MSG1…4 Message 1 to 4 in RACH procedure

The Random Access Channel (RACH) procedure is a key component of the initial access process in cellular communication networks. It is used by mobile devices to establish a connection with the network and request resources for communication. The RACH procedure involves four messages, known as MSG1, MSG2, MSG3, and MSG4. Each of these messages plays a specific role in the RACH procedure and is essential for successful communication.

Message 1 (MSG1)

MSG1 is the first message sent by a mobile device during the RACH procedure. Its primary purpose is to request access to the network and to inform the network of the device's timing and synchronization parameters. MSG1 contains several important pieces of information, including the cell identity, timing advance value, and random access preamble.

The cell identity is a unique identifier for the cell that the mobile device is attempting to access. The timing advance value is used to synchronize the timing of the mobile device with that of the network. The random access preamble is a short sequence of bits that serves as a unique identifier for the mobile device.

When a mobile device sends MSG1, it randomly selects a preamble from a predefined set of preambles. The network listens for these preambles and attempts to decode them to identify the mobile device. If the network successfully decodes the preamble, it sends a response to the mobile device, indicating that it has been granted access to the network.

Message 2 (MSG2)

MSG2 is the second message sent by a mobile device during the RACH procedure. It is sent in response to the network's acknowledgment of MSG1 and contains additional information that is required for successful communication. MSG2 includes the radio network temporary identifier (RNTI) and the uplink grant.

The RNTI is a unique identifier that is used by the network to identify the mobile device and allocate resources for communication. The uplink grant specifies the resources that are allocated for the mobile device to transmit its data to the network. Once the network receives MSG2, it verifies the information and acknowledges the receipt of the message.

Message 3 (MSG3)

MSG3 is the third message sent by a mobile device during the RACH procedure. Its primary purpose is to establish a connection with the network and to exchange control information. MSG3 contains several important pieces of information, including the uplink grant, the transport block size, and the message integrity check (MIC).

The uplink grant specifies the resources that are allocated for the mobile device to transmit its data to the network. The transport block size specifies the size of the data that the mobile device intends to transmit. The MIC is used to ensure the integrity of the message and to verify that it has not been modified during transmission.

When the network receives MSG3, it verifies the information and acknowledges the receipt of the message. The network may also send additional control information to the mobile device, such as the downlink grant, which specifies the resources that are allocated for the network to transmit data to the mobile device.

Message 4 (MSG4)

MSG4 is the fourth and final message sent by a mobile device during the RACH procedure. Its primary purpose is to confirm the successful establishment of the connection and to initiate data transmission. MSG4 contains several important pieces of information, including the transport block size and the message integrity check (MIC).

When the network receives MSG4, it verifies the information and acknowledges the receipt of the message. The network may also send additional control information to the mobile device, such as the downlink grant, which specifies the resources that are allocated for the network to transmit data to the mobile device.

Once the connection is established, the mobile device and the network can exchange data using the allocated resources. The mobile device can continue to use the allocated resources for a specified time period or until the resources are no longer needed. Once the time period or the data transmission is completed, the resources are released, and the connection is terminated.

It is important to note that the RACH procedure is a random access procedure, which means that multiple mobile devices may attempt to establish a connection with the network simultaneously. This can lead to collisions, where two or more devices send their preambles at the same time, causing interference and making it difficult for the network to decode the preambles.

To mitigate collisions, the RACH procedure uses a contention-based mechanism, where multiple devices randomly select a preamble from a predefined set of preambles and attempt to transmit their messages. If there is a collision, the network sends a negative acknowledgment (NACK) to the devices, indicating that they should retry the transmission using a different preamble.

In summary, the RACH procedure is a critical component of the initial access process in cellular communication networks. It allows mobile devices to establish a connection with the network and request resources for communication. The procedure involves four messages, MSG1, MSG2, MSG3, and MSG4, each of which plays a specific role in the process. By understanding the RACH procedure and its messages, network operators can ensure that their networks are operating efficiently and providing reliable communication services to their users.