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Menon, R, Gerstoft P, Hodgkiss WS.  2012.  Asymptotic eigenvalue density of noise covariance matrices. IEEE Transactions on Signal Processing. 60:3415-3424.   10.1109/tsp.2012.2193573   AbstractWebsite

The asymptotic eigenvalues are derived for the true noise covariance matrix (CM) and the noise sample covariance matrix (SCM) for a line array with equidistant sensors in an isotropic noise field. In this case, the CM in the frequency domain is a symmetric Toeplitz sinc matrix which has at most two distinct eigenvalues in the asymptotic limit of an infinite number of sensors. Interestingly, for line arrays with interelement spacing less than half a wavelength, the CM turns out to be rank deficient. The asymptotic eigenvalue density of the SCM is derived using random matrix theory (RMT) for all ratios of the interelement spacing to the wavelength. When the CM has two distinct eigenvalues, the eigenvalue density of the SCM separates into two distinct lobes as the number of snapshots is increased. These lobes are centered at the two distinct eigenvalues of the CM. The asymptotic results agree well with analytic solutions and simulations for arrays with a small number of sensors.

Cho, SE, Song HC, Hodgkiss WS.  2012.  Asynchronous multiuser underwater acoustic communications (L). Journal of the Acoustical Society of America. 132:5-8.   10.1121/1.4726029   AbstractWebsite

An asynchronous multiuser system is proposed to support multiple-access underwater communications without the use of code-division multiple-access or a feedback channel. The rich multipath channels experienced by spatially separated users will be sufficient to ensure separation of collided packets at the base station. The iterative receiver will employ a combination of adaptive time-reversal processing, matching pursuit, and successive interference cancellation in a block-wise fashion to achieve multiuser separability. Data collected during the KAM11 experiment are used to illustrate the system's capability in a dynamic, time-varying environment. (C) 2012 Acoustical Society of America. []

Cho, SE, Song HC, Hodgkiss WS.  2012.  Multiuser interference cancellation in time-varying channels. Journal of the Acoustical Society of America. 131:EL163-EL169.   10.1121/1.3678668   AbstractWebsite

In this letter, an adaptive time-reversal multichannel combiner is embedded within an iterative successive interference cancellation receiver. With the addition of matching pursuit, a sparse channel estimation technique, the combined receiver is shown to provide both temporal interference cancellation as well as spatial interference suppression in decoding simultaneous transmissions from separate users in a time-varying underwater acoustic environment. Experimental data collected during the KAM11 experiment illustrates that for a two-user multiple-access system, multiuser separation can be achieved. (C) 2012 Acoustical Society of America

Yardim, C, Gerstoft P, Hodgkiss WS.  2012.  Sequential geoacoustic inversion at the continental shelfbreak. Journal of the Acoustical Society of America. 131:1722-1732.   10.1121/1.3666012   AbstractWebsite

Environmental parameters can have large spatial and temporal variability in shelfbreak regions. The capability of sequential Bayesian filters in tracking this variation is investigated. Particle filtering (PF) is used to extract the environmental parameters and their uncertainties. The method tracks the environment with fewer particles relative to conventional geoacoustic inversion methods using successive independent inversions. As an example, data from the Shallow Water 2006 Experiment are processed. The PF approach first is used to track the source and the environment with little spatial variation just northwest of the shelfbreak. Then the strongly range-dependent shelfbreak region is analyzed and the PF results are compared to previous geoacoustic inversion studies from the region. (C) 2012 Acoustical Society of America. [DOI: 10.1121/1.3666012]

Song, HC, Hodgkiss WS.  2012.  Diversity combining for long-range acoustic communication in deep water. Journal of the Acoustical Society of America. 132:EL68-EL73.   10.1121/1.4731639   AbstractWebsite

A recent experiment showed that coherent long-range acoustic communication is feasible in deep water over a similar to 550km range between a source towed slowly at similar to 75m depth and a horizontal line array towed at 3.5 knots at similar to 200m depth. This letter further demonstrates that diversity combining mitigates channel fading and increases the output signal-to-noise ratio. Using sparse channel-estimate-based equalization, three transmissions are combined successfully to decode a 40 Hz bandwidth (230-270 Hz) 8 phase-shift-keying communication signal, achieving an effective data rate of 17 bits/s at similar to 550km range. (C) 2012 Acoustical Society of America

