RACH

TRAN_HUN

NW : RACH Preamble (RA-RNTI, indication for L2/L3 message size)","marks":[{"type":"fontSize","value":16}]}]}],"state":{}},{"type":"block","id":"K3AQ-1735029654074","name":"paragraph","data":{"version":1},"nodes":[{"type":"text","id":"MHX8-1735029654073","leaves":[{"text":"ii) UE  NW : L2/L3 message","marks":[{"type":"fontSize","value":16}]}]}],"state":{}},{"type":"block","id":"tZGf-1735029654078","name":"paragraph","data":{"version":1},"nodes":[{"type":"text","id":"g5DC-1735029654077","leaves":[{"text":"iv) Message for early contention resolution","marks":[{"type":"fontSize","value":16}]}]}],"state":{}},{"type":"block","id":"xJ0l-1735029660314","name":"paragraph","data":{"version":1},"nodes":[{"type":"text","id":"OXpA-1735029660313","leaves":[{"text":"","marks":[{"type":"fontSize","value":16}]}]}],"state":{}},{"type":"block","id":"FmVJ-1735029660485","name":"paragraph","data":{"version":1},"nodes":[{"type":"text","id":"ZOAU-1735029660484","leaves":[{"text":"Typical 'Contention Free' RACH Procedure is as follows :","marks":[{"type":"fontSize","value":16}]}]}],"state":{}},{"type":"block","id":"Lp1f-1735029669364","name":"paragraph","data":{"version":1},"nodes":[{"type":"text","id":"5tbv-1735029669363","leaves":[{"text":" i) UE  NW : RACH Preamble (RA-RNTI, indication for L2/L3 message size)","marks":[{"type":"fontSize","value":16}]}]}],"state":{}},{"type":"block","id":"jFHr-1735029669370","name":"paragraph","data":{"version":1},"nodes":[{"type":"text","id":"2JOp-1735029669369","leaves":[{"text":"iii) UE 为什么要RACH？
For sure, it is not for confusing you :), RACH has very important functionality especially in LTE (and in WCDMA as well). The main purpose of RACH can be described as follows.
i) Achieve UP link synchronization between UE and eNB
ii) Obtain the resource for Message 3 (e.g, RRC Connection Request)

什么时候RACH？
i) Initial access from RRC_IDLE
ii) RRC Connection Re-establishment procedure
iii) Handover (Contention Based or Non Contetion Based)
iv) DL data arrival during RRC_CONNECTED requiring random access procedure
    E.g. when UL synchronisation status is “non-synchronised”
v) 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.
vi) For positioning purpose during RRC_CONNECTED requiring random access procedure;
    E.g. when timing advance is needed for UE positioning

两种RACH方式：
Contention-based and Contention-free   竞争和非竞争
Typical 'Contention Based' RACH Procedure is as follows :
i) UE --> NW : RACH Preamble (RA-RNTI, indication for L2/L3 message size)
ii) UE <-- NW : Random Access Response (Timing Advance, T_C-RNTI, UL grant for L2/L3 message)
iii) UE --> NW : L2/L3 message
iv) Message for early contention resolution

Typical 'Contention Free' RACH Procedure is as follows :
i) UE <--NW : RACH Preamble (PRACH) Assignment
ii) UE --> NW : RACH Preamble (RA-RNTI, indication for L2/L3 message size)
iii) UE <--NW : Random Access Response (Timing Advance, C-RNTI, UL grant for L2/L3 message)

解决竞争接入的两种情况：

• 当终端发送相同的前导序列，随后网络侧分配了相同的 T_C-RNTI 身份标识，由于是相同小区，可能会在相同时间或频率问题发送L2/L3 message，由于两个相互干扰，NW侧解不出来，因为会重新选择前导序列；
• 当两个终端发送相同的前导序列，在MSG 3中会携带UE 的ID（40位随机数，或IMSI），重复概率极小，因此NW侧可以区分UE，进而发送MSG4 进针解决；
  

终端侧在何时何地发送RACH数据？
The, how PRACH Configuration Index is determined ? It is determined by SIB2 parameter prach-ConfigIndex.