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5G NR RACH Preamble Types: Long and Short Preambles

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The Random Access Channel (RACH) is a critical component in 5G New Radio (NR) networks, facilitating the establishment of communication between user devices (UEs) and the network. The RACH procedure allows UEs to initiate communication with the network when there is no existing connection. In 5G NR, two types of RACH preambles are defined: Long Preamble and Short Preamble.

1. Long Preamble:

Generation:

  • The Long Preamble is generated by the UE using a specific pseudo-random sequence and is transmitted over the PRACH (Physical Random Access Channel).
  • The generation involves spreading the sequence with a specified root sequence index and cyclic shift, ensuring orthogonal transmission with other UEs.

Structure:

  • The Long Preamble is longer in duration compared to the Short Preamble. It is divided into multiple preambles, each having a specific time duration and frequency resources.
  • The Long Preamble carries more information bits, making it suitable for scenarios where more initial information is required for synchronization.

Use Cases:

  • Long Preambles are typically used in scenarios with higher propagation delays, where the UE and the network may experience longer time delays during the transmission.

2. Short Preamble:

Generation:

  • The Short Preamble is also generated by the UE, but it uses a different pseudo-random sequence than the Long Preamble.
  • The generation process involves selecting a specific root sequence index and cyclic shift, ensuring orthogonal transmission with Long Preambles and other Short Preambles.

Structure:

  • Short Preambles have a shorter duration compared to Long Preambles, and they are also divided into multiple preambles with specific time and frequency resources.
  • Short Preambles carry less information compared to Long Preambles, making them suitable for scenarios where a quicker initial access is required.

Use Cases:

  • Short Preambles are used in scenarios where the propagation delay is expected to be shorter, enabling faster access to the network.

Common Aspects:

Detection and Synchronization:

  • Both Long and Short Preambles are designed to be easily detectable and synchronized by the network.
  • The network employs algorithms to detect the transmitted preambles and synchronize with the UEs.

Resource Allocation:

  • Time and frequency resources are allocated for both Long and Short Preambles, and the allocation is determined by the network.

Collision Handling:

  • Mechanisms are in place to handle collisions when multiple UEs transmit preambles simultaneously. This involves random backoff and contention resolution procedures.

The choice between Long and Short Preambles in 5G NR RACH is based on the specific requirements of the communication scenario, considering factors such as propagation delay and the amount of initial information needed for synchronization.