Radio Link Monitoring (RLM) and Radio Link Failure (RLF) in 5G-NR

The Radio Link Monitor (RLM) is a crucial component in ensuring reliable communication in 5G New Radio (NR) networks. It evaluates the quality of the radio link and interacts with the RRC layer to detect potential failures. Below is a breakdown of the RLM process.

Overview of RLM Process:

RLM Purpose: 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.
Indications: The RLM uses two key indications — ‘Out-of-Sync’ and ‘In-Sync’ — to determine the quality of the radio link.

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.

RLM Reference Signals

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.

Thresholds for Link Quality Evaluation

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.

Qout (Out-of-Sync Threshold):

• The UE’s Physical layer generates an ‘Out-of-Sync’ indication when the radio link quality for all monitored reference signals is worse than a specified 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
• Represents a worse quality where the reception is unreliable, signaling potential link failure.
• This indication is forwarded to the RRC layer for further action

Qin (In-Sync Threshold):

• The Physical layer generates an ‘In-Sync’ indication when the radio link quality for at least one monitored reference signal exceeds a better-than-threshold value, 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.
• Ensures that the downlink communication is stable and reliable.
• Corresponds to a quality where the Block Error Rate (BLER) for a hypothetical PDCCH transmission is below a critical threshold, meaning PDCCH reception is reliable.
• This indication is also forwarded to the RRC layer

Hypothetical PDCCH Transmission:

• PDCCH is assumed to transfer DCI Format 1_0 with CCE aggregation level of 4 and a CORESET spanning 2 symbols.
• Ensuring a lower aggregation level helps guarantee reliable downlink reception.

RRC Layer Detection of Radio Link Failure:

• The RRC layer uses the ‘Out-of-Sync’ and ‘In-Sync’ indications in conjunction with timers N310, T310, and N311 to detect Radio Link Failure.
• 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.
• The network can configure the UE with values for N310, T310, and N311 through the rlf-TimersAndConstants
• N310: Number of consecutive ‘Out-of-Sync’ indications required to start the timer T310.
• T310: Timer that counts until it expires, triggering a Radio Link Failure detection.
• N311: Number of consecutive ‘In-Sync’ indications required to stop and reset the T310 timer.
• Failure Detection: Radio Link Failure is detected when T310 expires, indicating the link is unstable.

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.

Impact of DRX Configuration on RLM

• The rate at which “Out-of-Sync” and “In-Sync” indications are generated depends on the DRX cycle duration and configuration.
• If the DRX cycle duration exceeds 320 ms, the indications are generated at a faster rate.
• Longer DRX cycles result in slower rate for these indications, while DRX disabled or fast cycle configurations lead to faster evaluation.
• When DRX is disabled, or the reference signal period for Beam Failure detection is minimal (i.e less than 10 ms), indications are generated at a slower rate.

Measurement Gaps and Restrictions:

• The UE may face restrictions on measuring CSI-RS or SSB simultaneously, especially in cases with different numerologies or frequency bands (FR1 and FR2).
• The network configures these parameters to ensure efficient RLM and avoid measurement conflicts.

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.

Indication Generation Frequency: The UE does not generate an indication after every evaluation period.

• If the radio link quality is between the thresholds Qout and Qin, no indication is generated.
• The rate of indication generation is influenced by the network’s configuration and the UE’s operational state.

RLM is critical for maintaining robust communication in 5G networks by continuously assessing downlink link quality. Network operators can configure RLM parameters to ensure that the UE responds appropriately to variations in link quality, thereby preventing radio link failures and optimizing performance.

Below section covers how SSB/CSI-RS based RLM is done by UE.

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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.

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Reference :

• 3GPP TS 38.133 5G NR Requirements for support of radio resource management
• 3GPP TS 38.213 5G NR Physical layer procedures for control