Reuse efficiency for non-uniform traffic distributions in CDMAsystems

An exact and simple method to evaluate the frequency reuse efficiency for the reverse link of cellular CDMA systems over a non-uniform traffic distribution is presented. This method is based on a traffic redistribution and decomposition process and on standard curves with which partial reuse efficiencies are obtained and conveniently added to give the final parameter     

Throughput availability in Markov systems

Explicit formulations for throughput availability, demand availability, on-stream availability, availability at a particular level and time, limiting throughput availability, and mean throughput to first failure are developed in terms of discrete and continuous parameter Markov chains. These measures are particularly adapted to continuous production or transportation systems. A numerical example illustrates the results     

Satellite systems in a VSAT environment

The next generation of satellites is discussed and a Ka-band payload is proposed which will provide mesh connectivity at high data rates (e.g. 1 Mbit/s) directly between very small aperture terminals (VSATs) using a maximum antenna diameter of 1.5 m. It is demonstrated that there are no significant differences between Ku-band and Ka-band VSAT operations apart from propagation margins, which are analysed statistically over 650 European cities. It is shown that margins required for Ka-band operation are not excessive. A Ka-band satellite payload is proposed, based on frequency division multiple access, multibeam switching with dynamically controllable transponder bandwidths and gain. The payload adapts to traffic-loading requirements in its provision of multibeam connectivity and capacity. The signal format, modulation and coding technique used for transmission is unconstrained, except in terms of bandwidth, and this will allow advances in modulation and coding to take place during the lifetime of the satellite in orbit     

虚拟交通环境中的雨滴仿真

为实现虚拟交通场景中的降雨实时仿真,设计出一种基于OpenGL的降雨仿真模型。现有雨的仿真模型主要关注降雨过程,而在虚拟交通环境中,通常还需要对雨滴在挡风玻璃上的运动进行仿真。利用OpenGL将场景渲染到纹理,运用几何方法计算雨滴粒子并将其映射到网格化的虚拟挡风玻璃平面,采用贴纹理的方法将纹理映射到虚拟挡风玻璃平面,建立了雨滴在挡风玻璃平面的仿真模型。实践表明,该方法的时间开销与场景、光照等因素无关,仅与车辆挡风玻璃上的雨滴数量相关。该方法在普通PC实验平台上渲染场景的帧速率超过25帧／秒,能够成功实现雨滴的实时仿真。     

虚拟交通环境中的雨滴仿真

为实现虚拟交通场景中的降雨实时仿真,设计出一种基于OpenGL的降雨仿真模型.现有雨的仿真模型主要关注降雨过程,而在虚拟交通环境中,通常还需要对雨滴在挡风玻璃上的运动进行仿真.利用OpenGL将场景渲染到纹理,运用几何方法计算雨滴粒子并将其映射到网格化的虚拟挡风玻璃平面,采用贴纹理的方法将纹理映射到虚拟挡风玻璃平面,建立了雨滴在挡风玻璃平面的仿真模型.实践表明,该方法的时间开销与场景、光照等因素无关,仅与车辆挡风玻璃上的雨滴数量相关.该方法在普通PC实验平台上渲染场景的帧速率超过25帧/秒,能够成功实现雨滴的实时仿真.     

Performance of a nonblocking space-division packet switch in a timevariant nonuniform traffic environment

The authors study the performance of a nonblocking space-division packet switch, given that the traffic intensities at the switch not only are nonuniform but also change as a function of time. A finite-state Markov chain is used as an underlying process to govern the time variation of traffic for the entire switch. The packet arrivals at each input form an independent Bernoulli process modulated by the underlying Markov chain. The output address of each packet is independently and randomly assigned with probability distributions, which are also modulated by the Markov chain. Provided that the traffic on each output is not dominated by individual inputs the service time of each output queue for sufficiently large switches can be characterized by an independent Markov modulated phase-type process. A matrix geometric solution for the resultant quasi-birth-death type queuing process is presented. The maximum throughput is obtained at the system saturation. The performance of the switch is numerically examined under various traffic conditions. A contention priority scheme to improve the switch performance is proposed     

Performance of a packet switch with crossbar architecture

A packet having a crossbar architecture with M inputs and N outputs is considered. Following earlier work, the theoretical capacity of such a switch is found in terms of the throughput of a closed queueing network. A state-dependent server model that approximates the rate at which the switch is transferring packets as a function of the work backlog (the number of packets queued at the switch inputs) is then developed. In this way, the model reflects the fact that such a switch tends to function more efficiently as the work backlog increases. This model yields an accurate method for approximating the entire distribution of the work backlog, as well as mean queue lengths and waiting times     

