Viscoacoustic prestack reverse time migration based onthe optimal time-space domain high-order finite-difference method

Prestack reverse time migration （RTM） is an accurate imaging method ofsubsurface media. The viscoacoustic prestack RTM is of practical significance because itconsiders the viscosity of the subsurface media. One of the steps of RTM is solving thewave equation and extrapolating the wave field forward and backward; therefore, solvingaccurately and efficiently the wave equation affects the imaging results and the efficiencyof RTM. In this study, we use the optimal time-space domain dispersion high-order finite-difference （FD） method to solve the viscoacoustic wave equation. Dispersion analysis andnumerical simulations show that the optimal time-space domain FD method is more accurateand suppresses the numerical dispersion. We use hybrid absorbing boundary conditions tohandle the boundary reflection. We also use source-normalized cross-correlation imagingconditions for migration and apply Laplace filtering to remove the low-frequency noise.Numerical modeling suggests that the viscoacoustic wave equation RTM has higher imagingresolution than the acoustic wave equation RTM when the viscosity of the subsurface isconsidered. In addition, for the wave field extrapolation, we use the adaptive variable-lengthFD operator to calculate the spatial derivatives and improve the computational efficiencywithout compromising the accuracy of the numerical solution.

Benchmarking of two three-dimensional numerical models in time/space domain to predict railway-induced ground vibrations

In the last 30 years,the scientific community has developed and proposed different models and numerical approaches for the study of vibrations induced by railway traffic.Most of them are formulated in the frequency/wave number domain and with a 2.5D approach.Three-dimensional numerical models formulated in the time/space domain are less frequently used,mainly due to their high computational cost.Notwithstanding,these models present very attractive characteristics,such as the possibility of considering nonlinear behaviors or the modelling of excess pore pressure and non-homogeneous and non-periodic geometries in the longitudinal direction of the track.In this study,two 3D numerical approaches formulated in the time/space domain are compared and experimentally validated.The first one consists of a finite element approach and the second one of a finite difference approach.The experimental validation in an actual case situated in Carregado(Portugal)shows an acceptable fitting between the numerical results and the actual measurements for both models.However,there are some differences among them.This study therefore includes some recommendations for their use in practical soil dynamics and geotechnical engineering.

Direct data domain approach to space-time adaptive processing

In non-homogeneous environment, traditional space-time adaptive processing doesn＇t effectively suppress interference and detect target, because the secondary data don＇ t exactly reflect the statistical characteristic of the range cell under test. A ravel methodology utilizing the direct data domain approach to space-time adaptive processing （ STAP ） in airbome radar non-homogeneous environments is presented. The deterministic least squares adaptive signal processing technique operates on a ＂snapshot-by-snapshot＂ basis to dethrone the adaptive adaptive weights for nulling interferences and estimating signal of interest （SOI）. Furthermore, this approach eliminates the requirement for estimating the covariance through the data of neighboring range cell, which eliminates calculating the inverse of covariance, and can be implemented to operate in real-time. Simulation results illustrate the efficiency of interference suppression in non-homogeneous environment.

A Krylov Space-Based Finite Element Time Domain Method for Broadband Frequency Domain Solutions

A Krylov space based time domain method for wave propagation problems is introduced. The proposed method uses the Arnoldi algorithm to obtain broad-band frequency domain solutions. This method is especially advantageous in cases where slow convergence is observed when using traditional time domain methods. The efficiency of the method is examined in several test cases to show its fast convergence in such problems.

LARGE TIME ASYMPTOTIC BEHAVIOR OF THE COMPRESSIBLE NAVIER-STOKES EQUATIONS IN PARTIAL SPACE-PERIODIC DOMAINS

In this article, we study the large time behavior of the 3-D isentropic compressible Navier-Stokes equation in the partial space-periodic domains, and simultaneously show that the related profile systems can be described by like Navier-Stokes equations with suitable ＂pressure＂ functions in lower dimensions. Our proofs are based on the energy methods together with some delicate analysis on the corresponding linearized problems.

Earthquake-Affected Time-Space Domain, Recurrence Interval and Effective Preparation Time of Earthquakes

The study shows that earthquake-affected time-space domain (ETSD), i.e. a time-space range in which strong earthquakes are unable to occur owing to the influence of a prior earthquake occurring, shows a hyperbolic margin curve in the t(time)-r(distance) coordinate plane, which has a maximum affected radius r 0 at t=0 and a maximum influence time t 0 (i.e. the in-situ recurrence interval of earthquakes) at r=0. Based on the time-distance distributions of posterior earthquakes relative to prior ones in the regions of North China, Northwest China, Qinghai-Xizang (Tibet) plateau and Southwest China, the optimized and 90%-confidence margin curves are estimated using optimization and statistical analysis methods. This indicates that the concept and method of ETSD with 3-dimension (time-distance-magnitudes) instead of those of “recurrence interval" with 1-dimension (time) or 2-dimension (time-magnitude) provides a new approach to understanding the fluctuation of seismic activities, estimating the effective earthquake-preparation time of potential hypocenters, and therefore improving the medium- and long-term prediction of strong earthquakes.

