Publications
Long-Range Underwater Acoustic Communications Using a Multi-Mode Decision Feedback Equalizer
Abstract
The underwater environment imposes challenging constraints limiting the effectiveness of long range Underwater Acoustic (UWA) communication systems. Two of the most severe constraints are: 1) the received Signal to Noise Ratio (SNR) is extremely low due to the long range from the source and 2) scattering due to internal waves increases the multipath spread, causing additional ISI. A successful long range UWA communications system must overcome these limitations. Freitag and Stojanovic [L. Freitag and M. Stojanovic, "Basin Scale Acoustic Communication: A Feasibility Study using Tomography M-sequences", Oceans 2001, pp. 2256-2261.] demonstrated 37.5 bits/second commu- nication capability with signals collected during the Acoustic Engineering Test (AET). The AET experiment had a 75 Hz source and a 20 channel receiving array separated by a distance of ≈ 3250 km. Due to the low received SNR and large channel spreads, a 20-channel Decision Feedback Equalizer (DFE) designed with a 180-tap filter on each channel was required. While it successfully equalized the AET signals, a multi-channel DFE with a total of 3600 taps is a computationally complex algorithm to implement. This paper proposes a multi-mode DFE with lesser computational complexity than the multi-channel DFE. The new approach uses a set of spatial filters to reduce the received pressure field to a set of propagating modes for the DFE to operate on. An individual mode signal has less spread than the received signal on a single channel, which consists of a sum of modes. Lower spreads require fewer taps, meaning a significant reduction in complexity. This paper presents results of testing the multi-mode equalizer on tomographic signals collected during the 1998 North Pacific Acoustic Laboratory (NPAL) billboard array experiment. During the NPAL experiment a 75 Hz source near Kauai transmitted M sequence signals to a 40-channel receiving array at a range of 3900 km. The received signals have exceptionally low SNR due to the proximity of shipping lanes and have large multipath spreads. This paper demonstrates that the multi-mode DFE, using only a subset of modes, can successfully equalize the received signal. Initial results indicate that the mode equalizer can achieve a 7.5 bits/second rate in the noisy NPAL environment. This paper calculates the average bit error rate achievable using different subsets of modes and compares the results with the multi-channel DFE.
[Work supported by an ONR Ocean Acoustics Entry-Level Faculty Award.]
This talk was presented at the 2005 Underwater Acoustic Signal Processing Workshop (UASP) , sponsored by the IEEE Providence Section in cooperation with the IEEE Signal Processing Society. The abstract may be found on the UASP website.