Calibration of tracking systems in a surgical environment

The purpose of this paper was to assess to what extent an optical tracking system (OTS) used for position determination in computer-aided surgery (CAS) can be enhanced by combining it with a direct current (DC) driven electromagnetic tracking system (EMTS). The main advantage of the EMTS is the fact that it is not dependent on a free line-of-sight. Unfortunately, the accuracy of the EMTS is highly affected by nearby ferromagnetic materials. The authors have explored to what extent the influence of the metallic equipment in the operating room (OR) can be compensated by collecting precise information on the nonlinear local error in the EMTS by using the OTS for setting up a calibration look-up table. After calibration of the EMTS and registration of the sensor systems in the OR the authors have found the average euclidean deviation in position readings between the DC tracker and the OTS reduced from 2.9±1.0 mm to 2.1±0.8 mm within a half-sphere of 530-mm radius around the magnetic field emitter. Furthermore the authors have found the calibration to be stable after re-registration of the sensors under varying conditions such as different heights of the OR table and varying positions of the OR equipment over a longer time interval. These results encourage the further development of a hybrid magnetooptical tracker for computer-aided surgery where the electromagnetic tracker acts as an auxiliary source of position information for the optical system. Strategies for enhancing the reliability of the proposed hybrid magnetooptic tracker by detecting artifacts induced by mobile ferromagnetic objects such as surgical tools are discussed     

Hot-spot traffic relief in cellular systems

By analyzing mathematical models, it is shown that combining channel borrowing with a coordinated sectoring or overlying scheme provides effective ways to handle hot-spots in the system. Blocking probabilities with these arrangements are derived, and the dynamic sharing with bias (DSB) rule is suggested for increasing the trunking efficiency. A simple handoff model is formulated and analyzed for comparing the probabilities of additional handoffs due to sectoring and overlaying of cells. With the nominal allocation of 60 channels per cell and a donor cell having a load of 30 Erlangs, numerical results show that at a blocking requirement of 1%, the traffic load in the hot-spot cell can be increased from 47 to 63 Erlangs with the use of the channel borrowing with the cell sectoring scheme: while with the use of the DSB rule, the load can be increased further to 71 Erlangs. A slightly higher load can be carried in the hot-spot cell with the use of cell overlaying arrangement     

Approximate availability analysis of VAXcluster systems

The availability of an n-processor VAX cluster system is determined using a hierarchical approach that allows: (1) a closed-form answer to an apparently difficult problem to be obtained, and (2) the determination of the optimal number of processors in the cluster for a given set of cluster parameters. The approach is a two-level hierarchical model in which the lower-level model is a nine-state Markov chain that is solved in a closed form. The nine-state Markov chain is then aggregated into a three-state device analogous to a diode. Subsequently, the system availability is computed by analyzing a simple network     

Delivering QoS requirements to traffic with diverse delaytolerances in a TDMA environment

The focus of this paper is on determining the call admission region and scheduling policies for a time-division multiple-access (wireless) system supporting heterogeneous real-time variable bit rate applications with distinct quality of service (QoS) requirements and traffic characteristics. The QoS is defined in terms of a maximum tolerable packet delay and dropping probability. A packet is dropped if it experiences excess delay. The call admission region is established for policies that are work-conserving (WC) and that satisfy the earliest due date (EDD) service criterion (WC-EDD policies). Such policies are known to optimize the overall system performance. In addition to the determination of the call admission region, this study leads also to the construction of scheduling policies that deliver any performance in the region established for WC-EDD policies. Finally, an upper bound on the call admission region that can be achieved under any policy (not limited to the WC-EDD policies) is determined     

Degradation of availability performance in dual-polarized satellitecommunications systems

Dual-polarized operation of satellite communications channels allows for an efficient use of the radio spectrum. A method for evaluating the availability of Ku- and Ka-band dual-polarized digital systems is described. It is based on long-term measurements of slant-path attenuation and depolarization caused by the troposphere and of their joint occurrence. The additional degradation of dual-polarized system availability performance with respect to single polarization operation is assessed, and its dependence on major link and system parameters is highlighted. It appears that satellite links characterized by high-frequency, low elevation angle, circular polarization, and high-level modulation schemes are particularly affected by the depolarization-induced interference. In that case, doubling their capacity by dual-polarization frequency reuse reduces dramatically their availability, assuming a fixed link margin, or alternatively requires prohibitive link margins to meet a given availability objective     

自然环境下交通标志牌的检测与识别

在社会经济飞速发展背景下,汽车的应用得到普及,已进入千家万户,汽车为人们出行带来了便利性,不过频发的交通事故也值得深思。而引起交通事故的原因之一即未及时检测到交通标志牌,故而探索出可快捷进行交通标志牌检测与识别的技术非常重要,该检测结合智能交通技术进行交通标志识别系统的设计,设计的识别方法即经提取标准图片与待识别图片的垂直、水平方位的灰度分布,完成Pearson相关系数计算工作,并通过一定阈值的设定,于待识别图片和所有标准图片里均作Pearson相关系数计算后,相关系数的值为最大,同时比阈值更大则认定成匹配成功来达到提醒与警示驾驶员的作用,将交通事故发生率控制到最低。     

