(12) INTERNATIONAL APPLICATION PUBLISHED UNDER THE PATENT COOPERATION TREATY (PCT)
`
`(19) World Intellectual Property
`Organization
`International Bureau
`
`I lllll llllllll II llllll lllll lllll lllll llll I II Ill lllll lllll lllll 111111111111111111111111111111111
`
`( 43) International Publication Date
`29 April 2004 (29.04.2004)
`
`PCT
`
`(10) International Publication Number
`WO 2004/036947 Al
`
`(51) International Patent Classification7:
`
`H04Q 7/38
`
`(21) International Application Number:
`PCT/EP2003/050688
`
`(22) International Filing Date: 3 October 2003 (03.10.2003)
`
`(25) Filing Language:
`
`(26) Publication Language:
`
`English
`
`English
`
`(30) Priority Data:
`02292562.2
`
`17 October2002 (17.10.2002) EP
`
`(71) Applicant (for all designated States except US): MO(cid:173)
`TOROLA INC [US/US]; 1303 E. Algonquin Road,
`Schaumburg, IL 60196 (US).
`
`(72) Inventor; and
`(75) Inventor/Applicant (for US only): AL BAKRI, Ban
`[GB/GB]; Motorola Limited, Thamesdown Drive,
`Groundwell, Swindon, Wiltshire SN25 4XY (GB).
`
`(81) Designated States (national): AE, AG, AL, AM, AT, AU,
`AZ, BA, BB, BG, BR, BY, BZ, CA, CH, CN, CO, CR, CU,
`CZ, DE, DK, DM, DZ, EC, EE, EG, ES, Fl, GB, GD, GE,
`GH, GM, HR, HU, ID, IL, IN, IS, JP, KE, KG, KP, KR,
`KZ, LC, LK, LR, LS, LT, LU, LV, MA, MD, MG, MK,
`MN, MW, MX, MZ, NI, NO, NZ, OM, PG, PH, PL, PT,
`RO, RU, SC, SD, SE, SG, SK, SL, SY, TJ, TM, TN, TR,
`TT, TZ, UA, UG, US, UZ, VC, VN, YU, ZA, ZM, ZW.
`
`(84) Designated States (regional): ARIPO patent (GH, GM,
`KE, LS, MW, MZ, SD, SL, SZ, TZ, UG, ZM, ZW),
`Eurasian patent (AM, AZ, BY, KG, KZ, MD, RU, TJ, TM),
`European patent (AT, BE, BG, CH, CY, CZ, DE, DK, EE,
`ES, Fl, FR, GB, GR, HU, IE, IT, LU, MC, NL, PT, RO,
`SE, SI, SK, TR), OAPI patent (BF, BJ, CF, CG, CI, CM,
`GA, GN, GQ, GW, ML, MR, NE, SN, TD, TG).
`
`Published:
`with international search report
`before the expiration of the time limit for amending the
`claims and to be republished in the event of receipt of
`amendments
`
`(74) Agent: LITCHFIELD, Laura; Motorola European Intel(cid:173)
`lectual Property Operations, Midpoint, Alencon Link, Bas(cid:173)
`ingstoke, Hampshire RG21 7PL (GB).
`
`For two-letter codes and other abbreviations, refer to the "Guid(cid:173)
`ance Notes on Codes and Abbreviations" appearing at the begin(cid:173)
`ning of each regular issue of the PCT Gazette.
`
`iiiiiiii ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~-
`!!!!!!!!
`(54) Title: A METHOD AND APPARATUS FOR HANDING OVER A SUBSCRIBER UNIT BETWEEN CELLULAR COM(cid:173)
`MUNICATION SYSTEMS
`
`PSTN
`COMMUNICA HON
`UNIT
`
`303
`
`- -
`
`- 2!!_1-i_ -
`
`-
`
`-
`
`-
`
`, r -
`1 1
`
`UMTS
`MOBILE
`SWITCH CENTRE
`
`213
`
`-
`
`-
`
`- LJ2...3-,
`
`GSM
`MOBilE
`SWITCH CENTRE
`221
`
`GSM
`BASE
`STATION CONTROLLER
`223
`
`1 1
`11
`11
`11
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`11
`11
`11
`11 -----
`11
`
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`
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`
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`
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`I
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`
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`
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`..........
`~ (57) Abstract: The invention relates to handover of a subscriber unit (301, 305) from a first cellular communication system (201)
`0
`to a second cellular communication system (203). The first communication system is for example a UMTS communication system
`~ supporting a plurality of connections, and the second communication system is for example a GSM communication system support-
`ing only a single connection. The handover comprises forming a handover connection (219) to the subscriber unit (301, 305) through
`0 the second communication system and handing one of the connections under the first communication system over to this connection.
`> Rather than dropping the remaining connections, they are entered into a holding state. After the handover, one of the connections on
`~ hold may be entered into an active state by associating the handover connection (219) with this connection.
`
`Ericsson v. IV II LLC
`Ex. 1021 / Page 1 of 36
`
`

