Showing papers by "Beijing University of Posts and Telecommunications published in 1999"

Coherent and incoherent crosstalk in WDM optical networks

Abstract: The impact of coherent and incoherent crosstalk on an optical signal passing through optical cross-connect nodes (OXC's) in wavelength division multiplexing (WDM) optical networks is studied, and the analytical expressions are given. Such crosstalk will be generated when the optical propagation delay differences of optical paths in an OXC do not exceed the coherent time of the lasers. While causing fluctuation of signal power, coherent crosstalk may cause noise or not, depending on the relationship between the optical propagation delay differences and the time duration of one bit of the signal. Incoherent crosstalk may cause very high noise power, because it can be a coherent combination of crosstalk contributions. The statistical impact of all crosstalk contributions on signal is studied by simulation, and the concept of quantile is proposed to relax the crosstalk specification requirement for components. The crosstalk specification requirements are then obtained for components used in WDM optical networks with different scales.

A high spectrum efficient multiple access code

Abstract: A new class of high spectrum efficient multiple access codes and its modifications are presented. These codes have some nice properties such that: (1) the sidelobes of their non-cyclic auto-correlations and cross-correlations can be simultaneously controlled to their minimum value +1 or -1. (2) For any auto-correlation or cross-correlation, from the main lobe (origin) to the 1st sidelobe, there exists a "zero correlation window" and the window width can also be controlled. The construction method of such a class of multiple access codes is presented. The application of such codes to a high data rate CDMA system with a much higher capacity than IS-95 is also discussed.

Performance evaluation of communication networks

Abstract: Simulation provides a practical methodology for performance evaluation of broadband communication systems. This paper first gives a general description of analytical and simulation methods and then proposes some simulation techniques combined with analytical predictions to improve the efficiency.

Theoretical analysis on polarization deviation and switch window optimization in nonlinear optical loop mirror demultiplexer

Abstract: This paper investigates theoretically two of the dominant issues on a nonlinear optical loop mirror (NOLM) demultiplexer: the sensibility to the polarization deviation between signal and control pulses and the optimization of the switch window width. The complete nonlinear Schrodinger equations concerning the different states of polarization between signal and control lights are firstly established to study the impact of the polarization deviation on the demultiplexed signal. Considering simultaneously the channel crosstalk and the timing jitter between signal and control pulses, the switch window width of NOLM is optimized to achieve the best demultiplexing performance. The theoretical analysis shows that the polarization deviation has to be controlled less than 20/spl deg/ within the bit error rate (BER) of 10/sup -9/ s. The optimal amount of the pulse walkoff is a little less than half of the slot width of the OTDM system.

ACK delay control for improving TCP throughput over satellite links

Abstract: TCP has been proved to work well in a variety of situations. However, in links of large bandwidth products, e.g., satellite links, TCP might underutilize the link and degrade the reliability. A number of revised algorithms have been proposed to improve TCP performance. In this paper, we propose a new algorithm to improve TCP throughput without modifying the TCP protocol. Since TCP uses acknowledgements (ACKs) to adjust the sending rate, the basic idea of our scheme is to delay (or shape) ACKs traveling through a node where the forward connection is congested. The algorithm aims to fully eliminate packet loss caused by buffer overflow. We implement this scheme in the intermediate nodes connecting satellite links and make a simulation study on its effect. In addition, we test the proposed algorithm in the real world. The results of both simulation and tests show that the proposed scheme improves the TCP throughput by effectively controlling the acknowledgment flow.

