外文翻譯----4g異構網(wǎng)絡的切換

1、<p>　　Handoffs in Fourth Generation Heterogeneous Networks</p><p>　　Nidal Nasser, University of Guelph Ahmed Hasswa and Hossam Hassanein, Queen’s University</p><p><b>　　4G異構網(wǎng)絡的切換<

2、/b></p><p>　　加拿大安大略省圭爾夫大學 加拿大安大略省皇后大學</p><p><b>　　ABSTRACT</b></p><p>　　As mobile wireless networks increase in popularity and pervasiveness, we are faced with the

3、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 c

4、onsidered 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 (inte</p><p><b>　　

5、摘要</b></p><p>　　移動無線網(wǎng)絡日益盛行之時，不同網(wǎng)絡之間融合的挑戰(zhàn)擺在面前。4G無線通信整合了大多數(shù)異構網(wǎng)絡的無線技術，在走向通用的無縫接入方面邁向了一大步。無縫移動通信面臨的主要挑戰(zhàn)之一是可靠的水平(網(wǎng)間)切換和垂直(網(wǎng)內)切換機制的有效性。有效的切換機制提高了服務質量提供了無縫移動通信。本文提出了新穎不同的切換并討論了4G切換的相關問題。本為給出了令人滿意的切換特性。切換判決，無線

6、鏈路傳輸和信道分配在整個切換過程中分步描述。本文還給出了一種能夠讓終端去分配權重給不同網(wǎng)絡參數(shù)的垂直切換判決函數(shù)。</p><p>　　INTRODUCTION</p><p><b>　　1.介紹</b></p><p>　　First, second- and third-generation mobile systems depended

7、 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 bandwidt

8、h 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.</p><p>　　首先，第二代第三代移動通信系統(tǒng)

9、依賴于無線頻譜的利用，這些頻譜要么被公共使用要么被批準在各個地區(qū)由一些小數(shù)量的服務提供商和網(wǎng)絡運營商使用。帶寬和覆蓋范圍的差異導致發(fā)展多網(wǎng)絡接口終端的必要性，多網(wǎng)絡接口終端可以使用不同網(wǎng)絡服務提供的多樣性。</p><p>　　The fourth generation (4G) of wireless communications is expected to integrate a potentially l

10、arge 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 cu

11、rrently 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 ban</p><p>　　4G無線通信融合了大多

12、數(shù)異構網(wǎng)絡的無線技術，在通用的無縫接入方面和通過無縫移動通信進行泛在計算方面邁向了一大步[1]。即使4G當前沒有明確定義，仍然有很多當前的展望描述了新一代無線技術的景象?；谝苿油ㄐ鸥‖F(xiàn)的趨勢，4G擁有更多的帶寬和數(shù)率以及更平滑快捷的切換，集中致力于減少差錯服務和允許通過多種無線網(wǎng)絡的無縫切換。關鍵概念是使用已存在的移動技術整合4G性能。網(wǎng)絡管理，對于不同的接入系統(tǒng)如垂直和水平切換還有無縫移動性、服務質量、可靠性和安全性，很有必要。&l

13、t;/p><p>　　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 ne

14、tworks 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.&

15、lt;/p><p>　　下面的文章內容如下。給出了一種新的用于切換的分類層次。對異構無線網(wǎng)絡中各種切換判決因子的全面研究進行解釋，解釋和定量地評估本文提到垂直切換函數(shù)(VHDF)。提供所述解決方案的性能評估及文章概要。</p><p>　　CLASSIFICATION OF HANDOFFS</p><p><b>　　2.切換的分類</b><

16、;/p><p>　　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

17、 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 eac

18、h other and overlap in such a way that, for the majority of time,any mobile devi</p><p>　　大體上，每個移動終端(節(jié)點)總是歸類于至少一種網(wǎng)絡接入點即基站。每一個基站的服務區(qū)定義為它的小區(qū)。每個小區(qū)的大小和外形依賴于網(wǎng)絡類型、基站大小、基站的傳輸和接收功率。通常相同類型的小區(qū)以這樣一種方式彼此相鄰和重疊，大多數(shù)時間內任一移動

