Research And Implementation Of OFDM Underwater Acoustic Communication Under Time-varying Channel

The 21st century witnesses rapid development of underwater acoustic communication,which has been widely used not only in the military field,but also in the business field,such as the development and utilization of marine resources,marine environment monitoring,marine surveying and mapping etc.However,the ocean channel with random time-space-frequency variation characteristic,limited bandwidth,multipath delay and Doppler shift,hiders the rapid development of underwater acoustic communication.Orthogonal Frequency Division Multiplexing(OFDM)technology,compared with single carrier communication,has the advantages of high spectrum utilization,strong anti-multipath ability and high transmission rate.But the multipath delay and the serious Doppler shift in the time-varying underwater acoustic channel seriously affect the performance of the OFDM underwater acoustic communication system.OFDM is very sensitive to Doppler shift.And if there is Doppler shift,the orthogonality between subcarriers will be destroyed,which will lead to a great reduction in demodulation performance.So,in order to ensure the accuracy of OFDM underwater acoustic communication and reduce the system error rate,we need to take some methods to eliminate the influence of Doppler shift.Therefore,the estimation and compensation of Doppler shift is an important part of OFDM underwater acoustic communication system.In this paper,OFDM underwater acoustic communication and its hardware implementation under time-varying channel are studied in combination with OFDM,time-frequency differential modulation and channel coding techniques,and simulation and sea trial are also carried out.The design of OFDM underwater acoustic communication system under time-varying channel is the main work of this thesis.As follows:1.Analyze the influence of time-varying channel and the application of time-frequency differential modulation in OFDM underwater acoustic communication.According to the characteristics of underwater acoustic channel,a Doppler shift estimation method based on bit error rate search is proposed.Via the simulation and sea trial show that the Doppler shift estimation method based on bit error rate search can accurately estimate the Doppler shift.After resampling,Doppler shift can be eliminated effetely.2.Analyze the common methods used for Doppler shift compensation.The polyphase filter-banks are usually used for interpolation and decimation to change the sampling rate,but the required number of filters is large and the implementation complexity is high.Considering the low complexity and simplicity of hardware implementation,a variable sampling rate hardware compensation scheme is proposed in this paper for Doppler shift compensation3.The OFDM underwater acoustic communication system based on STM32 is designed and implemented,which performance under time-varying condition is verified by sea trial.