Chulwon Seo

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 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.

Performance of Convolution Coding Underwater Acoustic Communication System on Frequency Selectivity Index

The convolution code(CC) of code rate 1/2 as a forward error correction (FEC) in Quadrature Phase Shift Keying (QPSK) is applied to decrease bit error rate (BER) by background noise and multipath in shallow water acoustic channel. Ratio of transmitting signal bandwidth to channel coherence bandwidth is defined as frequency selectivity index. BER and bit energy-to-noise ratio gain of transmitted signal according to frequency selectivity index are evaluated. In the results of indoor water tank experiment, BER is well matched theoretical results at frequency selectivity index less than about 1.0. And bit energy-to-noise ratio gain is also matched theoretical value of 5 dB. BER is effectively decreased at frequency selective multipath channel with frequency selectivity index higher than 1.0. But bit energy-to-noise ratio greater than a certain size in terms of CC weaving is effective in reducing bit errors. In the results, the defined frequency selectivity index in this study could be applied to evaluate a performance of CC in multipath channel. Also it could effectively reduced BER in a low speed underwater acoustic communication system without an equalizer.

Bias error of source level estimate in submerged vessel’s radiated noise measurement

Two standards, the American National Standards Institute/the Acoustical Society of America (ANSI/ASA) and the International Organization for Standardization/Publicly Available Specification (ISO/PAS) have been published for surface vessel radiated noise assessment but not for a submerged vessel. Their measurement uncertainties of random and bias error are addressed for components of the measurement system. In this study, the bias error for a submerged vessel source level estimate is interpreted on the basis of the sea surface coherent reflection coefficient and transmission anomaly as functions of frequency, grazing angle, and effective wave height. It is found that the bias error using the mean intensity in a vertical hydrophone array is approximately 3 dB at frequencies less than 2 kHz, but 1 dB at frequencies higher than 2 kHz for effective wave heights of 0.3 and 0.6 m. This result will contribute a future standard for submerged vessel radiated noise assessment.

Error Characteristics of Ship Radiated Noise Estimation by Sea Surface Scattering Effect

The ship radiated noise level fluctuates by the interference between direct and reflected paths. The effect of sea surface reflection path on interference depends strongly on sea surface roughness. This paper describes error characteristics of ship acoustic signature estimation by sea surface scattering effect. The coherent reflection coefficient which explains a magnitude of sea surface scattering and its resultant interference acoustic field is analyzed quantitatively as a function of a grazing angle, effective surface height, frequency, source-receiver range and depths of source and receiver. Theoretical interference acoustic field is compared with experimental result for two different sea surfaces and five different frequencies by changing source-receiver range. It is found that both matches well each other and a magnitude of interference acoustic field is decreasing by increasing a grazing angle, effective surface height, frequency, and depths of source and receiver and decreasing source-receiver range. For given experimental conditions, the transmission anomaly which is a bias error of ship acoustic signature estimation, is about a range of 1~3 dB. The bias error of an existing ship radiated noise measurement system is also analyzed considering wind speed, source depth and frequency.