Synchronization in networks with random interactions: theory and applications

Synchronization is an emergent property in networks of interacting dynamical elements. Here we review some recent results on synchronization in randomly coupled networks. Asymptotical behavior of random matrices is summarized and its impact on the synchronization of network dynamics is presented. Robert May's results on the stability of equilibrium points in linear dynamics are first extended to systems with time delayed coupling and then nonlinear systems where the synchronized dynamics can be periodic or chaotic. Finally, applications of our results to neuroscience, in particular, networks of Hodgkin-Huxley neurons, are included.     

定向传输水声通信网络介质访问控制协议

针对水声通信网络吞吐量低、通信隐蔽性差、隐藏终端等问题,提出一种定向传输水声通信网络介质访问控制(Medium Access Control,MAC)协议。协议中各节点采用定向模式传输数据,不需要邻节点位置等先验信息。通过发送节点顺序连续发送传输请求(Request To Send,RTS)信号、维护邻节点相对位置信息表及定向虚拟载波监测等技术实现网络高效无碰撞运行。本协议可有效解决水声通信网络中存在的由非对称增益导致的隐藏终端问题,增加网络覆盖范围,提高通信隐蔽性。仿真结果表明,所提MAC协议能够显著提高水声通信网络吞吐量性能。     

Optical networks, last mile access and applications

Free Space Optical (FSO) links can be used to setup FSO communication networks or to supplement radio and optical fiber networks. Hence, it is the broadband wireless solution for closing the “last mile” connectivity gap throughout metropolitan networks. Optical wireless fits well into dense urban areas and is ideally suited for urban applications. This paper gives an overview of free-space laser communications. Different network architectures will be described and investigated regarding reliability. The usage of “Optical Repeaters”, Point-to-Point and Point-to-Multipoint solutions will be explained for setting up different network architectures. After having explained the different networking topologies and technologies, FSO applications will be discussed in section 2, including terrestrial applications for short and long ranges, and space applications. Terrestrial applications for short ranges cover the links between buildings on campus or different buildings of a company, which can be established with low-cost technology. For using FSO for long-range applications, more sophisticated systems have to be used. Hence, different techniques regarding emitted optical power, beam divergence, number of beams and tracking will be examined. Space applications have to be divided into FSO links through the troposphere, for example up- and downlinks between the Earth and satellites, and FSO links above the troposphere (e.g., optical inter-satellite links). The difference is that links through the troposphere are mainly influenced by weather conditions similar but not equal to terrestrial FSO links. Satellite orbits are above the atmosphere and therefore, optical inter-satellite links are not influenced by weather conditions. In section 3 the use of optical wireless for the last mile will be investigated and described in more detail. Therefore important design criteria for connecting the user to the “backbone” by FSO techniques will be covered, e.g., line of sight, network topology, reliability and availability. The advantages and disadvantages of different FSO technologies, as well as the backbone technology are discussed in this respect. Furthermore, the last mile access using FSO will be investigated for different environment areas (e.g., urban, rural, mountain) and climate zones. The availability of the FSO link is mainly determined by the local atmospheric conditions and distance and will be examined for the last mile. Results of various studies will complete these investigations. Finally, an example for realizing a FSO network for the last mile will be shown. In this example FSO transmitters with light emitting diodes (LED) instead of laser diodes will be described. By using LEDs, problems with laser- and eye safety are minimized. Some multimedia applications (like video-conferences, live TV-transmissions, etc.) will illustrate the range of applications for FSO last mile networks.     

复射线法及其在水声技术中的应用

本文介绍复源点射线技术原理,概述各种复射线分析方法,并举例说明这种方法在水声技术中一些可能的应用,包括指向性声场的复射线分析,声辐射器阵列方向性的复射线模拟,近区和远区声场的复射线变换,以及声呐目标强度的复射线理论预估等。     

Compressed sensing with linear-in-wavenumber sampling in spectral-domain optical coherence tomography

We propose a novel method called compressed sensing with linear-in-wavenumber sampling (k-linear CS) to retrieve an image for spectral-domain optical coherence tomography (SD-OCT). An array of points that is evenly spaced in wavenumber domain is sampled from an original interferogram by a preset k-linear mask. Then the compressed sensing based on l1 norm minimization is applied on these points to reconstruct an A-scan data. To get an OCT image, this method uses less than 20% of the total data as required in the typical process and gets rid of the spectral calibration with numerical interpolation in traditional CS-OCT. Therefore k-linear CS is favorable for high speed imaging. It is demonstrated that the k-linear CS has the same axial resolution performance with ～30 dB higher signal-to-noise ratio (SNR) as compared with the numerical interpolation. Imaging of bio-tissue by SD-OCT with k-linear CS is also demonstrated.     