Preliminary investigation on error code rate of high speed digital circuitry caused by electrical contact failure

Abstract: It is found that one of the most important reasons causing high error code rates in digital communication systems is the contact failure of coaxial connectors installed in the systems Preliminary investigation into the failed connectors collected in-situ has revealed that the contact failure of coaxial connectors may be caused by many factors, such as corroded products or dust particles on the contact surfaces, and the change of normal contact force caused by relaxation of the spring elements, etc It is interesting to learn how the contact failure causes high and erratic contact impedance and results in error codes in digital communication systems which are still unknown The effects of tarnish materials on the contact impedance and also the influence of the contact failure on digital signal transmission are studied theoretically and experimentally Some of the results and tentative conclusions are presented in this paper

Failure analysis on bolt-type power connector's application

Abstract: Bolt-type power connectors are widely used in China due to their simple structure and low cost. However, investigation shows that this kind of power connectors needs continuous maintenance and frequent replacement. By analyzing the failed connectors that were collected from different cities and sites, high connection resistance of most samples was found. Clearly high temperature has been occurred at each connector. Bolts of nearly 45% of the testing connectors appeared loosening, which caused low contact pressure, evidently the bolts were stress relaxed. Even the bolts were rusted and locked, the strand wires could still rock in the grooves of the connectors. Sliding and fretting tracks were clearly observed and appeared different kinds of morphologies. Serious contaminants and corrosive film covered on the groove surfaces, which caused high temperature rise and further enhanced stress relaxation. Surfaces of almost 58% samples preserved obvious melting or welding appearance, some of them were even burnt out, evidently there were gaps between wires and connector. Therefore, the structure of the bolt type power connectors can hardly stand for the harsh environment in China. It is also found that the material used for making connectors was not fitted to the standard.

Cooperative software agents for workflow management system

Abstract: In the workflow management field, fast developments are taking place The research of the multi-agent system in distributed artificial intelligence provides a new implementation method for workflow management We propose a structure and organization of an agent-based workflow management system

3D wavelet video codec and its rate control in ATM network

Abstract: This paper presents a new video coding system based on the wavelet transform and its rate control scheme over ATM networks. First, the three dimensional (3D) wavelet transform is performed on the original image sequence, and an extension of set partitioning in hierarchical trees (SPIHT) algorithm is employed to quantize the wavelet coefficients. Then, the output rate of the coder is controlled at group of frame (GOF) scale, ensuring that it conforms to the parameters of a leaky bucket controller. Several leaky buckets with different sizes are discussed too. Our simulation shows the efficiency of this codec and the effectiveness of the proposed rate control scheme.

The application of multiwavelet transform to image coding

Abstract: This work presents a new image coding scheme based on multiwavelet filter banks. First, 2D multiwavelet decomposition is performed on original image. Then, several hierarchical trees are constructed in transform domain, and an extension of set partitioning in hierarchical trees algorithm is proposed to quantize multiwavelet coefficients. Our simulation shows that this scheme is effective and promising.

Island growth of corroded products on various plated surfaces after long-term indoor air exposure in China

Abstract: The environmental effects on several contact platings including gold, silver, nickel and tin were studied after investigating the morphology, composition and fretting behavior of natural corroded products, which were formed in Shanghai, China after long-term indoor air exposure. Corrosion formed on all of the testing coupons are found as discrete islands rather than continuous layers as predicted. Not only the protrusive core product but also one or more dark rings centrically surrounding the core are seen on gold plated coupons. Micro-observation on the ring shows that it is formed by accumulated and discrete islands similar to that of the core. The height of corroded products on the ring is much lower than that of the core. The composition of corroded product on the ring is similar to that of the core. Fretting across the dark ring also causes high contact resistance. Preliminary investigation shows that this may cause sudden and rapid high impedance and error codes in telecommunication systems.

Modulation instability in non-Kerr-like optical fibers near the zero dispersion point

Abstract: Modulation instability (MI) in non-Kerr-like optical fiber near the zero dispersion point is analyzed. Taking into account the fourth order dispersion, it is shown that new regions of MI are found. The action of the self-steepening isn't the same for each region of MI and the critical modulation frequency.