Modeling of Surface Waves in a Fluid Saturated Poro-Elastic Medium under Initial Stress Using Time-Space Domain Higher Order Finite Difference Method

In this present context, mathematical modeling of the propagation of surface waves in a fluid saturated poro-elastic medium under the influence of initial stress has been considered using time dependent higher order finite difference method (FDM). We have proved that the accuracy of this finite-difference scheme is 2M when we use 2nd order time domain finite-difference and 2M-th order space domain finite-difference. It also has been shown that the dispersion curves of Love waves are less dispersed for higher order FDM than of lower order FDM. The effect of initial stress, porosity and anisotropy of the layer in the propagation of Love waves has been studied here. The numerical results have been shown graphically. As a particular case, the phase velocity in a non porous elastic solid layer derived in this paper is in perfect agreement with that of Liu et al. (2009).

Design and Analysis of Spatial Modulation Based Orthogonal Time Frequency Space System

In this paper,we design a spatial modulation based orthogonal time frequency space(SMOTFS)system to achieve improved transmission reliability and meet the high transmission rate and highspeed demands of future mobile communications,which fully utilizes the characteristics of spatial modulation(SM)and orthogonal time frequency space(OTFS)transmission.The detailed system design and signal processing of the SM-OTFS system have been presented.The closed-form expressions of the average symbol error rate(ASER)and average bit error rate(ABER)of the SM-OTFS system have been derived over the delay-Doppler channel with the help of the union bounding technique and moment-generating function(MGF).Meanwhile,the system complexity has been evaluated.Numerical results verify the correctness of the theoretical ASER and ABER analysis of the SM-OTFS system in the high signal-to-noise ratio(SNR)regions and also show that the SM-OTFS system outperforms the traditional SM based orthogonal frequency division multiplexing(SM-OFDM)system with limited complexity increase under mobile conditions,especially in high mobility scenarios.

Gravitational Space-Time Quantization for Charged Wormholes and the Diophantine Uncertainty Relation

This research work proceeds from the assumption, which was still considered by Einstein, that the quantization of gravity does not require additional external procedures: quantum phenomena can be a consequence of the properties of the universal gravitational interaction, which maps any physical field upon the space-time geometry. Therefore, an attempt is made in this research work to reduce the quantization of physical fields in GRT to the space-time quantization. Three reasons for quantum phenomena are considered: Partition of space-time into a set of unconnected Novikov’s R- and T-domains impenetrable for light paths;the set is generated by the invariance of Einstein’s equations with respect to dual mappings;The existence of electric charge quanta of wormholes, which geometrically describe elementary particles in GRT. This gives rise to a discrete spectrum of their physical and geometric parameters governed by Diophantine equations. It is shown that the fundamental constants (electric charge, rest masses of an electron and a proton) are interconnected arithmetically;The existence of the so-called Diophantine catastrophe, when fluctuations in the values of physical constants tending to zero lead to fluctuations in the number of electric charges and the number of nucleons at the wormhole throats, which tend to infinity, so that the product of the increments of these numbers by the increment of physical constants forms a relation equivalent to the uncertainty relation in quantum mechanics. This suggests that space-time cannot but fluctuate, and, moreover, its fluctuations are bounded from below, so that all processes become chaotic, and the observables become averaged over this chaos.

多机器人协同路径的时域协作-势场A*规划

为了减小多机器人协同工作路径长度并提高路径平滑度,提出了基于时域协作-势场A*算法的多机器人协同规划方法。建立了机器人工作环境的栅格模型,分析了传统A*算法辅助信息不充分的问题。为了充分利用环境中的障碍物和目标点位置等先验信息,基于势场法将障碍物和目标点信息引入到A*算法辅助信息中,提出了基于势场A*算法的单机器人路径规划方法。使用三维时空图代替二维平面图,可以有效判断出多机器人碰撞问题,并根据碰撞类型制定了避撞策略。经验证,势场A*算法规划的路径长度和平滑度均优于传统A*算法、文献[11]蚁群算法;在多机器人协同路径规划中,在规划出各机器人最优路径基础上,可以有效解决机器人之间的碰撞问题。实验结果验证了时域协作-势场A*算法在多机器人协同路径规划中的有效性和优越性。

