In this post, we will try to describe the very basics of cell selection and reselection based on the 3GPP specifications of the main cellular protocols. This information is simplified for a reader who wants to understand the basics of this signaling procedure without diving into the details of 3GPP TS documents.
Idle Mode
Idle mode is the state during which the UE has no RRC connection attached to it. In this state the UE can perform a number of activities/processes including:
- PLMN Selection (Public Land Mobile Network) – Operator Selection
- Cell Selection and Reselection
- Location Registration
Initial Cell Selection Procedure
The basic procedure of the UE initial cell selection is described in the flow diagram below:
When the UE is powered ON, it selects the appropriate PLMN based on the USIM information (ie. IMSI, Roaming Agreements if EHPLMN), and it completes the Initial attachment and the PLMN Registration.
After the completion of the Initial Attachment, the UE jumps to another mode of operation called RRC Connected State and stays there until RRC inactivity timer expires (usually 20 seconds later).
PLMN Selection
As mentioned before, the PLMN Selection is based on the information stored in the USIM card provided by the Mobile Network Operator (MNO) to the user. This can be the Home PLMN or the Equivalent Home PLMN (EHPLMN). After the PLMN is selected the UE has to successfully be able to register to the Core Network (ie. EPC for the case of LTE or 5GC for the case of NR), or else it has to start the process all over again.
Initial Cell Selection
The UE performs the necessary procedures to collect the appropriate information from the surrounding cells in order to select the most appropriate cell to camp on.
Cell selection can occur in two cases
- When the UE transitions from Idle to RRC connected
- When the UE is Powered On and after the PLMN Selection
If there is stored information from a previous connection the UE attempts to select the previous cell otherwise it scans all the Frequency Bands to find an appropriate Channel and an appropriate cell.
In the case of LTE, and similarly, with 5G NR, the UE synchronizes with the cell by deriving the Synchronization signals from the Physical Cell Identity (PCI) of the cell. This is the Secondary Syncronization Signal (SSS) and the Primary Syncronization Signal (PSS).
After the Initial Synchronization with the cell, the UE finds and decodes the Master Information Block (MIB) and System Information Blocks (SIBs) which are broadcasted. These Information Blocks contain Information Elements (IE) with useful parameters and timers when it comes to Cell Reselection or other important Session Management and Mobility Management events such as Handover and Location Area Updates.
Cell Selection and Reselection Criteria
The picture below illustrates the procedure and the thresholds for a UE to perform a Cell Reselection.
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