Performance of maximal-ratio diversity systems in a correlatedNakagami-fading environment

The bit error rate (BER) performance of an M-branch maximal-ratio combiner (MRC) for the detection of signals in a correlated Nakagami-fading channel is analyzed. Coherent and incoherent detection of frequency shift-keying (FSK) and phase-shift keying (PSK) signals are considered. It is assumed that the fading parameters in each diversity branch are identical. The effect of correlation is studied by assuming two types of correlation among the quadrature components of the signals in each diversity branch. Outage probabilities are also calculated for the digital radio communication systems via the correlated fading channel     

Analysis of non-uniform cell destination distribution in virtual output queueing systems

We develop a discrete-time analytical queueing model for studying the performance of input-queued switches with non-uniform cell destination distribution. Virtual output queues are assumed at the ingress ports where cell arrivals are geometrically distributed and the service process is based on a work conserving random selection scheme. We consider the conditions for stability as basis for deriving closed-form expressions for the stationary queue size distributions from which the mean queue sizes and mean cell latencies are derived. We show that a very good agreement is obtained between simulation and analytical results.     

基于单目视觉的行车环境安全预警系统设计

文中设计了基于机器视觉的行车环境安全预警系统,采用包含视频图像采集、行车环境检测以及行车环境安全预警3个功能模块的系统结构,重点研究并提出了新的行车环境识别方法,包含车道线提取和前方车辆检测方法,并对系统硬件架构及软件流程和算法进行说明,实现高速公路行车环境的识别并进行危险警示。实验结果显示,系统能够准确地对前方车道线和车辆进行检测,实现设计效果。     

Reliability and availability analysis of transit systems

This paper presents three newly developed Markov models representing on-surface transit systems. Transit system reliability, steady-state availability, mean time to failure (MTTF) and variance of time to failure formulas are developed. Selective plots are shown for each model. These plots clearly exhibit the impact of various parameters on transit system reliability, steady-state availability, and MTTF.     

DELAY JITTER OF A PERIODIC CELL STREAM THROUGH AN ATM MULTIPLEXER IN A BURSTY TRAFFIC ENVIRONMENT

The periodic cell stream is a very important member among the input traffic sources in ATM networks. In this paper, a finite-buffered ATM multiplexer with traffic sources composed of a periodic cell stream, multiple i.i.d Bernoulli cell streams and bursty two-state Markov Modulated Bernoulli Process (MMBP) cell streams is exactly analyzed. The probability mass function of queuing delay, the autocorrelation and power spectrum of delay jitter for this periodic cell stream are derived. The analysis is used to expose the behavior of delay jitter for a periodic cell stream through an ATM multiplexer in a bursty traffic environment. The simulation results indicate that the analytical results are accurate.     

DRIFS-TDMA: a proposal for a satellite access distributed-control algorithm for multimedia traffic in a faded environment

Most demand assignment time division multiple access (TDMA) satellite access protocols use centralized-control access schemes, rather than distributed ones, because their simplicity and robustness usually compensates for the longer allocation delay. Starting from the fifo ordered demand assignment/information bit energy adapter (FODA/IBEA) centralized-control protocol, we studied two distributed-control protocols, named distributed allocation with request in fixed slots (DRIFS) and faded environments effective distributed engineering redundant signalling (FEEDERS) respectively, for accessing a geostationary satellite channel. Multimedia traffic and faded environments were considered in the study of both access schemes. This paper presents the DRIFS proposal, together with the recovery procedures from critical events, the handling of which is central to the discussion of a distributed satellite access protocol. Probabilities of such events are also estimated. © 1997 John Wiley & Sons, Ltd.     

Computation of minimal-order realizations of generalized state-space systems

In this paper we consider the problem of obtaining minimal-order representations of generalized state-space systems described by equations of the formE x(t)=A x(t)+B u(t),y(t)=C x(t)+D u(t) withE singular and det(sE–A)0. The underlying principle is that of removal of impulsive and exponential uncontrollable and unobservable modes. When this is followed by the removal of the remaining impulsive modes, we get a minimal-order generalized or standard state-space representation. Simple reduction procedures and numerical algorithms based on these principles are developed and illustrated by means of two numerical examples.P. Misra's research was supported in part by AFWAL, Wright Patterson Air Force Base under Grant F33615-88-C-3605. R. V. Patel's research was supported by NSERC of Canada under Grant A1345.