GB2408895A

GB2408895A - Network selection based upon subscriber's network identity

Info

Publication number: GB2408895A
Application number: GB0328317A
Authority: GB
Inventors: Howard Thomas; Maya Benson; Robert Davies; Gerard Foster
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 2003-12-05
Filing date: 2003-12-05
Publication date: 2005-06-08
Anticipated expiration: 2023-12-05
Also published as: GB2408895B; GB0328317D0

Abstract

A method of handover in a cellular communication system 2 wherein the selection of a handover candidate cell 12, 14, 16, 18 for a subscriber device 20 is made at least partly in dependence on a network identity associated with the subscriber device 20. A comparison may be made between the network identity associated with the subscriber device 20 and a network identity associated with candidate handover cells 12, 14, 16, 18 and the selection of a handover cell may be at least partly based upon a match between the compared network identities. Thus, for example, a user terminal 20 that is able to operate in both 2G (e.g. GSM) and 3G (e.g. UMTS) systems will handover to an operator's own 2G system upon leaving the operator's 3G coverage area rather than handover to a competing 2G system.

Description

METHOD OF NETWORK SELECTION

The present invention relates to a method of network selection. The invention is particularly applicable to, but is not limited to, a method of handover in a cellular communication system.

Many cellular communication systems are now available or are planned for the future. In a cellular communication system, such as the exemplary cellular communication system shown in Figure 1, the whole coverage area 2 of the communication system is divided into plurality of cells 4, 6, 8, 10, each cell 4, 6, 8, 10 having a respective serving base station 12, 14, 16, 18 to support communication with subscriber user terminals 20 within the cell. The term uplink 22 defines communications in the direction from a subscriber terminal to the base station: the term downlink 24 defines communications in the direction from the base station to the subscriber terminal.

Generally subscriber terminals 20 are able to move about, or roam, within the whole coverage area 2 of the communication system. As a subscriber terminal moves from one cell to another, a handover between the serving base station and the base station of the new cell is carried out. The necessity for a handover is determined, and a new cell is selected, on the basis of the respective signal levels received at the subscriber terminal 20. Handover enables a speech call or a data transfer, or more generally a communication connection between the subscriber terminal 20 and the communication system, to be maintained despite the movement of the subscriber terminal 20 between cells and through the communication system.

Generally current so-called 2n Generation (2G) cellular communication systems such as the Global System for Mobile communication (GSM) system provide for more or less continuous coverage across a wide geographic area, typically across an entire country. However, it is envisaged that communication networks that do not provide substantially continuous coverage across a wide geographical area will increasingly be deployed. An example of such a communication system is the so-called 3r Generation (3G) cellular communication systems such as the Universal Mobile Telecommunications System (UMTS). It is presently envisaged that UMTS will, at least initially, be deployed only in limited regions such as cities and along major transport routes such as motorways. Multitechnology subscriber terminals 20, i.e. user terminals that are able to operate on more than one system, such as both a 2G and a 3G system, will therefore be able to access a 3G system in 3G coverage areas but will rely on a 2G system for communication services in areas not covered by a 3G system.

Some 3G operators will not have a corresponding 2G network and, to enable such 3G network operators to offer national coverage to their subscribers, national roaming is being introduced. Thus in areas not covered by the 3G network the subscribers of such 3G operators will able to use the 2G network of another operator.

When such a subscriber moves from an area exclusively served by the 2G system to an area also served by the 3G system of the subscriber's network operator, it is desirable for the subscriber's network operator that the subscriber terminal is handed over to a neighbor cell in the 3G system of the subscriber's network operator.

Moreover, where an operator operates both a 2G system and a 3G system, the operator would prefer that a subscriber moving from an area served exclusively by the 2G system to an area also served by the operator's 3G system and a competing 3G system is handed over to a neighbor cell in the 3G system of the subscriber's network operator instead of to a neighbor cell in the competing 3G system.

Currently a cell for handover is selected on the basis of signal strength, and it is not possible to satisfy operator preferences in the selection of a cell for handover.

According to one aspect of the invention there is provided a method in a communication network of selecting a target communication system as claimed in claim 1.

