., a heavy rainfall in one study limited the FSO range to 6.7 km in a high-attenuation region) [48]. Temperature fluctuations, while not causing direct absorption, drive turbulence by creating refractive index gradients; hotter ambient conditions or urban heat can increase the strength of turbulence (\(C_{n}^{2}\)), leading to stronger scintillation. These environmental impairments translate to higher BER and outage probability if not accounted for. We incorporate the impact of such conditions through the Gamma-Gamma turbulence model parameters and a baseline attenuation (3 dB/km) in our simulations. Beam divergence and pointing errors also play a role: as the beam travels, it spreads, and any misalignment can result in power loss. A narrower beam (small divergence) can reduce geometric loss but is more sensitive to pointing jitter, whereas a wider beam is more robust to pointing errors but suffers higher diffraction loss.