Underwater high-speed wireless communication technology is rapidly advancing, driven by increased exploration for military and scientific activities in the oceans. Compared to traditional acoustic and radio frequency communications, underwater wireless optical communication offers higher bandwidth, speed, and confidentiality. However, end-to-end wireless optical communication faces limitations in transmission distance underwater and significant environmental constraints on radiation. Given the diverse underwater environment and potential obstructions that can interrupt long-distance communication links, employing an underwater wireless optical relay network communication system can extend communication distances and enable flexible networking. Yet, the underwater wireless optical relay communication system and associated relay forwarding protocol are still in the research and simulation phase, focusing mainly on transmissive transmission. Therefore, our study designs a hierarchical data processing structure and bootstrap sequences based on the ethernet data communication interface. We also design a serial-like character frame structure and a data forwarding protocol to address the challenges of long-distance underwater transmission of standard ethernet data and flexible networking communication.

To facilitate underwater wireless optical relay networking communication, we design an ethernet data wireless optical relay transmission protocol under a chain network topology, using the RJ45 standard ethernet interface and data transmission protocol. This design is geared towards realizing an underwater wireless optical relay communication system. First, considering the heterogeneous data types and rates between the 100 Gbit ethernet channel and the optical channel, we develop a data matching algorithm. By writing code and utilizing field programmable gate array (FPGA) hardware, we achieve the conversion from high-speed parallel electrical signals to low-speed serial optical signals. Moreover, addressing the issues of high power consumption and data distortion in underwater wireless optical communication systems, we design bootstrap sequences, a serial-like character frame structure, and a line coding algorithm. These innovations ensure reliable data transmission in the underwater relay communication system while significantly reducing system power consumption. Finally, we construct and test a three-node underwater wireless optical relay system, implementing the underwater wireless optical relay forwarding protocol and achieving end-to-end communication among the three nodes.

We establish an experimental environment for a three-node cascade relay system, converting 100 Mbit/s parallel ethernet data to 4 Mbit/s serial optical channel data. Theoretical analysis and experimental results indicate that the maximum ethernet frame length that can be received without loss is 400 B, given a storage capacity of 3300 B, an optical channel rate of 4 Mbit/s, and an ethernet electrical interface frame sending interval of 1 ms. As frame length increases, the probability of correctly receiving decreases rapidly due to limited storage capacity in the FPGA information processing unit. Increasing the time interval between transmitted frames or the optical channel rate improves the probability of correct frame reception. Additionally, the bootstrap sequence length correlates with receiving optical power and the optical channel rate. When the receiving optical power reaches its limit, the number of bytes in the bootstrap sequence increases linearly with the optical channel rate to achieve transmission without frame loss. This relationship exists because background optical noise affects high-speed signals more pronouncedly when the receiving optical power reaches its sensitivity limit, necessitating more bytes of the bootstrap sequence to counteract the background light effect. Further experiments revealed that, under similar conditions for correct frame reception probability, the terminal B node’s received optical power limit is 12.5 nW in a two-node end-to-end system, while the terminal C node’s limit is 17 nW in a three-node cascade relay system. This outcome effectively doubles the communication distance in the cascade relay system compared to the two-node system.

Our study introduces an innovative wireless optical data and relay forwarding protocol based on the ethernet RJ45 interface protocol to extend underwater wireless optical communication ranges. Utilizing high-power wireless optical signal transmission, high-sensitivity reception, and an FPGA system, we realize a three-layer structure for data processing, serial-type data processing, and channel codec layers. This structure facilitates the conversion and integration of different data types and rates, enhancing effective relaying and forwarding through parsing and reorganization of optical channel data. Our approach significantly boosts transmission distance without compromising the system performance of underwater wireless optical communication. Furthermore, we propose and implement a bootstrap sequence and a serial port-like character frame structure based on a layered data processing strategy, which reduces device energy consumption and enhances communication reliability.