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1、LTE 标准介绍,王斌,LTE的系统架构LTE的协议堆栈和功能划分LTE用户面PDCPRLCMACLTE 控制面LTE的关键技术LTE-A的关键技术中继技术载波聚合技术增强技术,3GPP标准组织与制定阶段,阶段1：需求阶段2：结构阶段3：详细实现RAN1 - 物理层RAN2 - L2与L3无线协议RAN3 - 结构与S1/X2接口RAN4 - RF与RRM性能要求RAN5 - 终端测试,无线网络结构,出于达到简化信令流程，缩短延迟的目的，E-UTRAN舍弃了UTRAN的RNC-NodeB结构，完全由eNB（基站）组成,LTE协议堆栈和功能划分,用户平面结构,在用户面，协议栈主要分为MAC、RL
2、C、PDCP。主要完成头压缩、加密、解密、调度、ARQ和HARQ的功能,控制平面,控制平面的底层协议，和用户平面相似，而上层的RRC层和非接入子层（NAS）是控制平面最重要的部分。,EPS Bearer Service Architecture,RRC子层,RRC子层主要承担广播、无线接口寻呼、RRC连接管理、无线承载控制（RBC）、移动性管理、UE测量上报和控制等功能。把RRC在网络侧终结于eNB，是网络的一个重大改变。,RRC的状态,RRC_IDLE：当UE不发起业务时，通常处于该状态。此时，eNB侧也没有UE的RRC上下文，只进行一些诸如监听寻呼、小区广播消息等操作，在eNB之内不存储R
3、RC上下文。RRC_CONNECTED：UE已经建立业务后，进入RRC连接状态，E-UTRAN具有该UE的上下文，并知道UE所在的小区；网络和UE之间进行数据传送；进行切换和邻区测量；以及控制UE进行非连续发送/接收（DTXDRX）。,非接入层,非接入层（NAS，Non-Access-Stratum），顾名思义，更多的是完成核心网对用户的移动性、呼叫控制和QoS管理功能，不属于接入网的范畴，这部分终结于GW。UE的NAS层状态和其所处的RRC状态有相应的关系,NAS层的状态,LTE_DETACHED：网络和UE侧都没有RRC实体，此时UE通常处于关机、去附着等状态LTE_IDLE：对应RRC的
4、IDLE状态。UE和网络侧存储的信息包括：给UE分配的IP地址、安全相关的参数（密钥等）、UE的能力信息、无线承载。此时UE的状态转移由基站或GW决定。3)LTE_ACTIVE：对应RRC连接状态；状态转移由基站或GW决定。,NAS层的状态,LTE_DETACHED：网络和UE侧都没有RRC实体，此时UE通常处于关机、去附着等状态LTE_IDLE：对应RRC的IDLE状态。UE和网络侧存储的信息包括：给UE分配的IP地址、安全相关的参数（密钥等）、UE的能力信息、无线承载。此时UE的状态转移由基站或GW决定。3)LTE_ACTIVE：对应RRC连接状态；状态转移由基站或GW决定。,层2的整体功
5、能描述,服务访问点（SAP）：同一系统中，相邻两层的实体进行通信的地方是服务访问点。物理层和MAC层之间的SAP提供传输信道。MAC层和RLC层之间的SAP提供逻辑信道。MAC层可以将多个逻辑信道（无线承载）复用到同一个传输信道（传输块）上。,下行的层2结构,上行的层2结构,MAC层的描述,复用和解复用（1）,复用和解复用（2）,RLC模式,AM模式： AM模式是为可靠性要求很高并且分组的长度可变的业务提出的。它的典型特征是支持ARQ和分组的切割和串接。M模式：UM模式是为可靠性要求不高的业务提出的。它的典型特征是支持分组的切割和串接，但不支持ARQ。TM模式：TM模式是直接将高层的分组传到下
6、层，在RLC层不封装协议头。在TM模式下，高层的数据是不分段的。目前，LTE仅仅确定了在随机接入阶段对于控制平面支持TM模式，。,RLC实体,RLC- concatenation, segment/resegment,图中给出了RLC数据的打包和分片过程，形象地给出了RLC SDU数据根据信道的质量好坏来进行串接还是分割。红色方块是PDCP PDU序号，蓝色方块是RLC PDU的序号。,RLC的AM和UM模式的数据发送过程,PDCP层,PDCP层的最重要功能是头压缩和加密功能，在这里需要特别提出的是：PDCP的序号是用来产生与加密相关的时变MASK，它不参与压缩和被加密,RRC概述,RRC连接
7、建立,RC的建立过程中要考虑3个方面的内容，即：移动和安全方面的控制、NAS信令的传输、冲突解决和路由等几个方面的问题,RRC重配置,Measurement configurationMobility control information (HO)NAS Radio resource configurationSRB/DRB configurationMAC configurationSPS configurationPhysical configurationSecurity (HO),RRC重新建立,LTE的关键技术,随机接入过程切换重传技术调度技术干扰协调技术,随机接入过程（1）,Pu
8、rpose Contention resolutionUplink synchronizationContention based random access procedure Initial access from RRC_IDLE RRC Connection Re-establishment procedure HandoverNon-contention based random access procedure,随机接入过程（2）,随机接入过程（3）,RA preamble (message1)：The UE randomly selects an RA preamble sequ
