随机接入学习

随机接入过程UE通过随机接入过程（Random Access Procedure）与cell建立连接并取得上行同步。只有取得上行同步，UE才能进行上行传输。随机接入的主要目的：1）获得上行同步；2）为UE分配一个唯一的标识C-RNTI。随机接入的使用场景随机接入过程通常由以下6类事件之一触发：(见36.300的10.1.5节)1) 初始接入时建立无线连接（UE从RRC_IDLE态到RRC_CONNECTED态）；2) RRC连接重建过程（RRC Connection Re-establishment procedure）；3) 切换（handover）；4) RRC_CONNECTED态下，下行数据到达（此时需要回复ACK/NACK）时，上行处于“不同步”状态；5) RRC_CONNECTED态下，上行数据到达（例：需要上报测量报告或发送用户数据）时，上行处于“不同步”状态或没有可用的PUCCH资源用于SR传输（此时允许上行同步的UE使用RACH来替代SR）；6) RRC_CONNECTED态下，为了定位UE，需要timing advance。随机接入过程还有一个特殊的用途：如果PUCCH上没有配置专用的SR资源时，随机接入还可作为一个SR来使用。随机接入过程有两种不同的方式：(1) 基于竞争（Contention based）：应用于之前介绍的前5种事件；(2) 基于非竞争（Non-Contention based）：只应用于切换，下行数据接收，定位。基于竞争的随机接入: UE随机选择preamble码发起 Msg1：发送Preamble码 eNB可以选择64个Preamble码中的部分或全部用于竞争接入 Msg1承载于PRACH上 Msg2：随机接入响应 Msg2由eNB的MAC层组织，并由DL_SCH承载 一条Msg2可同时响应多个UE的随机接入请求 eNB使用PDCCH调度Msg2，并通过RA-RNTI进行寻址，RA-RNTI由承载Msg1的PRACH时频 资源位置确定 Msg2包含上行传输定时提前量、为Msg3分配的上行资源、临时C-RNTI等 Msg3：第一次调度传输 UE在接收Msg2后，在其分配的上行资源上传输Msg3 针对不同的场景，Msg3包含不同的内容 初始接入：携带RRC层生成的RRC连接请求，包含UE的S-TMSI或随机数 连接重建：携带RRC层生成的RRC连接重建请求，C-RNTI和PCI 切换：传输RRC层生成的RRC切换完成消息以及UE的C-RNTI 上/下行数据到达：传输UE的C-RNTIMsg4：竞争解决Example:初始随机接入:Msg1:Random Access Preamble包含preamble index和发送preamble的时频域位置。Msg2: Random Access Response包含：根据占用的时频资源计算获得RA_RNTI,并下发一个T_C_RNTI.Msg3:RRCConnectionRequest Msg4: Contention Resolution on DLMsg4携带成功解调的Msg3消息的拷贝，UE将其与自身在Msg3中发送的高层标识进行比较，两者相同则判定为竞争成功, Msg2中下发的临时C-RNTI在竞争成功后升级为UE的C-RNTI.Msg5:RRCConnectionsetupC_RNTI用于标识RRC Connect状态的UEHandover(非竞争随机接入)基于非竞争的随机接入UE根据eNB的指示，在指定的PRACH上使用指定的Preamble码发起随机接入Msg0：随机接入指示对于切换场景，eNB通过RRC信令通知UE对于下行数据到达和辅助定位场景，eNB通过PDCCH通知UEMsg1：发送Preamble码UE在eNB指定的PRACH信道资源上用指定的Preamble码发起随机接入Msg2：随机接入响应Msg2与竞争机制的格式与内容完全一样，可以响应多个UE发送的Msg1标识类型应用场景获得方式有效范围是否与终端/卡设备相关RA-RNTI随机接入中用于指示接收随机接入响应消息根据占用的时频资源计算获得（0001003C）小区内否T-CRNTI随机接入中，没有进行竞争裁决前的CRNTIeNB在随机接入响应消息中下发给终端（003DFFF3）小区内否C-RNTI用于标识RRC Connect状态的UE初始接入时获得（T-CRNTI升级为C-RNTI）（003DFFF3）小区内否SPS-CRNTI半静态调度标识eNB在调度UE进入SPS时分配（003DFFF3）小区内否P-RNTI寻呼FFFE（固定标识）全网相同否SI-RNTI系统广播FFFF（固定标识）全网相同否初始接入和连接重建场景切换，上/下行数据到达场景竞争判定Msg4携带成功解调的Msg3消息的拷贝，UE将其与自身在Msg3中发送的高层标识进行比较，两者相同则判定为竞争成功UE如果在PDCCH上接收到调度Msg4的命令，则竞争成功调度Msg4使用由临时C-RNTI加扰的PDCCH调度eNB使用C-RNTI加扰的PDCCH调度Msg4C-RNTIMsg2中下发的临时C-RNTI在竞争成功后升级为UE的C-RNTIUE之前已分配C-RNTI，在Msg3中也将其传给eNB。竞争解决后，临时C-RNTI被收回，继续使用UE原C-RNTI/36.211The RA-RNTI associated with the PRACH in which the Random Access Preamble is transmitted, is computed as:RA-RNTI= 1 + t_id+10*f_idWhere t_id is the index of the first subframe of the specified PRACH (0 t_id 10), and f_id is the index of the specified PRACH within that subframe, in ascending order of frequency domain (0 f_id 6).这个例子中：RA_RNTI=2 (RA_RNTI =1+1+10*0=2)eNodeB通过广播SIB-2发送prach-ConfigIndex和prach-FrequencyOffset，从而确定该小区可用于传输preamble的时频资源集合。