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文献翻译 通信工程专业 中英文 蜂窝无线通信系统的仿真.doc 13页
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号 0座机电话号码
级 文献翻译
蜂窝无线通信系统的仿真
院（系）名称 信息工程学院
专业名称 通信工程
学生姓名 李彬
指导教师 赵春雨 日
蜂窝无线通信系统的仿真1.概述
人们开发出了许多无线通信系统，为不同的运行环境中的固定用户或移动用户提供了接入到通信基础设施的手段。当今大多数无线通信系统都是基于蜂窝无线电概念之上的。蜂窝通信系统允许大量移动用户无缝地、同时地利用有限的射频（radio frequency，RF）频谱与固定基站中的无线调制解调器通信。基站接收每一个移动台发送来的射频信号，并把他们转换到基带或者带宽微波链路，然后传送到移动交换中心（MSC），再由移动交换中心连入公用交换电话网（PSTN）。同样的，通信信号也可以从PSTN传送到基站，再从这里发送个移动台。蜂窝系统可以采用频分多址（FDMA）、时分多址（TDMA）、码分多址（CDMA）或者空分多址（SDMA）中的任何一种技术。
无线通信链路具有恶劣的物理信道特征，比如由于传播途径中有再大的障碍物，会产生时变多径和阴影。此外，无线蜂窝系统的性能还会受限于来自其他用户的干扰，因此，对干扰进行准确的建模就很重要。很难用简单的解析模型来描述复杂的信道条件，虽然有集中模型确实易于解析求解并与信道实测数据比较相符，不过，即使建立了完美的信道解析模型，再把差错控制编码、均衡器、分集及网络模型等因素都考虑再链路中之后，要得出链路性能的解析在绝大多数情况下任然是很困难的甚至是不可能的。因此，在分析蜂窝通信链路的性能时，常常需要进行仿真。
跟无线链路一样，对蜂窝无线系统的性能分析使用仿真建模时很有效的，这是由于在时间和空间上对大量的随机事件进行建模非常困难。这些随机事件包括用户的位置、系统中同时通信的用户个数、传播条件、每个用户的干扰和功率级的设置（power level setting）、每个用户的话务量需求等，这些因素共同作用，对系统中的一个典型用户的总的性能产生影响。前面提到的变量仅仅是任一时刻决定系统中的某个用户瞬态性能的许多关键物理参数中的一小部分。蜂窝无线系统指的是，在地理上的服务区域内，移动用户和基站的全体，而不是将一个用户连接到一个基站的单个链路。为了设计特定大的系统级性能，比如某个用户在整个系统中得到满意服务的可能性，就得考虑在覆盖区域内同时使用系统的多个用户所带来的复杂性。因此，需要仿真来考虑多个用户对基站和移动台之间任何一条链路所产生的影响。
链路性能是一个小尺度现象，它处理的是小的局部区域内或者短的时间间隔内信道的顺时变化，这种情况下可假设平均接收功率不变。在设计差错控制码、均衡器和其他用来消除信道所产生的瞬时影响的部件时，这种假设时合理的。但是，在大量用户分布在一个广阔的地理范围内时，为了确定整个系统的性能，有必要引入大尺度效应进行分析，比如在大的距离范围内考虑单个用户受到的干扰和信号电平的统计行为时，忽略瞬时信道特征。我们可以将链路级仿真看作通信系统性能的微调，而将系统级仿真看作时整体质量水平粗略但很重要的近似，任何用户在任何时候都可预计达到这个水平。
通过让移动台在不同的服务区内共享或者复用通信信道，蜂窝系统能达到较高的容量（比如，为大量的用户服务）。信道复用会导致公用同一信道的用户之间产生同频干扰，这是影响蜂窝系统容量和性能的主要制约因素之一。因此，在设计一个蜂窝系统时，或者在分析和设计消除同频干扰负面影响的系统方法时，需要正确理解同屏干扰对容量和性能的影响。这些影响主要取决于通信系统的状况，如共享信道的用户数和他们的位置。其他与传播信道条件关系更密切的方面，如路径损耗、阴影衰落（或叫阴影）、天线辐射模式等对系统性能的影响也很重要，因为这些影响也岁特定用户的位置而改变。本章我们将讨论在同频干扰情况下，包括一个典型系统中的天线和传播的影响。尽管本章考虑的例子比较简单，但提出的分析方法可以容易地进行扩展，以包括蜂窝系统的其他特征。
2.蜂窝无线系统
系统级描述：
如图1所示，通过把地理区域分成一个个称为小区的部分，蜂窝系统可以在这个区域内提供无线覆盖。把可用的频谱也分成很多信道，每个小区分配一组信道，每个小区中的基站都配备了可以同移动用户进行通信的无线调制解调器。从基站到移动台这个发送方向使用的射频信道称为前向信道，而从移动台到基站这个发送方向使用的信道称为反向信道。前向信道和反向信道共同构成了双工蜂窝信道。当使用频分双工（FDD,frequency division duplex）时，前向信道和反向信道使用不同的频率；当使用时分双工时（TDD,time division duplex）时，前向信道和反向信道占用相同的频率，但使
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通信的一些专业的英语缩写,求汉语请把这段话的英文缩写翻译成中文（4G移动通信方面的,很多缩写有多个意思,我有点搞不懂才问高手的）,对应一下就行了.“用户速率和MCS及占用的RB数量相关MCS取决于SINR值DOA 两个系统基站天线摆放时需要满足MCL大于61dB.c/I能力ICICPRACH用户的RAKE反馈、CQI反馈包括小区间的PRAcH干扰、PSS干扰、SSS干扰、PBCH干扰、PDCCH干扰、PHlCH干扰、PCFICH干扰等与部分频率复用和软频率复用对一组连续的PRB采用统一的资源使用和发射功率限制不同,全频率复用对时频资源的使用和发射功率的限制以PRB为单位PRAcH(物理随机接入信道)、Pu ccH(物理上行控制信道)sRs(探测用参考信号),下行调度用户数主要受限于PcFIcH信道、PHICH信道和PDCCH”当然了,这段话并不是一段话,只是便于理解,摘抄的组成了一段
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MCS主控装置（Master Control Set）RB无线电信标（Radio Beacon)SINR信号与干扰和噪声比（Signal
to Interference plus Noise Ratio）DOA波达方向（direction of arrival ）MCL最小耦合损耗（Minimum Coupling Loss）C/I - 保险证明书（Certificate Of Insurance）ICIC小区间干扰协调,又称“软频率复用”（Inter-Cell Interference Coordination）PRACH 物理随机接入信道(Physical Random Access Channel)RAKE不是英文缩写,因此也没有全名.一般在通讯领域指RAKE接收机.RAKE接收机是一种能分离多径信号并有效合并多径信号能量的最终接收机CQI信道质量指示符(Channel Quality Indicator)PSS分组交换服务（Packet Switching Service）SSS交换子系统（Switching Sub-system）PBCH物理广播信道(Physical Broadcast CHannel)PDCCH物理下行控制信道(Physical Downlink Control CHannel)PHlCH物理HARQ指示信道(Physical HARQ Indicator CHannel)PCFICH物理控制格式指示信道(Physical Control Format Indicator CHannel)PUCCH物理上行控制信道(Physical Uplink Control CHannel)PRB物理资源块（Physical Resource Block ）
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For adaptation of coding and modulation, our protocol can use an error count or iteration count, each of which can be obtained easily for each packet that is decoded correctly. A high rate CRC code is used to verify the correctness of the decoded packets. One way to obtain the error count is to encode the information bits from a correctly decoded packet and compare t保密 with the binary representations of the demodulator hard decisions for the packet. The intent is to count the number of binary symbol errors that would he occurred if there were no decoder in the receiver. The protocol also uses a demodulator statistic to adapt the power if further adaptation of coding and modulation cannot offset an increase that has occurred in the propagation loss. The error count for a packet that decodes correctly is the number of binary symbol errors at the output of the demodulator. The decoder can easily report the number of iterations that it made for a packet that has been decoded and verified for correctness. A demodulator statistic is used by our protocol for certain secondary modes of operation, such as the power-adjustment phase at the start of a session that will use a new frequency band. The iteration count is the erage number of decoder iterations per packet among the packets that decode correctly