Leroy, C, Lani S, Sabra KG, Hodgkiss WS, Kuperman WA, Roux P.  2012.  Enhancing the emergence rate of coherent wavefronts from ocean ambient noise correlations using spatio-temporal filters. Journal of the Acoustical Society of America. 132:883-893.   10.1121/1.4731231   AbstractWebsite

Extracting coherent wavefronts between passive receivers using cross-correlations of ambient noise (CAN) provides a means for monitoring the seismoacoustic environment without using active sources. However, using cross-correlations between single receivers can require a long recording time in order to extract stable coherent arrivals from CAN. This becomes an issue if the propagation medium fluctuates significantly during the recording period. To address this issue, this article presents a general spatio-temporal filtering procedure to enhance the emergence rate for coherent wavefronts extracted from time-averaged ambient noise correlations between two spatially separated arrays. The robustness of this array-based CAN technique is investigated using ambient shipping noise recorded over 24 h in the frequency band [250-850 Hz] on two vertical line arrays deployed 143 m apart in shallow water (depth 20 m). Experimental results confirm that the array-based CAN technique can significantly reduce the recording duration (e. g., from 22 h to 30 min) required for extracting coherent wavefronts of sufficient amplitude (e. g., 20 dB over residual temporal fluctations) when compared to conventional CAN implementations between single pairs of hydrophones. These improvements of the CAN technique could benefit the development of noise-based ocean monitoring applications such as passive acoustic tomography. (C) 2012 Acoustical Society of America. []

Angelico, BA, Jeszensky PJE, Burt PMS, Hodgkiss WS, Abrao T.  2012.  Comparison of multiple-input single-output single-user ultra-wideband systems with pre-distortion. European Transactions on Telecommunications. 23:240-253.   10.1002/ett.1528   AbstractWebsite

This paper presents a performance analysis of a baseband multiple-input single-output ultra-wideband system over scenarios CM1 and CM3 of the IEEE 802.15.3a channel model, incorporating four different schemes of pre-distortion: time reversal, zero-forcing pre-equaliser, constrained least squares pre-equaliser, and minimum mean square error pre-equaliser. For the third case, a simple solution based on the steepest-descent (gradient) algorithm is adopted and compared with theoretical results. The channel estimations at the transmitter are assumed to be truncated and noisy. Results show that the constrained least squares algorithm has a good trade-off between intersymbol interference reduction and signal-to-noise ratio preservation, providing a performance comparable to the minimum mean square error method but with lower computational complexity. Copyright (C) 2011 John Wiley & Sons, Ltd.

Karimian, A, Yardim C, Hodgkiss WS, Gerstoft P, Barrios AE.  2012.  Estimation of radio refractivity using a multiple angle clutter model. Radio Science. 47   10.1029/2011rs004701   AbstractWebsite

The common occurrence of a marine boundary layer in coastal regions can affect radio wave propagation significantly. Refractivity from clutter (RFC) techniques are aimed at estimating the refractivity profile of the ambient environment based on the received radar clutter. While most previous RFC work has not considered the dependence of RFC inversions on a variable grazing angle, this study investigates incorporation of the grazing angle information into the clutter model. A newly proposed clutter model based on multiple incident angles at each range is used here. The inversion performance of the multiple angle clutter model is compared to that of other models. Synthetic examples of a range-independent surface-based duct and a range-dependent evaporation duct are investigated for a S-band radar. Finally, a comparison of inversions on one set of experimental measurements from the SPANDAR 1998 data set is provided, using single and multiple grazing angle clutter models, and the previously used model based on grazing angle independent sea surface reflectivity.

Kang, T, Song HC, Hodgkiss WS.  2012.  Long-range multi-carrier acoustic communication in deep water using a towed horizontal array. The Journal of the Acoustical Society of America. 131:4665-4671.: ASA   10.1121/1.4711009   AbstractWebsite

During a recent long-range acoustic communication experiment carried out in deep water, multi-carrier Orthogonal Frequency Division Multiplexing (OFDM) communication signals were transmitted with a 50 Hz bandwidth (225–275 Hz) at various source-receiver ranges from 100 to 700 km. The experiment consisted of two mobile components: (1) a source towed slowly at a speed of 2–3 knots at ∼75 m depth and (2) a horizontal line array towed at 3.5 knots at a depth of ∼200 m. In addition to beamforming, an interleaver gain is exploited to compensate for low signal-to-noise ratio at the expense of data rate while providing diversity in the frequency domain. Error-free performance is shown at effective data rates of 15 and 7.5 bits/s at ranges of 550 km and 700 km, respectively, by combining interleaved repetitions with low-density parity-check coding after beamforming, demonstrating the feasibility of multi-carrier OFDM communications in deep water using a towed horizontal array.

