Research On Differential Frequency Division Multiplexing Based Underwater Acoustic Communication

As the users and the transimitted information through the acoustic channel are greatly increased,there are new challenges to high-speed wireless underwater acoustic(UWA)communication.Based on Orthogonal Frequency Division Multiplexing(OFDM)technology,the differential OFDM underwater acoustic communication,cyclic suffix technology and vector signal processing technology are studied in the paper to further improve the effective data transmitting rate and guarantee the communication quality.Meanwhile,Single Carrier Frequency Division Multiple Access(SC-FDMA)technology is applied in the UWA communication to solve the peak-to-Power average ratio(PAPR)problem of OFDM,and it can reduce the transmission power and power amplifier cost at UWA communication nodes.Firstly,differential modulation is proposed in Orthogonal Frequency Division Multiplexing(OFDM)based underwater acoustic(UWA)communication.It includes differential phase-shift keying(DPSK),multilevel differential amplitude and phase-shift keying(MDAPSK)in time domain and a time-frequency differential modulation.All these differential demodulations can against inter-symbol inference,avoid the phase ambiguity and reconstruct transmitted information without adding pilots to monitor the channel.That has the lower complexity of the system and the better spectrum utilization,which meet the need of the high transmitting rate in underwater wireless Ad Hoc Network.Furthermore,the time-frequency differential demodulation can realize narrow-band Doppler compensation for OFDM system.An acoustic vector sensor is used in the differential OFDM system,and4.8 dB gains can be obtained by combined processing pressure and velocity signals from the acoustic vector sensor to further improve the performance of the system.Mixed Excitation Linear Prediction(MELP)speech compression coding technology with 2.4 kbps-transmitting rate required was used in this differential OFDM based UWA communication system to achieve underwater speech communication.Secondly,a cyclic postfix is added as the guard interval for differential OFDM underwater communication and the application of cyclic postfix in under ice communication,time reversal mirror(TRM)channel equalization and single vector acoustic covering and focusing technology is studied.The structure of acoustic channel under ice is relatively stable,but it does exist that the first received signal is not the strongest.In this case,if only the cyclic prefix is added as the guard interval,the current symbol will be contaminated by the next one,which destroys the orthogonality among subcarriers and brings inter symbol interference(ISI).Thus the cyclic postfix is proposed and to be added to the guard interval.And it can ensure that the data for FFT operation is only from the current symbol and avoid the use of complex timing synchronization algorithm and equalizer to overcome the ISI.The effective channel for the TRM and signal vector covering and focusing based OFDM communication system is the TRM channel,which is the result of the inverse convolution of the actual channel and the estimated channel.The cyclic postfix is applied in the system to against the multipath inference caused by the side lobe before the main peak of TRM channel,and to improve the performance.Thirdly,Single Carrier Frequency Division Multiple Access(SC-FDMA)technology is applied in the UWA communication.It is similar to OFDMA,both of them divides the transmission bandwidth into multiple subcarriers for transmitting in parallel using inverse Fourier transform(IFFT)and serial to parallel conversion techniques.However the SC-FDMA system has another DFT transform module which transmits the signal in the form of single carrier,and it makes that SC-FDMA has lower peak-to-average power ratio(PAPR)and saves energy overhead.SC-FDMA is applied in UWA communication in this paper,and a SC-FDMA underwater communication system using channel estimation and TRM technology is proposed.Finally,the experiments in the lake,sea and under ice are carried out to verify the high data transmitting rate and robustness of the differential OFDM underwater communication system.The image and the MELP coded speech information are used as source information to be transmitted.And the results show all the proposed technology and scheme have effectiveness and robustness in the UWA communication.