【導(dǎo)讀】移動無線網(wǎng)絡(luò)日益盛行之時，不同網(wǎng)絡(luò)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(gòu)網(wǎng)絡(luò)的無線技術(shù)，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(nèi))切換機(jī)制的有效性。有效的切換機(jī)制提高。了服務(wù)質(zhì)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(guān)問題。本為給出了令人滿意的切換特性。無線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(quán)重給不同網(wǎng)絡(luò)參數(shù)的垂直切換判決函數(shù)。商和網(wǎng)絡(luò)運(yùn)營商使用。帶寬和覆蓋范圍的差異導(dǎo)致發(fā)展多網(wǎng)絡(luò)接口終端的必要性，多網(wǎng)絡(luò)接口終端可以使用不同網(wǎng)絡(luò)服務(wù)提供的多樣性。平滑快捷的切換，集中致力于減少差錯服務(wù)和允許通過多種無線網(wǎng)絡(luò)的無縫切換。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(wù)質(zhì)量、可靠性和安全性，很有必要。

　　

【正文】 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. 切換判決 —水平切換判決主要依賴于有將接收信號強(qiáng)度 反映的 信道質(zhì)量 和目標(biāo)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(qiáng)度可作為信道質(zhì)量的指示。如果相鄰基站的 RSS超過當(dāng)前基站的 RSS一個預(yù)設(shè)的門檻值時就執(zhí)行切換。 在垂直切換中，很多網(wǎng)絡(luò)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(shí) 上 (如上面的解釋 )， 水平切換只考慮信號強(qiáng)度和可用 信道 [8]。 下列特點(diǎn)是剛剛提到的品質(zhì)，對垂直切換判決非常重要。如圖 4，根據(jù)他們的適當(dāng)度和應(yīng)用性 進(jìn)行了分類。 Quality of Service: Handing over to a work 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 work can provide a higher assurance of continuous connectivity. 服務(wù)質(zhì)量： 在更好條件和更高性能是切換網(wǎng)絡(luò)通暢會提高服務(wù)水平。傳輸率，錯誤率和其他特點(diǎn)能夠被測量以至于來決定哪一個網(wǎng)絡(luò)可提供更高的持續(xù)連接保證。 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 work. Different broadband wireless Inter 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 work and thus handoff decision. 服務(wù)費(fèi)用： 用戶不同服務(wù)的費(fèi)用 是個 主要 問題，有時候成為網(wǎng)絡(luò)選擇的決定性因素。不同帶寬的無線互聯(lián)網(wǎng) 服務(wù)供應(yīng)商和蜂窩網(wǎng)絡(luò)服務(wù)運(yùn)營商會提供各種資費(fèi)計(jì)劃方案和選擇，這可能會影響用戶的網(wǎng)絡(luò)選擇和切換判決。 Security: Risks are inherent in any wireless technology. Some of these risks are similar to those of wired works。 some are exacerbated by wireless connectivity。 some are new. Perhaps the most significant source of risks in wireless works is that the technology’s underlying munications medium, the airwave, is open to intruders, making it the logical equivalent of an Ether port in the parking lot. 安全： 任何無線技術(shù)天生就具有風(fēng)險。 一些風(fēng)險和有線網(wǎng)絡(luò)是一樣的；一些風(fēng)險通過無線連接會加??；而一些風(fēng)險是 新產(chǎn)生的?；蛟S在無線網(wǎng)絡(luò)中最顯著的風(fēng)險源是技術(shù)的根本通信媒介，無線電波對入侵者是開放的，使得它邏輯上等同于停車場的以太網(wǎng)接口。 Power Requirements: Wireless devices operate on limited battery power. When the level decreases, handing off (or remaining connected)to a work 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： 無線設(shè)備 運(yùn)行在有限的電池電源上。當(dāng)功率需求水平下降時， 低功耗的 網(wǎng)絡(luò)切換 能延長使用時間。 例如，如果終端電池快要耗盡了，從 WLAN切換到 WWAN是明智的選擇。事實(shí)上是這樣，當(dāng)終端在蜂窩網(wǎng)絡(luò)中，終端大部分時間里是空閑的。 但是，給定一個不可知的不穩(wěn)定的傳輸特性的 WLAN，終端不能夠在數(shù)據(jù)包傳輸之間待命，因?yàn)檫@里沒有設(shè)定數(shù)據(jù)傳輸?shù)竭_(dá)的時間，數(shù)據(jù)包時偶發(fā)地到達(dá)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 work conditions. By permitting the user to choose a preferred work, the system is able to acmodate the user’s special requirements. 自主 切換： 通過 自主 切換， 用戶在垂 直切換判決中涉及到，并最終決定是否切換 ，不管網(wǎng)絡(luò)條件如何。通過許可用戶選擇網(wǎng)絡(luò)，系統(tǒng)可適應(yīng)用戶的特別要求。 Velocity: In vertical handoff, the velocity factor has a larger weight and imperative effect on handoff decision than in horizontal of the overlaid architecture of heterogeneous works, handing off to an embedded work when traveling at high speeds is discouraged since a handoff back to the original work would occur very shortly afterward. 速度： 速度因素在 垂直 切換判決中比水平切換有很大的權(quán)重和強(qiáng)制性的影響。因?yàn)楫悩?gòu)網(wǎng)絡(luò)的覆蓋結(jié)構(gòu)，當(dāng)終端高速移動時切換到內(nèi)嵌的網(wǎng)絡(luò) 中 是鼓勵這么做的， 因?yàn)?在 隨后很短的時間內(nèi)就會切換回原來的網(wǎng)絡(luò) 。 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 , a handoff made from one cell to another, referred to as intercell handoff,requires handoff rerouting operations to link the mobile’s current munication path to the new base station. 無線鏈路轉(zhuǎn)換 —— 無線鏈路轉(zhuǎn)換是切換過程的第二部分，它是指在新基站為一個呼叫建立連接鏈路。無線鏈路從舊基站轉(zhuǎn)換到新基站。如果無線鏈路轉(zhuǎn)換是在當(dāng)前服務(wù)小區(qū)內(nèi)，即小區(qū)內(nèi)切換，不需要新鏈路轉(zhuǎn)轉(zhuǎn)換操作。從一個小區(qū)到另一個小區(qū)的切換，即小區(qū)間切換，要進(jìn)行重選路由操作把終端當(dāng)前通信路徑連接到新基站。 Once a handoff procedure has menced,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 existing radio link。 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 (., due to rapid mobility). This type of handoff is used in most cellular works to date. Forward handoff, on the contrary, is a faster handoff process, but its problem is a drop in handoff reliability. 一旦需要進(jìn)行切換，切換方案會隨著把呼叫轉(zhuǎn)移到新鏈路的方法而改變。目前已知的方法是前向和后向切換。后向切換中，原服務(wù)基站為切換作準(zhǔn)備，并且直到新基站的控制單元確保資源分配才接入目的基站。在前向切換中切換過程由目標(biāo)基站啟動，且在切換的初始階段不依賴舊基站。這兩種方法都有各自的優(yōu)缺點(diǎn)，后向切換的優(yōu)點(diǎn)是信令信息通過現(xiàn)存的無線鏈路傳輸，因此，再切換的初始階段不需要確定一個新的信令信道。然而，它的缺點(diǎn)是當(dāng)信道質(zhì)量迅速惡化時切換有可能不成功。至今大多數(shù)蜂窩網(wǎng)中仍采用這種切換。相反，前向切換速度較快，但問題是它的切換可靠性較低。 Channel Assignment — The final handoff stage is channel assignment which consists of the allocation of resources at the new base station. If a new