WO2010130759A1

WO2010130759A1 - Mobile device roaming

Info

Publication number: WO2010130759A1
Application number: PCT/EP2010/056505
Authority: WO
Inventors: Dave Edwards
Original assignee: Evolved Intelligence Limited
Priority date: 2009-05-12
Filing date: 2010-05-11
Publication date: 2010-11-18
Also published as: GB0908126D0; GB2470194A; GB2470194B

Abstract

A number of methods are provided that relate to the roaming of mobile devices (2) from a home network (6). In particular, a method of operating a mobile device is provided in which the mobile device is attached to a network (16) other than the home network of the mobile device (2), the method comprising determining (68) whether the mobile device (2) is in a part of said other network (16) in which there may be coverage from the home network; and if it is determined that the mobile device (2) is in a part of said other network (16) in which there may be coverage from the home network (6), initiating a scan for a signal from the home network (6).

Description

MOBILE DEVICE ROAMING

Technical Field of the Invention

The invention relates to the operation of a mobile device near the edge of the coverage offered by their home network, and in particular to methods and devices for minimising the roaming of the mobile device in these areas.

Background to the Invention

Radio waves do not respect country borders. As a result, it is entirely possible for a mobile subscriber who lives near a border between two countries to be using their mobile device at home (in their home country), but actually be connected to a foreign network in the neighbouring country as if they were travelling abroad (i.e. they are considered to be roaming).

The mobile device may register with a foreign network even though coverage from the home network is actually still available. Once the mobile device has registered with the foreign network, it can sometimes take a long time for the mobile device to re-register with the home network. This results in unreasonable roaming costs for the mobile subscriber and operator, particularly if the subscriber has not in fact left their home country.

For a legitimate outbound mobile device (i.e. one that is crossing the border and that will need to roam to a foreign network in order to maintain service), this can mean that roaming costs are increased, as the mobile device may register with the foreign network earlier than necessary.

A similar problem occurs for an inbound roaming mobile device, in that the mobile device can stay connected to the foreign network for some time after they have returned to their home country (and the coverage of their home network). Again, this results in unnecessary roaming charges for both the subscriber and the home network operator.

Each of these scenarios can lead to a considerable revenue loss for the home network operator and an increase in customer complaints. These problems are particularly significant in areas where there is a land border between two countries, typically meaning that there is no clear demarcation between service areas provided by the home network and the foreign network. It is often the case that the coverage of one or more of these networks will extend into the geographic area of the other country.

Moreover, it will be appreciated that these problems are not necessarily restricted to international borders, as similar problems can occur within a single country, particularly when there are gaps in the coverage offered by a specific network operator (for example in France where the government requires that individual network operators provide coverage in sparsely populated areas). Here, a mobile device may lose the coverage of their home network, and they may roam to the network of another operator.

Therefore there is a need for a solution that allows the home network to retain the mobile device for as long as possible, and to recapture the mobile device at the earliest opportunity.

Summary of the Invention According to a first aspect of the invention, there is provided a method of operating a mobile device, the mobile device being attached to a network other than the home network of the mobile device, the method comprising determining whether the mobile device is in a part of said other network in which there may be coverage from the home network; and if it is determined that the mobile device is in a part of said other network in which there may be coverage from the home network, initiating a scan for a signal from the home network.

According to a second aspect of the invention, there is provided a mobile device configured to perform the method described above.

According to a third aspect of the invention, there is provided a network authentication device for use in a mobile device, the network authentication device comprising code that, when executed by a processor or the network authentication device or a processor of the mobile device, causes the mobile device to perform the method described above. According to a fourth aspect of the invention, there is provided a computer program product, comprising computer program code, that, when executed by a suitable computer or processor, performs the method described above.

According to a fifth aspect of the invention, there is provided a method of obtaining information about a network for use in the method described above, comprising the steps of moving a probe device having a network authentication device for a home network around a coverage area of a network other than the home network; operating the probe device such that it attaches to said other network via respective network nodes in said other network; operating the probe device to search for signals from the home network each time that the probe device is attached to said other network; and storing, in the probe device, the identity of any network node with which the probe device attaches, and an indication of whether signals have been detected from the home network.

According to a sixth aspect of the invention, there is provided a method of operating a network, the network being a home network for a mobile device, the method comprising receiving an update to a location of the mobile device, the mobile device being attached to a network other than the home network, the location update identifying a visitor location register in said other network with which the mobile device is associated; comparing the identified visitor location register to a list of visitor location registers that are associated with locations in said other network in which there may be coverage from the home network; and if the identified visitor location register is in said list, ensuring that the mobile device has a list of said locations stored therein.

These solutions enable the operator of the home network to keep their mobile subscribers attached to their network for a longer period than that which current practice provides, and will also enable the home network to recapture their returning roaming subscribers much sooner.

