Underwater Acoustic Digital Communication System

In this thesis, the characteristic of time-varying fading multipath channels for the underwater acoustic communication system is analyzed based on communication technique. The technique progresses that combat severe intersymbol interference on the underwater acoustic digital communication system are presented. The fundamental methods of digital modulation systems are presented. The power density spectrum, bandwidth and bandwidth efficiency of various modulations is analyzed. The performances of various modulations are in-depth compared. The choice of a modulation and demodulation technique for the special application condition is analyzed. The implement of digital modulations system is depicted. A digital modulation signal generator based on the PC is developed in C language. It consists of 2ASK, 4ASK, 8ASK, 2FSK, 4FSK, 8FSK, BPSK, DPSK, QPSK. QDPSK modulation signals, direct spread-spectrum sequence signals and bLlrst signals. It can provide flexible options of modulations signals for future experimentations. The trial of the generator system in reservoir is very successful. The importance of PSK coherent demodulation for the underwater channel communication is discussed. The theory and advantage of in-phase perpendicular loop BPSK demodulation are pointed out. The in-phase perpendicular loop BPSK demodulation circuit is designed and realized. The elliptical band pass pre-fllter for the BPSK demodulation is also implemented.