One of the most critical missions of sonar is to capture deep-sea pictures to depict sea floor and various objects, and provide an immense understanding of biology and geology in deep sea. Due to the poor condition of underwater acoustic channel, the captured sonar images very possibly suffer from several typical types of distortions before finally reaching to users. Unfortunately, very limited efforts have been devoted to collecting meaningful sonar image databases and benchmark reliable objective quality predictors. In this paper, we first generate a sonar image quality database (SIQD), including 840 images. All distorted images were collected without artificially introducing any distortions beyond those occurring during compression and transmission. The subjective quality assessment was conducted for gathering mean opinion score (MOS) to represent the image quality and existence of target (EOT) which describes whether the image is useful. Based on the built SIQD database, state-of-the-art general image quality metrics were found to poorly correlate with “ground-truth” MOS. As a consequence, this paper further develops a novel full-reference local entropy backed sonar image quality predictor (LESQP). The experimental results demonstrate the superiority of our LESQP metric over the available quality measures.

Subjective and objective quality evaluation of sonar images for underwater acoustic transmission

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.

/pdf/performance-evaluation-of-underwater-acoustic-communication-2hfvec74o8.pdf

Performance Evaluation of Underwater Acoustic Communication in Frequency Selective Shallow Water

Multiple Frequency-Shift Keying (MFSK) has been used widely for underwater acoustic communications due to its low complexity and channel robustness. However, the traditional MFSK has the limitation of a low bit rate compared with coherent acoustic communication. To increase the bit rate, this study designs a new MFSK with the concept of orthogonal frequency division multiplexing (OFDM). We also adopt a channel-concatenated coding to resist the multipath interference and design the iterative joint decoding. The channel-concatenated coding consists of a Hadamard code and a convolutional code. Correspondingly, the iterative joint decoding uses the Hadamard–Viterbi joint soft decoding framework with a newly designed branch metric, which uses the Hadamard structure. As an important preprocessing link for a received signal, frame synchronization and Doppler compensation are also described in detail in this study. Simulations and experiments are conducted to show the effectiveness of the proposed MFSK underwater acoustic communications.

/pdf/hadamard-viterbi-joint-soft-decoding-for-mfsk-underwater-1ebmab9z.pdf

Hadamard-Viterbi Joint Soft Decoding for MFSK Underwater Acoustic Communications

Compassing the spatial shape is sometimes performed to develop a surround speaker system or enhance a signal. Although the spatial shape is reconstituted from the boundary position, conventional methods offer arrays consisting of a large number of elements (microphones and loudspeakers). In this research, we developed a method of localizing the acoustic reflective boundary based on the direction-of-arrival (DOA) of direct and reflected sounds using a pair of microphones. This method offers the DOA values of direct and reflected arbitrary sound sources. Although the estimation involves some estimation errors caused by approximations in the calculation, the correction function was obtained from several conditions numerically. Results indicate that it is possible to localize the reflective boundary almost accurately (the average error ratio from the true value is below 10%) through the presented method.

Localization of Acoustic Reflective Boundary Using a Pair of Microphones and an Arbitrary Sound Source (Special Issue : Ultrasonic Electronics) -- (Ocean acoustics)

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.

Coherence Bandwidth Effects on Underwater Image Transmission in Multipath 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.

https://iopscience.iop.org/article/10.7567/JJAP.53.07KG02/pdf

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

https://iopscience.iop.org/article/10.7567/JJAP.52.07HG01/pdf

Effectiveness of Convolutional Code in Multipath Underwater Acoustic Channel

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.

/pdf/performance-of-convolution-coding-underwater-acoustic-45t0sezbjn.pdf

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

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.

https://iopscience.iop.org/article/10.7567/JJAP.54.07HG05/pdf

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

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