Showing papers by "Beijing University of Posts and Telecommunications published in 2002"

Performance of reliable transport protocol over IEEE 802.11 wireless LAN: analysis and enhancement

Abstract: IEEE 802.11 medium access control (MAC) is proposed to support asynchronous and time bounded delivery of radio data packets in infrastructure and ad hoc networks. The basis of the IEEE 802.11 WLAN MAC protocol is a distributed coordination function (DCF), which is a carrier sense multiple access with collision avoidance (CSMA/CA) with a binary slotted exponential back-off scheme. Since IEEE 802.11 MAC has its own characteristics that are different from other wireless MAC protocols, the performance of reliable transport protocol over 802.11 needs further study. This paper proposes a scheme named DCF+, which is compatible with DCF, to enhance the performance of reliable transport protocol over WLAN. To analyze the performance of DCF and DCF+, this paper also introduces an analytical model to compute the saturated throughput of WLAN. Compared with other models, this model is shown to be able to predict the behavior of 802.11 more accurately. Moreover, DCF+ is able to improve the performance of TCP over WLAN, which is verified by modeling and elaborate simulation results.

IEEE 802.11 distributed coordination function (DCF): analysis and enhancement

Abstract: Being a part of IEEE project 802, the 802.11 medium access control (MAC) is used to support asynchronous and time bounded delivery of radio data packets. It is proposed that a distributed coordination function (DCF), which uses carrier sense multiple access with collision avoidance (CSMA/CA) and binary slotted exponential backoff, be the basis of the IEEE 802.11 WLAN MAC protocols. This paper proposes a throughput enhancement mechanism for DCF by adjusting the contention window (CW) resetting scheme. Moreover, an analytical model based on Markov chain is introduced to compute the enhanced throughput of 802.11 DCF. The accuracy of the model and the enhancement of the proposed scheme are verified by elaborate simulations.

Chinese named entity identification using class-based language model

Abstract: We consider here the problem of Chinese named entity (NE) identification using statistical language model(LM). In this research, word segmentation and NE identification have been integrated into a unified framework that consists of several class-based language models. We also adopt a hierarchical structure for one of the LMs so that the nested entities in organization names can be identified. The evaluation on a large test set shows consistent improvements. Our experiments further demonstrate the improvement after seamlessly integrating with linguistic heuristic information, cache-based model and NE abbreviation identification.

Chaos-based secure communications in a large community.

Abstract: One-way coupled map lattices are used for cryptography in secure communication, based on spatiotemporal chaos synchronization. The sensitivity of synchronization between the encryption and decryption systems can be adjusted by varying the system size. With a suitable parameter combination, the cryptosystem can reach optimal trade-off of security and performance, i.e., it shows high security (resistant against the public-structure and known-plaintext attacks) together with fast encryption (and decryption) speed. An experiment of duplex voice transmission through university network is realized, which confirms the above advantages of our approach.

The Mobile IP handoff between hybrid networks

Abstract: Future mobile data networks will consist of several tiers, which overlap each other. We introduce two of the main technologies, GPRS network and IEEE 802.11 WLAN, which are likely to be widely deployed. We propose schemes to support mobility between these two networks. We also analyze the Mobile IP handoff between hybrid networks and propose our handoff scheme, which reduces handoff delay and packet loss, and prevents the ping-pong effect by utilizing a dwell timer and multi-tunnel technology.

A new self-adapt DCF algorithm

Abstract: This paper has first presented an in-depth analysis on the distributed coordination function (DCF) access mode of IEEE802.11 protocol. Based on the result of our study, we have concluded a new self-adapt wireless LAN MAC algorithm. Numerous simulation results have shown that the new algorithm can achieve the self-adapt character with the growing of node number and is superior to the original DCF algorithm in the characters concerned (e.g. throughput, fairness).