Yardim, C, Gerstoft P, Hodgkiss WS.  2011.  Range aliasing in frequency coherent geoacoustic inversion. Journal of the Acoustical Society of America. 130:EL154-EL160.   10.1121/1.3630021   AbstractWebsite

This paper discusses the effects of frequency selection on source localization and geoacoustic inversion methods that use frequency coherent objective functions. Matched-field processors based on frequency-coherent objective functions often have rapidly fluctuating range ambiguity surfaces. Insufficient sampling in frequency domain results in range aliasing terms that affect geoacoustic inversion. Range aliasing and its effects on source localization and environmental parameter inversion are demonstrated on data collected during the MAPEX2000 experiment. Guidance for frequency selection to avoid range aliasing is provided. (C) 2011 Acoustical Society of America

Isukapalli, Y, Song HC, Hodgkiss WS.  2011.  Stochastic channel simulator based on local scattering functions. Journal of the Acoustical Society of America. 130:EL200-EL205.   10.1121/1.3633688   AbstractWebsite

This letter addresses the limitations of the stochastic channel simulation approach employed in wireless channels when directly applied to underwater channels. First it is shown analytically why the simulation method fails when the correlated taps each have a different Doppler spectrum. Then, based on the idea of local scattering functions, we propose a simple solution to the problem of simulating a channel with correlated taps where the correlated taps occur in subgroups with each of the subgroups having a different Doppler spectrum, applicable to underwater channels. Our simulation approach is tested successfully on the KAM08 channel. (C) 2011 Acoustical Society of America

Cho, SE, Song HC, Hodgkiss WS.  2011.  Successive interference cancellation for underwater acoustic communications. IEEE Journal of Oceanic Engineering. 36:490-501.   10.1109/joe.2011.2158014   AbstractWebsite

In this paper, we introduce the addition of an iterative, successive interference cancellation (SIC) process to improve on a multiuser, single-input-multiple-output (SIMO) communications receiver using passive time reversal as a space-time preprocessor. Time reversal has been shown to apply the spatial degrees of freedom to enhance the signal-to-noise ratio (SNR) and suppress interference for a target user. With the introduction of SIC, the receiver can remove the residual interference experienced by each user while preserving the SNR gain achieved by time-reversal preprocessing. The SIC process is a decision-directed approach for removing multiuser interference at the receiver and is similar to the decision-feedback equalizer (DFE) for intersymbol interference (ISI) channels. The interference experienced by each user is estimated at the receiver using previously decoded symbols from interfering users. This estimate is scaled and synchronized before subtraction from the target user's signal after time-reversal combining. Since SIC is applied before symbol decoding, symbol estimates are improved as the process is allowed to iterate until a stationary point is reached. Following time-reversal combining and SIC, a DFE can mitigate any remaining ISI before symbol decisions are made. Data collected from two Focused Acoustic Fields experiments (FAF-05 and FAF-06) are used to demonstrate the performance of the proposed interference cancellation scheme. During the FAF-05 experiment, three users transmitted 16-quadrature amplitude modulation (QAM) symbols simultaneously over the 3-4-kHz frequency band to a 20-element receiving array deployed in 120-m-deep water at a range of 20 km. The FAF-06 experiment included the simultaneous transmissions of 8-QAM symbols from two users over the 9-21-kHz band to a 16-element receiving array in 92-m-deep water at a range of 2.2 km. For both of the examples, SIC is shown to improve the output SNR in the presence of strong interference over time-reversal processing alone. This translates to a significant bit error rate (BER) reduction from 1.53 x 10(-2) to 8.80 x 10(-4) for the FAF-05 data and from 1.77 x 10(-3) to error-free decoding for the FAF-06 data.

