Measurement accuracy of chromatic dispersion by the modulation phase technique

A simple measuring method for the chromatic dispersion measurement is described. This technique uses group delay measurement by modulation phase of laser diodes with different wavelengths. An experiment showed that this technique is capable of measuring the chromatic dispersion of a 42.3-km single-mode fiber. The accuracy of measurement is discussed in view of fiber length, modulation frequency, and the number of employed laser diodes. The accuracy is improved when a higher modulation frequency is used and a longer fiber is measured since the detected phase fluctuation depends little on the modulation frequency and very little on the fiber length. The estimated measurement error of chromatic dispersion was 0.1 ps/km/nm for a four-laser diode system.     

Maximizing the Mean Number of Communicating Vertex Pairs in Series-Parallel Networks

A communication network can be modelled as a probabilistic graph where each of b edges represents a communication line and each of n vertices represents a communication processor. Each edge e (vertex v) functions with probability Pe (pv). If edges fail independently with uniform probability p and vertices do not fail, the probability that the network is connected is the probabilistic connectedness and is a standard measure of network reliability. The most reliable maximal series-parallel networks by this measure are those with exactly two vertices of degree two. However, as p becomes small, or n becomes large, the probability that even the most reliable series-parallel network is connected falls very quickly. Therefore, we wish to optimize a network with respect to another reliability measure, mean number of communicating vertex pairs. Experimental results suggest that this measure varies with p, with the diameter of the network, and with the number of minimum edge cutsets. We show that for large p, the most reliable series-parallel network must have the fewest minimum edge cutsets and for small p, the most reliable network must have maximum pairs of adjacent edges. We present a construction which incrementally inproves the communicating vertex pair mean for many networks and demonstrates that a fan maximizes this measure over maximal series parallel networks with exactly two edge cutsets of size two.     

基于椭圆曲线数字签名算法的序列号软件保护方案

结合身份的认证,提出了一种基于椭圆曲线数字签名算法的序列号保护方案,该方案包括序列号的生成和验证两部分。软件代理商利用序列号生成方案生成序列号,并利用加壳机制将序列号验证部分嵌入到软件中,用户输入的序列号经过验证后才能安装该软件。由于序列号的安全性寓于私钥而不是算法本身的安全性,只要私钥不被泄漏,破解者无法根据序列号和公钥生成新的序列号。因此,该方案具有很好的安全性和有效性。     

基于相位-强度调制转换的光纤色散精确测量方法

在分析光纤色散导致的相位-强度调制转换的基础上,提出了一种利用电光相位调制的光纤色散扫频测量方法。方法的原理为光纤色散使相位调制信号获得附加相移并产生周期性衰落,从衰落曲线的特征性凹陷频率确定出光纤色散。实际运用中,由于凹陷频率附近的信号弱,因此噪声大且不稳定。为了解决这一问题,通过衰落曲线的多项式拟合,进一步提高凹陷频率和光纤色散的测量精度。实验中,对长光纤或者短光纤分别测试以验证本文方法对于不同色散的适应性。实验结果表明,本文方法的相对误差小于0.22%。使用矢量网络分析仪(VNA)和相位调制器进行测试,可工作于不同光波长,适用于测量不同种类的光纤的色散;并且可以利用简单的实验系统,实现光纤色散的大小和符号的测量。     

Synthesis and Number of all Series-Parallel Structures with n Components

A synthesis procedure and?as a by-product?the number of different redundant structures for series-parallel systems with n components is derived. The reliability graphs of these structures, especially their fault trees, are branched like true rooted trees in the sense of graph theory. The results might be useful for some reliability optimization procedures and other applications.     

