# A Review-Unguided Optical Communications: Developments, Technology Evolution, and Challenges
## 4 Underwater Optical Communication
### Underwater Optical Wireless Communication (UOWC)
All these studies show that high-capacity UOWC over moderate distances is possible. Figure 29: UWOC system architecture. Light propagation in water has been studied experimentally over the past 20 years by Duntley, and their report says that light with wavelengths between 450 nm and 550 nm had a comparatively low attenuation property in seawater [182]. Gilbert et al. validated this finding through experiment in 1966 [183]. The viability of optical communication in submerged and satellite terminals was proposed in 1976 and 1977 [184; 185]. In 1992, an argon-ion photon light ray operating at 514 nm was used to demonstrate a UOWC 50 Mbps transmission throughput over a range of 9 m [186]. In 1995, an LED-based UOWC system was theoretically analyzed for 1 Mbps for a 30 m distance and 10 Mbps for a 20 m distance [187]. Many researchers have shown interest in the field of UWOC over the years and have experimented with real-time UWOC systems for various military, ocean observing, Navy Internet of Underwater Things (IoUT), and other applications. The outage performance of a vertical diffused LoS UWOC link was recently systematically investigated [188]. To enhance the IoUT networks' dependability and power performance with UOWC, a special retransmission strategy based on analytical packets and an energy-collecting method was proposed [189]. A developed Markov chain model was used to assess system capacity, latency, and throughput in regard to energy use. The authors of [190] employed 16-QAM, QPSK, and 64-QAM OFDM modulation techniques in harbor II and clear ocean water to achieve ultra-high-definition quality and real-time video streaming over a 4.5 m UOWC link. The communication system was built on a bi-directional framework that allowed video to be transmitted in downlink and feedback control to be received in uplink. In [191], the BER performance of an OOK-based 5 m UOWC link was studied over air bubbles and weak turbulence, and it was improved using receiver diversity and the Bose-Chaudhuri-Hocquenghem (BCH) coding technique. The authors of [192] investigated and improved the performance of a 30 m UOWC system based on OOK at an information speed of 500 Mbps using MIMO and methods for signal encoding in IoUT applications. For IoUT applications, an M-QAM-modulated UOWC system in a river-ocean situation for programmable submerged optical sensor networks (OSNs) was proposed and studied [193]. The transmitter, receiver units, and the water channel are the three pieces of apparatus used in a UOWC link. The apparatus of typical UOWC schemes are depicted in Figure 29. The transmitter's four main components are a modulator, driving circuit, light source, and Tx optics. The optical beam is modulated with the input data and delivered, at a high data rate, across greater distances within the water. OOK, PPM, PAM, QAM-OFDM, and QAM-DMT are some of the modulation techniques used in the UOWC system. The transmitter's function is to perform E/O conversion in accordance with data input and to project precisely focused light rays in the water. The transmitter uses projection optics and a beam steering system to concentrate and direct the light signal in the direction of the receiver. The transmitted signal travels along the undersea channel, which includes a variety of features depending on the time and geographic place. The arriving signal is captured by the receiving optics at the receiver and transfers it to the sensor for O/E conversion. To retrieve the original input data, the electrical signal is passed to a processor and a demodulator. Laser transmission underwater is challenging due to the peculiar optical characteristics of the undersea environment. Diverse water bodies have different characteristics depending on their geographical location (from coastal waters near land to deep blue ocean) and the number of dissolved substances in them. The various water types considered in UOWC systems are a muddy harbor, a clean ocean, a coastline ocean, and pure sea.