Research On Multi-Carrier Frequency-Hopping Underwater Acoustic Mobile Communication Method

Shallow sea acoustic channel has become a very complex wireless channel due to its inherent time-space-frequency variable characteristics,strong multi-path interference,large environmental noise,limited transmission bandwidth and other characteristics.At the same time,the difference between the speed of the underwater motion platform and the propagation speed of underwater sound wave is small,which makes the Doppler effect more obvious and the information transmission more difficult.These factors bring great challenges to the effectiveness,universality and reliability of underwater mobile acoustic communication.Underwater acoustic Communication System based on Multi-Carrier Frequency-Hopping/Differential Phase Shift Keying,not only embodies the characteristics of high processing gain of spread spectrum communication,which has strong anti-multi-path and anti-multi-access disturbance,but also effectively integrates multi-carrier communication,which improves the spectrum utilization rate and channel capacity of the system,is suitable for realizing underwater acoustic mobile communication with serious intersymbol interference.This thesis focuses on the research of key technologies of mobile underwater acoustic communication based on MC-FH/DPSK,including underwater acoustic channel modeling,theory and simulation of MC-FH/DPSK system,theory and simulation of error correction coding technology.The lake test of mobile underwater acoustic communication is designed and completed,and the test results are analyzed in detail.The main research work of this thesis is as follows:1.The physical characteristics of underwater acoustic channel,such as transmission loss,marine environmental noise,multi-path propagation and Doppler effect,are researched theoretically to lay a theoretical foundation for underwater acoustic channel modeling.By comparing with wireless channel,the complex and changeable channel environment of underwater acoustic channel is highlighted.The time-varying multipath channel model are presented to provide a basis for the performance evaluation of underwater acoustic communication system.2.For the MC-FH/DPSK underwater acoustic communication system,the anti-multipath and anti-Doppler performance is researched,and the bit error rate performance of the system is studied in the measured channel.The simulation results compared with FH/4FSK system show that,the system’s ability of resisting multipath interference is basically unchanged,but the information transmission rate is increased by one time.When the communication data frame length is 840bit and the symbol width is 4ms,the tolerated Doppler velocity of the MC-FH/DPSK communication system can reach 0.5m/s.When the relative velocity of motion exceeds the velocity,the corresponding Doppler estimation and compensation measures are needed.In the measured channel,the bit error rate of the system can only reach 10^-2～10^-3magnitude.In order to obtain the bit error rate performance of 10^-5 magnitude or lower,appropriate error correcting coding technology is still needed.3.To solve the problem of high original bit error rate of MC-FH/DPSK underwater acoustic communication system,two codes,which near Shannon limit,Turbo code and LDPC code,are applied to the underwater acoustic communication system.Aiming at the complex problem of random construction of LDPC codes,QC-LDPC codes are constructed by using structured Quasi-Cyslic method.Aiming at the problem that the decoding performance is limited due to the poor randomness of QC-LDPC code,the H matrix of QC-LDPC code is improved by removing four and six rings.The simulation results show that,in the measured channel,the code length is 420bit,the coding efficiency _bR is 1/2,and the symbol duration time is 8ms,QC-LDPC code has a SNR gain of 3d B compared with LDPC code.When the code length is840bit,_bR is 1/2,and the symbol duration time is 8ms,When_bE/N ₀≥22d B,Turbo codes using SOVA decoding method can reduce the system bit error rate to less than 10^-5;while QC-LDPC codes using BP decoding method are in_bE/N₀≥20d B,the bit error rate performance below 10^-5 can be obtained.4.On the basis of theoretical analysis and performance simulation research,the lake test of underwater multi-carrier frequency-hopping mobile underwater acoustic communication is designed and implemented,and the test data under different Doppler speed and different distance channel conditions are processed and analyzed in detail.The experimental results show that:（1）the proposed QC-LDPC+MC-FH/DPSK decoding algorithm is effective;（2）In underwater acoustic multipath fading channels,when certain SNR conditions are reached,the performance of the original bit error rate of the MC-FH/DPSK communication system is generally on the order of 10^-2～10^-3;（3）The error correcting coding technology can effectively reduce the bit error rate of the system.For example,the QC-LDPC code adopted in the test can achieve a simulated Doppler speed of 30Kn at a distance of 3000m and a communication rate of 242bps for zero-error hydroacoustic communication in the communication test of finite frames.