Choosing the right signal: Doppler shift estimation for underwater acoustic signals

In this paper, we consider the problem of estimating the coarse Doppler shift ratio for underwater acoustic communication (UWAC). Since underwater the constant motion of nodes results in Doppler shifts that significantly distort received signals, estimating the Doppler shift and compensating for it is required for all UWAC applications. Different than for terrestrial radio-frequency where the Doppler effect is modeled by a frequency shift, due to the slow sound speed in water, the effect of transceiver motion on the duration of the symbol cannot be neglected. Furthermore, since the carrier frequency and the signal bandwidth are of the same order, UWAC signals are considered wideband and Doppler-induced frequency shifts cannot be assumed fixed throughout the signal bandwidth. Considering these challenges, we present a method for Doppler-shift estimation based on comparing the arrival times of two chirp signals and approximating the relation between this time difference and the Doppler shift ratio. This analysis also provides an interesting insight about the resilience of chirp signals to Doppler shift. Our simulation results demonstrate improvement compared to commonly used benchmark methods in terms of accuracy of the Doppler shift estimation at near- Nyquist baseband sampling rates.