`

`WO 2004/036947
`
`PCT /EP2003/050688
`
`A METHOD AND APPARATUS FOR HANDING OVER A SUBSCRIBER
`UNIT BETWEEN CELLULAR COMMUNICATION SYSTEMS
`
`5
`
`Field of the invention
`
`The invention relates to a method and apparatus for handing over a
`
`·subscriber unit between cellular communication systems and in particular
`
`to handovers from a third generation cellular communication system to a
`second generation cellular communication system.
`
`10
`
`Background of the Invention
`
`15
`
`FIG. 1 illustrates the principle of a conventional cellular communication
`
`system 100 in accordance with prior art. A geographical region is divided
`into a number of cells 101, 103, 105, 107 each of which is served by base
`station 109, 111, 113, 115. The base stations are interconnected by a fixed
`
`network which can communicate data between the base stations 101, 103,
`
`20
`
`105, 107. A mobile station is served via a radio communication link by the
`
`base station of the cell within which the mobile station is situated. In the
`example if FIG. l, mobile station 117 is served by base station 109 over
`radio link 1191 mobile station 121 is served by base station 111 over radio
`link 123 and so on.
`
`25
`
`As a mobile station moves, it may move from the coverage of one base
`
`station to the coverage of another, i.e. from one cell to another. For
`
`example mobile station 125 is initially served by base station 113 over
`
`radio link 127. As it moves towards base station 115, it enters a region of
`overlapping coverage of the two base stations 111and113 and within this
`
`30
`
`overlap region it changes to be supported by base station 115 over radio
`
`link 129. As the mobile station 125 moves further into cell 107, it
`
`Ex. 1021 / Page 2 of 36
`
`

`

`WO 2004/036947
`
`PCT /EP2003/050688
`
`continues to be supported by base station 115. This is known as a
`
`handover or handoff of a mobile station between cells.
`
`5
`
`10
`
`15
`
`A typical cellular communication system extends coverage over typically
`
`an entire country and comprises hundred or even thousands of cells
`supporting thousands or even millions of mobile stations. Communication
`from a mobile station to a base station is known as uplink, and
`communication from a base station to a mobile station is known as
`
`downlink.
`
`Currently the most ubiquitous cellular communication system is the 2nd
`
`generation communication system known as the Global System for Mobile
`
`communication (GSM). GSM uses a technology known as Time Division
`Multiple Access (TDMA) wherein user separation is achieved by dividing
`
`frequency carriers into 8 discrete time slots, which individually can be
`allocated to a user. A base station may be allocated a single carrier or a
`multiple of carriers. One carrier is used for a pilot signal which furthe1·
`contains broadcast information. This carrier is used by mobile stations for
`
`measuring of the signal level of transmissions from different base stations,
`
`20
`
`and the obtained information is used for determining a suitable serving
`
`cell during initial access or handovers. Further description of the GSM
`
`TDMA communication system can be found in 'The GSM System for
`
`Mobile Communications' by Michel Mouly and Marie Bernadette Pautet,
`Bay Foreign Language Books, 1992, ISBN 2950719007.
`
`25
`
`Currently, 3rd generation systems are being rolled out to further enhance
`
`the communication services provided to mobile users. The most widely
`
`adopted 3rd generation communication systems are based on Code Division
`Multiple Access (CDMA) wherein user separation is obtained by allocating
`
`30
`
`different spreading and sc1·ambling codes to different users on the same
`
`car1·ier frequency. The transmissions are spread by multiplication with the
`
`allocated codes thereby causing the signal to be spread over a wide
`
`2
`
`Ex. 1021 / Page 3 of 36
`
`

