MTSA-Net: A multiscale time self-attention network for ship radiated self-noise reduction

A technique is developed to detect underwater acoustic signals by utilizing the resultant surface perturbations. It uses light wave in air and uses acoustic wave under water. Because two best channels are combined together, it is an ideal method for remotely detecting underwater acoustic signals. The underwater acoustic signal can cause the perturbations at water-air interface. These perturbations result in an amplitude modulation effect on a laser beam reflected by the water surface. The modulated underwater laser signal can be detected by using a direct light intensity detecting method. Based on the theory of modulation of the intensity of laser, a detection system was developed in the laboratory. By analyzing the massive experimental data, the technique of laser detection of underwater acoustic signals is validated.

The advancement of Underwater acoustic communication (UAC) holds great promise for the Internet of Underwater Things (IoUT). However, a critical challenge to the development of UAC technology is the difficulty of obtaining sea trial data, such difficulty makes the emerging algorithms inconvenient to evaluate. In this paper, we propose a Shared Underwater Acoustic Communication Layer (SUACL) to collect underwater acoustic signal, the proposed SUACL is founded on the IoUT and users are allowed to remotely access the system to arrange underwater acoustic data transmission and reception via Internet. The SUACL system aims to enable the remote operation of communication units that are deployed in designated sea areas. The merit of the proposed SUACL is that the sea trial data can be obtained without risky and high expense, and the emerging underwater acoustic (UWA) technologies can be evaluated in time. The SUACL is composed of the non-underwater part which includes the application layer and fusion layers, and the above/under water part including the communication layer on which we focus. We provide a detailed description of the communication layer of the proposed SUACL system, such as the pre-amplifier with reconfigurable gain, the analog-to-digital converter with reconfigurable sampling rates, the reconfigurable transducers with different frequency bands, the reconfigurable number of hydrophones. The SUACL's reconfigurability options make it a versatile tool that can meet the diverse experimental needs for underwater communication research. In addition, we also describe the mechanisms of how experimental data is exchanged through the SUACL. Our experimental results demonstrate the feasibility and effectiveness of the proposed SUACL in a shallow-water environment.

The basic idea of this letter is concerned with the large Peak-to-Average Power Ratio (PAPR) problem in multi-carrier Under-Water Acoustic (UWA) communication systems that implement Orthogonal Frequency Division Multiplexing (OFDM). A Tangent Rooting (TanhR) companding technique is presented in this paper to reduce the PAPR in UWA communication systems. This technique achieves a noticeable reduction of the PAPR in UWA systems, while maintaining a good Bit Error Rate (BER) performance . Channel equalisation with Linear Minimum Mean Square Error (LMMSE) and Zero-Forcing (ZF) equalisers is considered in this paper. Simulation results ensure superiority of the proposed technique compared to the Selected Mapping (SLM) and the Partial Transmit Sequence (PTS) techniques from both the PAPR and BER perspectives.

