带有服务中断的周期到达队列的模拟仿真

文中运用连续时间离散化的思想,设计了一种带有服务中断的多服务器系统的仿真算法。将周期到达队列模型与服务中断结合起来,在高负荷的条件下,通过Matlab编程,模拟带有服务中断的Mt/M/n模型,为多服务台排队模型的模拟仿真提供了一种新思路。通过修改随机数产生方法和限制条件模型可以拓展到G/M/n+M,G/M/n/K+M模型等。     

广义数据包到达的时延优化研究

在无线传感网中,Choobkar算法与Dilmaghani算法分析仅适用于在较高优先级节点空时隙时,较低优先级的节点总是有数据包要发送的情况。为此,提出了一种广义数据包到达算法(GPA)的时延分析。算法适用于数据包到达的任何模式,其中包括在较高优先级节点空时隙的情况下,较低优先级的节点可能或可能不具有待发送数据包。通过仿真验证,对比Choobkar算法和Dilmaghani算法,GPA算法的时延分析在整个负载范围内具有较好的准确度。     

Characterizing the variability of arrival processes with indexes ofdispersion

The author proposes to characterize the burstiness of packet-arrival processes with indexes of dispersion for intervals and counts. These indexes, which are functions of the variance of intervals and counts, are relatively straightforward and can estimate and convey much more information than simpler indexes (such as the coefficient of variation) that are often used to describe burstiness quantitatively. The author defines and evaluates the indexes of dispersion for some of the simple analytical models that are frequently used to represent highly variable processes. The indexes for a number of measured point processes that were generated by workstations communicating to file servers over a local area network are also estimated. It is shown that nonstationary components in the measured packet-arrival data distort the shape of the indexes, and ways to handle nonstationary data are proposed. To show how to incorporate measures of variability into analytical models and to offer an example of how to model the measured packet-arrival processes, the author describes a fitting procedure based on the index of dispersion for counts for the Markov-modulated Poisson process     

Link quality estimation for arbitrary packet sizes over wireless links using packet reception events

The packet error rate of wireless links is known to increase with the length of packets. Yet, packet length is rarely taken into account in protocols and algorithms estimating the packet error rate. Still, it is an important factor that higher layer protocols need to be aware of. In this article, we systematically measure the relationship between packet length and packet error rate over a wide range of wireless links and technologies. On the basis of our measurements, we propose a simple empirical model that can capture this behavior. Using this model, multiple methods are proposed that can estimate the packet error rate for any packet length by sampling the link. We consider methods based on hello packets with controlled packet lengths as well as data packets, where the transmitted packet lengths cannot be controlled. We investigate the accuracy of the different estimation methods in various situations and show how they are able to predict the delivery ratio for different packet sizes.     

The superposition of periodic cell arrival streams in an ATMmultiplexer

The authors consider the queue arising in a multiservice network using ATM (asynchronous transfer mode) when a superposition of periodic streams of constant-length cells is multiplexed on a high-speed link. An exact closed formula is derived for the queue length distribution in the case where all streams have the same period, and tight upper and lower bounds are obtained on this distribution when the periods are different. Numerical results confirm that the use of a Poisson approximation (i.e. the M/D/1 queue) can lead to a significant overestimation of buffer requirements, particularly in the case of heavy loads. Buffer requirements for a mixture of different period streams can be accurately estimated from the upper bound on the queue length distribution. For given load, requirements increase with the number of long-period (i.e. low-bit rate) sources. The results are deduced from a novel characterization of the single-server constant service time queue, which should be useful in other applications     

Analysis of packet loss processes in high-speed networks

The packet loss process in a single-server queueing system with a finite buffer capacity is analyzed. The model used addresses the packet loss probabilities for packets within a block of a consecutive sequence of packets. An analytical approach is presented that yields efficient recursions for the computation of the distribution of the number of lost packets within a block of packets of fixed or variable size for several arrival models and several numbers of sessions. Numerical examples are provided to compare the distribution obtained with that obtained using the independence assumption to compute the loss probabilities of packets within a block. The results show that forward error correction schemes become less efficient due to the bursty nature of the packet loss processes; real-time traffic might be more sensitive to network congestion than was previously assumed; and the retransmission probability of ATM messages has been overestimated by the use of the independence assumption     

Using a packet switch for circuit-switched traffic: a queueingsystem with periodic input traffic

Consideration is given to the effects of time-multiplexed stream traffic on the performance of a store-and-forward packet switch. Substantially reducing the amount of buffering in the switch results in only a small probability that an existing circuit will be disrupted during the length of its connection. For example, with a circuit-switched frame of length 1000 and 100% loading, reducing the buffer size from 999 packets to 83 results in only a 10^-6 circuit-disruption probability     

SR ARQ packet delay statistics on markov channels in the presence of variable arrival rate

