At present, elevated pile-cap structures are extensively utilized in deep-water bridges. However, studies on seismic wave-induced hydrodynamic forces acting on circular pile-caps remain limited. In this study, numerical models of a circular pile-cap and a deep-water pier with identical cross-sectional dimensions are established and validated. Subsequently, the generation mechanisms and characteristics of hydrodynamic forces on circular pile-caps are comprehensively analyzed. The results indicate that the hydrodynamic force calculation methods for deep-water piers cannot be applied to pile-caps because the hydrodynamic forces on pile-caps are significantly influenced by free surface waves and bottom free end effects. Additionally, both the inertial force and the wave force are important components of the hydrodynamic force on the pile-cap, especially when the seismic excitation frequency is lower, i.e., 0.20 Hz <= f <= 0.40 Hz. The bottom free end effects not only reduce the wave force but also diminish the inertial force in the hydrodynamic force on the pile-cap. The contribution ratios of the inertial force and the wave force to the hydrodynamic force are approximately not influenced by the seismic excitation amplitude, and are weakly influenced by the aspect ratio, but are significantly influenced by the seismic excitation frequency.

Affine frequency division multiplexing (AFDM) is an emerging multicarrier modulation technique that processes signals in the discrete affine Fourier domain, enabling reliability for high-mobility communications. In order to further improve spectral efficiency (SE), this letter proposes an AFDM scheme with dual index modulation (DIM), referred to as AFDM-DIM. Specifically, information bits are transmitted not only by the traditional symbol constellation, but also jointly through the active states of subcarriers and the corresponding pre-chirp parameter configurations, which can enhance SE and improve robustness against doubly selective fading channels. We also develop two detector algorithms, i.e., maximum likelihood (ML) and joint minimum mean square error-ML (MMSE-ML) for the proposed AFDM-DIM system. We further derive an upper bound on the average bit error rate via pairwise error probability analysis based on ML detection, and compare the SE between the proposed AFDM-DIM scheme with the existing AFDM-index modulation (IM) systems. Simulation results demonstrate that the proposed AFDM-DIM scheme achieves superior performance compared to other existing AFDM-IM schemes.

The characteristics and influencing factors of the tsunami bore force on the multi-box girder are experimentally studied for the first time by using breaking solitary waves to simulate the tsunami bore. The factors influencing tsunami bore force such as wave height a, water depth h, and clearance Z are investigated. It is found that the tsunami bore force time history curve can be divided into three stages, that is, the impinging stage, the fluctuation stage, and the quasi-stable stage. The horizontal and vertical tsunami bore force maxima (Fx+ and Fz+, respectively) occur in the impinging stage. The girder model is decomposed into panels, and the contribution of each panel of the multi-box girder to the tsunami bore force maxima is significantly different from that of the T-girder because of the difference in the geometric shape of the upstream panel. The contribution of the side panels of the multi-box girder to Fz+ is larger than that of the T-girder, because the horizontal projection area of the side panels of the multi-box girder is larger than that of the T-girder. And Fx+ generally decreases with clearance and increases with wave height. However, Fx+first increases and then remains stable as the wave height increases when the clearance is smaller (e.g. less than 0.04m). When h=0.64m and Z=0.02m, and the wave height increases from 0.12 m to 0.21 m, the increment of Fx+ is 221.76%, and Fz+ increases with wave height, but significantly decreases with clearance. When h=0.61m and a=0.15m, the clearance increases from 0.01m to 0.07m, and the reduction of Fz+ is 87.56%.

