Doppler Estimators Using Pilot Tones for Ofdm Communication Signal in Shallow Water Environment

Underwater acoustic communications constitute a very difficult issue since the transmission channel is severely doubly dispersive. Indeed, radio waves suffer from strong attenuation so that the only solution is to use acoustical modems. Thus, achievable bandpass is very reduced (typically less than 20kHz) and narrowband signal approximation cannot be assumed since sound velocity is around 1500 m/s and frequency carrier is around 10kHz. A natural choice to fight against time dispersivity is the use of OFDM (Orthogonal Frequency Division Multiplex). Unfortunately, this choice increases the sensitivity to the Doppler effect. That is why the subject of this paper, the estimation of this Doppler effect, is so crucial. For this purpose, we use ZP-OFDM (OFDM with Zero Padding) modulation in order to consider independent time blocks with duration chosen such that the Doppler effect can be considered as approximately constant. Our study is restricted to the case of a single path. Firstly, Doppler is estimated thanks to a correlation between the received signal and a pilot signal. Then, another estimator is also studied, using the minimization of a cost function based on the exponential base of the OFDM signal.