RACH: Random Access procedure is used for uplink synchronization and RRC Connection.
UE will get downlink synchronization after successfully decoding SSB.
Before Rel 16, there was only 1 type of RACH that is similar to LTE i.e 4-step RACH.
But after Rel-16, 3GPP introduced new 2-step RACH procedure to reduce the latency of RACH Procedure.
So RACH procedure can be categorized as below:
1. Type-1 RACH (4-step-RACH)
2. Type-2 RACH (2-step-RACH)
Below image gives you a brief of the rach procedures.
In next chapters, we shall see both of the rach procedures in details.
Random Access Process event trigger scenarios:
1. Initial access: UE from RRC_IDLE state to RRC_CONNETTED state.
2. RRC connection re-establishment: So that the UE re-establishes the connection after the radio link fails (the new cell may or may not be the cell where the UE’s radio link failed).
3. Handover: UE in the RRC_CONNETED state it needs a new cell to establish uplink synchronization.
4. DL or UL data arrival during RRC_CONNECTED when UL synchronisation status is “non-synchronised”.
5. In the RRC_CONNETTED state, when uplink data arrives, and the UE does not have PUCCH resources for SR at this time.
6. SR failure: Re-obtain PUCCH resources through random access process.
7. Request by RRC upon synchronous reconfiguration (e.g. handover).
8. Access in RRC_INACTIVE state: UE will change from RRC_INACTIVE state to RRC_CONNETTED state.
9. To establish time alignment when SCell is added.
10. Request other System Information.
11. Beam failure recovery: When the UE detects a failure and finds a new beam, it will select a new beam.
From the above trigger points, Contention Based Random Access (CBRA) is used for the points 2, 3, 4, 5, 6, 8, 9, 10, 11.
Contention Free Random Access is applied for 3, 4, 7, 9, 10, 11.