对于编码和调制的自适应来说. The iteration count is the erage number of decoder iterations per packet among the packets that decode correctly。 其意图是计数如果在接收器中没有解码器时可能已经发生的二进制符号错误的数目，我们的协议可以采用一个错误计数或重复计数，它们中的每一种对于正确解码的每个包来说都可容易获得。对应于一个被正确解码的包的错误计数就是在解调器输出端处的二进制符号错误。得到错误计数的一种方法是将来自一个正确解码的包的信息位译码，并将它们与解调器对包的硬判决的二进制表示进行比对For adaptation of coding and modulation, our protocol can use an error count or iteration count, each of which can be obtained easily for each packet that is decoded correctly. A high rate CRC code is used to verify the correctness of the decoded packets. The protocol also uses a demodulator statistic to adapt the power if further adaptation of coding and modulation cannot offset an increase that has occurred in the propagation loss. One way to obtain the error count is to encode the information bits from a correctly decoded packet and compare t保密 with the binary representations of the demodulator hard decisions for the packet. The intent is to count the number of binary symbol errors that would he occurred if there were no decoder in the receiver，例如一个将采用新的频带的会话（作业阶段）开始时的功率调节方面。如果编码和调制的进一步自适应不能弥补传播损耗业已产生的增加，此协议还会采用一种解调器统计来适应此功率. The error count for a packet that decodes correctly is the number of binary symbol errors at the output of the demodulator。重复计数是在正确解码的所有包中每一个包的解码器重复（迭代） 的平均数。解码器会容易地报告出它为一个业已解码并已验证了准确性的包所进行的重复（迭代）数。一种解调器统计被我们的协议所采用而用于一定的二次（辅助）工作模式. The decoder can easily report the number of iterations that it made for a packet that has been decoded and verified fo......方式之一，以获取错误计数编码比特的信息从一个正确的解码包和比较它们的二进制申述的解调艰难的决定的数据包。高利率CRC码是用来验证正确的解码包。错误计数的数据包解码是正确的数目的二进制符号错误的输出解调器，会议将使用新的频段。其意图是，计数的二进制代码错误可能会出现如果没有解码器的接收器。迭代计数的平均数目迭代解码器每包之间的数据包解码正确。解码器可以很容易地报告迭代次数，它为一包已被破译和核实的正确性。解调统计是我们所使用的协议的某些中学的操作模式，如电源调整阶段开始时为适应编码和调制，我们的协议可以使用一个错误计数或迭代计算，每个可轻松地为每个数据包解码是正确的
告诉你方法吧！以后可以自己翻译！百度上输入 在线翻译，搜索到的第一个网站就是，把你的文章复制过去直接就出来中文了……
对于编码和调制的适应来说， 我们的草案能使用一个错误数或者迭代数，每个数都可以容易地从被正确地译解的信息包中获得。 一条高的比率科林斯无线电公司代码用来证实被译解的信息包的正确性。被正确地译解的信息包中的错误数据是在解调器输出时的二进制符号。 获得错误数据的一种方法是把被正确译解的信息包的讯息进行编码并用解调器的二进制进行比较。这样做的目的是如果在接收者里没有译码器，就去计算会发生的二进制的符号错误的数量。迭代数字是在译码器中每一个信息包被正确地译解出来的反覆出现的平均数。译码器能容易报告迭代数的数量,这样有利于那些已被正确的译解和检验的信息包。由於我们的协议,解调器统计被用于一些次要工作模式，例如在会议一开始时的功率调整阶段,将使用一根新频率带。如果更多的编码和调制的改写不能补偿已经在传播方面增加的损失，草案也使用解调器统计去改写功率 。
为适应编码和调制，我们的草案能使用一个错误数或者迭代数，每个数都可以轻松地从被正确地译解的数据包中获得。一条高的比率科CRC代码用来证实被译解的数据包的正确性。被正确地译解的数据包中的错误数据是在解调器输出时的二进制符号。 获得错误数据的一种方法是把被正确译解的数据包的信息进行编码并与解调器的二进制进行比较。这样做的目的是如果在接收者里没有译码器，就去计算会发生的二进制的符号错误的数量。迭代数字是在译码器中每一个信息包被正确地译解出来的反覆出现的平均数。译码器能轻松的报告迭代数的数量,这样有利于那些已被正确译解和检验的数据包。由于我们的协议,解调器统计被用于一些次要工作模式，例如在会议一开始时的功率调整阶段,将使用一根新频率带。如果更多的编码和调制的改写不能补偿已经在传播方面增加的损失，草案也使用解调器统计去改写功率 。 看到眼都花。。希望对你有帮助..
此答复请楼主绝对放心，我看你也是十级高手，定能明鉴。 COGNITIVE radios are ideally suited for use in dynamic spectrum access networks in which there may be large variations in channel conditions from one session to the next. Such variations are common in networks that operate in a fixed frequency band, but the variations are more severe if the frequency band is changed for consecutive sessions. Each radio in a dynamic spectrum access network must be aware of its communication environment, and it must provide the information that other radios need in order to communicate with it efficiently. This information should besimple, easy to derive, and easy to send to neighboring radios. We focus on the information needed to adapt the error-control coding, modulation, and transmitter power for half-duplex packet transmissions. 认知无线电能理想地适用于动态谱接入（访问）网，在这种网络中，信道条件从一个会话（作业阶段）到下一个会话可能存在很大变化。这样的变化在以固定频带工作的网络中是常见的，但是如果频带对连续的会话来说是变化的，那么这样的变化就会更严重。在一个动态谱接入网中的每一无线电必须感知到自己的通信环境，而且它必须提供其它无线电所需的信息，以便与其有效地通信。此信息应该是简单的，容易导出的，以及容易发送到临近无线电的。我们重点讨论为适应错误控制编码、调制、以及半双工包传输用发射机功率所需的信息。 A new session begins when one radio, referred to as the source, has a collection of packets to send to another radio, the destination. At the start of a new session, which may be in a different frequency band than the previous session, the protocol must adjust the transmitter power to provide reliable communications with minimal energy consumption and minimal interference to other radios. As the session progresses, the protocol must adjust the trans......