Song, HC, Cho S, Kang T, Hodgkiss WS, Preston JR.  2011.  Long-range acoustic communication in deep water using a towed array. Journal of the Acoustical Society of America. 129:EL71-EL75.   10.1121/1.3554707   AbstractWebsite

In September 2010 a long-range acoustic communication (LRAC10) experiment was carried out in deep water off the Southern California Coast. The experiment involved two mobile components: (1) a source towed slowly at a speed of 2-3 knots at similar to 75-m depth and (2) a horizontal line array towed at 3.5 knots at a depth of similar to 200 m. Phase-coherent communication sequences were transmitted in the frequency band of 200-300 Hz at various ranges (100-700 km). Initial analysis of the LRAC10 data demonstrates that an information rate of 50 bits/s can be achieved over similar to 550-km range using quadrature-phase shift-keying (QPSK) modulation and error-correction coding combined with beamforming. (C) 2011 Acoustical Society of America

Park, C, Seong W, Gerstoft P, Hodgkiss WS.  2011.  Fluctuating arrivals of short-range acoustic data. Journal of the Acoustical Society of America. 129:98-103.   10.1121/1.3514505   AbstractWebsite

Geoacoustic inversion using fluctuating signal observations can be challenging. The origin of these fluctuations needs to be understood so the signals can be used appropriately. A set of experiments [Tang et al., Oceanogr. 20(4), 156-167 (2007)] was carried out in shallow water near the New Jersey shelf break in summer 2006. Significant fluctuations in the direct path and surface-reflected arrivals of short-range chirp transmissions (1.1-2.9 kHz) were observed on a vertical line array. This paper explains the origin of these signal fluctuations through analysis of the arrival amplitudes. It is shown that the strong thermocline combined with an oscillating source motion due to ocean surface waves results in the signal fluctuations. (C) 2011 Acoustical Society of America. [DOI: 10.1121/1.3514505]

Kang, T, Song HC, Hodgkiss WS.  2011.  Multi-carrier synthetic aperture communication in shallow water: Experimental results. Journal of the Acoustical Society of America. 130:3797-3802.   10.1121/1.3652855   AbstractWebsite

Orthogonal frequency division multiplexing (OFDM) communications in the presence of motion is investigated using data collected from the Kauai Acomms MURI 2008 (KAM08) experiment, conducted off the western side of Kauai, Hawaii, in June-July 2008. The experiment involved a vertical array moored in 106111 deep shallow water and a source towed at a speed of 3 knots at ranges between 600 m and 6 km. In order to attain reliable communications with only a single receive element, a synthetic aperture approach is applied. After combining multiple transmissions, an error-free reception is achieved with a low-density parity-check code, confirming the feasibility of coherent synthetic aperture communications using OFDM. (C) 2011 Acoustical Society of America. [DOI: 10.1121/1.3652855]

Karimian, A, Yardim C, Gerstoft P, Hodgkiss WS, Barrios AE.  2011.  Refractivity estimation from sea clutter: An invited review. Radio Science. 46   10.1029/2011rs004818   AbstractWebsite

Non-standard radio wave propagation in the atmosphere is caused by anomalous changes of the atmospheric refractivity index. In recent years, refractivity from clutter (RFC) has been an active field of research to complement traditional ways of measuring the refractivity profile in maritime environments which rely on direct sensing of the environmental parameters. Higher temporal and spatial resolution of the refractivity profile, together with a lower cost and convenience of operations have been the promising factors that brought RFC under consideration. Presented is an overview of the basic concepts, research and achievements in the field of RFC. Topics that require more attention in future studies also are discussed.

Gerstoft, P, Hodgkiss WS.  2011.  Improving beampatterns of two-dimensional random arrays using convex optimization. Journal of the Acoustical Society of America. 129:EL135-EL140.   10.1121/1.3556896   AbstractWebsite

Sensors are becoming ubiquitous and can be combined in arrays for source localization purposes. If classical conventional beam-forming is used, then random arrays have poor beampatterns. By pre-computing sensor weights, these beampatterns can be improved significantly. The problem is formulated in the frequency domain as a desired look direction, a frequency-independent transition region, and the power minimized in a rejection-region. Using this formulation, the frequency-dependent sensor weights can be obtained using convex optimization. Since the weights are data independent they can be pre-computed, the beamforming has similar computational complexity as conventional beamforming. The approach is demonstrated for real 2D arrays. (C) 2011 Acoustical Society of America

Traer, J, Gerstoft P, Hodgkiss WS.  2011.  Ocean bottom profiling with ambient noise: A model for the passive fathometer. Journal of the Acoustical Society of America. 129:1825-1836. AbstractWebsite