Code routing technique in optical network

Abstract: This paper outlines the idea of a code routing technique in an optical packet switching network, which aims to combine the advantage of code division multiplexing (CDM) and wavelength multiplexing (WDM) We describe the principle of operation and propose our opinion on the three questions that are mainly considered in the system design Also, the functional blocks structure is described in greater detail with reference to the hardware implementation

ACK filtering on bandwidth asymmetry networks

Abstract: We study the effects of bandwidth asymmetry on end-to-end TCP performance and suggest a mechanism to improve it. Bandwidth asymmetry has been considered a mismatch in the bandwidths in two directions of data transfer. It is common in wireless networks, ADSL networks, etc. In a bandwidth asymmetry network, throughput achieved is not only dependent on the traffic and link characteristics of the forward direction, but significantly determined by those of the backward direction. Some mechanisms, has been proposed to overcome this problem. In our study, we focus on the ACK filtering algorithm, which decrease the rate of ACK on a constrained reverse link, taking advantage of the fact that TCP ACKs are cumulative.

Packet filtering in bridge

Abstract: Filtering of unauthorized packets is a common way to improve network security. Packet filtering is described as one of the firewall systems, which is generally implemented in gateways and routers. Packet filtering of gateways and routers is dangerous because an attacker may find the machine by its IP address. To resolve this security problem, we designed a packet filtering system without IP address. Using a personal computer and Linux as our system platform, we revise the operating system kernel to add filtering packet capability. Finally we implemented a secure packet filtering firewall and used it to protect our internal network from attack.

Cryptanalysis of convertible group signature

Abstract: It is proven, using two group centre cheats, that the convertible group signature scheme proposed by Kim et al. (see Asiacrypt'96, p. 311-21) is insecure.

Multimedia data modeling based on temporal logic and XYZ System

Abstract: This paper proposes a new approach to modeling multimedia data. The new approach is the multimedia data model based on temporal logic and XYZ System. It supports the formal specifications in a multimedia system. Using this model, we can not only specify information units but also design and script a multimedia title in an unified framework. Based on this model, an interactive multimedia authoring environment has been developed.

Adaptive call admission control for multi-class CDMA cellular systems

Abstract: We propose a novel adaptive call admission control (CAC) policy, which can adjust the CAC threshold dynamically according to the current traffic load in local and neighboring cells and handoff users density by employing the equivalent capacity demand estimation and fuzzy guard channel capacity estimator The results of computer simulation indicate the method is more efficient and simpler than previous policies

A general CAC approach using novel ant algorithm training based neural network

Abstract: We propose a neural network based approach for call admission control (CAC), which is applicable to very general traffic. In our approach, a feedforward neural network is used to predict whether a new call can be accepted. The input vector of the neural network consists of a set of data reflecting the first and second-order statistical properties of the input aggregate stream, and its dimension is independent of the number of traffic classes. In addition, we give a novel ant algorithm to train the neural network. Unlike the backpropagation (BP) algorithm often used, our training algorithm can realize global optimization. Simulations show the effectiveness of our approach.

Constrained constant modulus algorithm in multiuser detection

Abstract: A novel blind adaptive algorithm, constrained constant modulus algorithm (C-CMA) for multiuser detection is proposed in the paper. We analyze the convergence behavior of C-CMA in the noiseless case, from which an approximate relation between the constant in C-CMA and the power of the desired user, which guarantees the algorithm converges to the desired stationary point, is given. Simulations illustrate that both the constant and step size make a critical effect on the convergence, and it is better to select a suitable value to guarantee C-CMA converges to a desired stationary point while the convergence speed is bearable. It is also shown that if it is well initialized, C-CMA attains a better performance than the minimum output energy (MOE) algorithm in Honig et al. (1995). Therefore, it is a feasible approach to use the MOE algorithm firstly, after it reaches steady state, and transfer to C-CMA to improve the performance. Lastly, we compare the performance between C-CMA and CMA. It is shown that C-CMA has better performance in the low noise case. But the convergence behaviors of the algorithms are both sensitive to the constant and step size.

