# Designing an Adaptive Underwater Visible Light Communication System +
## 2 Literature Survey  
Efforts were made to optimize optical channels by varying system parameters or designing an adaptive system. [PERSON] et al. in [8] used an angle diversity technique in a receiver to overcome the signal-to-noise ratio (SNR) variation in free-space optical communication. [PERSON] et al. in [9] worked on adapting the beam divergence angle in free-space optics for high-speed trains. Despite considerable progress in free-space applications, few visible light communication (VLC) advancements for underwater environments have been made. The challenges of underwater communication, including light attenuation from scattering and absorption, complex optical propagation, and fluctuating environmental conditions, make implementing VLC underwater more difficult. [PERSON] et al. in [10] designed a prototype innovative transceiver for multiple users using on-off keying code division multiple access (OOK-CDMA). The proposed receiver offers a wide field of view and can estimate the angle of arrival, with the capability of estimating water quality through optical backscatter. This CDMA research could support the future networking of swarms of uncrewed underwater vehicles. [PERSON] et al. in [11] studied optical Gaussian beam divergence in the lowest-order transverse electromagnetic mode to address the misalignment of the transmitter and receiver. The paper demonstrated that, based on the angle of arrival and the cumulative normalized received power, there is a trade-off between power loss and channel bandwidth. [PERSON] et al. in [7] intended to provide an overview of current technologies and potential future developments in underwater optical communication. [PERSON] et al. in [12] reviewed recent advancements in underwater wireless optical communication (UWOC) systems and highlighted the challenge posed by oceanic turbulence, which causes scintillation and misalignment in underwater links. It proposes that using light-scattering-based, non-line-of-sight communication, scintillating-fiber photo receivers, and large photovoltaic cells can reduce the need for complex pointing and tracking systems. Methods like diffraction beam shaping techniques, relay-based methods, and SDRs were studied to determine the performance of the UWOC channel and how to make it more efficient by making the transmitter or receiver adaptive. [PERSON] et al. in [13] investigated  
Figure 1: The “transparent window” for light aquatic attenuation [7]. the performance of UWOC in channels with turbulence and misalignment. The diffraction beam shaping method was used to adjust the transmitter distance dynamically. The received power was adjusted through a liquid light crystal valve and a neutral density filter. [PERSON] et al. in [14] proposed a relay-based UWOC system to enhance communication link performance and expand the optical receiver's reception area. [PERSON] et al. in [15] proposed a dual-hop UOWC system where multiple amplify-and-forward relay nodes support the transmitter-receiver link. The bit error rate (BER) using OOK modulation was analyzed to assess the performance improvements of this relay system. The paper studied the impact of key system parameters, such as transmit power, the number of relays, and the spatial positioning of the attenuation spread, on the system's overall performance. [PERSON] et al. in [16] analyzed the factors that affect underwater optical communication systems and compared them with underwater acoustic systems regarding channel capacity, range, and energy efficiency. A multi-hop system was investigated to extend the UWOC range. [PERSON] and [PERSON], in the paper [17], designed a software-designed UWOC system based on Universal Software Radio Peripheral (USRP) and Python 3.7 capable of transmitting real-time videos through a water tank with a low error rate. [PERSON] et al. in [4] studied the effect of transceiver alignment on the UVLC system by manually changing the direction of the receiving Li-Fi with respect to the transmitter using an SDR.