Geoffrey F. Edelmann

An initial demonstration of underwater acoustic communication using time reversal

In July 1999, an at-sea experiment to measure the focus of a 3.5-kHz centered time-reversal mirror (TRM) was conducted in three different environments: an absorptive bottom, a reflective bottom, and a sloping bottom. The experiment included a preliminary exploration of using a TRM to generate binary-phase shift keying communication sequences in each of these environments. Broadside communication transmissions were also made, and single-source communications were simulated using the measured-channel response. A comparison of the results is made and time reversal is shown to be an effective approach for mitigating inter-symbol interference caused by channel multipath.

Robust time reversal focusing in the ocean

Recent time-reversal experiments with high-frequency transmissions (3.5 kHz) show that stable focusing is severely limited by the time-dependent ocean environments. The vertical focal structure displays dynamic variations associated with focal splitting and remerging resulting in large changes in focal intensity. Numerical simulations verify that the intensity variation is linked to the focal shift induced by phase changes in acoustic waves resulting from sound speed fluctuations due to internal waves. A relationship between focal range shift, frequency shift, or channel depth changes is illustrated using waveguide-invariant theory. Based on the analysis of experimental data and numerical simulations, methods for robust time-reversal focusing are developed to extend the period of stable focusing.

Spatial resolution of time-reversal arrays in shallow water

A series of time-reversal experiments was performed in shallow water including a range-dependent slope environment. Time-reversal arrays implemented with center frequencies of 445 and 3500 Hz achieved sharp focal regions up to ranges of 30 and 13 km, respectively in 110–130-m shallow water. In this paper, resolution expressions are derived using an image method to describe the focal sizes achieved with time-reversal arrays in various ocean environments. Analysis for the measured data indicates that the focal size approaches the diffraction limit of an array for given waveguide conditions, i.e., waveguide geometry and attenuation. The measured focal size has implications for the maximum achievable resolution of linear matched-field processing which is a computational implementation of the time-reversal process.

Echo-to-reverberation enhancement using a time reversal mirror

Reverberation from rough ocean boundaries often degrades the performance of active sonar systems in the ocean. The focusing capability of the time-reversal method provides a new approach to this problem. A time-reversal mirror (TRM) focuses acoustic energy on a target enhancing the target echo while shadowing the boundaries below and above the focus in a waveguide, thereby reducing reverberation. The resulting echo-to-reverberation enhancement has been demonstrated experimentally using a time-reversal mirror in the 3–4 kHz band in shallow water.

Beamforming using compressive sensing

Compressive sensing (CS) is compared with conventional beamforming using horizontal beamforming of at-sea, towed-array data. They are compared qualitatively using bearing time records and quantitatively using signal-to-interference ratio. Qualitatively, CS exhibits lower levels of background interference than conventional beamforming. Furthermore, bearing time records show increasing, but tolerable, levels of background interference when the number of elements is decreased. For the full array, CS generates signal-to-interference ratio of 12 dB, but conventional beamforming only 8 dB. The superiority of CS over conventional beamforming is much more pronounced with undersampling.

Underwater acoustic communication using time reversal

Time reversal, or phase-conjugation, refocuses energy back to a probe source location despite the complexity of the propagation channel. A probe source pulse is transmitted and a complicated multipath signal is measured by an array of source/receiver elements. The signal is time reversed and retransmitted into the ocean. The time-reversal process recombines this temporal multipath at the original probe source range and depth. The ability of time reversal to reduce dispersion and its simplicity of implementation makes it ideal for underwater acoustic communications, which must mitigate the inter-symbol interference caused by the time-varying multipath dispersion. Furthermore, time reversal focuses energy at the desired depth, thus mitigating the effects of channel fading. An experiment was conducted in June 2000 demonstrating that the time-reversal process recombined the temporal multipath resulting in reduced bit errors for communication. Communication sequences were transmitted over a distance of 10 km both in range independent and range dependent environments north of Elba Island, Italy. The range independent transmissions were made in 110-m deep water and the range dependent transmissions were made upslope from 110-m deep water into 40-m deep water. Single source transmissions were also measured in the same channels. Quantitative bit error results are shown for BPSK (binary phase shift keying) and QPSK (quadrature phase shift keying).This paper contains theoretical and experimental results on the application of the time-reversal process to acoustic communications in order to improve data telemetry in the ocean. A coherent underwater acoustic communication system must deal with the inter-symbol interference caused by the time-varying, dispersive, shallow-water ocean environment. An approach is demonstrated that takes advantage of the focal properties of time reversal. The spatial and temporal compression available at the time-reversal focus mitigates channel fading, reduces the dispersion caused by the channel, and increases the signal strength. Thus, a time-reversal communication system does not require spatial diversity at the receiver, i.e., an array of receiving sensors, but takes advantage of spatial diversity at the transmitter. The time-reversal communications system concept is demonstrated using experimental data collected in shallow water. Data telemetry bit rates of 500 bps (BPSK) and 1000 bps (QPSK) with bit error rates of 0 out of 4976 bits and 254 out of 9953 bits, respectively, were obtained when transmitting to a receiver at a distance of 10 km, with a carrier frequency of 3500 Hz, and a 500 Hz bandwidth. In a shallow-water upslope region, bit error rates of 15 out of 4976 bits and 14 out of 4976 bits were achieved over the same distance. In neither case was complex processing at the receiver used (i.e., channel equalization, error correction coding). Time-reversal transmissions are intercompared with single source and broadside transmissions and shown to have superior results in both range independent and dependent bathymetries. The time-reversal performance appears limited by self-generated inter-symbol interference. In addition, an initial look at the application of a single channel adaptive channel equalizer to received time-reversal communication sequences is presented. The same properties that are beneficial to a single channel receiver are also beneficial to adaptive channel equalization. A single channel RLS DFE equalizer is cascaded with the received time-reversal sequences and shown to further reduce scatter in the I/Q plane. The bit error rate decreased in all but one of the cases