Growing random networks with fitness

Three models of growing random networks with fitness-dependent growth rates are analysed using the rate equations for the distribution of their connectivities. In the first model (A), a network is built by connecting incoming nodes to nodes of connectivity k and random additive fitness η, with rate (k−1)+η. For η>0 we find the connectivity distribution is power law with exponent γ=〈η〉+2. In the second model (B), the network is built by connecting nodes to nodes of connectivity k, random additive fitness η and random multiplicative fitness ζ with rate ζ(k−1)+η. This model also has a power law connectivity distribution, but with an exponent which depends on the multiplicative fitness at each node. In the third model (C), a directed graph is considered and is built by the addition of nodes and the creation of links. A node with fitness (α,β), i incoming links and j outgoing links gains a new incoming link with rate α(i+1), and a new outgoing link with rate β(j+1). The distributions of the number of incoming and outgoing links both scale as power laws, with inverse logarithmic corrections.     

Compressed sensing and the reconstruction of ultrafast 2D NMR data: Principles and biomolecular applications

A topic of active investigation in 2D NMR relates to the minimum number of scans required for acquiring this kind of spectra, particularly when these are dictated by sampling rather than by sensitivity considerations. Reductions in this minimum number of scans have been achieved by departing from the regular sampling used to monitor the indirect domain, and relying instead on non-uniform sampling and iterative reconstruction algorithms. Alternatively, so-called "ultrafast" methods can compress the minimum number of scans involved in 2D NMR all the way to a minimum number of one, by spatially encoding the indirect domain information and subsequently recovering it via oscillating field gradients. Given ultrafast NMR's simultaneous recording of the indirect- and direct-domain data, this experiment couples the spectral constraints of these orthogonal domains - often calling for the use of strong acquisition gradients and large filter widths to fulfill the desired bandwidth and resolution demands along all spectral dimensions. This study discusses a way to alleviate these demands, and thereby enhance the method's performance and applicability, by combining spatial encoding with iterative reconstruction approaches. Examples of these new principles are given based on the compressed-sensed reconstruction of biomolecular 2D HSQC ultrafast NMR data, an approach that we show enables a decrease of the gradient strengths demanded in this type of experiments by up to 80%.     

VCSEL-based gigabit IR-UWB link for converged communication and sensing applications in optical metro-access networks

We report on experimental demonstration of an impulse radio ultrawideband (IR-UWB) based converged communication and sensing system. A 1550-nm VCSEL-generated IR-UWB signal is used for 2-Gbps wireless data distribution over 800-m and 50-km single mode fiber links which present short-range in-building and long-reach access network applications. The IR-UWB signal is also used to simultaneously measure the rotational speed of a blade spinning between 18 and 30 Hz. To the best of our knowledge, this is the very first demonstration of a simultaneous gigabit UWB telecommunication and wireless UWB sensing application, paving the way forward for the development and deployment of converged UWB VCSEL-based technologies in access and in-building networks of the future.     

Spatial networks with wireless applications

Many networks have nodes located in physical space, with links more common between closely spaced pairs of nodes. For example, the nodes could be wireless devices and links communication channels in a wireless mesh network. We describe recent work involving such networks, considering effects due to the geometry (convex, non-convex, and fractal), node distribution, distance-dependent link probability, mobility, directivity, and interference.     

Scale-free networks via attaching to random neighbors

Preferential attachment is considered one of the key factors in the formation of scale-free networks. However, complete random attachment without a preferential mechanism can also generate scale-free networks in nature, such as protein interaction networks in cells. This article presents a new scale-free network model that applies the following general mechanisms: (i) networks expand continuously by the addition of new vertices, and (ii) new vertices attach to random neighbors of random vertices that are already well connected. The proposed model does not require global-based preferential strategies and utilizes only the random attachment method. Theoretical analysis and numerical simulation results denote that the proposed model has steady scale-free network characteristics, and random attachment without a preferential mechanism may generate scale-free networks.     