A practical space-frequency block coded OFDM scheme for fast fading broadband channels

Abstract: Multi-antenna OFDM system calls the consideration of coding across antennas and OFDM tones. Derived from the design criteria for space-time block codes (STBC), the coding scheme termed space-frequency block codes (SFBC) is conceived to achieve the maximum diversity order for a given number of transmit and receive antennas. In this contribution, a practical space-frequency block coded OFDM scheme (SFBC-OFDM) is proposed for fast fading broadband channels. Besides, this novel scheme is rather flexible to combat the frequency-selective fading by decreasing the coding size. Simulation and analysis show that our scheme outperforms the STBC-OFDM scheme in the scenario with high vehicle speed and large delay spread when the number of subcarriers in the OFDM system is large.

Performance analysis on phase-encoded OCDMA communication system

Abstract: The performance of an asynchronous phase-encoded optical code-division multiple-access system is evaluated on the condition that the impact of fiber channel is neglected. Phase-encoded optical signal (pseudorandom optical signal with low intensity) is analyzed in the view of stationary random process. The pseudorandom optical signal with low intensity is seen as a sample function of a certain stationary random process which is ergodic in strict sense. The analysis results reveal that the variance of the corresponding random process is only inversely proportional to the code length while the root-mean-square width of the phase-encoded optical signal is proportional to the width of initial optical pulse and the code length F. The numerical results demonstrate that the better system performance can be achieved in case of larger code length and shorter initial optical pulse.

Consideration and research issues for the future generation of mobile communication

Abstract: With the explosive growth of the mobile communication and the expansion of Internet services, Mobile multimedia services, e.g. more than 2Mb/s, are expected for future mobile communication systems. As 3G is still not sufficient to support future mobile communication strategy and according to the historical indication of a generation revolution once a decade, it is high time to discuss the development of the future generation mobile communication systems. After illuminating the impetus force to driven the 4G research, this paper studies the vision and framework of 4G, describes the adaptability features of terminals, networks and applications in 4G system and proposes some technologies in the design of future mobile communication systems.

Data synchronization in the mobile Internet

Abstract: At present, people use many different devices to communicate each other and to store their important personal or business data. These data are stored in some of the devices and must be synchronized between the different devices while being updated Data synchronization specified by SyncML can realize to synchronize data between a wide range of devices and any net-work based data format. WAP Forum has adopted SyncML Specification as one of its most important mobile Internet services in WAP2.0. This paper focuses on the cooperation between wireless devices and data servers, and introduces several solutions on data synchronization, and also give the data format that suits Chinese, which will be applied to be the industry standard of China by China Mobile Internet Application protocol Special Group (CMIS).

The identification of joint parameters for modular robots using fuzzy theory and a genetic algorithm

Abstract: This paper discusses a technique for identifying the joint parameters of a modular robot in order to study the dynamic characteristics of the whole structure and to realise dynamic control. A method for identifying the joint parameters of the structure applying fuzzy logic combined with a genetic algorithm has been studied using a 9-DOF modular redundant robot. A Genetic Algorithm was used in the fuzzy optimisation, which helped to avoid converging to locally optimal solutions and made the results identified much more reasonable. The joint parameters of a 9-DOF modular redundant robot have been identified.

Call admission control in hierarchical cell structure

Abstract: In interference limited CDMA systems, call admission control plays a very important role because it directly controls the number of users. We introduce a interference-based CAC algorithm according to the characteristics of HCS. This algorithm adopts a global strategy in macro layer, while using a local strategy in the micro layer. Meanwhile, in order to make full use of resources in the macro layer, different threshold values in the two layers are used. Simulation results show that the performance has been greatly improved.

A time-efficient contributory key agreement scheme for secure group communications

Abstract: Group-oriented services are attractive to both customers and service providers since they allow for potential markets that exploit user collaboration and shared experiences. In order to protect the content of a group service, the content must be encrypted using a key shared by group members. With the advancement of network technologies, customers will access group services from different networks using different terminals. The computational capabilities of the group members will vary. Contributory key agreement schemes have been developed to support secure communication between many senders and many receivers, but do not consider that users might require different amounts of time to perform a round of key establishment. In this paper, we develop a contributory key agreement scheme to setup the customer group which achieves a smaller latency for group key generation than existing schemes. We then address the latency needed to rekey the group during member joins and departures. A protocol that determines the location to place a joining member is presented that considers the time needed for rekeying the joining member as well as the effect the joining member has upon the departure times of the other group members. Simulations are presented that compare our algorithms with existing schemes for different group sizes, as well as different user join rates.

