Note : See "Idle Mode Procedure" section first for the big picture.

Cell Selection

When you power on the mobile device, in most case the device is under a circumstance where it sees many base station (eNode B) around it. In some cases UE would be surrounded not by the multiple basestation from one system operator but by the multiple basestation from multiple system 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 base station (cell) to register, this specific decision making process is called 'Cell Selection'.

The Flow Chart in 36.304

Overall description of cell selection process is described in 36.304 Figure 5.2.2-1: RRC_IDLE Cell Selection and Reselection as follows. Of course the minimum condition is to meet Cell Selection Criterion .

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. To be honest, I don't know exact answer to the cases that I would give you here.

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.

The figure is reproduced below in two parts, because it does not fit on one screen. The two pictures are halves of one diagram rather than two diagrams, and the numbered circles are where they join. Circle 1 and circle 2 each appear in both halves.

36.304 Figure 5.2.2-1, upper half: the normal service part of the RRC_IDLE cell selection and reselection flow

The entry point sits at the top, labelled go here whenever a new PLMN is selected, and it leads into circle 1.
Circle 1 is the branch the whole page turns on. Its left arrow reads cell information stored for the PLMN and reaches Stored information Cell Selection. Its right arrow reads no cell information stored for the PLMN and reaches Initial Cell Selection.
Either box reaches Camped normally along an arrow labelled suitable cell found, and circle 2 is where those arrows meet.
Cell Reselection Evaluation Process sits below Camped normally. A trigger arrow runs down into it and a suitable cell found arrow runs back up, so reselection is drawn as a loop rather than as a step.
Cell Selection when leaving connected mode sits on the left, between Connected mode and Camped normally. Its arrows read return to idle mode and leave idle mode.
Any Cell Selection sits at the bottom right and collects three arrows. Two are labelled no suitable cell found and the third is the NAS rejection path, whose note names causes #12, #14, #15 and #25 as the exceptions.

36.304 Figure 5.2.2-1, lower half: the limited service part, entered when the UE has no USIM

This half is entered at the top right, where the label reads go here when no USIM in the UE.
An arrow labelled USIM inserted leaves Any Cell Selection into circle 1, and that is the same circle 1 as in the half above. Inserting a USIM is how a UE leaves the limited service half.
Camped on any cell is the counterpart of Camped normally. Its incoming arrows read acceptable cell found rather than suitable cell found, and that single word is the difference between the two halves.
Camped on any cell leaves through circle 2 along an arrow labelled suitable cell found, so a UE that finds a usable cell rejoins the upper half.
The connected mode box in this half is labelled Connected mode (Emergency calls only), which is the service level the whole half is limited to.
The same reselection loop appears here, and its escape arrow reads no acceptable cell found and returns to Any Cell Selection.

Keywords in the Flow Chart

Followings are the keywords that you have to know in order to understand the follow chart shown above. The two below carry most of the weight. Nearly every arrow in the figure is labelled with one of them. The difference between the two halves of the figure is the difference between these two words.

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. Refer to 23.122 for details), or:
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

Marked Paths for Three Cases

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

The red line starts at circle 1 and takes the right branch, the one labelled no cell information stored for the PLMN. It reaches Initial Cell Selection and then follows suitable cell found down to Camped normally, where a violet IDLE box is laid over the state. From there it leaves through leave idle mode to Connected mode, and the orange marker on that arrow reads Make Call.

The same 36.304 figure with a red line marking the Initial Cell Selection path

Case 2 : Stored Cell Selection

This red line differs from the one above at a single branch. At circle 1 it takes the left arrow, cell information stored for the PLMN, and reaches Stored information Cell Selection instead. Everything after that is identical: suitable cell found to Camped normally, then Make Call and leave idle mode to Connected mode.

The same figure with a red line marking the Stored Cell Selection path

Case 3 : Cell Reselection

This path reaches Camped normally the same way the one above does, and then stays in idle mode. Instead of leaving upward it goes down the trigger arrow into Cell Reselection Evaluation Process, which is outlined in red, and returns along suitable cell found. The orange marker on the trigger arrow reads Cell Power Change.

The same figure with a red line marking the Cell Reselection loop out of Camped normally

Reading the three together is what makes them worth the space. They differ in exactly two places. One is which branch leaves circle 1, and the other is whether the path leaves Camped normally upward into connected mode or downward into reselection.

The three marked paths differ at two branches only : which arrow leaves circle 1, and which direction the path leaves Camped normally.
Reselection is a loop and not a step : the trigger arrow goes down and the suitable cell found arrow comes back, so the UE returns to the state it started in.

What the Current Release Adds

The two definitions above are quoted accurately and they are shorter than the clause they come from. 36.304 v19.2.0 clause 4.3 carries a few more conditions, and two of them change which cells a UE may camp on. They are worth having beside the originals.

The clause opens by naming three levels of service rather than two. Limited service covers emergency calls together with ETWS and CMAS notifications, on an acceptable cell. Normal service is for public use on a suitable cell. Operator service is for operators only, on a reserved cell.

The table below sets the quoted conditions against the current ones.

	As quoted above	36.304 v19.2.0 clause 4.3
Acceptable cell, purpose	Enough to make an emergency call	Emergency calls, and receiving ETWS and CMAS notifications
Acceptable cell, scope	Not stated	Not applicable to the RRC_INACTIVE state
Suitable cell, USIM	Not stated	The UE shall have a valid USIM
Suitable cell, CSG	Not stated	For a CSG cell, the cell is a CSG member cell for the UE
Where barring is defined	Not stated	Clause 5.3.1
Where the criteria are defined	Not stated	Clause 5.2.3.2

The valid USIM line is the one that matters most. It explains the half of the figure that the definitions above do not mention. A UE with no USIM cannot reach a suitable cell by definition. The lower half of the figure is where such a UE belongs, and that half is entered at the label reading go here when no USIM in the UE.

The two clause pointers are worth following as well. Barring is decided in clause 5.3.1 and the cell selection criteria in clause 5.2.3.2. The two conditions the definitions state in one line each are themselves whole clauses.

The USIM requirement is what splits the figure in two : a suitable cell needs a valid USIM, so a UE without one is confined to the limited service half.
Both definitions end in conditions that are clauses of their own : cell barring is clause 5.3.1 and the cell selection criteria are clause 5.2.3.2.

Reference

One specification carries the figure and both definitions used on this page. The clause numbers sit beside each claim so the wording can be checked rather than taken on trust.

36.304 - Evolved Universal Terrestrial Radio Access (E-UTRA); User Equipment (UE) procedures in idle mode, v19.2.0. Figure 5.2.2-1 is the RRC_IDLE cell selection and reselection flow reproduced above in two halves. Clause 4.3 is Service types in Idle Mode, which defines the acceptable cell and the suitable cell. Clause 5.2.3 is the cell selection process and clause 5.2.3.2 is the criterion the definitions point at. Clause 4.3 and clause 5.2 were read.
23.122 - the page above points at it for the meaning of the registered PLMN. It was not read for this note, so nothing here rests on it.