9、ence from the set of sequences available in the cell and transmits it on an RA channelRA Response (message2) ：1) correct timing-advance。2) Corresponding RA response is received only if RA-RNTI and preamble index is matched RA Message (message3) : Message 3 contain one unique UE ID, which could be S-
10、TMSI or random id or C-RNTIContention Resolution (message4): 1) Contention is resolved if UE find its UE ID in message 4 which is sent in message 3 2) UE will only feedback ACK i.e. no NACKFallback scheme: If random access procedure fails due to e.g. loss of msg2 or failed msg3 or contention resolut
11、ion UE would start from step1 after some delay.,重传技术,HARQ与ARQ,HARQ采用停等协议，位于MAC层， RTT时间 是 8，进程数也是8HARQ分为：同步HARQ，异步HARQ，自适应HARQ，和非自适应HARQ ARQ位于RLC层，采用基于窗口的选择性ARQARQ与HARQ的交互：由于ARQ与HARQ实体位于同一节点中，发送实体如果得知HARQ传输错误，就会发送一个Local NACK 通知ARQ的发送实体。加速了错误的发现。,切换技术（1）,在LTE系统中，软切换方案已经被彻底抛弃，这是由于LTE沿用了HSDPA/HSUPA思想，放
12、弃了宏分集这种需要网络架构支持的技术。LTE现在只支持没有丢包的硬切换机制,切换技术（2）,切换技术（3）,源eNB只转发没有被UE成功接收的RLC_SDU，来减少空口资源的浪费,非连续接收（DRX）,LTE supports DRX to enable UE power savings by turning off some or all of its radio circuitry, thereby increasing the battery lifetime of the UEWhen the UE does not have an established radio-resource
13、 control (RRC) connection, it wakes up and monitors the paging channel every DRX cycle,DRX in RRC_CONNECTED,When the UE has an RRC connection, the DRX function is characterized by a DRX cycle, an on-duration period, and an inactivity timerThe UE wakes up and monitors the PDCCH at the beginning of ev
14、ery DRX cycle for the entire on-duration period. If no scheduling assignment is received, the UE falls asleep again.Whenever the UE receives an assignment from the network, it starts (or restarts) the inactivity timer and continues to monitor the PDCCH until the timer expiresNote that the HARQ opera
15、tion overrides the DRX function. Thus, the UE wakes up for possible HARQ feedback,DRX operation,调度技术（1）,dynamic packet scheduler（1）,dynamic packet scheduler（2）,Scheduling OperationsDifferent schedulers operate for the DL-SCH and UL-SCHDynamic (DL/UL)Addressed to C-RNTI on PDCCHthe UE monitors PDCCH-
16、subframes during active time,dynamic packet scheduler（3）,time-domain scheduler selects the users to be scheduled in the next TTIRB QosHARQ information,dynamic packet scheduler（4）,Frequency domain scheduler is limited to only handling the subset of users selected by the time-domain schedulerThe FDPS
17、principle exploits frequency selective fading by only scheduling users on the PRBs with high channel quality, while avoiding the PRBs where a user experiences deep fades.,Semi-persistent scheduling,The basic principle is to periodically allocate certain transmission resources for a particular userTh