Brief Description of the Drawings

The invention will now be described, by way of example only, with reference to the following drawings, in which:

Figure 1 illustrates the coverage of two networks near a border between two countries; Figure 2 is a block diagram of a mobile device;

Figure 3 is an illustration of a probe device and home and foreign network architectures;

Figure 4 is a flow chart illustrating a method performed by a SIM applet in accordance with an aspect of the invention;

Figure 5 is a flow chart illustrating a method performed by a management module in accordance with an aspect of the invention;

Figure 6 is a diagram illustrating the operation of the invention for an out-bound mobile device;

Figure 7 is a diagram illustrating the operation of the invention for a returning in-bound mobile device; and

Figure 8 is a diagram illustrating the operation of the invention when a mobile device unintentionally roams to another network.

Detailed Description of the Preferred Embodiments

Although the invention will be described below with reference to the operation of a mobile device near to an international border between two countries, it will be appreciated that, unless otherwise stated, the principles of the invention can be applied more generally to the operation of a mobile device near to the edge of the coverage area of the home network of the mobile device.

In addition, although the invention will be described with reference to a GSM, or GSM- based network, it will be appreciated by those skilled in the art that the invention can be applied to other types of network.

Figure 1 shows three mobile devices 2 that are located near part of a border 4 between two countries. The country hosting the home network 6 (which is operated by the network operator to which each of the mobile devices 2 subscribe) is shown to the left of the border 4. This country is referred to herein as the "home country" of the mobile devices. The home network 6 is also referred to as the Home Public Land Mobile Network (HPLMN) herein.

Part of the network architecture of the home network 6 is shown in Figure 1. Specifically, two base stations 8, 10 are shown, each defining a respective cell having a coverage area 12, 14 illustrated by a dashed line.

A network 16 operated by a different network operator is located in the country to the right of the border 4 (this country is referred to as a foreign country). This network 16 is a foreign network, and the mobile devices 2 can roam to this network 16, if necessary. The network 16 is referred to as the "visited network" or the VPLMN (Visited Public Land Mobile Network) when a mobile device 2 is roaming in that network.

Three base stations 18, 20, 22 that form part of the architecture of the foreign network 16 are shown in Figure 1. Each of the base stations 18, 20, 22 defines a respective cell having a coverage area 24, 26, 28 illustrated by a solid line.

It can be seen that the coverage area 12, 14, 24, 26, 28 of each cell extends across the border 4 into the other country, and that there are areas on both sides of the border 4 in which the coverage of the home network 6 overlaps with the coverage of the foreign network 16.

It will be appreciated that Figure 1 only illustrates a small part of each network 6, 16, and that there may be many more base stations and associated cells near the border 4 than those shown. Moreover, it will also be appreciated that there may be multiple networks in each country, each operated by a respective network operator. However, only one of these networks will be the "home network" of the mobile devices 2.

Each of the three mobile devices 2 illustrate one of the problems to be overcome by the invention. Mobile device 2A is located in the foreign country but is moving back towards the home country. As a result of this movement, the mobile device 2A, which is currently registered with the foreign network 16 as a roaming device, has just entered the coverage area 12 of base station 8 in the home network 6. As described above, it is desirable for this device 2A to register with the home network 6 as soon as possible, in order to reduce roaming charges. Mobile device 2B represents a device that is located in the home country, but near to the border 4, and that is within the coverage area of a cell in both the home network 6 (cell 12) and the foreign network 16 (cell 26). As described above, it is desirable for this device 2B to remain registered with the home network 6 whenever possible, in order to avoid incurring roaming charges.

Mobile device 2C is located in the home country but is moving towards the foreign country. As a result of this movement, the mobile device 2C, which is registered with the home network 6, has just entered the coverage area 28 of base station 22 of the foreign network 16 and is about to cross the border 4 into the foreign country. Although the movement of mobile device 2C will result in the device 2C leaving the coverage area of the home network 6 completely (and therefore becoming a roaming device of the foreign network 16), it is desirable for the device 2C to remain registered with the home network 6 for as long as possible to reduce roaming charges.

Figure 2 illustrates an exemplary mobile device 2. The mobile device 2 comprises an antenna 32, transceiver circuitry 34, a central processor 36, a power source 38, a memory 40, a display 42, a keypad 44, a network authentication device holder 46 and a removable network authentication device 48. In the following, the network authentication device will be referred to as a SIM or USIM. A mobile device 2 of this type is known in the art, and the function and interaction of each of these elements will be understood to those skilled in the art. Unless otherwise stated, each of these elements operates in a conventional manner, and they will not be described further herein.

