When you power on the mobile device, in most case the device is under a circumstance where it sees many base station (gNB) around it. In some cases UE would be surrounded not by the multiple basestation from one network operator but by the multiple basestation from different network operators.

Out of those many cells, UE can camp on (register) to only one cell. Then the question is which specific single cell the UE have to register. For this UE goes through a specific decision making process to pick up a specific Cell to register, this specific decision making process is called 'Cell Selection'.

Cell Selection process in NR is very similar to the Cell Selection Process in LTE. So the description in this page would be almost identical to the LTE Cell Selection page.

Overall description of cell selection process is described in 38.304 Figure 5.2.3: Cell Selection Process and 5.2.4 Cell Reselection Evaluation Process

Pretty complicated flow diagram, right ? But you will find it even more complicated if you try to associate this flow to the situation you come across when you test the device in real life. You see in this flow diagram many branches and loops which makes it difficult to follow the right path and other thing is that it is not so easy to figure the exact status of the UE when you turn on the UE.

Let me give you some examples in which UE needs to go through the cell selection process.

• i) You got a new phone from the shop and just inserted the USIM and power on the device.
• ii) You have been using a phone for a while, and power it off and then power on.
• iii) You have been using a phone for a while and switch it to Airplan mode and then switch back to the normal mode.
• iv) You have been using a phone for a while and switch it to Airplan mode and flew into another country and then switch back to the normal mode.
• v) You just turned on a phone and turn it off right away in the middle of registration and then turned it on.
• vi) Turned off the phone you have been using and pull out the battery and then put the battery back and turn it on.
• vii) Turned off the phone you have been using and pull out the battery, pull out the USIM and put a new USIM in and then put the battery back and turn it on.
• viii) Turned of the phone you have been using and leave it off for several days and then turn it on.
• ix) Turned of the phone you have been using and take out the USIM, and then turn it on without USIM.

Can you mark the path on this flow diagram for each of the cases listed above ? To be honest, I may be able to do it only a couple of the case and not for all.

If you have access to UE logging tools, try to collect field test log in various situation and analize the log according to this flow charts. Only practice would make you understand this clearly unless you are the protocol stack developer who implements this process in UE.

Following diagram is from 38.304 Figure 5.2.2-1. (This is exactly same as LTE Process in 36.304). This is the case when UE find a suitable cell.

Following is the case where UE fails to find a suitable cell and camp on to any cell that is available.

Followings are the keywords that you have to know in order to understand the follow chart shown above.

Acceptable Cell

Acceptable cell is the cell that is not enough to be a suitable cell, but meets the minimum condition at least to make an emergency call. The minimum conditions are

• The cell is not barred
• The cell selection criteria is met

Suitable Cell

The cell that the UE may camp on for a normal service. The E-UTRA and UTRA Suitable Cell criteria is defined in 36.304 4.3 Service types in Idle Mode as follows.

The cell is part of either:

• the selected PLMN (PLMN that has been selected by the NAS, either manually or automatically), or:
• the registered PLMN(PLMN on which certain Location Registration outcomes have occurred.
• a PLMN of the Equivalent PLMN list

according to the latest information provided by NAS:

• The cell is not barred;
• The cell is part of at least one TA that is not part of the list of "forbidden tracking areas for roaming"
• The cell selection criteria are fulfilled

Even though the flow chart shown above handles various situations, if you don't follow through each of the case one by one, it would be difficult for you to get any concrete understanding for each specific case. My recommendation is to mark a specific path for each specific case one by one as examples shown below.

Case 1 : Initial Cell Selection

This flow describes the Initial Cell Selection process in mobile networks, focusing on how the UE selects a suitable cell for communication. This is what happens in your mobile phone when you power on for the first time.

Followings are some highlights on this scenario

New PLMN Selected

When a new PLMN (Public Land Mobile Network) is selected, the UE first checks whether it has stored information about cells in this PLMN. If no cell information is stored for the selected PLMN, the UE proceeds to the Initial Cell Selection process.

• Initial Cell Selection: In this process, the UE scans all available frequency bands to find a suitable cell to camp on. If no suitable cell is found, the UE continues searching until a suitable cell is identified.

