# High-Speed Underwater Optical Wireless Communication with Advanced Signal Processing Methods Survey
## 1 Introduction
In addition, some key technologies, such as higher sensitivity receivers and more advanced signal modulation methods are also presented to improve the transmission capacity and performance of UOWC systems. In addition, some recent survey papers also provide a more focused review of key parts of the UOWC system, such as the network layer and underwater channel turbulence. In [2], in addition to the physical layer such as the channel characterization and modulation methods, the network layer issues, including the link configuration and budgets, multiple access schemes, relaying techniques, and potential routing algorithms are also presented. In addition to the absorption and scattering of the signal beam by the particles in the water, UOWC systems also face great challenges from underwater optical turbulence (UOT), which is physically caused by the fluctuation of water with random variations of temperature and pressure [22]. In [23], theoretical UOWC system models considering \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline  & **Acoustic Systems** & **Radio Frequency Systems** & **Optical Wireless Systems** \\ \hline
**Attenuation Distance** & Low (tens of kilometers) & High Short (tens of meters) & Moderate Limited (hundreds of meters) \\
**Carrier Frequency** & Low (10 Hz–1 MHz) & Moderate (30 Hz–300 MHz) & High (10\({}^{12}\) Hz–10\({}^{15}\) Hz) \\
**Bandwidth Data Rate** & Narrow (kHz) & Moderate (MHz) & Broad (MHz–GHz) \\
**Power Consumption** & Low (kbps) & Moderate (Mbps) & High High (Gbps) \\
**Transmission Latency** & High (1500 m/s physical propagation speed of sound wave) & Low (\(2.26\times 10^{8}\) m/s physical propagation speed of electromagnetic wave) & Low (\(2.26\times 10^{8}\) m/s physical propagation speed of optical wave) \\
**Performance-limiting factors** & Temperature, hydrostatic pressure, and the chemistry of water & Conductivity and permitivity & Absorption, scattering, turbidity, marine life blocking, and beam shaping \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparisons of three underwater wireless communication technologies. turbulent channels are summarized, together with underwater turbulence mitigation technologies in the physical layer, including aperture averaging, optical beam shaping, transmitter and receiver enhancement, and multiple-input multiple-output (MIMO) spatial diversity techniques. With the rapid development of UOWC technologies, increasingly more UOWC applications have begun to appear, which are captured by a few recent survey papers. For instance, in [24], the UOWC-based Internet of Underwater Things (IoUT) network is summarized, focusing on the medium access control (MAC) aspect. Moreover, AUVs, which are key technologies for the maritime industry are widely deployed for commercial, scientific, environmental, and defense applications. Thanks to high-speed data transmission, the UOWC technology has been widely considered in AUV application. In [3], a summary of swarm robotics techniques based on the LED type of UOWC is presented. Through the above literature review, due to the absorption and scattering of signal light caused by the complex underwater environment and the influence of underwater turbulence on channel stability, UOWC faces challenges of limited transmission distance and fluctuating transmission reliability [23]. In order to improve the transmission distance of UOWC and to achieve more stable system performance, a large number of techniques have been studied in the physical layer [8; 15; 16; 17; 18; 19], such as more advanced transmitter technologies (e.g., high-bandwidth Gallium nitride (GaN)-based mini-LEDs [25], two-stage-injection-locked technique [26; 27]), more sensitive receiver technologies (e.g., lensed array optical interface [28; 29], photomultiplier tubes (PMT) [30], and single-photon avalanche diode (SPAD) [31]), and more advanced UOWC spatial technologies (MIMO principles [32; 33; 34]).