• Journal of Navigation and Port Research
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• v.37 no.3
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• pp.269-273
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• 2013

Underwater acoustic channels are generally recognized as one of the most difficult communication media because of the multipath propagation, dispersion, and so on. MIMO (Multiple-input multiple-output) techniques have been actively pursued in underwater acoustic communications recently to increase the data rate over the bandwidth-limited channels. The transmit diversity techniques can be applied in this case, and one of them is Alamouti's scheme. In this paper the performances of the transmit diversity technique are evaluated via experiment. Two transmitters and two receivers were used in experiment, and the experiment was performed in indoor water tank. The error rate 5~8% was confirmed in experimental results, and these are the improved values than the error rate 14.8% for SISO(Single-input single-output) channel under same data rate condition.

• The Journal of the Acoustical Society of Korea
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• v.31 no.1
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• pp.1-10
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• 2012

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.

• The Journal of the Acoustical Society of Korea
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• v.38 no.1
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• pp.1-8
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• 2019

This paper estimated communication parameters according to underwater channel environment of lake for underwater acoustic communication. This paper calculated coherence time and coherence bandwidth through two experiments in actual lake environments. In both experiments, the chirp signal for channel estimation and the BPSK (Binary Phase Shift Keying) signal for calculating the bit error rate were transmitted. In each experiment, the distance between transmitter and receiver was 300 m to 400 m, and 500 m to 600 m. The coherence times calculated in experiment 1 and experiment 2 are 175 msec and 340 msec, and the coherence bandwidths are 10 Hz and 5.71 Hz, respectively. It is confirmed that the experimental results are more appropriate because the synchronization and the bit error rate performance are better only when the length of the synchronization signal and the interval of the pilot signal in the frame are shorter than the coherence time.

• Proceedings of the Acoustical Society of Korea Conference
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• autumn
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• pp.483-486
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• 2004

수중음향채널의 다중경로에 의한 수신신호의 시간확산(Time spread)은 ISI를 유발하여 수중통신시스템의 성능을 저하시킨다. 시간확산에 의한 ISI를 줄이고 전송율이 높은 코히어런트(coherent) 시스템을 적용하기 위해서는 다중경로 영향을 제거해야 한다. 본 연구에서 적용하는 수동 페이저 컨쥬게이션(passive phase conjugation) 기법은 시역전 기술로 프로브 신호에 의한 시역전을 수행하여 다중경로에 의한 ISI가 감소하여 비트 오류율이 적어진다. 아울러 수신신호의 처리가 간단하여 다중경로에 의한 시간확산에 비례하는 탭수의 증가로 수신신호 처리시간이 과다한 적응등화기 기법에 비해 실시간 시스템 구현에 유리한 기법이다. 수치모의 실험으로 제안하는 기법의 성능을 해석하였다.

• Journal of the Institute of Electronics Engineers of Korea TC
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• v.48 no.1
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• pp.1-8
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• 2011

In this paper, a water tank experiment using Binary Frequency Shift Keying (BFSK) method for text transmission performance by water surface fluctuation is conducted. Water surface fluctuation and delay spread which affect the channel coherence bandwidth is a limiting factor in underwater acoustic communication. The amplitude fluctuation and delay spread the smooth surface and fluctuation surface, were identified. The effective delay spread of both cases are 5ms, 4ms corresponding to the coherence bandwidth of 200Hz, 250Hz, respectively. The bit error rate of BFSK modulated text transmission is about $10^{-4}$ in less than 200bps in smooth surface but less than 250bps in fluctuation surface. Therefore, this experiment shows that the water surface fluctuation is important factor determining the performance of the underwater acoustic transmission.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.23 no.11
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• pp.1434-1442
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• 2019

In this paper, the channel change was continuously measured for 24 hours from July 5, 2017 on the coast near Deokjeok-do, Incheon. The underwater channel has various channel environment characteristics as the change in the time axis and the change in the frequency axis occurs in real time, and the underwater communication performance decreases due to the multipath fading and the Doppler effect. Therefore, in this study, we performed the OFDM system performance analysis in the underwater channel environment by applying the repetitive transmission diversity scheme in the time and frequency domain to improve the communication performance in the real-world underwater communication environment. Using the collected data, we compared the channel environment in the time and frequency domain and analyzed the BER performance according to the pilot spacing and the number of repetitive transmissions in the time and frequency axis.

• The Journal of the Acoustical Society of Korea
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• v.27 no.6
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• pp.286-295
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• 2008

Underwater acoustic communication has multi path error because of reflection by sea-level and sea-bottom. The multipath of underwater channel causes signal distortion and error floor. In this paper, we consider the use of various channel coding schemes such as RS code, convolutional code, cross-layer code and LDPC code in order to compensate the multipath effect in underwater channel. As shown in simulation results, characteristic of multipath error is similar to that of random error, so interleaver has little effect for error correcting. For correcting of error floor by multipath error, it is necessary strong channel codes like LDPC code that is similar to Shannon's limit. And the performance of concatenated codes including RS codes has better performance than using singular channel codes.

• The Journal of the Acoustical Society of Korea
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• v.24 no.8
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• pp.454-461
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• 2005

Time spread due to the multipath in underwater acoustic channel, induces ISI (Inter-Symbol Interference) which degrades the performance of the underwater acoustic communication system. The passive phase conjugation (PPC) which improves the signal to multipath interference ratio (SMR) and therefore reduces the frequency selectivity. is a diversity communication technique giving a less ISI under multipath fading channel. Its frequency selectivity depends on the number of receiver array sensors and time varying source to receiver range. In this study, frequency selectivity of the PPC and its effects on bit error of underwater acoustic communication is analyzed by numerical simulation.

• 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.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.20 no.5
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• pp.887-894
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• 2016

In the underwater acoustic communication channels, a multipath reflection becomes the cause of obstacle. To solve this problem, a rake receiver has been required for which one could take the time diversity. However, there is a concern about using incorrect path to recover signals with a high weighting value as underwater acoustic communication channels have severe time-variant property. In order to prevent these problem, a rake receiver is proposed which is based on BER(bit error rate) train sequence duration. The performance is evaluated through lake trials; there are three methods that are a proposed rake receiver, a conventional rake receiver, and a non-rake receiver. As a result, the number of bit errors in the proposed rake receiver, that of bit errors in the conventional rake receiver, and that of bit errors in the non-rake receiver is 8, 45, and 72, respectively.