Cell Re-selection procedure in WCDMA/GSM networks
Cell re-selection procedure is the process by which UE/Mobile finds the suitable cell to camp on. This is applied to any mobile supporting GSM or CDMA or HSPA or any other network. This situation arises when mobile station moves to new PLMN area and hence it need to find suitable cell within the new PLMN to camp on and secure access to the network.
The major criteria for mobile to camp on cell depends on RxQual(Receiver Quality or S/N ratio) and RxLev (receiver signal level in dBm) . It should be noted that path loss between Mobile and BTS/eNode must be less than threshold set by the operator(of that PLMN).
RxQual and RxLev at mobile depends on channel conditions and base station transmission power. In addition mobile terminal should use maximum posssible transmit power so that cell re-selection information can reach to the base station(BTS/eNode).
Cell re-selection can be either network originated or it cn be mobile terminal originated.
Example WCDMA to GSM Cell Re-Selection
1. Let us take example of dual mode supported(both GSM and WCDMA) mobile station. Scenatio is Mobile is camped on WCDMA cell and packet switched connection has been established (PS call in progress).
2. While in PS call, mobile does neighbour cell measurements of nearby WCDMA cells as well as GSM cells. Mobile in WCDMA mode does the measurements in compressed mode. compressed mode is the mode in which network creates gaps in between frame to perform measurements. During this period of gaps transmission and reception is put off and the period is dedicated for measurements.
3. As soon as mobile terminal finds received signal strength below the threshold, mobile is provided permission to select new cell(WCDMA or GSM cell).
4. If mobile re-selects WCDMA cell while in WCDMA mode,
then routing area update information as well as data packets waiting in the queue are easy to switched to new cell.
But if Mobile terminal selects GSM cell then mobile first establishes connection with GSM cell and
initiates procedure to perform routing area update.
This is taken care by core network backbone as core network is in connection with both GSM and WCDMA
systems with suitable interfaces as shown in the figure above.
Useful Links
CDMA Tutorial
GSM Tutorial
GSM mobile originated call flow
GSM Mobile Terminated Call flow
GSM protocol stack
GSM physical layer
GPRS protocol stack
GPRS tutorial