Kyu-Chil Park

Measurement and Analysis of Broadband Acoustic Propagation in Very Shallow Water

The results of an experiment conducted to understand the frequency dependence of acoustic propagation in very shallow water using a ship as an acoustic source are presented and the possible causes of the features appearing in the results are discussed. The result shows large propagation losses at a frequency of approximately 140 Hz. The results of numerical simulation including the shear property and layered structure of the bottom imply that the features might be caused by the shear property of the lower sediment that lies below the thin upper fluid sediment.

Performance Evaluation of Underwater Acoustic Communication in Frequency Selective Shallow Water

An underwater acoustic (UWA) communication in shallow water is strongly affected by the water surface and the seabed acoustical properties. Every reflected signal to receiver experiences a time-variant scattering in sea surface roughness and a grazing-angle-dependent reflection loss in bottom. Consequently, the performance of UWA communication systems is degraded, and high-speed digital communication is disrupted. If there is a dominant signal path such as a direct path, the received signal is modeled statistically as Rice fading but if not, it is modeled as Rayleigh fading. However, it has been known to be very difficult to reproduce the statistical estimation by real experimental evaluation in the sea. To give an insight for this scattering and grazing-angle-dependent bottom reflection loss effect in UWA communication, authors conduct experiments to quantify these effects. The image is transmitted using binary frequency shift keying (BFSK) modulation. The quality of the received image is shown to be affected by water surface scattering and grazing-angle-dependent bottom reflection loss. The analysis is based on the transmitter to receiver range and the receiver depth dependent image quality and bit error rate (BER). The results show that the received image quality is highly dependent on the transmitter-receiver range and receiver depth which characterizes the channel coherence bandwidth.

Coherence Bandwidth Effects on Underwater Image Transmission in Multipath Channel

The performance of an underwater acoustic communication system in shallow water is strongly affected by the water surface and the seabed acoustical properties. Time-variant water surface scattering and grazing-angle-dependent bottom reflection limit the received signal coherency. Consequently, the performance of underwater acoustic communication systems is degraded, and high-speed digital communication is disrupted. In this study, the authors consider two different multipath channels, such as a water tank and a river, and conduct experiments to quantify these effects on the image transmission using binary frequency shift keying (BFSK) modulation. The quality of the received image in a water tank experiment is shown to be affected by water surface scattering. In the 9-m-deep Nakdong river experiment, it is shown to be affected by grazing-angle-dependent bottom reflection. The analysis is based on the coherence bandwidth of the multipath channel response.

Underwater Acoustic Communication Channel Simulator for Flat Fading

Time-variant sea surface scattering and Doppler spreading due to source/receiver motion cause the signal fading stochastically in amplitude and phase fluctuation. Consequently, the performance of underwater acoustic communication systems are degraded, and high-speed digital communication is disrupted. In this study, a channel simulator for flat fading over time-variant sea surface scattering and Doppler spread in source/receiver motion is presented. Rayleigh and Rice fading models are adopted and their distributions are compared with the distribution measured in a water tank. The bit error rate of binary phase shift keying (BPSK) and binary frequency shift keying (BFSK) for flat fading is examined to evaluate the simulator performance.

Neural network approach for automatic classification of ship-radiated noise

Tonal signals from ship-radiated noise contain important information for a ships design, maintenance, operating condition diagnosis, and machinery monitoring. The signals mainly consist of two components–the speed-dependent component and the speed-independent component. Therefore, some procedures are required to detect and classify the tonal signals from ship-radiated noise. We apply two neural network approaches for the detection of a tonal signal by peak extraction, and the classification of the tonal signal by pattern recognition in some numerical experiments on a simulation signal and ship-radiated noise obtained from a real ship.

Comparison of EEG Characteristics between Dementia Patient and Normal Person Using Frequency Analysis Method

Nowadays our society is rapidly transforming into an aging society. A better understanding of dementia is a high priority in the aging society. Therefore our study is basically aimed at understanding characteristics of EEG signals from dementia patients. Firstly, we analyzed spontaneous EEG signals from normal persons and dementia patients to distinguish their characteristics. The EEG signals are recorded with 16 electrodes and we classified the EEG signals form the signals according to frequency band. To obtain the clean EEG signals, we used cross correlation function between two channels. From the analysis results, we can observe that the EEG characteristics from dementia patients are distinctly different from that from normal persons.

Performance of COFDM in Underwater Acoustic Channel with Frequency Selective Fading

In this paper, performance of COFDM (Coded Orthogonal Frequency Division Multiplexing) which is OFDM with a forward error correction code, is studied in frequency selective fading underwater acoustic communication channel. The OFDM is a multiplexing technique resistant to frequency selective multipath channel. In OFDM, a broadband information signal is transformed into several narrow band signals and transmits narrow band signals whose bandwidths are less than the channel coherence bandwidth. However, its performance is degraded in a specific narrow band signal due to its deep fading by multipath. To mitigate this degradation, COFDM which is OFDM with convolution code as a forward error correction code, is evaluated. The performance of COFDM is found to be better than that of OFDM in multipath channel.

Effectiveness of Convolutional Code in Multipath Underwater Acoustic Channel

The forward error correction (FEC) is achieved by increasing redundancy of information. Convolutional coding with Viterbi decoding is a typical FEC technique in channel corrupted by additive white gaussian noise. But the FEC effectiveness of convolutional code is questioned in multipath frequency selective fading channel. In this paper, how convolutional code works in multipath channel in underwater, is examined. Bit error rates (BER) with and without 1/2 convolutional code are analyzed based on channel bandwidth which is frequency selectivity parameter. It is found that convolution code performance is well matched in non selective channel and also effective in selective channel.

Performance comparison between packet and continuous data transmission using two adaptive equalizers in shallow water

Transmitted signals are markedly affected by sea surface and bottom boundaries in shallow water. The time variant reflection signals from such boundaries characterize the channel as a frequency-selective fading channel and cause intersymbol interference (ISI) in underwater acoustic communication. A channel-estimate-based equalizer is usually adopted to compensate for the reflected signals under this kind of acoustic channel. In this study, we apply two approaches for packet and continuous data transmission of the quadrature phase shift keying (QPSK) system. One is the use of a two-dimensional (2D) rotation matrix in a non-frequency-selective channel. The other is the use of two equalizers of types — the feed forward equalizer (FFE) and decision-directed equalizer (DDE) — with a normalized least mean square (NLMS) algorithm in a frequency-selective channel. The percentage improvement of packet transmission is notably better than that of continuous transmission.

Performance comparison of convolution and Reed–Solomon codes in underwater multipath fading channel

In underwater acoustic communication, the transmitted signal is affected by multipath fading and background noise. Therefore, the intersymbol interference (ISI) of underwater acoustic communication systems depends on the effects of these parameters. To cope with ISI due to multipath fading, various equalizers and orthogonal frequency division multiplexing systems have been developed. Forward error correction coding is also adopted to reduce the error by background noise. Here, the performances of convolution code (CC) and Reed–Solomon (RS) code are evaluated in a multipath fading channel. The CC shows better performance in frequency nonselective fading but the RS code shows better performance in frequency selective fading.