Topology Management in Directional Antenna-Equipped Ad Hoc Networks

With fully directional communications, nodes must track the positions of their neighbors so that communication with these neighbors is feasible when needed. Tracking process introduces an overhead, which increases with the number of discovered neighbors. The overhead can be reduced if nodes maintain only a subset of their neighbors; however, this may increase the length of paths between node pairs in the network. In this work, we study the tradeoffs between node degree and path stretch. We first design a topology control algorithm to optimize this tradeoff. Assuming that nodes communicate with their directional neighbors using circular directional transmissions, we model the original graph as a unit disk graph (UDG). Given a UDG G, our algorithm finds a sparse subgraph G' with a maximum degree of 6, and connecting each node pair u,v by a path of length hops_{G'}(u,v)=O(hops_G(u,v)+logDelta), where Delta is the maximum degree in G, hops_{G}(u,v) denotes length of the shortest path between u, v in G. We show that this result is near-optimal. Based on the insights gained from this design, we next construct a simpler, more practical scheme that integrates fully-directional neighbor discovery and maintenance with topology control strategy. We simulate both algorithms and compare their performances.     

整数剩余类环上的Walsh变换

本文提出了整数剩余类环上的Walsh变换,它们与一般的Walsh变换相比用于CDMA通信中有着更加优越的 性能。它们具有数据压缩的功能,提高了信息传输的可靠性。整数剩余类环上的运算具有速度快、无舍入误差的优点。     

Greedy Routing with Anti-Void Traversal for Wireless Sensor Networks

The unreachability problem (i.e., the so-called void problem) that exists in the greedy routing algorithms has been studied for the wireless sensor networks. Some of the current research work cannot fully resolve the void problem, while there exist other schemes that can guarantee the delivery of packets with the excessive consumption of control overheads. In this paper, a greedy anti-void routing (GAR) protocol is proposed to solve the void problem with increased routing efficiency by exploiting the boundary finding technique for the unit disk graph (UDG). The proposed rolling-ball UDG boundary traversal (RUT) is employed to completely guarantee the delivery of packets from the source to the destination node under the UDG network. The boundary map (BM) and the indirect map searching (IMS) scheme are proposed as efficient algorithms for the realization of the RUT technique. Moreover, the hop count reduction (HCR) scheme is utilized as a short-cutting technique to reduce the routing hops by listening to the neighbor's traffic, while the intersection navigation (IN) mechanism is proposed to obtain the best rolling direction for boundary traversal with the adoption of shortest path criterion. In order to maintain the network requirement of the proposed RUT scheme under the non-UDG networks, the partial UDG construction (PUC) mechanism is proposed to transform the non-UDG into UDG setting for a portion of nodes that facilitate boundary traversal. These three schemes are incorporated within the GAR protocol to further enhance the routing performance with reduced communication overhead. The proofs of correctness for the GAR scheme are also given in this paper. Comparing with the existing localized routing algorithms, the simulation results show that the proposed GAR-based protocols can provide better routing efficiency.     

The impact of network topology on delay bound in wireless Ad Hoc networks

We consider single channel wireless networks with interference constraint among the links that can be activated simultaneously. The traffic flows are assumed to be single hop. Delay performance of the well known throughput optimal maximum weight link scheduling algorithm has been studied recently. In this paper, we study the relation between network topology and delay of maximum weight link scheduling algorithm. First, we consider 1-hop interference model. Under this interference model, an upper bound for the average delay of packets is derived analytically in terms of edge chromatic number of the network graph. Then the results have been extended to the case of general interference model. Under this model of interference, an upper bound for delay as a function of chromatic number of conflict graph is derived. Since chromatic number and edge chromatic number are network topology parameters, the results show that how the upper bound of delay is affected by network topology. Simulation results confirm our analytical relations.     

图的最大权团的DNA计算

给定顶点赋权的无向图,图的最大权团问题是寻找每个顶点都相邻的顶点子集(团)具有最大权.这个问题是寻找无权图的最大团问题的推广.图的最大团和最大权团都是著名的NP-完全问题,没有非常有效的算法.1994年Adleman博士首先提出用DNA计算解决NP-完全问题,使得NP-完全问题的求解可能得到解决.本文给出了基于质粒技术的无向图的最大权团问题的DNA算法,依据Head T等的实验手段,本文提出的算法是有效并且可行的.     

