LDPC Decoding Techniques for Free-Space Optical Communications

There has been significant interest in free-space optical (FSO) communication by the research community in recent years. This is due to its high data rate, unlicensed spectrum, low cost, and immense security for FSO systems. Due to these advantages, FSO can have broader applications that extend from terrestrial to satellite communication. Atmospheric turbulence (AT) induced fading is a primary problem in the FSO link since it significantly impairs its performance. Atmospheric turbulence occurs due to the random variation of the air refractive index with time. Several statistical models are introduced to characterize the AT. The Log-normal (LN) model represents weak and moderate turbulence, and the Gamma-Gamma (G-G) model is employed for strong turbulence. These models are used with the effect of weather attenuation, geometric losses, and misalignment errors. One possible solution is channel coding, such as low-density parity-check (LDPC) codes. This paper proposed employing Weighted Bit Flipping (WBF) and Implementation Efficient Reliability Ratio Weighted Bit Flipping (IERRWBF) decoding techniques to improve FSO link performance. The results show a superior improvement in the bit error rate (BER) than the uncoded FSO system. In addition, the obtained results prove that the IERRWBF technique is more optimized than WBF from the point of the number of iterations, especially in weak and moderate turbulence FSO channels. In the calculation of decoders processing time, the WBF maintained lower decoding time than the IERRWBF technique, while in higher $E_{b}/N_{o}\text{s}$ , they have the same level. The same response for both techniques in the case of resultant throughput. Finally, both methods are evaluated from the point of convergence. IERRWBF technique achieved faster convergence than WBF in all FSO channels under study