New sub-optimal detection algorithm of layered space-time code

Abstract: As an important space-time code, the layered space-time (LST) code has been studying widely since it was firstly proposed by Foschini in 1996. To exploit its potential, the Bell Lab Layered Space-Time (BLAST) structure of the LST was proposed by Bell Lab. There are two types of BLAST architectures: vertical BLAST (V-BLAST) and diagonally BLAST (D-BLAST). Detection algorithms of V-BLAST were proposed by Golden (see Electronics Letters, vol.35, no.1, 1999). His detection process uses linear combination nulling and successive symbol cancellation (SSC) based on inversing and ordering. However, inversing (Moore-Penrose pseudoinverse) and ordering operation for each iteration bring a huge computation complexity. To detect M transmit antenna signals, Golden's detection algorithm needs M inversing and M ordering operations. Aiming at this shortcoming, a new detection algorithm for layered space-time code is proposed. This new sub-optimal detection algorithm is based on Greville inversing process, with two inversing and one ordering process. Simulation results show us the proposed sub-optimal scheme still has a good BER performance.

The solitonic features of a nonintegrable (3 + 1)-dimensional Jimbo-Miwa equation☆

Abstract: We investigate a nonintegrable (3+1)-dimensional Jimbo-Miwa equation by truncating the Painleve expansion and performing computerized symbolic computation. We obtain a new auto-Backlund transformation and a new family of the exact solitonic solutions. Sample solutions of this family are presented.

Comparison between the periodic and event-triggered compressed mode

Abstract: In the HCS system when different frequencies are applied, compressed mode is used so that interfrequency measurement can be obtained. When and how to trigger compressed mode is a crucial problem because the compressed mode may have a bad effect on the system performance: The performance of the periodic and the event-triggered compressed mode is compared with the help of system simulations. Moreover, the impact of some of the parameters is evaluated in this paper.

Spatial orders appearing at instabilities of synchronous chaos of spatiotemporal systems

Abstract: Various spatial orders introduced by the instabilities of synchronous chaotic state of spatiotemporal systems are investigated by considering coupled map lattice and chaotic partial differential equation. In particular, the motions of on-off intermittent states at the onset of the instabilities are studied in detail. The chaotic desynchronized patterns can be described by a simple universal form, including three parts: the synchronous chaos; a spatially ordered pattern, determined by the unstable mode of the reference synchronous chaos; and on-off intermittency of the scale of this given pattern.

Simple self-synchronization scheme for OTDM packet networks

Abstract: Summary form only given. In this paper, a simple self-synchronization scheme employing only one SOA and optical filter based on the self-phase modulation (SPM) induced frequency chirping is proposed. When an incoming OTDM packet composed of short optical pulses is introduced into the SOA, the SOA gain is saturated abruptly by the first pulse, and then recovers slowly. If the bit period is much shorter than the carrier lifetime, the gain will only recover partly when the second pulse arrives and be saturated again. Therefore, only the first pulse experiences larger gain and phase variations. As phase change always accompanies frequency chirping, the first pulse acquires larger frequency shifting than other pulses do. Behind SOA, if we arrange an optical band-pass filter (OBF) whose center frequency corresponds to the frequency shift of the first pulse, the first pulse can be extracted out from the OTDM packet. Here EDFA is exploited to boost the power of the OTDM packet signal to the required level if necessary.

Polarization mode dispersion in WDM systems

Abstract: The coupled nonlinear Schrodinger equations for two wavelength optical pulses in a birefringence fiber are given. The model for treating polarization mode dispersion in two channel (WDM) system is established. Based on this model, optical signal propagation behaviors in two channel WDM with polarization mode dispersion are numerically simulated. The influences of polarization mode dispersion on two channel WDM system are also analyzed.