The water sloshing in aqueducts under seismic excitations is essentially a sloshing problem of water in liquid storage tanks. However, the present calculation method for the lateral natural frequency of water within U-shaped aqueducts is extremely complicated, inefficient, and only valid for the first lateral natural frequency calculation. Based on an existing calculation method derived from the Rayleigh quotient for the natural frequency of water in 2D (two-dimensional) containers with arbitrary shapes, a simplified calculation method for the lateral natural frequency of water within U-shaped aqueducts is proposed. The Taylor series, along with the β and Γ functions, is used to simplify the complex non-elementary integrals as algebraic expressions, making this calculation method convenient and efficient enough to calculate the first four lateral natural frequency of water in U-shaped aqueducts. Furthermore, a numerical model established by OpenFOAM-9 is employed to validate the accuracy of the proposed method. Validation shows the proposed calculation method is 9 times as precise as the method specified in Chinese code for the first lateral natural frequency calculation, and it can also calculate the second- to fourth-order lateral frequencies of the water in U-shaped aqueducts precisely. Results also show when water level inside the U-shaped aqueduct is beyond the semicircle part, aqueduct width significantly influences water natural frequency, whereas water depth has a minor impact.

Orthogonal time frequency space (OTFS) modulation offers reliable communication in time- and frequency-selective channels and is a promising technique in underwater acoustic (UWA) communication. This letter proposes a deep learning receiver for UWA OTFS communication that addresses the Doppler squint effect (DSE). Conventional UWA OTFS systems tend to ignore DSE, which leads to significant performance degradation. The proposed receiver stacked convolutional neural network (CNN) and ResNet as an receiver, which is abbreviated as S-CNN-ResNet. To provide additional effective features to the network, first, the pilot data is processed using a CNN to obtain effective channel features, and then channel features are stacked together with the received data. Finally, the stacked data is input into the improved ResNet to recover the transmitted symbols. Specifically, to deal with the complex DSE UWA channel, the scheme employs the idea of data augmentation technique, utilizing CNN to capture the effective channel features from pilot data and then stack these with the received data. This approach expands the training features of the network input data, enhancing the network's learning capability. Subsequently, the ResNet focuses on the effective information of the input data through the residual structure, thereby facilitating accurate symbol recovery. Simulation results demonstrate that the proposed method achieves a lower bit error rate (BER) compared to single network receivers, cascaded network receivers, and classic algorithm-based receivers. Additionally, it strikes an effective balance between complexity and performance.

In recent years, tuned liquid dampers (TLDs) have attracted significant research interest; however, overall progress has been limited due to insufficient understanding of the mechanisms governing sloshing-induced loads. In particular, it remains unclear whether the water in aqueducts—common water-diversion structures in many countries—can serve as an effective TLD. This study investigates the generation mechanisms of sloshing loads during the first-order transverse resonance of water in a U-shaped aqueduct using a two-dimensional (2D) numerical model. The results reveal that, at the equilibrium position, the free surface difference between the left and right walls, the horizontal force on the aqueduct, and the fluctuating component of the vertical force all reach their maxima, with energy predominantly stored as potential energy. At the maximum displacement position, the surface difference and horizontal force drop to zero, while the fluctuating vertical force attains its minimum and energy shifts primarily to kinetic form. At this stage, static pressure is governed solely by the vertical convective acceleration, whereas at equilibrium it is closely linked to both the free surface difference and vertical local acceleration of the water. This dynamic energy exchange generates vertical force oscillations even when the free surface appears nearly symmetric.

