Design criteria and analysis procedures for a practical OFDM underwater communications system are presented. A new broadband underwater channel model with Doppler time scaling effect is developed. A two-dimensional MMSE scheme using time and frequency pilots is employed for channel estimation. Doppler frequency shift is estimated using cyclic prefix and is compensated by a digital sampling rate conversion procedure. Numerical simulations demonstrate that OFDM technique is robust with respect to frequency selectivity and time selectivity.
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