We propose an acoustic OFDM system for underwater communication, specifically for vertical link communications such as\nbetween a robot in the sea bottom and a mother ship in the surface. The main contributions are (1) estimation of time varying\nDoppler shift using continual pilots in conjunction with monitoring the drift of Power Delay Profile and (2) symbol-by-symbol\nDoppler compensation in frequency domain by an ICI matrix representing nonuniform Doppler. In addition, we compare our\nproposal against a resampling method. Simulation and experimental results confirm that our system outperforms the resampling\nmethod when the velocity changes roughly over OFDM symbols. Overall, experimental results taken in Shizuoka, Japan, show our\nsystem using 16QAM, and 64QAM achieved a data throughput of 7.5Kbit/sec with a transmitter moving at maximum 2 m/s, in a\ncomplicated trajectory, over 30m vertically.
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