A model is presented for the complete passive fathometer response to ocean surface noise, interfering discrete noise sources, and locally uncorrelated noise in an ideal waveguide. The leading order term of the ocean surface noise contribution produces the cross-correlation of vertical multipaths and yields the depth of sub-bottom reflectors. Discrete noise incident on the array via multipaths give multiple peaks in the fathometer response. These peaks may obscure the sub-bottom reflections but can be attenuated with use of minimum variance distortionless response (MVDR) steering vectors. The seabed critical angle introduces discontinuities in the spatial distribution of distant surface noise and may introduce spurious peaks in the passive fathometer response. These peaks can be attenuated by beamforming within a bandwidth limited by the array geometry and critical angle. (C) 2011 Acoustical Society of America. [DOI:10.1121/1.3552871]

Angelico, BA, Burt PMS, Jeszensky PJE, Hodgkiss WS, Abrao T.  2011.  Performance analysis of a single-user MISO ultra-wideband time reversal system with DFE. Telecommunication Systems. 46:333-342.   10.1007/s11235-010-9295-1   AbstractWebsite

This paper presents an analysis of the performance of a baseband multiple-input single-output (MISO) time reversal ultra-wideband system (TR-UWB) incorporating a symbol spaced decision feedback equalizer (DFE). A semi-analytical performance analysis based on a Gaussian approach is considered, which matched well with simulation results, even for the DFE case. The channel model adopted is based on the IEEE 802.15.3a model, considering correlated shadowing across antenna elements. In order to provide a more realistic analysis, channel estimation errors are considered for the design of the TR filter. A guideline for the choice of equalizer length is provided. The results show that the system's performance improves with an increase in the number of transmit antennas and when a symbol spaced equalizer is used with a relatively small number of taps compared to the number of resolvable paths in the channel impulse response. Moreover, it is possible to conclude that due to the time reversal scheme, the error propagation in the DFE does not play a role in the system's performance.

Raghukumar, K, Cornuelle BD, Hodgkiss WS, Kuperman WA.  2010.  Experimental demonstration of the utility of pressure sensitivity kernels in time-reversal. Journal of the Acoustical Society of America. 128:989-1003.   10.1121/1.3466858   AbstractWebsite

Pressure sensitivity kernels were recently applied to time-reversal acoustics in an attempt to explain the enhanced stability of the time-reversal focal spot [Raghukumar et al., J. Acoust. Soc. Am. 124, 98-112 (2008)]. The theoretical framework developed was also used to derive optimized source functions, closely related to the inverse filter. The use of these optimized source functions results in an inverse filter-like focal spot which is more robust to medium sound speed fluctuations than both time-reversal and the inverse filter. In this paper the theory is applied to experimental data gathered during the Focused Acoustic Fields experiment, conducted in 2005, north of Elba Island in Italy. Sensitivity kernels are calculated using a range-independent sound-speed profile, for a geometry identical to that used in the experiment, and path sensitivities are identified with observed arrivals. The validity of the kernels in tracking time-evolving Green's functions is studied, along with limitations that result from a linearized analysis. An internal wave model is used to generate an ensemble of sound speed profiles, which are then used along with the calculated sensitivity kernels to derive optimized source functions. Focal spots obtained using the observed Green's functions with these optimized source functions are then compared to those obtained using time-reversal and the inverse-filter. It is shown that these functions are able to provide a focal spot superior to time-reversal while being more robust to sound speed fluctuations than the inverse filter or time-reversal. (C) 2010 Acoustical Society of America. [DOI: 10.1121/1.3466858]

Song, HC, Hodgkiss WS, van Walree PA.  2010.  Phase-coherent communications without explicit phase tracking. Journal of the Acoustical Society of America. 128:969-972.   10.1121/1.3466860   AbstractWebsite

Phase-coherent communications typically requires a reliable phase-tracking algorithm. An initial phase estimate with training symbols allows a receiver to compensate for a motion-induced Doppler shift. Following the training period, however, explicit phase tracking can be avoided in time reversal communications that has been implemented on a block-by-block basis to accommodate time-varying channels. This is accomplished by a smaller block size and adaptive channel estimation using previously detected symbols on a symbol-by-symbol basis. The proposed time reversal approach without explicit phase tracking is demonstrated using experimental data (12-20 kHz) in shallow water. (C) 2010 Acoustical Society of America. [DOI: 10.1121/1.3466860]

