There are two main factors influencing the cell selection process. One factor is Cell Power and the other one is various PLMN configuration. I will focus on PLMN based selection here but in many cases you have to consider both Cell Power and PLMN at the same time and the selection process can be more complicated in this case.

PLMN selection is a NAS procedure, and 23.122 is the specification that owns it. The access stratum only scans the carriers and reports the PLMNs it finds, together with a signal quality flag that 25.304 defines for UTRA. The NAS then picks a PLMN from that report, and cell selection finds a suitable cell of the chosen PLMN. Let's compare the two selection modes first, then go through the order that Automatic mode follows, and then look at what the UE does once it is registered.

• What is the difference between Manual and Automatic mode ?
• In which order does Automatic mode try the PLMNs ?
• What does the UE do once it is registered ?
• Reference

What is the difference between Manual and Automatic mode ?

Usually many phones provide two options for you to select from for selecting a cell to camp on. One is Automatic and the other one is Manual. There is nothing for me to talk about 'Manual' mode since it is so straightforward. In manual mode, the UE camps on whatever network that user specified. If the UE cannot find the specified cell or rejected by the cell, it would just give up without trying anything else.

The issues come in when the mode is set to be 'Automatic'. 'Automatic' means "Select a cell based on a predefined rule". Then your question would be "What is the predefined rule?". Even though 3GPP provide overal guidelines, the detailed implementation would be up UE makers and the requirement from each specific network operator.

Manual mode still has a few rules of its own in 23.122, and they explain what you see on a phone. The UE first shows the user every available PLMN, including the ones on the forbidden PLMN lists. The list follows the same priority order as Automatic mode: the HPLMN or EHPLMN, then the user controlled list, then the operator controlled list, then the other PLMNs. The user picks one, and the UE registers on it.

"Give up" in the paragraph above means that the UE does not register on another PLMN by itself. It does not mean that the UE has no radio at all. If the user does not select a PLMN, the UE keeps the PLMN selected before the search. If that PLMN is gone too, the UE camps on any acceptable cell and enters the limited service state, where it can still make an emergency call.

23.122 also lists the few cases in which a UE in Manual mode registers on a different PLMN without the user. The first is a PLMN that the registered PLMN declares as an equivalent PLMN. The second is the user switching to Automatic mode. Further cases cover emergency calls that the current PLMN cannot complete, where the UE may leave Manual mode for the call.

• Manual mode fixes the PLMN, not the cell : The user picks a PLMN, and the normal cell selection of 25.304 still picks the cell inside it.
• A forbidden PLMN can still appear in the manual list : The UE shows it, and the user can try it. Automatic mode does not try a forbidden PLMN in steps i to v.
• Limited service is not the same as no service : A UE that cannot register on the chosen PLMN still camps on an acceptable cell for emergency calls.

In which order does Automatic mode try the PLMNs ?

Automatic mode has to answer one question with no help from the user: which PLMN comes first when several are on the air? The answer comes from the USIM lists and from the signal quality that the access stratum reports. The rule has a short form and a long form.

General guideline especially guidelines regarding "PLMN" is as follows :

i) Select the cell with HPLMN (Home PLMN) or EHPLMN(Equivalent HPLMN)

ii) If UE lose coverage for HPLMN or EHPLMN, then try find the cell with VPLMN.

More specifically, in Automatic mode, UE select the Cells with PLMNs in the following order

• i) either the HPLMN (if the EHPLMN list is not present or is empty) or the highest priority EHPLMN that is available (if the EHPLMN list is present);
• ii) each PLMN/access technology combination in the ‘User Controlled PLMN Selector with Access Technology’ data file in the SIM (in priority order);
• iii) each PLMN/access technology combination in the ‘Operator Controlled PLMN Selector with Access Technology’ data file in the SIM (in priority order);
• iv) other PLMN/access technology combinations with received high quality signal (GSM – RLA ≥ -85 dBm, UTRAN FDD - CPICH RSCP above -95 dBm) in random order;
• v) other PLMN/access technology combinations in order of decreasing signal quality.