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本科毕业论文翻译精华(4G异构网络的切换.翻译版)
Handoffs in Fourth Generation Heterogeneous NetworksNidal Nasser, University of Guelph Ahmed Hasswa and Hossam Hassanein, Queen’s University4G异构网络的切换加拿大安大略省圭尔夫大学 加拿大安大
略省皇后大学ABSTRACTAs mobile wireless networks increase in popularity and pervasiveness, we are faced with the challenge of combining a diverse number of wireless networks. The fourth generation of wireless communications is expected to integrate a potentially large number of heterogeneous wireless technologies in what could be considered a huge step forward toward universal seamless access. One of the main challenges for seamless mobility is the availability of reliable horizontal (intrasystem) and vertical (intersystem) handoff schemes. Efficient handoff schemes enhance quality of service and provide flawless mobility. This article presents different and novel aspects of handoff and discusses handoff related issues of fourth generation systems.Desirable handoff features are presented. Handoff decisions, radio link transfer, and channel assignment are described as stages of the complete handoff process. A vertical handoff decision function, which enables devices to assign weights to different network parameters, is also presented.摘要移动无线网络日益盛行之时，不同网络之间融合的挑战摆在面前。4G无线通信整合了大多数异构网络的无线技术，在走向通用的无缝 接入方面迈向了一大步。无缝移动通信面临的主要挑战之一是可靠的水平(网间)切换和垂直(网内)切换机制的有效性。有效的切换机制提高 了服务质量提供了无缝移动通信。本文提出了新颖不同的切换并讨论了4G切换的相关问题。本为给出了令人满意的切换特性。切换判决， 无线链路传输和信道分配在整个切换过程中分步描述。本文还给出了一种能够让终端去分配权重给不同网络参数的垂直切换判决函数。INTRODUCTION1.介绍First, second- and third-generation mobile systems depended on the employment of the radio spectrum that was either unlicensed (available for public use) or licensed for use by a very small number of service providers and network operators in each region. Differences in bandwidth and coverage areas have led to the necessity of developing multi-network interface devices (terminals) that are capable of using the variety of different network services provided. 首先，第二代第三代移动通信系统依赖于无线频谱的利用，这些频谱要么被公共使用要么被批准在各个地区由一些小数量的服务提供 商和网络运营商使用。带宽和覆盖范围的差异导致发展多网络接口终端的必要性，多网络接口终端可以使用不同网络服务提供的多样性。 The fourth generation (4G) of wireless communications is expected to integrate a potentially large number of different heterogeneous wireless technologies in what could be considered a huge step forward toward universal wireless access and omnipresent computing through seamless mobility [1]. Even though 4G is currently undefined,there are many current outlooks that delineate the vision of the new wireless technologies.Based on the emergent trends of mobile communication, 4G will have larger bandwidth, higher data rates, smoother and quicker handoff, and will focus on reducing faultless service and allowing seamless handoff across a multitude of wireless networks. The key concept is integrating the 4G capabilities with all of the existing mobile technologies. Network management will be necessary among different access systems in terms of horizontal (intrasystem)and vertical (intersystem) handoff as well as seamless mobility, quality of service, dependability,and security. 4G无线通信融合了大多数异构网络的无线技术， 在通用的无缝接入方面和通过无缝移动通信进行泛在计算方面迈向了一大步[1]。即使 4G当前没有明确定义，仍然有很多当前的展望描述了新一代无线技术的景象。基于移动通信浮现的趋势，4G拥有更多的带宽和数率以及更 平滑快捷的切换，集中致力于减少差错服务和允许通过多种无线网络的无缝切换。关键概念是使用已存在的移动技术整合4G性能。网络管 理，对于不同的接入系统如垂直和水平切换还有无缝移动性、服务质量、可靠性和安全性，很有必要。 The remainder of this article is organized as follows. We present a novel classification hierarchy for handoffs. A comprehensive study of various handoff decision factors in heterogeneous wireless networks is explained. We then explain and qualitatively evaluate the proposed vertical handoff decision function (VHDF). We provide a performance evaluation of the described solution.Finally, an article summary is given. 下面的文章内容如下。给出了一种新的用于切换的分类层次。对异构无线网络中各种切换判决因子的全面研究进行解释，解释和定量 地评估本文提到垂直切换函数(VHDF)。提供所述解决方案的性能评估及文章概要。CLASSIFICATION OF HANDOFFS2.切换的分类In principle, each mobile terminal (node) is, at all times, within range of at least one network access point, also known as a base station. The area serviced by each base station is identified as its cell. The dimensions and profile of every cell depend on the network type, size of the base stations,and transmission and reception power of each base station. Usually, cells of the same network type are adjacent to each other and overlap in such a way that, for the majority of time,any mobile device is within the coverage area of more than one base station. Cells of heterogeneous networks, on the other hand, are overlaid within each other. Therefore, the key issue for a mobile host is to reach a decision from time to time as to which base station of which network will handle the signal transmissions to and from a specific host and handoff the signal transmission if necessary. 大体上，每个移动终端(节点)总是归类于至少一种网络接入点即基站。每一个基站的服务区定义为它的小区。每个小区的大小和外形 依赖于网络类型、基站大小、基站的传输和接收功率。通常相同类型的小区以这样一种方式彼此相邻和重叠，大多数时间内任一移动终端 所处的覆盖面积超过一个基站。另一方面，异构网络的小区相互覆盖。因此，移动主机的关键问题是要时不时地做出选择，哪个网络的哪 个基站来处理具体移动主机的信号传输和接收以及必要时切换信号的传输。 We classify handoffs based on several factors as shown in Fig. 1. No longer is the network type the only handoff classification factor. Many more factors constitute categorization of handoffs including the administrative domains involved, number of connections and frequencies engaged. The following are categorization factors along with the handoff classifications that are based on them. 如图1所示，基于几种因素给出了切换的分类。网络类型不再是唯一的切换分类的因素。这个切换的分类因素有很多组成包含涉及的管 理区域、连接和使用的频率数。下图是基于它们的切换类型的分类因素。图 1：切换的分类树FIRST FACTOR: NETWORK TYPES INVOLVED This is the most common classification factor.Handoffs can be classified as either horizontal or vertical. Thisdepends on whether a handoff takes place between a single type of network interface or a variety of different network interfaces. Horizontal handoff: the handoff process of a mobile terminal between access points supporting the same network technology. For example,the changeover of signal transmission (as themobile terminal moves around) from an IEEE 802.11b base station to a geographically neighboring IEEE 802.11b base station is considered as a horizontal handoff process. Vertical handoff: the handoff process of a mobile terminal among access points supporting different network technologies. For example, the changeover of signal transmission from an IEEE 802.11b base station to an overlaid cellular network is considered a vertical handoff process.Horizontal and vertical handoffs are discussed in more detail in the following sections.2.1 因素一：涉及的网络类型：这是最普遍的分类因素。切换能被分为水平切换或者垂直切换。这主要看切换是发生在同种网络类型之间还是不同 网络类型之间。 水平切换：移动终端在支持相同网络技术的接入点之间的切换过程，例如，从802.11b基站到地理上相邻802.11b基站的信号传输的转 变就是水平切换过程。 垂直切换：移动终端在支持不同网络技术的接入点之间的切换过程，例如，从802.11b基站到重叠覆盖蜂窝网络的信号传输的转变就是 垂直切换过程。水平和垂直切换在后面的章节中会详细讨论。