0)
`
`Europaisches Patentamt
`
`European Patent Office
`
`Office européen des brevets
`
`@) Publication number:
`
`0 660 626 A2
`
`EUROPEAN PATENT APPLICATION
`
`©®|®©© Priority: 22.12.93 GB 9326189
`
`Application number: 94308827.8
`
`Date offiling: 30.11.94
`
`® Date of publication of application:
`28.06.95 Bulletin 95/26
`
`® Designated Contracting States:
`DE FR GB IT SE
`
`Applicant: NOKIA MOBILE PHONESLTD.
`P.O. Box 86
`
`SF-24101 Salo (Fl)
`
`&) Multi-mode radio telephone.
`
`@ int. cis, HO4Q 7/32, HO4M 1/72
`
`
`
`®) Inventor: Byrne, John Daniel
`3 Durrell Way
`Shepperton,
`Middlesex, TW17 8HR (GB)
`
`Representative: Frain, Timothy John
`Patent Department
`Nokia Mobile Phones
`
`St Georges Court
`St Georges Road
`Camberley, Surrey GU15 3QZ (GB)
`
`@) A radio telephone capable of being operated in
`error rate and user velocity. The radio telephone can
`be used in a radio telephone system adapted to co-
`more than one radio telephone system, comprising
`operate with at least one other radio telephone sys-
`communication means respectively associated with
`each of the more than one radio telephone system,
`tem, comprising user information exchange means
`respectively associated with each of the at least one
`monitoring means for monitoring signals of the more
`than one radio telephone system, and_selection
`other radio telephone system for exchanging user
`means
`responsive to said monitoring means for
`information signals between the radio telephone sys-
`fem and the at
`least one other
`radio telephone
`automatically selecting and re-selecting respective
`said communication means in accordance with the
`system, monitoring means for monitoring user in-
`formation signals of the radio telephone systems and
`selection means for automatically assigning and re-
`assigning a user to one of the radio telephone sys-
`tems in accordance with the user control signals
`fulfilling at least one predetermined criterion.
`
`signals of one of the more than one radio telephone
`system fulfilling at least one predetermined criterion.
`The predetermined criterion can be based on a
`number of features such as received signal strength,
`access rights, system requests, least BER or frame
`
`CELLULAR
`AUDIO
`
`CORDLESS
`CELLULAR
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`
`
`RECEIVER
`RECEIVER
`CELLULAR
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`CY CORDLESS
`
`TRANSMITTER
`TRANSMITTER
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`
`
`
`SERVICE
`AVAILABLE
`
`
`
`REGISTER:
`
`aut
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`MICRO-
`PROCESSOR
`
`
`
`
`
`2§aQ
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`EP0660626A2
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`SAMSUNG 1027
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`SAMSUNG 1027
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`EP 0 660 626 A2
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`invention relates to a radio tele-
`The present
`phone, and in particular fo a radio telephone op-
`erable for more than one system.
`Over the last ten years there has been a rapid
`growth in the use and availability of radio telephone
`systems. As part of this growth there has been a
`proliferation of different types of
`radio telephone
`system, offering the user a variety of services,
`geographical coverage and cost. Many of
`these
`different radio telephone systems cover the same,
`or a part of the same, geographical area as each
`other.
`
`radio systems operate on
`Typically, different
`different radio frequencies, utilise different modula-
`tion techniques, signaling techniques and intra-sys-
`tem protocols etc.
`to each other. Thus, a radio
`telephone designed for one system is generally
`unable to be used on another system. Thus if a
`user wishes to have access to more than one
`
`is necessary to have either more than
`system it
`one radio telephone or a radio telephone capable
`of operating in more than one system. Having more
`than one radio telephone is inconvenient for the
`user. Known radio telephones capable of operating
`in more than one system typically consist of little
`more than 2 separate phones combinedin a signal
`housing. The preference for operating in a particu-
`lar system is user defined as disclosed in US
`patent number 4 989 230.
`A particularly useful and appropriate environ-
`ment
`for multi-mode radio telephones is the re-
`cently available cellular and cordless telephone
`systems.
`In the prior art, cordless telephones are
`typically used in the home and office to allow the
`user
`to place and receive calls at any point
`throughout the house via an RF link with a home
`base station located within the house or office.
`
`Such cordless telephones are connected via the
`home base station to the user's telephone landline
`which in turn is connected to the Public Switched
`
`there are
`Telephone Network (PSTN). Further,
`known second generation cordless telephone sys-
`tems such as CT-2 or DECT which are digital
`systems. Such CT-2 or DECT systems extend be-
`yond conventional domestic operation of cordless
`telephones by allowing the user to establish an RF
`link between a CT-2 or DECT radio telephone and
`a base station in a more publicly accessible loca-
`tion e.g. outside of
`the user's home, a railway
`station, shopping mall or airport. Such base sta-
`tions are known as telepoint base stations and are
`linked to the PSTN in much the same way as a
`home base station. Some cordless, and in particu-
`lar DECT radio telephones, are now able to receive
`calls via telepoint base stations whereas hitherto
`they were only able to place them. A description of
`such a system can be found in PCT international
`patent application WO 92/03006. Thus, placing and
`
`receiving calls whilst geographically roaming is
`possible in cordless telephone systems.