The list above is the classic form of 23.122 clause 4.4.3.1.1. Steps i to iii come from the USIM, so the home operator decides them. Steps iv and v come from the radio, so the UE decides them from its measurements. In steps ii and iii, the UE should search only the access technologies that the list entry names. In steps iv and v, the UE searches all access technologies it supports before it decides.

Step iv needs a definition of a high quality signal. The current 23.122 no longer gives the numbers itself. Instead, it points to the access stratum specification of each RAT, and the table below collects those thresholds.

The UTRA scan works on the strongest cell of each carrier. The UE reads the PLMN identities of that cell. If the cell meets the criterion, the UE reports each PLMN to the NAS as a high quality PLMN without the RSCP value. Otherwise, it reports the PLMN together with its CPICH RSCP, and the NAS uses that value to order step v.

The current 23.122 v20.0.0 also adds to the classic list. In step iii, the operator controlled list can now be stored in the ME, for example after steering of roaming. Steps iv and v skip the PLMNs in the "Operator controlled lower selection-priority PLMN selector with access technology" list. A new step v-0 tries those PLMNs after step v. Further steps after v-0 cover disaster roaming for E-UTRAN and NG-RAN. When the lower selection-priority list is empty and no disaster condition applies, the order is the same as the classic list above.

• The USIM decides the first three steps : EHPLMN, the user controlled list and the operator controlled list are all files on the USIM. So the operator can change the order without touching the UE.
• Random order in step iv is on purpose : Among the high quality PLMNs outside the USIM lists, no PLMN is preferred, so the UE picks one at random.
• The thresholds are for PLMN selection only : -95 dBm CPICH RSCP only sorts the PLMNs. Whether a cell is suitable to camp on is still decided by the S criterion of 25.304.

What does the UE do once it is registered ?

Selection does not end with the first registration. A roaming UE in Automatic mode keeps looking for a better PLMN, and a UE that loses coverage has to decide where to return. The two modes behave differently here, and this is where most field complaints about roaming come from.

At switch on, or after a loss of coverage, the UE first tries the registered PLMN or an equivalent PLMN. The UE runs the mode procedure of the previous section only when that fails. There is one exception in Automatic mode. If the UE finds an EHPLMN, it may register there and not return to the registered PLMN. The operator controls this option through the SIM.

In Automatic mode, a UE on a VPLMN also searches for a higher priority PLMN periodically. The period is timer T. The range below applies to a UE that does not use EC-GSM-IoT, Category M1 or Category NB1. T runs from 6 minutes to 8 hours in 6 minute steps, or it can say that no periodic search is made. The value comes from the EF HPPLMN file on the USIM, and the default is 60 minutes when the file holds no value. The UE makes these periodic attempts only in Automatic mode and only when it is roaming.

The table below puts the two modes side by side for the situations above.

• The registered PLMN comes first after power on : Neither mode starts from the HPLMN. Both start from the registered PLMN or an equivalent PLMN.
• A roaming UE returns home only through timer T : In Automatic mode, the periodic search is the mechanism that brings the UE back to a higher priority PLMN.
• Manual mode has no periodic search : A user who picked a VPLMN by hand stays on it until the user changes the mode or the PLMN.

Reference

• 3GPP TS 23.122 v20.0.0 : Non-Access-Stratum (NAS) functions related to Mobile Station (MS) in idle mode
• 3GPP TS 25.304 v19.1.0 : User Equipment (UE) procedures in idle mode and procedures for cell reselection in connected mode
• 3GPP TS 36.304 v19.2.0 : Evolved Universal Terrestrial Radio Access (E-UTRA); User Equipment (UE) procedures in idle mode
• 3GPP TS 43.022 v19.1.0 : Functions related to Mobile Station (MS) in idle mode and group receive mode
• 3GPP TS 31.102 v19.5.0 : Characteristics of the Universal Subscriber Identity Module (USIM) application