SECOND FACTOR: FREQUENCIES ENGAGEDIntrafrequency handoff: the handoff process of a mobile terminal across access points operating on the same frequency. This type of handoff is present in code-division multiple access (CDMA) networks with frequency-division duplex (FDD). Interfrequency handoff: the handoff process of a mobile terminal across access points operating on different frequencies. This type of handoff is present in CDMA networks with time-division duplex (TDD) and is the only handoff type supported in GSM cellular systems.2.2 因素二：使用的频率频间切换：移动终端穿越在相同频率上运行的接入点之间的切换过程。这种切换的典型例子就是FDD的CDMA网络。 频内切换：移动终端穿越在不同频率上运行的接入点之间的切换过程。这种切换的典型例子就是TDD的CDMA网络和只支持GSM蜂窝 系统的切换类型。THIRD FACTOR: NUMBER OF CONNECTIONS INVOLVED Handoffs can be classified as hard, soft, or softer.Hard handoff: In a hard handoff the radio link to the old base station is released at the same time a radio link to the new base station is established. In other words, using hard handoff,a mobile node is allowed to maintain a connection with only one base station at any given time. Soft handoff: Contrary to hard handoffs, in a soft handoff a mobile node maintains a radio connection with no less than two base stations in an overlapping handoff region and does not release any of the signals until it drops below a specified threshold value. Soft handoffs are possible in situations where the mobile node is moving between cells operating on the same frequency. Softer handoff: A softer handoff is very similar to a soft handoff, except the mobile terminal switches connections over radio links that belong to the same access point.2.3 因素三：涉及连接数 切换被分为硬切换，软切换和更软切换。硬切换：在硬切换中，目标基站无线链路建立的同时释放原基站的无线链路。也就是说，使用硬切换时，在任何时间，移动节点只能 同一个基站保持连接。 软切换：与硬切换相比，在软切换中，移动节点在重叠覆盖的切换区内和不少于2个基站保持连接，并且不会释放任何信号直到某信 号低于给定的阀门值。当移动节点在相同频率上运行的小区间移动时可能会发生软切换。 更软切换：除了移动终端在同一个接入节点的无线链路上交换连接这点特性之外，更软切换同软切换很类似。FOURTH FACTOR: ADMINISTRATIVE DOMAINS INVOLVED An administrative domain is a group of systems and networks operated by a single organization ofadministrative authority. Administrative domains play a significant role in 4G wireless networks as different networks, each controlled by different administrative authorities, become available. Consequently, the classification of handoffs in terms of administrative domains is a crucial issue. Intra-administrative handoff: a handoff process where the mobile terminal transfers between different networks (supporting the same or different types of network interfaces) managed by the same administrative domain. Inter-administrative handoff: a handoff process where the mobile terminal transfers between different networks (supporting the same or different types of network interfaces) managed by different administrative domains.2.4 因素四：涉及的管理区域 管理区域是由同一个管理者管理的一组系统和网络。管理区域在 4G 无线网络如不同的网络中扮演重要的角色，不同的网络都有不同的管理者。因此，根据管理区域划分的切换类型是一个很重要的研究问题。 管理域之内的切换：移动节点在相同管理域管理的不同网络(支持相同或不同类型网络接口)之间切换过程。 管理域之间的切换：移动节点在不同管理域管理的不同网络(支持相同或不同类型网络接口)之间切换过程。FIFTH FACTOR: NECESSITY OF HANDOFF Handoffs can be classified based on need.Obligatory handoff: In some situations it is necessary for the mobile terminal to transfer the connection to another access point in order to avoid disconnection. Voluntary handoff: In other situations transfer of connection is optional and may or may not improve the quality of service.Voluntary and obligatory handoffs are discussed in more details later on.2.5 因素五：切换的必要性 基于需要而划分的切换强制切换：在有些情况下，为了避免掉线，有必要将移动终端的连接转移到另一个接入点。 自愿切换：在其他情况下，连接的转移是可选择的，对于提高服务质量不确定。自愿切换和强制切换在以后的章节中详细讨论。SIXTH FACTOR: USER CONTROL ALLOWANCE Handoffs can be classified as proactive or passive.Proactive handoff: In a proactive handoff the mobile terminal’s user is allowed to decide when to handoff. The handoff decision can be based on a set of preferences specified by the user.Proactive handoff is expected to be one of the radical features of 4G wireless systems. Passive handoff: The user has no control over the handoff process. This type of handoff is the most common in first-, second-, and third-generation wireless systems.2.6 因素六：用户控制允许 切换可分为主动的和被动的。主动切换：在主动切换中，移动终端用户可以选择何时切换。这种切换判决基于用户事先设定好的参数。主动切换是4G无线网络系统 的基本特点之一。 被动切换：用户不控制切换过程。这种类型的切换在前3代无线通信系统中最为普遍。HANDOFFS IN 4G HETEROGENEOUS NETWORKS3.4G异构网络的切换As discussed above, handoffs can be defined as the transition of signal transmission between different cells. A handoff scheme is required to preserve connectivity as devices move about, and at the same time curtail disturbance to ongoing transfers. Therefore, handoffs must exhibit low latency, sustain minimal amounts of data loss, as well as scale to large networks. Handoff schemes have been thoroughly researched and deployed in cellular systems, also known as wireless wide area networks (WWANs), and are escalating in importance in other networks, such as wireless LANs (WLANs), as research in 4G wireless communications increases in popularity. Handoffs can be classified as either horizontal or vertical,as depicted in Fig. 2. 如上面讨论的，切换可以定义为不同小区之间传输信号的转移。切换机制要求当设备移动时保持连接性，同时对正在进行的转移减少 干扰。因此，切换必须表现出低的反应时间，保持最小的数据丢失和测量大的网络。切换机制在蜂窝网络中已经彻底地研究和配置过，比 如无线广域网(WWANs)，在无线局域网(WLANs)中的重要性也逐渐提高。如图2，切换可划分为水平切换和垂直切换。 Horizontal handoff is the changeover of signal communication from one base station to a geographically neighboring base station supporting the same technology, as the user roams about. Horizontal handoff is also referred to as intra-technology handoff. Every time a mobile cellular host crosses from one cell into a neighboring cell (supporting the same technology),the network routinely and automatically exchanges the coverage responsibility from one base station to another. Each base station change, as well as the exchange procedure or method is known as horizontal handoff. In a properly operating network, handoff takes place smoothly and efficiently, without gaps in communications and without uncertainty as to which base station should be dealing with the mobile node. Mobile users need not get involved in order for horizontal handoff to take place nor do they have to sense the handoff process or identify which base station is managing the signals at any certain time. 水平切换是从一个基站到地理上相邻且支持相同技术基站的信号传输的转变，如用户漫游时。水平切换也被当作技术内切换提及。每 当一个移动蜂窝主机从一个小区穿越相邻小区(支持相同技术)，网络通常会自动地从一个基站到另一个基站交换覆盖责任。每一次基站的 改变，还有交换手续和方法都认为是水平切换。在一个运行良好的网络中，切换会平滑而有效的发生，没有通信上的不通畅，也没有不确 定性，即不知道哪个基站应该接管这个移动节点。移动用户不需要涉及水平切换的发生，对切换过程也没有感觉，在任何时间也不涉及识 别到底哪个基站正在处理信号。 Horizontal handoff is the most widespread definition of handoff due to the extensive research that has taken place in this field in the last several years. Vertical handoff, on the other hand is a more recent and exciting scheme that promises to transfigure the way we communicate.While horizontal handoff is a handover among base stations in service by the same wireless network interface, vertical handoff takes place between different network interfaces that usually represent different technologies [2]. Vertical handoff architectures and schemes will play a major role in the IEEE 802.21 standard and shall pave the road for emergence of 4G overlay multinetwork environments.水平切换是范围最为广泛的切换定义，是由于其在这个领域里的广泛研究在最近很多年一直在进行。另一方面，垂直切换更多的是最 近提出而令人振奋的切换机制，让大家感觉将来的通信方式会很美好。