在这个脚本中SIB2：prach-ConfigIndex：51，DL/UL configuration：1，对于与36.211表Preamble format:4, PRACH density value, ：1 ，：（0,0,0,*）则UE在子帧1上发送Preamble，频域：=0，起始RB=0 /根据下面的公式Nsp=2，选1号公式For preamble format 4, the frequency multiplexing shall be done according towhereis the system frame number and whereis the number of DL to UL switch points within the radio frame.Each random access preamble occupies a bandwidth corresponding to 6 consecutive resource blocks for both frame structures.小结：时域上占的连续的subframe数：1、2、3、UpPTS；占据子帧内的所有slot和所有symbol。配置时需要考虑小区的覆盖范围以及资源的使用（preamble越大，可用于传输上行数据的资源就越少）。对于TDD而言，通过prach-ConfigIndex查表Table 5.7.1-3和Table 5.7.1-4得到preamble format以及可以用于四元组，其中 、确定时域上可用于传输preamble的系统帧和子帧号，从而确定可选的时域资源。通过prach-FrequencyOffset得到，并与共同确定了可选的频域资源（TDD在某个子帧上可能存在多个频域资源，所以是可选择的）。小结：频域上占6个连续的RB。对TDD而言，每个子帧可以有多个PRACH资源，这是因为TDD中每个系统帧的上行子帧数更少，从而要求每个子帧发送更多的RA请求。在TDD中，每个10ms的系统帧内至多可发送6个RA请求。（见36.211的5.7.1-3的）对TDD而言，preamble在时域上的配置也是通过prach-ConfigIndex来指定的，且对应的表为36.211的Table 5.7.1-3和Table 5.7.1-4。其中表示UE在一个10ms的系统帧内有多少次随机接入的机会。/36.30010.1.5Random Access ProcedureThe random access procedure is characterized by:-Common procedure for FDD and TDD;-One procedure irrespective of cell size;The random access procedure is performed for the following six events:-Initial access from RRC_IDLE;-RRC Connection Re-establishment procedure;-Handover;-DL data arrival during RRC_CONNECTED requiring random access procedure;-E.g. when UL synchronisation status is “non-synchronised”;-UL data arrival during RRC_CONNECTED requiring random access procedure;-E.g. when UL synchronisation status is non-synchronised or there are no PUCCH resources for SR available.-For positioning purpose during RRC_CONNECTED requiring random access procedure;-E.g. when timing advance is needed for UE positioning;Furthermore, the random access procedure takes two distinct forms:-Contention based (applicable to first five events);-Non-contention based (applicable to only handover, DL data arrival and positioning).Normal DL/UL transmission can take place after the random access procedure.10.1.5.1Contention based random access procedureThe contention based random access procedure is outlined on Figure 10.1.5.1-1 below:Figure 10.1.5.1-1: Contention based Random Access ProcedureThe four steps of the contention based random access procedures are:1)Random Access Preamble on RACH in uplink: -There are two possible groups defined and one is optional. If both groups are configured the size of message 3 and the pathloss are used to determine which group a preamble is selected from. The group to which a preamble belongs provides an indication of the size of the message 3 and the radio conditions at the UE. The preamble group information along with the necessary thresholds are broadcast on system information.2)Random Access Response generated by MAC on DL-SCH:-Semi-synchronous (within a flexible window of which the size is one