Measurement event A4 in 5G NR

Event A4 is trigerred when Neighbour becomes better than threshold

In 5G NR networks, effective mobility management is crucial to ensure that devices (User Equipment or UE) stay connected with optimal signal quality as they move through various coverage areas. Measurement Event A4 is a critical event used to trigger handover when a neighboring cell offers better signal quality than a defined threshold.

What is Measurement Event A4?

Measurement Event A4 is defined in 3GPP specifications for 5G NR and is triggered when a neighboring cell’s signal quality surpasses a pre-set threshold, making it a candidate for the UE to consider for handover. This threshold-based event helps in mobility procedures that rely more on network load balancing than signal coverage alone.

Trigger Condition for Event A4:

Event A4 is triggered when the signal measurement of a neighboring cell (Meas_neigh), adjusted by frequency and cell-specific offsets (O_neigh,freq and O_neigh,cell), exceeds the threshold by a hysteresis margin (Hyst). The condition is:

Mn + Ofn + Ocn – Hys > Thresh

This condition ensures that the neighboring cell provides a strong enough signal, factoring in network-specific offset adjustments and the hysteresis value to avoid frequent handover oscillations.

Cancellation Condition for Event A4:

Once Event A4 is triggered, it remains active until the neighboring cell’s measurement drops below the threshold with the hysteresis value added back:

 Mn + Ofn + Ocn + Hys < Thresh

This ensures that the handover only occurs when the neighboring cell’s signal consistently meets the required quality, improving connection stability and reducing unnecessary handovers.

The variables in the formula are defined as follows:

• Mn is the measurement result of the neighbouring cell, not taking into account any offsets.
• Ofn is the measurement object specific offset of the neighbour cell (i.e. offsetMO as defined within measObjectNR corresponding to the neighbour cell).
• Ocn is the measurement object specific offset of the neighbour cell (i.e. cellIndividualOffset as defined within measObjectNR corresponding to the neighbour cell), and set to zero if not configured for the neighbour cell.
• Hys is the hysteresis parameter for this event (i.e. hysteresis as defined within reportConfigNR for this event).
• Thresh is the threshold parameter for this event (i.e. a4-Threshold as defined within reportConfigNR for this event).
• Mn is expressed in dBm in case of RSRP, or in dB in case of RSRQ and RS-SINR.
• Ofn, Ocn, Hys are expressed in dB.
• Thresh is expressed in the same unit as Mn.

eventA4                                     SEQUENCE {
            a4-Threshold                                MeasTriggerQuantity,
            reportOnLeave                               BOOLEAN,
            hysteresis                                  Hysteresis,
            timeToTrigger                               TimeToTrigger,
            useWhiteCellList                            BOOLEAN

Key Parameters Configurable for Event A4

Event A4 involves several key parameters that network operators can configure to fine-tune mobility and load balancing behavior:

1. Offsets (Frequency and Cell Specific):

• Both frequency-specific and cell-specific offsets can be configured, with values ranging between -24 dB and +24 dB. These offsets help adjust the UE’s perception of neighboring cell quality based on operator priorities and deployment scenarios.

1. Hysteresis:

• The hysteresis value prevents rapid toggling between cells when the signal quality fluctuates around the threshold. It can be configured between 0 and 15 dB in 0.5 dB steps, with values between 0 and 30 representing the signaled values. Higher hysteresis values reduce unnecessary handovers by requiring a greater difference in signal quality before a switch.

1. Threshold:

• The threshold for triggering Event A4 varies based on the measurement type:
• RSRP (Reference Signal Received Power): -156 to -31 dBm
• RSRQ (Reference Signal Received Quality): -43 to 20 dB
• SINR (Signal-to-Interference-plus-Noise Ratio): -23 to 40 dB
• These thresholds are mapped to values between 0 and 127, allowing a wide range of signal quality levels to be used as triggers.

1. Time-to-Trigger (TTT):

• TTT defines the duration for which the trigger condition must hold before the event is reported. This prevents triggering based on short-term fluctuations. TTT can be set from values between 0 ms to 5120 ms, with options like 40, 64, 100, and more. A longer TTT means that the UE will only report the event after consistently detecting a neighboring cell’s superiority for a longer period.

The Role of Event A4 in Mobility Management

Event A4 serves two primary purposes:

1. Load Balancing:

• In dense urban areas, where load balancing is essential to prevent individual cells from becoming congested, Event A4 helps redirect UEs to neighboring cells with better signal quality. This allows network operators to manage load across cells effectively, improving the overall quality of service (QoS).

1. Handover Optimization:

• By defining conditions under which the UE should consider neighboring cells, Event A4 optimizes handover decisions. This is particularly useful in scenarios where a UE moves through areas with similar coverage levels but requires seamless transitions to maintain performance.

Configuring Event A4 for Optimal Network Performance

Configuring Event A4 involves balancing mobility, handover efficiency, and load management:

• Hysteresis and Offset Settings: Setting appropriate hysteresis and offset values can reduce handover “ping-ponging” between cells, ensuring that only significant signal differences trigger handovers.
• Threshold Adjustment: Selecting the correct RSRP, RSRQ, or SINR threshold helps maintain coverage while avoiding handovers to cells that do not offer substantial improvement.
• Time-to-Trigger: Adjusting TTT allows the network to be resilient to short-term signal variations, only triggering handovers when the neighboring cell consistently provides better quality.