Research And Implementation Of FRFT-based Frame Synchronization Method For Underwater Acoustic Communication

Underwater acoustic communication is pivotal in marine technology and has good prospects for application.In underwater acoustic communication,synchronization is the primary task at the receiver end of the communication system,which is crucial for the subsequent demodulation process.The underwater acoustic channel has severe multipath propagation,doppler effect,and limited bandwidth,which makes the synchronization process of underwater acoustic communication more challenging.The frame synchronization method based on matching correlation has high probability of missing and false detection and low synchronization accuracy under complex underwater acoustic channel conditions,and the frame synchronization method based on time-frequency analysis is computationally intensive and not applicable to underwater nodes powered by battery with limited energy.Therefore,in order to improve the synchronization accuracy and communication quality in underwater acoustic communication,the thesis carries out the research work of frame synchronization in underwater acoustic communication and verifies the effectiveness of the proposed frame synchronization method by using the self-developed underwater acoustic communication device.The paper also investigates the use of cross-layer approach to solve the problem of selective frequency fading caused by multipath effect in the underwater acoustic channel,which leads to the degradation of communication quality.To address the problem of insufficient accuracy of matching-related frame synchronization methods,the paper proposes a FRFT-based frame synchronization method for underwater acoustic communication.In the proposed method,a rough estimate of the starting point of frame synchronization is first obtained by using the double energy window method,and then the data after the rough estimate of the starting point of frame synchronization is subjected to the noise-reducing FRFT transform to determine the starting point of the data frame according to the peak position,while the dichotomous method is introduced in the process of sliding FRFT window to reduce the computational effort of the synchronization process.The simulation results show that the communication system constructed by the adopted synchronization method can effectively reduce the mean square error of frame synchronization in the underwater acoustic channel.In addition,the underwater acoustic communication using frequency shift keying modulation is susceptible to frequency bias,which leads to the degradation of communication quality.The paper considers the joint estimation of frame synchronization and frequency bias,and then uses the estimated frequency bias to compensate for the demodulation process.However,the above estimation process is a very computationally intensive two-dimensional search problem.In this paper,the PSO algorithm is introduced to optimize the two-dimensional FRFT process;meanwhile,the PSO is set to variable inertia weights and the u-domain is set to discrete domain and the p-domain is set to continuous domain in the two-dimensional PSO-FRFT search process.The computational speed is further improved.The simulation results show that the frequency bias compensation can effectively reduce the system BER.Considering the frequency selective fading problem caused by multipath effect of underwater acoustic channel,the paper proposes a cross-layer communication method based on channel estimation to improve the quality of underwater acoustic communication.In the proposed method,channel estimation is first performed using the leading signal to find the frequency range where the multipath effect causes frequency fading;then the estimation result is returned to the transmitter through the acknowledgement character,which is used to adjust the frequency range at the transmitter.The simulation results show that the proposed method can reduce the system BER.Finally,the proposed frame synchronization method and the cross-layer-based communication quality improvement method are implemented in the self-developed underwater acoustic communication device,and the sea trial is conducted in the East China Sea.The results of the sea trial show that the proposed and implemented frame synchronization method can correctly complete the frame synchronization process,and the BER of the system can be as low as8.59×10^-3,which is about 13.14%lower than the BER of the system based on the traditional FRFT frame synchronization method when the two nodes are 1.8km apart and the data is transmitted at 1kbps,and the cross-layer-based communication quality improvement method can further reduce the BER by about 10%when the transmission rate is constant and the two nodes are 1km apart.further reduce the BER by about 10%.