Cell Re-selection in WCDMA/GSM Networks

Cell re-selection is the process where a User Equipment (UE), also known as a mobile phone, identifies the most suitable cell to connect to and “camp on”. This applies to devices supporting various network technologies like GSM, CDMA, HSPA, and others.

This situation typically arises when a mobile device moves into a new Public Land Mobile Network (PLMN) area. Consequently, the device needs to find an appropriate cell within the new PLMN to connect to and gain access to the network services. The primary factors determining a mobile’s choice of cell are:

• RxQual (Receiver Quality): This represents the signal-to-noise ratio.
• RxLev (Receiver Signal Level): This is the received signal strength, measured in dBm.

It’s important to note that the path loss between the mobile device and the base transceiver station (BTS) or evolved NodeB (eNodeB) must be less than a threshold defined by the PLMN operator. Both RxQual and RxLev at the mobile device depend on the prevailing channel conditions and the base station’s transmission power.

Furthermore, the mobile terminal should use the maximum possible transmit power to ensure that cell re-selection information reaches the base station effectively. Cell re-selection can be initiated either by the network or by the mobile terminal itself.

Example: WCDMA to GSM Cell Re-Selection

Let’s consider a mobile device that supports both GSM and WCDMA networks.

1. Scenario: The mobile is currently connected to a WCDMA cell, and a packet-switched (PS) connection (i.e., a data call) is in progress.

2. Neighbor Cell Measurements: While the PS call is active, the mobile continuously measures the signal strength and quality of neighboring WCDMA cells and GSM cells. In WCDMA mode, these measurements are performed in compressed mode.

   • Compressed Mode: This mode allows the network to create brief “gaps” in the data transmission. During these gaps, the mobile suspends its regular transmission and reception activities and dedicates the period to performing measurements of neighboring cells.

3. Signal Strength Threshold: When the mobile detects that the received signal strength from the current WCDMA cell falls below a predefined threshold, it’s granted permission to select a new cell (either WCDMA or GSM).

4. Re-selection Process:

   • If the mobile re-selects another WCDMA cell while in WCDMA mode, the routing area update information, along with any data packets waiting to be transmitted, can be easily switched to the new cell.
   • However, if the mobile selects a GSM cell, it first establishes a connection with the GSM cell. Then, it initiates a procedure to perform a routing area update.

This entire process is managed by the core network backbone, which maintains connections with both GSM and WCDMA systems through suitable interfaces, as illustrated in the figure above.