Measurement report Event A3 in 5G NR

Measurement reporting is a critical aspect of mobility management in 5G NR (New Radio) networks. Among the various events defined by 3GPP, Event A3 plays a crucial role in enabling seamless handovers between cells. In this blog post, we will delve into the key aspects of Measurement Report Event A3, answer common questions, and provide a structured overview of how it operates in 5G NR networks.

What is Measurement Report Event A3 in 5G NR?

In simple terms, Measurement Event A3 is triggered when a neighboring cell’s signal quality becomes better than the current serving cell’s signal quality by a specified offset. The current serving cell is known as the “Special Cell” (SpCell), and this event primarily functions in both intra-frequency and inter-frequency handovers. Event A3 allows the UE to switch to a neighboring cell with a better signal, which helps maintain strong connectivity, especially in high-mobility scenarios.

Trigger conditions

Event A3 is triggered under the following condition:

• The signal strength (e.g., RSRP or RSRQ) of a neighboring cell is higher than the serving cell by a predefined threshold (A3 offset).

For example, if the serving cell’s signal strength is -95 dBm and a neighboring cell’s signal is -85 dBm with an A3 offset of 3 dB, the event will be triggered.

Key Concepts of Event A3

• Special Cell (SpCell): The primary serving cell of the UE, which could either be the Master Cell Group (MCG) or the Secondary Cell Group (SCG).
• Neighboring Cell: The target cell being evaluated by the UE for potential handover based on signal quality.
• Offset: A configurable threshold that determines when the neighboring cell’s signal should be considered better than the SpCell’s signal. This value can be positive or negative, allowing flexibility in defining the handover trigger.

Parameters for Event A3

1. A3 Offset: Defines the difference in signal quality between the serving and neighboring cells.
2. Hysteresis: Helps prevent unnecessary handovers by adding a margin to the A3 offset.
3. Time-to-Trigger (TTT): Specifies the duration for which the condition must be met before the event is reported.
4. Measurement Quantity: Indicates whether RSRP, RSRQ, or another parameter is used for comparison.

These parameters allow the network to fine-tune the conditions under which Event A3 is triggered, reducing unnecessary handovers.

Note: You can test out Measurement Report Event A3 using the Measurement Events Simulator.

ASN.1 Structure for Event A3

asn1
eventA3                                     SEQUENCE {
    a3-Offset                               MeasTriggerQuantityOffset,
    reportOnLeave                           BOOLEAN,
    hysteresis                              Hysteresis,
    timeToTrigger                           TimeToTrigger,
    useWhiteCellList                        BOOLEAN
}

How Does Event A3 Work?

Event A3 operates based on two main conditions:

1. Entering Condition (A3-1): This condition is satisfied when the neighboring cell’s signal strength is significantly better than the serving cell’s, factoring in a specific offset value.
2. Leaving Condition (A3-2): This condition is met when the neighboring cell’s signal strength drops below the serving cell’s by a predefined margin.

Let’s take a closer look at the formulas used to define these conditions:

• Entering Condition (A3-1): Mn + Ofn + Ocn - Hys > Mp + Ofp + Ocp + Off This inequality means the neighboring cell’s adjusted signal strength (Mn) must exceed the serving cell’s adjusted signal strength (Mp) by a specific offset value (Off) and margin (Hys).
• Leaving Condition (A3-2): Mn + Ofn + Ocn + Hys < Mp + Ofp + Ocp + Off This inequality signifies that the neighboring cell’s adjusted signal strength is no longer better than the serving cell’s, so the UE stays connected to the current serving cell.

Breaking Down the Formula Components

Each parameter in the above formulas has a distinct role:

• Mn: Measurement result of the neighboring cell (in dBm or dB).
• Mp: Measurement result of the serving cell (in dBm or dB).
• Ofn: Offset specific to the neighboring cell.
• Ocn: Offset specific to the neighboring cell’s frequency.
• Ofp: Offset specific to the serving cell.
• Ocp: Offset specific to the serving cell’s frequency.
• Hys: Hysteresis value that prevents unnecessary handovers.
• Off: Offset value that determines how much better the neighboring cell’s signal must be to trigger a handover.