In recent years, tuned liquid dampers (TLDs) have emerged as a focal point of research due to their remarkable potential for structural vibration mitigation. Yet, progress in this field remains constrained by an incomplete understanding of the fundamental mechanisms governing sloshing-induced loads in liquid-filled containers. Aqueducts present a distinctive case, as the capacity of their contained water to function effectively as a TLD remains uncertain. To address this gap, the present study investigates the generation mechanisms of sloshing loads under non-resonant cases through a two-dimensional (2D) computational fluid dynamics (CFD) model developed in ANSYS Fluent. The incompressible Reynolds-Averaged Navier-Stokes (RANS) equations are solved, while the Volume of Fluid (VOF) method captures the evolution of the air-water interface. Turbulent flow behavior is modeled using the RNG k-s approach. The ensuing results reveal the dynamic characteristics of the horizontal force (Fh) and the fluctuating component of the vertical force (Fvf). Fh is predominantly governed by the inertia of the deep-water region and its phase varies coherently with the aqueduct's acceleration. With increasing excitation amplitude (A) and frequency (f ), the contribution of deep-water inertia to Fh intensifies markedly, accounting for 82.6-92.1% of the total horizontal load at an excitation amplitude of 0.15 m and frequencies of 1.0-1.6 Hz. The extreme values of F Fvf arise primarily from asymmetric static pressures induced by free-surface fluctuations, which are further amplified when wall gaps appear at large amplitudes (A >= 10 cm) and high frequencies (f >= 1.4 Hz). Unlike resonant cases dominated by free-surface resonance, non-resonant sloshing loads are principally driven by deep-water inertia and motion-induced surface asymmetry.

Research on characteristics of tsunami forces acting on conical islands and acting on cylindrical structures placed on top of a conical island, as well as the generation mechanisms of the tsunami forces, is limited. The total tsunami force acting on conical islands and its generation mechanisms are studied in cases with different initial water depths and relative wave heights. The results indicate that the peak values of the positive and negative tsunami forces, which increase with both initial water depths and relative wave heights, have almost the same magnitude. The initial submerged state of the conical island does not change the variation trend of tsunami force peaks with relative wave height. Then the tsunami forces on the structures located at different positions on the conical island are compared and analyzed, and the results show that when the conical island is not submerged at the initial state, the front cylinder has the largest peak value; when the conical island is submerged at the initial stage, the rear cylinder has the largest peak value.

The hydrodynamic force generated by the interaction between deep-water pier and water under earthquake exerts a significant influence on the dynamic response of the pier. At present, there is a lack of characterization methods to measure the influence degree of pier-water interaction on the dynamic response of the deep-water pier. Based on the structural dynamics theory and considering the coupling effect of pier-water interaction, the dynamic equilibrium equations for the deep-water pier under earthquake are established, and the solutions are deduced and validated by the finite element simulation results. Then the comprehensive analysis of influence mechanism is conducted. Further, the displacement influence coefficients and internal force influence coefficients are proposed to evaluate the influence degree of hydrodynamic force on dynamic response of deep-water piers. Validation shows that the proposed coefficients have high accuracy, indicating the important application of the proposed coefficients in seismic design of deep-water piers in real practice.

Energy-efficient computation is an inevitable trend for unmanned aerial vehicles (UAV)-enabled wireless powered mobile edge computing (MEC), while it has not been investigated when the hybrid passive and active transmissions (ATs) are considered for Internet of Things (IoT) nodes' task offloading. In this paper, we study the computation energy efficiency (EE) fairness among IoT nodes in a UAV-enabled wireless powered MEC network with hybrid passive and ATs, where the UAV serves as a dynamic energy source to support IoT nodes for backscatter communication (BackCom) and AT. Specifically, we formulate an optimization problem to maximize the computation EE of the worst IoT node by jointly optimizing the UAV's transmit power and trajectory, the IoT nodes' BackCom time and reflection coefficients, the IoT nodes' AT power and time, as well as the IoT nodes' local computing time and frequencies. The formulated problem is highly non-convex and difficult to be solved optimally. To address it, we first obtain the closed-form expressions for the UAV's transmit power and the IoT nodes' local computing time by means of the proof by contradiction to simplify the problem, and then propose a Dinkelbach-based iterative algorithm to obtain the solution of other optimization variables. Specifically, based on the Dinkelbach's method, the original fractional problem is transformed into the problem with the subtractive objective function. Then we further decouple the transformed problem into two subproblems based on the block-coordinated-decent (BCD) method and solve the transformed problem by the proposed BCD-based iterative algorithm, where the above two subproblems are solved by means of the existing convex optimization tools and the proposed successive convex approximation (SCA)-based iterative algorithm alternatively. Simulation results show that the proposed algorithms have a fast rate of convergence and that the proposed scheme outperforms other baseline schemes in terms of the computation EE fairness.