Park, C, Seong W, Gerstoft P, Hodgkiss WS.  2010.  Geoacoustic inversion using backpropagation. IEEE Journal of Oceanic Engineering. 35:722-731.   10.1109/joe.2010.2040659   AbstractWebsite

This paper presents inversion results of the 2006 Shallow Water Experiment (SW06) data measured on a vertical line array. A low-frequency (100-900 Hz) chirp source was towed along two tracks (circle, straight line) at 30-m depth. For the inversions, a three-step optimization scheme is applied to the data using very fast simulated reannealing (VFSR). The objective function is defined by the energy of the backpropagated signal from the array to the source. At each step, water-column sound-speed profile (SSP), experimental geometry, and geoacoustic parameters are inverted successively. An environmental model is employed consisting of a linear segmented SSP in the water column, a sediment layer, and a half-space. The geometric parameter inversion results show good agreement with in situ measurements. Finally, the estimated geoacoustic parameters show that the experimental site near the vertical line array (VLA) is fairly homogeneous in bottom properties consisting of a 21-m-thick sediment layer with sound speed of around 1600 m/s over a hard basement whose sound speed is approximately 1750 m/s.

Song, HC, Kim JS, Hodgkiss WS, Kuperman WA, Stevenson M.  2010.  High-rate multiuser communications in shallow water. Journal of the Acoustical Society of America. 128:2920-2925.   10.1121/1.3488309   AbstractWebsite

Passive multiuser communications in shallow water previously was demonstrated in the 3-4 kHz band using a time reversal approach. This paper extends those experimental results in three respects. First, a larger bandwidth at higher frequency (11-19 kHz) is employed allowing for the use of various symbol rates (or bandwidths). Second, two different shaping pulses are examined: a raised cosine filter and LFM (linear frequency modulation) chirp. Third, the adaptive time reversal approach with spatial nulling is applied to suppress the crosstalk among users. It is shown that the use of a larger bandwidth is beneficial along with the time reversal receiver which can handle significant intersymbol interference with minimal computational complexity. In addition, adding each user degrades the performance by about 4 dB for the benefit of linear increase in data rate. It is demonstrated that an aggregate data rate of 60 kbits/s can be achieved with a 7.5 kHz bandwidth (a spectral efficiency of 8 bits/s Hz) by three users distributed over 4.2-m depth at a 2.2 km range in shallow water using 16-QAM (quadrature amplitude modulation). (C) 2010 Acoustical Society of America. [DOI: 10.1121/1.3488309]

Yardim, C, Gerstoft P, Hodgkiss WS.  2010.  Geoacoustic and source tracking using particle filtering: Experimental results. Journal of the Acoustical Society of America. 128:75-87.   10.1121/1.3438475   AbstractWebsite

A particle filtering (PF) approach is presented for performing sequential geoacoustic inversion of a complex ocean acoustic environment using a moving acoustic source. This approach treats both the environmental parameters [e.g., water column sound speed profile (SSP), water depth, sediment and bottom parameters] at the source location and the source parameters (e.g., source depth, range and speed) as unknown random variables that evolve as the source moves. This allows real-time updating of the environment and accurate tracking of the moving source. As a sequential Monte Carlo technique that operates on nonlinear systems with non-Gaussian probability densities, the PF is an ideal algorithm to perform tracking of environmental and source parameters, and their uncertainties via the evolving posterior probability densities. The approach is demonstrated on both simulated data in a shallow water environment with a sloping bottom and experimental data collected during the SWellEx-96 experiment. (C) 2010 Acoustical Society of America. [DOI: 10.1121/1.3438475]

Song, HC, Kim JS, Hodgkiss WS, Joo JH.  2010.  Crosstalk mitigation using adaptive time reversal. Journal of the Acoustical Society of America. 127:EL19-EL22.   10.1121/1.3280234   AbstractWebsite

Although the spatial focusing property of the conventional time reversal approach facilitates multiuser communications, there always is residual crosstalk between users. A recent paper [Kim and Shin, J. Acoust. Soc. Am. 115, 600-606 (2004)] proposed an adaptive active time reversal approach for simultaneous multiple focusing with minimal interference. This letter applies the adaptive approach to passive time reversal, multiuser communications for additional suppression of crosstalk among users. Experimental data at 3.5 kHz with a 1-kHz bandwidth demonstrate as much as 6.5-dB improvement per user in terms of output signal-to-noise ratio for three-user communications over a 20-km range in 120-m deep shallow water.