According to a second aspect of the invention there is provided a network element in a communication system as claimed in claim 12.

For a better understanding of the present invention, and to show how it may be brought into effect, reference will now be made, by way of example, to the accompanying drawings in which: Figure 1 shows an exemplary cellular communication system; Figure 2 is a schematic outline of elements in an exemplary communication system network in which the present invention may be implemented; Figure 3 illustrates a method in accordance with a first embodiment; Figure 4 illustrates a method in accordance with a second embodiment Figure 5 illustrates a method in accordance with a third embodiment Figure 6 illustrates a method in accordance with a fourth embodiment.

Embodiments will now be described in the context of a handover from a 2G system to a 3G cellular communication system. However, it will be apparent that the inventive concepts are more generally applicable and may for example also be applied in the context of a selection of other existing types of networks or cells, such as Wireless Local Area Networks (WLAN), or future generations of wireless communication technologies. For example, a subscriber device may be attached to a WLAN or registered on or attached to a cellular network when a preferred WLAN is detected, and the invention may be applied to preferred network selection in this situation. In addition, the inventive concepts may be applied to a handover between two similar-technology systems, such as a 3G to 3G handover or a 2G to 2G handover.

In accordance with an aspect of the inventive concept, a target communication system is selected from a plurality of possible candidate communication systems based on network identity information associated with the subscriber device. This inventive concept is particularly applicable to the selection of a candidate handover cell from a plurality of possible handover cells belonging to different networks. In this context a "possible handover cell" is one that would be an acceptable handover candidate using for example radio conditions criteria. For example, the list of neighbor cells in a measurement report would be considered to be possible handover cells.

Thus handover cells may be selected based on the operator preferences relating to the handling of different subscribers. As an example, an operator having a 2G and a 3G system may arrange that all subscriber devices identified as belonging to the operator are handed over to a cell in their 3G system instead of to a 3G competing system. Alternatively or additionally an operator may arrange that all subscriber devices identified as belonging to a competing operator or another system are handed over to a different cell, for example a 3G cell belonging to the competing operator or system. This might be required, for example, to fulfill terms of an agreement between the 2G/3G operator and the competing 3G operator to allow access to the 2G network for 2G network roaming.

Figure 2 is a schematic outline of elements in an exemplary communication system network in which the present invention may be implemented.

A subscriber device (MS) 2 is within a cell served by a base transceiver station (BTS) 4 and is able to communicate with the BTS 4 over a radio link 6.

Specifically the subscriber device may exchange both signaling or other control information and user data with the BTS 4 over the radio link 6.

The BTS 4 is operably coupled with a base station controller (BSC) 8. The BSC 8 controls the operation of the BTS 4.

The BSC 8 is operably coupled to a mobile switching center (MSC) 10 which in turn is operably coupled to an external network 12, a Visitor Location Register (VLR) 14 and a Home Location Register (HLR) 16. The MSC 10 controls the switching and routing of communications for the MS 2 to and from the external network 12 or other MSCs within the communication network and executes various other high level management functions with regard to the service provided to the MS 2 by the communication network.

The HLR 16 and the VLR 14 store various types of subscriber information about subscribers belonging to the communication network, in the case of the HLR 16, and about subscribers belonging to another communication network that are currently registered on the this communication network, in the case of the VLR 14.

As will be appreciated, for clarity only a few network elements have been shown and a typical network will contain many more network elements. In particular, typically a BTS 4 will support a large number of MSs; a single BSC 8 typically will control more than one BTS 4; an MSC 10 will be operably coupled to more than one BSC 8; and an MSC 10 will also be operably coupled to other MSCs to provide switching and routing functions within the communication network.

In addition, the network elements are not intended to be specific to any network and the use of the terms base transceiver station, base station controller, mobile switching center etc are not intended to be limit the invention to a particular communication system, but are instead intended merely to refer to generic functional elements. Furthermore, the invention is applicable to the communication of both speech and of general user data and is applicable to both circuit switched and packet switched networks and the absence of network elements, for example network elements supporting packet networks, is not intended to exclude the application of the present invention to such networks.

