Doppler tracking and compensation for underwater acoustic channels using shift-orthogonal pilot sequences

Abstract

A novel time-domain Doppler tracker and compensator, using shift-orthogonal OFDM pilot sequences, was developed, and assessed in simulations and early in-water trials. The objective is to address large Mach numbers like those experienced in communicating autonomous underwater vehicles (AUV). To start, the Doppler estimator extracts pilots in the received signal to produce Mach number estimates for every received signal sample. Given the large number of estimator samples, a Doppler tracker is proposed to reduce the number and simultaneously track Mach number variations. This novel tracker reduces the computational load on the compensator and makes it possible to implement on AUVs. Then, the Doppler compensator's first stage resamples the received signal using the Mach number estimates from the tracker's output. Since the estimates vary with time, the resampling performed is time-varying. Then, the final stage estimates the residual Doppler shift from the estimator and eliminates it with a phase rotation. The proposed tracker and compensator receiver outperform most existing compensators as measured through the mean-squared error, is more computationally efficient and addresses higher Mach numbers. Each element of the proposed receiver is tested in simulations and the results are shown to agree with theory. Comprehensive at-sea trials are next.