In this letter we investigate the packet delay statistics of a fully reliable selective repeat ARQ scheme by considering a discrete time Markov channel with non-instantaneous feedback and assigned round-trip delay m. Our focus is on studying the impact of the arrival process on the delay experienced by a packet. An exact model is introduced to represent the system constituted by the transmitter buffer, the m round-trip slots, and the channel state. By means of this model, we evaluate and discuss the delay statistics and we analyze the impact of the system parameters, in particular the packet arrival rate, on the delay statistics     

Linear autoregressive processes with periodic structures as models of information signals

Linear autoregressive processes with periodic structures have been considered. Certain properties of such processes that can be used in developing the recognition algorithms of different types of information signals were also presented.     

Performance analysis of a dynamic handoff scheme in wireless networks with heterogeneous call arrival processes

The Guard Channel Scheme (GCS) and Handoff Queueing Scheme (HQS) are the popular and practical strategies to prioritize handoff calls in wireless cellular networks. A key issue of giving handoff calls the higher priority is how to achieve a tradeoff among the handoff call blocking probability, new call blocking probability and handoff delay. This paper extends GCS and HQS and presents an efficient handoff scheme that dynamically manages the channels reserved for handoff calls depending on the current status of the handoff queue. A three-dimensional Markov model is developed to analyze the performance of this scheme and investigate the desirable performance tradeoff. The Poisson process and Markov-Modulated-Poisson-Process (MMPP) are used to model the arrival processes of new and handoff calls, respectively. The accuracy of this model is evaluated through the extensive comparison of the analytical results to those obtained from discrete-event simulation experiments. Performance measures in terms of the mean number of calls in the system, aggregate response time, aggregate call blocking probability, handoff call blocking probability, new call blocking probability and handoff delay are evaluated. The analytical model is used to investigate the effects of the number of channels originally reserved for handoff calls, the number of dynamic channels, and the ratio of the rate of handover calls to the aggregate arrival rate on the system performance.     

Throughput of DS‐CDMA/unslotted ALOHA radio networks with Markovian arrival processes

Previous works on the throughput analysis of the direct sequence‐code division multiple access/unslotted ALOHA radio network all used the Poisson arrival process (PAP). However, the interarrival times of PAP are independent, so it is not suited to model today's Internet and multimedia traffic, which have correlated interarrival times. We are motivated to use the Markovian arrival process (MAP), a more general input traffic model that captures the correlation of interarrival times. We are the first to analyze the throughput of the direct sequence‐code division multiple access/unslotted ALOHA radio network with MAP. We propose the use of MAP, which encompasses the PAP as a special case. The new MAP model basically generalizes the current traffic and queuing models of multimedia in wireless networks. Copyright © 2011 John Wiley & Sons, Ltd.     

A six-junction GaAs laser power converter with different sizes of active aperture

We investigate a novel GaAs-based laser power converters (LPCs) grown by metal-organic chemical vapor deposition (MOCVD), which uses a single monolithic structure with six junctions connected by tunnel junctions to obtain a high output voltage. The LPCs with diameters of active aperture of 2 mm and 4 mm were fabricated and tested. The test results show that under an 808 nm laser, two LPCs both show an open circuit voltage of above 6.5 V. A maximum power conversion efficiency of 50.2% is obtained by 2 mm sample with laser power of 0.256 W, and an output electric power of 1.9 W with laser power of 4.85 W is obtained by 4 mm sample. The performances of the LPCs are deteriorated under illumination of high flux, and the 4 mm sample shows a higher laser power tolerance.     

A method of analysis of periodic processes in nonautonomous discrete-time systems with quantization

A method of analysis of stationary processes in systems with an arbitrary number of quantization levels in the presence of periodic external signals is proposed. The method is based on the representation of stationary oscillations in the form of an invariant set of nonlinear discrete point mappings. The spectral composition of the response of a system with an arbitrary period coupled with the discretization period through a rational relationship is calculated with the use of the discrete Fourier transform. The discrete Fourier transform is applied also to calculate nonlinear distortions. It is suggested to use the dependence of the ratio of the first-harmonic amplitude of the system’s steady-state response to the amplitude of the input-signal harmonic on the frequency of the input harmonic signal as the amplitude-frequency characteristic of a nonlinear system. In addition, it is suggested to use the dependence of the difference between the argument of the first harmonic of the system’s response and the initial phase of the input signal on the frequency of the input harmonic signal as the phase-frequency characteristic of the system. The usage of the method is illustrated by its application for the analysis of processes in first-and second-order circuits.     

