Technical details of the Random Access Channel (RACH) procedure, which is a crucial part of the initial access process in cellular communication networks. The RACH procedure allows mobile devices to establish a connection with the network and request resources for communication. We’ll explore each of the four messages involved in this procedure: MSG1, MSG2, MSG3, and MSG4.
- Message 1 (MSG1):
- Purpose: MSG1 is the first message sent by a mobile device during the RACH procedure.
- Objective: It requests access to the network and informs the network about the device’s timing and synchronization parameters.
- Contents:
- Cell Identity: A unique identifier for the cell the mobile device aims to access.
- Timing Advance Value: Used to synchronize the device’s timing with that of the network.
- Random Access Preamble: A short sequence of bits serving as a unique identifier for the mobile device.
- Process:
- The mobile device randomly selects a preamble from a predefined set.
- The network listens for these preambles and attempts to decode them to identify the mobile device.
- If successful, the network responds, granting access to the network.
- Message 2 (MSG2):
- Purpose: MSG2 is the second message sent by the mobile device.
- Objective: It responds to the network’s acknowledgment of MSG1 and provides additional information.
- Contents:
- Radio Network Temporary Identifier (RNTI): A unique identifier used by the network to allocate resources.
- Uplink Grant: Specifies the resources allocated for the mobile device to transmit data.
- Process:
- Upon receiving MSG2, the network verifies the information and acknowledges its receipt.
- Message 3 (MSG3):
- Purpose: MSG3 is the third message sent by the mobile device.
- Objective: It establishes a connection with the network and exchanges control information.
- Contents:
- Uplink Grant: Specifies resources for the mobile device’s data transmission.
- Transport Block Size: Indicates the size of data the device intends to transmit.
- Message Integrity Check (MIC): Ensures message integrity during transmission.
- Process:
- The network verifies the information in MSG3 and acknowledges its receipt.
- Message 4 (MSG4):
- Purpose: MSG4 is the fourth and final message sent by the mobile device.
- Objective: It confirms successful connection establishment and initiates data transmission.
- Contents:
- Transport Block Size: Specifies the data size for transmission.
- MIC: Ensures message integrity.
- Process:
- MSG4 is only for the UE (User Equipment) that passed the check in MSG3.
