Radio Link Monitoring (RLM) is one of the important procedures in LTE. It is used to keep track of the radio link condition so that appropriate steps can be taken if Radio Link Failure (RLF) is declared. The figure below (taken from T-doc R2-133859) captures the RLM process:
From the same T-doc:
PCell radio link monitoring is to determine whether the PCell radio link should be considered as failed (i.e. radio link is worse than Qout for time period determined by N310). If so, UE performs two actions; 1) stopping autonomous uplink transmission by releasing SPS, CQI, SRS, SR and 2) starting cell selection procedure to find a cell providing acceptable radio link.
In LTE Rel-1o, the concept of Primary Cell (PCell) and Secondary Cell (SCell) was introduced to support Carrier Aggregation. The PCell is more or less like the serving cell in the non-CA case. The functions provided by the PCell are summarized below (see Section 7.5 of 3GPP TS36.300 for more details).
– Provides Security inputs
– Provides NAS mobility functions
– Have always Uplink and Downlink resources: Carrier frequency (FDD) or UL/DL subframes (TDD)
– Used for PUCCH transmission
– Used for RRC connection re-establishment
– Used for Radio Link Monitoring
– Can be changed only by Handover
– Cannot be deactivated
– Cannot be cross scheduled
– Semi-persistent resources can only be configured for the PCell
– UE acquires system information of PCell from the Physical Broadcast Channel (PBCH)