Low complexity multipath and Doppler compensation for direct- sequence spread spectrum signals in underwater acoustic communication

摘要

A method to achieve low complexity and low error decoding of direct-sequence spread spectrum signals is demonstrated using measured underwater acoustic communications. Unlike terrestrial radio, the ocean has pronounced and variable multipath (echoes) as well as strong Doppler due to the relatively slow sound speed. The severity of the decoding problem, as analyzed on experimental data, is characterized for both stationary and moving sources. Here, a method based on measuring and updating a passive conjugate matched filter is shown. This method produced low bit-error rates, even in cases with channel fading, by mitigating variable multipath effects. Additionally, Doppler estimation is accomplished by correlating each received symbol with a filter bank comprised of the most recent matched filter shifted by various Doppler offsets. Upon finding the best match, the received symbol is shifted, correlated, and decoded. This method is demonstrated on at-sea experimental data and shown to be a computationally efficient and robust method to receive low SNR. Error free results are shown down to -5 dB input signalto-noise ratio. The method is then further improved by 2 dB by allowing for non-constant Doppler shift functions within a symbol period, thereby improving Doppler estimation. Published by Elsevier Ltd.