19、終端所處的覆蓋面積超過一個基站。另一方面，異構網(wǎng)絡的小區(qū)相互覆蓋。因此，移動主機的關鍵問題是要時不時地做出選擇，哪個網(wǎng)絡的哪個基站來處理具體移動主機的信號傳輸和接收以及必要時切換信號的傳輸。</p><p>　　We classify handoffs based on several factors as shown in Fig. 1. No longer is the network type the on

20、ly 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 fac

21、tors along with the handoff classifications that are based on them.</p><p>　　如圖1所示，基于幾種因素給出了切換的分類。網(wǎng)絡類型不再是唯一的切換分類的因素。這個切換的分類因素有很多組成包含涉及的管理區(qū)域、連接和使用的頻率數(shù)。下圖是基于它們的切換類型的分類因素。</p><p><b>　　圖1：切換的分

22、類樹</b></p><p>　　FIRST FACTOR: </p><p>　　NETWORK TYPES INVOLVED This is the most common classification factor.Handoffs can be classified as either horizontal or vertical. This depends on wh

23、ether a handoff takes place between a single type of network interface or a variety of different network interfaces.</p><p>　　Horizontal handoff: the handoff process of a mobile terminal between access point

24、s 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

25、 is considered as a horizontal handoff process.</p><p>　　Vertical handoff: the handoff process of a mobile terminal among access points supporting different network technologies. For example, the changeover

26、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.&l

27、t;/p><p><b>　　2.1因素一：</b></p><p>　　涉及的網(wǎng)絡類型：這是最普遍的分類因素。切換能被分為水平切換或者垂直切換。這主要看切換是發(fā)生在同種網(wǎng)絡類型之間還是不同網(wǎng)絡類型之間。</p><p>　　水平切換：移動終端在支持相同網(wǎng)絡技術的接入點之間的切換過程，例如，從802.11b基站到地理上相鄰802.11b基站的信號傳

28、輸?shù)霓D變就是水平切換過程。</p><p>　　垂直切換：移動終端在支持不同網(wǎng)絡技術的接入點之間的切換過程，例如，從802.11b基站到重疊覆蓋蜂窩網(wǎng)絡的信號傳輸?shù)霓D變就是垂直切換過程。水平和垂直切換在后面的章節(jié)中會詳細討論。</p><p>　　SECOND FACTOR: FREQUENCIES ENGAGED</p><p>　　Intrafrequency

29、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

30、).</p><p>　　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 duple

31、x (TDD) and is the only handoff type supported in GSM cellular systems.</p><p>　　2.2因素二：使用的頻率</p><p>　　頻間切換：移動終端穿越在相同頻率上運行的接入點之間的切換過程。這種切換的典型例子就是FDD的CDMA網(wǎng)絡。</p><p>　　頻內切換：移動終端穿越在不同頻

32、率上運行的接入點之間的切換過程。這種切換的典型例子就是TDD的CDMA網(wǎng)絡和只支持GSM蜂窩系統(tǒng)的切換類型。</p><p>　　THIRD FACTOR: </p><p>　　NUMBER OF CONNECTIONS INVOLVED Handoffs can be classified as hard, soft, or softer.</p><p>　

33、　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 mai

34、ntain a connection with only one base station at any given time.</p><p>　　Soft handoff: Contrary to hard handoffs, in a soft handoff a mobile node maintains a radio connection with no less than two base stat

35、ions 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 opera

36、ting on the same frequency.</p><p>　　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.

