# Design of Encoding Algorithm for Underwater Wireless Optical Communication Based on Spinal Code
###### Abstract  
The marine environment is complex and variable, with the absorption and scattering effects of seawater and turbulence causing significant attenuation of received optical signals and introducing random jitter, which limits the communication range and stability of underwater wireless optical communication systems. This paper presents the Superposition UEP-Spinal Code structure, which utilizes unequal error protection (UEP) to adjust the transmission performance of different types of information in underwater composite data communication by adjusting the superposition weighting factors in the encoding algorithm. This encoding method enhances the noise immunity of important data, and with the same bandwidth utilization, the overall decoding complexity is reduced by 13.3% compared to the previously improved Spinal code encoding algorithm. The results show that the Superposition UEP-Spinal Code provides a more stable, reliable, and efficient communication solution for underwater wireless optical communication systems with randomly varying channel conditions. underwater wireless optical communication; rateless code; spinal code; unequal error protection  
29 January 20281 March 2025 7 March 2025  
in long-distance transmission, its low bandwidth, noise interference, poor information security, and impact on aquatic life limit its application scope [7]. In 1963, [PERSON] et al. discovered that blue-green light in the wavelength range of 450 nm to 550 nm experiences the least attenuation in water, which led to the development of underwater wireless optical communication (UWOC) technology [8]. Compared to underwater electromagnetic wave communication, the attenuation of optical signals in water is significantly reduced, which helps to increase transmission distance [9]; compared to underwater acoustic communication, this technology can greatly enhance link bandwidth while reducing power consumption, size, and cost [10]. Currently, UWOC has been widely applied in ocean observation networks, effectively improving marine exploration efficiency and underwater operational safety [11]. [PERSON] et al. proposed a multi-hop, free-space, optical-underwater wireless optical communication (FSO-UWOC)-integrated system method to address the safe transportation and tracking of cargo and missiles in naval and marine applications [12]. [PERSON] et al. introduced a visible light communication-based polarization division multiplexing (VLC-PDM) technique to tackle the performance issues of UWOC systems under harsh underwater conditions and varying chlorophyll levels [13]. [PERSON] et al. proposed a hybrid acoustic-optical communication method to solve the problem of autonomous underwater vehicle positioning and tracking control [14]. [PERSON] et al. developed a UAV (unmanned aerial vehicle) dual-hop hybrid RF (radio frequency)-UWOC (underwater optical communication) system to optimize UAV altitude and communication performance [15]. However, the marine environment is complex and variable. The absorption and scattering effects of seawater can cause severe attenuation of received optical signals, limiting the communication range of underwater wireless optical communication systems. Additionally, ocean turbulence effects can lead to random fluctuations in received optical signals, affecting the stability of underwater wireless optical communication systems. The incremental transmission mechanism of rateless codes is particularly suitable for scenarios where channel conditions are random or difficult to predict. In this transmission mechanism, the sender does not transmit a fixed number of encoded symbols but adjusts flexibly based on the channel conditions; the receiver, on the other hand, recovers the information by collecting a sufficient number of channel output symbols. Once decoding is successful, the receiver only needs to send a simple feedback signal, and the sender will stop transmission. Among these, Spinal codes, a rateless coding scheme with capacity-achieving properties, were proposed by [PERSON] et al. [16].