###### Abstract  
Modern cellular systems rely on high-capacity and low-latency optical networks to meet ever-increasing data demands. Centralized Radio Access Network (C-RAN) architectures offer a cost-effective approach for deploying mobile infrastructures. In this work, we propose a flexible and cost-efficient fronthaul topology that combines Wavelength Division Multiplexing (WDM) passive optical networks (PONs) with free-space optical (FSO) links. To enhance overall system performance, we introduce Low-Density Parity Check (LDPC) decoding, which provides robust error-correction capabilities against atmospheric turbulence and noise. Our system transmits 20 Gbps, 16-QAM intensity-modulated orthogonal frequency-division multiplexing (OFDM) signals, achieving a substantial reduction in bit error rate (BER). Numerical results show that the proposed WDM-PON-FSO architecture, augmented with LDPC decoding, maintains reliable transmission over 2 km under strong turbulence conditions.  
f 2025 7 April 2025 14 April 2025  
Unlike traditional forward error correction (FEC) techniques, such as convolutional or Reed-Solomon codes, which primarily correct random bit errors, LDPC codes excel in correcting bursty errors that occur due to atmospheric turbulence in FSO links. FSO channels suffer from intensity fluctuations caused by refractive index variations, leading to deep fading events that create clusters of errors in received data. LDPC's iterative decoding mechanism mitigates these error bursts by leveraging soft decision decoding and message-passing algorithms to reconstruct lost information. Moreover, LDPC's structured parity-check matrices allow for flexible code rate adaptation, making them highly effective under varying turbulence conditions, where signal-to-noise ratios fluctuate unpredictably.  
Recent studies have demonstrated novel LDPC variants (e.g., nonbinary LDPC, protograph-based LDPC, and generalized LDPC) that substantially improve bit error rate (BER) under strong turbulence or log-normal/Gamma-Gamma channels [12; 13; 14; 15].  
Accordingly, merging WDM-PON with FSO and LDPC decoding holds substantial promise for next-generation C-RAN fronthaul. Previous studies on hybrid WDM-PON-FSO systems have demonstrated significant gains in capacity, often achieved through multi-wavelength designs or advanced modulation and coding techniques [16; 17; 18; 19]. However, many existing solutions still face critical challenges that hinder widespread deployment. This paper proposes a WDM-PON Free-Space Optical (FSO) system utilizing LDPC decoding and orthogonal frequency-division multiplexing (OFDM) modulation to enhance the throughput, reliability, and overall robustness of C-RAN fronthaul networks. Specifically, we:  
* Present a system architecture that integrates multi-wavelength passive optical distribution with atmospheric wireless access, leveraging OFDM to improve spectral efficiency and resilience against channel impairments in 5G/6G deployments.
* Implement advanced LDPC codes optimized for strong turbulence conditions, achieving significant BER improvements by mitigating the impact of fading and signal distortions inherent in FSO links.
* Conduct comprehensive simulations and theoretical analyses to evaluate system performance under severe turbulence conditions.  
By harnessing the combined strengths of WDM-PON, OFDM, FSO, and LDPC-coded optical transmission, our study addresses key performance and cost challenges, paving the way for next-generation C-RAN solutions that scale efficiently under diverse environmental conditions and traffic demands.  
The remainder of this paper is organized as follows. Section 2 discusses existing methods from the literature, highlighting the limitations our approach addresses. Section 3 presents the proposed system architecture, and Section 4 details the simulation setup and key performance findings. Finally, Section 5 concludes the work and outlines future research directions.