37、</p><p>　　2.3因素三：涉及連接數(shù) 切換被分為硬切換，軟切換和更軟切換。</p><p>　　硬切換：在硬切換中，目標基站無線鏈路建立的同時釋放原基站的無線鏈路。也就是說，使用硬切換時，在任何時間，移動節(jié)點只能同一個基站保持連接。</p><p>　　軟切換：與硬切換相比，在軟切換中，移動節(jié)點在重疊覆蓋的切換區(qū)內和不少于2個基站保持連接，并且不會釋放任何

38、信號直到某信號低于給定的閥門值。當移動節(jié)點在相同頻率上運行的小區(qū)間移動時可能會發(fā)生軟切換。</p><p>　　更軟切換：除了移動終端在同一個接入節(jié)點的無線鏈路上交換連接這點特性之外，更軟切換同軟切換很類似。</p><p>　　FOURTH FACTOR:</p><p>　　ADMINISTRATIVE DOMAINS INVOLVED An administr

39、ative domain is a group of systems and networks operated by a single organization of administrative authority. Administrative domains play a significant role in 4G wireless networks as different networks, each controlled

40、 by different administrative authorities, become available. Consequently, the classification of handoffs in terms of administrative domains is a crucial issue.</p><p>　　Intra-administrative handoff: a handof

41、f process where the mobile terminal transfers between different networks (supporting the same or different types of network interfaces) managed by the same administrative domain.</p><p>　　Inter-administrativ

42、e 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.</p><p>

43、　　2.4因素四：涉及的管理區(qū)域 管理區(qū)域是由同一個管理者管理的一組系統(tǒng)和網(wǎng)絡。管理區(qū)域在4G無線網(wǎng)絡如不同的網(wǎng)絡中扮演重要的角色，不同的網(wǎng)絡都有不同的管理者。因此，根據(jù)管理區(qū)域劃分的切換類型是一個很重要的研究問題。</p><p>　　管理域之內的切換：移動節(jié)點在相同管理域管理的不同網(wǎng)絡(支持相同或不同類型網(wǎng)絡接口)之間切換過程。</p><p>　　管理域之間的切換：移動節(jié)點在不同管理

44、域管理的不同網(wǎng)絡(支持相同或不同類型網(wǎng)絡接口)之間切換過程。</p><p>　　FIFTH FACTOR: </p><p>　　NECESSITY OF HANDOFF Handoffs can be classified based on need.</p><p>　　Obligatory handoff: In some situations it is

45、necessary for the mobile terminal to transfer the connection to another access point in order to avoid disconnection.</p><p>　　Voluntary handoff: In other situations transfer of connection is optional and ma

46、y or may not improve the quality of service.Voluntary and obligatory handoffs are discussed in more details later on.</p><p>　　2.5因素五：切換的必要性 基于需要而劃分的切換</p><p>　　強制切換：在有些情況下，為了避免掉線，有必要將移動終端的連接轉移到

47、另一個接入點。</p><p>　　自愿切換：在其他情況下，連接的轉移是可選擇的，對于提高服務質量不確定。自愿切換和強制切換在以后的章節(jié)中詳細討論。</p><p>　　SIXTH FACTOR: </p><p>　　USER CONTROL ALLOWANCE Handoffs can be classified as proactive or passive.

48、</p><p>　　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

49、handoff is expected to be one of the radical features of 4G wireless systems.</p><p>　　Passive handoff: The user has no control over the handoff process. This type of handoff is the most common in first-, se

50、cond-, and third-generation wireless systems.</p><p>　　2.6因素六：用戶控制允許 切換可分為主動的和被動的。</p><p>　　主動切換：在主動切換中，移動終端用戶可以選擇何時切換。這種切換判決基于用戶事先設定好的參數(shù)。主動切換是4G無線網(wǎng)絡系統(tǒng)的基本特點之一。</p><p>　　被動切換：用戶不控制切換

51、過程。這種類型的切換在前3代無線通信系統(tǒng)中最為普遍。</p><p>　　HANDOFFS IN 4G HETEROGENEOUS NETWORKS</p><p>　　3. 4G異構網(wǎng)絡的切換</p><p>　　As discussed above, handoffs can be defined as the transition of signal tra

