【導讀】代數(shù)字系統(tǒng)比第一代有更高的容量和更好的語音質(zhì)量。此外,由于在各個國。在第二代蜂窩系統(tǒng)中,有兩個是比較廣泛部署的,他們分別是GSM(全。球移動通信系統(tǒng))和CDMA。相比于1G的模擬系統(tǒng),2G系統(tǒng)主要支持語。在后來發(fā)布2G版本的標準中,主要介紹了其支持數(shù)據(jù)傳輸?shù)哪芰?。倡議的IMT-2021為向3G的演進鋪平了道路。GSM和CDMA形成了自己獨立的3G合作伙伴項目,使IMT-2021標準發(fā)展成基于CDMA技術(shù)的標準。3GPP的3G標準被稱為寬帶。載波,其被稱為CDMA2021-3X。實踐中的支持率遠遠低于當時聲稱的標準。因此其需要作出一系列認真努力來提。平行于HRPD系統(tǒng),3GPP2同時也制定了一個聯(lián)合的語音。IEEE系列標準被正式。Wi,LET,IMT-2021要求的UMB滿足,因此它們可以滿足IMT-2021標準。OFDMA上行接入,因此要增加上行覆蓋。由于上行MIMO未受聘在首次發(fā)布的LTE標準,因此上行峰值。善外,LTE系統(tǒng)提供兩到四倍較高的細胞相對推出6HSPA系統(tǒng)的頻譜效率。三個系統(tǒng)的性能有所不同,因此預計將有微小的差別。率,同時支持各自的早期版本的向后兼容性。其中幾個增強,包括支持一個大于

　　

【正文】 al of supporting packetswitched traffic with seamless mobility, quality of service QoS and minimal latency. A packetswitched approach allows for the supporting of all services including voice through packet connections. The result in a highly simplified flatter architecture with only two types of node namely evolved NodeB eNB and mobility management entity/gateway MME/GW. This is in contrast to many more work nodes in the current hierarchical work architecture of the 3G system. One major change is that the radio work controller RNC is eliminated from the data path and its functions are now incorporated in eNB. Some of the benefits of a single node in the access work are reduced latency and the distribution of the RNC processing load into multiple eNBs. The elimination of the RNC in the access work was possible partly because the LTE system does not support macrodiversity or softhandoff. In this chapter, we discuss work architecture designs for both unicast and broadcast traffic, QoS architecture and mobility management in the access work. We also briefly discuss layer 2 structure and different logical, transport and physical channels along with their mapping. Network architecture All the work interfaces are based on IP protocols. The eNBs are interconnected by means of an X2 interface and to the MME/GW entity by means of an S1 interface as shown in Figure . The S1 interface supports a manytomany relationship between MME/GW and eNBs [1] The functional split between eNB and MME/GW is shown in Figure . Two logical gateway entities namely the serving gateway SGW and the packet data work gateway PGW are defined. The SGW acts as a local mobility anchor forwarding and receiving packets to and from the eNB serving the UE. The PGW interfaces with external packet data works PDNs such as the Inter and the IMS. The PGW also performs several IP functions such as address allocation, policy enforcement, packet filtering and routing. The MME is a signaling only entity and hence user IP packets do not go through MME. An advantage of a separate work entity for signaling is that the work capacity for signaling and traffic can grow independently. The main functions of MME are idlemode UE reachability including the control and execution of paging retransmission, tracking area list management, roaming, authentication, authorization, PGW/SGW selection, bearer management including dedicated bearer establishment, security negotiations and NAS signaling, etc. Evolved NodeB implements NodeB functions as well as protocols traditionally implemented in RNC. The main functions of eNB are header pression, ciphering and reliable delivery of packets. On the control side, eNB incorporates functions such as admissioncontrol and radio resource management. Some of the benefits of a single node in the access work are reduced latency and the distribution of RNC processing load into multiple eNBsFigure . Network architecture. Figure . Functional split between eNB and MME/GW. The user plane protocol stack is given in Figure . We note that packet data convergence protocol PDCP and radio link control RLC layers traditionally terminated in RNC on the work side are now described in Section . Figure . User plane protocol.