6G新型时延多普勒通信范式:OTFS的技术优势、设计挑战、应用与前景

在未来的通信网络中,被广泛期待的第6代移动通信系统(The Sixth Generation of Mobile Communications System,6G)技术将面临诸多挑战,其中包括在高速移动场景下的超高可靠通信问题。正交时频空间(Orthogonal Time Frequency Space,OTFS)调制技术克服了传统通信系统在高速移动环境下多径和多普勒效应的影响,为实现6G超高可靠通信提供了新的可能性。该文首先介绍了OTFS的基本原理、数学模型、干扰与优势分析。然后,归纳分析了OTFS技术在同步、信道估计、信号检测技术上的研究现状。接着,从车联网、无人机、卫星通信、海洋通信4个典型应用场景分析了OTFS的应用趋势。最后,从降低多维匹配滤波器、相位解调和信道估计、硬件实现的复杂度和提高对时频资源的高度利用4个角度探讨了未来研究OTFS需要克服的困难和挑战。

汽车组合仪表有意电磁干扰空时域联合分离系统设计

有意电磁波与汽车仪器电磁传输波产生的混合干扰,降低了汽车组合仪表的稳定运行能力,造成仪表设备失灵;为避免上述情况的发生,设计汽车组合仪表有意电磁干扰空时域联合分离系统;在步进电机驱动电路中,设置汽车组合仪表控制器、分离电源与PCB电路板、有意电磁干扰数字传感器;分配有意电磁波空时域源地址,通过计算干扰信号平坦度的方式,确定脉冲参数的取值范围,实现汽车组合仪表有意电磁的空时域干扰效应分析;估算有意电磁波干扰信号的数目,按照信号分解的处理原则,求解联合分离修正函数,实现电磁干扰的联合分离;在设置频率范围在100 MHz~1 GHz之间、幅度范围为0.5~5 V、脉冲宽度范围为10~100 ns区间后,通过实验结果表明,设计系统应用后可以在混合信号中,按照空域、时域标准的不同分离有意电磁波,且分离后的信号波长、信号频率依然在1.0μm、300 Hz左右,不会因混合干扰降低汽车组合仪表的稳定运行能力。

不同域下基于UNet的三维地震数据重建

由于自然因素的限制以及经济条件的约束,采集到的地震数据通常存在地震道的缺失,需对缺失数据进行插值。然而,目前基于深度学习的方法大多在某一固定的域下完成地震数据的插值。为了对比不同域下地震数据重建结果的差异,本文基于UNet网络,对比了五种不同域下的地震数据插值方法,分别为:t-x(time-space)域,时间切片,f-x(frequency-space)域,频率域按炮切片以及三维方法。在仿真数据集和真实数据集上进行了实验,通过三维重建结果,二维切片,不同缺失率的地震数据重建信噪比以及五种方法的耗时对比,全面展示了几种方法在地震数据重建问题上的差异。实验结果显示,五种方法在一定程度上均可重建缺失数据,其中三维方法可以取得最高的重建精度,和基于f-x域的方法相比,重建信噪比提高了近2 dB;但三维方法以三维数据块作为输入训练网络,所以最为耗时,网络一轮的训练时间比其它方法多70~170 min。此外,对不同缺失率的地震数据重建结果显示,三维方法和基于f-x域的方法在重建高缺失率的地震数据时更有优势。

新型连续B/J类功率放大器设计

为满足无线通信技术对高性能功率放大器的需求,提出一种新型宽带高效率B/J类功率放大器的设计方法。区别于传统连续B/J模式,设计中引入变量因子重塑其时域电压波形表达式,推导出扩展的阻抗设计空间。扩展的阻抗设计空间增加了阻抗匹配的灵活度,拓展了带宽,同时仿真表明可获得与传统连续B/J类功率放大器相同的功率和效率。基于此新型阻抗匹配技术,设计了一款工作频段为1.5~2.5 GHz的GaN HEMT宽带功率放大器。测试结果表明,在该频率范围内,输出功率大于40.6 dBm,漏极效率为72.4%~79.1%,增益为10.6~11.9 dB表明,所提出方法具有有效性.

基于尺度分析的CMA-GFS全球能量评估

全球模式能量循环和能量转换规律可准确反映模式动力和物理过程相互作用的物理机制,是诊断大气环流特征的重要方法。基于混合时空域能量循环框架,采用尺度分析方法,利用2022年中国气象局全球数值预报系统(CMA Global Forecast System,CMA-GFS)全球预报产品及欧洲中期天气预报中心第5代再分析资料(ECMWF reanalysis version 5,ERA5),考察CMA-GFS不同尺度下的能量蓄能及转换特征,以此诊断模式的误差来源。结果表明:CMA-GFS可有效预报大气能量循环基本特征,但其对斜压性的高估导致平均环流有效位能偏强,且具有随预报时效逐渐增长的趋势。定常和瞬变涡动能量分别受行星尺度和天气及以下尺度分量主导。涡动有效位能误差由模式斜压性决定,其中CMA-GFS的定常涡动有效位能偏高而瞬变涡动有效位能偏低。定常和瞬变涡动动能均存在系统性低估,负误差主要集中在副热带急流和极夜急流中心附近,偏强的正压输送使更多能量向平均环流转换,涡动能量偏弱。CMA-GFS的4种涡动能量在冬季预报偏低,而在夏季偏高或略偏低,严重削弱了季节变化影响。

