Differential OFDM Employing AFC for Fast Time-Varying Doppler Shifts in Underwater Acoustic Communications

This paper proposes differential orthogonal frequency division multiplexing (OFDM) employing automatic frequency control (AFC) for fast time-varying Doppler shifts in underwater acoustic communications (UWACs). There exist the following two problems in UWACs: 1) severe time and frequency selectivities, i.e., double selectivity; 2) fast time-varying Doppler shifts. With respect to the problem 1), the proposed scheme employs differential encoding/differential detection (DE/DD) for time selectivity, and it also employs OFDM for frequency selectivity. With respect to the problem 2), the proposed scheme employs a multiple open-loop frequency estimation (MOLFE), which has a good trade-off between frequency coverage and estimation accuracy in a short observation time. Next, this paper proposes a simple time-varying Doppler shift simulation model for autonomous underwater vehicle (AUV) turning. Finally, computer simulation results confirm that the proposed differential OFDM employing MOLFE has excellent performance in doubly-selective channels with fast time-varying Doppler shifts.