Performance evaluation of very long span direct detection intensityand polarization modulated systems

In this paper long distances very high capacity NRZ optical transmission systems adopting direct detection are considered in links with a low average chromatic dispersion. Particular attention is devoted to polarization modulated (PM-DD) systems. Polarization modulated systems results to he more degraded with respect to intensity modulated (IM-DD) systems by the light depolarization induced by the interplay among the Kerr effect, the ASE noise of the optical amplifiers and the polarization mode dispersion. The light depolarization is particularly strong in conditions of large spectral broadening that are met when the chromatic dispersion value is maintained very low along the link. On the other hand the use of a fluctuating chromatic dispersion with a mean value equal to zero, whereas the local dispersion is different from zero, shows the double advantage to reduce the chromatic dispersion impairments and to limit the spectral broadening. The advantages offered by this dispersion management technique have been already shown in several experiments for IM-DD systems: in this work we show that this technique is very important also for PM-DD systems since the limitation in the spectral broadening reduces the light depolarization. We show that adopting a suitable dispersion management and an opportune preamplifier optical filter transmissions at 5 Gb/s can be attained in transoceanic links by means of FM-DD systems     

Frequency-domain equalisation for optical transmission systems

The first optical single-carrier (SC) transmission system to employ frequency-domain equalisation (FDE) is proposed; 25 Gbit/s SC with FDE is demonstrated in severe chromatic dispersion conditions. The results show that SC with FDE is very tolerant of the chromatic dispersion.     

Chromatic dispersion measurement in 1.55 μm narrow-band regionusing a tunable external-cavity laser

Measurement of the chromatic dispersion of an 80.6-km-long, concatenated, dispersion-shifted, single-mode fiber (DSF) with a tunable 1.55-μm external-cavity laser diode, using the phase-shift technique at 1.55 μm over 80-nm bandwidth, is discussed. It is shown that the technique does not need intricate curve-fitting equations or a large number of laser sources with specified wavelengths. As a result, the measurement configuration and procedure are relatively simple. The technique is useful for measuring the chromatic dispersion of future advanced fibers such as dispersion flattened fibers with various refractive index profiles     

光纤色散测量概述

介绍了三种最为成熟的光纤色散测量方法:时延法、相移法和干涉法,并从色散测量原理、测量装置、适用范围以及优缺点等几方面进行了论述.时延法测量精度不高,目前较少采用;相移法测量精度高,但测量仪器价格昂贵,测量成本高;干涉法既能满足一定的测量精度要求(时间分辨率可达1ps),又控制了测量成本,是普遍采用的测量方法.     

基于模糊聚类的信源个数检测新算法

信源个数检测方法的研究是目前阵列信号处理领域的一个研究热点和重点,它的测量精度直接影响到许多高分辨测向算法的精确度。然而现有的检测方法对于不同的噪声环境非常敏感;而且随着快拍数的减少性能迅速下降。基于以上的限制与缺陷,该文提出了一种基于模糊c均值聚类的信源个数检测方法。在两种不同噪声环境下的仿真结果表明该方法的有效性和鲁棒性。     

基于源信号数目估计的欠定盲分离

该文利用欠定盲分离下稀疏源信号的特点,估计源信号的数目且恢复源信号。通常在用两步法来解决欠定盲分离时,首先利用K-均值算法对观测信号聚类估计出混叠矩阵,最后利用最短路径法来恢复源信号,但是在以往的算法中,第1步估计混叠矩阵时,通常假设源信号数目是已知的,从而进行K-均值聚类,而事实上源信号数目根本无法知道,因此对源信号数目的估计对两步法有很重要的影响。因此本文提出了一种新的两步法算法,其中第1步利用稀疏源信号反映在观测信号中的特征来准确地估计出稀疏源信号的数目,且能得到混叠矩阵,从而恢复源信号。最后的仿真结果,以及与通常的K-均值聚类算法对比的仿真结果说明了此算法的可行性和优异的性能。     

