Synchronization, Doppler and channel estimation for OFDM underwater acoustic communications

Designing transmission systems often requires to account for possible fast variations of environment propagation properties. In particular, for acoustic underwater communications, the channel is often highly frequency selective and highly non-stationary. In addition, the relative movement between the transmitter and the receiver generates Doppler Effect and underwater acoustic communication is challenging due to possibly large Doppler range. Doppler results in an expansion-compression of the transmitted signal in the time domain. At the receiver side, it is suitable to compensate its effect before any further processing, such as frame synchronization, equalization. On another hand, orthogonal frequency division multiplexing (OFDM) is a very attractive multi-carrier modulation to mitigate inter-symbol interference (ISI). But one main difficulty is to synchronize the OFDM symbol. The purpose of this paper is to present new techniques for Doppler shift estimation, OFDM symbol synchronization and channel estimation with the help of a dual training sequence.