水平切换是处于相同无线网络接口服务的基站之间的切换，而垂直 切换发生在不同网络接口之间，这些接口通常代表了不同的技术[2]。垂直切换体系和机制在IEEE 802.21标准中将扮演主要角色，并为4G 重叠多网络环境的融合铺平道路。 There are two types of vertical handoffs:upward and downward. An upward vertical handoff [3] is roaming to an overlay with a larger cell size and lower bandwidth such asWANs (cellular networks), and a downward vertical handoff is roaming to an overlay with a smaller cell size and larger bandwidth. Downward vertical handoffs are less time critical, since a mobile device can always remain connected to the upper overlay and not handoff at all. 有两种垂直切换类型：向上和向下。向上垂直切换[3]是漫游到一个覆盖更大带宽较低的区域如蜂窝网络，向下垂直切换是漫游到一个 覆盖更小带宽较大的区域。向下垂直切换的时间急迫性较小，因为移动设备能总是保持同更大覆盖网络的连接而可以不切换。SEAMLESS HANDOFFIn one of the revolutionary drivers for 4G, technologies will complement each other to provide ubiquitous high-speed wireless connectivity to mobile terminals [4]. In such an environment, it will be necessary to support seamless handoffs of mobile terminals without causing disruption to their ongoing sessions. As a result, the need for seamless handoff across the different wireless networks is becoming increasingly important.Whereas wired networks regularly grant high bandwidth and consistent access to the Internet,wireless networks make it possible for users to access a variety of services even when they are moving. Consequently, seamless handoff, with low delay and minimal packet loss, has become a crucial factor for mobile users who wish to receive continuous and reliable services. One of the key issues that aid in providing seamless handoff is the ability to correctly decide whether or not to carry out vertical handoff at any given time. This could be accomplished by taking into consideration two key issues: network conditions for vertical handoff decisions and connection maintenance [5]. These two schemes need to be tightly coupled in order to move seamlessly across different network interfaces. To attain positive vertical handoff, the network state ought to be constantly obtainable by means of a suit-able handoff metric. In multinetwork environments,this is very challenging and hard to achieve as there is not a single factor that can provide a clear idea of when to hand off. Signal strength, which is the chief handoff metric measured in horizontal handoffs, cannot be utilized for vertical handoff decisions due to the overlay nature of heterogeneous networks and the different physical techniques used by each network。Thus, a natural question arises as to what factors should be considered in the handoff decision. In the next sections we discuss desirable handoff features, horizontal and vertical handoff procedures,and newly proposed vertical handoff characteristics in detail, and explain their significance in 4G handover schemes. 3.1 无缝切换 4G的革命性驱动之一是，技术之间会相互弥补以致于提供泛在的高速无线移动终端连接[4]。在这样一个环境中，有必要支持移动终端 的无缝切换而没有对正在进行的任务造成中断。因此，穿越不同无线网络之间无缝切换的需求变得日益重要。而有线网络能够有规则地获 得大的带宽和持续的接入到因特网，无线网络对移动的用户来说将使得他们接入到各种服务成为可能。因此，低延时和最小丢包率的无缝 切换已成为至关重要的因素，尤其是对那些想接收持续而可靠服务的移动用户来说。提供无缝切换的关键辅助之一是能够在任何时间正确 地决定是否执行垂直切换。完成这个问题要考虑两个问题：垂直切换判决的网络条件和连接维持[5]。这两个问题对于能够在不同网络接口 间无缝移动来说是紧紧联系一起的。为获得实际的垂直切换，网络状态应该能够被合适的公制切换手段时时地获得。信号强度在水平切换 中是主要的切换公制计量，但是不能够应用在垂直切换判决中，因为异构网络的覆盖特性和不同网络使用的不同物理技术。因而，一个很 自然的问题出现，切换判决应该考虑什么样的因素。下章节将讨论令人满意的切换特性，水平和垂直切换手续，详细地讨论刚刚提到的垂 直切换特点，阐述他们在4G切换机制中的重要性。DESIRABLE HANDOFF FEATURESAn efficient handoff algorithm can achieve many desirable features by trading off different operating characteristics. Some of the major desirable features of any handoff algorithm [6] are described below (refer to Fig. 3). 3.2 令人满意的切换特点 一种有效的切换算法能可以通过权衡不同的运行特点以达到很多令人满意的特点。任何一种切换算法的某些主要的得意特点描述如下 (参考图3)。 Reliable: A handoff algorithm should be reliable.This means that the call should have good quality after handoff. Many factors help in determining the potential service quality of a candidate base station. Some of these factors include signal-to-interference ratio (SIR), signal-to-noise ratio (SNR), received signal strength (RSS), and bit error rate (BER). Many more critical factors are discussed in the next section. 3.2.1 可靠性：切换算法应当是可靠的。这意味着切换后通话应该有很好的质量。很多因素有助于决定候选基站的潜在服务质量。这些因素包括信干比(SIR)、信噪比(SNR)、接收信号强度(RSS)、误码率(BER)。很多更重要的因素在下节中讨论。 Seamless: A handoff algorithm should be fast so that the mobile device does not experience service degradation or interruption. Service degradation may be due to a continuous reduction in signal strength or an increase in co-channel interference (CCI). Service interruption may be due to a “break before make” or hard handoff approach of handoff being exercised in the network. 3.2.2 无缝连接：切换算法应当很快以致移动设备不会经历服务质量下降或中断。服务下降可能是由于信号强度的持续下降或者共道 干扰(CCI)的增加。服务中断可能是由于“断了再连”或者网络中实施的硬切换方法。 Interference prevention: A handoff algorithm should avoid high interference. Co-channel interference is caused by devices transmitting on the same channel. This is usually caused by a neighboring detrimental source that is operating on the same channel. Interchannel interference,on the other hand, is caused by devices transmitting on adjacent channels. Both CCI and interchannel interference may severely limit the transfer rates of a wireless network. WLANs suffer from interference more than WWANs; the reason for this is the fact that most networking products available at present follow IEEE 802.11standards, which operate in the unlicensed 2.4 and 5.2 GHz bands. As a result, devices operating in these ranges are open to all different kinds of noise and interference coming from a variety of sources such as other legitimate 802.11 networks to Bluetooth devices or cordless phones operating in the same band as the wireless network. Cellular networks have managed over the years to fix many of their interference problems, and current generations infrequently suffer from interference. Interference in WLANs has up to now been a major issue, although as WLAN technology advances interference should eventually be fully eliminated. Nonetheless,before vertical handoff from WWAN to WLAN takes place, examining the network to make certain that CCI or interchannel interference does not exceptionally degrade the network is a key concern. 3.2.3 干扰防止：切换算法应该可以避免高干扰。共道干扰是由于终端在相同的信道中传输信号引起。这通常由运行在相同信道上相 邻的有害源引起。另一方面，信道间干扰由终端在相邻信道中传输信号引起。CCI和邻道干扰会严重地限制了无线网络的传输率。无线局 域网比无线广域网要受到更多的干扰。原因是，大多数现在已用的网络产品都遵循802.11标准，这个标准是运行在公用的2.4GHz和5.2GHz 频带上。因此，运行在这个频带上的设备对所有不同种类的噪音和干扰都是敞开的，这些噪音和干扰有各种来源如其他合法的802.11网络 到蓝牙设备或者运行在相同频带上的无绳电话。蜂窝网络经过多年的管理已经解决了很多的干扰问题，虽然随着WLAN技术的进步，干扰 会最终完全消除，在从WWAN到WLAN的切换发生前，对网络检查以确保CCI或者临道干扰不会特别地降低网络的服务质量是要重点关注 的。 Load balancing: A handoff algorithm should balance traffic in all cells, whether of the same or different network type. This helps to eliminate the need for borrowing channels from neighboring cells that have free channels, which simplifies cell planning and operation, and reduces the probability of new call blocking. 3.2.4 负载平衡：切换算法应该可以均衡小区流量，不管是相同或者不同网络类型。这有助于消除从相邻小区借用空闲信道的必要， 这可以简化小区规划和运行和减少新通话调度的可能性。 