`However, cordless telephone systems are low
`power systems and each base station provides
`telecommunications within only approximately a
`150 metre radius of the base station, dependent
`upon the terrain and any man-made objects which
`could interfere with signalling between a cordless
`telephone and the base station. Such systems are
`generally only used in areas of high user density
`and thus tend to be limited to urban areas. This
`
`clearly restricts the geographical mobility of a CT-
`2, DECTor the like cordless telephone user.
`In the prior art there are known so called cel-
`lular radio telephone systems having base stations
`which cover a wide geographical area (cell), eg 35
`km diameter. However,
`in urban areas where the
`user density is high the cells are much smaller and
`operate at lower powersto facilitate frequency re-
`use thereby increasing the communication channel
`density.
`Cellular systems have advantages over cord-
`less systems since they allow a user to place and
`receive calls over a large area. Additionally they
`are suitable for use in moving vehicles. This is
`because cellular telephone systems have sophisti-
`cated handover procedures to facilitate switching
`betweencells as a user's vehicle crosses from one
`
`the cells are larger
`to another. Furthermore,
`cell
`than in cordless systems and thus handovers occur
`less often , even if
`travelling in a vehicle. This
`ensures continuity of service and is particularly
`important during a call. However,
`the cost of a
`cellular telephone call
`is significantly greater than
`the cost of a cordless telephone call, since cord-
`less telephone calls are made by wayof the user's
`land line PSTN link and cost the same as landline
`
`calls, whilst cellular telephone calls are made by
`way of expensive cellular base stations and cellular
`switching equipment and cost much more than
`land line telephone calls.
`In order for a user to be able to utilise both
`
`cellular and cordless telephone systems via a sin-
`gle radio telephone handset a so-called cellular
`cordless telephone (CCT) has been proposed in
`US patent US 4 989 230. Both the cellular system
`and the cordless system are monitored by the CCT
`for incoming calls and the CCT automatically en-
`ters a user defined preferred mode for answering
`an incoming call
`if
`the system corresponding to
`that mode is available. When placing a call
`the
`userinitiates the call as either cellular or cordless
`
`In
`and the CCT connects the call appropriately.
`order for the user to be able to make a choice the
`
`available systems can be displayed on the CCT.
`When using the CCT,
`the user has to make the
`decision as to which telephone system is used.
`Furthermore, the prior art CCT disclosed in US 4
`
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`EP 0 660 626 A2
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`989 230 requires the user to select transfer of a
`cordless call
`to the cellular system should the
`cordless signals deteriorate (eg the user moves out
`of range of a cordless base station or the user's
`velocity increases). Additionally, optimisation of the
`available systemsis likely not to be achieved if the
`user is left to decide which system is to be used.
`One of the disadvantages of the CCT disclosed in
`US 4 989 230 is that a user might not know if a
`signal
`is deteriorating until
`it
`is too late and the
`ongoing call is lost. For example, a visual indication
`of poor signal shown on the CCT is likely to be
`missed by a user, since in use the CCT will be
`placed at
`the side of
`the user's head and thus
`visual
`indications would not be visible to the user.
`Other indications such as an audible tone or alarm
`
`would interfere with the user's ongoing call, and
`would be annoying andirritating to the user.
`A first aspect of the present invention provides
`a radio telephone operable in more than one radio
`telephone
`system,
`comprising
`communication
`means respectively associated with each of
`the
`more than one radio telephone system, monitoring
`means for monitoring signals of the more than one
`radio telephone system, and selection means re-
`sponsive to said monitoring means for automati-
`cally selecting and re-selecting respective said
`communication means in accordance with the sig-
`nals of one of the more than one radio telephone
`system fulfilling at
`least one predetermined cri-
`terion, and a second aspect of the invention pro-
`vides a method for operating a radio telephone in
`more than one radio telephone system, comprising,
`monitoring signals of the more than one radio tele-
`phone system determining whether the signals of
`the more than one radio telephone system fulfil at
`least one predetermined criterion, and automati-
`cally selecting and re-selecting for which of
`the
`more than one radio telephone system the radio
`telephone is operable in accordance with the sig-
`nals fulfilling the at
`least one predetermined cri-
`terion.