`

`WO 2004/036947
`
`PCT /EP2003/050688
`
`bandwidth. At the receiver, the codes are used to de-spread the received
`
`signal thereby regenerating the original signal. Each base station has a
`code dedicated for a pilot and broadcast signal, and as for GSM this is
`
`5
`
`used for measurements of multiple cells in order to determine a serving
`cell. An example of a communication system using this principle is the
`Universal Mobile Telecommunication System (UMTS), which is currently
`being deployed. Further description of CDMA and specifically of the
`Wideband CDMA (WCDMA) mode of UMTS can be found in 'WCDMA for
`UMTS', Harri Holma (editor), Antti Toskala (Editor), Wiley & Sons, 2001,
`
`10
`
`ISBN 0471486876.
`
`In a UMTS CDMA communication system, the communication network
`
`comprises a core network and a Radio Access Network (RAN). The core
`
`network is operable to route data from one part of the RAN to another, as
`
`15 well as interfacing with other communication systems. In addition, it
`performs many of the operation and management functions of a cellular
`communication system, such as billing. The RAN is operable to support
`wireless user equipment over a radio link being part of the air interface.
`The RAN comprises the base stations, which in UMTS are known as Node
`
`20 Bs, as well as Radio Network Controllers (RNC) which control the Node Bs
`and the communication over the air interface.
`
`Whereas the GSM system was originally developed with a view to mainly
`
`supporting voice services, UMTS has from the outset been developed to
`
`25
`
`provide a wide plurality of different services including different voice
`
`communication services and data services. Different services can be set up
`
`having different characteristics and specifically with different Quality of
`
`Service parameters such as different delays, data rates, target error rates
`etc. Further, UMTS provides for a plurality of services and connections to
`
`30
`
`be set up between different mobile stations.
`
`3
`
`Ex. 1021 / Page 4 of 36
`
`

`

`WO 2004/036947
`
`PCT /EP2003/050688
`
`The deployment ofUMTS will initially be in islands of coverage wherein
`UMTS base stations provide coverage of selected areas and regions.
`However, it is expected that most UMTS systems will not provide full or
`extensive coverage for some time, and therefore it is planned that the gaps
`in UMTS coverage will be covered by 2nd Generation Systems such as
`
`5
`
`GSM. Consequently, multimode mobile stations capable of operating with
`
`both GSM and UMTS are being developed as is handover methods
`between the two systems.
`
`10 However, GSM provides limited services in comparison to UMTS and the
`
`handover methods therefore comprise reducing the services and quality of
`
`service provided to a user when handing over to GSM. Hence, the
`
`handover causes a reduced service level for a user therefore a system for
`handov~rs providing improved support of services, functionality and/or
`performance would be an advantage.
`
`15
`
`Summary of the Invention
`
`20 Accordingly the Invention seeks to mitigate, alleviate or eliminate one or
`
`more of the above mentioned disadvantages singly or in any combination.
`
`Accordingly, there is provided a method of handing over a subscriber unit
`
`from a first cellular communication system. supporting a plurality of
`
`25
`
`connections of the subscriber unit to a second cellular communication
`
`system; the method comprising the steps of: entering .at least a first
`
`connection of said plurality of connections into a holding state; forming a
`
`handover connection to the subscriber unit through the second
`
`communication system; handing over a second connection of said plurality
`
`30
`
`of connections to the second cellular communication system by associating
`
`the second connection with said handover connection; entering said at
`
`4
`
`Ex. 1021 / Page 5 of 36
`
`