Continuous random networks from graphs to glasses

The direct connection between the structure of amorphous materials, modelled geometrically by graphs, and their physical properties is demonstrated. The structural and physical differences between glasses and crystals are emphasized.     

Complex ray method and its applications to underwater acoustics

In this paper,the principle of complex source point and various analyt-ical methods of complex ray are summarized.Some possible applications ofcomplex ray method to underwater acoustics are proposed,including complexray analysis of directional acoustic fields,complex ray simulation of directivityof acoustic radiator arrays,complex ray transformation of acoustic fields fromnear zone to far zone,as wel1 as theoretical prediction of sonar target strengthby complex ray method.     

Clique percolation in random networks

The notion of k-clique percolation in random graphs is introduced, where k is the size of the complete subgraphs whose large scale organizations are analytically and numerically investigated. For the Erdos-Rényi graph of N vertices we obtain that the percolation transition of k-cliques takes place when the probability of two vertices being connected by an edge reaches the threshold p(c) (k) = [(k - 1)N](-1/(k - 1)). At the transition point the scaling of the giant component with N is highly nontrivial and depends on k. We discuss why clique percolation is a novel and efficient approach to the identification of overlapping communities in large real networks.     

A compressed sensing receiver for UWB impulse radio in bursty applications like wireless sensor networks

We propose a novel receiver for Ultra-Wide-band Impulse-Radio communication in Wireless Sensor Networks, which are characterized by bursty traffic and severe power constraints. The receiver is based on the principle of Compressed Sensing, and exploits the sparsity of the transmitted signal to achieve reliable demodulation from a relatively small number of projections. The projections are implemented in an analog front-end as correlations with tractable test-functions, and a joint decoding of the time of arrival and the data bits is done by a DSP back-end using an efficient quadratic program. The proposed receiver differs from extant schemes in the following respects: (i) It needs neither a high-rate analog-to-digital converter nor wide-band analog delay lines, and can operate in a significantly under-sampled regime. (ii) It is robust to large timing uncertainty and hence the transmitter need not waster power on explicit training headers for timing synchronization. (iii) It can operate in a regime of heavy inter-symbol interference (ISI), and therefore allows a very high baud rate (close to the Nyquist rate). (iv) It has a built-in capability to blindly acquire and track the channel response irrespective of line-of-sight/non-line-of-sight conditions. We demonstrate that the receiver’s performance remains close to the maximum likelihood receiver under every scenario of under-sampling, timing uncertainty, ISI, and channel delay spread.     

分段匹配追踪式Karhunen-Loeve非相干字典语音压缩感知

压缩感知(Compressed Sensing,CS)理论突破了经典采样定理的理论边界,为信号压缩提供了另一种途径。基于CS理论框架,做了两方面工作:为提高语音字典对信号的匹配性,设计了一种基于K-L展开的非相干语音字典;针对现有匹配追踪(MP,OMP)算法的不足,提出分段匹配追踪(Segment MP,SegMP)算法。首先对语音自相关函数进行建模并估计模型参数,构造语音自适应非相干字典,然后采用SegMP对语音稀疏向量分段观测,获得多个低维矢量,最后结合模型参数重建字典并重构信号,实现了语音压缩感知。语音测试结果表明:相比现有方案,本文方案对信号的稀疏表示更为精准,具有更好的重构质量,且降低了计算复杂度。      

Cross over of recurrence networks to random graphs and random geometric graphs

Recurrence networks are complex networks constructed from the time series of chaotic dynamical systems where the connection between two nodes is limited by the recurrence threshold. This condition makes the topology of every recurrence network unique with the degree distribution determined by the probability density variations of the representative attractor from which it is constructed. Here we numerically investigate the properties of recurrence networks from standard low-dimensional chaotic attractors using some basic network measures and show how the recurrence networks are different from random and scale-free networks. In particular, we show that all recurrence networks can cross over to random geometric graphs by adding sufficient amount of noise to the time series and into the classical random graphs by increasing the range of interaction to the system size. We also highlight the effectiveness of a combined plot of characteristic path length and clustering coefficient in capturing the small changes in the network characteristics.