cell selection part-1

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In this section, we will learn about cell selection basics for SA and LTE.

What is cell selection?

If a device is under circumstances where UE needs to be switched on or after UE has left RRC connected mode or UE returns to a coverage area, it sees many base stations around it. Many a times UE might be surrounded by multiple base stations from different operators. So from this many base stations, UE can register to only one cell. So to register to a specific cell UE goes through a dedicated process. This specific decision making process where UE decides to which cell to camp on is known as cell selection.

Is the initial cell selection procedure reacquired in the case of NSA?

When UE is connected to MCG that time only cell selection procedure is reacquired.
Note: ENDC cells (SCG) are not used for initial cell selection.

What is the cell selection procedure for LTE and SA?

Cell selection can be performed by one of the following two procedures:

A] Initial cell selection:

LTE: In this procedure device does not have prior knowledge of which RF channels are LTE carriers. The device will scan all RF channels in the supported LTE bands according to its capabilities to find a suitable cell. The device needs to search for the strongest cell on each carrier frequency. As soon as suitable cell is found the cell will be selected.

SA: Same as LTE, the device scans all RF channels within its supported frequency bands (supported SA Band). This Scanning is based upon the synchronization raster. The device assumes that a burst of SSBs is transmitted every 20 ms, during the initial cell selection procedure. The device searches for one or more SSB at each candidate carrier frequency which has GSCN (Global Synchronization Channel Number).

At specific GSCN after finding one or more SSBs, the UE identifies the strongest cell and proceeds to decode the System Information. If device discovers a set of SSBs which do not have any associated System Information then, the PBCH can provide information which directs the UE towards another set of SSBs.

B] Cell selection by leveraging the stored information:

LTE: Stored information of carrier frequencies is required and optionally the information on cell parameters is also required, from previously received measurement control information elements or from previously detected cells. Once the device has found a suitable cell the device will select it. Initial Cell Selection procedure will be started if no suitable cell is found.

SA: In this case device uses the stored information to support the cell selection procedure.  This can also include carrier frequencies which the device has previously camped on. This also includes cell parameters from previously received measurement control information or previously detected cells. 
The Base Station can use the RRC Release message to direct the Device towards a specific carrier, in case of cell selection after leaving RRC Connected mode. If the cell selection is based on store information is unsuccessful the device uses the initial cell selection procedure.

Why is raster required?

This raster is relatively coarse to reduce the number of candidate carrier frequencies and hence reduce the delay generated by band scanning.

What happens if stored information base cell selection is not successful?

A device uses the initial cell selection procedure if cell selection based upon stored information is unsuccessful.

What are the types of cell selection?

There are 3 types of cell selection:
1) Initial cell selection.
2) Stored information cell selection.
3) Cell selection when leaving RRC connected mode.

What is suitable cell?

Suitable cell can be defined as cell in which:

  • Cell should be not barred. Device can find this info from MIB in the case of SA and from SIB1 in the case of LTE.
  • Belongs to the PLMN selected by the NAS layer, the registered PLMN (previously) or an equivalent PLMN.
  • Cell belongs to at least one tracking area which is not forbidden
  • The device should satisfy the cell selection criteria.
In the next section, we will learn more about Cell selection and its basic. So keep reading……. And Stay tune……………
Pinal Dobariya………..



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