An underwater acoustic 64QAM OFDM communication system with robust Doppler compensation

This paper proposes an underwater acoustic OFDM communication system to transmit image or movie data from deep see AUV etc. to surface ship. In order to mitigate Doppler shift effect caused by transmitter and/or receiver movement, the OFDM receiver possesses additional time domain signal processing such as signal shrink-expansion processing and Doppler shift compensation capabilities. The outputs of the four OFDM receivers are further combined by frequency domain diversity combiner with Maximum-Ratio Combining (MRC) algorithm to improve Signal to Noise ratio. The proposed communication system is verified by Ocean measurement at Shizuoka prefecture in Japan. Our system utilized 20-28[KHz] ultrasonic OFDM channel and subcarrier spacing of 100[Hz]. Total number of sub-carriers is 81. QPSK / 16QAM / 64QAM modulations are used. By performing experiment in the ocean at depth of 1000m, BER of approximately less than 1E-2 has been successfully obtained using 64QAM without any error correction.