Cell Selection Process

There are two kinds of cell selection.

Normal cell selection

Stored list cell selection

In fact, the process might be different for different MSs

 

If SIM card of MS does not store any BCCH, it will search all the 124 RF channels and 374 more GSM1800 channels for dual-band MS, measure the receiving signal level of each. The whole process lasts about 3s to 5s. During this, MS obtains at least 5 measurement samples from each channel. Then MS tunes to the carrier with strongest receiving level, and judge whether it is BCCH(by searching for FCCH). If yes, MS tries to decode SCH and enable it synchronous with that BCCH, and reads system information on it. If MS can correctly read the system information and verify this cell belongs to the selected PLMN and C1 is larger than 0 and cell priority is normal, then MS conducts location update. After passing, MS resides in that cell. Otherwise, MS will tune to second BCCH and go on with the same procedure. After trying the strongest 30 (single-band) or 40 (dual-band) carriers, if it is impossible to access, MS will try to access the cell with low cell selection priority level. If still unsuccessful, MS will use PLMN allowed on SIM card to try on. If fail again, MS will go on monitoring all RF channels and find out a cell(Signal is the strongest. C1 is larger than 0. Access is not prohibited.) without considering the PLMN , and MS enters emergency call mode (service bar mode).

Note:1. When MS access level is prohibited by this cell, cell selection algorithm is not affected. If the condition is fulfilled, MS will still reside in this cell.

2. MS belongs to the PLMN selected, but is prohibited from access, or C1<0. Then MS obtains BA table from this cell and search BCCH  according to this BA.

When MS is powered off, it will store some BCCH carrier information. When powered on, it will first search the BCCH carrier that have been stored. If MS can decode the BCCH data of this cell but cannot reside, it will check the BCCH in BA table of this cell. If it still cannot pass, MS will start the cell selection process without BCCH table.