Can A4 trigger handover from 5G NR to LTE?

Yes. This is an inter-RAT handover. In 5G SA networks, the gNB can configure A4-based inter-RAT measurements where the neighbor is an LTE cell. If the LTE cell exceeds the A4 threshold, the UE reports it and the network can execute a handover from NR to LTE. This is commonly used as a fallback mechanism when NR coverage is lost.

mmWave & 5G

A4 Event

Q: What is the difference between A4 and A3?

A3 is a relative comparison: neighbor must exceed serving by an offset. A4 is an absolute threshold: neighbor must exceed a fixed RSRP value regardless of serving cell quality. This makes A4 ideal for inter-frequency scenarios where direct RSRP comparison across bands is unreliable due to different path loss characteristics. A network can use A4 to push users to a capacity layer whenever that layer is available, regardless of serving cell strength.

Q: Does an A4 Event interrupt the data connection?

The handover itself causes a brief interruption of 30-80 ms for inter-frequency handovers, slightly longer than intra-frequency A3 handovers (20-50 ms). The additional delay comes from the receiver retuning to the target frequency and performing random access. However, PDCP layer buffering and reordering ensure no user-visible packet loss in most cases.

/ay-four ee-vent/

An A4 Event is a 3GPP measurement report in LTE and 5G NR triggered when a neighbor cell on a different frequency exceeds an absolute RSRP threshold. Unlike A3 (which compares neighbor vs. serving), A4 evaluates only the neighbor against a fixed value, making it the primary tool for inter-frequency load balancing. By setting the A4 threshold to match the minimum viable signal on a capacity layer (e.g., -95 dBm on 3.5 GHz), engineers can offload users from congested coverage bands to high-speed capacity bands whenever the capacity layer is available.

Category: mmWave & 5G

3GPP Spec: TS 38.331

Action: Inter-freq handover

Understanding the A4 Event

Multi-layer networks deploy several frequency carriers at each cell site: a low-band coverage layer (e.g., 700 MHz), a mid-band capacity layer (e.g., 3.5 GHz), and sometimes a mmWave hotspot layer. The A4 Event provides the mechanism to push users from the coverage layer to the capacity layer purely based on whether the capacity layer signal is strong enough, regardless of how strong the current serving cell is.

This is critical for load balancing. Even if a user has excellent -70 dBm RSRP on the 700 MHz layer, the network may want to move them to 3.5 GHz to free up low-band capacity for indoor users who have no other option. A4 enables this by triggering purely on neighbor cell quality, not serving cell quality.

A4 Event Trigger Condition (3GPP TS 38.331)

Entry condition:
M_n + O_cn − H_ys > Thresh

Leave condition:
M_n + O_cn + H_ys < Thresh

Where:
M_n = neighbor cell measurement (RSRP/RSRQ/SINR)
O_cn = cell-individual offset for neighbor
H_ys = hysteresis (0-15 dB)
Thresh = absolute threshold (e.g., -95 dBm)

Note: Serving cell quality M_s is not evaluated. A4 triggers even if serving is excellent.

A3 vs A4 vs A5 Comparison

Event	Serving Evaluated?	Neighbor Evaluated?	Comparison Type	Primary Use
A3	Yes	Yes	Relative (offset)	Intra-freq handover
A4	No	Yes	Absolute threshold	Inter-freq load balancing
A5	Yes	Yes	Dual absolute thresholds	Coverage-triggered inter-freq

Common Questions

Frequently Asked Questions

What is the difference between A4 and A3?

A3 is a relative comparison: neighbor must exceed serving by an offset. A4 is an absolute threshold: neighbor must exceed a fixed RSRP value regardless of serving cell quality. This makes A4 ideal for inter-frequency scenarios where direct comparison across bands is unreliable due to different path loss characteristics.

Does an A4 Event interrupt the data connection?

The inter-frequency handover causes a brief interruption of 30-80 ms, slightly longer than intra-frequency A3 handovers (20-50 ms). The additional delay comes from receiver retuning and random access on the target frequency. PDCP buffering ensures no user-visible packet loss in most cases.

Can A4 trigger handover from 5G to LTE?

Yes. In 5G SA networks, the gNB can configure A4-based inter-RAT measurements where the neighbor is an LTE cell. If the LTE cell exceeds the threshold, the network can execute a handover from NR to LTE as a coverage fallback mechanism.

Related Terms

← A3 Event A5 Event →