A transceiver architecture for ultrasonic OFDM with adaptive Doppler compensation

We propose an acoustic OFDM system for underwater wireless communication in vertical direction, between a mother ship in the surface and a robot at the sea bottom. In our experiments, the velocity changes roughly over time, though it is not very high. In addition, Doppler does not manifest itself explicitly in time compression/expansion, for example, a velocity of 1(m/s) causes a frequency offset of 16 (Hz), but a small compression/expansion of 1(sample/an OFDM symbol). Therefore, we propose an adaptive Doppler compensation consisting of two stages without resampling. The first stage is phase de-rotation to compensate a common frequency offset before FFT processing. The second stage is non-uniform Doppler compensation by an ICI matrix before channel estimation. To boost the range of frequency offset estimation, we use continual pilots in conjunction with monitoring the drifting of power delay profile over time. Experimental results taken in Shizuoka, Japan show our system using QPSK, and 16QAM achieved a data throughput of 7.5(Kbit/sec) with a transmitter moving at maximum 2(m/s), over 30m vertically.