LTE Measurement Events Overview
LTE measurement events are the trigger conditions the UE uses to send measurement reports to the network. They are used for handover, inter-frequency mobility, inter-RAT mobility, carrier-management features, WLAN-related mobility, and selected advanced scenarios.
The event families are defined in 3GPP TS 36.331 clause 5.5.4. This page brings those event families together in one practical reference so the event names, mobility role, and tuning impact can be checked quickly.
Why Measurement Events Matter
- They decide when the UE reports changing radio conditions.
- They strongly affect handover timing.
- They influence mobility stability, ping-pong, late handover, and drop behavior.
- Wrong thresholds, hysteresis, or timeToTrigger settings can create real field issues.
Event Family Overview
The table below summarizes the main LTE measurement event families and their practical role. The exact event meanings come from TS 36.331 clause 5.5.4.
| Event | Meaning | Main use case | Priority |
|---|---|---|---|
| A1 | Serving becomes better than threshold | Recovery or reduced mobility pressure | Medium |
| A2 | Serving becomes worse than threshold | Serving-cell deterioration detection | High |
| A3 | Neighbor becomes offset better than serving | Intra-LTE handover | Highest |
| A4 | Neighbor becomes better than threshold | Threshold-based neighbor reporting | High |
| A5 | Serving becomes worse than threshold1 and neighbor becomes better than threshold2 | Controlled handover | Highest |
| A6 | Neighbor becomes offset better than SCell | Carrier aggregation and SCell logic | Medium |
| B1 | Inter-RAT neighbor becomes better than threshold | LTE to other RAT mobility | High |
| B2 | Serving becomes worse than threshold1 and inter-RAT neighbor becomes better than threshold2 | Inter-RAT handover | High |
| C1 | CSI-RS resource becomes better than threshold | CSI-RS based reporting | Medium |
| C2 | CSI-RS resource becomes offset better than reference CSI-RS | Advanced CSI-RS comparison | Medium |
| W1 / W2 / W3 | WLAN event family | LTE-WLAN interworking | Medium |
| V1 / V2 | Channel busy ratio events | Specialized scenarios | Low |
| H1 / H2 | Aerial UE height events | Aerial UE cases | Low |
| D1 / D2 | Distance-based events | Positioning and advanced mobility | Low |
| T1 | Time-based conditional event | Advanced conditional logic | Low |
Practical Event Buckets
A-series events
Used for LTE serving-cell and neighbor-cell comparison.
B-series events
Used for inter-RAT mobility decisions.
C-series events
Used for CSI-RS-based reporting behavior.
W-series events
Used for WLAN-related mobility and interworking.
V / H / D / T events
Used for specialized and advanced scenarios.
Which Events Matter Most in Real LTE Networks
A3 is usually the most important event for intra-LTE handover tuning. A5 is also widely used where tighter control of serving and neighbor thresholds is needed. A2 is useful for detecting serving-cell degradation. B1 and B2 are the most important for inter-RAT mobility. A6 matters more in carrier aggregation and SCell scenarios. The C, W, V, H, D, and T families are usually more specialized.
This priority order is editorial guidance based on practical usefulness. The event definitions themselves come from TS 36.331.
How LTE Measurement Events Work
The UE does not report randomly. It reports according to the measurement logic configured by the network.
| Term | Role in event reporting |
|---|---|
| Measurement object | Defines what the UE should measure, such as serving or neighbor frequencies, cells, or RATs. |
| Report configuration | Defines the event type and its reporting behavior, including thresholds, hysteresis, timeToTrigger, amount, and interval. |
| Measurement identity | Maps a measurement object to a report configuration so the UE knows which logic applies to which measured target. |
| Thresholds | Define the absolute level a serving cell, neighbor cell, or other target must cross. |
| Offsets | Define comparison margins between serving and neighbor conditions. |
| Hysteresis | Reduces event flapping by requiring extra margin before the condition is considered satisfied or cleared. |
| timeToTrigger | Requires the event condition to remain true for a configured time before the report is sent. |
| Report amount and report interval | Control how often and how many reports the UE returns after the event has been triggered. |
Common Mobility Tuning Problems
- Too many A3 reports causing ping-pong between similar neighbors.
- Handover happening too late because thresholds or timeToTrigger are too conservative.
- Too many A5-triggered reports caused by poor threshold planning.
- B1 or B2 not triggering properly for inter-RAT mobility.
- A6 behavior being misunderstood in SCell or carrier-aggregation scenarios.
- The event looking correct while the measId and reportConfig mapping is wrong in the actual RRC configuration.
Event Comparison
Event A3 vs A5
- A3 compares the serving cell and neighbor cell by using an offset relationship.
- A5 uses two explicit thresholds: one for serving degradation and one for neighbor suitability.
- A3 is often simpler for classic intra-LTE handover logic.
- A5 gives tighter threshold-based control when the network wants stronger gating.
Event B1 vs B2
- B1 is satisfied when the inter-RAT neighbor alone crosses its configured threshold.
- B2 needs both serving degradation and a suitable inter-RAT neighbor condition.
- B2 is usually more conservative because it depends on two conditions instead of one.
Event A2 vs A4
- A2 describes serving-cell deterioration.
- A4 describes a neighbor cell crossing its own threshold.
- A2 and A4 answer different questions, so they should not be treated as substitutes.
Links to Event Pages
Core Event Pages
Advanced Event Pages
FAQ
What are LTE measurement events?
LTE measurement events are configured trigger conditions that tell the UE when to send a Measurement Report. They are not random reports; they are controlled by RRC configuration.
What is the difference between LTE Event A3 and A5?
A3 is based on a neighbor cell becoming offset better than the serving cell. A5 is based on two thresholds, one for serving degradation and one for neighbor suitability.
Which LTE measurement event is used for handover?
A3 is often the most common intra-LTE handover trigger. A5 is also widely used where tighter control of thresholds is needed.
When is LTE Event B2 used?
B2 is used for inter-RAT mobility when the serving LTE cell degrades and the target RAT neighbor also becomes good enough.
What causes too many LTE Event A3 reports?
Low hysteresis, short timeToTrigger, or aggressive offsets can make the UE report too often and can contribute to ping-pong behavior.
What is hysteresis in LTE measurement events?
Hysteresis is a margin added around an event condition to prevent reports from toggling too quickly when radio values fluctuate.
What is time-to-trigger in LTE measurement reporting?
timeToTrigger is the period for which the event condition must remain true before the UE sends the report.