In accordance with the invention, when the mobile device 2 is attached to, or otherwise roaming in, a visited network 16 (i.e. any network other than the home network 6), the mobile device 2 can determine whether it is in part of the visited network 16 in which there may be coverage from the home network 6, and if so, the mobile device 2 can perform a scan to try and detect a signal from the home network 6. If the mobile device 2 detects a signal from the home network 6, the mobile device 2 can then register with the home network 6.

This method allows the mobile device 2 to register with the home network 6 as soon as (or shortly after) coverage from the home network 6 becomes available. Moreover, this method also has the advantage that if the mobile device 2 does attach to a VPLMN 16 while coverage from the home network 6 is temporarily unavailable, the mobile device 2 will reattach to the home network 6 as soon as possible.

In preferred embodiments of the invention, in order for the mobile device 2 to be able to determine whether it is in part of the visited network 16 in which there may be coverage from the home network 6, it is necessary to determine some information about the architecture and cell structure of the foreign network 16.

In particular, a (or a number of) probe devices can gather information about the architecture and cell structure of foreign networks in an area along the border 2 of two countries (or, more generally, in an area at the edge of the coverage area of the home network 6).

A probe device 50 and various parts of the architecture of the home and foreign networks are shown in Figure 3. The components of the probe device 50 are generally conventional, and can correspond to those shown in the mobile device 2 in Figure 2. In some embodiments, the probe device can correspond to a device as described and shown in any of United Kingdom Patent nos. 2416271 , 2416280 or 2413738.

The probe device 50 is moved around in the coverage area of the foreign network 16 near to the border 4 (for example by placing the probe device 50 on a vehicle), and gathers information on each cell (such as cells 24, 26, 28) in the foreign network 16 in which the probe device 50 is located. The probe device 50 also measures and stores information on the strength of any signals from the home network 6 while in cells 24, 26, 28.

The probe device 50 is provided with a subscriber identity module (SIM) 52 for the home network 6, and the probe device 50 performs a registration operation with the foreign network 16. The probe device 50 is forced to manually register with the PLMN of interest (that which the measurements are required for). While the probe device 50 is registered with the foreign network 16 or home network 6, the probe device 50 identifies and stores (in memory 54) the identity of the cell (8, 10, 24, 26 or 28) with which it is communicating, in terms of its Cell ID and Location Area Code (LAC), and also, possibly, the signal strength, and GPS location of the probe device 50 when the measurement is made. As known in GSM, a group of base stations/cells in a network form part of a Location Area, which is identified by a Location Area Code that is broadcast to any mobile device communicating with one of those cells. Each Location Area is under the control of several Base Station Controllers (BSC) 56 (illustrated in Figure 3), and there are usually multiple BSCs in a network under the control of a Mobile Switching Centre (MSC) 58.

A Visitor Location Register (VLR) 60 is associated with the MSC 58 in the network 16, and this stores information about all mobile devices that are currently registered with the MSC/foreign network 16. Whenever the MSC 58 is notified of a new mobile device in its network (such as when the probe device 50 or mobile device 2 registers with the network 16), the MSC 58 adds an appropriate record to the VLR 60. In addition to creating a new record in the VLR 60, the MSC 58 also provides a Location Update (LU) signal (shown by arrow 62 in Figure 3) to the home network 6 of the mobile device 2 (or probe device 50).

This Location Update signal is used to update a Home Location Register (HLR) 64 in the home network 6 (which is associated with a home network MSC 66), so that the home network 6 knows where its mobile subscriber/device 2 is located.

In accordance with an aspect of the invention, a Location Update capture unit 68 is used in the home network 6 between the home network MSC 66 and the foreign network MSC 58 to capture the Location Update signals resulting from the operation of the probe device 50. The Location Update capture unit 68 can identify the LU signals resulting from the operation of the probe device 50 by the IMSI information sent with the LU signal.

The captured Location Update signals are stored by the unit 68 in a database 70. The LU capture unit 68 and database 70 form part of a management module 72 that also includes a controller 74. The LU capture unit 68 also passes the Location Update signal to the MSC 66, so that the HLR 64 can be updated.

Thus, while the probe device 50 is gathering the LAC and Cell ID information of the base stations 18, 20, 22 in the VPLMN 16 and the associated coverage information from the HPLMN 6, the Location Update signals resulting from the movement of the probe device(s) 50 are captured by the LU capture unit 68 and stored in the database 70. The information gathered by the probe device(s) 50 can be uploaded to the database 70 and combined with the corresponding Location Update signals. In particular, the Location Updates are used to map the VLR global title (GT) to the radio layer LAC and Cell ID gathered by the probe device(s) 50.

The controller 74 can also process the measurements from the probe device 50 to determine an approximate location of each base station 18, 20, 22 in the foreign network 16. The approximate location can be determined from the Cell ID, LAC, signal strength and GPS location of the probe device 50 at each measurement. In particular, the controller 74 can use a weighted triangulation method (weighted using the measured signal strength) to determine the location of the relevant base station 18, 20, 22.

