## 2 Related Work  
Wavelength Division Multiplexing-Passive Optical Network (WDM-PON) technologies have evolved into core solutions for high-bandwidth broadband access. Early approaches focused on point-to-multipoint architectures with dedicated wavelengths per user [3], while subsequent innovations introduced a range of cost-saving measures, e.g., reflective semiconductor optical amplifiers (RSOAs) [4] and advanced arrayed waveguide gratings [20]. Researchers have also explored integrating next-generation standards (e.g., coherent 100 G) to push capacity further [21; 22; 23]. Meanwhile, multi-wavelength data comb generation [24] and dynamic resource allocation [25; 26] underscore WDM-PON's adaptability in heterogeneous networks.  
FSO channels, on the other hand, provide attractive \"last-mile\" or \"fronthaul\" solutions due to their high data rates and license-free spectrum [7]. However, atmospheric turbulence remains a dominant impairment, often modeled by log-normal or Gamma-Gamma fading [8; 14; 27]. Environmental factors (fog, rain, misalignment) can trigger large fluctuations in received signal intensity [16], thus degrading link availability. To bolster reliability, hybrid architectures have emerged: pairing FSO with millimeter-wave or RF backups [28], employing cooperative relay networks [15], or adopting advanced automatic repeat requests [29] and prediction techniques [30].  
Recent work has consistently shown that Low-Density Parity-Check (LDPC) codes are promising for bridging performance gaps in atmospheric optical channels [27]. Owing to flexible rate adaptation and near-capacity performance, LDPC codes can significantly reduce BER, even in strong turbulence or multi-hop FSO links [31; 32; 33]. Multiple decoding algorithms, such as weighted bit flipping, min-sum updates, or protograph extension, have been proposed to handle large block sizes and high data rates [27; 34].  
Moving beyond discrete deployments, a rising body of literature integrates FSO links directly into WDM-PON backbones to harness high capacity and flexible coverage. Some studies focus on resilience (e.g., fiber-FSO path redundancy or ring-based self-healing topologies) [18; 35; 36; 37], while others target improved spectral efficiency and modulation (e.g., OFDM, quadrature amplitude modulation) [8; 17; 38].  
Although a handful of proposals integrate LDPC-coded designs with WDM-PON-FSO architectures, the majority either overlook strong atmospheric fading or rely on baseline coding that fails to fully exploit the iterative decoding gains of LDPC algorithms [29; 39; 40]. Similarly, attempts to unify high-speed, multi-wavelength PON distribution with robust FSO wireless links for C-RAN remain relatively sparse [22].  
Notwithstanding the abundant research on (1) WDM-PON for broadband access, (2) FSO for fast, flexible wireless links, and (3) LDPC coding for improved optical communications, a coherent approach that addresses harsh turbulence, ensures robust error correction, and seamlessly aligns with C-RAN's capacity and latency requirements is still lacking. In particular:  
* Many studies do not fully incorporate advanced LDPC decoders optimized for strong turbulence channels, especially at high line rates.
* System-level demonstrations of WDM-PON-FSO fronthaul, optimized for 5G and beyond, remain limited.
* While some works propose rate-compatible or probabilistic shaping schemes [27; 41; 42; 43; 44], few solutions explicitly combine them with hybrid FSO topologies for cellular fronthaul.  
By systematically integrating these components, our research aims to fill the existing gaps, offering a WDM-PON FSO system utilizing LDPC decoding that meets the stringent demands of cellular C-RAN fronthaul. In what follows, we detail the proposed architecture, describe the LDPC-based error correction scheme, and provide comprehensive results illustrating how this approach achieves enhanced reliability, even under pronounced atmospheric turbulence.