# Research on Underwater Wireless Optical Communication Channel Model and Its Application
## 2 Modeling of the Communication Channel of UWOC
### Analysis of \"Effective Communication Space\" in UWOC  
The application of optical wireless communication in an underwater environment has its own peculiarities, where there is strong interference in the water. In particular,seawater's disturbance often renders optical communication unstable and difficult to control. For example, when an AUV is equipped with an optical wireless communication device conducting data exchange, the navigational trajectory of the AUV is always changing as the vehicle moves. The continued change in the trajectory means that the relative position between different terminals of optical communication devices keeps changing, including the alignment angle and range. In practical optical communications, while the detected power decreases with increasing distance, the data rate typically remains constant over a certain range. However, beyond a specific range threshold, the weakening of the detected signal can lead to an increased error rate, necessitating a reduction in data rate or the use of more sophisticated error correction techniques to maintain communication integrity. On top of that, entering the buffer space with instability from the stable space is very limited. Consequently, accurately knowing where the \"effective communication space\" is and avoiding leaving the space when using optical communication are significant for keeping communications stable and continuous. The term \"effective communication space\" refers to the specific space within which the same type of optical wireless device can achieve reliable communication under varying water quality conditions. The optical communication equipment itself does not give an indicator of the \"effective communication space\", but determining this space is crucial for the successful implementation of optical wireless communication in underwater equipment like AUVs. Therefore, this paper establishes a model of the communication space, which is expected to contribute to the practical application of UWOC technology. By obtaining a communicable space, underwater vehicles can plan and control their navigation trajectory and target area ranges more effectively, ensuring underwater optical communications are always in a valid state. When determining the effective communication space, the position of the receiver in relation to the transmitter needs to be taken into account. In practical scenarios, the receiver and transmitter may not be perfectly aligned but have an angular shift or deviation, requiring a re-analysis of the received optical signal, as shown in Figure 2. \(\Phi\) is the diffusion angle (full angle) of the transmitter's emitted beam, and \(\theta\) is the angle between the transmitter and receiver lens center line and the transmitter pointing vector. The variable \(d\) corresponds to the projected length of the transmitter-receiver spacing about the transmitter pointing vector, which indicates the range between the transmitter plane and the receiver plane. The underwater wireless optical communication link loss during this process is investigated as follows:  
1. Geometrical loss: Owing to the effects of diffraction, not all incident beams necessarily reach the receiving plane. Furthermore, even among the beams capable of reaching the receiving plane, some may fail to enter the photodetector due to the consideration  
Figure 2: Link geometry relationship of UWOC. of the receiving aperture's dimensions. Thus, when exclusively accounting for beam divergence, the optical power received at the receiver's end is the product of the optical power emitted at the transmitter's end and the ratio of the receiving aperture area to the area of optical intensity in the receiver's range. Consequently, this ratio represents the geometric loss caused by system geometric factors. 2. Multipath Effect: When light is emitted from a transmitter and travels towards a receiver, a portion of the light follows the direct path (the minimum-distance path).