When does the gNB issue a BSRP trigger?

The gNB issues a BSRP trigger when it needs updated beam quality information from the UE, typically in three scenarios: during initial beam refinement after the UE has completed cell access via SSB (the gNB sends CSI-RS on multiple narrow beams and triggers the UE to report which beams are best), during beam tracking when the serving beam quality has changed (detected via periodic CSI-RS measurements or HARQ feedback degradation), and before inter-cell handover preparation when the gNB needs to identify the best beam on the target cell for conditional handover configuration.

How does the UE respond to a BSRP trigger?

Upon receiving the DCI-triggered BSRP, the UE measures the L1-RSRP of each configured CSI-RS or SSB resource within the specified measurement window (typically 1 to 5 ms). It then reports the beam indices and L1-RSRP values of the best N beams (N configured by RRC, typically 1 to 4) via uplink control information (UCI) on PUCCH or PUSCH in the designated reporting slot. The report includes the CRI (CSI-RS Resource Indicator) or SSBRI (SSB Resource Indicator) and a quantized L1-RSRP value (7-bit, 0.5 dB resolution). The total procedure from trigger to report completion takes 2 to 8 slots depending on measurement window and reporting slot offset.

How does BSRP differ from periodic beam reporting?

Periodic beam reporting (configured via RRC) continuously measures and reports beam quality at fixed intervals (5, 10, 20, 40, or 80 ms), consuming uplink resources even when beam conditions are stable. BSRP (aperiodic) is triggered on-demand by the gNB only when needed, saving uplink overhead during stable conditions but potentially introducing latency when beam conditions change rapidly. In practice, mmWave deployments use periodic reporting with coarse resolution (SSB-based) for baseline tracking and supplement with BSRP triggers for fine beam refinement (CSI-RS-based) when the gNB detects that a beam switch may be needed.

5G NR Beam Management

BSRP Trigger

/bee-ess-ar-pee trig-ur/ — Beam Search & Reporting Procedure

A signaling mechanism in 5G NR by which the gNB requests the UE to perform an aperiodic beam measurement and report L1-RSRP for candidate beams. Conveyed via DCI on PDCCH, the trigger directs the UE to measure specific CSI-RS or SSB resources and report the best beam indices with L1-RSRP values within a configured time window, enabling on-demand beam refinement for mmWave beam management.

Category: 5G NR Beam Management

Signaling: DCI on PDCCH

Latency: 2 to 8 slots

Understanding BSRP Triggers

5G NR beam management relies on continuous measurement and reporting of beam quality. Periodic reporting (SSB-based) provides coarse baseline tracking at 5 to 80 ms intervals. When the gNB detects potential beam degradation (via HARQ feedback or periodic reports), it needs finer, more immediate beam information. The BSRP trigger provides this on-demand capability: the gNB sends a DCI message on PDCCH directing the UE to measure a specific set of narrow CSI-RS beams and report the results within 2 to 8 slots.

The UE measures L1-RSRP on each configured resource, selects the best N beams (typically 1 to 4, configured by RRC), and reports their CRI/SSBRI and quantized L1-RSRP (7-bit, 0.5 dB resolution) via UCI on PUCCH or PUSCH. The gNB uses this information to update the TCI state for PDSCH/PDCCH, switching the serving beam to the best candidate. This aperiodic approach saves uplink overhead during stable conditions while providing rapid beam refinement when needed.

BSRP Procedure Timeline

Trigger (DCI on PDCCH):
gNB → UE: DCI format 0_1/1_1 with BSRP request
Includes: CSI-RS resource set ID, reporting config

Measurement Window:
1 to 5 ms (UE measures L1-RSRP per resource)

Report (UCI on PUCCH/PUSCH):
UE → gNB: CRI/SSBRI (best N beams) + L1-RSRP
Resolution: 7-bit L1-RSRP (0.5 dB steps, −140 to −44 dBm)

Total Latency: 2 to 8 slots (~0.3 to 1.1 ms at 120 kHz SCS)

Beam Reporting Mode Comparison

Mode	Trigger	Periodicity	Overhead	Latency	Best For
Periodic (SSB)	RRC configured	5 to 80 ms	Continuous	Fixed interval	Coarse baseline tracking
Semi-Persistent (CSI-RS)	MAC CE activated	5 to 80 ms	When active	Fixed interval	Active beam monitoring
Aperiodic (BSRP)	DCI on PDCCH	On-demand	Only when needed	2 to 8 slots	Fine beam refinement

Common Questions

Frequently Asked Questions

When does the gNB issue a BSRP?

Three scenarios: initial beam refinement after cell access (CSI-RS on narrow beams after SSB coarse alignment), beam tracking when quality changes (HARQ degradation detected), and pre-handover preparation (identify best beam on target cell for CHO configuration).

How does the UE respond?

Measures L1-RSRP on configured CSI-RS/SSB resources within 1 to 5 ms window. Reports best N beam indices (CRI/SSBRI) and quantized L1-RSRP (7-bit, 0.5 dB resolution) via UCI on PUCCH/PUSCH. Total trigger-to-report: 2 to 8 slots.

BSRP vs periodic reporting?

Periodic reports continuously at fixed intervals (5 to 80 ms), consuming uplink resources even when stable. BSRP triggers on-demand, saving overhead but adding latency when conditions change rapidly. mmWave deploys both: periodic SSB for baseline, BSRP CSI-RS for fine refinement.

Related Terms

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