756 IEEE JOURNAL OF OCEANIC ENGINEERING, VOL. 35, NO. 4, OCTOBER 2010 Passive Time Reversal Acoustic Communications Through Shallow-Water Internal Waves Aijun Song, Member, IEEE, Mohsen Badiey, Member, IEEE, Arthur E. Newhall, Member, IEEE, James F. Lynch, Fellow, IEEE, Harry A. DeFerrari, and Boris G. Katsnelson Abstract—During a 12-h period in the 2006 Shallow Water Experiment (SW06), binary phase shift keying (BPSK) signals at the carrier frequencies of 813 and 1627 Hz were propagated over a 19.8-km source–receiver range when a packet of strong internal waves passed through the acoustic track. The communication data are analyzed by time reversal processing followed by a single-channel decision feedback equalizer. Two types of internal wave effects are investigated in the context of acoustic commu- nications. One is the rapid channel fluctuation within 90-s data packets. It can be characterized as decreased channel coherence, which was the result of fast sound-speed perturbations during the internal wave passage. We show its effect on the time reversal receiver performance and apply channel tracking in the receiver to counteract such fluctuation. The other one is the long-term (in the scale of hours) performance degradation in the depressed waveguide when the internal waves passed through the acoustic track. Even with channel tracking, the time reversal receiver expe- riences average 3–4-dB decrease in the output signal-to-noise ratio (SNR). Such long-term performance degradation is explained by the ray approximation in the depressed waveguide. Index Terms—Acoustic communications, decision feedback equalizers, internal waves, time reversal processing. I. INTRODUCTION U NDERWATER acoustic channels are challenging for dig- ital communications due to excessive multipath spread [1]. Further, the variability of the ocean environment can cause fluctuations of acoustic channels that can result in additional limitations on digital communications [2]–[4]. Among the dynamic ocean processes, internal waves can cause significant variation of acoustic pulses propagating in shallow water [5]. There have been a number of experimental and modeling efforts to investigate acoustic fluctuations in the presence of internal Manuscript received February 03, 2009; revised December 23, 2009, March 17, 2010, and June 25, 2010; accepted June 26, 2010. Date of publication November 09, 2010; date of current version November 30, 2010. This work was supported by the U.S. Office of Naval Research (ONR) Code 322OA under Grants N00014-07-1-0546 and N00014-06-1019. Associate Editor: H. Song. A. Song and M. Badiey are with the College of Earth, Ocean, and Environ- ment, University of Delaware, Newark, DE 19716 USA (e-mail: ajsong@udel. edu). A. E. Newhall and J. F. Lynch are with the Department of Applied Ocean Physics and Engineering, Woods Hole Oceanographic Institution, Woods Hole, MA 02543 USA. H. A. DeFerrari is with the Division of Applied Marine Physics, University of Miami, Miami, FL 33149 USA. Boris G. Katsnelson is with the Physics Department, Voronezh State Univer- sity, Voronezh 394006, Russia. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JOE.2010.2060530 waves. For example, internal waves were reported causing in- tensity fluctuations of acoustic pulses during the 1995 Shallow Water Acoustics in Random Media (SWARM-95) Experiment [6] and these fluctuations were later modeled [7], [8]. The coherence properties of acoustic pulses in an internal wave field have also been studied [9]–[12]. Both coherence and intensity fluctuations of the acoustic channel can affect the performance of coherent underwater acoustic communications. However, only limited efforts have been reported to relate these effects to the performance of acoustic communications. The only notable example is [13], where signal temporal coherence in several experiments has been summarized. Sample demodulation results at the carrier frequency of 400 Hz have been used to show effects of the decreased signal coherence on the decision feedback equalizer (DFE). The decrease of signal coherence has been attributed to the presence of internal waves, which were referred to as the common physical processes in the experimental sites. Time reversal is a pulse compression technique which fo- cuses on sound transmissions that have been spread by propaga- tion in a multipath environment without prior knowledge of the medium [14]. After being first demonstrated in the 1990s [15], both active [16] and passive [17] time reversals have been inves- tigated for acoustic communications. More recently, the time re- versal approach has been combined with DFEs to suppress the residual intersymbol interference (ISI) [18], [19] for carrier fre- quencies of 3–5 kHz or below. Channel and phase tracking has also been incorporated into time reversal receivers to achieve reliable communications in dynamic ocean environments at a higher central frequency (12 kHz) [4]. During the 2006 Shallow Water Experiment (SW06) [20], concurrent environmental observations and acoustic measure- ments were made to study internal wave effects on coherent un- derwater acoustic communications. During a 12-h communica- tion period over a 19.8-km acoustic track, a packet of strong in- ternal waves was observed by ship radars and thermistor strings. Binary phase shift keying (BPSK) signals at the carrier frequen- cies of 813 and 1627 Hz are analyzed by time reversal pro- cessing followed by a single-channel DFE. Two types of internal wave effects are investigated in the con- text of passive time reversal acoustic communications. One is the rapid channel fluctuation observed within 90-s data packets. It can be characterized as decreased channel coherence, which was the result of fast sound-speed perturbations during the in- ternal wave passage. We show its effect on the time reversal receiver performance and apply channel tracking in the com- munication receiver to counteract such rapid fluctuation. The 0364-9059/$26.00 © 2010 IEEE