The deep-water bridge would suffer more serious damage under earthquake than that standing in air. The larger blocking ratio has remarkable effect on the added mass coefficient, which deserves a further and comprehensive study. The generation mechanism of block effect is analyzed by using the numerical simulation software ANSYS Fluent. Results show that the recirculation zone with focus decreases the pressure on the rear surface of the cylinder, results in the peak value of the in-line force does not occur synchronously with the peak value of acceleration. Results also indicate that the change of the position and intensity of the recirculation zone with focus, as well as the change of the water flow around the cylinder surface, are the generation mechanism of the block effect, the block effect has a 10% influence on the hydrodynamic force. The added mass coefficient changing rule with blocking ratio is then discussed elaborately, and the modification approach to the present added mass coefficient calculation method is suggested. Finally the physical experiments are conducted to validate the modification approach, results shows the modification approach is accurate and can be used both in the further study and in real practice.

The bandwidth and power resources in underwater acoustic sensor networks (UASNs) are severely limited. By adopting adaptive resource allocation technique, the network capacity and energy efficiency of UASNs can be improved. In this paper, we model the underwater acoustic (UWA) soft frequency reuse (SFR) network as a multi-agent system, and propose a multi-agent deep Q network based resource allocation (MADQN-RA) method. The system state is designed as outdated feedback channel state information (CSI) sequences, considering the time-varying and long propagation delay features of UWA channel. By establishing an effective joint reward expression, the intelligent agents can mapping the relationship of state-action and reward in time-varying UWA channel and make corresponding resource allocation decisions. Furthermore, to improve the learning efficiency, a dynamic state length method is proposed with the specific design of multi-stage experience buffer. The pre- training method is also combined for further improvement of system efficiency. Simulation results show that the system performance of the proposed methods is better than other learning-based methods and channel prediction-based methods, and is closer to the theoretical optimal value.

Considering the challenges posed by the significant propagation delays inherent in underwater cognitive acoustic sensor networks, this paper explores the application of multi-agent deep reinforcement learning for the design of multiple access protocols. We deal with the problem of sharing channels and time slots among multiple sensor nodes that adopt different time-slotted MAC protocols. The multiple intelligent nodes can independently learn the strategies for accessing available idle time slots through the proposed multi-agent deep reinforcement learning (DRL) based multiple access control (MDRL-MAC) protocol. Considering the long propagation delay associated with underwater acoustic channels, we reformulate proper state, action, and reward within the DRL framework to address the multiple access challenges and optimize network throughput. To mitigate the decision deviation stemming from partial observability, the gated recurrent unit (GRU) is integrated into DRL to enhance the deep neural network’s performance. Additionally, to ensure both the maximization of network throughput and the maintenance of fairness among multiple agents, an inspiration mechanism (IM) is proposed to inspire the lazy agent to take more actions to improve its contribution to achieve multi-agent fairness. The simulation results show that the proposed protocol facilitates the convergence of network throughput to optimal levels across various system configurations and environmental conditions.

Sliding window decoding (SWD) algorithm is an applicable and efficient way to decode spatially coupled low-density parity-check (SC-LDPC) codes. In the conventional SWD algorithm, the edges involved in the overlapping region between two adjacent decoding windows will maintain the updated log-likelihood-ratios (LLRs) in the current decoding window to the next decoding window. Error accumulation may occur under worse channel condition. To address up this problem, we propose an improved SWD algorithm by reserving more reliable LLRs to the next decoding window, denoted as IRSWD algorithm. Specifically, we first reserve the total LLRs for the variable nodes in the overlapping region of the current decoding window and compare them with the channel LLRs. Then choose more reliable LLRs as the initial LLR information and prior information for these variable nodes in the next decoding window. Simulation results show that the proposed IRSWD algorithm performs better than the conventional SWD algorithm for different SC-LDPC codes. Moreover, we also calculate the LLRs of the variable nodes in one decoding window under IRSWD algorithm and SWD algorithm respectively and the comparison results verify that the initial LLRs of the variable nodes in the overlapping region under the proposed IRSWD algorithm are more reliable than those under the conventional SWD algorithm.

