Abstract:
A method for performing handover of a mobile station communicating in a first call via a first network to communication in a second call via a second network, comprising: generating a request for handover; establishing the second call between the first network and the mobile station via the second network; and transferring data communication between the mobile station and the first network from the first call to the second call.

Description:
This application claims the benefit of the earlier filed International Application No. PCT/IB99/01412, International Filing Date, 27 Jul. 1999, which designated the United States of America, and which international application was published under PCT Article 21(2) in English as WO Publication No. WO 00/07402. 
   FIELD OF THE INVENTION 
   This invention relates to a system of handover for mobile stations, for example in a cellular radio telecommunications network. 
   BACKGROUND OF THE INVENTION 
     FIG. 1  shows schematically the configuration of a typical cellular radio telecommunications network. The network comprises a number of base-stations (BSs)  1 ,  2 ,  3  etc. Each base-station has a radio transceiver capable of transmitting radio signals to and receiving radio signals from the area of a cell  4 ,  5  etc. next to the base-station. By means of these signals the base-station can communicate with a mobile station (MS)  6  in that cell, which itself includes a radio transceiver. Each base station is connected to a mobile system controller (MSC)  7 , which is linked in turn to the public telephone network  8 . By means of this system a user of the MS  6  can establish a telephone call to the public network  8  via the BS in whose cell the MS is located. 
   The location of the MS could be fixed (for example if it is providing radio communications for a fixed building) or the MS could be moveable (for example if it is a hand portable transceiver or “mobile phone”). When the MS is moveable it may move between cells of the cellular radio system. As it moves from one cell (the “old cell”) to another cell (the “new cell”) there is a need to hand it over from communication with the BS of the old cell to the BS of the new cell without dropping the call due to a break in communications between the mobile station and the network. This process is known as handover. A need can also arise to hand over a MS whose location is fixed, for example if atmospheric conditions affect its communications with the old BS and call quality can be improved by handing it over to another BS or if there is a need to free up capacity of the old BS. 
   In a conventional cellular radio system handover is controlled automatically by the MSC. Handover can be initiated by the MS or be network dependent, for example, on the quality of the signalling between the MS and the old and new BSs. 
   When a new cellular network is being introduced it can take some time to install all the base-stations and associated apparatus. Therefore, there is a delay before the new network provides full geographical coverage.  FIG. 2  illustrates the situation: an existing cellular network provides full geographical coverage by means of cells  20 - 27  but the new cellular network provides incomplete geographical coverage by means of only cells  28  and  29 . This presents a significant commercial problem for the operator of the new network. If the new network is launched for use before its geographical coverage is complete then customers will be dissatisfied by its inferior coverage to the old network. However, the cost of the infrastructure of the new network is high and no return can be gained on it until it is in use. 
   It has been proposed to tackle this problem by allowing mobile stations using the new network to be handed over to cells of the old network when they move outside the coverage of the new network. For instance, when a mobile station moves from  30  to  31  in  FIG. 2  it could be handed over from the base station of cell  28  (in the new network) to that of cell  21  (in the old network). However, the base-stations of cells  28  and  21  are in different networks and are therefore not linked by a common MSC, so conventional handover processes cannot be used. One solution to this could be to modify the old network to allow it to support inter-network handover. However, modifying the old network would be expensive and inconvenient. 
   There is therefore a need for a new method for handover between two telecommunications networks. 
   SUMMARY OF THE INVENTION 
   According to the present invention there is provided a method for performing handover of a mobile station communicating in a first call via a first network to communication in a second call via a second network, comprising: generating a request for handover; establishing the second call between the first network and the mobile station via the second network; and transferring data communication between the mobile station and the first network from the first call to the second call. 
   The method preferably also comprises the step of releasing the first call after data communication between the mobile station and the first network has been transferred from the first call to the second call. The said data communication is suitably communication of user data such as speech or other communication information. 
   The request for handover may be generated by the mobile station or the first network. Preferably the one of those entities generating the request transmits a message to the other of those entities to request the handover. 
   The mobile station may originate the second call. In that case it is preferred that the first network transmits to the mobile station data indicating an identification for the handover operation. Subsequently the mobile station may transmit to the second network data indicating that identification; and when the second call has been established the second network may transmit to the first network data indicating that identification. In response to receiving the identification in this way the first network may initiate transfer of the data communication from the first call to the second call. 
   The first network may originate the second call. In that case it is preferred that the mobile station transmits its identification in the second network (e.g. its MSISDN in the second network) to the first network and the first network uses that identification in originating the second call. 
   The geographical coverage of the second network may suitably be greater than that of the first network, at least in the region of the mobile station&#39;s location. 
   The first and second networks may suitably be cellular telephone networks. The mobile station may suitably be capable of communicating by radio with the first and second networks. The first and second calls may both be telephone calls. The mobile station may, for example, be a radio telephone. 

