隨機(jī)接入學(xué)習(xí)

1、隨機(jī)接入過程UE通過隨機(jī)接入過程（Random Access Procedure）與cell建立連接并取得上行同步。只有取得上行同步，UE才能進(jìn)行上行傳輸。隨機(jī)接入的主要目的：1）獲得上行同步；2）為UE分配一個唯一的標(biāo)識C-RNTI。隨機(jī)接入的使用場景隨機(jī)接入過程通常由以下6類事件之一觸發(fā)：(見36.300的節(jié))1) 初始接入時建立無線連接（UE從RRC_IDLE態(tài)到RRC_CONNECTED態(tài)）；2) RRC連接重建過程（RRC Connection Re-establishment procedure）；3) 切換（handover）；4) RRC_CONNECTED態(tài)下，下行數(shù)據(jù)到達(dá)（

2、此時需要回復(fù)ACK/NACK）時，上行處于“不同步”狀態(tài)；5) RRC_CONNECTED態(tài)下，上行數(shù)據(jù)到達(dá)（例：需要上報測量報告或發(fā)送用戶數(shù)據(jù)）時，上行處于“不同步”狀態(tài)或沒有可用的PUCCH資源用于SR傳輸（此時允許上行同步的UE使用RACH來替代SR）；6) RRC_CONNECTED態(tài)下，為了定位UE，需要timing advance。隨機(jī)接入過程還有一個特殊的用途：如果PUCCH上沒有配置專用的SR資源時，隨機(jī)接入還可作為一個SR來使用。隨機(jī)接入過程有兩種不同的方式：(1) 基于競爭（Contention based）：應(yīng)用于之前介紹的前5種事件；(2) 基于非競爭（Non-Cont

3、ention based）：只應(yīng)用于切換，下行數(shù)據(jù)接收，定位。基于競爭的隨機(jī)接入: UE隨機(jī)選擇preamble碼發(fā)起 Msg1：發(fā)送Preamble碼 eNB可以選擇64個Preamble碼中的部分或全部用于競爭接入 Msg1承載于PRACH上 Msg2：隨機(jī)接入響應(yīng) Msg2由eNB的MAC層組織，并由DL_SCH承載 一條Msg2可同時響應(yīng)多個UE的隨機(jī)接入請求 eNB使用PDCCH調(diào)度Msg2，并通過RA-RNTI進(jìn)行尋址，RA-RNTI由承載Msg1的PRACH時頻 資源位置確定 Msg2包含上行傳輸定時提前量、為Msg3分配的上行資源、臨時C-RNTI等 Msg3：第一次調(diào)度傳輸

4、UE在接收Msg2后，在其分配的上行資源上傳輸Msg3 針對不同的場景，Msg3包含不同的內(nèi)容 初始接入：攜帶RRC層生成的RRC連接請求，包含UE的S-TMSI或隨機(jī)數(shù) 連接重建：攜帶RRC層生成的RRC連接重建請求，C-RNTI和PCI 切換：傳輸RRC層生成的RRC切換完成消息以及UE的C-RNTI 上/下行數(shù)據(jù)到達(dá)：傳輸U(kuò)E的C-RNTIMsg4：競爭解決Example:初始隨機(jī)接入:Msg1:Random Access Preamble包含preamble index和發(fā)送preamble的時頻域位置。Msg2: Random Access Response包含：根據(jù)占用的時頻資源計

5、算獲得RA_RNTI,并下發(fā)一個T_C_RNTI.Msg3:RRCConnectionRequest Msg4: Contention Resolution on DLMsg4攜帶成功解調(diào)的Msg3消息的拷貝，UE將其與自身在Msg3中發(fā)送的高層標(biāo)識進(jìn)行比較，兩者相同則判定為競爭成功, Msg2中下發(fā)的臨時C-RNTI在競爭成功后升級為UE的C-RNTI.Msg5:RRCConnectionsetupC_RNTI用于標(biāo)識RRC Connect狀態(tài)的UEHandover(非競爭隨機(jī)接入)基于非競爭的隨機(jī)接入UE根據(jù)eNB的指示，在指定的PRACH上使用指定的Preamble碼發(fā)起隨機(jī)接入Msg0

6、：隨機(jī)接入指示對于切換場景，eNB通過RRC信令通知UE對于下行數(shù)據(jù)到達(dá)和輔助定位場景，eNB通過PDCCH通知UEMsg1：發(fā)送Preamble碼UE在eNB指定的PRACH信道資源上用指定的Preamble碼發(fā)起隨機(jī)接入Msg2：隨機(jī)接入響應(yīng)Msg2與競爭機(jī)制的格式與內(nèi)容完全一樣，可以響應(yīng)多個UE發(fā)送的Msg1標(biāo)識類型應(yīng)用場景獲得方式有效范圍是否與終端/卡設(shè)備相關(guān)RA-RNTI隨機(jī)接入中用于指示接收隨機(jī)接入響應(yīng)消息根據(jù)占用的時頻資源計算獲得（0001003C）小區(qū)內(nèi)否T-CRNTI隨機(jī)接入中，沒有進(jìn)行競爭裁決前的CRNTIeNB在隨機(jī)接入響應(yīng)消息中下發(fā)給終端（003DFFF3）小區(qū)內(nèi)否C-