Non-zero dispersion shifted fiber with low dispersion slope

Abstract: A new non-zero-dispersion-shifted fiber design is presented These fibers, with low dispersion slope of 003 ps/nm/sup 2/-km and small negative dispersion from 1530 to 1620 nm, are ideal transmission medium for use in broad band DWDM systems

Study on electromagnetic field in the end region of large generator by FEM

Abstract: The attractive model of analyzing a hydraulic generator's end region field is proposed. The stator core's end packets are included, the anisotropic and nonlinear stator core are considered. A new method to treat the boundary conditions is presented. Compared with the existing models, the model can calculate the field in the end packets of the stator core more accurately. The calculated results check closely with those obtained from measurement. The result has been applied to analyze the influence of structure and some recommendations in order to avoid local over-heating are proposed.

Exact numerical analysis of resonant-cavity-enhanced photodetectors with matrix simulation

Abstract: Great efforts have been made on the research and development of resonant cavity enhanced photodetectors since the structure was proposed by Katsumi Kishino and M. Selim Unlu in 1991. Based on all these works, a new analysis method, considering the effect of the refractive index differences between different layers of the structure on the characteristics of the device with matrix simulation, is presented in this paper. With such an analytical way, we can analyze the effects of the dielectric stacks, which forming the mirrors, on the response of the photodetector directly. And the standing wave effects of the detector's responses can be accurately calculated. In comparison with the former method established by Katsumi Kishino and M. Selim Unlu, our analysis leads to little difference in calculated maximum quantum efficiency but an obvious one in calculated optical response spectrum, for small inter-layer refractive index differences (< +/- 0.6). Whereas for large inter-layer refractive index differences, remarkable differences in both aspects of the above mentioned characteristics are resulted. Our analysis shows better agreement with experiments.

Fractal based texture analysis for retrieval of image data

Abstract: In this paper, directional fractal dimensions are computed for a special class of images with lines and curves Then the texture variation coefficient is defined and a simple algorithm for detecting singularity fields of an image is proposed Experiments show that 92% singularity fields are correctly recognized by the proposed algorithm, and this will greatly help with content-based retrieval of texture images from a large database

Frame structure and MAC protocol of ATM-based PON system

Abstract: Reference to ITU-T recommendation G.983 about APON, a new frame structure and a MAC protocol are presented. The most important features of this scheme are easy to access STM services and good cell delay (CD) and cell delay variation (CDV) performance.

A new reliable signalling transport protocol RSTP for voice over IP

Abstract: This paper investigates the issue of transport protocol for voice over IP signalling. A new reliable signalling transport protocol (RSTP) for VoIP is presented, after discussing the feasibility of utilizing existing transport protocols over IP (TCP/UDP) for VoIP signalling. The presented RSTP in this paper is a very lightweight reliable transport protocol with low delay, low overhead and high performance. Furthermore, it is easy to be implemented.

Soft handoff design and realization for C-CDMA systems

Abstract: Handoff is one of the key techniques in the cellular mobile communication systems. In this paper a method for the soft handoff design and realization for C-CDMA Systems is presented in detail. The method introduced here can be exploited in practical cellular mobile communication systems.

Parameter optimization for pilot symbol aided coherent demodulation

Abstract: In the forward link of many WCDMA proposals for IMT-2000, pilot symbol aided coherent demodulation (PSACD) is employed. The more pilot symbols there are, the more accurate the channel estimation is, but this will cause more information loss. This paper analyzes this trade-off by deriving the analytical error performance and performance loss due to pilot symbol aided coherent demodulation. Thus we can optimize the space between two neighbor pilot symbols.

On the testing quality of random and pseudo-random sequences for permanent and intermittent faults

Abstract: In this paper, the natures of random and pseudo-random input sequences and their influence on permanent and intermittent fault detecting are analyzed. The aliasing fault coverage between the pseudo-random and random sequences is estimated. The activity probability features of the intermittent faults are considered. The self-test circuits of the intermittent faults are illustrated. The experimental results based on real circuits are obtained through simulation. The mathematical analysis and experimental results show that the quality of the pseudo-random testing is better than that of the random testing for permanent and intermittent faults. The Markov chain models are used in obtaining the input sequence length needed for determining if a circuit fault is intermittent or permanent.