Why and when RACH?

Why 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
•  Handover
•  DL/UL Data arrival (In  RRC Connected )

 Whenever TA requirement is there

Types of RACH procedure

There are two types of RACH Procedure :

1. Contention Based RACH Procedure (CBRA).
2. 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

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

MSG1

The UE randomly selects an RA preamble Which contain RA-RNTI and transmit on PRACH

MSG2

• 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
• MCS
• back off indicator MAC header

MSG3

• 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.

MSG4

• 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

MSG5

UE will send the RRC Connection Setup message to initiate further signaling.

Categories: techlteworld

Tags: CBRA, CFRA, Contention resolution, Group A and Group B, LTE, MSG1, MSG2, MSG3, MSG4, MSG5, Preambles, RA-RNTI, RACH, Random Access Procedure in LTE, RAR