不同电极尺寸对光电二极管电流的影响

基于Sentaurus TCAD 软件对n-on-p型Hg1-xCdxTe红外探测器器件结构进行建模,并在不同电极尺寸条件下对器件的光电流进行仿真。通过仿真发现随着像元电极尺寸的减小,光电二极管反向电流也逐渐减小。针对这一现象从金属层对冶金结电势分布的影响和被吸收的光子数目Q两个角度进行了分析。金属层对半导体材料表面的电势分布具有调制作用,随着电极尺寸的减小,二极管的反向电流减小;随着像元电极尺寸的减小,被吸收的光子数目减小,导致光电二极管反向电流减小;以上两个方面都会引起光电二极管电流随着电极尺寸的减小而减小。     

基于正交空分复用的多用户调度策略

为了消除多用户MIMO下行系统的多用户间干扰以及改进系统的误码率性能,研究了块对角化预编码与几何均值分解的联合方案(BD-GMD).针对BD-GMD系统的资源分配和用户调度,对比了等功率分配和注水算法对系统的影响,并基于系统容量最大化,提出了一种根据用户信道的子空间特性的低复杂度的用户调度算法.此外,对比分析了穷举搜索算法和传统的贪婪算法.数值仿真结果表明,文中所提出的基于正交投影的多用户调度算法充在保证系统容量的同时降低了算法复杂度.     

Modeling (almost) periodic moving average processes using cyclicstatistics

Estimating parameters of almost cyclostationary non-Gaussian moving average (MA) processes using noisy output-only data is considered. It is shown that second-order cyclic correlations of the output are generally insufficient in uniquely characterizing almost periodically time-varying MA(q) models, while third-order and higher order cumulants can be used to estimate their model parameters within a scale factor. Both linear and nonlinear identification algorithms for fixed and time-varying order q(t) are presented. Statistical model order determination procedures are also derived. Implementation issues are discussed and resistance to noise is claimed when the signal of interest has cycles distinct from the additive noise. Simulations are performed to verify the theoretical results     

基于有限反馈的数据流数自适应SDMA

针对目前绝大部分有限反馈自适应SDMA由于使用固定满波束发送策略而导致在稀疏网络中严重的用户间干扰的问题,该文提出了一种基于有限反馈的数据流数自适应SDMA,该方案能通过自适应地选择并行发送波束的数目来达到有效控制用户间干扰的目的。具体实现如下：通过用户的有限反馈信息,以及信道的二阶统计信息,基站端使用约束最大似然估计的方法估计出使用不同数目发送波束的发送模式所支持的最大和吞吐量,从而自适应地选择最优的发送模式。仿真结果表明：该文提出的方案能在增加很少反馈量的前提下显著改善现有SDMA方案的性能。     

Multiplexing and double band filtering with common-multimodecavities

A novel multiplexing and double band filtering (DBF) method is established. DBF refers to two-port equipment which serves two transmit frequency channels simultaneously. The methods build on frequency-independent resonance modes, excited within a common multimode cavity (CMC), which are assigned to different channel filters. The design principles are presented for cylindrical CMCs by way of example. It is shown that combining CMCs with advanced coupling methods produces a multitude of new design variants. Initial experimental results are presented for a diplexer and DBF equipment that are both realized with only two double-dual TE₁₁₂/TM₁₁₀ CMCs and a novel iris-providing four intercavity couplings-to perform two four-pole elliptic function filter responses     

Estimation of equivalent bandwidth without modeling arrival processes for connection admission control in an ATM multiplexer

The traffic modeling is a key element in analyzing and simulating communications network. Most of the previously studied approaches to the connection admission control (cac) are based on an assumed arrival process model. However, there are some limits to apply the assumed model to the real environment. In this paper, we study a connection admission control (cac) method without assuming arrival processes in an atm multiplexer when homogeneous traffics are multiplexed. To obtain the equivalent bandwidth (eb) without modeling arrival processes, the concept of equivalent bandwidth convergence (ebc) is proposed and implemented by using the upper bound of the first derivative of cell loss rate (clr) for the load. With the ebc concept, one can avoid modeling of the arrival process and implement the cac simply and effectively. We show by numerical examples the convergence process of the average clr and the eb for voice and video traffics, and the robustness of the ebc concept.     

Diversity and Multiplexing Tradeoff of Spatial Multiplexing MIMO Systems With CSI

Following the seminal work of Zheng and Tse, this paper investigates the fundamental diversity and multiplexing tradeoff of multiple-input-multiple-output (MIMO) systems in which knowledge of the channel state at both sides of the link is employed to transmit independent data streams through the channel eigenmodes. First, the fundamental diversity and multiplexing tradeoff of each of the individual substreams is obtained and this result is then used to derive a tradeoff optimal scheme for rate allocation along channel eigenmodes. The tradeoff of spatial multiplexing is finally compared to the fundamental tradeoff of the MIMO channel and to the one of both space only codes and V-BLAST which do not require channel state information (CSI) at the transmit side.