`

`WO 2004/036947
`
`PCT /EP2003/050688
`
`least first connection i11to an active state by associating the at least first
`connection with the handover connection.
`
`5
`
`10
`
`An advantage provided by the invention is that upon handover from a first
`to a second communication system, the handover can be performed for just
`a single connection w bile still maintaining all connections of the first
`communication system. This is especially advantageous when the second
`communication system is only able to support one connection, as it allows
`
`for the remaining connections to be maintained rather than dropped. The
`
`connections put into a holding state can be put into an active state using
`the handover connection and thus without requiring more than one
`connection in the second cellular communication system. If the subscriber
`unit is subsequently handed back to the UMTS communication system, it
`allows for the connections cm·rently in a holding state to immediately be
`
`15
`
`re·established. Hence, an improved se1·vice is provided to the subscriber
`
`units in a dual cellular communication system which efficiently
`accommodates for the different capabilities, services and performance of
`
`the different communication systems.
`
`20 According to a different feature of the invention, the step of entering said
`
`at least first connection into an active state comprises switching the
`
`handover connection from being with the second connection to being with
`the at least first connection. Switching between connections provide for a
`
`simple and efficient implementation suitable for cellular communication
`
`25
`
`systems.
`
`According to another feature of the invention, the method further
`comprises the step of selecting the second connection from the plurality of
`
`connections in response to at least one characteristic of at least one of the
`
`30
`
`plurality of connections. Advantageously, the connection having a
`
`characteristic most suited for being associated with the handover
`
`connection can thus be selected as the second connection.
`
`5
`
`Ex. 1021 / Page 6 of 36
`
`

`

`WO 2004/036947
`
`PCT /EP2003/050688
`
`According to another feature of the invention, the characteristic is related
`
`to a type of connection and preferably the step of selecting comprises
`
`5
`
`selecting a data service connection in preference to a voice service
`connection as the second connection. This provides the advantage of
`minimising the service impact by putting connections into a holding stage
`
`as a voice service typically can be continued after the connection has been
`
`entered into the active state without unacceptable inconvenience.
`
`10 According to another feature of the invention, the at least one
`characteristic comprises at least one characteristic chosen from the group
`consisting of: a priority; a data rate; a propagation characteristic; an error
`
`rate; a transaction identifier; and a time of setup of at least one of the
`
`plurality of connections. These parameters provide efficient and practical
`parameters for selection the second connection.
`
`15
`
`According to another feature of the invention, the at least first connection
`is a data connection and the method comprises the steps of storing data of
`
`20
`
`the at least first connection in memory when the at least first connection
`is in the holding state; and communicating the data stored in said memory
`when the at least first connection enters the active state. This allows for a
`
`data service to be put on hold without any loss of data. The data source for
`
`the data service need not stop the data transmission or be informed of the
`
`holding state as the data communicated is buffered and forwarded to the
`
`25
`
`subscriber unit when the data connection ente1·s the active state.
`
`According to anothe1· feature of the invention, the at least first connection
`is a data connection and the method comprises the steps of sto1·ing data of
`the at least first connection in memory when the at least first connection
`
`30
`
`is in the holding state; and the subscriber unit retrieving the stored data
`
`from the memory by setting up a separate data call. This allows for a data
`service to be put on hold without any loss of data. The data source for the
`
`6
`
`Ex. 1021 / Page 7 of 36
`
`

`

`WO 2004/036947
`
`PCT /EP2003/050688
`
`data service need not stop the data transmission or be informed of the
`
`holding state as the data communicated is stored and retrieved by the
`subscriber unit. The user of a subscriber unit may have control of when to
`
`receive the stored data and this does not require an active connection.
`
`According to another feature of the invention, the method further
`
`comprises the step of notifying a user of the subscriber unit of which of the
`plurality of connections are in a holding state. Thereby the user is
`
`informed of the current status and thus may take appropriate action.
`
`According to another feature of the invention, at least one of the plurality
`of connections is between the subscriber unit and a second communication
`
`unit and the method further comprises the step of notifying a user of the
`second communication unit of which of the plurality of connections are in a
`holding state. Advantageously, the user of the second communication
`
`system is informed of the current status and thus may take appropriate
`
`action
`
`According to another feature of the invention, the notification is by means
`of a voice communication if at least one of the plurality of connections is a
`voice service connection. This is provides an efficient means for
`
`notification which is easily interpreted by a user.
`
`According to another feature of the invention, the step of selecting the first
`connection is in response to a parameter set by an operator of at least one
`of the first or second cellular communication systems. Advantageously, the
`
`operator may control parameters influencing the selection of the second
`
`connection thereby allowing the operator increased control over the
`
`communication systems and services provided.
`
`According to another featui·e of the invention, the step of selecting the
`second connection is in response to a parameter set by a user of the
`
`5
`
`10
`
`15
`
`20
`
`25
`
`30
`
`7
`
`Ex. 1021 / Page 8 of 36
`
`

