What is Handover
Handover is a procedure for transferring the service of a mobile device (phone) from one cell or radio access technology to another. The switch occurs seamlessly or with a minimally noticeable pause. The name comes from the English word “handoff” — “transfer.”
Handover ensures continuity of communication when a subscriber is in motion, distributes load between cells, and maintains quality of service (QoS) in 2G, 3G, 4G LTE, and 5G NR networks.
Why Handover Is Needed
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Ensuring Subscriber Mobility
The user changes their location. The signal from the serving cell drops below an acceptable threshold (its level is measured in dBm), while the signal from a neighboring cell grows. If the switch does not occur, the connection will drop — this is what is known as a Call Drop.
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Load Balancing
Even when a subscriber is stationary, a cell may become overloaded. Handover transfers some subscribers to a neighboring, less loaded cell, optimizing the use of frequency resources. This is called a Load Balancing (LB) Handover.
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Interference Optimization and Quality of Service
Modern networks, especially 5G NR, use adaptive beams. If channel quality (SINR) drops due to interference, the network may perform a handover to another cell or even another frequency (Inter-frequency HO) to maintain the required SLA (speed, latency).
Types of Handover
Two main types are distinguished based on connection interruption:
| Type | How It Works | Where It Is Used |
| Hard Handover | Break-Before-Make: the current connection is terminated, after which a new one is established | GSM, frequency changes in WCDMA/LTE |
| Soft Handover | Make-Before-Break: the device maintains communication with 2+ base stations simultaneously; disconnection only occurs after the new channel has stabilized | UMTS/WCDMA (voice) |
Modern 4G/5G networks use Seamless Hard Handover with minimal delay (near-zero interruption time), making it imperceptible to the user.
Types of Handover by Network Interaction
Inter/Intra-Frequency
Here the type of switch depends on the carrier frequency.
| How It Works | How It Occurs | Example | |
| Intra-Frequency | Switching between cells on the same frequency | The subscriber measures neighboring cells without a reception pause, retuning filters to a different PCI | A phone call while walking or driving |
| Inter-Frequency | Switching to a cell on a different frequency (e.g., from 1800 to 900 MHz in LTE) | To measure a different frequency, the network reserves windows — Measurement Gaps (GAP) — during which data transmission briefly pauses | Low-frequency indoor coverage |
Inter/Intra-System (IRAT)
| How It Works | Example | |
| Intra-System | The radio access technology does not change; service is transferred within the same system | LTE → LTE, 5G (NR) → 5G (NR), UMTS → UMTS |
| Inter-RAT (IRAT) | Switching between network generations (e.g., 4G → 3G or 5G → 4G) | 4G → 3G or 5G → 4G where there is no continuous coverage |
IRAT also includes switching between a mobile network and Wi-Fi — this is covered by the VoWiFi / Wi-Fi Calling technologies.
In this case, handover maintains a call or data transfer when moving from a cellular network to Wi-Fi and back. Wi-Fi is considered an untrusted (non-3GPP) network, so connecting a subscriber directly to the operator’s core is not possible — this requires the ePDG (Evolved Packet Data Gateway) gateway.
It creates a secure (IPsec) connection and ensures a smooth transition between Wi-Fi and LTE/5G. If the gateway performs poorly, a call will simply drop when transitioning from Wi-Fi to a cellular network — which is why voice quality in VoWiFi depends directly on it.
