William S. Hodgkiss

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

Improvement of Time-Reversal Communications Using Adaptive Channel Equalizers

The spatial and temporal focusing properties of time-reversal methods can be exploited for undersea acoustic communications. Spatial focusing mitigates channel fading and produces a high signal-to-noise ratio (SNR) at the intended receivers along with a low probability of interception elsewhere. While temporal focusing (compression) reduces significantly intersymbol interference (ISI), there always is some residual ISI depending upon the number of transmitters, their spatial distribution (spatial diversity), and the complexity of the channel. Moreover, a slight change in the environment over the two-way propagation interval introduces additional ISI. Using multilevel quadrature amplitude modulation (M-QAM) in shallow water, we demonstrate that the performance of time-reversal communications can be improved significantly by cascading the received time series with an adaptive channel equalizer to remove the residual ISI

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.

Impact of ocean variability on coherent underwater acoustic communications during the Kauai experiment (KauaiEx)

During the July 2003 acoustic communications experiment conducted in 100m deep water off the western side of Kauai, Hawaii, a 10s binary phase shift keying signal with a symbol rate of 4kilosymbol∕s was transmitted every 30min for 27h from a bottom moored source at 12kHz center frequency to a 16 element vertical array spanning the water column at about 3km range. The communications signals are demodulated by time reversal multichannel combining followed by a single channel decision feedback equalizer using two subsets of array elements whose channel characteristics appear distinct: (1) top 10 and (2) bottom 4 elements. Due to rapid channel variations, continuous channel updates along with Doppler tracking are required prior to time reversal combining. This is especially true for the top 10 elements where the received acoustic field involves significant interaction with the dynamic ocean surface. The resulting communications performance in terms of output signal-to-noise ratio exhibits significant change o...

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

Spatial diversity in passive time reversal communications

A time reversal mirror exploits spatial diversity to achieve spatial and temporal focusing, a useful property for communications in an environment with significant multipath. This paper presents the impact of spatial diversity on passive time reversal communications between a single probe source and a vertical receive array using at‐sea experimental data, while the probe source is either fixed or moving at about 4 knots. The performances of two different approaches are compared: (1) time reversal alone and (2) time reversal combined with adaptive channel equalization. In the presence of source motion, the motion‐induced Doppler shift is coarsely estimated using a decision‐feedback phase‐locked loop with a training sequence and then the received time series is resampled prior to the demodulation process. The time‐varying channel responses due to source motion require an adaptive channel equalizer such that time reversal combined with the equalizer outperforms time reversal alone by up to 13 dB as compared to 5 dB for a fixed source case. The experimental results around 3 kHz with a 1‐kHz bandwidth illustrate that even two or three receivers (i.e., 2‐ or 4‐m aperture) can provide resonable performance at 4.2‐ and 10‐km ranges in a 118‐m deep water.