# Recent Progress on Underwater Wireless Communication Methods and Applications
## 3 Wireless Communication System Applied in Underwater Devices
### Application and Limitations of Single Mode Underwater Communication Technology  
The evolution of submersibles exemplifies the convergence of various technological fields. With advancements in materials science, control algorithms, and related innovations, submersibles have transformed from traditional, large-scale, manned, propeller-driven vehicles into compact, unmanned bionic platforms [81]. These next-generation underwater machines are increasingly capable of handling complex tasks such as resource exploration, environmental monitoring, and cooperative operations with clusters of AUVs. However, the reliance on single mode underwater communication technologies, particularly traditional acoustic systems, is encountering significant limitations [82]. These systems struggle to meet modern requirements for high data transmission rates, low power consumption, and enhanced communication security, highlighting the urgent need for more advanced and diversified communication solutions. Among single-mode technologies, optical wireless communication has attracted considerable attention for its potential to achieve high-speed underwater links. For instance, ref. [74] investigates both simulation and experimental results of optical communication strategies for AUVs. The study models underwater light propagation using Lambert's law, incorporating water absorption and scattering coefficients to predict received optical power distribution. Channel capacity is evaluated via the Shannon-Hartley theorem, revealing that water turbidity critically limits transmission performance. Although increasing light source power extends communication distance, the improvement is marginal in turbid waters. The authors suggest future research directions including the use of non-ideal light source models and the design of multi-directional receivers to enhance the robustness and adaptability of optical wireless systems in underwater environments. Meanwhile, [PERSON] et al. [83] presents a low-complexity acoustic coherent communication system specifically designed for small AUVs, addressing typical limitations associated with underwater acoustic channels. Through differential binary phase-shift keying (DBPSK) modulation and a direct delay-tuning Doppler compensation scheme, the system achieves reduced computational burden, improving real-time communication performance. The incorporation of a least-mean-square (LMS) adaptive equalizer effectively mitigates multipath interference, a common issue in single-mode acoustic links. Simulations and field experiments conducted under varying conditions validate the system's ability to achieve low bit error rates and maintain reliable communication performance in challenging underwater environments. When integrating underwater wireless communication systems into AUVs, practical considerations such as transceiver size, energy consumption, heat dissipation, and antenna alignment become critical. For instance, low-frequency RF systems typically require large antennas, which are unsuitable for compact or biomimetic AUVs. Similarly, optical systems often demand precise pointing and alignment, which imposes mechanical constraints on maneuverable vehicles. Power consumption is another limiting factor--high-speed optical or active acoustic systems can drain onboard batteries rapidly, reducing operational time. To address this, ongoing research focuses on duty-cycled communication, ultra-low-power modulation protocols, and lightweight, modular transceivers that balance performance with AUV mobility. Beyond general-purpose communication, certain applications demand specialized designs. For environmental monitoring in ecologically sensitive zones, communication systems must minimize acoustic and electromagnetic disturbances. Low-power optical systems operating in the blue-green spectrum and biodegradable sensor nodes have been proposed to ensure minimal environmental impact. In parallel, stealth applications require low-emission communication methods.