18、e time pattern for semi-persistent scheduled resources is initially configured via the RRC protocolThe base station can always override semi-persistent scheduling in the TTI by dynamically scheduling the same user via the PDCCH.The main advantage of semi-persistent scheduling is that no explicit PHY
19、 signaling on the PDCCH is required for every transmission, resulting in a reduction of the downlink control signaling overhead,干扰协调技术,LTE的演进,2008,2009,2010,2011,2007,ITU-R WP5D,LTE,Core spec approval,TSG-RANmeeting,Test spec and CRs,Study Item,Work Item,IMT-Advanced,Proposals for Evaluation,Circula
20、r Letter,#394 Mar-,#4027 May-,#419 Sep-,#422 Dec-,#43,#44,#45,#46,#47,#48,#49,#50,#51,#52,#38&WS26 Nov-,LTE-A是在LTE版本8/版本9的基础之上进一步演进和增强， LTE-A将能够支持全业务数据传输速率，即：在高速情况下支持高达100Mb/s的峰值速率的移动接入，以及在游牧/本地无线接入等低速移动环境下支持1Gb/s的峰值速率。,LTE-A,中继技术载波聚合技术多点传输技术增强技术,LTE-A的关键技术,中继技术,Type of Relays (1),Type 1 relays are
21、non-transparent to the UE, have their own physical identity, and transmit all the necessary physical channels to appear as a regular eNB to all the UE.Type 2 relays do not have their own physical identity and are transparent to the UE (i.e., the UE is unaware of their existence)。,Type of Relays (2),
22、Type 1 relays (Outband relaying operation) does not pose any challenges at the lower layers of the LTE protocol stack No air interface changes are required to support this kind of relay.Type 1 (Inband relaying operation) requires the separation of backhaul and access links either in time or in space
23、. Spatial separation between access and backhaul links can be attained by sophisticated implementation and deployment,ARCHITECTURE AND PROTOCOLS,ARCHITECTURE,On one hand, it supports normal eNB functionality, such as terminating the radio protocols and the base-station-to-core network (S1) and inter
24、-base station (X2) interfaces。On the other hand, an RN supports a subset of UE functionalitya proxy concept has been developed that minimizes the impact on network nodes：from a core network perspective, the RN is seen as a normal eNB and from an RN perspective, the DeNB mimics a regular core network
25、.,Relay Startup and Attach,At power up, the RN can first attach to the network using the normal attach procedure for UE and retrieve its initial configuration parameters (e.g., the list of available DeNBs).The RN connects to a DeNB selected from the previous list. The startup procedure is the same,
26、except that this time the RN signals its identity to the chosen DeNB using a specific RN indicator at radio connection setup. Based on this information, the DeNB now also acts as a gateway, and it selects an MME that supports relay functionality. After the MME successfully sets up the context and th