The MSC 10 and the BSC 8 are adapted to implement the method of the invention, for example as described below with reference to the exemplary embodiments. It is anticipated that this adaptation may be by way of a modification of the software code controlling the operation of the MSC 10 and the BSC 8 respectively. However, in alternative embodiments this adaptation may be by way of addition of a separate module for implementing the described method of the invention.

In addition, the BSC 8 may be adapted to store network identity information associated with subscriber devices to facilitate the implementation of the embodiments of the invention. Thus for example, the BSC 8 may be provided with register or a table in a memory for storing network identity information associated with subscriber devices. This memory may be provided as a separate memory device, but will more commonly be implemented in part of another BSC memory device.

As indicated above, in accordance with the exemplary illustrated embodiment, a handover cell is selected based on network identity information associated with a subscriber device. Thus initially the network identity information associated with a subscriber device must be obtained. A method of obtaining network identity information associated with a subscriber device in accordance with a first embodiment will now be described with reference to Figure 3. In the first embodiment, MS 2 is in a 2G network cell in an area in which there is also 3G coverage.

The method shown in Figure 3 starts with a call set up using standard procedure applicable to the communication network, step s2. Once the call set-up is complete step s4-yes the MSC 10 is adapted to send an IDENTITY REQUEST message to the MS 2, step s6. In response to the IDENTITY REQUEST message, the MS 2 returns, step s8, an IDENTITY RESPONSE message to the MSC 10, containing the International Mobile Subscriber Identity (IMSI) associated with the subscriber device. It should be noted that support for this message exchange is mandatory in the GSM system (GSM specification 24.008 4.3.3). However, there is currently no requirement for the MSC 10 to initiate the message exchange immediately after call set up.

It is possible in some embodiments of the invention for the MSC 10 to be adapted to send the IDENTITY REQUEST message immediately after call set up for all calls made on the 2G network. However, in other embodiments the MSC may be adapted to sent the IDENTITY REQUEST message only in respect of MSs that are wishing to initiate a 3G call; or only in respect of 3G capable MSs; or only in respect of 3G capable MSs that have a particular subscription; or only in respect of some other subset of MSs, as appropriate.

Generally a message such as the IDENTITY RESPONSE message sent from the MS 2 to the MSC 10 is passed transparently by the BSC 8. However, in this first embodiment the BSC 8 intercepts the IDENTITY RESPONSE message sent from the MS2 to the MSC 10, step s10. The BSC 8 may intercept the IDENTITY RESPONSE message for example by detecting it directly or by examining all messages within a certain time period after call set up. Once the BSC 8 has intercepted the IDENTITY RESPONSE message, the BSC 8 can extract the IMSI. Part of the IMSI is formed from the Mobile Network Code (MNC) and there is a direct relationship between the MNC and the Public Land Mobile Network (PLMN) identity, thus enabling the BSC 8 to determine a network identity associated with the MS 2, in this case the PLMN code, from the MS identity information i.e. the IMSI. Generally, the BSC 8 will store the network identity associated with the MS 2, i.e. the PLMN code, in a storage area for this purpose 8 1 within the BSC 8, as indicated above. However, clearly other forms of network identity codes or flags may also be used in other embodiments.

Thus, as described above, in the exemplary embodiment illustrated in Figure 2, the network identity information associated with a subscriber device is obtained and stored by the BSC 8.

A handover method in accordance with an embodiment of the invention will now be described with reference to Figure 3. In this exemplary handover method, network identity information associated with the MS 2, for example the PLMN identity information obtained as described above with reference to Figure 2, is used by the BSC 8 in determining handover of the MS 2. As will be appreciated by a skilled person, the handover method described above may follow directly after call set up, i.e. directly after the method as described above with reference to Figure 2, or alternatively may occur independently at some later time. In either case, network identity information associated with the MS 2 is available to the BSC 8 as described above.

