Random Access Preamble & Random Access Response in LTE?

 In LTE, the Random Access Procedure (RACH) is used by the UE (User Equipment) to request access to the network. It involves two key messages:

·         Random Access Preamble (Msg1) - Sent by UE

·         Random Access Response (RAR) (Msg2) - Sent by eNodeB

1.       Random Access Preamble (Msg1) – Sent by UE

Sent on: PRACH (Physical Random Access Channel)

Purpose: UE uses this message to initiate the Random Access procedure and request uplink synchronization.

Contains:

1.       Preamble ID – A randomly selected number (from a set of 64 preambles).

2.       Preamble Transmission Power – Initial power for PRACH transmission.

3.       RA-RNTI (Random Access Radio Network Temporary Identifier) – Derived from Subframe number and PRACH index.

Failure Cases:

·         If the eNodeB does not receive this preamble, the UE will retry with increased power (power ramping).

·         If the maximum attempts are reached, Random Access Failure occurs.

2. Random Access Response (RAR) (Msg2) – Sent by eNodeB

• Sent on: PDCCH (Physical Downlink Control Channel) & PDSCH (Physical Downlink Shared Channel)
• Purpose: eNodeB responds to the UE with timing and resource allocation information.
• Contains:
1. Preamble ID – Identifies which UE the response is for.
2. Timing Advance (TA) Command – Helps the UE align its uplink transmission timing.
3. UL Grant (Uplink Resource Allocation) – Specifies uplink resources for Msg3.
4. Temporary C-RNTI (Cell-RNTI) – A temporary identifier for the UE.
• Failure Cases:

• If UE does not receive RAR within T300 Timer, it retries the process.
• If no response after multiple attempts, RACH failure occurs.

Debugging RACH Failures in QXDM?

RACH Trigger and RACH Attempt?

RACH Trigger: The UE initiates a RACH attempt when:

1. Initial Access: When the UE powers on and tries to attach to the network.
2. RRC Connection Re-establishment: After a radio link failure.
3. Handover: During handover when moving between cells.
4. UL Synchronization Lost: When uplink timing synchronization is lost.
5. On-demand Scheduling: When the UE does not have an uplink grant.

RACH Attempt: RACH follows a 4-step process (Contention-Based RACH)

Step 1: PRACH Preamble Transmission

• UE selects a random preamble from a set and transmits it on the PRACH (Physical RACH).
• The transmission is non-synchronized.

Step 2: Random Access Response (RAR)

• The eNodeB responds with:
• Timing Advance (TA)
• Temporary C-RNTI
• Uplink Grant (UL resources for Msg3)

Step 3: RRC Connection Request (MSG3)

• UE sends an RRC Connection Request.
• If it’s an Attach Request, it carries NAS messages.
• eNodeB forwards it to MME.

Step 4: RRC Connection Setup & Attach Procedure

• If accepted, the UE receives RRC Connection Setup.
• If contention occurs, the UE may need to retransmit.
• After setup, Attach Request & PDN Connectivity Request proceed.

RACH Messages in QXDM:-