Beam Failure detection and recovery is a very important procedure in 5G NR. I will try to explain the details in this post with as many details as possible.
I will start with giving an overall idea in simple terms to understand the concept and then go deeper in how it functions .
So why do we need Beam failure detection since there is a Radio link failure detection procedure already ?
Beam failure detection and recovery is an L1/L2 procedure and it is transparent to the RRC layer and it is much faster, whereas Radio link failure is an upper layer procedure i.e. RRC layer and it is much slower compared to Beam Failure procedure.
In Higher frequency beam-based communications the radio link between UE and gNB is susceptible to blockage and degradation which can suddenly interrupt the communication link. Imagine a situation where a user is sitting in a corner coffee shop playing a movie, Now if a truck stops outside the coffee shop and blocks the transmission between UE and gNB, and the UE is not able to perform Beam switch. So at the UE, there should be a mechanism to detect such sudden and rapid changes in the communication link and simultaneously recover from it, and the UE can do this with the help of the Beam failure recovery procedure (BFR) in 5G-NR.
Bands operating is FR2 are susceptible to beam failure due to the characteristics of radio wave propagation in higher frequency. So the chances of Beam failure are high in the Frequency range 2.
BFR is a combination of PHY and MAC procedure without any higher layer signaling. On a High level beam failure recovery procedure consists of below four steps
- Beam failure detection
- Best Beam selection (SSB or CSI-RS)
- Beam Failure recovery request
- Beam Failure recovery response
Beam failure detection
In order for UE to detect Beam failure at the Physical layer, there should be a mechanism where the UE can measure, detect and report a Beam failure.
The Physical layer uses a threshold to detect and trigger a Beam failure indication to MAC every time it detects that the quality of the received signal dropped below a certain threshold. The Indications from PHY to MAC layer are called Beam Failure instances (BFIs).
The Physical layer triggers a ‘Beam Failure instance‘ to the MAC layer when the quality of the Reference signals that UE is monitoring falls below a certain threshold i.e Qout_LR. Where the threshold Qout_LR is defined as the level at which the downlink radio link cannot be reliably received and shall correspond to t out-of-sync block error rate (BLERout) belonging to the hypothetical PDCCH transmission.