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Due to the open underwater channels and untransparent network deployment environments, underwater acoustic networks (UANs) are more vulnerable to hostile environments. Security research is also being conducted in cryptography, including authentication based on asymmetric algorithms and key distribution based on symmetric algorithms. In recent years, the advancement of quantum computing has made anti-quantum attacks an important issue in the field of security. Algorithms such as lattice and SPHINCS+ have become a research topic of interest in the field of security. However, within the past five years, few papers have discussed security algorithms for UANs to resist quantum attacks, especially through classical algorithms. Some existing classical asymmetric and symmetric algorithms are considered to have no prospects. From the perspective of easy deployment in engineering and anti-quantum attacks, our research focuses on a comprehensive lightweight security framework for data protection, authentication, and malicious node detection through the Elliptic Curve and Hash algorithms. Our mechanism is suitable for ad hoc scenarios with limited underwater resources. Meanwhile, we have designed a multi-party bit commitment to build a security framework for the system. A management scheme is designed by combining self-certifying with the threshold sharing algorithm. All schemes are designed based on certificate-less and ad hoc features. The proposed scheme ensures that the confidentiality, integrity, and authentication of the system are well considered. Moreover, the scheme is proven to be of unconditional security and immune to channel eavesdropping. The resource and delay issues are also taken into consideration. The simulations considered multiple variables like number of nodes, attackers, and message length to calculate proper values that can increase the efficiency of this scheme. The results in terms of delay, delivery ratio, and consumption demonstrate the suitability of the proposal in terms of security, especially for malicious node detection. Meanwhile, the computational cost has also been controlled at the millisecond level. [ABSTRACT FROM AUTHOR]

Poly(ε-caprolactone-co-trimethylene carbonate) (PCLTMC) is a biodegradable copolymer, which is widely used for medicine and tissue repair systems, and its morphology, degradation and mechanical properties have been extensively investigated. However, its crystallization behavior with different copolymerization ratios has not been studied systematically, which is of great significance for the study of property regulation and an understanding of the crystallization behavior of random copolymers. In this paper, the relationship between the crystallization temperature Tc, melting point temperature Tm and crystallinity Xc of copolymers with different molar ratios was determined by differential scanning calorimetry (DSC) using a non-isothermal crystallization method. The results show that the melting point and crystallinity of the copolymer decrease with an increase in TMC composition. The equilibrium melting point of the copolymers was predicted by an isothermal crystallization method, using the Hoffman–Weeks extrapolation method and crystallization rate growth fitting method both based on Lauritzen–Hoffman theory. It is proved that Lauritzen–Hoffman theory is of great significance for guiding the kinetic behavior of PCLTMC crystallization. [ABSTRACT FROM AUTHOR]

Generalized frequency division multiplexing (GFDM) is a newly introduced technique for the wireless fifth-generation (5G) standard based on multicarrier filter bank theory, which has the advantage of flexibility in setting the number of subcarriers and subblocks. The application of GFDM in underwater acoustic (UWA) communication can take full advantage of the limited spectral resources, which is a prime limitation in UWA communication and will promote the development of UWA network technology. However, the multicarrier communication technique utilized in radio 5G communication offers difficulty in channel estimation, and the influence of a channel cannot be ignored especially in the UWA communication field. Therefore, GFDM cannot be implemented directly in UWA communication; to solve this problem, a system combining chaotic sequence spread spectrum technology with GFDM is proposed, which is a novel technique with high spectrum efficiency. Simulation and experimental results verified the effectiveness of the proposed system.

It is difficult for the conventional image compression method to achieve good compression effect in the underwater acoustic image (UWAI), because the UWAI has large amount of noise and low correlation between pixel points. In this paper, fractal coding is introduced into UWAI compression, and a fractal coding algorithm based on interest region is proposed according to the importance of different regions in the image. The application problems of traditional quadtree segmentation in UWAIs was solved by the range block segmentation method in the coding process which segmented the interest region into small size and the noninterest region into large size and balanced the compression ratio and the decoded image quality. This paper applies the classification, reduction codebook, and correlation coefficient matching strategy to narrow the search range of the range block in order to solve the problem of the long encoding time and the calculation amount of encoding process is greatly reduced. The experimental results show that the proposed algorithm improves the compression ratio and encoding speed while ensuring the image quality of important regions in the UWAI.