Dynamic system level simulation for radio resource management of TD-SCDMA high speed downlink packet access

Abstract: TD-SCDMA, proposed by China Wireless Telecommunications Standard Group (CWTS), is the low chip rate (LCR) TDD CDMA proposal in 3G The Internet services will generate highly asymmetric data flow The aim of TD-SCDMA downlink packet access (HSDPA) is to provide very high packet throughput and low packet delay in the downlink using the newly introduced high speed downlink shared channel (HS-DSCH) This paper presents the methodology we used to model HSDPA for TD-SCDMA in OPNET

New DCT computation algorithm for VLIW architecture

Abstract: A new DCT computation algorithm is presented, which is based on an existing fast DCT algorithm and has fully utilized the parallelism of the VLIW (very long instruction word) architecture of DSP to achieve high speed performance. Simulation results show that this method can reduce computing cycles by more than 70% and 20%, compared with normal and fast DCT computation algorithms, respectively.

A policy-based solution for management of enhanced network services

Abstract: Traditional network management, with stress on monitoring rather than control, can not meet the needs of emerging multiservice networks with QoS guarantees We propose a policy-based solution to support delivery and management of network infrastructure and enhanced network services The article describes requirements for management of enhanced network services, hierarchical policy architecture, and provides an overview of policy-based management framework We discuss policy-driven service management with emphasis on QoS management, dynamic service provisioning, and Internet pricing and billing

Performance analysis of an adaptive modulation system over Nakagami-m fading channels

Abstract: This paper analyzes the performance of the adaptive modulation system over Nakagami-m fading channels, which utilizes four modulation modes. The optimization criterion of the setting of the modulation switching levels is to maintain the target BER which is desired by many data services. Under this condition, the performance analysis of the throughput and the transmission outage probability of the adaptive modulation system are presented. We do so by adopting the numerical analysis methods. Finally, the simulation results show that the adaptive modulation system can theoretically achieve good performance with high throughput and low transmission outage probability.

Properties of corrosion stains on thin gold plating

Abstract: Surface analysis of gold platings with different thicknesses after 3, 8, 15 and 22 months indoor air exposure was carried out. Some attractive phenomena are described in this paper. Corrosion products on gold plating are observed as the core products surrounded by different sizes of corrosion stains under an optical microscope. The stain appears as numerous tiny islands under SEM observation, which is spread and accumulated as rings or part of rings. The pore corrosion on thin gold plating is much more serious than that on the thick gold plating. It is found that contact resistance changes from point to point along a straight line through the stain area, and the values are much higher than normal in the whole stain area no matter where the testing probe is located inside the stain. It is reasonable to count the whole stain area at the contact region as failure surface area. Ratio of corrosion stain area to the sampling area versus exposure time matches Weibull probability distribution curve. Calculation leads to prediction of the reliability of contacts. Gold plating thickness below 0.5 /spl mu/m may appear high risk in this environment. Dust particles are also deposited on gold plating during indoor air exposure. In some corrosion stain areas, elements of dust are found. Dust may enhance the corrosion and also cause high contact resistance.

Link performance of TD-SCDMA systems

Abstract: In this paper, we start with the architecture in TD-SCDMA, and outline the main aspects of its important algorithms, including channel estimation, joint detection, and smart antenna. Our emphases are put on a new efficient channel estimation algorithm which brings performance near that brought by ideal channel estimation and the fast algorithms of JD which greatly reduce the computational complexity. Finally, computer simulations by COSSAP are presented to evaluate the link level performance of the TD-SCDMA system under various multipath Rayleigh fading channel environments. 12.2 kbit/s data service is mainly adopted. The numerical results show that our schemes can bring more accurate channel estimation, lower computational complexity, which benefit the efficiency of the TD-SCDMA systems.

'Proper acceleration' of a null geodesic in curved spacetime

Abstract: Given a null geodesic in Minkowski spacetime, there exists a one-parameter family of observers in 'hyperbolic' motion which approaches the null geodesic as the parameter x0 approaches zero. It is well known that the proper acceleration of the observers in the family approaches infinity as their world line approaches the null geodesic. The main purpose of this paper is to generalize this result to future-complete null geodesics in curved spacetimes.