FR2856879A1 - Public land mobile network selection acceleration method for use by mobile terminal, involves searching suitable cell in priority order, from system information specific to network that is previously used by mobile terminal - Google Patents

Abstract

The method involves storing, in a mobile terminal, system information specific to a public land mobile network (PLMN) that is previously used by the terminal. The system information contains parameters used for selection/reselection of cells, characteristics of common channels used in the cells, and an identity of location area of the terminal. A suitable cell is searched from the system information in an order of priority. An independent claim is also included for a multimode mobile communication terminal.

Description

METHOD FOR ACCELERATING THE SEARCH OF A NETWORK OF

TELECOMMUNICATION THROUGH A MOBILE TERMINAL

DESCRIPTION

TECHNICAL AREA
The invention relates to the telecommunications field and more specifically relates to a method of accelerating the search for a network by a mobile terminal.
The invention also relates to a multimode mobile communication terminal comprising means for implementing the method.

STATE OF THE PRIOR ART
When a user of a mobile terminal travels in geographic areas covered by separate telecommunications networks (PLMN, for Public Land Mobile Network), each time coverage is lost on the current network, the mobile terminal must search for a new network offering the best communication conditions.
This new network can be chosen according to automatic or manual mode. In automatic mode, a search order is given to the terminal. This order can be given periodically or can be generated punctually each time the terminal loses network coverage. The terminal then searches for networks present in the environment. Then, via the SIM card (for Subscriber Identity Module), the operator gives priority of selection to one of the networks among the networks found.
In manual mode, the user selects a network from a list of networks available in the environment.
When a network is selected, the terminal tries to register. If this fails, the terminal attempts to register on another network. If successful, the terminal can obtain services from this network.
After registration, the terminal can obtain information on its radio environment from the cell on which it is camped. This information is conveyed by the control channel BCCH (for Broadcast Control Channel) and notably includes the frequency band used in the PLMN or in the PLMN region (BA_Range), a list of neighboring cell frequencies (ARFCN, for Absolute Radio Frequency Number), the list of beacon frequencies for neighboring cells BA list for the GSM system (GSM for Global System For mobile, and the list of frequencies carrying scrambling codes for neighboring 3G cells 3G_Cell_Reselection-list for the UMTS system.
In the remainder of this description, H-PLMN (for Home PLMN) will denote the current public network (GSM or UMTS for example), V-PLMN (for Visited PLMN) the network in which the terminal is re-switched on after an extinction. , R-PLMN (for Registred PLMN) the network in which the mobile terminal is registered and RAT (for Radio Access Technology) the radio access technology implemented in an H-PLMN or a V-PLMN.
The 3GPP (Third Generation Partnership Group) standard defines the cell selection procedure for a GSM network by the following steps:
When the mobile is switched on, the physical layer L1, AS (for Access Stratum) performs the following tasks:
A- If the R-PLMN (for Registered PLMN) is such that RAT = GSM, a search for a suitable cell with PLMN = R-PLMN is carried out among: - the frequency of the last cell and / or, - set of frequencies of the last BA_LIST and / or, - the range of frequencies of the last BA_Range and / or, - the set of frequencies supported by the mobile.
B- The physical layer informs layer 3 (Non AS) of the search result, that is, that a suitable cell is found and that its PLMN = R-PLMN or that no cell of the R-PLMN is not found.
C- If the R-PLMN is such that RAT = UTRA, a search for a suitable cell with PLMN = R-PLMN is carried out among: - the frequency and scrambling code of the last cell and / or, - l 'set of frequencies of the last FDD_Frequency_list and / or, - the frequency range of the last FDD_frequency_range and / or, - all of the frequencies supported by the mobile.
D- The physical layer informs layer 3 (Non AS) of the search result, that is to say that a suitable cell is found and its PLMN = R-PLMN, or that no cell of the R-PLMN is not found.

