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Radio Link Monitoring – What is new SSB Based and CSI-RS Based RLM in 5G


When a UE  is connected to the network, it requires to continuously monitor the Radio link for reliable communication, the process is know as Radio Link Monitoring. The UE monitor’s the downlink link quality based on the reference signal to measure the downlink radio link quality for the serving cell.

Types of Radio Link Monitoring

SSB based radio link Monitoring

When UE evaluating the downlink radio link quality,  it monitors 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

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.


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