# High-Speed Underwater Optical Wireless Communication with Advanced Signal Processing Methods Survey
## 2 Overview of UOWC Systems  
### System Architecture  
Before introducing the typical UOWC system, it is essential to provide an overview of the UOWC network architecture. As shown in Figure 1, AUVs can use the UOWC system to communicate with divers and optical base stations (OBSs) in real time. Moreover, the underwater monitor (UM) can use underwater cameras to transmit real-time video signals through UOWC techniques to OBSs and finally to relevant departments to monitor water quality and deter poachers. Furthermore, the underwater defense system can use UOWC sensor technologies to detect enemies in time and take a counter measurement. At the same time, OBSs connect with the central OBS on the water surface to form a complete underwater sensor network. The central OBS can further communicate with satellites and ships by the RF link, truly realizing the integrated underwater-above-water-satellite communication network. The typical UOWC system, which includes the transmitter, the medium, and the receiver, is shown in Figure 2. In the transmitter part, the signal is generated and mapped to transmitted symbols before being loaded into a digital-to-analog converter (DAC). The most widely used symbol modulation techniques used are on-off keying (OOK) [39; 40; 41; 42; 43] and pulse position modulation (PPM) [44; 45; 46; 47]. OOK is the simplest modulation method and is widely used together with direct detection. However, the OOK modulation is susceptible to interference in the complex underwater environment [8; 13]. PPM is beneficial for long-distance UOWC communication since it is more power efficient. However, the PPM suffers from the disadvantage of low spectral efficiency. To achieve a high data rate transmission, more advanced signal modulation methods, such as multi-level pulse amplitude modulation (PAM) [27; 35; 48; 49; 50] and quadrature amplitude modulation (QAM) [51; 52; 53; 54; 55], have been applied in recent studies. PAM uses multiple power levels to modulate information, and hence, provides higher spectral efficiency. However, due to PAM requiring a higher SNR for correct symbol detection, the power consumption is high. Furthermore, QAM further improves the data rate by exploring quadrature features. However, due to its high implementation complexity, QAM modulation has high cost limitations [15]. Figure 1: UOWC network architecture. After symbol mapping, the digital signal is loaded into a DAC. After amplification (AMP), the signal is then modulated to the optical carrier. There are two optical transmitters that are widely used in UOWC. The first one is LED, which has the advantages of a wide optical beam and low cost. Hence, the LED-based UOWC system is widely used in short-range applications with a large signal coverage area (tens of meters transmission distance [10; 41; 49; 56]). However, the modulation bandwidth is typically limited to tens of MHz [10; 57] or hundreds of MHz [58], restricting the transmission data rate [54; 58]. Compared with LED, the LD transmitter has the advantage of broad modulation bandwidth over the GHz range [11; 27; 59; 60; 61]. Hence, the transmission data rate of LD-based UOWC systems can reach tens of Gpbs [27; 52; 53; 62]. Moreover, the narrow laser beam also enables a longer link distance of hundreds of meters [38]. However, due to the narrow laser beam, the transmission performance degrades sharply by underwater scattering and turbulence [8]. In addition, LD transmitters also have a higher cost compared to LEDs. To modulate symbols to the optical carriers, two methods are widely used. The most common method in UOWC systems is direct modulation, which uses a bias tee to combine the electrical signal generated by the DAC with the DC bias, which is then connected to the optical transmitter [30; 63; 64]. The advantage of direct optical modulation is simple and efficient. However, the modulation bandwidth is limited and is only capable of intensity modulation.