# A Review-Unguided Optical Communications: Developments, Technology Evolution, and Challenges
## 4 Underwater Optical Communication
### Underwater Optical Wireless Communication (UOWC)
The major challenges in UOWC systems are absorption and scattering, turbulence, alignment, and background noise. Other significant factors influencing the performance of a UOWC system include beam spreading, multipath interference, and physical obstruction. The interaction of each photon with seawater particles during light beam propagation in the submerged atmosphere causes absorption and scattering [194]. The overall attenuation due to absorption and scattering effects is described by the extermination coefficient \(c(\lambda)=a(\lambda)+b(\lambda)\), where \(a(\lambda)\) denotes the absorption coefficient and \(b(\lambda)\) denotes the scattering coefficient. Depending on the wavelength of the source and water type, these coefficients can vary [194]. The path loss \(h_{\alpha}\) is the function of distance \(L\) and wavelength \(\lambda\), as stated using the Beer-Lambert law in Equation (24) [195],  
\[h_{\alpha}(\lambda,L)=\exp^{-\varepsilon(\lambda)L}. \tag{24}\]  
The changes of refractive indices of the channel along the transmission distance produces distortions in the data-carrying optical beam. This change is due to the changes in the underwater environment's salinity, density, and temperature, and they cause large fluctuations in signal intensity at the receiver. This is referred to as scintillation, and it has a significant impact on UOWC's performance. There is no generic model for underwater turbulence in the undersea atmosphere available due to its dynamic character, as there is for FSO communication. Due to the similarity of the subsequent submerged fluctuation with the physical process of air optical instability, i.e., randomized heat variations and moderate pressure are the primary causes for both [195], traditional air-turbulence theory can be used for underwater turbulence study. Light radiation fluctuation is modeled using the log-normal and gamma-gamma models as it travels across a chaotic undersea environment [1; 196]. The PDF of the log-normal and gamma-gamma distributions is  
\[f(h)=\frac{1}{\sqrt{2\pi\sigma_{h}^{2}}}\frac{1}{h}\exp\left(-\frac{\left(\ln \left(\frac{h}{h_{0}}\right)+\sigma_{h}^{2}/2\right)^{2}}{2\sigma_{h}^{2}} \right),h\geq 0, \tag{25}\]  
where \(\sigma_{h}^{2}\) is the scintillation index and \(h_{0}\) is the received signal intensity. \[f_{h}(h)=\frac{2(\alpha\beta)^{(\alpha+\beta)/2}}{\Gamma(\alpha)\Gamma(\beta )\overline{h}}\left(\frac{h}{\overline{h}}\right)^{\frac{(\alpha+\beta)}{2}- 1}K_{\alpha-\beta}\left[2\sqrt{\alpha\beta\left(\frac{h}{\overline{h}}\right) }\right],h\geq 0, \tag{26}\]  
where \(h\) is the average signal attenuation due to turbulence, and \(\alpha\) and \(\beta\) are the actual number of large-scale and small-scale dispersion environmental currents, respectively. \(\Gamma(x)\) is the Gamma function, and \(K_{m}(x)\) is the \(M_{th}\) order Bessel functionality of the second type. Because the optical beam is so narrow, maintaining the LoS in UOWC is critical for a reliable optical link. Submarine vehicles, ocean tides, and other tumultuous factors create movement, thus maintaining a continuous and reliable link inviting regular tracking between transceivers. Background noise must be considered when designing the UOWC link. Noise is heavily dependant on the wavelength and location of the operation. The deep sea is generally less noisy than a harbor. Most of the underwater origins of noise are described as Gaussian profile and continuous spectrum. The transmitted optical signal will be significantly affected by the noise in addition to the extreme scattering, absorption, and other effects generated in the undersea atmosphere. This undesirable effect will have a direct influence on the BER of UOWC systems. Many mitigation strategies, such as robust modulation, channel coding (FEC, BCH, Reed Solomon, orthogonal space-time block code, low-density parity-check), diversity, optical amplification, etc., are commonly used to reduce the impact of underwater channel noise and maintain required BER performance in lower SNR underwater environments.