or more TTI) with message 1;-No HARQ;-Addressed to RA-RNTI on PDCCH;-Conveys at least RA-preamble identifier, Timing Alignment information, initial UL grant and assignment of Temporary C-RNTI (which may or may not be made permanent upon Contention Resolution);-Intended for a variable number of UEs in one DL-SCH message.3)First scheduled UL transmission on UL-SCH:-Uses HARQ;-Size of the transport blocks depends on the UL grant conveyed in step 2 and is at least 80 bits.-For initial access:-Conveys the RRC Connection Request generated by the RRC layer and transmitted via CCCH;-Conveys at least NAS UE identifier but no NAS message;-RLC TM: no segmentation;-For RRC Connection Re-establishment procedure:-Conveys the RRC Connection Re-establishment Request generated by the RRC layer and transmitted via CCCH;-RLC TM: no segmentation;-Does not contain any NAS message.-After handover, in the target cell:-Conveys the ciphered and integrity protected RRC Handover Confirm generated by the RRC layer and transmitted via DCCH;-Conveys the C-RNTI of the UE (which was allocated via the Handover Command);-Includes an uplink Buffer Status Report when possible.-For other events:-Conveys at least the C-RNTI of the UE.4)Contention Resolution on DL:-Early contention resolution shall be used i.e. eNB does not wait for NAS reply before resolving contention-Not synchronised with message 3;-HARQ is supported;-Addressed to:-The Temporary C-RNTI on PDCCH for initial access and after radio link failure;-The C-RNTI on PDCCH for UE in RRC_CONNECTED;-HARQ feedback is transmitted only by the UE which detects its own UE identity, as provided in message 3, echoed in the Contention Resolution message;-For initial access and RRC Connection Re-establishment procedure, no segmentation is used (RLC-TM).The Temporary C-RNTI is promoted to C-RNTI for a UE which detects RA success and does not already have a C-RNTI; it is dropped by others. A UE which detects RA success and already has a C-RNTI, resumes using its C-RNTI.10.1.5.2Non-contention based random access procedureThe non-contention based random access procedure is outlined on Figure 10.1.5.2-1 below:Figure 10.1.5.2-1: Non-contention based Random Access ProcedureThe three steps of the non-contention based random access procedures are:0)Random Access Preamble assignment via dedicated signalling in DL:-eNB assigns to UE a non-contention Random Access Preamble (a Random Access Preamble not within the set sent in broadcast signalling).-Signalled via:-HO command generated by target eNB and sent via source eNB for handover;-PDCCH in case of DL data arrival or positioning.1)Random Access Preamble on RACH in uplink: -UE transmits the assigned non-contention Random Access Preamble.2)Random Access Response on DL-SCH:-Semi-synchronous (withi