A UE when it is connected to the network, it needs to continuously monitor the Radio link for reliable communication. The UE monitor’s the downlink link quality based on the reference signal in order to detect the downlink radio link quality of the camped cell.
In 5G-NR radio link monitoring Is applicable for
- PCell in SA NR, NR-DC and NE-DC operation mode
- PSCell in NR-DC and EN-DC operation mode.
In 5G-NR the UE monitor’s the downlink radio link quality based on the reference signal configured as RLM-RS resource(s) in order to detect the downlink radio link quality of the PCell and PSCell. If there are no RLM-RS resource(s) configured then the UE will monitor the current SSB for the downlink radio link quality.
The network can configure below RLM-RS resources
- SSBs only
- CSI-RSs only or
- Combination of SSBs and CSI-RSs
The UE is expected to perform RLM on the camped active downlink bandwidth part(BWP) and it is not expected to perform measurements outside the DL BWP.
When the UE starts to monitor the reference signals RLM-RS as configured by the network, How will it determine whether the quality of the measured signal is good or worst, there should be criteria or threshold to determine when the signal is past the worst threshold and when it is below the worst threshold.
3GPP Has defined two thresholds that define the entry and exit criteria when a UE can declare radio link failure and when it can exit the RLF. The same thresholds are defined for LTE and 5G-NR but the reference signals measured for RLM are different in LTE and NR.
In LTE UE measures the Cell-Specific reference signal (CRS) for downlink radio link monitoring whereas
In NR UE measures either the SSB or CSI-RS ( if configured) for downlink radio link monitoring.
Threshold Qout: It is defined as the level at which the downlink radio link cannot be reliably received and shall correspond to out-of-sync block error rate (BLERout) which is the 10% block error rate of a hypothetical PDCCH transmission
Threshold Qin: It is defined as the level at which the downlink radio link quality can be significantly more reliably received than at Qout and shall correspond to in-sync block error rate (BLERin) which is 2% block error rate of a hypothetical PDCCH transmission
The out-of-sync block error rate (BLERout) and in-sync block error rate (BLERin) are determined from the network configuration via parameter rlmInSyncOutOfSyncThreshold signaled to the UE via RRC message.
When UE is not configured with rlmInSyncOutOfSyncThreshold from the network i.e the IE is omitted in the RRC message, UE determines out-of-sync and in-sync block error rates from Configuration #0 from the below table.
How many RLM-RS the UE can monitor and how often will the UE have to monitor these resources?
3GPP Specs has defined the number of RLM-RS UE that needs to monitor per Frequency range basis as the number of SSB varies per frequency range. UE shall be able to monitor up to N-RLM RLM-RS resources of the same or different types in each corresponding carrier frequency range, depending on a maximum number Lmax of SSBs per half frame.
SSB based radio link monitoring
When evaluating the downlink radio link quality the UE has to monitor the reference signal over a period of time to determine whether if it’s meeting the threshold Qout or the threshold Qin. The monitoring time is in milliseconds and the time varies if the DRX is configured and Measurement gaps along with smtc1 and smtc2 are configured by the network.
3GPP Specifications has specified the UE requirements as below
UE shall be able to evaluate whether the downlink radio link quality on the configured RLM-RS resource estimated over the last TEvaluate_out_SSB [ms] period becomes worse than the threshold Qout_SSB within TEvaluate_out_SSB [ms] evaluation period.
UE shall be able to evaluate whether the downlink radio link quality on the configured RLM-RS resource estimated over the last TEvaluate_in_SSB [ms] period becomes better than the threshold Qin_SSB within TEvaluate_in_SSB [ms] evaluation period.
Further reading –> 3GPP TS 38.133 Section 8.1.2.2
In the case of FR1
When the SSB for RLM is in the same OFDM symbol as CSI-RS for RLM, Beam Failure Detection (BFR), Candidate Beam detection(CBD) or L1-RSRP measurement, and
- If SSB and CSI-RS have same SCS, then the UE shall be able to measure the SSB for RLM without any restriction;
- If SSB and CSI-RS have different SCS,
- If UE supports simultaneousRxDataSSB-DiffNumerology, UE shall be able to measure the SSB for RLM without any restriction;
- If UE does not support simultaneousRxDataSSB-DiffNumerology, UE is required to measure atleast one of the refrences signals i.e either SSB for RLM and CSI-RS.
In the case of FR2
when the SSB for RLM measurement on one CC coincides with the same OFDM symbol as CSI-RS for RLM, BFD, CBD or L1-RSRP measurement on the same Carrier or different Carriers in the same band, then the UE is expected to measure both the reference signals i.e SSB for RLM and CSI-RS.
A longer measurement period for SSB-based RLM is expected, and no requirements are defined in the specifications for both FR1 and FR2 case.
CSI-RS based radio link monitoring
The UE should be configured with CSI-RS-based resources to monitor RLM-RS on the Pcell or the Spcell.
The CSI-RS Configured for the RLM-RS should be inside the active DL BWP during the entire evaluation period. UE is not expected to perform radio link monitoring measurements on the CSI-RS configured as RLM-RS if the CSI-RS is not in the active TCI state of any CORESET configured in the UE active BWP.
UE shall be able to evaluate whether the downlink radio link quality on the configured RLM-RS resource estimated over the last TEvaluate_out_CSI-RS ms period becomes worse than the threshold Qout_CSI-RS within TEvaluate_out_CSI-RS ms evaluation period.
UE shall be able to evaluate whether the downlink radio link quality on the configured RLM-RS resource estimated over the last TEvaluate_in_CSI-RS ms period becomes better than the threshold Qin_CSI-RS within TEvaluate_in_CSI-RS ms evaluation period.
As in the case of SSB-based measurements, the monitoring time is in milliseconds and the time varies if the DRX is configured and Measurement gaps along with smtc1 and smtc2 are configured by the network.
The UE is required to be capable of measuring CSI-RS for RLM without measurement gaps and the UE is required to perform the CSI-RS measurements with measurement restrictions
When the SSB transmission for RLM coincides with the CSI-RS transmission i.e SSB and CSI-RS is on the same OFDMA symbol then the UE is not expected to receive CSI-RS for RLM in the PRBs that overlap with an SSB. This scenario is applicable for both FR1 and FR2.
And when the CSI-RS for RLM coincides with another CSI-RS transmission configured for RLM, BFD, CBD or L1-RSRP measurement, the UE shall be able to measure the CSI-RS for RLM without any restriction.
In the case of FR1
If the SSB and the CSI-RS have the same SCS and are within the same active BWP then the UE shall be able to perform CSI-RS measurement without restrictions.
- If the UE supports simultaneousRxDataSSB-DiffNumerology the UE shall be able to perform CSI-RS for RLM measurement without restrictions.
- If the UE does not support simultaneousRxDataSSB-DiffNumerology, UE is required to measure one of but not both CSI-RS for RLM and SSB. Longer measurement period for CSI-RS based RLM is expected, and no requirements are defined
In the case of FR2
when the CSI-RS for RLM measurement on one CC is in the same OFDM symbol as SSB for RLM, BFD, or L1-RSRP,the UE is required to measure one of but not both CSI-RS for RLM and SSB
when the CSI-RS for RLM measurement on one CC is in the same OFDM symbol as another CSI-RS for RLM, BFD, CBD, or L1-RSRP measurement on the same CC or different CCs in the same band then
- The UE is expected to measure either one of the CSI-RS but not both.