No Suitable Cell Found

If no suitable cell is found during the Stored Information Cell Selection or Initial Cell Selection, the red arrow shows that the UE reverts to performing Any Cell Selection.

• Any Cell Selection: The UE tries to find any available cell, even if it is not part of the selected PLMN, as long as it allows for emergency or limited services.

Registration Rejection

If the UE finds a suitable cell and camps on it in the Idle state, it attempts to register with the network. If the network rejects the registration with specific causes (e.g., cause #12, #14, #15, or #25), the UE exits the idle state and re-enters the Cell Selection process .

Leaving Idle Mode (Triggering Connected Mode)

While in the Idle state, if the UE is required to make a call or perform any uplink transmission (e.g., mobile-originated call or data session), it transitions from the Idle state to Connected Mode (red arrow). This action triggers a transition to actively communicate with the network.

Case 2 : Stored Cell Selection

This scenario focuses on how the UE utilizes stored information to select a suitable cell for communication.

Followings are some highlights on this scenario

New PLMN Selected

When the UE switches to a new PLMN (Public Land Mobile Network), it first checks for stored cell information associated with that PLMN. If cell information is stored, the UE enters the Stored Information Cell Selection process (red arrow). This step enables the UE to quickly identify a suitable cell without initiating a full scan. If no suitable cell is found using stored information, the UE proceeds to the Initial Cell Selection process.

No Suitable Cell Found

If the Stored Information Cell Selection process fails to identify a suitable cell, the UE moves to Any Cell Selection (red arrow). In this fallback scenario, the UE attempts to camp on any available cell, even if it is not part of the selected PLMN. This ensures that the UE can access emergency or limited services if no preferred cell is available.

Registration Rejection

If the UE successfully finds a cell and enters the Idle state, it attempts to register with the network. If the network rejects the registration with specific NAS causes (e.g., #12, #14, #15, or #25), the UE exits the idle state and returns to the Cell Selection process (red arrow). This rejection indicates that the selected PLMN or cell does not meet the UE's requirements, prompting the UE to restart the selection process.

Transition to Connected Mode

While in the Idle state, if the UE initiates a call or requires an uplink communication, it transitions to Connected Mode (red arrow). This involves switching to an active communication state where the UE can transmit and receive data with the network.

Case 3 : Cell Reselection

This scenario describes how the UE evaluates and switches to a more suitable cell while in the Idle state

Triggering Cell Reselection

When the UE is in the Idle state and camped on a cell, it continuously monitors the network for better cell options. A trigger for the Cell Reselection Evaluation Process occurs in scenarios such as:

• Change in cell power: The UE detects a degradation in signal quality (e.g., reference signal power or quality drops below a threshold).
• Network-triggered re-evaluation: The network may instruct the UE to reselect based on specific policies or conditions.

Cell Reselection Evaluation Process

Upon detecting a trigger, the UE enters the Cell Reselection Evaluation Process (red arrow). This involves:

• Scanning neighboring cells and evaluating their signal quality based on predefined criteria such as RSRP (Reference Signal Received Power) or RSRQ (Reference Signal Received Quality).
• Comparing the signal quality of the current cell with neighboring cells.
• Identifying a suitable cell that offers better service or higher priority, based on PLMN and frequency priorities provided by the network.

If a suitable cell is found during this process, the UE reselects the new cell and camps on it.

No Suitable Cell Found

If the Cell Reselection Evaluation Process fails to find a suitable cell, the UE remains camped on the current cell and continues monitoring until conditions improve or another trigger occurs. If the current cell becomes entirely unsuitable (e.g., due to coverage loss), the UE transitions to Any Cell Selection (red arrow), where it attempts to connect to any available cell, even outside the current PLMN.

Registration Rejection

If the UE camps on a suitable cell but the network rejects the registration request with specific causes (e.g., #12, #14, #15, or #25), the UE is forced to reinitiate the selection process. This may involve returning to Cell Reselection or moving to Any Cell Selection, depending on the conditions (red arrow).

Transition to Connected Mode

If the UE is required to initiate communication (e.g., a call or data transmission), it leaves the Idle state and transitions to Connected Mode (red arrow). This ensures the UE can actively communicate with the network.