The above process should be repeated for each network that is available at the edge of the coverage area of the home network 6, as, typically, in the border situation illustrated in Figure 1 , there will be a number of foreign networks available in the foreign country, each operated by a respective network operator, and it is possible for a roaming subscriber to use any one of those networks.

The above process should also be repeated periodically in order to take account of any changes in the architecture of the foreign network(s) (such as by the introduction or removal of a base station) or in their configuration.

The resulting data stored in the database 70 will provide a map of the coverage provided to mobile devices by the HPLMN 6 and any foreign networks 16 in the relevant border region 4. As indicated above, this coverage and location data will include the SS7 GT address of VLRs 60 in the foreign network 16, as well as the Location Area Codes of BSCs 56 and Cell IDs of base stations 18, 20, 22.

This information can then be used to identify areas where mobile devices 2 should be able to attach to the home network 6.

In particular, this information identifies those VLRs 60 in the foreign network 16 that are associated with cells having some overlapping coverage from the HPLMN 6. In a preferred embodiment of the invention, the data on the coverage of the foreign networks 16 in the border region 4 stored in the database 70 is split into two categories. Cells in the foreign network 16 that are in a location close to a cell or cells in the HPLMN 6 and that should have good coverage from the HPLMN 6 can be placed into the first category, while cells in the foreign network 16 that are at the edge of the possible coverage area of the HPLMN 6 and therefore might not have good coverage from the HPLMN 6 can be placed into the second category. The cells in the database 70 can be distinguished by a flag that is set based on whether the cell is in the first category or the second category. Coverage from the HPLMN 6 that is considered to be "good" is that sufficient for a mobile device 2 to attach to the HPLMN 6. Note that these may be "good" coverage even where the signal strength and/or signal quality of the HPLMN 6 is less than that currently experienced by the mobile device 2 in the VPLMN 16.

Thus, in accordance with an aspect of the invention, when a mobile device 2 is attached to, or otherwise roaming in, a visited network 16 (i.e. any network other than the home network 6), the mobile device 2 can use the above information to determine whether it is in part of the visited network 16 in which there may be coverage from the home network 6.

In accordance with a particular aspect of the invention, a list of cells 24, 26, 28 in the VPLMN 16 where there is or may be coverage from the HPLMN 6 is stored on the SIM or USIM 48 of the mobile device 2, and a program executes on the SIM 48 to determine whether the mobile device 2 is currently located in one of the cells 24, 26, 28 in the list, and, if so, causes the mobile device 2 to start scanning for a signal from the HPLMN 6.

In particular, prior to use of the mobile device 2 or SIM 48, a "Location Status Event" (as defined by the 3GPP document "Specification of the SIM Application Toolkit (SAT) for the Subscriber Identity Module - Mobile Equipment (SIM-ME) interface") is added to the mobile device's "Current Event List", which causes the mobile device 2 to inform the SIM 48 of a change or update to the location information.

A SIM applet is installed in the SIM 48 to check the location information returned by the Location Status Event against a known list of VPLMN border cell sites stored in the memory of the SIM 48. The operation of this SIM applet will be described in more detail below with reference to the flow chart in Figure 4.

The operation of the SIM applet is triggered by a change or update to the Location Status stored in the SIM 48 as a result of the Location Status Event (step 101 ).

The SIM applet compares the updated or changed Location status (which comprises a Location Area Code and a Cell ID with which the mobile device 2 is communicating) with the list of cells in the VPLMN 16 stored in the memory of the SIM 48 (step 102).

If the location information does not match a VPLMN cell site stored in the SIM 48 (i.e. the mobile device 2 is not in a cell of the VPLMN 16 that has been identified as having overlapping coverage with the HPLMN 6), the SIM applet determines whether a timer used to trigger a search for a "higher priority" PLMN (HPPLMN) is set to the default value for the VPLMN 16 (step 103). The parameter used to indicate the value for the timer is denoted _EFHPPLMN. A default value for the HPPLMN timer can be of the order of 30 minutes.

If the timer is set to the default value, the SIM applet takes no further action (step 104) until a further Location Status update occurs (step 101 ). If the timer is not set to the default value, the SIM applet sets the timer to the network default value (step 105), which has previously been stored in the SIM 48 by the operator of the HPLMN 6.

In step 106, if the location information does match a VPLMN cell site stored in the SIM 48, it is determined whether the VPLMN cell site that the mobile device 2 is located in is a cell site in which there is likely to be good coverage from the HPLMN 6 (i.e. is the cell site labelled as being in the first category defined above?).

