RACH in LTE (Random Access Procedure in LTE)

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: LTE,5G,TELECOMMUNICATION

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