# Recent Progress on Underwater Wireless Communication Methods and Applications
## 2 Main Methods of Underwater Wireless Communication
### Underwater Wireless Optical Communication
Absorption and scattering are caused by the interaction of photons with particlesin water, where absorption is caused by the transfer of energy by photons to other particles, and scattering is caused by the collision of photons with other particles. Generally, the attenuation coefficient \(c(\lambda)\) can be used to describe the attenuation of light in water and be expressed as  
\[c(\lambda)=a(\lambda)+b(\lambda), \tag{1}\]  
where \(a(\lambda)\) and \(b(\lambda)\) represent the absorption coefficient and scattering coefficient with the unit of m\({}^{-1}\) respectively, and \(\lambda\) indicates that the attenuation coefficient is related to the wavelength. The value of the attenuation coefficient \(c(\lambda)\) will vary with the type and depth of water [9]. Typical values of absorption coefficient, scattering coefficient and attenuation coefficient related to the four main water types [68] are as shown in Table 4.  
To address these, research has focused on developing robust channel models, turbulence compensation techniques, and system integration approaches such as multi-input multi-output (MIMO) and adaptive optics [65; 66]. Techniques such as RGB wavelength division multiplexing (WDM) combine illumination and communication functionalities, while FPGA-based QAM-OFDM systems improve BER and extend transmission distances [63; 66]. Moreover, experimental setups have demonstrated the effects of underwater turbulence on communication performance, leading to the development of new mitigation strategies, such as spatial diversity and aperture averaging, to counteract signal degradation [65; 67]. Conventional UWOC systems based on LED/LD and NRZ-OOK modulation typically achieve 100 Mbps to 1 Gbps over clear-water distances up to 20-30 m. However, they suffer from rapid attenuation and poor performance in turbid conditions.Research innovations such as QPSK coherent detection [64], wavelength division multiplexing, and turbulence-aware adaptive optics [63; 66] now enable stable Gbps-level links even under moderate scattering. The performance evolution is summarized in Table 5.  
Different communication modes have their own advantages and disadvantages, and a single communication mode cannot completely cover a variety of working conditions, Table 6 shows the characteristic of different methods. Presently, UWOC has started integrat \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Water Types** & \(a(\lambda)\) **(m\({}^{-1}\))** & \(b(\lambda)\) **(m\({}^{-1}\))** & \(c(\lambda)\) **(m\({}^{-1}\))** \\ \hline Pure sea water & 0.041 & 0.003 & 0.044 \\ Clear ocean water & 0.114 & 0.037 & 0.151 \\ Coastal ocean water & 0.179 & 0.219 & 0.398 \\ Turbid harbour water & 0.366 & 1.824 & 2.190 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Typical \(a(\lambda)\), \(b(\lambda)\), and \(c(\lambda)\) for different water types. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Modulation Scheme** & **Light Source** & **Data Rate** & **Transmission Distance** & **Reference** \\ \hline OOK & 450 nm LD & 15 Gbps & 20 m & [67] \\ NRZ-OOK & 520 nm LD & 2.70 Gbps & 34.5 m & [62] \\ IM/DD-OFDM & 405 nm LD & 1.45 Gbps & 4.8 m & [61] \\ NRZ-OOK & 520 nm LD & 500 Mbps & 100 m & [63] \\ QPSK & 532 nm LD & 8 Gbps & 0.6 m (scattering conditions) & [64] \\
16-QAM-OFDM & 450 nm LD & 4.8 Gbps & 5.4 m & [69] \\ DPIM & 470 nm LED & 0.6 Mbps & 9 m & [70] \\
16-QAM-OFDM & 450 nm LED & 161.3 Mbps & 2 m & [71] \\
128-QAM OFDM & 685 nm LD & 1.324 Gbps & 6 m & [72] \\
32-QAM OFDM & 685 nm LD & 4.883 Gbps & 6 m & [72] \\ OOK-NRZ & 520 nm LD & 2.3 Gbps & 7 m & [73] \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Summary of recent UWOC works. ing with other communication technologies such as acoustic and RF systems. This integration significantly enhances its utility by enabling hybrid underwater networks that leverage the unique advantages of each method.