Why and when RACH?
- To Achieve Uplink synchronization between UE and eNB.
- To Obtain the resource for RRC Connection Request (Message 3).
The next question comes in mind is “ When RACH “ ?
The answer is during below occasions as per 3gpp spec
- Initial access
- RRC Connection Re-establishment procedure
- DL/UL Data arrival (In RRC Connected )
Whenever TA requirement is there
Types of RACH procedure
There are two types of RACH Procedure :
- Contention Based RACH Procedure (CBRA).
- Non-Contention Based RACH Procedure /Contention free RACH Procedure (CFRA).
Contention Based RACH Procedure (CBRA)
(applicable to all five events)
- Synchronization initiated by UE using common preambles during any one of the following: Initial access, radio link lost, handover, or UL synchronization lost
- Collisions possible: Up to two groups of random-access preambles for contention-based access
(applicable to only handover and DL data arrival)
Non-Contention Based RACH Procedure /Contention free RACH Procedure (CFRA)
- Synchronization initiated by the eNB, using dedicated preambles during handovers
- Up to 1 group of reserved random-access preambles
RACH related parameter is obtained by UE by reading SIB-2 Parameters
RACH Preambles have 4 possible format
- There are total 64 preambles for each cell.
- Preamble available are divided into 2 groups.(Group A and Group B)
- Total 6PRBs are used to send RACH Preamble
LTE Random Access Procedure
The UE randomly selects an RA preamble Which contain RA-RNTI and transmit on PRACH
- The eNodeB transmits the RA Response on the DL-SCH channel
- Derives RA-RNTI
- Calculate TC-RNTI Calculate Timing advance
- Uplink Resource Grant
- Hoping flag
- back off indicator MAC header
- UE saves TC-RNTI from RAR.
- Channel –UL-SCH
- UE does not have a permanent identity till now, so it will have a random number as the UE identity.
- The random UE identity is included in the RRC connection request.
- UE starts the T300 timer.
- UE receive the RRC Connection Setup message in MSG4.
- RRC Connection Setup message carries CRNTI for further communication with network.
- Channel –DL-SCH
- Contention resolution
UE will send the RRC Connection Setup message to initiate further signaling.