• The Journal of Korean Institute of Communications and Information Sciences
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• v.34 no.3C
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• pp.243-250
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• 2009

In this paper, a closed-form expression of the ergodic channel capacity for a narrow-band time-reversal communication scheme is analytically formulated. In the time-reversal communication scenario, a transmitter sends a signal and a so-called time-reversal array receives the signal. Then, the received signal is reversed in the time do main and resent to the original transmitter. Here, one transmitter and an antenna array for the time-reversal array are assumed. Since the spacing between the array elements is large, the signals received by each antenna element can be considered independent. For simplicity, the communication channel is assumed stationary, whose properties are not changed for the time-reversal process. Based on the obtained formulation, the channel capacities for the time-reversal and the conventional channels are compared.

• The Journal of the Acoustical Society of Korea
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• v.27 no.3E
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• pp.104-111
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• 2008

Recently, a method for robust time reversal focusing has been introduced to extend the period of stable focusing in time-dependent ocean environments [S. Kim et al., J. Acoust. Soc. Am. 114, 145-157, (2003)]. In this study, concept of focal-size broadening based on waveguide invariant theory in an ocean time reversal acoustics is described. It is achieved by imposing the multiple location constraints. The signal vector used in multiple location constraints are found from the theory on waveguide invariant for frequency band corresponding the extended focal range. The broadening of foci in an ocean waveguide can play an important role in the application of time reversal processing, particularly to the underwater acoustic communication with moving vehicles. The proposed method is demonstrated in the context of the underwater acoustic communication from the transmit/receive array (TRA) to a slowly moving vehicle.

• The Journal of the Acoustical Society of Korea
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• v.35 no.5
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• pp.389-394
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• 2016

MIMO (Multiple-Input-Multiple-Output) in underwater acoustic communication has distortion of received signal because of ISI (Inter-Symbol Interference) and crosstalk among transmitters. Time-reversal mirror was used for compensating of signal distortion, but it has a limit in eliminating crosstalk effectively. This paper proposes a time-reversal mirror based on GSC (Generalized Sidelobe Canceller) for removing crosstalk. The FAF05 (The Focused Acoustic Forecasting 05) experimental data has been used to verify the suggested method by comparison with the conventional time-reversal for communication performance, and it is demonstrated that the suggested method produces better communication performance results than conventional time-reversal.

• The Journal of the Acoustical Society of Korea
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• v.37 no.1
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• pp.21-30
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• 2018

A passive time-reversal technique can improve error performance of the underwater communication system by reducing influence of inter-symbol interferences, which is caused by a multipath channel response. The passive time-reversal communication system equipped with numerous receivers generally can obtain superior error performance since larger diversity gain can be obtained as the number of available received signal increased. In this paper, we analyze the optimal number and combination of receivers that can approximately achieve the best error performance when using the limited number of receivers. For this analysis, we use communication data collected during SAVEX15 (Shallow-water Acoustic Variability Experiment 2015) carried out in the south-western part of Jeju Island from May 14 to May 28, 2015. Analysis results show that there are depths of energy concentration due to the channel characteristics in which the underwater sound channel are present, and the passive time-reversal technique using the limited number of the receivers can derive near-optimal communication performance if the receivers for time-reversal processing are located at the depths where energy is concentrated.

• The Journal of the Acoustical Society of Korea
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• v.33 no.6
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• pp.392-399
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• 2014

The underwater acoustic communication is characterized by doubly spread channels, which are the delay spread due to multiple paths and the doppler spread due to environmental fluctuations or a moving platform. An equalizer is used to remove the inter-symbol interferences that the delay spread causes, but an equalizer doesn't use an acoustic environment such as a multipath. However, a passive time-reversal mirror is simpler than an equalizer because a matched filter is implemented numerically at the receiver structure along with one-way propagation. In this paper, a passive time-reversal mirror is applied to remove interferences due to a multipath in sea-going experimental data in East Sea in Oct. 2010 and improved communication performance is confirmed. The performance is verified by comparing the signal-to-interference plus noise ratio before/after passive time-reversal mirror. It is also performed independently of the passive time-reversal mirror and adaptive equalizer and the bit error rate is compared to verify the performance of underwater acoustic communication.