7、RNTI用于標(biāo)識RRC Connect狀態(tài)的UE初始接入時獲得（T-CRNTI升級為C-RNTI）（003DFFF3）小區(qū)內(nèi)否SPS-CRNTI半靜態(tài)調(diào)度標(biāo)識eNB在調(diào)度UE進(jìn)入SPS時分配（003DFFF3）小區(qū)內(nèi)否P-RNTI尋呼FFFE（固定標(biāo)識）全網(wǎng)相同否SI-RNTI系統(tǒng)廣播FFFF（固定標(biāo)識）全網(wǎng)相同否初始接入和連接重建場景切換，上/下行數(shù)據(jù)到達(dá)場景競爭判定Msg4攜帶成功解調(diào)的Msg3消息的拷貝，UE將其與自身在Msg3中發(fā)送的高層標(biāo)識進(jìn)行比較，兩者相同則判定為競爭成功UE如果在PDCCH上接收到調(diào)度Msg4的命令，則競爭成功調(diào)度Msg4使用由臨時C-RNTI加擾的PDCCH調(diào)度

8、eNB使用C-RNTI加擾的PDCCH調(diào)度Msg4C-RNTIMsg2中下發(fā)的臨時C-RNTI在競爭成功后升級為UE的C-RNTIUE之前已分配C-RNTI，在Msg3中也將其傳給eNB。競爭解決后，臨時C-RNTI被收回，繼續(xù)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 s

9、ubframe 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).這個例子中：RA_RNTI=2 (RA_RNTI =1+1+10*0=2)eNodeB通過廣播SIB-2發(fā)送prach-ConfigIndex和prach-FrequencyOffset，從而確定該小區(qū)可用于傳輸preamble的時頻資源集合。在這個腳本中SIB

10、2：prach-ConfigIndex：51，DL/UL configuration：1，對于與36.211表Preamble format:4, PRACH density value, ：1 ，：（0,0,0,*）則UE在子幀1上發(fā)送Preamble，頻域：=0，起始RB=0 /根據(jù)下面的公式Nsp=2，選1號公式For preamble format 4, the frequency multiplexing shall be done according towhereis the system frame number and whereis the number of DL to

11、UL switch points within the radio frame.Each random access preamble occupies a bandwidth corresponding to 6 consecutive resource blocks for both frame structures.小結(jié)：時域上占的連續(xù)的subframe數(shù)：1、2、3、UpPTS；占據(jù)子幀內(nèi)的所有slot和所有symbol。配置時需要考慮小區(qū)的覆蓋范圍以及資源的使用（preamble越大，可用于傳輸上行數(shù)據(jù)的資源就越少）。對于TDD而言，通過prach-ConfigIndex查表Tabl

12、e -3和得到preamble format以及可以用于四元組，其中 、確定時域上可用于傳輸preamble的系統(tǒng)幀和子幀號，從而確定可選的時域資源。通過prach-FrequencyOffset得到，并與共同確定了可選的頻域資源（TDD在某個子幀上可能存在多個頻域資源，所以是可選擇的）。小結(jié)：頻域上占6個連續(xù)的RB。對TDD而言，每個子幀可以有多個PRACH資源，這是因為TDD中每個系統(tǒng)幀的上行子幀數(shù)更少，從而要求每個子幀發(fā)送更多的RA請求。在TDD中，每個10ms的系統(tǒng)幀內(nèi)至多可發(fā)送6個RA請求。（見36.211的-3的）對TDD而言，preamble在時域上的配置也是通過prach-Co

13、nfigIndex來指定的，且對應(yīng)的表為36.211的Table -3和。其中表示UE在一個10ms的系統(tǒng)幀內(nèi)有多少次隨機(jī)接入的機(jī)會。/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:-Ini

14、tial 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 sync

15、hronisation 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

16、:-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 proc

17、edure is outlined on Figure .1-1 below:Figure .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 si

18、ze 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 broadca

19、st 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 gra

20、nt 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 i

21、s 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 R

22、equest 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

23、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-

24、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, echoe

25、d 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 s

26、uccess 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 .2-1 below:Figure .2-1: Non-contention based Random Access ProcedureThe three steps of the non-contention based random

27、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 (with