The BSC 8 may apply the handover method only to MSs associated with certain network identities. Thus, for example, the network operator of the serving network may allow MSs belonging to a number of other networks to roam onto the network, but may wish to apply selective handover only to some of those other networks, for example because the network only has a relevant agreement to selectively handover the MS with a subset of the other network operators.

Thus, the BSC 8 may have available to it a list of network identities, for example PLMN codes in the present embodiment, for which the method of this embodiment is to be applied. In general it is envisaged that such a list of network identities to which the method should be applied would be consistent across a network. However, in some embodiments it is envisaged that the list of network identities may change across a network. In particular, it is envisaged that the list of network identities to which the method can be applied may be different at network edges bordering other networks, in particular for example in country border areas.

In step s12 the BSC 8 determines whether the network identity associated with the MS 2 matches a network identity on the list of network identities to which the method of the invention is to be applied that is held by the BSC 8.

If the network identity of the MS 2 does not match a network identity on the list held by the BSC 8, step s1 2-n, the handover procedure proceeds in a conventional fashion, step s14. Thus the BSC 8 may, for example, select a handover candidate cell based on the received signal strength.

If, however, the network identity of the MS 2 does match a network identity on the list held by the BSC, step s12-y, the handover procedure in accordance with the exemplary embodiment continues in accordance with step s16 of the described exemplary embodiment.

As will be known to a skilled person, a MS 2 periodically reports to its serving base station information identifying a number (typically 6 in a GSM system) of neighbor cells from which the MS 2 is receiving the strongest signal, allowing the BSC 8 to maintain an up to date list of possible handover candidates for each MS. Part of the information identifying the neighbor cells contains information about the network identity of the neighbor cells. Specifically, in the illustrative embodiment the PLMN code identity for the neighbor cells may be obtained from the information reported by the MS 2. Thus the BSC 8 is aware of the communication system to which the neighbor cells belong.

Thus in step s16 the BSC 8 compares the network identity associated with the MS 2 with the network identity of candidate handover cells in the neighbor list for the MS 2 maintained by the BSC 8, and determines whether there are one or more matches, step s18.

The present invention has been described with the assumption that the selection of a handover cell should be made such that the MS 2 is handed over to a cell of its home operator network, where available. Thus in the exemplary embodiment a match between the network identity associated with the MS and the network identity of the neighbor cell is determined. However, the inventive concept of the present invention is not restricted only to a matching relationship and in principle could be applied to any relationship between the network identity associated with the MS 2 and the network identity of the candidate handover cells.

In the exemplary described embodiment, if no match is found step s1 8-n, the method returns to the comparison of the network identity associated with the MS 2 with the network identity of candidate handover cells in the neighbor list for the MS 2 maintained by the BSC 8 in step s16. Thus should the handover candidates reported by the MS change so as to include a candidate handover cell with a matching network identity, the method of the invention can then be applied to handover to the new candidate cell.

If there are one or more matches, step s1 8-y, it is possible to handover the MS to a new cell on the basis of the network identity of the MS. Therefore the BSC 8 initiates a standard handover process to perform a handover of the MS 2 to the best matching handover candidate, step s20. Typically this will be the network- matching handover candidate cell having the highest received signal strength by the MS 2, although other possible ways of selecting a handover candidate should a number of candidate cells meet the network- selection handover criteria are possible.

It is envisaged that once the target handover cell has been selected in accordance with the inventive principles set out above, the actual handover process to the selected handover candidate will proceed as normal. Thus in the exemplary case of a handover from a 2G system to a 3G system a GSM to UMTS Radio Access Network (UTRAN) handover is standardized in the standards specifications 08.08 clause 3.2.1.9 and 25. 413 clause 9.2.1.25, and enables one target RNC to be specified, which specifies a single cell id. Thus the BSC 8 will sent a HANDOVER REQUIRED message to its associated MSC 10 and the MSC 10 will send a HANDOVER REQUEST message to the MSC 10 of the target cell.

In step s22 it is determined whether the handover was successful. If the handover was not successful, the BSC 8 may initiate a second handover attempt.