PCL and PLA have good biocompatibility and degradability, which are widely used in biomedical fields. Here, PDLLA, PLLA and PCL porous microspheres with different crystallinities were prepared using the single emulsion solvent evaporation method. The effects of polymer crystallinity, molecular weight, concentration and temperature on the morphology of microspheres were systematically studied, and flat porous and concave porous microspheres were accurately prepared. Petroleum ether, n-dodecane and paraffin liquid were used as alkane porogens. DSC and XRD showed that the crystallinity of PDLLA, PLLA, and PCL increased in turn. Porous microspheres with different morphologies were prepared using three different porogens. The results showed that the polymer microspheres with different crystallinities have different morphologies, including concave porous and flat porous microspheres. Different types of porogens have the greatest influence on the morphology of PLLA microspheres while having little effect on PCL and PDLLA. The larger the molecular weight of PDLLA microspheres, the larger the particle size. The phase separation trend of low molecular weight PCL and high concentration porogen increased, and it facilitated easy formation of hollow porous and bowl-shaped microspheres, which affected the integrity of the microspheres. The mechanism of the microsphere morphology can be analyzed and attributed to two main processes, namely polymer phase separation and organic solvent volatilization. This study provides an effective strategy for the precise preparation of microspheres with a porous morphology. [ABSTRACT FROM AUTHOR]

Software-defined measurement (SDM) is a simple and efficient way to deploy measurement tasks and collect measurement data. With SDM, it is convenient for operators to implement fine-grained network-wide measurements at the flow level, from which many important functions can benefit. The prior work provides mechanisms to distribute flows to monitors, such that each monitor can identify its non-overlapped subset of flows to measure, and a certain global performance criterion is optimized, such as load balance or flow coverage. Many applications of network management can benefit from a function that can find large flows efficiently, such as congestion control by dynamically scheduling large flows, caching of forwarding table entries, and network capacity planning. However, the current network-wide measurements neglect the diversity of different flows as they treat large flows and small flows equally. In this paper, we present a mechanism of accuracy-aware collaborative monitoring (ACM) to improve the measurement accuracies of large flows in network-wide measurements at the flow level. The structure of the sketch is an approximate counting algorithm, and a high-measurement accuracy can be achieved by merging the results from multiple monitors with sketches, which is termed as collaborative monitoring. The core idea of our method is to allocate more monitors to large flows and achieve the load balance to provide accuracy-aware monitoring. We modeled our problem as an integer–linear programming problem, which is NP-hard. Thus, we propose an approximation algorithm, named the improved longest processing time algorithm (iLPTA); we proved that its approximation ratio is (1 2 + n l) . We propose a two-stage online distribution algorithm (TODA). Moreover, we proved that its approximation ratio is (1 + n l − 1) . The iLPTA is an offline approximation algorithm used to assign monitors for each flow, which prove the validity and feasibility of the core idea. The TODA is an online algorithm that attempts to achieve the load balance by selecting the monitor with the smallest load to a large flow. Our extensional experiment results verify the effectiveness of our proposed algorithms. [ABSTRACT FROM AUTHOR]

As low-frequency and broadband acoustic emission capability is beneficial to the detection range and acoustic communication speed of small scale autonomous underwater vehicles (AUV), this type of transducer is required, especially in cases of complex acoustic environments. A broadband bender transducer with cavity baffle that suits small scale AUV is proposed. Rather than additional benders, a passive cavity baffle, which would be capable of providing mutual radiation and a fluid cavity mode, is introduced to a single bender. The bending resonant frequency is reduced by the mutual radiation between the bender and the cavity baffle, the cavity baffle extends the lower limit of the available frequency band of the transducer, the liquid resonant frequency behind the former expands the higher limit, then the cavity baffle bender transducer fills the role of radiating low-frequency and broadband emissions through multimode coupling. The finite element method is used to analyze the acoustic performance of the transducer under different baffle conditions. Then, a prototype of the broadband cavity baffle bender transducer is developed according to the optimized parameters of simulation. The acoustic parameters of the prototype were measured in an anechoic pool. The resonant frequency measured in water of the bender itself is 3 kHz, and the −3dB bandwidth is 560 Hz. The prototype test results show that the cavity baffle scheme can improve the −3dB bandwidth of the bender from 560 Hz to 1000 Hz, which fundamentally realizes the expectations of the prototype design. [ABSTRACT FROM AUTHOR]

In UWAC (underwater acoustic communication), UWA (underwater acoustic) channels change rapidly due to varying environment conditions. AMC (adaptive modulation and coding) is an efficient technique to improve system efficiency by changing transmission parameters according to channel conditions in UWA channels. In this paper, we propose an environmental cognition orthogonal frequency division multiplexing (OFDM) UWAC system and compare it with AMC algorithm with six transmission modes together with three threshold detection algorithms. Simulation and experimental results show the effectiveness of the proposed system. The system will play an important role in future communication networks which can significantly improve the efficiency of the system. [ABSTRACT FROM AUTHOR]

