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What are the two types of RACH procedure ?

In 5G, the Random Access Channel (RACH) process is crucial for initial access, allowing devices to establish a connection with the network. There are two main types of RACH procedures: Contention-Based and Non-Contention-Based. Each serves different scenarios and requirements within the 5G system.

Contention-Based RACH

Contention-Based RACH is the most common procedure for initial access, especially when a device is connecting to the network for the first time or after losing synchronization. In this method, multiple devices may attempt to access the network at the same time using the same set of resources. The process involves four main steps:

  1. Preamble Transmission: The device randomly selects a preamble from a set provided by the network and transmits it on the RACH.
  2. Random Access Response (RAR): The network responds to the detected preambles, providing timing advance and resource allocation for the next step.
  3. Scheduled Transmission: The device sends a message (Msg3) using the allocated resources.
  4. Contention Resolution: If multiple devices selected the same preamble, the network resolves the contention by responding only to the device whose message was correctly received.

This method is efficient for handling a large number of devices but can result in collisions if multiple devices choose the same preamble. The contention resolution step is essential to ensure only one device succeeds in accessing the network.

Non-Contention-Based RACH

Non-Contention-Based RACH is used in scenarios where the network can directly assign a unique preamble to a specific device. This eliminates the possibility of collisions, as no other device will use the same preamble at the same time. The typical use cases include:

  • Handover: When a device moves from one cell to another, the target cell can assign a dedicated preamble for the device to use during the handover process.
  • Scheduled Access: For certain IoT or machine-type communication scenarios, the network may schedule access for devices and assign preambles accordingly.

The procedure is simpler and faster since there is no need for contention resolution. The device transmits the assigned preamble, receives a response, and proceeds with the connection setup.

Summary

  • Contention-Based RACH: Used for general access, supports many devices, but may require contention resolution due to possible collisions.
  • Non-Contention-Based RACH: Used for specific, scheduled, or handover scenarios, avoids collisions by assigning unique preambles.

The choice between these two procedures depends on the network's requirements, device type, and the specific scenario (e.g., initial access, handover, or scheduled communication).

Further Readings