# A Review-Unguided Optical Communications: Developments, Technology Evolution, and Challenges
## 4 Underwater Optical Communication
### Underwater Optical Wireless Communication (UOWC)
Direct LoS, diffused LoS, retroreflector-based LoS, and NLoS are the four major types of UWOC link configurations [197]. A 25 Gbps UWOC device using a two-stage infusion 680 nm red-light vertically hollow surface-emitting laser broadcaster and a laser beam amplifier to increase the magnified beam width is shown across a 5 m highly turbid beach water connection [198]. In a 10 m underwater network, a cost-effective UOWC system based on FPGA with data speeds up to 25 Mbps is achieved [199]. DMT with only a 450 nm GaN light beam and adjustable bit energy loading is used to demonstrate a 7.33 Gbps speed for a 15 m UOWC system. At the receiver, post-nonlinear equalization, relying on the Volterra reduced collection model, was utilized to relieve the UOWC system's nonlinear degradation. Using a nonlinear equalization device, performance is increased with an adaptable bit-power load by 18% [200]. Mixed RF or FSO/UOWC (RF-UOWC, FSO-UOWC) schemes have received much interest recently because they address severe constraints for high bandwidth and efficient data transfer in the sea environment. To sustain the cooperative surveillance of the undersea environment, the relay sends data to the terminus through a UOWC network. Real-time data transmission between underwater terminals and land users is possible with the information transmission technique via a water surface floating buoy. The authors of [201] investigated the BER and outage performance of a cooperative RF-UOWC connection network with focusing faults across a hyperbolic tangent log-normal distribution platform. In Ref. [202], the authors examined the outage routine of a hybrid UOWC-FSO link under oceanic instability circumstances. A 9 Gbps 82 m four-level PAM-based FSO-plastic optical fiber (POF)-UOWC convergent system was demonstrated for environmental monitoring, disaster prevention, and manufacturing industry applications [203]. In Ref. [204], an RF network was simulated by using generalized-K distribution, while the UOWC network was modelled using a combination of the exponential generalized Gamma distribution. For underwater optical wireless sensor networks (UOWSNs) and IoUT applications, a DPSK-based dual-hop UOWC-FSO convergent system was presented [205]. The collected sensor data was sent to a decode-and-forward relay using UOWC links modelled as gamma-gamma distribution and the signal was transmitted to the terrestrial destination via an FSO link with a Malaga distribution model. To create high-speed network connectivity, a decode-and-forward multi-hop converging FSO-UOWC network link was developed [206]. The performance of a reconfigurable intelligent surface-assisted dual-hop mixed RF-UOWC system was investigated in [207]. A reconfigurable intelligent surface (RIS) is a new and cheap device which attempts to boost performance parameters by boosting the receiving signal's intensity. In Ref. [208], a cooperative triple-hop RF-FSO-UOWC link for UAV-assisted underwater sensor networks was developed. A direct transmission scenario and retro-reflection scenario are provided, with water-to-air and air-to-water interaction implications for the FSO uplink and downlink communications between the AUV and the UAV. The IoT is an endeavor that aims to integrate a wide range of applications that involve communicating and connecting information content as well as system intelligence to create new business/commercial opportunities. The IoUT is a relatively new technology that is regarded as a subset of the IoT. It has many applications, as well as the smart worldwide networking of underwater devices, ocean environment monitoring, disaster prevention, etc. The integration of RF/FSO/mmW/UOWC systems, as shown in Figure 30, is extremely useful for connecting space, terrestrial, and underwater things/devices to networks. A local underwater Li-Fi network which enables communication for light autonomous underwater vehicles, the IoUT, and UOWSNs is also an emerging research topic. Figure 30: Communication network architecture for underwater, air, and space connectivity.