`

`WO 2004/036947
`
`PCT /EP2003/050688
`
`subscriber unit. Advantageously, the user is thus provided with means for
`
`influencing which connection should initially be in an active state.
`
`5
`
`According to another feature of the invention, if the handover to the
`second cellulal' communication system is unsuccessful at least one of the
`plurality of connections is re-established through the first communication
`
`system. Hence, the advantage of increased resilience and reduced dropped
`
`connection rates is achieved as a connection is only dropped if the
`handovei· to the second communication system fails and no support for the
`connection is available in the first communication system.
`
`According to another feature of the invention, the second cellular
`
`communication system comprises a master switch center comprising
`functionality for selecting the second connection out of the plurality of
`connections. This provides for a very efficient implementation suitable for
`most cellular communication systems.
`
`According to another feature of the invention, the method is operated in a
`
`single integrated master switch centre for the first cellular communication
`system and the second cellular communication system. This provides for
`the advantage of allowing for an efficient and relatively low complexity
`implementation wherein the characteristics and functionality of both
`communication systems can be considered and utilised.
`
`The second communication system may be operable to only support one
`connection for each served subscriber unit and the connections are
`preferably circuit switched connections.
`
`10
`
`15
`
`20
`
`25
`
`Preferably, the. first cellular communication system is a Third Generation
`30 Cellular Communication System such as a Universal Mobile
`Telecommunication System (UMTS).
`
`8
`
`Ex. 1021 / Page 9 of 36
`
`

`

`WO 2004/036947
`
`PCT /EP2003/050688
`
`Preferably the second cellular communication system is a Second
`
`Generation Cellular Communication System such as the Global System for
`
`Mobile communication (GSM) cellular communication system.
`
`5 According to a second aspect of the invention, there is provided an
`
`apparatus for handing over a subscriber unit from a first cellular
`communication system supporting a plurality of connections of the
`subscriber unit to a second cellular communication system; the apparatus
`comprising: means for entering at least a first connection of said plurality
`of connections into a holding state; means for forming a handover
`
`10
`
`connection to the subscriber unit through the second communication
`
`system; means for handing over a second connecti~n of said plurality of
`
`connections to the second cellular communication system by associating
`
`the second connection with said handover connection; means for entering
`
`15
`
`said at least first connection into an active state by associating the at
`
`least first connection with the handover connection.
`
`Brief Description of the Drawings
`
`20
`
`An embodiment of the invention will be described, by way of example only,
`with reference to the drawings, in which
`
`FIG. 1 is an illustration of a cellular communication system in accordance
`25 with the prior art;
`
`FIG. 2 is an illustration of an architecture of two cellular communication
`
`systems to which an embodiment of the invention may apply; and
`
`30
`
`FIG. 3 is an illustration of handover situation to which a prefened
`
`embodiment of the invention may be applied.
`
`9
`
`Ex. 1021 / Page 10 of 36
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`

`

`WO 2004/036947
`
`PCT /EP2003/050688
`
`Detailed Description of a Preferred Embodiment of the Invention
`
`The invention will in the following be described with specific reference to
`
`5
`
`handovers between the UMTS communication system standardised by the
`
`Third Generation Partnership Project (3GPP) and the GSM
`communication system standardised by the European Telecommunication
`Standards Institute (ETSI), but it will be apparent that the invention is
`not limited to this application but is equally applicable to many other
`
`10
`
`communication systems and handovers.
`
`FIG. 2 is an illustration of architectures of cellular communication
`
`systems 201, 203 to which an embodiment of the invention may apply.
`
`15 A first communication system 201 is a UMTS communication system
`
`supporting a plurality of UMTS subscriber units. The second
`
`communication system 203 is a GSM communication system supporting a
`plurality of GSM subscriber units. In addition both communication
`
`systems 201, 203 are able to support a number of multi mode subscriber
`units comprising both GSM and UMTS functionality. A subscriber unit
`
`20
`
`may typically be a wireless user equipment, a mobile station, a
`
`communication terminal, a personal digital assistant, a laptop computer,
`
`an embedded communication processor or any communication element
`
`communicating over the ail' interface.
`
`25
`
`The UMTS communication system comprises a UMTS Mobile Switch
`
`Centre (MSC) 205 which among other things is in charge of switching the
`
`incoming data streams from different subscriber units to the appropriate
`output streams for the ca'll destination. Specifically, the MSC 205 may
`
`30
`
`switch the data to another MSC. Further, the MSC 205 is, in the example
`
`shown, a Gateway· MSC comprising a gateway interface for interfacing to
`
`an external network which in this case is a Public Standard Telephone
`
`10
`
`Ex. 1021 / Page 11 of 36
`
`

