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With the goal of achieving high stability and reliability to support underwater point-to-point communications and code division multiple access (CDMA) based underwater networks,a direct sequence spread spectrum based underwater acoustic communication system using dual spread spectrum code is proposed.To solve the contradictions between the information data rate and the accuracy of Doppler estimation,channel estimation,and frame synchronization,a data frame structure based on dual spread spectrum code is designed.A long spread spectrum code is used as the training sequence,which can be used for data frame detection and synchronization,Doppler estimation,and channel estimation.A short spread spectrum code is used to modulate the effective information data.A delay cross-correlation algorithm is used for Doppler estimation,and a correlation algorithm is used for channel estimation.For underwater networking,each user is assigned a different pair of spread spectrum codes.Simulation results show that the system has a good anti-multipath,anti-interference,and anti-Doppler performance,the bit error rate can be smaller than 10 6 when the signal-to-noise ratio is larger than-10 dB,the data rate can be as high as 355 bits/s,and the system can be used in the downlink of CDMA based networks.