Sign in Subscribe

MSG3 (Message 3)

Last updated on 


Message 3 (MSG3) is a specific message in the LTE (Long-Term Evolution) and 5G radio communication protocols. It plays a crucial role in the Random Access Procedure, which is used by User Equipment (UE) to establish a connection with the base station (eNodeB in LTE or gNodeB in 5G) when the UE initially powers on, reattaches to the network, or needs to request resources for uplink data transmission. Here's a detailed technical explanation of MSG3 and its role in the Random Access Procedure:

1. Random Access Procedure Overview:

  • The Random Access Procedure allows UEs to request access to the network or resources for uplink transmission. It's essential when a UE is initially connecting to the network or when it has data to send.

2. Trigger for MSG3:

  • UE Initiation: The UE initiates the Random Access Procedure when it needs to access the network.
  • Examples: This can happen during power-up, after network reselection, or when the UE has data to send.

3. MSG3 Generation:

  • Content: MSG3 is a specific uplink message generated by the UE.
  • Content Elements: It includes crucial information, such as the identity of the UE (C-RNTI), the preamble used for access, and the timing advance value to align the transmission in time.

4. Preamble Transmission:

  • Preamble Selection: Before sending MSG3, the UE selects a preamble from a predefined set of possibilities. The preamble helps the network identify the UE and assign resources.
  • Frequency Domain: The UE also selects the frequency-domain resources (frequency resource blocks) for the message.

5. Synchronization:

  • Timing Advance: The UE adjusts its transmission timing based on the timing advance value included in MSG3 to align with the network's expectations.

6. Transmission Timing:

  • Contention Resolution Timer: After transmitting MSG3, the UE starts a contention resolution timer, awaiting a response from the network.
  • Retransmission: If the UE doesn't receive a response within the timer's duration, it may need to retransmit MSG3.

7. Network Reception:

  • Preamble Detection: The network's base station continuously monitors the radio channel for incoming preambles from UEs.
  • Preamble Recognition: When a preamble is detected, the network identifies the UE and assigns it a temporary identity (Temporary C-RNTI).

8. Contention Resolution:

  • Multiple UEs: If multiple UEs transmit preambles simultaneously, a contention may arise, and the network may request retransmissions from the UEs.
  • RAR (Random Access Response): The network responds to successful MSG3 reception with a Random Access Response message, assigning resources to the UE for further communication.

9. Network Allocation:

  • Resource Allocation: The network allocates uplink resources (UL grant) for the UE based on the received MSG3 and other scheduling considerations.
  • Temporary C-RNTI: The UE uses the assigned Temporary C-RNTI to identify itself in subsequent communications.

10. Uplink Data Transmission:

  • After Successful MSG3: Following a successful Random Access Procedure, the UE can transmit its intended uplink data to the network using the allocated resources.

MSG3 is a critical part of the Random Access Procedure in LTE and 5G networks, allowing UEs to initiate communication with the network and request the necessary resources for uplink transmission. It enables efficient and flexible access to the network, particularly in scenarios where multiple UEs need to establish connections simultaneously.