Demonstration at sea of the decomposition-of-the-time-reversal-operator techniquea)

This paper presents a derivation of the time reversal operator decomposition (DORT) using the sonar equation. DORT is inherently a frequency-domain technique, but the derivation is shown in the time-frequency domain to preserve range resolution. The magnitude of the singular values is related to sonar equation parameters. The time spreading of the time-domain back-propagation image is also related to the sonar equation. Noise-free, noise-only, and signal-plus-noise data are considered theoretically. Contamination of the echo singular component by noise is shown quantitatively to be very small at a signal-to-noise ratio of 0dB. Results are shown from the TREX-04 experiment during April 22 to May 4, 2004 in 94m deep, shallow water southwest of the Hudson Canyon. Rapid transmission of short, 500Hz wide linear frequency modulated beams with center frequencies of 750, 1250, 1750, 2250, 2750, and 3250Hz are used. Degradation caused by a lack of time invariance is found to be small at 750Hz and nearly complete a...

Comparison of a subrank to a full-rank time-reversal operator in a dynamic ocean

This paper investigates the application of time-reversal techniques to the detection and ensonification of a target of interest. The focusing method is based on a generalization of time-reversal operator techniques. A subrank time-reversal operator is derived and implemented using a discrete set of transmission beams to ensonify a region of interest. In a dynamic ocean simulation, target focusing using a subrank matrix is shown to be superior to using a full-rank matrix, specifically when the subrank matrix is captured in a period shorter than the coherence time of the modeled environment. Backscatter from the point target was propagated to a vertical 64-element source-receiver array and processed to form the sub-rank time-reversal operator matrix. The eigenvector corresponding to the strongest eigenvalue of the time-reversal operator was shown to focus energy on the target in simulation. Modeled results will be augmented by a limited at-sea experiment conducted on the New Jersey shelf in April-May 2004 measured low-frequency backscattered signal from an artificial target (echo repeater).

A method for robust time-reversal focusing in a fluctuating ocean

In recent years, the authors have demonstrated time-reversal mirrors (TRM) in the ocean. A focus of up to 30 km was achieved with low frequency (445 Hz) transmissions and the focal structure could be maintained over several days at a range of 15 km. However, the stable focus was limited to less than an hour with high frequency (3.5 kHz) transmissions due to the sensitive response of high frequency sound propagation to the medium fluctuations. In this study, an approach for robust time-reversal focusing is investigated based on a method developed for matched-field processing. Instead of using a single probe source pulse, the method makes use of several probe source pulses obtained over a certain period of time where each ping represents a different propagation condition of the medium. The back-propagation from a TRM weighted by a linear combination of the dominant singular vectors obtained from the signal matrix leads to stable focusing for a longer period of time than that with a single probe pulse. The proposed method is useful in non-static propagation conditions and when frequent probe signals are not available.

Forward‐scatter barrier with a time‐reversal mirror

The classic difficulty in constructing an acoustic trip line barrier is that the forward‐scattered field from the target must be extracted from the (usually) much more intense direct blasting arriving beam, i.e., ‘‘looking into the sunlight effect.’’ During the time‐reversal experiment conducted in May 2000, we investigated the forward‐scatter barrier concept using a six‐element transponder at 65‐m depth drifting along with a ship, which traversed the trip line between two moored vertical arrays separated by 5 km in 110‐m water depth. A 10‐ms cw pulse from a probe source (PS) located at the bottom of the vertical receive array (VRA) is received and time‐reversed at the source/receive array (SRA). The transmitted time‐reversed signal is then refocused at the PS location of the VRA. This time‐reversed signal is also captured by the transponder simulating the forward‐scatter target and is retransmitted to the VRA with various amplitudes simulating different forward‐scatter target strengths. With an appropria...