`

`WO 2004/036947
`
`PCT /EP2003/050688
`
`Network (PSTN) 207. The MSC furthe1· comprises functionality for
`
`performing various maintenance and operational functions including
`
`authentication and billing.
`
`5 The UMTS MSC 205 is connected to a plUl'ality of UMTS Radio Network
`controllers (RNCs) 209 of which one is shown in FIG. 2. The RNC 209 is
`
`connected to a number of UMTS base stations, which are known as Node
`
`Bs 211, 213, of which two are shown in FIG. 2. The RNC 209 performs
`
`many of the control functions related to the air interface of the UMTS
`
`10
`
`communication system 201 including radio resource management and
`muting of data to and from appropriate Node Bs 211, 213. An RNC and
`associated Node Bs is known as a Radio Network System (RNS).
`
`The Node Bs 211,213 comprise much of the functionality required for
`
`15
`
`supporting subscribe1· units over the radio air inte1·face. As such, the Node
`
`Bs comprise radio transmitters, receivers and the required control
`
`circuitry for communicating with a plurality of subscriber units. FIG. 2
`
`illustrates three UMTS subscriber units 215, 217, 219 communicating
`
`with one of the Node Bs 213. The communication is by means of radio
`
`20
`
`communication in accordance with the Technical Specifications for the
`
`UMTS ~ommunication system. Typically, each Node B will support a high
`
`number of subscriber units simultaneously.
`
`The GSM communication system 203 comprises a GSM Mobile Switch
`
`25
`
`Centre (MSC) 221 which generally performs the same functions in the
`
`GSM communication system 203 as the UMTS MSC 205 performs for the
`UMTS communication system 201. It thus provides switching, operations
`
`and maintenance functions as well as interfacing to external
`
`communication systems.
`
`30
`
`The GSM MSC 221 is connected to a GSM Base Station Controller (BSC)
`
`223 which provides control functionality for the base stations including
`
`11
`
`Ex. 1021 / Page 12 of 36
`
`

