OMNeT++网络仿真器的设计原理分析

简要介绍离散事件模拟器OMNeT 相对于其他仿真器的一些特点,着重介绍其对网络拓扑描述、网络事件处理以及仿真结果分析的支持,并对仿真程序的实现给出了具体步骤。最后结合对ALOHA协议的仿真实例,使用Plove工具分析了网络的吞吐性能。     

改进ALOHA算法在RFID多目标识别中的应用

RFID系统中,由于多标签引起的冲突一直是影响系统性能的大问题。ALOHA算法是运用比较普遍的一种防冲突算法,在标签数目较少的情况下,现有的ALOHA算法也有较高的效率,但是随着标签数目的急剧增加,算法时间也成指数往上增长,本文提出一种改进的ALOHA算法,在标签总数未知的情况下,动态估计标签数目,以选择合适的窗口大小,获得较高的系统效率。仿真结果表明,跟传统ALOHA算法相比,性能有明显改善。     

First Passage Time Analysis in Wireless Sensor Networks for SPR and MPR Systems

In this paper, we study the performance of Slotted ALOHA in an elementary process in wireless sensor networks, in which sensors are activated by rare events and transmit packets to a single sink node within single hop. The time cost of the process can be viewed as a first passage time and we give the expectation of time and energy cost which is dependent on both the size of active nodes and their transmission rate by a first passage time analysis. Meanwhile, given the size of active nodes n, we demonstrated that the optimal transmission rate λ_min(n) by which the first passage time attains its minimum, is nearly . Our result indicates that time cost of the process in which each node transmits packet with transmission rate λ_min(n) increases nearly linearly with n increasing. We present the maximum likelihood estimate (MLE) of the size of active nodes from the viewpoint of sink nodes so that activated nodes can adjust its transmission rate to improve the performance of the process. Energy dissipation and time cost of the procedure in the channel model of Multi Packet Reception (MPR) are considered also. Numerical results indicate that both time cost and energy consumption of the procedure in MPR channel is superior to that in Single Packet Reception (SPR) channel while the transmission rate near or less than the optimal values with which time costs attain their minimum. Otherwise the energy dissipation in MPR channel is more than that in SPR channel although comparison of time cost in the above two channels reveals the superiority of MPR channel.

低轨卫星移动通信中扩频S—ALOHA协议吞吐性能分析

由于扩频具有良好的抗多径干扰的能力，因此，在地面移动无线系统中被认为是一种较好的多址方式。但在低轨卫星移动系统中，其传播条件恶劣，各种因素对系统的吞吐性能的影响各不一样，本文分析了多径衰落，扩频增益，同步捕获等因素对吞吐性能的影响，得出了解析式，计算了数值曲线并进行了讨论     

具有多重码的预约Slotted ALOHA协议性能分析

本文对具有多重码的预约Slotted ALOHA协议进行了性能分析。它是基于帧的协议且在一个时隙内有多重码可用来传输分组信息。文中利用离散时间,离散状态和Markov链来分析一个小区上行链路的话音分组吞吐量和数据分组延时性能,并提出了两种改进碰撞解决的方法,得到了较好的系统特性。     

Performance evaluation of random power capture on mobile communications

Power assignment schemes are man‐made methods to enhance the capture effect of radio communications. In a previous study, Wen and Yang investigated the combined capture effect of the fixed power assignment scheme, Rayleigh fading, and near–far effect on the performance of packet radios. The performance analysis was limited to an infinite population environment. This assumption is reasonable for a conventional packet radio system with a vast service area. However, for a cellular mobile system, a finite population model should be used. In this paper, we analyse the combined natural and man‐made capture effect on the performance of a cellular system with finite population in each cell. A random power assignment scheme is adopted to produce the man‐made capture. The system throughput and delay are carried out by a Markov model. Some numerical calculations are used to demonstrate the degree of performance improvement. Copyright © 2001 John Wiley & Sons, Ltd.     

基于分组动态帧时隙ALOHA防碰撞算法研究

为解决射频识别系统中多标签防碰撞问题,在现有ALOHA算法的基础上提出了一种改进的分组动态帧时隙ALOHA算法。当大量标签同时进入阅读器识别范围内时,算法通过设置一个阈值把要响应的标签分成两组,符合条件的一组去响应阅读器,不符合条件的暂时不响应,该算法通过分组限制响应标签数量达到较高的识别效率。仿真结果表明,该算法在标签数大于256甚至更多时识别效率也能维持在相对较高的数值。     

RFID系统防碰撞算法的研究及其改进

射频识别技术中,防碰撞问题是提高系统读取效率的关键问题。文章在传统防碰撞算法的基础上,分析了ALOHA的防碰撞算法和二进制搜索算法。并提出一种改进的动态二进制搜索算法。利用曼彻斯特编码可以准确识别碰撞位的特性,当读写器检测到碰撞位之后,仅需要记录最高碰撞位和次高碰撞位的位置,并设定这两个位置上的比特数作为下次查询命令,从而使系统的传输数据量、传输时间大大减少,改进后的算法比二进制搜索算法更具优势。     

基于信号强度的分组帧时隙ALOHA防碰撞算法

针对现有ALOHA防碰撞算法存在系统吞吐量低下的问题,利用射频识别阅读器和标签之间的信号能量传输特点,对帧时隙ALOHA算法( FSA)进行改进,提出一种分组帧时隙ALOHA防碰撞算法。根据接收信号能量的强度将标签均匀分布到帧时隙中进行分组应答,从而降低标签碰撞概率。仿真实验结果表明,在标签数与帧时隙数之比小于1.8的情况下,该算法最大系统吞吐量可达50%,优于FSA算法及碰撞分组算法。     

一种改进的帧时隙ALOHA防碰撞算法研究与实现

防碰撞算法是RFID系统中需要解决的关键技术。首先对帧时隙ALOHA防碰撞算法进行了分析,针对帧时隙ALOHA算法的局限性,提出了一种基于分组机制的动态帧时隙算法,并在.NET平台下对原算法和改进算法进行了仿真实验,仿真结果表明,改进后算法可以提高RFID系统吞吐率。