## 2 Seawater Channel Modelling Elements
### UOWC System Layout
#### 2.1.3 Optical Receiver Unit  
The temporal profile of the received optical pulse (\(I_{opt}(t)\)) is shaped by the photons that are incident on the optical receiver (Rx) surface. After collecting the incident optical signal power (\(P_{opt}\)) on Rx's photodetector surface, the detector backend converts the \(P_{opt}\) into an electronic signal (\(P_{sig}\)) [50]. The photodetector electronic backend circuit includes a preamplifier, amplifier, and filter/equaliser, and it is responsible for re-shaping/converting the output signal to an optimum signal profile as an input to the decision unit, making the bit decisions with the lowest possible reception probability of error (Pe). Furthermore, for binary optical receivers, the location of the decision threshold plays a vital role in securing the lowest Pe.  
Different types of photodetectors are available, and the most used ones in many fields are photodiodes [51]. Photodiodes are commonly deployed in optical communications due to their minute size, high sensitivity, suitable materials, and fast response time. Among the types of photodiodes are an Avalanche Photodiode (APD) and a Positive Intrinsic Negative (PIN) photodiode. Generally, photodiodes are unsuitable for long-distance UOWCs because of their high detection threshold and noise intensity, limiting their practical application [52]. Owing to the rapid attenuation of water, the optical link distance for standard detecting equipment and techniques is less than 100 m [49; 53]. APDs are structurally similar to the PIN photodiodes but can work at a greater level of reversed bias. The PIN and APD are Germanium waveguide photodiodes. These photodiodes thrive on the strong optical absorption of Germanium in the fibre optics communication window. Their use in optical applications is based on the hetero-epitaxial growth of Germanium on Silicon [54]. PIN diodes are commonly employed in free space optical (FSO) systems with a range of a few kilometres or less.  
The fundamental disadvantage of PIN photodiodes is that thermal noise severely limits Rx performance. APDs are commonly utilised for long-distance communications because they give current gain through the impact ionisation process. As a result, the APD gain is often tuned with respect to the power of the received signal to improve the optical SNR. The benefit of APD occurs at the cost of higher implementation complexity. The disadvantages of APD are that they require a much higher operating voltage, low reliability, and high reverse bias for operation, and their output is not linear and has extra noise, mimicking random events that cause secondary photoelectrons to form. Its reverse biassing requires a lot of operating voltage, which mandates the deployment of specific electronic circuits. As a result, it raises the power consumption of the Rx [54]. The authors of [55] investigated three distinct systems involving long, short, and hybrid transmission areas utilising PIN and APD receivers. They have also been applied in many UOWC experiments, such as in [56; 57]. Recent studies have concentrated on the potential applicability of the single-photon avalanche diode (SPAD) method to UOWC systems. This physical property enables one photon to generate a substantial avalanche of electrons. Such a method is also known as Geiger's method. Further research towards Rx sensors based on SPAD is still needed [58].