Handovers in LTE

Types of Handover in LTE

Handover in LTE is a process where an ongoing connection is transferred from one cell to another without service interruption. It ensures seamless mobility when users move across different network areas.

---

1️⃣ Inter-Frequency Handover (Different LTE Frequencies)

📌 Definition:

• Happens when the UE moves between two LTE cells operating on different frequencies (EARFCNs).
• Required when a network operates on multiple LTE frequency bands (e.g., Band 3 and Band 5).
• The UE must retune to the new frequency while maintaining an active session.

✅ Example:
📍 A user is on a VoLTE call in LTE Cell A (EARFCN 100, Band 3 - 1800 MHz).

• As the user moves, Cell A’s signal weakens.
• The network triggers a handover to LTE Cell B (EARFCN 200, Band 5 - 850 MHz).
• The UE switches to the new frequency without dropping the call.

📝 Use Case: When a user moves from an urban area (higher frequency band) to a rural area (lower frequency band) where the coverage is better.

---

2️⃣ Intra-Frequency Handover (Same LTE Frequency)

📌 Definition:

• Happens when a UE moves between two LTE cells on the same frequency (EARFCN is unchanged).
• Typically occurs within the same LTE band when users move between two neighboring eNBs.
• More common than inter-frequency handovers because LTE networks prefer keeping users on the same frequency.

✅ Example:
📍 A user is streaming a video on LTE Cell A (EARFCN 100, Band 3 - 1800 MHz).

• The signal weakens as the user moves towards LTE Cell B, operating on the same EARFCN.
• The network triggers an intra-frequency handover, and the UE switches to Cell B.
• The video stream continues without interruption.

📝 Use Case: When a user is moving within a city where multiple LTE towers use the same frequency.

---

3️⃣ Inter-RAT Handover (Between LTE and Other RATs: 3G, 2G, 5G)

📌 Definition:

• Occurs when a UE moves from LTE to a different Radio Access Technology (RAT), such as 3G (UMTS), 2G (GSM), or 5G (NR).
• Required when LTE coverage is poor, or when a legacy system (e.g., 3G) is needed for voice calls (CSFB).

✅ Example:
📍 A user is on a call in an LTE network without VoLTE support.

• The user moves into an area where LTE coverage is weak, but 3G is available.
• The network triggers an Inter-RAT handover from LTE to UMTS (3G).
• The call continues on the 3G network without dropping.

📝 Use Case: When a network wants to offload users to another RAT due to congestion or coverage limitations.

---

4️⃣ Intra-RAT Handover (Within the Same RAT but Different Cells)

📌 Definition:

• Happens when the UE moves between two cells in the same RAT (e.g., LTE to LTE or 3G to 3G).
• Includes both intra-frequency and inter-frequency LTE handovers.

✅ Example:
📍 A user is in a video call while moving through a city.

• The network moves the user from LTE Cell A to LTE Cell B, either on the same or different frequency.
• The call continues seamlessly without interruption.

📝 Use Case: When a user is moving within an LTE network, and the network ensures smooth connectivity by switching between LTE cells.

---

5️⃣ X2 Handover (Fast Handover Between eNBs)

📌 Definition:

• X2 handover happens when the source eNB directly communicates with the target eNB via the X2 interface.
• Used for handovers within the same LTE network when both eNBs are connected via the X2 interface.
• Reduces signaling delay and improves efficiency compared to S1 handover (which involves the MME/S-GW).

✅ Example:
📍 A user is watching YouTube while moving between two LTE towers.

• The serving eNB A detects poor signal and sends a handover request to eNB B over the X2 interface.
• eNB B accepts, and the UE switches to it without involving the EPC (MME/S-GW).
• The video continues streaming without buffering.

📝 Use Case: Used in dense LTE networks (e.g., urban areas) to provide a fast and seamless handover between cells.

---

🚀 Summary of All Handover Types

Handover Type	Definition	Example	Service Impact
Inter-Frequency Handover	LTE to LTE on different frequencies (EARFCN change)	Moving from Band 3 (1800 MHz) to Band 5 (850 MHz)	No service interruption
Intra-Frequency Handover	LTE to LTE on the same frequency (same EARFCN)	Moving from LTE Cell A (EARFCN 100) to LTE Cell B (EARFCN 100)	No service interruption
Inter-RAT Handover	LTE to another RAT (3G, 2G, 5G)	Moving from LTE to 3G for a voice call	No service interruption
Intra-RAT Handover	LTE to LTE handover (includes intra- and inter-frequency)	Switching between LTE cells while on a video call	No service interruption
X2 Handover	Handover between eNBs using the X2 interface	Moving from eNB A to eNB B via X2 while streaming video	No service interruption

---

🎯 Key Takeaways

✔ Inter-Frequency Handover – LTE to LTE but on different frequencies.
✔ Intra-Frequency Handover – LTE to LTE on the same frequency.
✔ Inter-RAT Handover – LTE to 3G/2G/5G.
✔ Intra-RAT Handover – LTE to LTE, including inter- and intra-frequency.
✔ X2 Handover – LTE to LTE using the X2 interface for faster handovers.