# High-Speed Underwater Optical Wireless Communication with Advanced Signal Processing Methods Survey
## 1 Introduction  
The ocean covers more than 70 percent of the surface of our planet [1]. Human exploration of the ocean has not stopped since ancient times. With the rapid development of science and technology, human exploration of the ocean has gradually deepened. The invention and optimization of a large range of underwater applications such as underwater wireless sensor networks [2] and autonomous underwater vehicles (AUVs) [3] have become key factors. Underwater wireless communication (UWC) technologies, which are summarized in Table 1, have become the cornerstone of these underwater applications. With the need for real-time underwater communications, high-speed and long-distance transmission is more in demand than ever in UWC technologies. Traditional UWC mainly relies on underwater acoustic communication (UAC) technology, which has been explored in transmitting data for long distances reaching up to several tens of kilometers [4], exploring the low attenuation property enabled by the physical properties of sound waves propagating in water. However, UAC suffers from a low data rate limitation due to the low modulation bandwidth (only tens of kHz) [5, 6]. The propagation speed of acoustic waves in the underwater channel is also low (only 1500 m/s), leading to a latency of about 0.67 s per kilometer [7]. Moreover, the power consumption is typically high (tens of watts [8]). Compared to UAC, underwater radio frequency (RF) communication suffers from a high attenuation coefficient due to the low conductivity of electromagnetic waves in water, which leads to a highly limited transmission distance (only a few meters to tens of meters) [9]. Thus, RF communication is not adopted in UWC. To overcome the conventional UWC limitations, UOWC has been proposed and widely studied since it has great potential to achieve a higher data rate reaching Gbps, thanks to the large modulation bandwidth (exceeding MHz [10] and even GHz [11], typically limited by the transceiver). Moreover, the physical communication latency is much shorter due to the high propagation speed of light in the underwater channel [12]. These high-speed and low-latency advantages can enable many real-time applications. Furthermore, UOWC is also cost-effective and power-effective compared to UAC and RF communication, which benefits from low-cost and low-power transceivers such as light-emitting diodes (LEDs) and photodiodes (PDs) [13]. Although the UOWC technology has these advantages, the transmittance of the optical wave is limited compared with the acoustic wave (only hundreds of meters in the tap water channel [14]), and due to the shorter wavelength, the optical signal also experiences more complex underwater propagation channels. Hence, improving the transmission distance, data rate, and system stability of the UOWC system becomes a research focus. In recent years, a number of surveys and summary papers on UOWC have been published, which are summarized in Table 2. In [8; 15; 16; 17; 18], brief overviews and recent advances of UOWC are presented, focusing on the UOWC channel characterization, modulation methods, and coding technologies. In addition to an overview of recent UOWC achievement, in [19], a summary of transmitter and receiver technologies is presented. Moreover, the UOWC channel model and the impact of underwater turbulence are discussed. Due to the complex underwater environment, accurate theoretical UOWC channel models are the basis for designing and optimizing practical UOWC systems. Therefore, the available UOWC models to investigate the communication performance, such as the transmission range and data rate, are summarized in [20; 21]. With the continuous optimization and improvement of theoretical models, many practical UOWC systems have been further designed and studied experimentally. In [13], a detailed summary of UOWC experimental demonstrations with both laser diode (LD) and LED transmitters in recent years is presented.