Improving performance: The number of handoffs should be minimized. Excessive horizontal or vertical handovers lead to heavy handoff processing loads and poor communication quality. In a handoff scenario, the more handoff attempts, the greater the chances that a call will be denied access to a channel, resulting in a higher handoff call dropping probability. 3.2.5 提高性能：切换数量应该最小化。过多的水平切换或垂直切换导致繁重的切换过程负担和差劲的通信质量。在切换场景中，切 换尝试越多，呼叫被拒绝接入信道的可能性就越大，从而导致更高的切换呼叫丢失可能性。 A high number of horizontal handoff attempts may result in more delay in the processing of handoff requests, which will cause signal strength to decrease over a longer time period to a level of unacceptable quality. In addition, the call may be dropped if a sufficient SIR is not achieved. Handoff algorithms require network resources to connect the call to a new base station. Thus, minimizing the number of handoffs reduces the switching load. Unnecessary handoffs should be prevented, especially when the current base station is able to provide the desired service quality without interfering with other mobile devices and base stations. 太多数量的水平切换尝试会导致切换请求处理的更长延时，这会迫使信号增强以减少过长处于低服务质量的时间。此外，如果充足的 SIR信干比没有达到的话呼叫就会放弃。切换算法要求网络资源将呼叫与目标基站连接起来。因而，最小化切换数量会减少交换负载。不 必要的切换应该防止，尤其是当当前基站可以提供满意的服务质量而没有干扰其他移动设备和基站时。 In a heterogeneous networking environment the challenge of choosing the “best” network is a major issue. An internal LAN with a weak signal inside a limestone building may yield better performance than a WAN with a strong signal.Handover between the different network tiers can lead to a very different quality of service being available to the mobile terminal, such as handover from a WLAN operating at 11 Mb/s to a GSM network operating at 9.6 kb/s. There may also be other factors such as economic discrepancies that do not occur insome networks charge per minute or byte.在异构网络环境中，选择最佳网络的挑战是主要问题。与强信号的无线局域网相比，一个在石灰石建筑内的弱信号局域网内会提供更 好的性能。不同网络之间切换等级会对移动终端造成非常不同的服务质量，如从WLAN的11 Mb/s 切换到 GSM 的9.6 kb/s。这或许有其他 因素如经济差异，这差异不会发生在个人网络中，一些网络的收费是按分钟或按流量的。HANDOFF PROCESSBoth horizontal and vertical handoff processes consist of a three phases: handoff decision, radio link transfer and channel assignment [7]. In this section we discuss each phase and describe the role it plays in fourth generation handoffs. 3.3 切换过程 水平切换和垂直切换处理都由三部分组成：切换判决，无线链路转移和信道分配[7]。在这节中，讨论每一个部分并描述其在4G切换中 扮演的角色。 Handoff Decisions ― Horizontal handoff decisions mainly depend on the quality of the channel reflected by the received signal strength and the resources available in the target cell. Many systems are interference limited, meaning that signal strength is an adequate indication of channel quality. A handoff is made if the RSS from a neighboring base station exceeds the RSS from the current base station by a predetermined threshold value. In vertical handoffs, many network characteristics have an effect on whether or not a handoff should take place. Most of these characteristics were not needed in horizontal handoffs. In fact (as explained above), only signal strength and channel availability are considered in horizontal handoffs [8]. The following characteristics are newly proposed qualities which are particularly important for vertical handoff decision. In Fig. 4,we classify these characteristics and categorize them depending on their relevance and applicability. 3.3.1 切换判决―水平切换判决主要依赖于有将接收信号强度反映的信道质量和目标基站可用的资源。很多系统的干扰是有限的，意 味着信号强度可作为信道质量的指示。如果相邻基站的RSS超过当前基站的RSS一个预设的门槛值时就执行切换。 在垂直切换中，很多网络特性对是否执行切换都有影响。这些特性的大部分在水平切换是不必要的。事实上(如上面的解释)，水平切 换只考虑信号强度和可用信道[8]。下列特点是刚刚提到的品质，对垂直切换判决非常重要。如图4，根据他们的适当度和应用性进行了分 类。Quality of Service: Handing over to a network with better conditions and higher performance would usually provide improved service levels. Transmission rates, error rates, and other characteristics can be measured in order to decide which network can provide a higher assurance of continuous connectivity. 服务质量：在更好条件和更高性能是切换网络通畅会提高服务水平。传输率，错误率和其他特点能够被测量以至于来决定哪一个网络可提供更高的持续连接保证。 Cost of Service: The cost of the different services to the user is a major issue, and could sometimes be the decisive factor in the choice of a network. Different broadband wireless Internet service providers (WISPs) and cellular service providers may well provide a variety of billing plans and options that will probably influence the customer’s choice of network and thus handoff decision. 服务费用：用户不同服务的费用是个主要问题，有时候成为网络选择的决定性因素。不同带宽的无线互联网服务供应商和蜂窝网络服 务运营商会提供各种资费计划方案和选择，这可能会影响用户的网络选择和切换判决。 Security: Risks are inherent in any wireless technology. Some of these risks are similar to tho some are exacerbated by w some are new. Perhaps the most significant source of risks in wireless networks is that the technology’s underlying communications medium, the airwave, is open to intruders, making it the logical equivalent of an Ethernet port in the parking lot. 安全：任何无线技术天生就具有风险。一些风险和有线网络是一样的；一些风险通过无线连接会加剧；而一些风险是新产生的。或许 在无线网络中最显著的风险源是技术的根本通信媒介，无线电波对入侵者是开放的，使得它逻辑上等同于停车场的以太网接口。 Power Requirements: Wireless devices operate on limited battery power. When the level decreases, handing off (or remaining connected)to a network with low power consumption can provide elongated usage time. For instance, if a device’s battery is nearly exhausted, handing over from a WLAN to WWAN would be a smart decision. This is due to the fact that when operating in a cellular WWAN, the device is idle for most of the time. However, given the unpredictable and erratic nature of transmissions with WLANs, handsets are unable to standby between packet transmission since there is no set time for the arrival and transmission of data and packets arrive sporadically. 功率需求：无线设备运行在有限的电池电源上。当功率需求水平下降时，低功耗的网络切换能延长使用时间。例如，如果终端电池快 要耗尽了，从WLAN切换到WWAN是明智的选择。事实上是这样，当终端在蜂窝网络中，终端大部分时间里是空闲的。但是，给定一个不 可知的不稳定的传输特性的WLAN，终端不能够在数据包传输之间待命，因为这里没有设定数据传输到达的时间，数据包时偶发地到达的。 Proactive Handoff: By proactive handoff,the users are involved in the vertical handoff decision and have the final decision on whether or not to handoff, regardless of the network conditions. By permitting the user to choose a preferred network, the system is able to accommodate the user’s special requirements. 自主切换：通过自主切换，用户在垂直切换判决中涉及到，并最终决定是否切换，不管网络条件如何。通过许可用户选择网络，系统 可适应用户的特别要求。 Velocity: In vertical handoff, the velocity factor has a larger weight and imperative effect on handoff decision than in horizontal handoffs.Because of the overlaid architecture of heterogeneous networks, handing off to an embedded network when traveling at high speeds is discouraged since a handoff back to the original network would occur very shortly afterward. 速度：速度因素在垂直切换判决中比水平切换有很大的权重和强制性的影响。因为异构网络的覆盖结构，当终端高速移动时切换到内 嵌的网络中是鼓励这么做的，因为在随后很短的时间内就会切换回原来的网络。 Radio Link Transfer ― Radio link transfer,the second part of the handoff process, refers to the task of forming links to a call at the new base station. The radio link is transferred from the old to the new base station. If the radio link transfer is within the currently serving cell, referred to as intracell handoff, no new link transfer operations are required.However, a handoff made from one cell to another, referred to as intercell handoff,requires handoff rerouting operations to link the mobile’s current communication path to the new base station. 