Low-Complexity UAC Modem and Data Packet Structure

In underwater acoustic communication (UAC), the propagated signal undergoes severe multipath and Doppler distortions. The high overall complexity of UAC receivers is essentially due the Doppler estimation, channel estimation and equalization techniques required to deal with these effects. In this paper, we propose a novel data packet structure for transmission in underwater acoustic channels. This data structure allows the development of relatively simple estimation and equalization techniques, thus resulting in a low-complexity modem design. The data packet is based on single-carrier modulation with superimposed data and pilot symbols. The pilot symbol sequence is repeated within the packet, thus allowing application of the multi-branch autocorrelation Doppler estimation, possessing a low complexity and high accuracy. The data rate in the packet can easily be adjusted depending on the propagation conditions. The received packet is processed in the frequency domain, thus allowing low-complexity channel estimation and equalization. More specifically, in the example design, the channel estimation is based on local cubic B-splines. The modem has been evaluated using numerical simulation and experiments in a water tank, demonstrating successful performance of our design.