Measurement Report: Event A4 in 5G NR

Measurement reporting is an essential part of radio resource management (RRM) and mobility in 5G NR. While some measurement events compare the serving cell to a neighbour cell (like Event A3), others evaluate cells against an absolute threshold. Event A4 is exactly that—it triggers when a neighbour cell's signal quality becomes better than a predefined absolute threshold.

What is Measurement Event A4?

In 5G NR, Event A4 is defined as the condition where the measurement result of a neighbour cell becomes better than an absolute threshold. Unlike Event A3, which relies on relative comparisons (neighbour vs. serving), Event A4 only cares about the absolute quality of the neighbour cell itself.

Primary Use Cases for Event A4

1. Load Balancing: If a serving cell is congested, the network can configure the UE to search for neighbour cells that meet a minimum quality threshold (Event A4) to offload traffic.
2. Inter-Frequency or Inter-RAT Handover: Moving the UE to another frequency or a different Radio Access Technology (like LTE) when a good enough candidate is found, regardless of the current serving cell's quality.
3. Directed Cell Reselection: Guiding a UE to a preferred frequency layer.

Triggering Conditions for Event A4

The conditions for entering and leaving Event A4 involve the measured signal quality (such as RSRP, RSRQ, or SINR), frequency offsets, cell-specific offsets, and hysteresis.

Entering Condition (A4-1)

The UE sends a measurement report when the neighbour cell's quality exceeds the threshold for a configuration duration known as Time-to-Trigger (TTT).

$$M_n + Of_n + Oc_n - Hys > Thresh$$

Leaving Condition (A4-2)

The UE stops reporting or indicates it is leaving the condition when the neighbour cell's quality drops below the threshold (accounting for hysteresis) for the TTT duration.

$$M_n + Of_n + Oc_n + Hys < Thresh$$

Where:

• $M_n$: The measurement result of the neighbour cell (excluding any offsets).
• $Of_n$: The frequency-specific offset of the frequency of the neighbour cell.
• $Oc_n$: The cell-specific offset of the neighbour cell.
• $Hys$: The hysteresis parameter for this event (used to prevent ping-ponging).
• $Thresh$: The absolute threshold parameter for this event.

Key Parameters for Configuring Event A4

To properly configure Event A4, network engineers define several parameters via RRC signaling:

• Threshold: The absolute value (in dBm for RSRP or dB for RSRQ/SINR) that the neighbour cell must exceed.
• Hysteresis (Hys): A margin (in dB) applied to the threshold to prevent rapid toggling of the event when the signal fluctuates near the threshold.
• Time-to-Trigger (TTT): The continuous time period for which the entering or leaving condition must be met before the UE sends the report.

FAQ: Measurement Event A4

1. What is the difference between Event A3 and Event A4?

Event A3 triggers when a neighbour cell becomes an offset better than the serving cell (it's a relative comparison). Event A4 triggers when a neighbour cell becomes better than an absolute threshold, regardless of how good or bad the serving cell is.

2. Can Event A4 be used for Intra-Frequency handovers?

While technically possible, Event A4 is mostly used for inter-frequency or load-balancing purposes. For general intra-frequency mobility (finding the best cell on the same frequency), Event A3 is widely preferred because it directly compares the neighbour to the serving cell.

3. What happens if multiple neighbour cells meet the Event A4 threshold?

The UE will report all neighbour cells that satisfy the A4 conditions within the measurement report, up to the maximum number of reported cells configured by the network (maxReportCells). The network then decides which cell to hand over to.

4. How does Hysteresis help in Event A4?

Hysteresis creates a buffer zone. It requires the neighbour cell to be significantly above the threshold to trigger the event, and significantly below the threshold to leave the event. This prevents the UE from sending a flood of measurement reports if the signal quality is fluctuating right at the threshold boundary.