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Radio Link Failure (RLF)

 

To be honest I don't think I saw any clear/explicit definition of this term. My personal definition of Radio Link Failure is "Physical Layer(especially low PHY) break" and in most case this failure is unintentional.

 

When UE Report RLF ?

 

Then the next question would be "How UE or eNodeB can detect this kind of Radio Link Failure ?". Unfortunately we don't have any clear answer to this question either. So detailed detection implementation is up to UE maker and eNodeB maker, but we can think of several guidelines.

 

UE may assume that Radio Link is broken in the following setuation.

  • The measured RSRP is too low (under a certain limit)
  • It failed to decode PDCCH due to power signal quality (e.g, low RSRP, RSRQ)
  • It failed to decode PDSCH due to power signal quality (e.g, low RSRP, RSRQ)

However, detailed mechanism to RLF is upto the chipset implementation. So the individual RLF detection mechanism may vary chipset-to-chipset.

 

eNodeB may assume that that Radio Link is broken in the following setuation.

  • SRS Power (SINR) from UE is much lower than what eNB configured for the UE
  • eNodeB couldn't detect (see) any NACK nor ACK from UE for PDSCH.

 

Does UE or eNodeB declare "Radio link Failure" whenever it sees the problem described above, even for only one subframe ?

No, it is not. In most case, this kind of problem should happen for a certain period of time consecutively and a couple of timers and parameters are involved in the criteria setup. (See T311, n310)

 

 

What UE tries to do when RLF Happens ?

 

Then the next question would be "What UE does when it detects Radio Link Failure ?" or "What eNodeB does when it detects Radio Link Failure ?".

The most typical procedure is to go through RRC Connection Restablishement procedure.

 

< Attemp to recover Radio Link while Out of SYNC (Radio Link Failure) - RRC Connection ReEstablishment >

 

 

 

< When UL Data arrives from higher layer while Out of SYNC (Radio Link Failure) >