In recent years, underwater sensor networks can be used for environment monitoring, disaster prevention, and military surveillance. When acoustic 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.
This thesis presents constructing transmitter and receiver by using a direct sequence spread spectrum techniques to DPSK (differential phase shift keying) scheme in underwater acoustic communication. Using direct sequence spread spectrum is helpful to overcome the jamming unlike other spread spectrum methods and has strong security between user. Since DPSK signal can be demodulated if the receiver knows only the phase difference between the adjacent bits, DPSK receiver structure has the advantage of being simplified. In the conventional receiver, two adjacent symbols of transmitted signal before despread are passed to the transition correlator that detects data by comparing maximum correlation outputs. At this time, the error for maximum value of the correlator output may increase because of low SNR (signal-to-noise ratio) or high Doppler shift frequency according to the underwater channel. In this thesis, a new method is proposed for accurate detection result. The proposed method uses the peaks’ width as well as the its magnitude among outputs produced by the correlator for BER performance enhancement. The performances of the proposed method was evaluated by simulation and sea trial data.