Turbulence mitigation in FSO based on turbo coding and equalization

Abstract

Free space optical (FSO) communication has gained considerable interest for various applications because of its high speed, security, and low power consumption, which seem like promising advantages for the future. FSO systems are severely constrained by turbulence produced by air temperature and density fluctuations, resulting in scattered and distorted light beams. The capacity and bit error rate (BER) of an FSO link can be influenced by fading brought on by turbulence. Turbulence raises the BER, especially under conditions of moderate and high turbulence. The proposed model suggests using a turbo encoder with an intensity modulator at the FSO transmitter and combining maximum likelihood estimation (MLE), zero-forcing equalization (ZFE), and a turbo decoder for the direct detection receiver. However, MLE does an accurate channel estimate, which minimizes fading and turbulence. ZFE improves the quality of the link even further, while turbo coding lowers the bit error rate by increasing redundancy and greatly enhancing error-correcting capability. The Gamma–Gamma turbulence model simulation showed the proposed system’s superiority over other techniques, which achieved a BER of 10−5 with a signal-to-noise ratio (SNR) of 16 dB for moderate and strong turbulence, respectively. Consequently, even though turbo coding adds redundancy to the signal, it can achieve substantial coding gains that increase FSO capacity.