Mode Equalization at Megameter Ranges

Abstract

Low frequency underwater sound propagation over ranges of 3.5 megameters or more has a complicated multipath arrival structure with early steep angle-arrivals, followed by an energetic finale composed of the lower order acoustic modes. Internal waves produce time-varying multipath and induce frequency-selective fading in the received signals. The low mode arrivals are strongly affected by internal waves, making it difficult to obtain precise travel time measurements for these signals. An equalizer along with suitable spatial filters, mitigates the multipath of the lower order modes. The Signal to Noise Ratio (SNR) measured at the output of the equalizer is used as an observable to localize modes, make time of arrival (TOA) measurements and measure the multipath spread of the modes. Results for the new mode equalizer on a simulated channel are presented. The mode equalizer is also tested on one of the North Pacific Acoustic Laboratory (NPAL) receptions.

Stochastic Models and Robust Estimation for Broadband Acoustic Mode Signals

“…As a part of this project, the problem of equalizing these signals has been studied. Tarun Chandrayadula presented a paper on the design of mode equalizers at the September 2005 IEEE/MTS Oceans Conference [10]. He also presented a talk on the multi-mode decision feedback equalizer for underwater acoustic communications at the 2005 Underwater Acoustic Signal Processing Workshop [11].…”

“…Motivated by this earlier work, we began exploring the design of equalizers for the low mode signals. In 2004-2005, we implemented a DFE-based equalizer for the low modes [10]. In tomography (as opposed to communications), the transmitted symbols are known.…”

“…When the equalizer synchronizes to a time instant that enables it to combine all the multipaths in the signals, the decoded symbols will have a low mean square error, thus a high SNR. Chandrayadula presented a paper at the 2005 IEEE/MTS Oceans Conference [10] demonstrating that equalizers can be designed for the low mode signals in the NPAL receptions. Figure 3 shows examples of SNR vs. synchronization time curves for modes 1 and 10 in one of the NPAL-98 receptions (for the 40-element VLA located off California).…”

“…The plots on the right show the output SNR vs. synchronization time curves obtained from the mode equalizer. See the paper by Chandrayadulaand Wage for more details about these results[10].…”