How does the UE detects RLF?

RAN
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How does the UE detects RLF and report Out of Synchronization indications based on specific timers and parameters?

  • The UE’s physical layer sends an ‘Out-Of-Sync’ signal to the RRC Layer when the quality of all monitored reference signals drops below a certain level (Qout).
  • The BTS and UE have specific targets, settings, and timers, the UE gets its targets and settings, starts monitoring and comparing, and only declares RLF when it detects a problem based on the specified timers and thresholds.
  • The Base Station configures the UE with a pair of Block Error Rate (BLER) targets:
  • BLERout corresponds to a quality level (Qout) at which the radio link is categorized as being unreliable.
  • BLERin corresponds to a quality level (Qin) at which the radio link is considered reliable.
  • The UE RRC Layer uses the ‘Out-Of-Sync’ indications in combination with N310, T310 and N311 to detect Radio Link Failure.
  • Where the timer T310 started if the RRC Layers receives N310 consecutive Out of Sync indications.
  • The Timer t310 is stopped and reset if the RRC Layer subsequently receives N311 consecutive In Sync.
  • Radio Link Failure is detected if T310 expires, and the Transmitted of UL Data is turned off after the timer t310 is expired.
  • After T310 expiry, The UE will start RLF reestablishment procedure

Let’s make things clearer with examples. In the examples below, we’ll assume the following:
BLER out target = 10% and BLER in = 2%
N310 =3, n311 = 2 and T310 = 1 Second

  • Assume that the current DL BLER is 11% or more for the UE. In this scenario, if the Out-Of-Sync indications reach the n310 value, the T310 timer starts, giving the UE a chance to sync back.
  • If the UE doesn’t hit the BLER-In target within the set thresholds during the T310 timer, the RLF Layer steps in.
  • When the T310 Timer runs out without success, the UE declares a radio link failure, leading to the initiation of the RRC Reestablishment Procedure.

How to check related UE related timers’ configuration from layer 3 messages❓

  • UE SIB1 in 5G SA and SIB2 in 4G.
  • RRC Configuration “spCellConfig” in 5G NSA.

Study Source: NR in Bullets

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One question: what’s Qin and Qout indicator update rate (frequency)?