Beam Failure Recovery (BFR) — Telecom Resource

← MAC Overview RACH → DRX → CA → DC → Sidelink → Beamforming → ☆ NTN MAC ☆ Cell-Free

1. Beam Failure Detection (BFD)

UE monitors the active beam's L1-RSRP against a threshold. When BLER exceeds 10%, a Beam Failure Indication (BFI) is generated. When the BFI counter reaches beamFailureInstanceMaxCount, beam failure is declared.

📡 BFD Counter — BFI Accumulation to Beam Failure

Inject BFI events to see the counter accumulate. When it reaches maxCount, beam failure is declared and BFR triggers.

maxCount:

Parameter	Values	Purpose
beamFailureInstanceMaxCount	1-10	BFI count to declare beam failure
beamFailureDetectionTimer	10-500 ms	BFI counting window
RSRP threshold	Configurable	Min beam quality (maps to ~10% BLER)

2. Candidate Beam Identification (CBD)

After beam failure, UE scans candidate beams from the candidateBeamRS-List (up to 64 entries). The beam with highest L1-RSRP above rsrp-ThresholdBFR is selected.

🔍 Candidate Beam Scan — Find the Best Alternative

See multiple beams with different RSRP values. The UE selects the strongest candidate above threshold.

RSRP threshold (dBm): -80

CBD latency: SSB-based ~10-20 ms (depends on SSB periodicity), CSI-RS-based ~5-10 ms (more frequent). UE reports selected beam ID (SSBRI or CRI) in the BFR request.

3. Full BFR Procedure

The complete BFR flow: Detect → Identify → Request → Response. Can be step-through or auto-play. Total time: ~30-80 ms vs ~200-500 ms for handover.

🔄 BFR Procedure — Detect → CBD → Request → Response

Step through the full BFR procedure. Watch the timeline and see how fast beam recovery is compared to handover.

Request type:

Stage	PUCCH BFR	RACH BFR
Detection	20-40 ms	20-40 ms
CBD	5-20 ms	5-20 ms
Request	2-5 ms	10-15 ms
Response	2-5 ms	2-5 ms
Total	~30-70 ms	~40-80 ms

4. BFR vs Handover Latency

⚡ BFR vs Handover — Side-by-Side Latency

Compare recovery times: BFR completes in tens of ms vs handover in hundreds of ms vs RLF in seconds.

BFR (~50 ms)

Beam switch within same cell. MAC-triggered. No RRC reconfig needed. Fastest recovery.

Handover (~300 ms)

Cell change. RRC reconfiguration + RA on target. Moderate interruption.

RLF (~2 sec)

Full RRC re-establishment. Cell selection + security. Longest interruption.

5. Practical Field Considerations

Scenario	maxCount	Timer	CBD Source
Indoor mmWave	2	20 ms	CSI-RS (fast)
Urban pedestrian	4	50 ms	SSB
High-speed vehicle	6	100 ms	SSB + CSI-RS
Fixed wireless (CPE)	10	500 ms	SSB

Common pitfall: maxCount too low → false BFR on transient fades → unnecessary beam switches. maxCount too high → slow detection → prolonged outage. Tune to environment dynamics.

6. 6G Evolution — Predictive BFR, THz, RIS-Assisted

Predictive BFR

AI detects degradation before failure. Proactive beam switch. Near-zero BFR events.

THz Ultra-Fast BFR

<5 ms recovery. PUCCH-only. Pre-configured backup beams for instant switch.

RIS-Assisted Recovery

RIS provides alternative path when direct beam blocked. Beam "rerouting" vs switching.

🚀 BFR Evolution — 4G to 6G

Compare beam recovery capabilities across generations.

Generation:

Dimension	4G (LTE)	5G NR	6G (Target)
Beam recovery	None (RLF only)	BFR (~50 ms)	Predictive (<5 ms)
Method	RRC re-establish	RACH + PUCCH	AI-preemptive
Blockage handling	Full outage	Fast beam switch	RIS rerouting
Frequency	Sub-6 GHz	Sub-6 + mmWave	+ THz