`

`WO 2004/036947
`
`PCT /EP2003/050688
`
`channel allocation, channel configuration management and handover
`control. The BSC 223 is connected to a plurality of GSM base stations 225,
`227 known as Base Transceiver Stations (BTSs) in GSM. The BTSs 225,
`
`227, of which two are shown in FIG. 2, comprise the functionality required
`
`5
`
`for communicating over the GSM air interface to GSM subscriber units
`
`including the required radio transmitters, receivers and control
`
`functionality. In the example of FIG. 2, each of the illustrated BTSs 225,
`227 are shown to communicate with a single GSM subscriber unit 229,231,
`but typically each BTS 225, 227 simultaneously communicate with a high
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`number of subscriber units.
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`In the example of FIG. 2, one of the UMTS subscriber units 219 is a dual
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`mode subscriber unit which is operable to communicate according to both
`the UMTS and the GSM communication standards. If the dual mode
`subscriber unit 219 moves out of the coverage area of the supporting Node
`B 213 it will attempt to handover to another Node B, and thus remain on
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`the UMTS communication system 201. However, it may be preferred, for
`example because the dual mode subscriber unit 219 is moving into an area
`
`with insufficient UMTS coverage, to handover to a GSM BTS. In this case,
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`it will be attempted to set up a suitable communication link between the
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`dual mode subscriber unit 219 and a GSM BTS 225 as illustrated by the
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`connection 238 in FIG. 2.
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`However, as the two communication systems 201, 203 are not identical
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`they have different characteristics and capabilities. In particular, the GSM
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`cellular communication system 203 does not provide the same variety and
`performance of services as can be achieved in a UMTS communication
`system 201. Specifically, it is possible to set up a plurality of connections
`to a subscriber unit in a UMTS cellular communication system. Hence, in
`30 UMTS, each connection is terminated in the subscriber unit. The plurality
`
`of connections may have different characteristics, and thus different
`
`services may simultaneously be set up for a subscriber unit including for
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`example a mix of voice and data services that may further have different
`characteristics. However, in GSM only one physical connection is allowed
`for each subscriber unit on the interface between the GSM MSC 221 and
`the BBC 223, and therefore only one simultaneous connection is set up to
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`the dual mode subscriber unit 219 through the GSM communication
`system 203.
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`In accordance with a preferred embodiment of the invention, a subscriber
`
`unit 219 having a plurality of associated connections is handed over from a
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`first cellular communication system, such as the UMTS communication
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`system 201, to a second cellular communication system, such as the GSM
`communication system 203, by the performing the steps of entering at
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`least a first connection of said plurality of connections into a holding state;
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`forming a handover connection to the subscriber unit 219 through the
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`second communication system 203; handing over a second connection of
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`said plurality of connections to the second cellular communication system
`203 by associating the second connection with said handover connection;
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`and entering the at least first connection into an active state by
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`associating the at least first connection with the handover connection.
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`FIG. 3 is an illustration of handover situation to which a preferred
`
`embodiment of the invention may be applied. For clarity and brevity, only
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`the elements of the cellular communication systems of FIG. 2 which are
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`directly involved in supporting a specific subscriber unit are shown. In the
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`example of FIG. 3, a subscriber unit 301 is communicating over three
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`circuit switched connections with a second communication unit 303. The
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`second communication unit 303 is in this example a PSTN communication
`unit, which is connected to the UMTS cellular communication system
`
`through the PSTN Gateway 207. The second communication unit 303 may
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`30
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`thus for example be a bridge confe1·ence system or in the specific example
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`may be a network unit of the PSTN supporting three separate telephone
`
`connections.
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`It will be apparent, that the second communication unit 303 could be any
`suitable communication unit and could be associated with the cellular
`communication system in any suitable way. As such, the second
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`communication unit could be a communication unit associated with a
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`different external network, such as the internet, or could be associated
`with the first communication system, such as for example a second UMTS
`
`subscriber unit. Further, it will be apparent that instead of a single second
`communication unit, the subscriber unit 301 could be in communication
`10 with a plurality of communication units, and that these further could be
`different communication units having different characteristics,
`capabilities and associations with the cellular communication system(s).
`As such, the second communication unit 303 could be associated with the
`second communication system.
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`20
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`25
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`In the specific example of FIG. 3, the subscriber unit 301 is
`communicating through three circuit switched voice connections. Hence,
`
`the subscriber unit 301 is simultaneously supporting three voice services.
`
`For example, the subscriber unit 301may in this case be a conference
`bridge unit that maintains a conference between four users through the
`three connections. In order to support the three circuit switched
`connections, at least three connections are established for the subscriber
`
`unit 301 between the subscriber unit 301 and the serving Node B 213,
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`between the serving Node B 213 and the serving RNC 209, between the
`
`serving RNC 209 and the UMTS MSC 205, between the UMTS MSC 205
`and the PSTN Gateway 207 and between the PSTN gateway 207 and the
`PSTN communication unit 303.
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`In the present example, the subscriber unit 301 moves out of the coverage
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`30
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`area of the UMTS communication system 201. In the illustrated example,
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`the subscriber unit 301 is a dual mode subscriber unit and therefore it
`may be attempted to set up a communication with the GSM
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`communication system 203. For this reason, the subscriber unit 301
`
`measures the pilot signals (such as the BCCH broadcast transmissions) of
`the neighbouring GSM BTSs in the same way as a conventional GSM
`mobile station does. In addition, the subscriber unit 301 measures beacon
`signals of the neighbouring UMTS Node Bs. These measurements are
`reported back to the UMTS cellular communication system, wherein it is
`determined if a handover is necessary, and if so to which base station the
`handover should be directed to. If the reported measurements indicate
`
`that no suitable UMTS handover candidate is available but that the
`subscriber unit may be supported by a GSM BTS, a handover is made to
`this BTS. The selection of which base station to handover to is preferable
`
`determined in the same manner as for a standard GSM mobile station.
`Specifically the base station being reported with the highest measured
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`BCCH signal level is chosen.
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`Whe