太赫兹MIMO系统中基于SRCGAN的空时频信道估计方案

为了能有效利用THz-MIMO系统的多维信道特性,提出一种基于SRCGAN的THz-MIMO系统信道估计方案。在该方案中,由预估计模块获得的初始空时域信道响应矩阵被视作一张二维的低分辨率图像,利用SRCGAN网络提取太赫兹信道的空时特性进行空时域信道补全获得完整的信道信息,然后相邻子载波之间的频率相关性作为SRGAN提供的条件信息提升信道估计精度。为了增强SRCGAN网络对时变信道预测的鲁棒性,在线上估计阶段,基于最大均方误差准则采用梯度下降算法对输入的预估计信道信息矩阵进行迭代更新。仿真结果证明了基于SRCGAN的空时频信道估计方案性能的优越性,以及利用信道“空时频”的相关性提升估计精度的有效性。

Direction-of-arrival estimation based on direct data domain (D3) method

A direction-of-arrival （DOA） estimation algorithm based on direct data domain （D3） approach is presented. This method can accuracy estimate DOA using one snapshot modified data, called the temporal and spatial two-dimensional vector reconstruction （TSR） method. The key idea is to apply the D3 approach which can extract the signal of given frequency but null out other frequency signals in temporal domain. Then the spatial vector reconstruction processing is used to estimate the angle of the spatial coherent signal source based on extract signal data. Compared with the common temporal and spatial processing approach, the TSR method has a lower computational load, higher real-time performance, robustness and angular accuracy of DOA. The proposed algorithm can be directly applied to the phased array radar of coherent pulses. Simulation results demonstrate the performance of the proposed technique.

一种改进的稀疏恢复直接数据域STAP方法

稀疏恢复空时自适应处理(Sparse Recovery Space-Time Adaptive Processing, SR-STAP)方法可以利用少量训练快拍数据估计出较为准确的杂波协方差矩阵(Clutter Covariance Matrix, CCM)。然而,复杂的杂波环境对运动目标的检测不利。为了改善检测性能,本文提出了一种改进的稀疏恢复直接数据域(Direct Data Domain, D3)STAP方法。首先构造待检测快拍数据对应的全局空时导向字典,对雷达检测回波数据进行稀疏恢复得到稀疏向量,然后根据先验知识从稀疏向量中挑选出杂波和目标元素,实现二者的分离,最后对目标和杂波进行功率谱估计和滤波处理。实验表明,本文所提方法可以在只有一个检测样本的条件下分离目标和杂波,避免杂波分布的非均匀性问题,提高运动目标的检测性能。

移动机器人跨域跃质关键技术综述

人类探索陆域、天域、空域、海域等空间的脚步从未停止,该过程蕴含了多域-多介质环境的复杂科学问题。移动机器人是实现跨域跃质环境下运动能力的典型代表,未来将走向更广的陆面、更高的蓝天、更远的深空、更深的海底,对移动机器人能够在多域-多介质环境中始终保持高性能提出了挑战。介绍了移动机器人在能量域-时空域-信息域的科学融合,及其在探索新域新质道路上所发挥的越来越重要的作用。归纳了移动机器人在支撑与牵引技术、速度与稳定技术、交互与协同技术等方面的基础理论研究应用及国内外发展现状。对移动机器人学科重点解决的多域-多介质机器人既有机动性和作业智能性问题提出建议,以期能够完成人类所不可及的任务,真正实现机器人装备体系在多域-多介质环境下的广泛应用。

基于“导航战”的北斗卫星定位抗干扰策略

为了进一步加强北斗卫星定位系统的抗干扰应用,立足“导航战”理念,提出电磁干扰环境下北斗卫星定位抗干扰策略:在总结北斗卫星信号结构和特性的基础上,查找北斗接收机面临人为干扰的薄弱环节,并重点对压制式、欺骗式和组合式干扰的机理、特点等进行分析;结合“导航战”3种属性进行逆向思维,从可用信号增强、干扰信号抵消、其他定位辅助等3个角度设计抗干扰总体策略;最后针对接收机抗干扰,提出一种“圆形阵列天线/空时域滤波”的压制式干扰对抗策略,以及惯导辅助定位、基于宽窄巷组合的载波相位差分等2种欺骗式干扰对抗策略。研究结果可为“导航战”理念实战转化和接收机抗干扰相关研究提供参考。