3.3.2 无线链路转换――无线链路转换是切换过程的第二部分，它是指在新基站为一个呼叫建立连接链路。无线链路从旧基站转换到 新基站。如果无线链路转换是在当前服务小区内，即小区内切换，不需要新链路转转换操作。从一个小区到另一个小区的切换，即小区间 切换，要进行重选路由操作把终端当前通信路径连接到新基站。 Once a handoff procedure has commenced,handoff schemes can vary in the approach they take to transfer a call to a new link. The two approaches taken are known as forward and backward handoffs. In backward handoff the old serving base station prepares the handoff, and no access to the target base station is made until the control unit of the new base station has confirmed the allocation of resources. In forward handoff the process is initiated by means of the target base station without relying on the old base station during the preliminary phase of the handoff process. Each of these two methods has its pros and cons. The advantage of backward handoff is that the signaling information is transmitted through an therefore,the establishment of a new signaling channel is not required during the initial stages of the handoff process. The drawback, however, is that the handoff process may be unsuccessful if the link quality of the serving base station was rapidly deteriorating (e.g., due to rapid mobility). This type of handoff is used in most cellular networks to date. Forward handoff, on the contrary, is a faster handoff process, but its problem is a drop in handoff reliability. 一旦需要进行切换，切换方案会随着把呼叫转移到新链路的方法而改变。目前已知的方法是前向和后向切换。后向切换中，原服务基站为切换作准备，并且直到新基站的控制单元确保资源分配才接入目的基站。在前向切换中切换过程由目标基站启动，且在切换的初始阶 段不依赖旧基站。这两种方法都有各自的优缺点，后向切换的优点是信令信息通过现存的无线链路传输，因此，再切换的初始阶段不需要 确定一个新的信令信道。然而，它的缺点是当信道质量迅速恶化时切换有可能不成功。至今大多数蜂窝网中仍采用这种切换。相反，前向 切换速度较快，但问题是它的切换可靠性较低。 Channel Assignment ― The final handoff stage is channel assignment which consists of the allocation of resources at the new base station. If a new call is admitted to access the network, a call admission control (CAC) algorithm will make a decision to accept or reject it according to the amount of available resources vs. QoS requirements, and the effect on QoS of existing connections that may occur as a result of the new connection. Channel assignment is part of CAC and resource management, and therefore is not discussed in detail here. 3.3.3 信道分配――切换的最后阶段是信道分配，它是由新基站的资源分配组成。如果一个新的呼叫准许接入网络，呼叫准许接入算 法根据当前可用的资源，QoS要求，和它对当前连接的QoS影响（新连接可能会对现存的连接产生影响）来决定是否接受这个呼叫。 信道 分配是CAC和资源管理的一部分，因此这里不做进一步详细讨论。VOLUNTARY AND OBLIGATORY HANDOFFS3.4 自愿切换和强制性切换 Different handoff forms can be distinguished based on need. To elucidate this point, consider the following handoff scenario. A crowded town would usually contain several base stations in the city center, and there would be immense amounts of coverage overlap between neighboring base stations. Assuming that one of these base stations becomes fully overloaded to the extent that adding more nodes would degrade its performance and quality of service, it becomes beneficial for some of the terminals to be reassigned to another base station in order to relieve the jammed base station. In this situation a handoff is preferable but not mandatory. On the contrary, if a user was driving on a highway with dispersed base stations and negligible coverage overlapping, it becomes crucial for h otherwise, the connection will be broken. 不同的切换方式是可以依照需要来区分的，为了说明这点，假设下面的切换场景。在城市的中心通常有很多的基站，在相邻的基站之 间通常有大量区域信号覆盖重叠了，假设一个基站已经是完全饱和了，如果加入更多的节点会导致性能和服务质量的下降，可以通过将其 中一些节点重新分配到另一个基站来缓解拥塞。在这种的情形下切换是可取的担不是强制行的。相反，如果一个用户在高速行驶，脱离了 以前的基站进入了邻基站，这适合切换就是必须的，否则连接就会断开。 A vertical handoff could be voluntary or obligatory,depending on the direction of handoff.When handing over from a WLAN to a cellular network, delay in the handoff transmission region must be short, so the preferred handoff point is the first time the signal strength degrades. In this case the handoff is obligatory or the connection is lost. Obligatory handoffs are also referred to as forced vvertical handoffs (FVHs). On the other hand, since a cellular network covers a wide area and the handoff time is not critical, the preferred handoff point from cellular to WLAN is the first time the signal strength in the WLAN reaches an acceptable level. The handoff is voluntary in that case, and the handoff time is not critical. 垂直的切换可以是自愿的也可以是必须要做的，主要是取决于切换的指示。当从WLAN中切换到蜂窝网中，切换的时延必须要短，那 么首选的切换时刻就是信号强度减弱的时候。在这种下如果是切换是强制的，那么连接就会中断。强制的切换同样会涉及到FVHs,另一方 面，由于蜂窝网覆盖的区域很大，切换时刻不是很关键，那么首选的由蜂窝网向区域网切换时刻是信号强度在区域网中达到可以接收水平 的第一时刻，这种切换就是自愿的，切换的时间是很关键。 In many cases measured metrics, such as SNR and BER, will signify that a specific network is reliable and provides high quality of service。In several of the currently proposed vertical handoff schemes [9], this would routinely lead to a vertical handoff to that network. Nonetheless,one essential factor frequently neglected is the position and movement direction of the mobile node. Occasionally, this motion could indicate that the person will probably promptly leave this network’s coverage area. Consequently, although the newly encountered network is in healthy condition, handoff to it would almost immediately be followed by another reverse handoff back to the original network. Such vertical handoffs are ineffectual, overwhelm the network with fruitless communications, and may at times irritate the user. 在许多公制测量情况下，如SNR和BER，意味着这种特定的网络是可靠的，能够提供高质量的服务。在目前提出的垂直切换机制中[9]， 这会很正常地导致一个网络的垂直切换。但是，移动节点的位置和移动方向是经常被忽略的重要的因素。有时候，节点的运动可能会显示 这个用户可能会离开这个网络的覆盖区域。因此，尽管新的网络是在一个健康的情况下，立即切换到这个新网络中，同样的可能立即被切 换到旧的网络中去，是整个网络是无效的，可能会激怒用户。A VERTICAL HANDOFF DECISION FUNCTIONIn this section we propose a vertical handoff decision function (VHDF). VHDF is used to measure the improvement gained by handing over to aparticular network i. It is aimed at making use of some of the parameters from Fig.4 in order to make wiser handoff decisions.As discussed earlier, handoff decision parameters help determine which of the available networks is best suited for data transfer. Because of their importance, we choose the following network parameters for VHDF: ? Cost of service (C): The cost of the different services to the user is a major issue, and can sometimes be the decisive factor in the choice of a network. ? Security (S): When the information exchanged is confidential, a network with high encryption is preferred. ? Power consumption (P): Vertically handing off to a high power consuming network is not desirable if the mobile terminal’s battery is nearly exhausted or the battery’s lifetime is relatively short. ? Network conditions (D): Available bandwidth is used to indicate network conditions and is a major factor, especially for voice and video traffic. ? Network performance (F): In some cases interference or unstable network connections might discourage a handoff decision.For more information on the above mentioned parameters, please refer to an earlier section.4.垂直切换判决函数在这节中，提出了垂直切换函数(VHDF)。VHDF通过切换到特定的网络i来测量得到的改进。目的在于利用图4中的一些参数以做出更 明智的切换判决。