Sign in Subscribe

RACH Failure reasons in 5G-NR

Last updated on 


RACH (Random Access Channel) is a fundamental mechanism in 5G NR (New Radio) for initial access and uplink synchronization. When a user equipment (UE) tries to establish a connection with the network, it sends a RACH preamble, and the network responds by assigning a temporary identifier and providing resources to the UE.

  1. Collision in Preambles:
    • In 5G NR, different UEs use the same set of preambles, making it possible for collisions to occur when multiple UEs try to access the network simultaneously.
    • The Random Access Procedure in 5G NR has been enhanced with various features like Preamble Transmissions (PTs), preamble diversity, etc., to mitigate collision probability. However, under heavy load or interference, collisions can still happen.
  2. Contention Resolution Timer Expiry:
    • After sending a RACH preamble, the UE waits for a response from the network (MSG2). If the UE does not receive the expected response within a specified Contention Resolution Timer (CRT), it considers the attempt as failed.
    • This timer is essential to ensure that the network can handle collisions and allocate resources effectively. If the timer expires before contention resolution, the UE will try the RACH procedure again.
  3. Physical Layer Issues:
    • Radio frequency (RF) interference, poor signal quality, or weak coverage can lead to RACH failures. The UE might not decode the network's response correctly, or the network may not decode the UE's preamble correctly due to these issues.
    • In some cases, the uplink synchronization might not be achieved due to issues like multipath propagation, signal attenuation, or interference from neighboring cells.
  4. Incorrect Timing Advance (TA):
    • Timing Advance is crucial for uplink synchronization. If the UE's timing is not correctly aligned with the network's timing, it can lead to RACH failures.
    • Incorrect TA values might cause the UE's transmission to arrive outside the network's expected timing window, leading to missed RACH preambles or unacknowledged MSG2 messages.
  5. UE Configuration Issues:
    • Improper UE configuration, such as incorrect RACH configurations, transmission power settings, or parameter values, can result in RACH failures.
    • For example, if the UE uses an inappropriate preamble format, transmission power, or frequency, it may not be recognized by the network, leading to failed RACH attempts.
  6. Overload and Congestion:
    • In scenarios with high UE density or network congestion, the RACH procedure may experience increased collision probabilities, contention resolution delays, and overall failure rates.
    • The network might be unable to allocate resources promptly, process RACH requests efficiently, or manage contention among UEs effectively, resulting in RACH failures.