Underwater acoustic sensor networks have recently attracted considerable attention as demands on the Internet of Underwater Things (IoUT) increase. In terms of efficiency, it is important to achieve the maximum communication coverage using a limited number of sensor nodes while maintaining communication connectivity. In 2017, Kim and Choi proposed a new deployment algorithm using the communication performance surface, which is a geospatial information map representing the underwater acoustic communication performance of a targeted underwater area. In that work, each sensor node was a vertically separated hydrophone array, which measures acoustic pressure (a scalar quantity). Although an array receiver is an effective system to eliminate inter-symbol interference caused by multipath channel impulse responses in underwater communication environments, a large-scale receiver system degrades the spatial efficiency. In this paper, single-vector sensors measuring the particle velocity are used as underwater sensor nodes. A single-vector sensor can be considered to be a single-input multiple-output communication system because it measures the three directional components of particle velocity. Our simulation results show that the optimal deployment obtained using single-vector sensor nodes is more effective than that obtained using a hydrophone (three-channel vertical-pressure sensor) array.

The underwater acoustic sensor network (UWASN) is a system that exchanges data between numerous sensor nodes deployed in the sea. The UWASN uses an underwater acoustic communication technique to exchange data. Therefore, it is important to design a robust system that will function even in severely fluctuating underwater communication conditions, along with variations in the ocean environment. In this paper, a new algorithm to find the optimal deployment positions of underwater sensor nodes is proposed. The algorithm uses the communication performance surface, which is a map showing the underwater acoustic communication performance of a targeted area. A virtual force-particle swarm optimization algorithm is then used as an optimization technique to find the optimal deployment positions of the sensor nodes, using the performance surface information to estimate the communication radii of the sensor nodes in each generation. The algorithm is evaluated by comparing simulation results between two different seasons (summer and winter) for an area located off the eastern coast of Korea as the selected targeted area.

Underwater acoustic communication is heavily influenced by intersymbol interference caused by the delay spread of multipaths. In this article, communication sequences transmitted from a drifting source were received by a fixed acoustic vector receiver system consisting of an accelerometer-based vector sensor and a pressure sensor, which can measure the three-directional components of vector quantity and pressure at a point. The underwater acoustic communication experiment was conducted in water approximately 30 m deep off the south coast of Geoje Island, South Korea, in May 2017 during the Korea Reverberation Experiment. Acceleration signals received by the vector sensor were converted to pressure-equivalent particle velocities, which were then used as input for a four-channel communication system together with acoustic pressure. These four channels have multipaths with different amplitudes but the same delay times, providing directional diversity that differs from the spatial diversity provided by hydrophone arrays. To improve the communication performance obtained from directional diversity, the Multichannel Combined Bidirectional Block-based Time Reversal Technique was used, which combines bidirectional equalization with time-reversal diversity and block-based time reversal that was robust against time-varying channels. Communication performance was compared with the outcomes produced by several other time reversal techniques. The results show that the Multichannel Combined Bidirectional Block-based Time Reversal Technique using a vector sensor achieved superior performance under the environmental conditions considered in this article.

Japanese anchovy (Engraulis japonicus) is an economically important species found in the coastal waters of the northwestern Pacific Ocean. To conduct effective acoustic surveys on the spawning or fishing grounds of this species, the target strength (TS, dB re 1 m²) during its growth stages must be understood. However, knowledge regarding the TS of juvenile Japanese anchovies is limited. To address this knowledge gap, we measured the TS and proposed equation models for juvenile Japanese anchovies while describing the TS differences between young and adult anchovies. We evaluated 21 live anchovies using 38, 120, and 200 kHz transducers. TS models were fitted for the juvenile stage of anchovies, considering different frequencies. The mean TS values of young and adult anchovies were similar to those reported in previous TS studies, whereas the TS values of juvenile anchovies were significantly lower. Thus, applying the same TS equation based on the acoustic characteristics of anchovies at different growth stages, especially for juvenile anchovies, may not be appropriate. These findings underscore the importance of adopting a suitable TS equation specific to the growth stages of Japanese anchovies for accurate data analysis in acoustic surveys conducted on spawning and fishing grounds.

Time reversal processes have been used to improve communication performance in the severe underwater communication environment characterized by significant multipath channels by reducing inter-symbol interference and increasing signal-to-noise ratio. In general, the performance of the time reversal is strongly related to the behavior of the $q$ -function, which is estimated by a sum of the autocorrelation of the channel impulse response for each channel in the receiver array. The $q$ -function depends on the complexity of the communication channel, the number of channel elements and their spacing. A $q$ -function with a high side-lobe level and a main-lobe width wider than the symbol duration creates a residual ISI (inter-symbol interference), which makes communication difficult even after time reversal is applied. In this paper, we propose a new parameter, $E_q$ , to describe the performance of time reversal communication. $E_q$ is an estimate of how much of the $q$ -function lies within one symbol duration. The values of $E_q$ were estimated using communication data acquired at two different sites: one in which the sound speed ratio of sediment to water was less than unity and one where the ratio was higher than unity. Finally, the parameter $E_q$ was compared to the bit error rate and the output signal-to-noise ratio obtained after the time reversal operation. The results show that these parameters are strongly correlated to the parameter $E_q$ .

Passive acoustic monitoring is an observation method for detecting and characterizing ocean soundscapes, and it has recently been used to observe underwater marine life. The brown croaker ( Miichthys miiuy ) is an important fish species in the Northwest Pacific Ocean that produces biological sounds. In this study, the sounds of 150 adult brown croakers were recorded continuously for three weeks using a self-recording hydrophone. The acoustic parameters of their calls, choruses and vocalization patterns were analysed using environmental factors from the ocean. The brown croaker’s call sound with zero peak sound pressure level was 150.8 dB, but the chorus sound was relatively high at 161.3 dB. The vocalization of the sounds occurred daily around sunset and dusk and was associated with decreased spawning activity when the water temperature decreased below approximately 25°C. The acoustic characteristics of the brown croaker’s sounds will help improve ocean soundscape management to protect the marine ecosystem and identify spawning and fishing grounds.