Cell Selection

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

That figure appears further down this page. Before you get there, here is the same thing stripped down to its spine, with the connected-mode and reselection branches left out :

                       +--------------------------------+
                       |    new PLMN selected by NAS    |
                       +--------------------------------+
                                        |
                 +----------------------+----------------------+
cell info stored |                                             | no cell info stored
                 |                                             |
                 v                                             v
   +---------------------------+                 +---------------------------+
   |    Stored Information     | no suitable cell|       Initial Cell        |
   |      Cell Selection       |---------------->|         Selection         |
   +---------------------------+                 +---------------------------+
                 |                                             |        |
                 | suitable cell found                         |        | no suitable cell found
                 +----------------------+----------------------+        |
                                        v                               |
                          +--------------------------+                  |
                          |     CAMPED NORMALLY      |                  |
                          |      normal service      |                  |
                          +--------------------------+                  |
                                        |                               |
                                        |  registration rejected,       |
                                        |  EXCEPT with cause            |
                                        |  #12, #14, #15 or #25         |
                                        |                               |
                                        v                               v
                  +--------------------------------------------------------+
                  |                   Any Cell Selection                   |<------------ no USIM
                  +--------------------------------------------------------+
                                              | acceptable cell found
                                              v
                                +--------------------------+
                                |    CAMPED ON ANY CELL    |
                                |     limited service      |
                                +--------------------------+

Everything else in the full figure is detail hung off those decisions. Naming them explicitly makes the real chart much less frightening :

Do I have stored information for this PLMN ? - decides whether the UE takes the fast path or the slow path.
Did I find a SUITABLE cell ? - note that the two procedures do not fail the same way. If Stored Information Cell Selection finds nothing it falls sideways into Initial Cell Selection and tries again properly. Only when Initial Cell Selection also finds nothing does the UE drop to Any Cell Selection.
Did I find an ACCEPTABLE cell ? - decides whether the fallback gives emergency-only service. This is also where a UE with no USIM enters the chart, as the second half of the figure shows.
Was my registration rejected ? - and here the figure repays careful reading, because its condition is an exception rather than a trigger. See the reject causes section below.

Notice that the two words doing all the work here are suitable and acceptable. They are not synonyms and the whole lower half of the chart exists because they differ. Your definitions of both are further down this page.

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.

Walking the nine cases through the flow chart

Let me try to answer that question, because it turns out that once you ask the right question of each case, most of them fall out cleanly. And the right question is not "what did the user do?" - it is :

What stored information does the UE still have, and is it valid for the PLMN it is now trying to select ?

Every one of the nine cases is really a different answer to that one question. Here they are side by side.

	What the UE still has	Which branch it takes	Where it ends up
i)	Nothing of its own. Only the band lists the manufacturer preconfigured	Initial Cell Selection	Camped normally, but by the slowest route - a full scan of every supported band
ii)	Last camped frequency and PLMN, held in non-volatile memory	Stored Information	Camped normally, quickly. The everyday case
iii)	Everything, and it never even lost power - the context is still in RAM	Stored Information	Usually back on the very same cell. The fastest of all nine
iv)	Stored information, but all of it describes another continent	Stored Information fails -> Initial Cell Selection	Camped normally on a roaming partner, after a visible delay. The path everyone has watched at an airport
v)	Possibly the frequency, but no registered PLMN and no valid 5G-GUTI - registration never finished	Stored Information (usually)	Finds the cell fast, then has to register from scratch. See the note below - this one is genuinely implementation dependent
vi)	The same as ii). Pulling the battery clears RAM, not non-volatile memory	Stored Information	Identical outcome to ii). See the note below on why this case and iii) are worth comparing
vii)	Stored information that belongs to the previous subscriber	Depends on the new USIM's PLMN - see below	Registration is fresh either way, because the SUPI changed
viii)	Stored information, still valid if you have not moved, but a long expired registration	Stored Information	Cell found quickly ; registration is a new one because the UE was implicitly deregistered days ago
ix)	No USIM, so there is no PLMN to select and stored information is irrelevant	Any Cell Selection	Camped on any cell - limited service, emergency calls only. Never "camped normally"

Four of these deserve a bit more than a table row.

