How Cell Selection in GSM
Once the mobile is switched on and the registered home PLMN was selected (e.x. SMART), it will next search for a BCCH frequency list, stored in its memory or in its SIM card. The list can have up to 32 BCCH frequencies for the mobile to scan. This reduces the time of cell selection, compared to scanning the whole frequency band. If this feature is turned off at the switch then the mobile has to scan the entire frequency band for the strongest BCCH carrier.
The BCCH frequency list is called BA (BCCH Allocation) list and there are 2 types, Active and Idle.
Idle is a list of BCCH used for scanning when the mobile is in an idle mode and Active is a list of BCCH used during mobile busy mode.
Why 2 List ?
- When the mobile is in idle mode it may want to scan a longer list of BCCH and tune to the strongest
- whereas when in Active mode the list of BCCH should be shorter (correspond to defined neighbors) so that the mobile will scan the short list and get a more accurate signal strength measurements to achieve better handover performance. It is also to reduce the time spend by the mobile to decode the BSIC.
RF Tips : ACTIVE MODE LIST SHOULD NOT BE MORE THAN 15 BCCH FREQUENCIES.
If there is no BA list stored in the Mobile or SIM card then the mobile will scan all the 124 GSM channels and 374 DCS channel and arrange the frequencies in a DESCENDING order of signal strength. It will take the mobile 3 to 5 seconds to scan the whole band.
After which it will tune to the strongest frequency. The mobile will check if this is a BCCH carrier by looking out for the frequency correction burst send by the FCCH (Frequency Correction Channel).
If it is the BCCH carrier than mobile tunes to this carrier to read the SCH (Synchronization Channel) for the BSIC parameter.
Next it will read the BCCH for system information like CGI (Cell Global identity), LAI (Location Area Identity), BCCH carriers of the neighboring cells (BA List), maximum output power allowed in the cell and other broadcast messages like barred cell.
Next the mobile will compare if the selected cell belongs to a forbidden PLMN stored in its SIM card. It will look at the 2 digit Network Mobile Code transmitted by the BCCH on the LAI (Location Area Identity). If those 2 digits ware registered as forbidden in the SIM card then the mobile will not select this cell. The mobile then will tune to the second strongest BCCH carrier and subsequently does the same process over and over again until it finds the right cell.
Once it finds the right cell it will start using the BA (BCCH allocation) list transmitted by the BCCH carrier for cell reselection, will be discussed later.
Lets say the chosen PLMN is correct, able to read the FCH, SCH and BCCH and the chosen Cell is accessible (no cell barring),
DOES THIS MEAN THAT THE MOBILE NOW CAN CAMP ON THIS SITE ? NO !!!
There is one last criteria called c1 cell selection Criterion which must be calculated by the mobile and If the c1 value is greater than 0 then the mobile Can camp on this cell or else the next cell with c1 > 0 Will be selected