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)