Jong Rak Yoon

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.

Multiple-Symbol Encapsulated Orthogonal Frequency Division Multiplexing Communication in Multipath Underwater Acoustic Channel

In this paper, multiple-symbol encapsulated orthogonal frequency division multiplexing (MSE-OFDM) is proposed to be employed in a multipath fading channel. Simulation results demonstrate that this method is particularly suitable for channels with long impulse response. MSE-OFDM combined with the spatial diversity technique yields a significant improvement in bandwidth efficiency even in a low signal-to-noise ratio (SNR) environment.

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.

Orthogonal Frequency Division Multiplexing Performance Dependency on Pulse Shape in Underwater Channel

In this study, we propose pulse shapes to decrease the inter-carrier interference (ICI) caused by the frequency offset in orthogonal frequency division multiplexing (OFDM). Several pulse shapes have been employed to reduce ICI, and simulations demonstrate that the linear pulse has improved performance in reducing ICI due to the frequency offset.

Adaptive Equalization for Underwater Acoustic Communication in Multipath Channel

Adaptive equalizers are always realized in an underwater acoustic time-dispersive channel. In this paper, we propose that for channels having a large eigenvalue spread, the equalizer first adopts recursive least square (RLS) algorithm during the training period and then employs least mean square (LMS) algorithm after switching to decision direct mode. The simulation results demonstrate this equalizers superior performance. Finally, the equalizer performance in the presence of the receivers drifting has been investigated and it is found that an equalizer works well if channel coherence remains high.

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.