Study Of Underwater Wireless Optical Communication System Using 16-ary QAM Technique Based On FPGA

Underwater wireless optical communication?UWOC?technology has the advantages of high bandwidth,low power consumption and compact size,which can provide strong technical support for real-time,high-speed and short-range underwater wireless communication.It will play an important role in the fields of marine resources exploitation,marine environmental monitoring and study on marine geological structure,which attracts most countries as one of the most important communication technologies in the world.In order to improve data rates,robustness and practicability of UWOC system,this paper summarizes the study of UWOC systems'architecture,the analysis of light absorption and scattering effects in seawater,the comparison of laser diode and light emitting diode in the transmitter and details of devices in receiver,and implements an real-time UWOC system based on Field Programmable Gate Array?FPGA?,combining the 16 Quadrature Amplitude Modulation?16QAM?technology which can efficiently provide high data rates.The 16QAM demodulation technology with improved carrier recovery algorithm is also proposed,which can strengthen the robustness of the 16QAM modulation system.Owing to the practical point of view,signal processing is realized based on FPGA to construct the whole UWOC system,which can improve the practicability of the system.The achievements and related work of this paper are organized as follows.Firstly,the principle of 16QAM modulation and demodulation technology is deeply studied.Aiming at the advantages and disadvantages of several different phase detection algorithms in carrier recovery module of demodulation system,an improved carrier recovery algorithm is proposed,which integrates directional-decision and polar decision based phase detector algorithms.Then 16QAM scheme is simulated by using MATLAB software.The bit error rate is analyzed with different signal-to-noise ratio?SNR?,and the performance of improved carrier synchronization algorithm is verified.Secondly,this paper improves the practicability of UWOC system based on FPGA,then elaborates the whole system and global clock allocation scheme in detail,and analyzes the implementation process of data source,serial-to-parallel conversion,constellation mapping,up-conversion shaping filter,carrier generation and other modules in transmitter.Further,the parameter calculation and the implementation scheme of the improved carrier recovery loop,bit synchronization loop,threshold decision,demapping,parallel-to-serial conversion and bit error rate statistics modules in the receiver are explained.Finally,based on the MODELSIM simulation results,a practical underwater wireless optical communication system is built and tested in different water turbidity.Experimental results show that this system can achieve data rates up to 50 Mbit/s over a distance of 3 m underwater with a bit error rate?BER?of 7.11×10^-44 in artificial seawater.This BER is below the forward error correction?FEC?target of 3.8×10^-3.Thus,this system can provide reliable high-speed communications over short to moderate distance point-to-point UWOC links.