In order to improve the efficiency of chemical preparation of YBa2Cu3O7 s (YBCO) with long taps, the pyrolysis of traditional process preparing precursor film needs to be reduced to one step, so the crystallization process should be directly connected to the former without the cooling of pyrolysis. Replacing copper trifluoroacetate with copper isooctanoate can alleviate the centralized heat release and the contraction of the precursor film during pyrolysis. In this study,a suitable amount of boric acid was added into the precursor solution with total cation concentration of 1.0 mol/L. After a continuous heat treatment YBCO superconducting film was obtained. Utilizing the flow state of boric acid at 200-500 °C? boron oxide was doped into YBCO film which becomes denser and smoother by filling some pores. Compared with that one prepared by ?doped precursor solution under the same conditions, the former shows higher superconductivity. [ABSTRACT FROM AUTHOR]

• Simple homopolymers replace complex copolymers as underwater adhesive materials. • Adhesives show better adhesion under water compared to the air environment. • The adhesive strength is much greater than that reported in the previous literature. The underwater adhesive materials inspired by marine mussels have been a research hotspot. As is known to all, interface adhesion properties, mechanical properties, rapid response properties are important indicators for the application of underwater adhesive materials. In this study, we report that Poly(N-(3,4-Dihydroxyphenethyl) methacrylamide) (PDMA), a simple dopamine-based homopolymer, replaces complex binary or multiple copolymers as an underwater adhesive material. The influence factors of PDMA were investigated by universal tensile dynamometer, water contact angle tester, X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared reflection (FTIR). Meanwhile, the underwater self-assembly performance of PDMA was performed. Compared with the air group, there are more phenolic hydroxyl groups on the surface of the adhesive in the underwater group, and the adhesive performance is better. In summary, PDMA can be regarded as an alternative substitution material for underwater adhesives. [ABSTRACT FROM AUTHOR]

To solve the problem of the high peak-to-average power ratio (PAPR) in Orthogonal Frequency Division Multiplexing (OFDM) for the underwater acoustic communication system, the paper offers a method of reducing PAPR which combines the amplitude limiting and the improved nonlinear transformation. Traditional amplitude limiting technique can reduce PAPR in OFDM system effectively, at the cost of reducing the bit error rate (BER). However the companding transformation has far less computation complexity than SLM or PTS technologies and can improve the BER performance compared to the amplitude limiting technique simultaneously. The paper combines these two kinds of techniques, takes full use of advantages of the two method, and puts forward a low-complexity scheme choosing parameters that are more appropriate to the underwater acoustic field, with the result of improved BER performance even in lower SNR. Both simulation and experiment results show that the new method which combines clipping and companding transformation can effectively reduce the PAPR in the underwater acoustic OFDM communication system and improve the BER performance simultaneously. [ABSTRACT FROM AUTHOR]

A novel adaptive filtering (AF) algorithm is proposed for group-sparse system identifications. In the devised algorithm, a novel mixed error criterion (MEC) with two-order logarithm error, p-order errors and group sparse constraint method is devised to give a resistant to the impulsive noise. The proposed group-sparse MEC can fully use the known group-sparse characteristics in the cluster sparse systems, and it is derived and analyzed in detail. Various simulations are presented and analyzed to give a verification on the effectiveness of the developed group-sparse MEC algorithms, and the simulated results shown that the developed algorithm outperforms the previously developed sparse AF algorithms for identifying the systems. [ABSTRACT FROM AUTHOR]

Distinctive features of underwater communication channel pose significant challenges to effective underwater acoustic communication. Due to bandwidth limitation, orthogonal frequency division multiplexing is widely used for its high spectrum efficiency. However, orthogonal frequency division multiplexing also has its shortcomings, one of which is the relatively high peak-to-average power ratio, which leads to saturation in the power amplifier and consequent distortion of the signal. Clipping is the most commonly used method to address the high peak-to-average power ratio; however, it introduces additional noise resulting in degradation of the system’s performance. This article proposes a compressed sensing technique for mitigation of the clipping noise, which exploits pilot and data tones instead of reserved tones, thus making it distinct from the previous works and improves data rate. Moreover, in contrast with previous works, the channel is also estimated using compressed sensing technique, which provides more accurate channel characteristics for estimating the clipping noise than traditional methods like least square or minimum mean squared error. The better performance of the proposed Iterative compressed sensing algorithm is proved in simulations as well as in a pool experiment using acoustic wave sensors. [ABSTRACT FROM AUTHOR]