Seongil Kim

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

Geoacoustic model of the transverse acoustic variability experiment area in the northern East China Sea

Transverse Acoustic Variability Experiment (TAVEX) is a joint ROK-US acoustic experiment for testing transverse acoustic variability in the northern limit of the East China Sea. TAVEX was performed in a water depth of 70–90 m around 32°30 to 40N, 125°20 to 30E, about 150 km southwest of Cheju Island, in August 2008. Purpose of this paper is to provide geoacoustic models of the TAVEX area, integrated and detailed as available. Two geoacoustic models of three layers were reconstructed, located at 32°40N/125°30E and 32°30N/125°30E of the northern East China Sea, respectively. The models were based on the data of the high-resolution subbottom and air-gun seismic profiles with cores. Geoacoustic data of the cores were extrapolated down to a depth of 100 m of the geoacoustic models. For actual underwater experiments, the P-wave speed of the models was compensated to in situ depth below the sea floor using the Hamilton method.

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.

Equalizer Mode Selection Method for Improving Bit Error Performance of Underwater Acoustic Communication Systems

The linear and decision-feedback equalization can mitigate time-varying intersymbol interference (ISI) caused by time-varying multipath propagation for underwater acoustic channels. The perfect elimination of interference components, however, is difficult using the linear equalization and the decision feedback equalizer has an error propagation problem. To overcome these shortcomings, this paper proposes an equalizer mode selection method using training sequences. The proposed method selects an equalization mode corresponding to the signal-to-noise ratio (SNR). If the SNR is low, the proposed system operates the linear equalizer for preventing the error propagation and if the SNR is high, the decision feedback equalizer for eliminating the residual ISI. Therefore, the proposed method can improve the error performance compared to the conventional equalizers. The computer simulation shows the proposed method improves the bit error performance using practical underwater channels responses acquired from the sea experiment.

Extraction of an Underwater Transient Signal Using Sound Mask-filter

ABSTRACT: An underwater transient signal is distinguished from an ambient noise. Database for the underwater transient signal is required since the underwater transient signal shows various characteristics depending on acoustic features. In the paper, hence, sound mask-filter was applied to extract the transient signals which exist temporally and locally in the ocean. The standard signal was chosen and cross-correlated with the raw signal. A mask-filter for a transient signal was obtained using the threshold which was decided by the maximum likelihood method in the envelope of the cross-correlated signal. Using the sound mask-filter, the transient signal of a sea catfish { Galeichthys felis (Linnaeus)} was extracted from the underwater ambient noise. Similarly, the man-made signal was added into the noise and it was extracted by the same method. We also have demonstrated the significance of the transient signal through comparing th e extracted signals depending on the standard signal. In the results, the proposed method, sound mask-filtering, could be utilized as a database construction of the transient signals in underwater noise. Particularly, this study would be useful to extract the wanted signal from arbitrary signals.

Classification of Transient Signals in Ocean Background Noise Using Bayesian Classifier

In this paper, a Bayesian classifier based on PCA (principle component analysis) is proposed to classify underwater transient signals using order LPC (linear predictive coding) coefficients as feature vector. The proposed classifier is composed of two steps. The mechanical signals were separated from biological signals in the first step, and then each type of the mechanical signal was recognized in the second step. Three biological transient signals and two mechanical signals were used to conduct experiments. The classification ratios for the feature vectors of biological signals and mechanical signals were 94.75% and 97.23%, respectively, when all 16 order LPC vector were used. In order to determine the effect of underwater noise on the classification performance, underwater ambient noise was added to the test signals and the classification ratio according to SNR (signal-to-noise ratio) was compared by changing dimension of feature vector using PCA. The classification ratios of the biological and mechanical signals under ocean ambient noise at 10dB SNR, were 0.51% and 100% respectively. However, the ratios were changed to 53.07% and 83.14% when the dimension of feature vector was converted to three by applying PCA. For correct, classification, it is required SNR over 10 dB for three dimension feature vector and over 30dB SNR for seven dimension feature vector under ocean ambient noise environment.

Optimal Acoustic Search Path Planning in Realistic Environments Based on Genetic Algorithm

The measure of effectiveness for detection of sonar system is directly related to the Cumulative Detection Probability (CDP) while its operation. In order to maximize CDP, there exists an optimal search path for given environment and sonar systems. Recently, with the advance of modeling and simulation methods, it has become possible to access the optimization problem more systematically. For this purpose, a program package named as OASPP (Optimal Acoustic Search Path Planning) was developed based on Genetic Algorithm (GA) and detection algorithm with environmental models. In this paper, the simulation results for a set of cases are presented and discussed, for which cases the intuitive solution are known.

Optimal Acoustic Search Path Planning Based on Genetic Algorithm in Continuous Path System

The design of efficient search path to maximize the Cumulative Detection Probability(CDP) is mainly dependent on experience and intuition when the searcher detect the target using SONAR in the ocean. Recently with the advance of modeling and simulation methods, it has been possible to access the optimization problem more systematically. In this paper, a method for the optimal search path calculation is developed based on the combination of the genetic algorithm with detection algorithm. We consider the continuous system for search path, space, and time, and use the movement direction of the SONAR as a gene of the genetic algorithm. The developed algorithm, OASPP(Optimal Acoustic Search Path Planning), is shown to be effective, via a simulation, in finding the optimal search path for the case where the intuitive solution exists.

Performance comparison of FFT-based and GCC-PHAT time delay estimation schemes for target azimuth angle estimation in a passive SONAR array

Target object detection and azimuth angle estimation are very important issues in the passive SONAR array systems. However, in coastal areas due to cost and efficiency issues, there are cases where a proximity sensor with relatively low sensitivity to reception is employed than a high-performance array sensor. In these cases, it is desirable to use low-complexity signal processing techniques for efficiently exploiting received signals. In this paper, we compare the performance of GCC-PHAT and low-complexity FFT-based time delay estimation techniques to efficiently estimate the target azimuth angle in a passive SONAR array. It is observed that the performance of the time delay estimation methods is dependent upon the underwater environment.

Improvement of Engine Stall by Load Increment on Tracked Armored Vehicles

Currently, there are many kinds of tracked armored vehicles in service and they have encountered various environment and situations. So there are many obstacles to operate them improperly such as an engine stall. The causes of engine stall are an insufficient fueling, a mixture of air-fuel or vapor lock, and load increment which results from a rapid steering or increasing a viscosity of lubricant by low temperature. In this paper, engine stall by load increment due to a rapid steering or increasing of lubricant viscosity on tracked armored vehicles is analyzed, the ways to prevent it are applied, and their degrees of improvement are evaluated.