iii) versus vi) - the case pair that tests what "stored" means. These two look almost the same and they end up on the same branch, but they are not identical, and the difference is a good thing to have straight. Airplane mode never removes power, so the UE keeps everything including the volatile context in RAM - it often re-camps on the same cell within a second. Pulling the battery destroys RAM but leaves non-volatile memory untouched, and the stored frequency and PLMN live in non-volatile memory. So both take the stored information branch, but iii) starts with more in hand and is quicker. If you ever see a device that does a full band scan after a battery pull, that is a bug or a deliberately wiped NV, not the normal behaviour.

iv) - why the airport case is slow. This is the only case in the list where the UE genuinely does the work twice. It has stored information, so it takes the fast branch first and tries the frequencies it knew - and every one of them is silent, because they belong to a network thousands of kilometres away. Only after that fails does it fall through to Initial Cell Selection and scan properly. On top of the scan, NAS then has to do PLMN selection among networks it has never seen, and may be rejected by some of them before finding one that accepts roaming. The minute or two you spend staring at "Searching..." after landing is these three things in sequence.

vii) - what actually matters is the PLMN, not the USIM. It is tempting to say "new USIM, therefore start from scratch", but that is not quite it. Stored information is held per PLMN. So :

If the new USIM belongs to a different PLMN, there is no stored information for the newly selected PLMN, and the UE takes Initial Cell Selection.
If the new USIM belongs to the same PLMN - a replacement SIM from the same operator, say - the stored information is still perfectly valid for that PLMN, and the UE takes Stored Information Cell Selection.

Either way NAS registration starts fresh, because the subscriber identity has changed and any 5G-GUTI the UE was holding belongs to someone else.

ix) - the case that shows why "acceptable" exists. Without a USIM there is no selected PLMN, and read your own definition of a suitable cell further down this page : a suitable cell must be part of the selected, registered or equivalent PLMN. With no USIM, no cell in the world can be suitable. So the "SUITABLE cell found?" answer is permanently no, the UE goes to Any Cell Selection, and it camps on an acceptable cell in the limited service state. That is exactly why you can still dial an emergency number on a phone with no SIM in it. If you want one case to demonstrate the acceptable / suitable distinction to someone, this is the one.

NOTE : case v) is the one I would not state confidently, and it is worth saying so rather than inventing an answer. Whether the UE committed the frequency to non-volatile memory during those few seconds before it was switched off is implementation dependent - some stacks write it as soon as they camp, others only once registration completes. So v) lands on the stored information branch on some devices and on initial cell selection on others. This is precisely the kind of case where the only honest answer is to go and read the log, which is what you recommend next.

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

Read that list again and notice how short it is. There is nothing about PLMN, nothing about tracking areas, nothing about whether you have a subscription. That is the entire point : an acceptable cell is the lowest bar the standard is willing to define, because the thing it protects is the ability to make an emergency call. A phone with no SIM, a phone roaming where it has no agreement, a phone whose subscription has been cut off - all of them can still find an acceptable cell, and that is deliberate.

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

NOTE : the text above is quoted from 36.304 as you say, and 38.304 carries the same definition across to NR essentially word for word. This is one of the places where "NR is very similar to LTE" is not an approximation - the criteria really are the same, which is why the LTE page and this page can share a description.

Putting the two definitions side by side is the quickest way to see what is actually being asked :

Condition	Acceptable	Suitable	Barred
Cell is not barred	required	required	fails here
Cell selection criteria (S-criteria) met	required	required	-
Cell belongs to the selected / registered / equivalent PLMN	not required	required	-
TA is not in the forbidden roaming list	not required	required	-
Service you get	emergency only (limited service)	everything (normal service)	none

So every suitable cell is also an acceptable cell, but not the other way round. The two extra conditions that a suitable cell must satisfy are both about permission rather than about radio quality - is this network mine, and am I allowed to be in this area. The radio conditions are identical in both columns.

