5G NR / Mobility

B1 Event

Q: What triggers a B1 event report?

A B1 event fires when the measured signal quality of a neighbor cell on a different frequency exceeds a configured absolute threshold. The entry condition is: Mn + Ofn + Ocn - Hys > Thresh. Mn is the neighbor measurement (RSRP or RSRQ). Ofn is a frequency-specific offset. Ocn is a cell-specific offset. Hys is the hysteresis value. Thresh is the absolute threshold configured by the network. The UE must sustain the entry condition for a time-to-trigger (TTT) duration before sending the report, preventing false triggers from fast fading.

Q: Why would a network operator use B1 instead of B2 for inter-frequency handover?

B1 is simpler: it only checks whether the neighbor cell is good enough (above a threshold). B2 adds a second condition: the serving cell must also be bad (below a different threshold). Operators use B1 when they want to steer traffic to a preferred frequency layer regardless of current serving cell quality. For example, an operator might configure B1 to move users to a newly deployed n78 (3.5 GHz) carrier whenever it becomes available, even if the current n1 (2.1 GHz) connection is perfectly fine. B2 is used when the handover should only happen if the current connection is degrading.

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A B1 Event is an inter-frequency or inter-RAT measurement event defined in 3GPP TS 38.331 that triggers when a neighbor cell operating on a different carrier frequency (or a different radio access technology like LTE) exceeds a configured signal quality threshold. When the UE's measurement of the neighbor cell's RSRP or RSRQ stays above the threshold for the configured time-to-trigger duration, the UE sends a measurement report to the serving gNB, which uses it to initiate inter-frequency handover preparation.

Category: 5G NR Mobility

3GPP Spec: TS 38.331, TS 38.133

Measurement: RSRP, RSRQ, or SINR

Understanding the B1 Event

In a multi-frequency 5G network, a UE may be camped on one carrier (say n1 at 2.1 GHz) while a better carrier (n78 at 3.5 GHz) becomes available as the user moves into coverage. The B1 event is the mechanism the network uses to detect this situation. The gNB configures the UE with a B1 measurement configuration that says: "Measure n78 SSB, and if any cell on n78 has RSRP above -100 dBm for more than 320 ms, send me a report." When the UE triggers the event, the gNB prepares the handover to the target cell on n78.

How B1 Differs from Other Events

The 5G NR measurement event framework uses a systematic naming convention. A-series events (A1 through A6) measure cells on the same frequency as the serving cell. B-series events compare the serving cell against cells on a different frequency or different RAT. The B1 event is the simplest B-series event: it checks only whether the neighbor is good enough, without considering the serving cell's quality.

B1 Entry Condition

B1 Event:
A B1 Event is an inter-frequency or inter-RAT measurement event defined in 3GPP TS 38.331 that triggers when a neighbor cell operating on a different...

Key specifications:
1 a | 2.1 GHz | 78 a | 3.5 GHz | 1 m | -100 dB

Power: P(dBm) = 10log(P_mW), 0dBm = 1mW

5G NR Measurement Events Comparison

Event	Type	Condition	Typical Use Case
A1	Intra-freq	Serving > Threshold	Stop inter-freq measurements (serving is good)
A2	Intra-freq	Serving < Threshold	Start inter-freq measurements (serving is weak)
A3	Intra-freq	Neighbor > Serving + Offset	Intra-frequency handover
A5	Intra-freq	Serving < Thresh1 AND Neighbor > Thresh2	Conditional intra-freq handover
B1	Inter-freq/RAT	Neighbor > Threshold	Inter-frequency handover (neighbor-only)
B2	Inter-freq/RAT	Serving < Thresh1 AND Neighbor > Thresh2	Conditional inter-frequency handover

Key Equations

Decibel conversion:
Power: dB = 10log(P₂/P₁)
Voltage: dB = 20log(V₂/V₁)

dBm to watts:
P(W) = 10^{(dBm−30)/10}
0 dBm = 1 mW, +30 dBm = 1 W

Wavelength:
λ = c/f = 300/f(MHz) meters

Comparison

Aspect	B1 Event Spec	Typical Range	Impact	Design Note
Primary function	The B1 event is the mechanism the networ...	Application-dep.	Critical	Verify in sim
Operating range	How B1 Differs from Other Events The 5G...	Application-dep.	Critical	Verify in sim
Performance	A-series events (A1 through A6) measure...	Application-dep.	Critical	Verify in sim
Integration	B-series events compare the serving cell...	Application-dep.	Critical	Verify in sim
Trade-off	The B1 event is the simplest B-series ev...	Application-dep.	Critical	Verify in sim

Common Questions

Frequently Asked Questions

What is the difference between A-series and B-series measurement events?

A-series events (A1 through A6) handle intra-frequency measurements, where both the serving and neighbor cells are on the same carrier frequency. B-series events (B1 and B2) handle inter-frequency and inter-RAT measurements, where the neighbor cell is on a different frequency or technology like LTE. B-series events require measurement gaps, temporarily tuning the receiver away from the serving frequency to measure the neighbor, which introduces a small interruption to data throughput.

What triggers a B1 event report?

A B1 event fires when the neighbor cell's measured signal quality exceeds a configured absolute threshold. The entry condition is: Mn + Ofn + Ocn - Hys > Thresh. The UE must sustain the entry condition for a time-to-trigger (TTT) duration, typically 40 to 640 ms, before sending the report. This prevents false triggers from fast fading. The network can also configure a report-on-leave option to notify when the condition is no longer met.

Why use B1 instead of B2 for inter-frequency handover?

B1 only checks whether the neighbor cell is good enough (above a threshold). B2 adds a second condition: the serving cell must also be bad. Operators use B1 to steer traffic to a preferred frequency layer regardless of current serving cell quality. For example, moving users to a newly deployed n78 (3.5 GHz) carrier whenever it becomes available, even if the current n1 connection is fine. B2 is for conditional handover when the current connection is degrading.

Related Terms

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