52、nsmission 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,

53、 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</p><

54、;p>　　如上面討論的，切換可以定義為不同小區(qū)之間傳輸信號的轉移。切換機制要求當設備移動時保持連接性，同時對正在進行的轉移減少干擾。因此，切換必須表現(xiàn)出低的反應時間，保持最小的數(shù)據(jù)丟失和測量大的網(wǎng)絡。切換機制在蜂窩網(wǎng)絡中已經(jīng)徹底地研究和配置過，比如無線廣域網(wǎng)(WWANs)，在無線局域網(wǎng)(WLANs)中的重要性也逐漸提高。如圖2，切換可劃分為水平切換和垂直切換。</p><p>　　Horizontal ha

55、ndoff 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-

56、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

57、base station to another. Each base station chan</p><p>　　水平切換是從一個基站到地理上相鄰且支持相同技術基站的信號傳輸?shù)霓D變，如用戶漫游時。水平切換也被當作技術內切換提及。每當一個移動蜂窩主機從一個小區(qū)穿越相鄰小區(qū)(支持相同技術)，網(wǎng)絡通常會自動地從一個基站到另一個基站交換覆蓋責任。每一次基站的改變，還有交換手續(xù)和方法都認為是水平切換。在一個運行良好的網(wǎng)絡

58、中，切換會平滑而有效的發(fā)生，沒有通信上的不通暢，也沒有不確定性，即不知道哪個基站應該接管這個移動節(jié)點。移動用戶不需要涉及水平切換的發(fā)生，對切換過程也沒有感覺，在任何時間也不涉及識別到底哪個基站正在處理信號。</p><p>　　Horizontal handoff is the most widespread definition of handoff due to the extensive research

59、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 han

60、dover among base stations in service by the same wireless network interface, vertical handoff takes place between different network interfaces that usually represent different technologi</p><p>　　水平切換是范圍最為廣泛

61、的切換定義，是由于其在這個領域里的廣泛研究在最近很多年一直在進行。另一方面，垂直切換更多的是最近提出而令人振奮的切換機制，讓大家感覺將來的通信方式會很美好。水平切換是處于相同無線網(wǎng)絡接口服務的基站之間的切換，而垂直切換發(fā)生在不同網(wǎng)絡接口之間，這些接口通常代表了不同的技術[2]。垂直切換體系和機制在IEEE 802.21標準中將扮演主要角色，并為4G重疊多網(wǎng)絡環(huán)境的融合鋪平道路。</p><p>　　There a

62、re 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

63、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.<

64、;/p><p>　　有兩種垂直切換類型：向上和向下。向上垂直切換[3]是漫游到一個覆蓋更大帶寬較低的區(qū)域如蜂窩網(wǎng)絡，向下垂直切換是漫游到一個覆蓋更小帶寬較大的區(qū)域。向下垂直切換的時間急迫性較小，因為移動設備能總是保持同更大覆蓋網(wǎng)絡的連接而可以不切換。</p><p>　　SEAMLESS HANDOFF</p><p>　　In one of the revolutio

65、nary 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 mo

66、bile 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 g

67、rant high bandwidth and consistent ac</p><p><b>　　3.1 無縫切換</b></p><p>　　4G的革命性驅動之一是，技術之間會相互彌補以致于提供泛在的高速無線移動終端連接[4]。在這樣一個環(huán)境中，有必要支持移動終端的無縫切換而沒有對正在進行的任務造成中斷。因此，穿越不同無線網(wǎng)絡之間無縫切換的需求變得日益重要。而