E- After receiving the message by L3:
- In the event of failure to search for R-PLMN, L3 requests a search for PLMN by giving priority to RAT (RAT = GSM or RAT = UTRA). - If successful, the registration procedure on the R-PLMN is activated.
F- If Ll receives a PLMN search request, Ll performs a full search for all suitable cells and their PLMN. L3 then starts the registration on the highest priority PLMN found.
G- If no correct PLMN is found, the mobile is camping on any PLMN in limited service (emergency calls only).
When the terminal switches off:
If RAT = GSM, depending on what is used when switching on, the mobile memorizes: - the frequency of the last cell and / or, - the set of frequencies of the last BA_LIST and / or, the frequency range of the last BA_Range and / or, of all the frequencies supported by the mobile.
If RAT = UTRA, depending on what is used at ignition, the mobile can memorize: - the frequency of the last cell and / or, - all the frequencies of the last FDD Frequency list and / or, - the range of frequencies of the last FDD frequency range and / or, - of all the frequencies supported by the mobile.
The duration of the GSM search is typically 30s for all the frequencies supported by a mobile. A problem stems from the fact that a failure to search for R-PLMN means that the duration of this procedure can be greater than 35 seconds. in real communication conditions. Furthermore, the introduction of the GSM / WCMA Dual-Mode system (WCMA, for Wideband Code Division Multiple Access) will increase this duration to too high values.
The object of the invention is to reduce the duration of the search phase for a new network when the device is switched on when the terminal is located in a geographical area covered by a telecommunications network different from that of the area where it was off.

STATEMENT OF THE INVENTION
The invention recommends a method of accelerating the selection by a mobile terminal of a telecommunications network in the event of inter-system roaming. In particular when the terminal is switched on in a different geographical area and for which the last PLMN has no coverage.
The method according to the invention comprises the following steps: - memorizing in the terminal the system information specific to the telecommunication networks previously used by the terminal, and each time the network coverage is lost, - priority search for a suitable cell from the system information previously memorized.
In a preferred embodiment, said information includes in particular: - the parameters used for cell selection / reselection, - the characteristics of the common channels used in the current cell, - the information for controlling the measurements to be carried out by the mobile terminal, - the identity of the location areas to which the cell belongs.
Preferably, this information is grouped in a database which is updated each time the terminal is switched off.
The invention is implemented in a multimode communication terminal comprising:
means for memorizing the system information specific to the current network, and - means for prioritizing the search for a suitable cell from the system information memorized each time the terminal is switched off.

BRIEF DESCRIPTION OF THE DRAWINGS
Other characteristics and advantages of the invention will emerge from the description which follows, taken by way of nonlimiting example, with reference to the appended figures in which: - Figure 1 is a general block diagram illustrating the main steps of the method according to the invention. - Figure 2 is a flowchart illustrating an embodiment of the invention.

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS
The invention will now be described, by way of example, in a particular implementation in which a user of a dual mode GSM / UMTS mobile terminal moves from an area covered by the GSM network (respectively UMTS) to an area covered by the UMTS network (respectively GSM).
Remember that the PLMN selection phase involves the terminal finding a network from which it can obtain service. Two types of measurements are carried out at the level of the procedures of the RR layer (for Radio Resource control). Either the terminal has no stored information concerning its environment and then it must scan all the frequencies it supports to select the best cells, or the terminal has a priori knowledge of its radio environment and then it can test first this environment.
The invention therefore recommends a method of accelerating the selection by a mobile terminal of a network according to the procedure described schematically in FIG. 1.
Step 1 of this procedure consists in building a database comprising system information from the different networks in which the terminal is likely to be used.
Step 2 consists of constantly updating this database according to the movements of the user of the terminal.
Step 3 consists in using the database created to reduce the duration of the cell search during a network change.
1) Constitution of a database Step 1 of FIG. 1 consists in storing in a flash type persistent memory (Flash RAM in English) associated with the terminal, records relating to the PLMN in which the user normally receives a service. via the terminal and in which the terminal is not in a limited state of service. Each time the terminal is switched off, these records are updated.
For a dual mode GSM / UMTS mobile, the records are stored in the form of the table below.