Design guidelines for routing protocols in ad hoc and sensor networks with a realistic physical layer

We present guidelines on how to design network layer protocols when the unit disk graph (UDG) model is replaced by a more realistic physical layer model. Instead of merely using the transmission radius in the UDG model, physical, MAC, and network layers share the information about a bit and/or packet reception probability as a function of distance between nodes. We assume that all nodes use the same transmission power for sending messages, and that a packet is received when all its bits are correctly received. The MAC layer reacts to this probabilistic reception information by adjusting the number of acknowledgments and/or retransmissions. We observe that an optimal route discovery protocol cannot be based on a single retransmission by each node, because such a search may fail to reach the destination or find the optimal path. Next, we discuss that gaining neighbor knowledge information with "hello" packets is not a trivial protocol. We describe localized position-based routing protocols that aim to minimize the expected hop count (in case of hop-by-hop acknowledgments and fixed bit rate) or maximize the probability of delivery (when acknowledgments are not sent). We propose a guideline for the design of greedy position-based routing protocols with known destination locations. The node currently holding the message forwards it to a neighbor (closer to the destination than itself) that minimizes the ratio of cost over progress, where the cost measure depends on the assumptions and metrics used, while the progress measures the difference in distances to the destination. We consider two basic medium access layer approaches, with fixed and variable packet lengths. This article serves as a preliminary contribution toward the development of network layer protocols that match the assumptions and criteria already used in simulators and ultimately in real equipment.     

Extracting the cliques from a neighbourhood system

A method is proposed for obtaining the local clique set from a neighbourhood system. The Markov random field model, which is used extensively in image processing, is defined with respect to a neighbourhood system. The mathematical interpretation of the model is defined with respect to the corresponding clique set. A systematic method is presented for extracting the complete local clique set from any neighbourhood system on which a Markov random field may be defined     

Separability and topology control of quasi unit disk graphs

A deep understanding of the structural properties of wireless networks is critical for evaluating the performance of network protocols and improving their designs. Many protocols for wireless networks—routing, topology control, information storage/retrieval and numerous other applications—have been based on the idealized unit-disk graph (UDG) network model. The significant deviation of the UDG model from many real wireless networks is substantially limiting the applicability of such protocols. A more general network model, the quasi unit-disk graph (quasi-UDG) model, captures much better the characteristics of wireless networks. However, the understanding of the properties of general quasi-UDGs has been very limited, which is impeding the designs of key network protocols and algorithms. In this paper, we present results on two important properties of quasi-UDGs: separability and the existence of power efficient spanners. Network separability is a fundamental property leading to efficient network algorithms and fast parallel computation. We prove that every quasi-UDG has a corresponding grid graph with small balanced separators that captures its connectivity properties. We also study the problem of constructing an energy-efficient backbone for a quasi-UDG. We present a distributed local algorithm that, given a quasi-UDG, constructs a nearly planar backbone with a constant stretch factor and a bounded degree. We demonstrate the excellent performance of these auxiliary graphs through simulations and show their applications in efficient routing.     

Waveform degradation due to dispersion effects in an optical CPFSKsystem

Waveform degradation due to polarization and chromatic dispersions in a single-mode fiber is calculated for a coherent CPFSK signal. For a single-mode fiber with polarization dispersion of ⩽1 ps, chromatic dispersion almost dominates the system. However, if a fiber has polarization dispersion of more than a few picoseconds and a chromatic dispersion of less than 0.1 ps/km/nm, which can be attained by using a dispersion-shifted fiber and/or by electric dispersion compensation, polarization dispersion will restrict transmission capacity. For instance, polarization dispersion of 5 ps will restrict a bit rate by ~60 Gb/s when chromatic dispersion is fully reduced using a dispersion-shifted fiber or applying electrical equalization