The 5G communication specially at higher frequencies or mmWave is using Massive MIMO with Beamforming. Beamforming is a signal processing technique that allows gNB send targeted beams of data to users, reducing interference and making more efficient use of the frequency spectrum with improved spectral efficiency.
When user is indoor or moving, the radio link between the UE and gNB is susceptible to blockage and degradation of RF signal which can suddenly interrupt the communication link result in Beam Failure. So to detect the Beam failure at right time, the UE should have some mechanism to measure such sudden and rapid changes in the communication link and simultaneously recover from it to continue the services. The UE does this with the help of Beam failure Recovery (BFR) procedure. BFR procedue is a combined effort of UE PHY and MAC procedure without involvement of any higher layer signaling.
Beam Failure and Radio Link Failure
We should not get confuse/consider Beam Failure and Radio Link Failure (RLF) are same. In a multi beam sceranio, Radio Link Failure occurs when radio problems within a cell cannot be solved by the recovery procedures or UE is not able to find any suitable beam and Random Access procedure to any beam could not successful to recover the connection failure between gNB and UE. Where as Beam Failure occurs when UE lost the link from one Beam but it is able to make link to anothe beam and able to perform successful Random Access. Following fiugre shows an example of beam failure and radio link failure can be detected. For beam failure, there are still beam pairs that may be able to reestablish connection between the gNB and the UE.
- In case of RLF, the Link Recovery may deal with connection failure related to a beam pair, and BFR may be able to reestablish connection by switching the beam pairs used for communication. Instead, RLF may be identified, among other factors, because BFR was not able to reestablish connection. This implies that the cell currently connected to the UE cannot provide connectivity and there shall be cell reselection. For cell reselection, RRC connection reestablishment is clearly necessary.
- Where as in BFR – Beam Recovery, generally occurs more often than RLF, since the probability that connection through one beam pair within a cell is unsuitable is lower than the probability that connection through all beam pairs within the same cell is unsuitable.
High Level Beam Failure and Recovery Procedure
At a High level beam failure recovery procedure includes following four steps:
- Beam Failure detection
- Best Beam selection (SSB or CSI-RS)
- Beam Failure Recovery Request
- Beam Failure Recovery Response
Beam Failure Detection
Beam failure detection is a combined L1/L2 procedure where L1 provides the MAC layer indications of beam failure instances (BFIs). The MAC layer counts the indications and declares failure when configured maximum number of BFI indications has been reached. So whenever the PHY layer detects that the RSRP of the Reference signal of the serving beam goes below the threshold i.e 10% BLER of a hypothetical PDCCH, it triggers a Beam failure instance (BFI) and sends it to MAC.
MAC layer starts a timer as soon as it receives BFI and it keeps incrementing the counter by 1 for every BFI. When a certain threshold of BFI is reached i.e when BFI_COUNTER >= beamFailureInstanceMaxCount, MAC will trigger Beam Failure and will start the recovery procedure.
There can be a scenario, where L1 stops sending beam failure instances to MAC or the signal quality has improved and L1 no longer detects any issues. In handle such cases, the UE MAC layer has a timer running everytime PHY reports a BFI, for every occurrence, it increments BFI_COUNTER by 1 and restarts the timer. If there are no beam failure instances received at MAC and the timer expires, the MAC layer will reset the BFI_COUNTER and assume that there are no more beam failure instances.”
RRC Parameters for Beam Failure Detection
Beam Failure Recovery
The UE is provided with a set of resources for the recovery procedure in the BeamFailureRecoveryConfig via RRC message .The Beam Recovey happens by performing RACH on the best candidate beam selected during the Beam failure recovery procedure.
A UE Can perform below two RACH procedures for beam failure recovery request
- Contention Free Random Access – CFRA RACH
- Contention Based Random Access – CBRA RACH
- CBRA for Beam Failure Recovery : A UE will perform CBRA RACH due to fone of the following reasons
- If UE is not configured with CFRA RACH resources
- If UE has been configured with CFRA RACH but was unable to perform CFRA RACH due to the unavailability of the candidate beams
- If CFRA RACH was unsuccessful and UE has to fall back to CBRA RACH.
So when a UE performs CBRA RACH for the recovery procedure, it is performed as a Normal Contention based RACH procedure.
- UE is in RRC Connected mode
- UE detects Beam Failure at the PHY layer.
- UE performs Candidate beam selection for the recovery procedure.
- UE triggers the RACH process and sends a RACH preamble to gNB.
- gNB sends a Random Access Response(RAR) and UL grant for msg3 within the RAR window.
- UE sends MAC CE(msg3) for contention resolution.
- gNB sends a UL grant to UE
- Beam Failure recovery Complete
- CFRA for Beam Failure Recovery : Below is the Procedure for Contention Free RACH procedure for Beam Failure recovery. In the case of CFRA Beam failure recovery request, the UE will receive RAR scrambled with C-RNTI instead of RA-RNTI is a regular RACH process. The gNB will also configure a dedicated CORSET config for Beam Failure recovery which the UE monitors after transmitting the RACH preamble.
- In short, it includes following steps:
- UE is configured with CORSET-BFR and also configured with dedicated preambles to be used for BFR.
- In case UE detects a Beam Failure, It starts the Best Beam selection procedure
- If the Best beam is within any of the beams configured in the beam failure recovery config, the UE selects CFRA else it selects the CBRA RACH procedure. For example, In this case, UE selects CFRA RACH
- UE triggers RACH with a dedicated preamble
- UE Monitors the CORSET-BFR for PDCCH scrambled with C-RNTI- UE Receives the RAR scrambled with C-RNTI
- Beam Failure recovery Complete
RRC Parameters for Beam Failure Recovery
Article by Syed Syed Mohiuddin