如之前的讨论，切换判决参数有助于决定哪个可用的网络最适合数据传送。因为他们的重要性，VHDF选择下列网络参 数： ?服务费用(C):不同服务的费用对用户来说是大问题，有时候成为网络选择的决定性因素。 ?安全(S):当信息交换是保密时，具有高编密码的网络是首选。 ?功率消耗(P):垂直切换到高功率消耗网络是不可取的，如果移动终端的电池近要耗尽或者电池寿命相对很短。 ?网络条件(D):可用的带宽常用来指示网络条件，是一个主要因素，尤其是语音和视频流量。 ?网络性能(F):在一些情况下干扰或者不稳定的网络连接会阻碍切换判决。有关提及参数的更多信息，参阅前面的章节。 As the mobile roams across different networks,VHDF is evaluated for all accessible networks.The network with the highest calculated value for VHDF is the most desirable for the user based on specified preferences. The network quality Qi, which provides a measure of the appropriateness of a certain network i, is measured via the function： 当移动终端在不同网络间漫游时，VHDF 对所有可接入的网络都是可评估的。基于用户特定的参数选择，VHDF 评估值最高的网络对 用户来说是最让人满意的。网络质量 Qi 提供了某一网络合适的测量方法，通过下列函数测得：In order to allow for different circumstances,there is an apparent necessity to weigh each factor relative to the magnitude it endows on the vertical handoff decision. Therefore, a different weight is introduced: 为了顾及不同的网络环境，明显有必要衡量相关因素的作用在垂直切换判决上的大小，因此，引入不同的权重：where ?c, ?s, ?p, ?d, and ?f are weights for each of the network parameters. The values of these weights are fractions (i.e., they range from 0 to 1). Furthermore, all five weights add up to 1.0.Each weight is proportional to the significance of a parameter to the vertical handoff decision.The larger the weight of a specific factor, the more important that factor is to the user and vice versa. These weights are obtained from the user via a user interface. Even though we could add the different factors in the VHDF to obtain network quality Qi, each network parameter has a different unit, which leads to the necessity of normalization. The final normalized equations for n networks are 这里?c, ?s, ?p, ?d和 ?f 分别是网络参数的权重。这些权重都是小数 (0到1之间)。而且，所有5个权重加起来为1。每一个权重在垂直 切换判决参数的重要性是成比例的。特殊因素的权重越大，因素对用户的重要性越大，反之则反。这些权重通过用户界面从用户获得。虽 然在VHDF中加入了不同的因素以获得网络质量Qi，但是每一个网络参数都有不同的单位，这使得标准有其必要性。最后n个网络的标准化 公式如下：Assume that the mobile terminal detects a new network. It calculates the network quality Qi for its current network and for the newly detected network. The weights would already have fixed (but different) values that assign priorities to the various characteristics. VHDF simply calculates Qi based on Eq. 3. The network with the highest Qi is the preferred network. If the newly detected network receives a higher Qi, vertical otherwise, the device remains connected to the current network. For a more thorough description and implementation of this vertical handoff decision scheme, please refer to [10]. 假设移动终端侦测到新的网络。终端就为当前的网络和新侦测到的网络计算网络质量Qi。权重已经有固定值(但是不同)，这些固定值 分配优先权给不同的特点。VHDF简单地用公式3计算Qi。Qi值最高的网络是首先网络。如果新网络接收一个更高的Qi，垂直切换发生；否 则，终端同当前网络保持连接。更详细的描述和垂直切换的执行判决机制，参阅[10]。PERFORMANCE EVALUATION5.性能评估The performance of VHDF is evaluated by simulation,and the results are presented and discussed in this section. A simulation model for evaluating VHDF was developed in Network Simulator (NS-2).The simulated network topology is shown in Fig.5. This network topology is set up in NS-2 such that the mobile terminal and stationary server are both wirelessly connected via two heterogeneous network interfaces. Network A represents a WWAN and has a low bandwidth of 384 kb/s.Network B, on the other hand, has a higher bandwidth of 1 Mb/s and represents a WLAN. Nonetheless, each connection is independent of the other. The mobile terminal shares the networks’bandwidth with various other background traffic sources. As the mobile terminal moves about, the amount of backgro consequently, the performance of both networks will also vary. Therefore, it is impossible to predict which of the two networks will provide higher bandwidth or lower cost and so on. In fact, remaining connected to a single network will not necessarily provide the best possible performance.Network B has larger capacity and can therefore better tolerate background traffic and maintain higher performance. Network B is also considered the more expensive of the two networks due to its higher QoS. VHDF的性能通过仿真来评估，其结果在本节予以给出和讨论。 评估VHDF的仿真模型在NS-2中有披露。 仿真的网络拓扑结构如图5显示。 网络拓扑结构建立在NS-2中以致移动终端和静态服务器都是 无线连接于2个异构网络接口。网络A代表WWAN，有384 kb/s的低带宽。网络B代表WLAN有1Mb/s带宽。但是，每种连接都是独立的。移 动终端分享网络带宽和不同其他背景的流量资源。当移动终端移动时，背景流量的是变化的；因此，两个网络的性能也是变化的。因而， 不可能预测两个网络中哪一个提供更高的带宽、更低的开销等等。事实上，同单一网络保持连接并不见得会提供最可能好的性能。网络B 有较大的容量因此更能容忍背景流量和保持较好的性能。网络B的费用也认为更贵因为其较好的QoS。 NS-2’s exponential traffic (expo-traffic) source is used to generate background traffic.Separate traffic is generated for each of the two networks. We classify the traffic into “none,”“light,” “heavy,” and “oscillating.” The mobile terminal has several connection strategies to select when the VHDF is calculated: it can remain connected to network A or hand off to network B. Initially, the mobile terminal is connected to the server through network A’s interface (i.e., the primary path is the path from the mobile terminal to the interface on network A).The secondary path is from the mobile terminal to network B’s interface. Figure 6 shows the effect of background traffic on overall network throughput. In this experiment,the user’s preference is to get the highest possible QoS by receiving the maximum amount of bandwidth, regardless of other factors such as usage cost, security, and power consumption.Therefore, the user sets the VHDF to NS-2的指数流量资源常常产生背景流量。个别的流量产生于两个网络中。将流量分类为“没有”“轻微”“严重”和“振荡”。移动 终端有几种连接策略以选择何事计算VHDF：终端同网络A连接或者切换到网络B。最初，移动终端通过网络A接口同服务器连接(例如，最 初的路径是从终端到网络A接口)。第二路径从终端到网络B接口。 图6显示了背景流量对整个网络吞吐量的影响。在这个试验中，用户的优先选择是获得最可能好的QoS，通过接收最大数量的带宽而不 管其他因素如使用费用，安全和功率消耗。因此，用户设定的VHDF是：By implementing VHDF, the system manages to increase throughput by up to 57.9 percent in individual cases. VHDF increased throughput by 99.1 percent compared to the scheme that remains connected to network A. When the background traffic varies, using the VHDF based scheme shows a significant improvement over remaining connected to network A. This is due to the VHDF-based scheme’s ability to hand off several times with low handoff latency and guaranteed high bandwidth. In general, it is apparent from the results obtained that VHDF significantly helps bring about smarter handoff decisions and boost network throughput.通过执行VHDF，在个案中系统设法增加了57.9%的吞吐量。同保持网络A连接机制的比较，VHDF增加99.1%的吞吐量。当背景流量变 化时，使用VHDF显示出终端与网络A保持连接上有显著的提高。总的来说，从获得的结果显示出，VHDF能明显的有助于更明智的切换判 决和提高网络吞吐量。SUMMARYThe integration of heterogeneous wireless networks is one of the most anticipated features of fourth-generation systems. This is expected to be a huge step toward universal seamless access and omnipresent computing. One of the main challenges for seamless mobility is the availability of reliable horizontal (intrasystem) and vertical (intersystem) handoff schemes. Horizontal and vertical handoffs are integral components of future fourth-generation communications. This article presents a tutorial on the different aspects of handoffs, and discusses handoff design and performance related issues. Some of the most desirable features of handoff schemes are discussed.Handof