This may be done by sending a second HANDOVER REQUIRED message in respect of another match on the original list, for example. However, in view of the likely signaling delays involved in determining failure of the handover attempt, it is preferred that if handover is not successful, step s22-n, the process returns to step s16 so that the determined network identity associated with the MS can be compared with an up-to- date candidate handover list.

If the handover attempt is successful, step s22-y, the current handover process stops s24. However, the skilled person will understand that in practice multiple handovers may occur in succession, and in practice it is likely that the operation will loop back to await the start of another handover, as such a handover becomes necessary.

Thus in accordance with the inventive concepts described with reference to Figure 4, the selection of a handover candidate cell for a MS is made at least partly in dependence on a network identity associated with the MS. In particular, the selection of a candidate handover cell may be made at least partly in dependence on a comparison between the network identity associated with the MS and a network identity associated with candidate handover cells. The selection of a handover cell may at least partly depend upon a match between the network identity associated with the MS and the network identity associated with the candidate handover cells, but in some embodiments may also depend on some other relationship. The network identity may for example be a PLMN code. In addition the selection may also be determined by other factors, for example signal strength, such that if two or more reported candidate cells are available for selection based on the comparison between network identities, the candidate cell with the highest reported received signal may be selected.

The network identity associated with the MS may be stored in the BSC 8 and may be obtained by use of the IDENTITY REQUEST mechanism as described above with reference to Figure 3. However, alternative methods by which the BSC 8 may obtain the network identity associated with a MS will now be described with reference to Figures 5 and 6 Firstly an alternative method by which the network identity associated with the MS can be obtained by the BSC on set up of the call is described with reference to Figure 5. Again, in this embodiment, MS 2 is in a 2G network cell in an area in which there is also 3G coverage.

The method shown in Figure 4 starts with a call set up using standard procedure applicable to the communication network, step s26. Once the call set-up is complete step s28-yes the MSC 10 is adapted to access the HER or the VLR, as appropriate to access subscription information relating to the MS 2, step 30.

Such subscription information relates, among other things, to the IMSI of the MS 2.

Thereafter the MSC 10 is adapted to send a COMMON IDENTITY message, containing the IMSI of the MS 2, to the BSC 8, step 32. The COMMON IDENTITY message is already standardized to support dual-mode subscriber devices.

As mentioned above, part of the IMSI is formed from the Mobile Network Code (MNC) and there is a direct relationship between the MNC and the Public Land Mobile Network (PLMN) identity, thus enabling the BSC 8 to determine a network identity associated with the MS 2, step 34. In this exemplary case the network identity is the PLMN code of the MS 2. Generally, the BSC 8 will store the network identity associated with the MS 2 in a storage area for this purpose within the BSC 8, as indicated above.

Thus, as described above, in the exemplary embodiment illustrated in Figure 5, the network identity information associated with a subscriber device is obtained during call set up and stored by the BSC 8.

In some situations, the MS may not have undergone call set-up within the currently serving cell, but may already have undergone a handover from the cell in which call set up occurred, possibly via handovers from a plurality of other cells. In this situation it is important to ensure that the network identity information associated with the MS that was obtained on call set up, for example by way of the process described with reference to either Figure 3 or Figure 5 above, is passed to a new serving base station during the normal handover procedure.

This may be achieved using the method shown in Figure 6, for example. Thus in step s36 a handover is triggered. This may be a handover from a 2G cell to a 2G cell or a handover from a 2G cell to a 3G cell in accordance with the prior art, or may be a handover in accordance with the inventive concepts of the invention, as described above with reference to Figure 4.

The messaging associated with the handover has the effect of transferring network identity information to the target BSC 8, step s38. This can be done in the context of a GSM/UMTS system by requiring the HANDOVER REQUEST message sent from the MSC 10 of the originally serving BSC 8 to the MSC 10 of the target BSC 8 to include the IMSI of the MS, and requiring the MSC 10 of the target BSC 8 to forward the IMSI information to the target BSC 8.

Once the handover is complete, step s40, the new BSC 8 can extract the network identity information and can store the network identity in the BSC memory, as described above.

Thus Figure 6 illustrates an exemplary method of transferring the network identity associated with the MS during a handover of the MS.