27、e DeNB sets up bearers for S1 and X2, the RN will set up S1 and X2 connections with its DeNB,Bearer Management,Bearers for the RN can be activated and modified by the DeNB. This procedure is the same as the normal network-initiated beareractivation/modification procedure for UE, with the exception t
28、hat in this case gateway functionality is embedded within the DeNB,Handover,As it is built on base stations functionality, LTE relaying supports legacy mobility in RRC connected state. The DeNB, aware of a certain set of UE being attached to the RN, proxies the relevant S1 messages between the RN an
29、d the MME (S1 handover) and, correspondingly, the X2 messages between the RN and the target base station (X2 handover).,载波聚合技术,ITU IMT-Advanced要求系统的最大带宽不小于40MHz。考虑到现有的频谱分配方式和规划，很难找到足以承载IMT-Advanced系统带宽的整段频带。基于以上考虑，LTE-Advanced提出了载波聚合的概念，它的根本目的是将多个相对窄带的载波聚合成一个更宽的频谱，从而满足LTE-A的要求。载波聚合可以分为连续载波聚合以及频带内和频带
30、间的非连续载波聚合，最大聚合带宽为100MHz。,载波聚合技术（2）,component carrier,Each UE unit has a single serving cell that provides all necessary control information and functions, such as non-access stratum (NAS) mobility information, security input, RRC connection maintenance, etc. This serving cell is referred to as the p
31、rimary cell (PCell)If more than one CC is configured for a user, the additional CCs are denoted secondary cells (SCells) for the user.,PROTOCOL STACK,RRM CONSIDERATIONS (1),CC configuration,CC activation / de-activation,Cell activation/deactivation is a mechanism aiming to reduce UE power consumptio
32、n in LTE-A. To further reduce UE battery consumption, an SCell in LTE-A carrier aggregation can be activated or deactivatedFor a deactivated SCell, UE does not receive any downlink signal, nor does the UE transmit any uplink signalFor an activated SCell, UE performs normal activities for downlink re
33、ception and uplink transmissionServing cell activation/deactivation is performed independently for each SCellActivation/deactivation is not applicable for the PCell since the functions provided by the PCell require it to always remain activated when the UE has an RRC connection to the network.,Dynam
34、ic packet scheduler,LTE-Advanced PS is allowed to schedule users across multiple CCs. the scheduling could be done in parallel for the different CCs, including some coordination to ensure fairness and joint control for users scheduled on multiple CCsLTE-A includes enhancements allowing the base stat