   
     BRIEF DESCRITPTION OF THE DRAWINGS 
     The present invention will now be described by way of example with reference to the accompanying drawings, in which: 
       FIG. 1  shows schematically the configuration of a typical cellular radio telecommunications network; 
       FIG. 2  shows coverage of two overlapping telecommunications networks; 
       FIG. 3  illustrates a handover process; 
       FIG. 4  illustrates information flow for a mobile station triggered handover with a mobile terminated call; 
       FIG. 5  illustrates information flow for a network triggered handover with a mobile terminated call; 
       FIG. 6  illustrates information flow for a mobile station triggered handover with a mobile originated call; and 
       FIG. 7  illustrates information flow for a network triggered handover with a mobile originated call. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The handover process illustrated in  FIG. 3  allows a mobile station  39  to be handed over between two telecommunications networks. In this illustration the mobile station moves from cell  40  to cell  41 . Cell  40  is the cell of base-station  42  in network NW 1 . Base-station  42  is connected to the public telephone network  43  via an MSC  44  of network NW 1 . Cell  41  is the cell of base-station  45  in network NW 2 . Base-station  45  is connected to the public telephone network  43  via an MSC  46  of network NW 2 . 
   Initially the MS  39  is in communication with BS  42  and MSC  44  by means of a call  47  using the protocol of network NW 1 . When the MS moves away from the BS  42  it reaches a zone  48  where cells  40  and  41  overlap and the MS can communicate with both BS  42  and BS  45 . Whilst the MS is in that overlap zone  48  it can be handed over from BS  42  to BS  45 . The handover can be initiated by the mobile station, for example if it detects a greater signal strength or a lesser error rate for communications with BS  45  than with BS  42 . Alternatively the handover can be initiated by the network, for example if it detects that communications between base-station  42  and mobile station  39  have a signal strength that falls below or an error rate that rises above pre-set thresholds, or if it is desired to free up capacity of base station  42  by handing the MS  39  off to BS  45 . 
   The mobile station is capable of maintaining a call with a base-station of network NW 1  at the same time as maintaining a call with a base-station of network NW 2 . Dual band mobile stations of this general type are well-known. Such mobile stations can make the normal location updates to both networks. 
   Once handover has been initiated, by the mobile station or the network, a second call  49  is established between the mobile station  39  and the MSC  46 . This call passes over network NW 1  via the base station  45 . Meanwhile the original call  47  is maintained. Thus at this stage there are two calls in progress at once from the mobile switching centre of network NW 2  to the mobile station  39 . Once the second call  49  has been established network NW 1  routes the data formerly being carried by the original call  47  over the second call  49 . Then the original call  45  can be released. The mobile station  39  then communicates with the mobile switching centre  44  of the network NW 1  to which it was originally connected, only via a base station of network NW 2 . Thus the mobile station has in effect been handed off to network NW 2 , although the call is still routed through the MSC  44  of network NW 1 . If the call passed onward to the public telephone network  43  from the MSC  44  (rather than to another mobile station in network NW 1 , for example) then this arrangement could be inefficient. To overcome this there could be Provided means for optimising the routing of the call after handover has taken place, for example by routing the call to the public telephone network directly from the MCS of network NW 2  rather than via that of NW 1 . 
     FIGS. 4 to 7  illustrate possible ways of implementing this type of handover. 
     FIG. 4  shows the signalling used for a mobile station triggered handover when mobile terminated calls are used. Initially the mobile station is in communication with network NW 1  over call  47 . When the MS  39  discovers that inter-system handover is required it sends a handover request  50  to the MSC  44  of network NW 1 . This handover request includes information that defines the identification number of the mobile station (its MSISDN) in network NW 2 . Then the MSC  44  of network NW 1  makes a new call (call  49 ) to that MSISDN number. This could be done using ISUP or TUP signalling. A request  51  for this new call reaches network NW 2  which sets up the call with the mobile MS  39  in the usual way for a mobile terminated call—for example by means of page and page_response messages  52 ,  53 . The new call  49  is then set up (at  54 ). When the new call  49  has been set up network NW 2  returns the normal message (e.g. ISUP_connect message  55 ) to indicate this fact to the network NW 1  that originated the call  49 . The network NW 1  can then issue a message (indicated as a handover_confirmation message  56 ) to the mobile station to confirm that handover can now be made. Then the mobile station and the network NW 1  connect call  47  to call  49  (at  57 ) so that user data that would formerly have been carried over call  47  is carried instead over call  49 . Once all user data is being carried over call  49  then call  47  can be released (at  58 ). Handover is then complete. 
     FIG. 5  shows the signalling used for a network triggered handover when mobile terminated calls are used. Initially the mobile station is in communication with network NW 1  over call  47 . When the network NW 1  discovers that inter-system handover is required it sends a handover request  60  to the mobile station  39 . In response to this handover request the mobile station  39  returns an acknowledgement signal (indicated as handover_request_acknowledgement signal  61 ) that includes information that defines the identification number of the mobile station (its MSISDN) in network NW 2 . Then the handover proceeds as described above in the scenario of  FIG. 4  following receipt by network NW 1  of the handover request  50 . The MSC of network NW 1  makes a new call (call  49 ) to that MSISDN number. This could be done using ISUP or TUP signalling. A request  62  for this new call reaches network NW 2 , which sets up the call with the mobile MS  39  in the usual way for a mobile terminated call—for example by means of page and page_response messages  63 ,  64 . The new call  49  is then set up (at  65 ). When the new call  49  has been set up network NW 2  returns the normal message (e.g. ISUP_connect message  66 ) to indicate this fact to the network NW 1  that originated the call  49 . The network NW 1  can then issue a message (indicated as a handover_confirmation message  67 ) to the mobile station to confirm that handover can now be made. Then the mobile station and the network NW 1  connect call  47  to call  49  (at  68 ) so that user data that would formerly have been carried over call  47  is carried instead over call  49 . Once all user data is being carried over call  49  then call  47  can be released (at  69 ). Handover is then complete. 