This paper studies the perfermance of an ambient backscatter communication (AmBackCom) network with hybrid long and short packets, where long packets are used for the cellular transmission while multiple Internet-of-Things (IoT) nodes take turns to backscatter short packets via backscatter communication (BackCom). Considering the energy causality constraint of each IoT node and the joint decoding error between BackCom and the cellular transmission, we derive the closed-form expressions of the block error rate (BLER) for each IoT node’s BackCom, as well as the upper and lower bounds of the outage probability for the cellular transmission. Simulation results verify the correctness of the derived expressions and show the impacts of key parameters on the BLER of BackCom and the outage performance of the cellular transmission. Specifically, there exists an optimal power reflection coefficient that minimizes the average BLER of all the IoT nodes and the average BLER decreases with the short-packet blocklength until it reaches a certain value. Besides, the outage probability of the cellular transmission is well bounded by the derived upper and lower bounds, which become tighter when the long-packet data bits increase.

Based on a comprehensive analysis of the advantages and disadvantages of existing tsunami resistant fairings for bridge girders, two novel types of fairings are proposed: the upper arc fairings (UA- alpha) and the lower arc fairings (LA- alpha). In addition, the exceedance impulse of horizontal force is suggested as a new indicator for evaluating tsunami resistant capability of fairings. In numerical simulations, comparative analysis is conducted among the upper arc fairings (UA- alpha), lower arc fairings (LA- alpha), and existing L-shape fairings (L- W ) to evaluate their tsunami resistance capabilities. Results show that the UA- 120 and L-1.00H H are the best among the upper arc fairings (UA- alpha) and L-shape fairings (L- W ), respectively. The differences between tsunami resistance capability of the lower arc fairings (LA- alpha) are inapparent, and LA- 60 demonstrating relatively superior performance. Physical experiments further indicate that the UA-120 and L- 0.50H H fairings can averagely reduce the exceedance impulse of horizontal force 76.88% and 76.24% respectively. And the LA-60 fairing can averagely reduce the exceedance impulse of horizontal force 45.08%.

Spatially coupled low density parity check (SC-LDPC) are prominent candidates for future communication standards due to their "threshold saturation" properties. To evaluate the finite-length performance of SC-LDPC codes, a general and efficient finite-length analysis from the perspective of the base matrix is proposed. We analyze the evolution of the residual graphs resulting at each iteration during the decoding process based on the base matrix and then derive the expression for the error probability. To verify the effectiveness of the proposed finite-length analysis, we consider the SC-LDPC code ensembles constructed by parallelly connecting multiple chains (PC-MSC-LDPC). The analysis results show that the predicted error probabilities obtained by using the derived expression for the error probability match the simulated error probabilities. The proposed finite-length analysis provides a useful engineering tool for practical SC-LDPC code design and for analyzing the effects of the code parameters on the performances.

Herein, one partial repetition extension method is proposed to construct the rate-compatible spatially coupled low-density parity-check (RC-SCLDPC) codes. For each position of the base SCLDPC code, a certain proportion of the variable nodes are first selected randomly and then repeated a certain number of times. By adjusting the selection proportions and the repetition times, a family of RC-SCLDPC codes with arbitrary rates from 0 to the rate of the base SCLDPC code can be obtained and the rate-compatibility is realized. Threshold analysis results show that all member codes in the proposed RC-SCLDPC code family display capacity-approaching thresholds over the binary erasure channel and additive white Gaussian noise channel. Finite length simulation results also confirm their excellent thresholds. Moreover, the decoding complexity can be significantly decreased using this partial repetition extension method.