RACH Procedure in LTE - PART1

The random access procedure is characterized by:
- Common procedure for FDD and TDD;
- One procedure irrespective of cell size and number serving cells when CA is configured;

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