If the cell site is labelled as being in the first category (i.e. likely to be good coverage from the HPLMN 6), a "hard" HPLMN search process is initiated in the mobile device 2. In particular, in step 107, the SIM applet triggers a background scan for the HPLMN 6 by deleting files on the SIM 48 that indicate the network with which the mobile device 2 is, or was last, registered. In preferred embodiments, the SIM applet deletes the _EFLOCI and _EFPS-LOCI files on the SIM 48 that specify the network identity and the location area in this network that the mobile device 2 was last registered with.

Also in step 107, the SIM applet sends a REFRESH (STK) command to the mobile device 2 to instruct the mobile device 2 to restart SIM initialisation. This REFRESH command causes the mobile device 2 to perform a background search for a network.

As is known, a mobile device 2 or SIM 48 will have a list of "preferred" or "priority" networks or network operators to which the mobile device 2 should attach, if multiple networks are available in a particular area. Usually, the mobile device 2 or SIM 48 will have the home network 6 at the top of this list (i.e. as the network with the highest priority). Thus, on receipt of the REFRESH command, the mobile device 2 executes a background search for signals from Home Networks (HPLMN) or Equivalent Home Networks (EHPLMN) other than the network the mobile device 2 is registered with.

If the mobile device 2 detects a signal from the home network 6, the mobile device 2 attaches to the home network 6 automatically. In this way, the SIM applet will return the mobile device 2 to the home network 6, if it is available, as quickly as possible.

If the mobile device 2 attaches to the HPLMN 6 as a result of the search, the SIM applet takes no further action (step 109).

However, if the mobile device 2 does not attach to the HPLMN 6 as a result of the search (i.e. it does not detect a signal from the home network 6), the SIM applet proceeds to step 1 11.

The SIM applet also proceeds to step 1 11 if the VPLMN cell information "Hard" search flag in step 106 is set to "false" (i.e. the cell site is in the second category defined above), and a "soft" HPLMN search process is initiated in the mobile device 2.

In step 1 1 1 , the SIM applet sets the value of the HPPLMN timer (_EFHPPLMN) to a low, or its lowest, value (say, 6 minutes) in order to cause the mobile device 2 to perform a higher priority PLMN search within the time specified by the timer value. It will be noted that, in this embodiment, as the SIM applet cannot directly instruct the mobile device 2 to perform the HPPLMN search, it is necessary to trigger this search indirectly by adapting the value of the HPPLMN timer.

On expiry of the HPPLMN search timer, the mobile device 2 executes a background search for signals from Home Networks (HPLMN) or Equivalent Home Networks (EHPLMN) other than the network the mobile device 2 is registered with.

If the mobile device 2 detects a signal from the home network 6, the mobile device 2 attaches to the home network 6 automatically, and the HPPLMN timer is deactivated (the timer becomes unnecessary as the mobile device 2 is connected to the highest priority network). In this way, the SIM applet will return the mobile device 2 to the home network 6, if it is available, as quickly as possible.

If the mobile device 2 attaches to the HPLMN 6 as a result of the search, the SIM applet takes no further action (step 115) until a further Location Status update occurs (step 101 ).

The SIM applet allows the mobile device 2 to search for a HPPLMN a predetermined number of times (for example 3) by allowing the shortened timer to count down to zero that number of times.

In particular, if the mobile device 2 does not attach to the HPLMN 6 in step 113, an initial attach attempt counter is incremented by one (step 1 17).

If the number of permitted initial attach attempts has not been exceeded (step 119), the method returns to step 1 11 and repeats. However, if the number of permitted initial attach attempts has been exceeded (at step 1 19), the SIM applet sets the _EFHPPLMN timer value to an intermediate value (step 121 ), i.e. less than the network default value, but higher than the value set in step 1 11 (for example 15 minutes). This reduces the frequency of the attach attempts and therefore helps to preserve the battery life of the mobile device 2. However, if during this time the mobile device 2 moves into a cell site that has a high probability of HPLMN coverage (i.e. the "hard" search flag is set to "true), the "hard" HPLMN search process will be initiated by the SIM applet executing steps 101 , 102, 106 and 107. If the mobile device 2 manages to attach to the HPLMN as a result of a HPPLMN search triggered using the intermediate timer value (step 123), the SIM applet takes no further action (step 125) until a further location status update occurs (step 101 ).

If the mobile device 2 does not attach to the HPLMN 6 in step 123, a second attach attempt counter is incremented by one (step 126).

If the number of permitted second attach attempts has not been exceeded (step 127), the method returns to step 121 and repeats. However, if the number of permitted second attach attempts has been exceeded (at step 127), the SIM applet resets the _EFHPPLMN timer value to the default value (step 128). This helps to preserve the battery life of the mobile device 2.

The SIM applet then takes no further action until a further location status update occurs (step 101 ).