In simpler terms: These parameters are just numbers used to compare signal strengths and determine whether a handover should happen.

Key Points to Remember about Event A3

1. Entering Condition: Neighboring cell’s signal strength must exceed the serving cell’s by a certain margin.
2. Leaving Condition: Neighboring cell’s signal strength falls below the serving cell’s by a defined value.
3. Offsets and Hysteresis: These values ensure the UE doesn’t switch cells too often, preventing frequent handovers that can disrupt connectivity.

Summary

1. Purpose: Event A3 helps manage handovers by comparing the signal strength of neighboring cells to the serving cell.
2. Conditions: It uses two main conditions—entering and leaving—to decide if a handover should happen.
3. Parameters: Several offset values and a hysteresis value control when the event is triggered, ensuring efficient mobility management.
4. Avoiding Unnecessary Handover: The hysteresis parameter helps avoid rapid switching between cells, maintaining stable connections.
5. Handover Optimization: Reduces call drops and ping-pong effects, ensuring stable connections during mobility.
6. Configuration: Managed via RRC signaling, allowing dynamic adjustment based on network conditions.
7. Challenges: Includes risks of ping-pong effects and handover failures if not properly configured.
8. Use Cases: Inter-cell handovers, load balancing, and dual connectivity scenarios.

By understanding and configuring Event A3 effectively, network operators can ensure seamless mobility management and an optimal user experience in 5G NR networks.

Measurement Event A3 Configurability

Legend:

• RAT: Radio Access Technology
• MCG: Master Cell Group
• SCG: Secondary Cell Group
• FR1: Frequency Range 1
• FR2: Frequency Range 2

FAQ's

1. How Does Event A3 Optimize Handover Performance?

Event A3 enables efficient handovers by balancing between the serving and neighboring cells. It ensures that a handover only occurs when there is a significant and sustained improvement in signal quality. This helps:

• Reduce call drops or interruptions.
• Improve user experience, especially in high-mobility scenarios.
• Enhance network resource utilization by keeping UEs connected to the best possible cell.

2. What Challenges are Associated with Event A3?

While Event A3 is effective, it comes with some challenges:

• Ping-Pong Effects: If the A3 offset is not properly configured, UEs may switch back and forth between cells.
• Handover Failures: In scenarios where the TTT is too short, the network might trigger handovers prematurely, leading to failed handovers.

To mitigate these challenges, network operators must carefully adjust the A3 offset, TTT, and hysteresis values based on specific deployment scenarios.

3. How is Event A3 Configured in 5G NR?

The configuration of Event A3 is done via RRC (Radio Resource Control) signaling messages between the gNB (5G base station) and the UE. The gNB sets the measurement configuration, including the A3 offset, hysteresis, and TTT. UEs then use this configuration to determine when to report Event A3 to the network.

4. What are Some Use Cases of Event A3?

Event A3 is widely used in scenarios such as:

• Inter-cell Handover: Triggering handovers between cells of the same or different frequency layers.
• Load Balancing: Ensuring that UEs are distributed evenly across cells, avoiding congestion in any single cell.
• Dual Connectivity: Deciding when a UE should connect to an additional cell for increased bandwidth.

5. Why is Event A3 Important in 5G Mobility?

Event A3 plays a key role in making handover decisions to maintain network stability and avoid ping-pong effects, which occur when frequent handovers degrade the user experience. By ensuring that a handover is triggered only when a neighboring cell’s quality is consistently better than the serving cell, it helps the network maintain efficient mobility management.

eventA3 SEQUENCE {
  a3-Offset           MeasTriggerQuantityOffset,
  reportOnLeave       BOOLEAN,
  hysteresis          Hysteresis,
  timeToTrigger       TimeToTrigger,
  useWhiteCellList    BOOLEAN
}