H. C. Song

Phase conjugation in the ocean: Experimental demonstration of an acoustic time-reversal mirror

An experiment conducted in the Mediterranean Sea in April 1996 demonstrated that a time-reversal mirror (or phase conjugate array) can be implemented to spatially and temporally refocus an incident acoustic field back to its origin. The experiment utilized a vertical source–receiver array (SRA) spanning 77 m of a 125-m water column with 20 sources and receivers and a single source/receiver transponder (SRT) colocated in range with another vertical receive array (VRA) of 46 elements spanning 90 m of a 145-m water column located 6.3 km from the SRA. Phase conjugation was implemented by transmitting a 50-ms pulse from the SRT to the SRA, digitizing the received signal and retransmitting the time reversed signals from all the sources of the SRA. The retransmitted signal then was received at the VRA. An assortment of runs was made to examine the structure of the focal point region and the temporal stability of the process. The phase conjugation process was extremely robust and stable, and the experimental resu...

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.

A LONG-RANGE AND VARIABLE FOCUS PHASE-CONJUGATION EXPERIMENT IN SHALLOW WATER

A second phase-conjugation experiment was conducted in the Mediterranean Sea in May 1997 extending the results of the earlier time-reversal mirror experiment [Kuperman et al., J. Acoust. Soc. Am. 103, 25–40 (1998)]. New results reported here include (1) extending the range of focus from the earlier result of 6 km out to 30 km, (2) verifying a new technique to refocus at ranges other than that of the probe source [Song et al., J. Acoust. Soc. Am. 103, 3234–3240 (1998)], and (3) demonstrating that probe-source pulses up to 1 week old can be refocused successfully.

Underwater acoustic communications using time reversal

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

A time-reversal mirror with variable range focusing

Recent time‐reversal mirror (TRM) experiments conducted in the Mediterranean Sea demonstrated variable range focusing using the frequency‐range invariant property in a shallow‐water waveguide [Song et al., J. Acoust. Soc. Am. 102, 3171(A) (1997)]. The technique involves retransmitting the data at a shifted frequency according to the desired change in focal range in almost real‐time fashion. As opposed to the variable range focusing, there does not appear to be an environmentally independent elegant solution for variable depth focusing. However, a method based on knowledge of the environment, which translates to being able to compute the acoustic modal structure, permits a focal depth shift. The method involves projecting out the important modes received at the vertical source–receive array (SRA) and appropriately altering their excitation to focus at another depth. In the May 1997 experiment, some limited results demonstrated that this method is feasible. Comparison of data with simulations and theoretica...

Iterative time reversal in the ocean

The efficiency of an iterative time reversal process in the ocean has been demonstrated in an experiment conducted in the Mediterranean Sea during April 1996. [See Kuperman et al. in this session.] The iterative time reversal process consisted of initiating a 50‐ms pulse with equal amplitudes on all elements from SRA to VRA, capturing the signal at 75‐m depth on a single hydrophone of VRA, and retransmitting it from SRT at the same depth to SRA, reversing the received signal in time and retransmitting it from all the SRA sources back to VRA. This process was repeated many times over an hour. Although this was not the intended mode of operation of iterative phase conjugation as in the free‐field case with many scatterers [Prada et al., J. Acoust. Soc. Am. 90, 1119–1129 (1991)], it shows that an iterative process can enhance the focusing of a single target in the ocean. Comparisons of data with simulations and theoretical analysis will be presented and discussed.

Multiple-input-multiple-output coherent time reversal communications in a shallow-water acoustic channel

A recent time reversal (TR) experiment demonstrated that multiple foci can be projected from an array of sources to the same range but at different depths. This multiple input/multiple output process can potentially increase the information data rate. This paper presents experimental results of coherent TR communications (binary phase-shift keying, quaternary phase-shift keying, and 8-quadratic-amplitude modulation) at 3.5 kHz with a 1-kHz bandwidth where different messages were sent simultaneously to either two or three different depths at an 8.6-km range in a 105-m-deep water

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.

Multiuser Communications Using Passive Time Reversal

A recent paper (Song , IEEE Journal of Oceanic Engineering, vol. 31, no. 2, pp. 170-178, 2006) demonstrated multiple-input-multiple-output (MIMO) communications in shallow water using active time reversal where the time reversal array (i.e., base station) sent different messages to multiple users simultaneously over a common bandwidth channel. Passive time reversal essentially is equivalent to active time reversal with the communications link being in the opposite direction. This paper describes passive time reversal communications which enables multiple users to send information simultaneously to the time reversal array. Experimental results at 3.5 kHz with a 1-kHz bandwidth demonstrate that as many as six users can transmit information over a 4-km range in a 120-m-deep water using quaternary phase-shift keying (QPSK) modulation, achieving an aggregate data rate of 6 kb/s. Moreover, the same data rate has been achieved at 20-km range by three users using 16 quadrature amplitude modulation (16-QAM).