Suitable cell is defined:

Srxlev > 0
Squal > 0

Then, questions arise: what is Srxlev and Squal?

Srxlev = Qrxlev_meas – Qrxlevmin – Pcompensation

Qrxlevmin is the minimum received signal level defined for the cell (broadcast information). Qrxlevmin ranges from -115dBm to -25dBm.
Here, we use RSCP (power measured by a receiver on a particular physical communication channel. It is used as an indication of signal strength, as a handover criterion, in downlink power control, and to calculate path loss).

Pcompensation = Max (UE_TXPWR_MAX_RACH – P_MAX, 0)

UE_TXPWR_MAX_RACH is the broadcast value of the maximum RACH power and P_MAX is the UE maximum TX power.

Qrxlev_meas is the measured RSCP value at the device.
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Squal = Qqual_meas – Qqualmin

Qqualmin is the minimum received signal quality defined for the cell. (broadcast value).
Here we use CPICH Ec/Io( measurement defined as the ration of the received energy per chip and the total receiver interference plus noise power spectral density.)

Qqual_meas is the received CPICH Ec/Io at the receiver.

contains information on the cell selection and reselection parameters that the UE need to use in connected mode.

If SIB3 is not present it can be also be used for UEs in idle mode.

contains information on the cell selection and reselection parameters that the UE need to use in idle mode.

If SIB4 is not present it can be also be used for UEs in connected mode.

1. initial cell selection

2. cell selection based on stored information

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1. Initial cell selection procedure occurs. That is, UE tries to find a cell (not necessarily the best cell, but a usable cell) for it to camp on the cell.

For a given frequency, UE does the below until it finds a working cell:

There are roughly four steps for initial cell selection:

1. search for primary synchronization channel (P-SCH):

chip and slot synchronization is done

2. frame synchronization via secondary synchronization channel (S-SCH):

3.  find the primary scrambling code from the common pilot channel(CPICH):

4.  tune to Primary common control physical channel (P-CCPCH) and decode the system information;

check whether it is a suitable cell for camping ( PLMN code is broadcast)

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Once you successfully camped on a cell, you can get the neighbor list from the system information and tries to camp on the best cell.

2. Frequency and scrambling code may be saved on the phone. The UE may try to synchronize into the cell after switch on, and if it fails, it may do the initial cell selection as above.

1. UE measures the received power of CPICH channel from different cells (called the Monitored Set) and selects the one with the highest one as the serving cell.

2. The decision to add or remove the Active Set depends on the ratio between the received CPICH level of the best cell and other cells.