Data Packet Structure and Modem Design for Dynamic Underwater Acoustic Channels

In underwater acoustic (UWA) communications, the propagated signal undergoes severe Doppler and multipath distortions. The Doppler estimation/compensation and channel estimation/equalization techniques are required to deal with these distortions and contribute significantly to the overall complexity of UWA modems. In this article, we propose a data packet structure for high data rate transmission in time-varying UWA channels with the channel dynamic modeled by velocity and acceleration between the transmitter and the receiver. The data packet consists of superimposed data and periodic pilot sequences. The superposition allows achievement of a high spectral efficiency for data transmission. The repeated pilot symbols allow the use of a low-complexity multibranch autocorrelation method for coarse estimation of Doppler parameters related to the velocity and acceleration. To refine these estimates, we further propose a low-complexity fine Doppler estimator based on dichotomous iterations. We also present a low complexity frequency domain channel estimator exploiting the channel sparsity. The proposed modem design has been evaluated using simulation and practical sea trials. The experiments demonstrate high detection performance of the proposed design, in particular in comparison with a more traditional design that ignores the acceleration between the transmitter and the receiver.