Research On Underwater Acoustic-Optical Cooperative Communications Experiment System

Underwater wireless sensor network is widely used in underwater environment monitoring,underwater detection and other fields,which is of great significance to the development,utilization and protection of marine resources.Underwater acoustic communication technology is the most mature underwater wireless communication technology.It has the advantages of long communication distance and high reliability,but also has the defects of low transmission rate and obvious multipath effect,so it is difficult to meet the increasing bandwidth demand of underwater sensor network.Underwater visible light communication is a new communication mode with high speed,low delay and low energy consumption.It has great application prospects in the construction of a new type of underwater sensor network,but there are also problems such as the need for optical alignment of the communication link and the stability of which is easily affected by water quality.From the perspective of enhancing the stability and reliability of underwater visible light communication,this thesis proposes an underwater acoustic-optical cooperative communications experiment system scheme.The underwater acoustic communication does not require strict line-of-sight transmission.It provide auxiliary alignment and automatic repeat request(ARQ)functions for optical communication with its advantages of long distance and high stability.Based on this scheme,this thesis designs and implements an experiment system that includes underwater visible light modem,underwater acoustic modem,and remotely operated vehicle(ROV).In the part of the visible light modem,the optical communication transceiver circuit is designed based on the blue LED and the silicon-based PIN photodiode.The RS(255,239)and 8B/10B cascaded channel coding and decoding system is realized based on FPGA.The Raspberry Pi is used as the main control circuit of the system.After testing,the underwater visible light modem can realize communication with a transmission distance of7.6m and a transmission rate of 5Mbps in a laboratory pool,and the error rate after error correction is lower than 10^-6.In addition,with the help of the ROV with 4 degrees of freedom movement ability,the system can realize the automatic alignment function of optical communication assisted by underwater acoustic communication.Inspired by the traditional ARQ protocol,this thesis proposes an error-free data transmission protocol for underwater acoustic-optical cooperative communication.The protocol transmits large-capacity data through optical communication,and realizes the packet loss feedback function with the help of underwater acoustic communication,so that the two-way underwater optical communication link does not need to be established.While achieving strong error control performance,the difficulty of optical alignment of the system is further reduced.In order to improve the throughput performance of the system as much as possible,five ARQ schemes are designed,corresponding to different packet loss feedback retransmission strategies.Through simulation,the throughput performance of each scheme under different data size and packet loss rate is analyzed and compared,and experiments are carried out on the designed hardware system platform.The result of experiments shows that the protocol can ensure error-free data transmission in the underwater optical communication channel with a certain packet loss rate.The designed hybrid error correction system scheme has better error control performance combining forward error correction channel coding and automatic repeat request mechanism.