In the above-described embodiments of the invention, the network identity associated with the MS is provided to and may be stored in the BSC 8, to enable selection of a suitable handover candidate for the MS, when necessary, dependent on the network identity of the MS 2. However, it will be understood that alternatively in some embodiments it would be possible for the BSC 8 to provide the MSC 10 with information relating to candidate handover cells, in particular network identity information associated with candidate handover cells to enable the MSC 10 to select a handover candidate based on the network identity associated with the MS.

Thus a method of handover has been described in which handover selection decision for a subscriber device is made at least partly dependent on network identity information associated with the subscriber device. This enables different handover treatment of subscriber devices associated with different operators or networks. This is of particular benefit in the context of handover from 2G to 3G systems, but is also applicable to other sorts of handovers or transfers.

Claims

1. Method in a communication network of selecting a target communication system for a subscriber device, wherein the target communication system is selected from a plurality of possible candidate communication systems at least partly based on a network identity associated with the subscriber device.

2. The method as claimed in claim 1 wherein the method relates to a method of selecting a handover candidate cell for the subscriber device.

3. The method as claimed in claim 2 wherein the network identity associated with the subscriber device is compared with a network identity associated with potential handover cells and a cell is selected as a candidate handover cell at least partly based on the results of the comparison.

4. The method as claimed in claim 3 wherein a potential handover cell is preferentially selected as a candidate handover cell when the network identity associated with the subscriber device corresponds with the network identity associated with the potential handover cell.

5. The method as claimed in claim 2 or 3 further comprising the step of obtaining the network identity associated with the subscriber device.

6. The method as claimed in claim 5 wherein the step of obtaining the network identity associated with the subscriber device comprises receiving network identity information associated with the subscriber device on handover.

7. The method as claimed in claim 5 wherein the step of obtaining the network identity associated with the subscriber device comprises intercepting a message from the subscriber device to another network element of the communication system and obtaining network identity information therefrom.

8. The method as claimed in claim 5 wherein the step of obtaining the network identity associated with the subscriber device comprises receiving a message containing identity information from another network element of the communication system.

9. The method as claimed in claim 7 or 8 where the other network element is a mobile switching center (MSC).

10. The method as claimed in one of claims 3-9 where the network identity associated with the potential handover cell is obtained from cell identity information in a measurement report of a subscriber device.

11.A storage medium storing processor-implementable instructions for comtrolling a processor to carry out the method of any proceeding claim

12.A network element in a communication system, comprising storage means for storing, in use, a network identity associated with at least one subscriber device associated with the network element; and communication system selector for selecting a target communication system for the subscriber device from a plurality of possible candidate communication systems at least partly based on a network identity associated with the subscriber device.

13. The network element as claimed in claim 12 wherein the communication system selector is a handover selector for selecting a handover candidate cell for the subscriber device.

14.The network element as claimed in claim 13 wherein the handover selector compares the network identity associated with the subscriber device with a network identity associated with potential handover cells, and selects a cell as a candidate handover cell at least partly based on the results of the comparison.

15.The network element as claimed in claim 14 wherein the handover selector preferentially selects a potential handover cell as a candidate handover cell when the network identity associated with the subscriber device corresponds with the network identity associated with the potential handover cell.

16.The network element as claimed in claim 14 or 15 further comprising means for obtaining the network identity associated with the subscriber device.

17. The network element as claimed in claim 16 wherein the means for obtaining the network identity associated with the subscriber device comprises means for receiving network identity information associated with the subscriber device on handover.

18. The network element as claimed in claim 16 wherein the means for obtaining the network identity associated with the subscriber device comprises means for intercepting a message from the subscriber device to another network element of the communication system and obtaining network identity information therefrom.

19.The network element as claimed in claim 16 wherein the means for obtaining the network identity associated with the subscriber device comprises means for receiving a message containing identity information from another network element of the communication system.

20.The network element as claimed in claim 12-19, comprising means for obtaining the network identity associated with the potential handover cell from cell identity information in a measurement report of a subscriber device.

21.The network element as claimed in claim 12-20 where the network element is a base station.