In recent years, underwater sensor networks can be used for environment monitoring, disaster prevention, and military surveillance. Unlike terrestrial communication, underwater acoustic communication utilizes sound waves. The sound speed is dependent on the water temperature and the salinity. The sound transmission path is changed by refraction according to the gradient of vertical sound velocity. When sound waves are passed through the underwater, they are affected by attenuation, reflection of bottom and surface, scattering, ambient noise, and the Doppler effect caused by movement of the transmitter and the receiver. Of these factors, the Doppler effect affects the correlation time of the channel related to the transmission efficiency. These influences thus cannot be neglected in the design of underwater acoustic communication systems. To increase transmission efficiency, estimation the changing Doppler frequency and compensation for distortion are needed. In this thesis, Doppler shift frequency estimation methods to use in the time varying channel proposed. Two kinds of methods are proposed. One is a Doppler estimation algorithm that has stable performance as a part of the communication system. From an overall communication system perspective, the other is a demodulation scheme that is different from the conventional communication system.
The ambiguity function and decision-directed estimation are presented. Also, a more accurate Doppler estimation method that is based on decision-directed estimation is proposed, and a re-modulation method that can update the channel parameters for the equalizer operation and estimate the Doppler frequency during the data frame using two Doppler estimation techniques is proposed. A simulation based on the Bellhop modeling and sea experiment was carried out to compare the performance of the proposed method with a conventional non-recursive Doppler estimation. When the channel coding technique was not applied, the uncoded bit error rate performance of the proposed method was improved about maximum 50.3 % compared with conventional method.