As the memory available in the SIM 48 is limited, it is generally not possible to store the cell information for all cells in all foreign networks 16 along a border 4 (even just the list of cells that have overlapping coverage from the HPLMN 6). Therefore, it is necessary to store a subset of the cell information in the SIM 48.

Firstly, only those VLRs 60 near the border 4 that have some coverage from the HPLMN 6 can be stored in the SIM 48. If a bordering VLR 60 has no coverage at all from the HPLMN 6, then it is treated the same as if it were a VLR 60 situated away from the border 4.

Furthermore, the subset stored in the SIM 48 may be one of several that are defined based on the last known location of the mobile device 2 in the VPLMN 16.

Figure 5 illustrates a method of maintaining the cell list in the SIM 48 by the management module 72. In step 131 , the controller 74 in the management module 72 receives a Location Update signal for the mobile device 2 via the Location Update capture unit 68.

In step 132, the controller 74 determines if the global title (GT) of the VLR indicated in the Location Update signal is a VLR that is known to have cells with overlapping coverage from the HPLMN 6. If the VLR does not have any cells with overlapping coverage from the HPLMN 6, the controller 74 takes no further action (step 133).

If the VLR does have cells with overlapping coverage with the HPLMN 6, the controller 74 determines if the VLR has more than one associated cell list (step 134). That is, a VLR will have more than one cell list if the data relating to the VPLMN cell sites within that VLR service area is too large to be stored in a memory of a SIM 48 or a SIM file having a predefined maximum size.

If the VLR does have more than one cell list, the controller 74 needs to determine the location of the mobile device 2 more precisely in order to provide the mobile device 2 with an appropriate list of cell sites. Therefore, in step 135, the controller 74 sends a Provide Subscriber Information (PSI) message to the mobile device 2 to obtain the Cell ID of the cell currently serving the mobile device 2.

On receipt of this Cell ID, the controller 74 determines if that cell is known to border the HPLMN 6 (step 136). If the cell does not border the HPLMN 6, the controller 74 takes no further action (step 137).

If the cell is a known border cell of the HPLMN 6, the controller 74 proceeds to step 138.

The controller 74 also proceeds to step 138 if the VLR is found to only have one cell list in step 134.

At step 138, the controller 74 compares a list of the last-known VPLMN VLR cell sites for the mobile device 2 (based on a previous Location Update signal for the mobile device 2) with a list that is required according to the current VLR (and current Cell ID if the VLR has more than one cell list) of the mobile device 2.

The controller 74 then determines whether the SIM 48 of the mobile device 2 contains the correct or most appropriate list of cell sites. The controller 74 does this by storing a table of version numbers (and network) and the subscriber's IMSI details, and by updating this table whenever a new list of cell sites is sent to the mobile device 2. If the SIM 48 does contain the correct list, the method ends (step 141 ). If the SIM 48 does not contain the correct list, then the SIM 48 is updated with the correct list. Preferably, this update is performed using over-the-air (OTA) programming (Class 2 binary SMS).

Where the VLR has more than one cell list, the controller 74 selects a subset of the VLR cell data to send to the mobile device 2 based on the Cell ID currently serving the mobile device 2. In one embodiment, subsets of VLR cell data are predefined, and the controller 74 selects the subset that contains the currently serving Cell ID. However, in an alternative embodiment, the controller 74 dynamically selects a subset of the VLR cell data from all of the VLR cell data based on the currently serving Cell ID. For example, the controller 74 can select the data for the currently serving Cell ID and cells neighbouring the currently serving Cell ID.

Thus, the solution described above ensures that mobile devices 2 remain with the HPLMN 6 for as long as possible when they are located in areas that border another country, and that in-bound roaming mobile devices 2 are recaptured by the HPLMN 6 as soon as possible.

It will be appreciated by those skilled in the art that in alternative embodiments in which it is possible to store a complete list for all border cell sites in the SIM 48 or memory 40 of the mobile device 2, it is not necessary to collect the Location Update information when the probe devices 50 are being used, or to implement the method described above with reference to Figure 5.

Three examples of the operation of the invention will now be presented with reference to Figures 6, 7 and 8.

Figure 6 is an illustration of part of the border between France and Germany, where a road runs near to the international border 4 between two towns in France, A and B. A subscriber to a French mobile network is driving along the road between A and B, and coverage from the (home) French network is temporarily lost. Therefore, the mobile device 2 will roam to a German network, which has coverage in this part of France. Once this occurs, it is possible that the French subscriber will not return to his home network until he reaches his destination, town B, where there is no coverage from the German network.

However, as the gap in coverage from the French network covers a small part of the road between A and B, the mobile device 2 could make use of the French network for most of the journey between A and B.