Submarine hydrophone arrays sample the underwater acoustic pressure field in space and time to sense the presence of sources of radiated sound and to extract tactical information from the received sounds. The outputs of the hydrophones are combined by a spatial filter (or beamformer) so that signals from a chosen direction are coherently added while the effects of noise and interference from other directions are reduced by destructive interference. The spatial filter appropriately weights the hydrophone outputs prior to summation so as to enhance the output signal-to-noise ratio, thereby improving the detection, classification, localization and tracking performance of the passive sonar system onboard the submarine. By processing real acoustic data from a submarine hull-mounted array, it is shown that an optimal (adaptive) frequency-domain spatial filter (based on inversion of the observed cross spectral matrix for each frequency bin) enhances the detection of underwater acoustic signals, when compared with conventional spatial filtering (or delay-and-sum beamforming). The degree of improvement, however, depends on the method used to normalize the cross spectral matrix. Superior detection performance for a submarine hull-mounted array occurs when the observed cross spectral matrix for each frequency of interest is normalized using the average of all the single hydrophone output powers at each frequency. It is also shown that the self-noise analysis of a submarine towed array is facilitated by processing the element-level data in the frequency-wave number domain with a constrained optimal spatial filter, which is also referred to as a Minimum Variance Distortionless Response (MVDR) beamformer. This spatiotemporal filtering method is employed to separate signals (surface ship contacts) and tow vessel noise components (direct path and multipath arrivals) according to their directions of propagation. This method is also found to separate spatially-correlated self-noise components that propagate within the towed array structure at a speed that is slower than the speed of sound travel in water. A comparison of the frequency-wave number power spectra estimated using conventional and optimal spatial filtering methods shows that the MVDR spatial filter enables the various sources of acoustic energy that are sensed by the array to be more clearly delineated in frequency-wave number space. The MVDR spatial filter is a data-adaptive spatial filter that is observed to suppress spatial leakage, to enhance the spatial resolution of a towed array through narrower beamwidths, and to provide superdirective array gain at frequencies well below the design frequency of the towed array. Frequency-wave number analysis with optimal spatial filtering is a powerful diagnostic tool for studying the self-noise characteristics of a towed linear array of hydrophones.

T400 conventional underwater acoustic (UWA) wakeup signals detection methods for both underwater cooperative nodes and non-cooperative nodes are FFT based signal detection and matched-filtered based signal detection. While there is a big performance degradation of these conventional frequency-offset sensitive methods due to the relative motion between nodes, complex underwater noise environment, channel time-varying and other interference sources. This paper proposes a dynamic time warping (DTW) algorithm to detect the wake-up signal in UWA communication networks, which is frequency-offset insensitive and performs better than the conventional methods in severe frequency-offset environment by simulations based on the true UWA channel measured from sea trial.

To address the inability to quantitatively analyze the performance of the direct sequence spread spectrum (DSSS) communication system in the presence of short-period pseudo-noise (PN) sequences, this study derives the interference components resulting from correlation processing of single-frequency interference. Given the short period of the spread spectrum sequence used in underwater acoustic systems, a theoretical expression is proposed for analyzing the anti-single-frequency interference performance of DSSS communication systems in underwater acoustics. This expression combines the interference power output by the system to enable quantitative analysis. Simulations are conducted to validate the accuracy of the theoretical expression. The results reveal that the interference sequence generated by correlation processing assumes either a direct current (DC) or sinusoidal distribution. The bit error rate (BER) of the system is influenced by signal-to-interference ratio (SIR), interference frequency offset, phase deviation, and the structure of the spreading code itself. The BER exhibits regular variation with changes in the interference phase. In all working conditions where the interference frequency offset is an integer multiple of the bit bandwidth, single-frequency interference with a frequency offset equivalent to the bit bandwidth has the greatest impact on the system. The concise theoretical expression presented in this study has significant practical value as it allows for quick prediction of variation in the anti-single-frequency interference performance of underwater acoustic DSSS communication systems using short-period PN sequences.

Pre-set secret keys are generally used to encipher and decipher the message in communication systems, including underwater acoustic (UWA) systems. Communication is not secure if the keys are compromised, so we need to generate an adaptive shared secret key. The secret keys are generated adaptively using correlated channel estimation, which reckons on the randomness in the environment and does not require the secrecy of data transferred during the process. This concept is extended in UWA channels with orthogonal frequency-division multiplexing (OFDM) systems and implements a protocol to generate secret keys adaptively based on the channel frequency response (CFR). A secret key generated from the CFR estimation manages the channel coding system, namely a turbo-code. This scheme allows to jointly performing an error control and adaptive coding in only one step. This scheme allows an adaptive security coding without change of the hardware structure of the transceiver devices. In our analysis, BER is used as a measure of the performance of channel estimation and mutual information is used as a measure of secretkey performance.