Table 1

At each instant, there is only one record corresponding to a RAT, an MNC and a country code MCC for (Mobile country Code). For example, there will only be one record for:
GSM / France / orange or for UMTS / UK / Vodafone.
2) Updating a database Updating the database consists of adding information relating to the current PLMN. Thus, if the terminal is camped on GSM / France / Orange and this record did not exist in the table, then this field is created. If the field already exists, only the frequency information is updated using the latest available information (current cell, list of neighboring frequencies broadcast by the network or frequency range to be used).
If the maximum number of records is reached when a new record is inserted, the oldest record in the table is replaced by the new record.
3) Operation of the database.
FIG. 2 illustrates an exploitation of the database in a preferred embodiment of the invention.
In operation, the normal cell search procedure is modified when the terminal is switched on (step 4).
In step 6, the physical layer (L1, AS-Access Stratum) performs the following tasks:
a- If the R-PLMN (Registered PLMN) is such that RAT = GSM, search for a suitable cell with PLMN = R-PLMN among: - the frequency of the last cell and / or, the set of frequencies of the last BA_LIST and / or, the frequency range of the last BA_Range.
If the search fails, then the database records are used (step 8).
In step 10, a test consisting in checking whether an MCC has been read during the search for R-PLMN is executed.
If so, the search is restricted to records displaying the corresponding MCC, excluding networks from other countries (step 12).
Otherwise, step 8 is executed and the restriction is implemented on the first reading of an MCC. Thus, the record corresponding to the RPLMN which has already been searched is excluded from the search.
For each record of the same RAT as the R-PLMN and in the order of the most recent to the oldest records, the frequencies indicated in the table are sought.
This procedure is performed with the records of the other RAT.
If a suitable cell is found from one of the records (step 14), the normal search procedure ends and L3 is informed (step 16) of the presence of the PLMN.
Layer L3 then controls the registration of the terminal on this PLMN if it belongs to the list registered in the SIM card. Otherwise, L3 starts a full PLMN search. This is the case, for example, if the SIM card has been changed.
If none of the records is suitable, then the physical layer informs layer 3 (Non AS) of the search result and the search procedure is performed on all the frequencies supported by the mobile.
If the R-PLMN (Registered PLMN) is such that RAT = UTRA, the search for a suitable cell with the R-PLMN is carried out among: - the frequency of the last cell and / or, - the set of frequencies of the last FDD Frequency_list and / or, - the frequency range of the last FDD_frequency range and / or, - of all the frequencies supported by the mobile.
The physical layer informs layer 3 (Non AS) that a suitable cell is found and its PLMN = R-PLMN, or that no cell of the R-PLMN is found.
Thus, for users who do not travel, no change is observed since only the BCCH information of the R-PLMN will be filled. On the other hand, for users who travel frequently from one country to another, the information in the database makes it possible to speed up the selection time.
Thanks to the invention, the PLMN selections are accelerated as the database is built and enriched.
In another embodiment of the invention, it is possible to avoid the learning phase by personalizing the terminal with an existing BCCH information base.
In a variant, a choice can be made between storing only BA_List or only BA_range.
With only BA_list, the optimal region is small, but the selection time is very fast.
With BA_range, the selection region may be very large, but the selection time may be long.
In another alternative embodiment, the two previous approaches are combined so as to check first the ARFCNs of the BA_list, then in case of failure to check the BA_Range.
When traveling within the same country, instead of replacing the current PLMN information with the news at the time of extinction, frequencies are added to the existing ones.
Thus, if a PLMN covers several zones, the BCCH information is valid for all these zones.

Claims (3)

2. Method according to claim 1, characterized in that said information comprises in particular: - the parameters used for cell selection / reselection, - the characteristics of the common channels used in the cell, - control information for the measurements to be made by the mobile terminal, - the identity of the location areas to which the cell belongs. 3. Method according to claim 2, characterized in that the stored system information is grouped in a database which is updated each time the terminal is switched off. 4. Multimode communication terminal, characterized in that it comprises: means for storing in the terminal system information specific to the current network and - Means for firstly searching for a suitable cell from the stored system information.