As the identities of the cells in the German network where there is also radio coverage from the home French network are stored on the SIM of the French subscriber's mobile device, whenever the mobile's location information is updated the SIM applet checks the updated location information against the list stored on the SIM. If the French subscriber's location is found to be in an area (such as the road between A and B) where there is a high probability of home network coverage, then a "hard" home network search is triggered. If this search finds the subscriber's home (French) network then the mobile device will attach and return to the home network. Alternatively, if the subscriber's location is found to be in an area where there might be coverage from the home network, a "soft" home network search, comprising a Higher Priority PLMN search can be triggered.

Figure 7 is an illustration of part of the border between the Netherlands and Germany, along with a part of the road network between Meppen in Germany and Emmen in the Netherlands. This example demonstrates the benefits of the invention for a returning in-bound mobile subscriber.

In this example, a subscriber to a Dutch mobile network is driving from Meppen (in Germany) back to their home in Emmen (in The Netherlands). In conventional operation, the Dutch subscriber's mobile device would remain registered with a German network until the mobile device performed either a Periodic Location Update, a normal Location Update or the device lost coverage from the German network. This means that even though the mobile device may have returned to a place with coverage from the home network, the Dutch subscriber will remain registered with the more expensive German network.

However, using the invention, as the Dutch subscriber drives towards Emmen, the mobile device records each change in serving cell information. The serving cell location information is sent to the SIM and is processed by the SIM applet. If the SIM applet determines that the current location is in an area where there may be radio coverage from the home network, a "soft" home network search is triggered, which comprises a background Higher Priority PLMN search. If this search finds the Dutch subscriber's home network, then the mobile device will attach and register with the home network. Alternatively, if the current location is an area where there is a high probability of home network coverage, a "hard" home network search can be triggered.

Figure 8 is an illustration of part of the border between France and Germany, with a subscriber's residence being located in France, but near to the border. This example demonstrates the benefits of the invention for a generally static subscriber that accidentally roams to a foreign network.

The French mobile network subscriber lives within the geographic border of their HPLMN country (France), in a rural or isolated location that has poor coverage from their HPLMN. However, Germany has a town on the border which provides excellent VPLMN coverage in the French subscriber's home location.

Conventionally, this situation results in the HPLMN subscriber's mobile roaming to the VPLMN whilst the subscriber is in his home but it will often return after a period to the HPLMN when the subscriber moves a few meters within his house. This is a frustrating, un-desirable and potentially expensive situation for the subscriber.

The invention provides that if the subscriber's mobile device has legitimately attached to the VPLMN (due to loss of HPLMN radio coverage) then the SIM applet will ensure that the subscriber's mobile device returns to the HPLMN as soon as possible.

As described above, this is achieved by either manipulating the mobile device's HPPLMN background search timer using the SIM applet or by deleting the location information stored on the SIM and causing the mobile device to initialize with the SIM, after determining that the mobile subscriber could have radio coverage from the HPLMN in his current location.

Thus the invention will minimize the time (and therefore expense to the HPLMN and subscriber) that the mobile device is attached to a border VPLMN. Therefore, a solution has been provided that allows the home network to retain the mobile device for as long as possible, and to recapture the mobile device at the earliest opportunity.

Although the invention has been described in terms of a SIM applet that is to be loaded into a SIM for a mobile device, it will be appreciated by those skilled in the art that the invention can be implemented in alternative ways. For example, the processor in the mobile device could be configured to execute one or both of the methods described above (the process in the SIM applet and the process in the management module). Moreover, the processor in the mobile device could be configured to directly initiate the "soft" home network search comprising the background scan for the HPPLMN as soon as it is determined that the mobile device is in a cell of the VPLMN that may also have coverage from the HPLMN (rather than adapting a timer value).

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments.

Variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or other unit may fulfil the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. A computer program, such as the SIM applet, may be stored/distributed on a suitable medium, such as an optical storage medium or a solid- state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems. Any reference signs in the claims should not be construed as limiting the scope.

Claims

1. A method of operating a mobile device, the mobile device being attached to a network other than the home network of the mobile device, the method comprising: determining whether the mobile device is in a part of said other network in which there may be coverage from the home network; and if it is determined that the mobile device is in a part of said other network in which there may be coverage from the home network, initiating a scan for a signal from the home network.

2. A method as claimed in claim 1 , further comprising the step of: if a signal from the home network is found, attaching the mobile device to the home network.

3. A method as claimed in claim 1 or 2, wherein the step of initiating a scan comprises reinitialising the mobile device with a network authentication device associated therewith.

4. A method as claimed in claim 3, wherein the step of initiating a scan further comprises deleting data stored in the network authentication device, the data indicating the current or last network the mobile device was registered with.

5. A method as claimed in claim 1 or 2, wherein the step of initiating a scan comprises reducing a timer value associated with the scan, such that the time between scans is reduced.