Laser remote sensing underwater acoustical signal is one effective way to recognize underwater targets and realize on-board active scanning, acquisition, pointing and tracking over a wide range. By the research of the theoretical and the simulation of PC, one fundamental experiment system is constructed, which validates the feasibility of laser detection of underwater acoustic signal. The size and location of the diaphragm of the detection system are particularly discussed and one optimum diaphragm aperture was developed. The affection of the incidence beam width is also analyzed. By selecting the very incident angle and beam width, we can get the best performance. These conclusions has the important guiding significance to the technology of laser remote sensing underwater acoustic signal. As a whole laser remote sensing underwater acoustic signal has its own advantages and opens up broad possibilities for the practical application. Of course it still has something to do to improve its performance, which worth doing further studies.

<p>The key observation behind our scheme is that in contrast to the observed noise samples, which are assumed to be independent, the signal’s samples are actually statistically dependent. We utilize this dependence, as well as the prior knowledge of L , by formalizing the problem as a bipartite factor graph, where samples correspond to variable nodes, and the factors favor sequences of at least L variables. We then iterate along the factor graph and make detection decisions per sample, based on the resulting beliefs. This setup resembles the architecture of the low-density parity-check coding scheme [7], where message nodes and check nodes are analog to variable nodes and factor nodes, respectively. A legitimate codeword is a sequence of at least L variables, rejecting shorter transients. The novelty in our approach is, therefore, the utilization of local statistical dependence of the signal’s samples over time scales larger than L to reduce false alarms which otherwise exists due to noise transients.</p>

An attention-based multi-scale convolution network for intelligent underwater acoustic signal recognition

Modulation recognition of non-cooperation underwater acoustic communication signals using principal component analysis

© 2017 University of Western Australia. The orthogonal frequency-division multiplexing (OFDM) technology receives increasing attention in underwater acoustic (UA) communications. This paper presents a real-time OFDM-based UA communication system, implemented using the National Instruments CompactDAQ device and the LabVIEW software. The system design including both the transmitter and receiver is discussed. The performance of this real-time system is verified through a UA communication experiment conducted recently in a tank. Compared with conventional digital signal processor (DSP)-based design, the proposed implementation simplifies the prototype design and reduces the software development time.

Optimizing the number of relays for energy efficient multi-hop covert underwater acoustic cooperative networks

In the using of FBG(Fiber Bragg Grating) developed in home land, we designed and manufactured united FBG acoustic transducers the first in Korea. they are being applied to multi-point signal detection of FBG Hydrophone used Hopper WDM(national patent NO 10-1502954) in the underwater. On united FBG transducers manufactured, we made an demonstrated on respective frequency response peculiarities in the underwater and analyzed the special characters. As the experimental result on frequency response peculiarities, we made it possible underwater acoustic detection on united FBG acoustic transducers type to maximum 30Hz~2.5KHz. it's the optimum conditions of 1.2KHz frequency in detection. And for the purpose of realization on multi-point signal detection on wide scope in the underwater, in the using of WDM(Wavelength Division Multiplexing) method and passive band-pass filter system, established arrays system and succeeded in multi-point underwater acoustic signal detection to the frequency 200Hz~1.3KHz out of the two united type FBG transducers. Additionally, it would be possible directivity detection for the object of its source as the intensity of detection signal varies with the sound source's direction and angle. From now on we prepared a new moment on the practical use study on FBG hydrophone in the future.

Underwater Acoustic (UWA) communication systems support low data rate, and have bad performance as compared to traditional wireless systems. This is due to attenuation, ambient noise, water salinity, temperature, etc. Hence, channel coding is introduced in order to enhance the UWA system performance. This paper presents a joint Frequency Domain Equalization (FDE) and Low Density Parity Check (LDPC) coding scheme in UWA Orthogonal Frequency Division Multiplexing (OFDM) in order to reduce the effect of the UWA system poor link. The proposed UWA system performance is evaluated via extensive simulations over different channels. Simulation results show that the proposed equalization with LDPC codes gives a good improvement in the UWA system performance.