68、有線網(wǎng)絡能夠有規(guī)則地獲得大的帶寬和持續(xù)的接入到因特網(wǎng)，無線網(wǎng)絡對移動的用戶來說將使得他們接入到各種服務成為可能。因此，低延時和最小丟包率的無縫切換已成為至關重要的因素，尤其是對那些想接收持續(xù)而可靠服務的移動用戶來說。提供無縫切換的關鍵輔助之一是能夠在任何時間正確地決定是否執(zhí)行垂直切換。完成這個問題要考慮兩個問題：垂直切換判決的網(wǎng)絡條件和連接維持[5]。這兩個問題對于能夠在不同網(wǎng)絡接口間無縫移動來說是緊緊聯(lián)系一起的。為獲得實際的垂直切換，

69、網(wǎng)絡狀態(tài)應該能夠被合適的公制切換手段時時地獲得。信號強度在水平切換中是主要的切換公制計量，但是不能夠應用在垂直切換判決中，因為異構網(wǎng)絡的覆蓋特性和不同網(wǎng)絡使用的不同物理技術。因而，一個很自然的問題出現(xiàn)，切換判決應該考慮什么樣的因素。下章節(jié)將討論令人滿意的切換特性，水平和垂直切換手續(xù)，詳細地討論剛剛提到的垂直切換特點，闡述他們在4G切換機制</p><p>　　DESIRABLE HANDOFF FEATURES&

70、lt;/p><p>　　An 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

71、 below (refer to Fig. 3).</p><p>　　3.2 令人滿意的切換特點</p><p>　　一種有效的切換算法能可以通過權衡不同的運行特點以達到很多令人滿意的特點。任何一種切換算法的某些主要的得意特點描述如下(參考圖3)。</p><p>　　Reliable: A handoff algorithm should be reliable

72、.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 (SI

73、R), signal-to-noise ratio (SNR), received signal strength (RSS), and bit error rate (BER). Many more critical factors are discussed in the next section.</p><p>　　3.2.1 可靠性：切換算法應當是可靠的。這意味著切換后通話應該有很好的質量。很多因素有助

74、于決定候選基站的潛在服務質量。這些因素包括信干比(SIR)、信噪比(SNR)、接收信號強度(RSS)、誤碼率(BER)。很多更重要的因素在下節(jié)中討論。</p><p>　　Seamless: A handoff algorithm should be fast so that the mobile device does not experience service degradation or interrup

75、tion. 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 hand

76、off being exercised in the network.</p><p>　　3.2.2 無縫連接：切換算法應當很快以致移動設備不會經(jīng)歷服務質量下降或中斷。服務下降可能是由于信號強度的持續(xù)下降或者共道干擾(CCI)的增加。服務中斷可能是由于“斷了再連”或者網(wǎng)絡中實施的硬切換方法。</p><p>　　Interference prevention: A handoff alg

77、orithm 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. Inte

78、rchannel 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 inter

79、ference more than WWAN</p><p>　　3.2.3 干擾防止：切換算法應該可以避免高干擾。共道干擾是由于終端在相同的信道中傳輸信號引起。這通常由運行在相同信道上相鄰的有害源引起。另一方面，信道間干擾由終端在相鄰信道中傳輸信號引起。CCI和鄰道干擾會嚴重地限制了無線網(wǎng)絡的傳輸率。無線局域網(wǎng)比無線廣域網(wǎng)要受到更多的干擾。原因是，大多數(shù)現(xiàn)在已用的網(wǎng)絡產(chǎn)品都遵循802.11標準，這個標準是運行在

80、公用的2.4GHz和5.2GHz頻帶上。因此，運行在這個頻帶上的設備對所有不同種類的噪音和干擾都是敞開的，這些噪音和干擾有各種來源如其他合法的802.11網(wǎng)絡到藍牙設備或者運行在相同頻帶上的無繩電話。蜂窩網(wǎng)絡經(jīng)過多年的管理已經(jīng)解決了很多的干擾問題，雖然隨著WLAN技術的進步，干擾會最終完全消除，在從WWAN到WLAN的切換發(fā)生前，對網(wǎng)絡檢查以確保CCI或者臨道干擾不會特別地降低網(wǎng)絡的服務質量是要重點關注的。</p><