35、ion to send a scheduling grant on one CC for scheduling the user on another CC, which is referred to as cross-CC scheduling as the scheduling grant and the corresponding data transmission takes place on different CCs.The cross-CC scheduling functionality offers additional system flexibility for furt
36、her optimizing control and data channel performance across multiple CCs,多点传输,CoMP技术本质上是通过多小区MIMO技术解决这种干扰，其基本思路是利用空间信道上的差异来进行信号传输，从处理干扰的角度可以分为两种方式一是将其他小区的干扰信号转化为本小区用户的有用信号，这种技术即为联合处理技术另一种方式是通过基站端进行合理的空域调整，即通过预编码使得信号在空间上相互隔开，减小不同小区间的干扰，从而用户的链路质量得到保证，这种技术即为协同波束赋形。,联合处理,在联合处理方案中，协作小区集合内的全部小区在相同的无线资源块中发送
37、相同或者不同的数据到终端，即多个协作小区在同一时刻发送数据到同一个UE。通过联合传输方式，将原来LTE系统中不同小区间的干扰信号变成有用信号，从而减少小区间干扰，提升用户的接收信号质量，最终达到提升系统性能的目的动态小区选择是CoMP JP中的特例，仍然是多个小区联合服务于一个UE，但是在一个时刻PDSCH传输仅来自一个点，其他传输点的相同资源要空出来不传输任何数据,协同调度协同波束赋形,在该技术方案中，一个用户的数据同时只有一个小区传输；通过多个小区协作的调度来控制降低干扰。协作小区集合内的其他小区可以利用相同的无线资源块为不同UE服务，协作小区集合内各小区发送信号需要根据对其它小区信号的干
38、扰进行协调，尽可能地减少对其它小区的干扰。协作小区间通过协调发送信号波束的方向，有效地将干扰比较大的波束避开，这样，通过窄的波束，就能够有效降低小区间的干扰，提升接收信号的质量。,干扰协调增强,为丁更好地提供系统容量、改善覆盖并卸载宏蜂窝面临的流量压宏蜂窝中还会部署有大量的微小区(pico)以及家庭基站(femto)，这些pico和femto采用与宏蜂窝相同的频率从而形成异构的网络环境。在上述环境中宏小区微小区以及家庭基站之间将形成较强的同频下扰。,干扰协调增强,为了抑制上述场景的干扰在LTE R10版本中，为下行子帧引入了近空白子帧（ABS：Almost Blank Subframe）的概念
39、。干扰者尽量不在ABS子帧中调度本小区的用户。对于Macro-pico的场景, Macro是干扰者，其可以将某些子帧配置为ABS子帧。 Macro UE只会被调度在Macro小区的非ABS子帧上。而pico-eNB可以将pico边缘那些受Macro干扰较大的UE调度在与Macro小区ABS子帧所相对应的pico小区子帧上,针对机器通信的接入机制增强,在未来，MTC的应用可能会激增，成为与人和人通信并驾齐驱的主流通信方式。如果不对目前主要针对人与人通信方式而设计的移动通信网络进行相应的增强，那么将会显著降低移动通信网络的效率，不但不能服务好MTC通信，而且也会使人与人之间的通信受到很大的影响。由
40、于MTC通信与人-人通信间通信方式不同，移动通信网络需要在核心网侧和无线网侧针对MTC通信都进行相应的增强。,无线网拥塞备选解决方案（1）,接入控制方案。在LTE中，可以通过接入等级限制(Access Class Barring)抑制过多的流量，避免拥塞。现行的LTE系统，具有16个接入等级。LTE小区会广播一个限制因子和接入等级限制时间，当UE接入时，UE抽取一个随机数，将这个随机数和限制因子做比较。如果这个随机数小于限制因子，UE开始随机接入过程。此方案建议为MTC设备引入一个新的接入等级，或者为MTC设备引入一个特定的限制因子。,无线网拥塞备选解决方案（2）,资源划分方案。大量MTC设备
41、的同时接入会增大RACH信道的负载及RACH前导碰撞的概率，同时也会影响正常UE的工作，使正常UE的碰撞概率增加。可以将一些RACH资源用于MTC，其他RACH资源用于正常UE，这样，MTC冲突概率的增加不会影响正常UE。MTC特定的随机接人回退方案。此方案的主要思想是为MTC引入一个与普通UE不同的回退方案。比如，初始接入时即有一个非零的回退值；另外，回退值的范围也可以比普通UE的更广。,无线网拥塞备选解决方案（3）,MTC特定时隙接入方案。此方案为MTC设备定义一个接入时隙，每个MTC设备只能在特定的接入时隙内接入，其具体的接人时隙可以通过其标识（如IMSI号）来决定。Pull方案。对于某
42、些MTC应用，如智能抄表，可以不让MTC设备自动连接到网络来发送它们的数据，即不允许它们发起主叫业务，而是通过MTC服务器控制的方法，让MTC服务器触发MME来寻呼相应的MTC设备，只有被寻呼的设备才能接入网络发送它们的数据,核心网拥塞解决方案（1）,在“RRC Connection Request”消息中引入一个新的连接原因（Establishment Cause）：“Delay Tolerant Access”。当UE采用此cause发起RRC连接时，网络就可以判定此UE目前正在使用MTC应用，可以容忍较长时间的延迟接入。因而当网络拥塞时，eNB可以直接使用“RRC Connection Rejeet”拒绝此UE的接入,核心网拥塞解决方案（2）,在NAS层引入一个增强的等待计时器“eWaitTimer”，当网络决定拒绝UE的MTC应用接入时， 其会在“RRC Connection Reject”或“RRC Connection Release”中反馈一个“eWaitTime”的值，范围从O秒到4096秒。UE接收到此值后将其传给NAS层并触发“eWaitTimer”，在这段时间内UE的MTC应用都不会发起RRC连接。,