     FIG. 6  shows the signalling used for a mobile station triggered handover when mobile originated calls are used. Initially the mobile station  39  is in communication with network NW 1  over call  47 . When the MS  39  discovers that inter-system handover is required it sends a handover request  70  to the MSC of network NW 1 . The network NW 1  replies with a handover request acknowledgement message  71  to the mobile station. This handover request acknowledgement message includes information that defines a handover number assigned by the network NW 1  to this handover operation. The mobile station  39  then makes a new call (call  49 ) to the network NW 2 , for example by means of a CC_setup message  72 . In connection with that call the mobile station reports to the network NW 2  the handover number that it has been given. This could be done by means of the CC_setup message itself. The new call  49  is then set up (at  73 ). When the new call  49  has been set up network NW 2  returns a message (e.g. ISUP_connect message  74 ) to indicate this fact to the network NW 1  that originated the new call  49 . As part of this message or otherwise the network NW 2  also reports to the network NW 1  the handover number associated with the new call. On receiving this handover number from the network NW 2  the network NW 1  knows that the handover can be completed by means of the new call  49 . The network NW 1  can then issue a handover confirmation message  75  to the mobile station to confirm that handover can now be made. Then the mobile station and the network NW 1  connect call  47  to call  49  (at  76 ) so that user data that would formerly have been carried over call  47  is carried instead over call  49 . Once all user data is being carried over call  49  then call  47  can be released (at  77 ). Handover is then complete. 
     FIG. 7  shows the signalling used for a network triggered handover when mobile originated calls are used. Initially the mobile station  39  is in communication with network NW 1  over call  47 . When the network NW 1  discovers that inter-system handover is required it sends a handover request  80  to the mobile station  39 . This handover request includes information that defines a handover number assigned by the network NW 1  to this handover operation. The mobile station  39  then replies with a handover request acknowledgement message  81  to the network NW 1  and makes a new call (call  49 ) to the network NW 2 , for example by means of a CC_setup message  82 . In connection with that call the mobile station reports to the network NW 2  the handover number that it has been given. This could be done by means of the CC setup message itself. The new call  49  is then set up (at  83 ). When the new call  49  has been set up network NW 2  returns a message (e.g. ISUP_connect message  84 ) to indicate this fact to the network NW 1  that originated the new call  49 . As part of this message or otherwise the network NW 2  also reports to the network NW 1  the handover number associated with the new call. On receiving this handover number from the network NW 2  the network NW 1  knows that the handover can be completed by means of the new call  49 . The network NW 1  can then issue a handover confirmation message  85  to the mobile station to confirm that handover can now be made. Then the mobile station and the network NW 1  connect call  47  to call  49  (at  86 ) so that user data that would formerly have been carried over call  47  is carried instead over call  49 . Once all user data is being carried over call  49  then call  47  can be released (at  87 ). Handover is then complete. 
   One situation where this handover process could usefully be implemented is when a mobile station is being handed over from a network with incomplete geographical coverage to one with greater geographical coverage. For example, the network NW 1  from which the mobile station is being handed over could be a localised network or a network in the course of construction; the network NW 2  to which the mobile station is being handed over could be a more extensive or more established network. The networks could be of different core network types. One specific example is where one of the networks (e.g. network NW 1 ) is a 2G network such as a PDC network and the other network (e.g. network NW 2 ) is a 3G network such as an IMT-2000 network. 
   In the case of both mobile terminated calls ( FIGS. 4 and 5 ) and mobile originated calls ( FIGS. 6 and 7 ) there is no need for any change from conventional signalling in the network to which the mobile station is being handed over. This makes the handover method very convenient to implement since there is no need to modify the existing network. Thus only one of the networks needs specifically to support the handover method. In the case of mobile originated calls ( FIGS. 6 and 7 ), since the handover number could be an E.164 or E.163 number, to which the mobile station is calling, conventional signalling in network NW 2  can support the return of that number to the network NW 1 . 
   When the network NW 2  has a greater coverage than network NW 1  it is likely that the need to hand over from network NW 1  to network NW 2  will be more common than the need to hand over from network NW 2 . Thus any inability to handover easily from network NW 2  to network NW 1  (i.e. in the opposite direction from that described in detail above), for example because such handover is not supported by network NW 2 , may not be significant. 
   The present invention may include any feature or combination of features disclosed herein either implicitly or explicitly or any generalisation thereof, irrespective of whether it relates to the presently claimed invention. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention.