Once the PLMN is selected, the next step is to select a suitable cell inside that PLMN to camp on.
Then after camping on, UE will frequently checks the system information, does measurements on serving cell and neighboring cells. And if there is better cell to camp on, then it will camp to that cell by re-selection criteria.
Different type of Cell Category as given in specification:
acceptable cell: An “acceptable cell” is a cell on which the UE may camp to obtain limited service (originate emergency calls and receive ETWS notifications).
suitable cell: A “suitable cell” is a cell on which the UE may camp on to obtain normal service.
barred cell: A cell is barred if it is so indicated in the system information
reserved cell: A cell is reserved if it is so indicated in system information
Different Types of services in Idle Mode
There are 3 different types of Service Mode as defined by specification:
1. Limited Service: It is for emergency calls and ETWS notification
2. Normal Service: For public use
3. Operator Service: For operators on reserved cell.
Steps involved in Cell Search Procedure:
1. UE must obtain PCI with the help of PSS and SSS.
2. PSS and SSS are discussed in detain in previous chapter
3. Once UE performs gets PSS and SSS, it will calculate PCI by using “3 x cell group number + cell number” formula.
4. UE will get the Downlink Reference SIgnal to determine signal quality
5. UE will read MIB to get more detail about that cell.
Steps Involved in Cell Selection Procedure
There are 2 different types of cell selection Procedure:
1. Initial Cell Selection:
In this procedure, UE will not be having any idea of RF channels. UE will be in this state when it is switched on.
UE will scan all the RF channels in eNB bands and find a suitable cell.
UE need to select only the strongest cell on each carrier frequency.
Once UE finds this cell, it will be selected.
2. Stored Information Cell Selection:
In this procedure, UE will search the stored information of carrier frequencies and information on cell parameters received from IE’s or previously detected cells.
If there are no suitable cell found, then it will have to do Initial Cell Search Procedure.
When a UE goes to RRC Connected to RRC Idle mode, it must find a suitable cell to camp on.
If there is a stored information, then it will camp on the suitable cell or it will do Initial Cell Selection procedure.
Cell Selection Criterion
For a cell to be selected by UE, it must meet the Cell Selection Criterion.
UE will select a cell whose Srxlev and Squal are greater than 0.
Srxlev = Qrxlevmeas – (Qrxlevmin + Qrxlevminoffset) – Pcompensation
3GPP References: TS 36.304