81、;p>　　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 fre

82、e channels, which simplifies cell planning and operation, and reduces the probability of new call blocking.</p><p>　　3.2.4 負載平衡：切換算法應該可以均衡小區(qū)流量，不管是相同或者不同網(wǎng)絡類型。這有助于消除從相鄰小區(qū)借用空閑信道的必要，這可以簡化小區(qū)規(guī)劃和運行和減少新通話調度的可能性。<

83、/p><p>　　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 scenari

84、o, 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.</p><p>　　3.2.5 提高性能：切換數(shù)量應該最小化。過多的水平切換或垂直切換導致繁重的切

85、換過程負擔和差勁的通信質量。在切換場景中，切換嘗試越多，呼叫被拒絕接入信道的可能性就越大，從而導致更高的切換呼叫丟失可能性。</p><p>　　A high number of horizontal handoff attempts may result in more delay in the processing of handoff requests, which will cause signal st

86、rength 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

87、to a new base station. Thus, minimizing the number of handoffs reduces the switching load. Unnecessary handoffs should be prevented, especially w</p><p>　　太多數(shù)量的水平切換嘗試會導致切換請求處理的更長延時，這會迫使信號增強以減少過長處于低服務質量的時間。此外

88、，如果充足的SIR信干比沒有達到的話呼叫就會放棄。切換算法要求網(wǎng)絡資源將呼叫與目標基站連接起來。因而，最小化切換數(shù)量會減少交換負載。不必要的切換應該防止，尤其是當當前基站可以提供滿意的服務質量而沒有干擾其他移動設備和基站時。</p><p>　　In a heterogeneous networking environment the challenge of choosing the “best” networ

89、k 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 qu

90、ality 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</p><p>　　在異

91、構網(wǎng)絡環(huán)境中，選擇最佳網(wǎng)絡的挑戰(zhàn)是主要問題。與強信號的無線局域網(wǎng)相比，一個在石灰石建筑內的弱信號局域網(wǎng)內會提供更好的性能。不同網(wǎng)絡之間切換等級會對移動終端造成非常不同的服務質量，如從WLAN的11 Mb/s 切換到 GSM 的9.6 kb/s。這或許有其他因素如經(jīng)濟差異，這差異不會發(fā)生在個人網(wǎng)絡中，一些網(wǎng)絡的收費是按分鐘或按流量的。</p><p>　　HANDOFF PROCESS</p><

92、;p>　　Both 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

93、fourth generation handoffs.</p><p><b>　　3.3 切換過程</b></p><p>　　水平切換和垂直切換處理都由三部分組成：切換判決，無線鏈路轉移和信道分配[7]。在這節(jié)中，討論每一個部分并描述其在4G切換中扮演的角色。</p><p>　　Handoff Decisions — Horizontal

94、 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

95、 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.</p><p>　　In verti

96、cal 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 str

97、ength 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 t

98、hese characteristics and categorize them depending o</p><p>　　3.3.1 切換判決—水平切換判決主要依賴于有將接收信號強度反映的信道質量和目標基站可用的資源。很多系統(tǒng)的干擾是有限的，意味著信號強度可作為信道質量的指示。如果相鄰基站的RSS超過當前基站的RSS一個預設的門檻值時就執(zhí)行切換。</p><p>　　在垂直切換中，很多

99、網(wǎng)絡特性對是否執(zhí)行切換都有影響。這些特性的大部分在水平切換是不必要的。事實上(如上面的解釋)，水平切換只考慮信號強度和可用信道[8]。下列特點是剛剛提到的品質，對垂直切換判決非常重要。如圖4，根據(jù)他們的適當度和應用性進行了分類。</p><p>　　Quality of Service: Handing over to a network with better conditions and higher per

100、formance 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 connectivit