# High-Speed Underwater Optical Wireless Communication with Advanced Signal Processing Methods Survey
## 2 Overview of UOWC Systems
### System Architecture
To overcome these limitations, external optical modulation methods can be used; the Mach-Zehnder Modulator is widely employed in LD-based UOWC systems [65; 66; 67]. After signal generation, the optical beam propagates through the underwater channel. The two main physical phenomenons that cause signal loss in the underwater channel are absorption and scattering, as shown in Figure 3. When the optical beam with optical power \(P_{i}\) at a wavelength \(\lambda\) propagates in the water, a small part is absorbed, denoted by \(P_{u}\), and another part is scattered, denoted by \(P_{s}\). The remaining part \(P_{t}\) reaches the receiver [8]. Generally, the attenuation coefficient \(c(\lambda)\) is the sum of absorption coefficient \(a(\lambda)\) and scattering coefficient \(b(\lambda)\)[68]:  
\[c(\lambda)=a(\lambda)+b(\lambda). \tag{1}\]  
Figure 3: Geometry of optical beam propagation underwater. Figure 2: General architecture of UOWC system. The propagation path loss \(L_{p}\), as a function of wavelength \(\lambda\) and distance \(d\), is then given as [69]  
\[L_{p}(\lambda,d)=e^{-c(\lambda)d}. \tag{2}\]  
The attenuation coefficient \(c(\lambda)\) is highly dependent on the optical wavelength [70], where the range of 450-550 nm (blue and green lights) has a much smaller attenuation coefficient compared to other wavelengths. Hence, the blue and green wavelength bands are typically used in UOWC systems. Another factor that affects the attenuation coefficient is the type of water. Since the UOWC technology is typically applied in the ocean, the attenuation coefficient of seawater has become a key research focus. In [71], it was found that the absorption coefficient \(a(\lambda)\) is determined by organic pigments (chlorophyll, carotenoids, pheophytin, and chlorophyllide) produced by aquatic plants in water, and the scattering coefficient \(b(\lambda)\) is dependent on the density and volume of particles in water. However, ocean water at different depths and locations has differences in sunlight strength and temperature, causing differences in the organic pigment levels and the density and volume of particles. Therefore, in previous studies, the water channel is typically divided into three categories: clear ocean, coastal ocean, and turbid harbor. The typical values of absorption and scattering coefficients of the three different ocean water types are concluded in Table 3[8]. In UOWC experiments, many researchers use tap water with a lower attenuation coefficient (\(c\approx 0.07\) m\({}^{-1}\)) [26; 27; 72]. To achieve experimental results closer to practical systems, the chlorophyll base of a seawater channel was investigated in recent UOWC studies [73; 74; 75]. As a very common phenomenon in water, bubbles will also affect the performance of UOWC systems. There have been some recent studies investigating UOWC bubble channels [76; 77; 78]. In addition, near the ocean surface and harbor, coastal ocean water and turbid harbor water channels have always been a huge challenge for UOWC communications since these two kinds of water often have high levels of organic pigments and particles, leading to a large attenuation coefficient [8; 13]. In recent years, there has been research focusing on the improvement in coastal and turbid harbor water channels [35; 60; 79]. After passing through the underwater channel, an optical interface is typically used at the receiver to reduce the impact of the background light (sunlight) and to focus signal light, such as optical filters to suppress the background light [80; 81; 82], and the lens array to collect more signal light [28; 29]. Then, the signal light is detected by the optical detector. The PIN photodiode (PIN PD), which has the advantages of fast response time, low cost, and good tolerance to ambient light [8], is widely employed [50; 51; 53; 54; 55; 56; 58; 59; 60; 62; 83; 84; 85]. The avalanche photodiode (APD) has also been utilized due to the higher internal gain [10; 28; 35; 38; 41; 42; 49; 52; 64; 86; 87; 88; 89].