The three cell selection procedures

The flow chart at the top has three named procedures in it, and their names are unfortunately similar enough to blur together. They are genuinely different things :

Procedure	When it runs	What the UE actually does	Speed
Initial Cell Selection	No stored information for the PLMN being selected	Scans all RF channels in every band it supports and, on each carrier, looks for the strongest cell	slowest
Stored Information Cell Selection	Stored carrier / cell information is available for this PLMN	Goes straight to the frequencies it already knows, and only falls back to a scan if none of them work	fast
Any Cell Selection	Neither of the above found a suitable cell	Looks for any acceptable cell, of any PLMN, and camps on it for emergency service	fallback

The distinction people most often get wrong is the first two. They are not "quick scan" and "full scan" of the same procedure - they are chosen by a single yes/no question at the top of the chart, and the UE only ever runs the slow one if the fast one has nothing to work with or has already failed.

Camped states and what service you actually get

The flow chart ends in a state, not in an event, and there are three of them. Being able to name which one a device is in is most of what you need when reading a field log :

State	Reached from	What the UE can and cannot do
Camped normally	A suitable cell was found	Everything : registration, paging, calls, data. Monitors paging and system information, and runs cell reselection normally
Camped on any cell	Any Cell Selection found an acceptable cell	Emergency calls and emergency alerts (ETWS / CMAS) only. Still monitors paging and SI, and keeps looking for a suitable cell in the background
Any cell selection state	Not even an acceptable cell yet	No service at all. The UE is still searching. This is what "No Service" on the screen actually means

NOTE : "camped on any cell" is the state that surprises people in test, because the phone looks alive - it has bars, it shows a network - but nothing except an emergency call will work. If you are debugging a "phone has signal but no data" report, the first thing worth checking is whether the UE is camped normally or camped on any cell.

The NAS reject causes, and the word "except"

These four cause values appear three times in the case walkthroughs below, so they are worth pinning down - and the flow chart says something slightly different from what you might expect. Look closely at the label on the arrow running from Camped normally to Any Cell Selection in 38.304 Figure 5.2.2-1 :

     NAS indicates that registration on selected PLMN is rejected
                ( except with cause #12, #14, #15 or #25 )
                                    |
                                    v
                          Any Cell Selection

The word is except. These four causes are not the ones that push the UE out to Any Cell Selection - they are the four that are excluded from doing so. Any other reject cause is what sends the UE down that arrow into limited service.

Once you read it that way the grouping makes sense. All four of these say "not here" rather than "not you" :

Cause	Meaning	Why it is an exception
#12	Tracking area not allowed	The PLMN is still fine, only this tracking area is not. The UE adds the TAI to its forbidden list and looks for a cell in a different TA of the same network
#14	EPS services not allowed in this PLMN	Scoped to this one PLMN, so the sensible response is to go back to PLMN selection rather than give up on normal service altogether
#15	No suitable cells in tracking area	Again the tracking area is the problem, not the network. The UE searches other tracking areas of the same PLMN
#25	Not authorized for this CSG	A closed subscriber group cell the UE is not a member of. One specific cell is closed to it, nothing wider

So if the rejection is local - one tracking area, one CSG cell, one PLMN - the UE keeps hunting for a cell it can use normally. Only a rejection saying something more fundamental drops it to Any Cell Selection and emergency-only service. The exception list exists precisely so that a UE does not fall all the way to limited service over a problem it could fix by moving one cell to the left.

NOTE : these four numbers come from the EMM cause list in 24.301, which is what the 36.304 flow this page is built on refers to. In 5GS the equivalent list is the 5GMM causes in 24.501, and the numbering is not a straight copy - #12 and #15 carry over with the same meanings, while #14 and #25 are specific to EPS and to CSG respectively and have no direct NR counterpart. If you are reading an NR log rather than an LTE one, check the value against 24.501 before assuming it means the same thing.

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.

This is case i) from the list above - and it is the only one of the nine that reaches this path with genuinely nothing stored. Cases iv) and vii) can also end up here, but they arrive by a different route : they try the stored information branch first and only fall through to Initial Cell Selection when it fails.

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.

This is by far the most common path in real life - cases ii), iii), vi) and viii) from the list above all take it, and so does vii) when the new USIM happens to belong to the same PLMN.

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.

None of the nine cases above start here, and that is the point worth ending on. Cell selection is what a UE does when it has no serving cell ; cell reselection is what it does when it already has one and is deciding whether to move. Every one of the nine cases is a power-on or a mode change, so they are all selection. Reselection is what happens afterwards, for as long as the UE stays idle - and it is the process that quietly keeps the phone on a sensible cell while it sits in your pocket.