6. A method as claimed in claim 5, wherein if a signal from the home network is not found, repeating the scan for a signal from the home network.

7. A method as claimed in claim 6, wherein if a signal from the home network is not found after repeating the scan a predetermined number of times; the method comprises increasing the timer value associated with the scan to a higher value; or restoring the timer value associated with the scan to a default value.

8. A method as claimed in claim 5, 6 or 7, wherein in the event that it is determined that the mobile device is not in a part of said other network in which there may be coverage from the home network, setting the timer value associated with the scan to a default value.

9. A method as claimed in claim 5, 6, 7 or 8, wherein the scan comprises a higher priority public land mobile network search.

10. A method as claimed in claim 1 or 2, wherein, if it is determined that the mobile device is in a part of said other network in which there may be coverage from the home network, the method further comprises: determining whether there is a high probability of coverage from the home network in said part; wherein, if it is determined that there is a high probability of coverage from the home network, the step of initiating a scan comprises reinitialising the mobile device with a network authentication device associated therewith; and if it is not determined that there is a high probability of coverage from the home network, the step of initiating a scan comprises reducing a timer value associated with the scan, such that the time between scans is reduced.

11. A method as claimed in claim 10, wherein, if, following the step of reinitialising the mobile device with a network authentication device associated therewith, a signal from the home network is not found, the method further comprises initiating a further scan for a signal from the home network by reducing a timer value associated with the scan, such that the time between scans is reduced.

12. A method as claimed in any preceding claim, wherein the step of determining comprises: comparing the location of the mobile device to a set of locations in said other network in which there may be coverage from the home network.

13. A method as claimed in claim 12, wherein the step of comparing comprises comparing an identifier of a cell in said other network with which the mobile device is communicating to a set of identifiers of cells in said other network in which there may be coverage from the home network.

14. A method as claimed in claim 13, wherein it is determined that the mobile device is in a part of said other network in which there may be coverage from the home network in the event that the identifier of the cell in said other network with which the mobile device is communicating is also in the set of identifiers.

15. A method as claimed in claim 12, 13 or 14, wherein, when the mobile device attaches to said other network, the method further comprises the step of: receiving a list of the set of locations in said other network.

16. A mobile device configured to perform the method claimed in any of claims 1 to 15.

17. A network authentication device for use in a mobile device, the network authentication device comprising code that, when executed by a processor of the network authentication device or a processor of the mobile device, causes the mobile device to perform the method claimed in any of claims 1 to 15.

18. A computer program product, comprising computer program code, that, when executed by a suitable computer or processor, performs the method as claimed in any of claims 1 to 15.

19. A method of obtaining information about a network for use in the method claimed in any of claims 1 to 15, comprising the steps of: moving a probe device having a network authentication device for a home network around a coverage area of a network other than the home network; operating the probe device such that it attaches to said other network via respective network nodes in said other network; operating the probe device to search for signals from the home network each time that the probe device is attached to said other network; and storing, in the probe device, the identity of any network node with which the probe device attaches, and an indication of whether signals have been detected from the home network.

20. A method as claimed in claim 19, wherein the step of storing comprises storing a Cell ID and Location Area Code for each network node in said other network to which the probe device attaches.

21. A method as claimed in claim 19 or 20, wherein the method further comprises the steps of: placing a capture unit in the network architecture of the home network, such that the capture unit receives updates to the location of the probe device that are provided to the home network by said other network; and combining the information stored by the probe device in the step of storing with the location updates.

22. A method of operating a network, the network being a home network for a mobile device, the method comprising: receiving an update to a location of the mobile device, the mobile device being attached to a network other than the home network, the location update identifying a visitor location register in said other network with which the mobile device is associated; comparing the identified visitor location register to a list of visitor location registers that are associated with locations in said other network in which there may be coverage from the home network; and if the identified visitor location register is in said list, ensuring that the mobile device has a list of said locations stored therein.

23. A method as claimed in claim 22, wherein the locations comprise cells in said other network.

24. A method as claimed in claim 22 or 23, wherein the step of ensuring comprises determining whether the mobile device already has said list stored therein, and, if the mobile device does not already have said list stored therein, transmitting said list to the mobile device.

25. A method as claimed in claim 22, 23 or 24, wherein if the mobile device has insufficient storage to store said list of locations therein, the method further comprises the steps of: sending a request to the mobile device to obtain the location of the mobile device; and providing a subset of said list of locations to the mobile device, the subset of said list comprising at least the location of the mobile device.

26. A method as claimed in claim 25, wherein the location of the mobile device comprises an identifier of a cell in said other network with which the mobile device is communicating.

27. A method of operating a network as claimed in any of claims 22 to 26, further comprising the steps of operating a mobile device as claimed in any of claims 1 to 15.