6.3 UE in the Idle Mode


6.3.1  Overview


After being switched on or when roaming, the UE needs to find the network and establish contact with the network in the first place. It can get the network services only in this way. Therefore, activities of the UE in the idle mode are essential to the UE. This section describes the procedure for the UE to find the network and establish contact with the network.
In the UE idle mode, this process falls into PLMN selection and reselection, cell selection and reselection, and location registration.

After being switched on, the UE shall first select a PLMN, and then select a cell belonging to this PLMN. When finding such a cell, the UE can learn the information of the neighboring cells via the system message (broadcast). Thus, it can select the cell with the best signal quality from all these cells to camp on. Then the UE initiates the location registration procedure (attach or location update). After that, the UE camps on the cell. The residence of the UE has four functions as follows:
l   The UE can receive the system messages broadcast by the PLMN.
l   The UE can initiate the random access procedure in the cell.
l   The UE can receive the paging from the network.
l   The UE can receive the cell broadcast service.
After the UE camps on the cell and its registration succeeds, the signal strengths of the current cell and the neighboring cells keep changing as the UE moves. Therefore, the UE needs to select the most suitable cell. This is the cell reselection procedure. The most suitable cell may not be the cell with the best signal quality currently. For example, the UE is at the border of a cell, and moves between two cells that belong to different LAs or RAs. In this case, the UE keeps initiating the location update, thus wasting the network resources and the UE’s energy. Therefore, the reselection of a cell from all the cells shall follow a certain rule that is detailed later.
When finding out that the cell selected during the cell reselection belongs to another LA or RA, the UE shall initiate the location update procedure so that the network can get the latest location information of the UE. It detects the change of the LA or RA via the SIB1 in the system broadcast message.
If the location registration or update fails, for example, the network rejects the UE, or the UE is outside the coverage of the current PLMN, the UE can conduct the PLMN reselection to select another available PLMN.

6.3.2  PLMN Selection and Reselection


The purpose of PLMN selection and reselection is to select the best PLMN available (that is, the PLMN that can provide normal services). To achieve this purpose, the UE will maintain a PLMN list that is sorted by priority, and search the PLMNs from high priority to low priority, thus finding the PLMN with the highest priority. In addition, there are two modes of PLMN selection and reselection: Automatic and manual. Simply, the automatic PLMN selection is the process in which the UE automatically selects a PLMN by the PLMN priority, while the manual PLMN selection is the process in which all the currently available PLMNs are presented to the subscriber for the subscriber to select one of them.

6.3.3  Cell Selection and Reselection


The UE needs to select the cell after selecting the PLMN. It should select the cell with the best signal quality that belongs to the PLMN.
First, if some information related to the PLMN, such as frequency and scramble, is stored in the UE, the UE will use the information to conduct cell search (stored information cell selection). In this way the UE can find the network quickly. In most cases, the UE is switched off and on again in the same location, for example, UE switch-off at night and switch-on in the morning. Therefore, such information is stored in the SIM card or the non-volatile memory of the UE.

1. Cell selection

The cell selection procedure is as follows:
1)      Search for a cell
The cell search aims to find a cell, although the cell may not belong to the selected PLMN. The cell search procedure is as follows (it is certain that a frequency should be locked in the first place):
The UE synchronizes the slot via the primary SCH. After that, it shall synchronize the frame. The frame synchronization is implemented by using the synchronization code of the secondary SCH. This procedure also determines the scramble group of the cell. Then the UE finds the scramble with the maximum result value by associating each scramble in the scramble group with the CPICH. Thus the primary scramble is determined.
It is obvious that if the UE has already known some information of the cell, such as the frequency or even the primary scramble, the above procedure can be greatly speeded up.
2)      Read broadcast channel
The UE obtains the scramble of PCCPCH in Step 1) above. The PCCPCH channel code is already known and is unique in the entire UTRAN. The UE now can read the information in the broadcast channel.
l   After reading the MIB, the UE can decide whether the PLMN currently found is the desired PLMN, since the MIB has the PLMN identity field. If yes, the UE will find the other SIBs and get their contents based on the scheduling information of the other SIBs in the MIB. If not, the UE has to search for the next frequency and start the procedure over again (starting from the cell search).
l   If the current PLMN is the PLMN to be found by the UE, then the UE reads SIB3 and gets “Cell selection and re-selection info”. It calculates whether the cell residence standard is satisfied with the acquired information. If the standard is satisfied, then the UE considers the cell as a suitable cell. It then camps on the cell, reads other necessary system information, and then initiates the location registration procedure.
If the above conditions are not satisfied, the UE then reads SIB11 and obtains the neighboring cell information. Then it can calculate and decide whether the neighboring cell satisfies the cell selection and residence standards.
When finding any neighboring cell satisfying the cell residence standard, the UE will camp on the cell, read the other necessary system information, and then initiate the location registration procedure.
If it does not find a cell satisfying the cell residence standard, it will consider that there is no PLMN coverage, and proceed with the PLMN selection and reselection procedure.

2. Cell reselection

In the idle mode, the UE needs to monitor the signal quality of the current cell and the neighboring cell so as to select the best cell to provide the service. This is the cell reselection procedure. If the cell reselection condition is satisfied within the Treselection time, the UE will select the cell, camp on the cell, and read its broadcast message. By now, the cell reselection ends.

3. Cell selection after the UE leaves the connected mode

When transiting from the connected mode to the idle mode, the UE needs to conduct the cell selection to find a suitable cell. This selection procedure is the same as the common cell selection procedure. The only difference is that the cell selected at this time is cell used in the connected mode. If a suitable cell cannot be found from these cells, the stored information cell selection mode should be adopted.

6.3.4  Location Registration


For these procedures, see MM and GMM procedures. 

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