• The Journal of the Acoustical Society of Korea
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• v.25 no.5
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• pp.239-245
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• 2006

Time-reversal processing (TRP) has been shown as an effective way to focus in both time and space. The temporal focusing properties have been used extensively in underwater acoustics communications. Recently. adaptive time-reversal processing (ATRP) was applied to the simultaneous multiple focusing in an ocean waveguide. In this study. multiple focusing with ATRP is extended to the underwater acoustic communication algorithm for multiple receiving locations. The developed algorithm is applied to the underwater acoustic communication to show, via simulation and real data, that the simultaneous self-equalization at multiple receiving locations is achieved.

• Journal of the Institute of Electronics Engineers of Korea TC
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• v.49 no.8
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• pp.52-57
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• 2012

We propose a weighted time-reversal prefilter for indoor wireless communication systems. In the indoor wireless communication environments, the bit error rate (BER) performance is significantly degraded by the delay spread. The conventional schemes have complex receivers to recover deterioration of the BER. The proposed time-reversal prefilter simplifies the structure of receivers, minimizes the inter-symbol interference (ISI) and maintains the peak power level of the received signal. The simulation results show that the weighted time-reversal prefilter improves the BER performance in comparison with the conventional time-reversal prefilter.

• Journal of the Institute of Electronics Engineers of Korea TC
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• v.48 no.5
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• pp.33-39
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• 2011

We studied the performance of a two-user time reversal multiple input single output scheme combined with the shifted transmission technique in a variable rate back-off scenario, called shifted time reversal (TR), that minimizes both intersymbol interference and multiuser interference. We compare the bit error rate performance of the shifted TR scheme to both full-rate TR and full back-off TR schemes and demonstrate its superiority to shifted zero forcing scheme when the guard interval is larger than one.

• The Journal of the Acoustical Society of Korea
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• v.32 no.2
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• pp.116-123
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• 2013

This paper represents generation of time-varying underwater acoustic channels by performing scattering simulation with time-varying ocean surface and Kirchhoff approximation. In order to estimate the time-varying ocean surface, 1D Pierson-Moskowitz ocean power spectrum and Gaussian correlation function were used. The computed scattering coefficients are applied to the amplitudes of each impulse of BELLHOP simulation result. The scattering coefficients are then compared with measured doppler spectral density of signal components which were scattered from ocean surface and the correlation time used in the Gaussian correlation function was estimated by the comparison. Finally, bit-error-rate and channel correlation simulations were performed with the generated time-varying channel based on passive time-reversal communication scenario.

• The Journal of the Acoustical Society of Korea
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• v.35 no.3
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• pp.167-174
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• 2016

Underwater communication is difficult to increase the communication capacity because the carrier frequency is lower than that of radio communications on land. This is limited to the bandwidth of the signal under the influence of the characteristics of an ocean medium. As the high transmission speed and large transmission capacity have become necessary in the limited frequency range, the studies on MIMO (Multiple Input Multiple Output) communication have been actively carried out. The performance of the MIMO communication is lower than that of the SIMO (Single Input Multiple Output) communication because cross-talk occurs due to multiusers along with inter symbol interference resulting from the channel characteristics such as delay spread and doppler spread. Although the adaptive equalizer considering multi-channels is used to mitigate the influence of the cross-talk, the algorithm is normally complicated. In this paper, time reversal mirror technique with the characteristic of a self-equalization will be applied to simplify the compensation algorithm and relieve the cross-talk in order to improve the communication performance when the signal transmitted from two channels is received over interference on one channel in the same time. In addition, the performance of the MIMO communication based on the time reversal mirror is verified using data from the SAVEX15(Shallow-water Acoustic Variability Experiment 2015) conducted at the northern area of East China Sea in May 2015.