Cell Selection, Cell Reselection, Redirection, and Handover in LTE
These are different mobility procedures in LTE. Below is a detailed explanation with examples.
1️⃣ Cell Selection (First Time Camping on a Cell)
Definition:
- When a UE is powered ON, it searches for a suitable LTE cell to camp on.
- UE selects a cell based on signal strength and quality.
- Happens only in idle mode.
- UE reads SIB1 and SIB2 to confirm if it can register.
Example of Cell Selection:
📌 A mobile phone is powered ON in an LTE network.
- It scans all available LTE frequencies.
- It finds Cell A (EARFCN 100) with strong RSRP and selects it.
- UE camps on Cell A and is now ready for paging and registration.
2️⃣ Cell Reselection (UE Switching Between Cells in Idle Mode)
Definition:
- When a UE is in idle mode, it continuously measures neighboring cells.
- If another cell has better signal strength, UE moves to that cell.
- No network signaling is involved (UE-driven).
Types of Cell Reselection:
- Intra-frequency reselection: Switching between two LTE cells on the same frequency (EARFCN).
- Inter-frequency reselection: Switching between two LTE cells on different frequencies (EARFCN change).
- Inter-RAT reselection: Switching from LTE to 3G, 2G, or NR (5G).
Example of Cell Reselection:
📌 A UE is camped on Cell A (EARFCN 100, Band 3).
- The signal becomes weak as the user moves away.
- UE detects Cell B (same EARFCN) with stronger signal and switches to it.
3️⃣ Redirection (Network-Directed Movement to Another Frequency or RAT)
Definition:
- Happens during call setup or data transfer initiation.
- Network instructs UE to move to a different frequency or RAT (3G, 2G, 5G).
- Used to reduce congestion or optimize traffic load.
Example of Redirection:
📌 A UE requests VoLTE call setup on LTE.
- The network detects LTE is congested.
- The network redirects UE to UMTS (3G) for voice call handling.
- UE moves from LTE Cell A (EARFCN 100, Band 3) → UMTS Cell B (UARFCN 300).
4️⃣ Handover (Seamless Switch Between Cells in Connected Mode)
Definition:
- When a UE is in connected mode, it may experience signal degradation.
- Network decides to move UE to a better cell without service interruption.
- Network-controlled, unlike reselection (which is UE-controlled).
Types of Handover:
- Intra-frequency handover: LTE to LTE (same frequency).
- Inter-frequency handover: LTE to LTE (different frequency).
- Inter-RAT handover: LTE to 3G/2G/5G.
Example of Handover:
📌 A user is on a VoLTE call in Cell A (EARFCN 100, Band 3).
- The signal weakens as the user moves.
- eNB triggers a handover to Cell B (same EARFCN, different eNB).
- The call continues without drop as UE moves from Cell A → Cell B.
Comparison Table: Selection, Reselection, Redirection, Handover
| Feature | Cell Selection | Cell Reselection | Redirection | Handover |
|---|---|---|---|---|
| Mode | Idle | Idle | Connected | Connected |
| Control | UE-driven | UE-driven | Network-driven | Network-driven |
| Trigger | UE powers ON | Better neighboring cell detected | Network instructs UE to move | Network decides based on measurements |
| Network Involvement | No | No | Yes | Yes |
| Interruption | No | Yes (small delay) | Yes (UE moves to new frequency before continuing) | No (seamless transition) |
| Example | Phone turns ON and selects a cell | UE moves to a stronger cell while idle | UE is redirected to another frequency for call setup | UE moves to another eNB during a VoLTE call |
Key Takeaways
✔ Cell Selection: First-time camping after UE power ON.
✔ Cell Reselection: UE switches to a better idle mode cell (UE-driven).
✔ Redirection: Network moves UE to another RAT or frequency for call/data handling.
✔ Handover: Seamless switch between cells during an active connection (network-driven).
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