PLMN Selection - Automatic vs Manual
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.
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.
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.