`
`These have the advantage that the radio tele-
`phoneis not only operable for more than one radio
`telephone system, but that it
`is able to automati-
`cally select and re-select which of
`the available
`radio systems fo use. The selection can be based
`on any predetermined criterion or combination of
`criteria. For example, the radio telephone can be
`set to operate on the cheapest system available
`(e.g. cordless system). Such criteria are likely to be
`factory set, but optionally could be user program-
`mable. A further advantage is that the radio tele-
`phone user can know of incoming calls on systems
`other than the one for which the radio telephone is
`presently operable, and can manually switch to
`operate in the appropriate system thereby overrid-
`ing the automatic selection. Of course,
`if the radio
`
`telephoneis already in a call then an indication that
`there is an incoming call can be made to the user
`(eg call waiting indication). Such a system would
`be of particular benefit if incoming calls from sys-
`tems other than the one in which the radio tele-
`
`phone is currently operable were not forwarded to
`that system. An additional advantage is that auto-
`matic inter system handover would be possible,
`based on the predetermined criterion. For example,
`if the user were travelling out of a system service
`area and the service breaks down, a radio tele-
`phone operable in accordance with the present
`invention may automatically handover to a system
`having a good service (e.g. cordless to cellular).
`Similarly,
`if a user was engagedin a call on a high
`cost system and a low cost system became avail-
`able such a radio telephone may automatically
`handoverthe call to the low cost system. Examples
`of inter-system handover are disclosed in co-pend-
`ing British Patent Applications No. 9320814.8, No.
`9320815.8 and No. 9326169.1.
`
`the first and
`In a preferred embodiment of
`second aspects of the invention there is provided a
`visual
`indication of the one of the more than one
`
`radio telephone system. This has the advantage
`that the user is informed of what radio systems are
`available. Thus,
`if the only radio systems available
`are ones which the user does not wish to use, the
`radio telephone can be turned off. This would avoid
`the problem of receiving calls on particularly ex-
`pensive radio systems, if these radio systems were
`the only onesavailable.
`Preferably, at least one predetermined criterion
`is a user indicated preference for one of said more
`than one radio telephone system. This has the
`advantage that
`the user can pre-programme the
`radio telephone to select certain types of
`radio
`system, eg the cheapest available. Alternatively,
`the user indicated preference could be a manually
`indicated preference during operation of the radio
`telephone. Such a manually indicated preference
`could override
`any predetermined criteria and
`would give the user
`instantaneous control over
`which system the radio telephone is operable in at
`a given momentof time.
`Suitably, at
`least one predetermined criterion
`can be one or a combination of the following re-
`quirements that the selected radio system is one
`for which:
`
`i) the received signal strength at the radio tele-
`phone is greatest;
`ii) the radio telephone has accessrights;
`iit) the selected radio telephone system requests
`selection:
`iv) the bit error rate, frame error rate or the like
`is the lowest; and
`v) the user velocity is appropriate.
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`These have the advantages in that the system
`for which there is a high probability of successful
`communication can be selected. Furthermore, the
`radio telephone can be set to select only those
`systems for which it has access rights thereby
`avoiding possible extra high costs for using a ser-
`vice for which no prior access arrangements have
`been mace.
`
`nals fulfil at least one predetermined criterion, and
`automatically assigning and re-assigning a user to
`one of the radio telephone systems in accordance
`with the user
`information signals fulfilling the at
`least one predetermined criterion.
`These have the advantage that users are auto-
`matically assigned or reassigned to radio telephone
`systems dependent upon certain criteria being ful-
`filled. These criteria can be set by the radio sys-
`tems themselves, or can be based on instructions
`from users. Since the radio telephone system ex-
`changes user information between itself and other
`radio systems,
`the assigning and re-assigning of
`users to particular radio telephone systems can be
`madeeasily and quickly.
`Additionally, calls from users can be routed
`through different radio telephone systems based on
`the predetermined criteria and the user information.
`Figure 1
`is a block diagram of a cellular cord-
`less telephone system embodying the present
`invention;
`Figure 2 is a block diagram of a cellular cord-
`less telephone embodying the present invention;
`Figure 3 is a flow diagram showing the oper-
`ational steps taken under control of the micro-
`processor fo monitor for system availability and
`connect the CCT to an appropriate system;
`Figure 4 is a flow diagram showing the oper-
`ational steps taken under control of the micro-
`computerwhilst a call is in progress; and
`Figure 5 is a block diagram of a combined
`DECT/GSM network.
`
`a community cordless base station and such base
`stations may be located throughout an urban area,
`or common user area such as a railway station,
`shopping mall or airport,
`for providing a shared
`telephone service to CCTs 200.
`In such a case, the
`cordless base station 118 may include additional
`
`4
`
`the monitoring means moni-
`Advantageously,
`tors the signals intermittently. Thus, less power is
`consumedbythe radio telephone's monitoring pro-
`cess which results in prolonged battery life. Fur-
`thermore, signals required to carry out the monitor-
`ing process are used less often which allows more
`time for the radio telephone to process signals for
`the currently operating radio system,
`in particular
`when a call
`is in progress. Intermittent monitoring
`is particularly effective during periods when no
`calls are in progress.
`Typically, the radio system in which the radio
`telephone is operating will be a TDMA system and
`it would be advantageous if the monitoring process
`were carried out during a period of TDMA inactivity
`(eg unused slots). Thus, there would be none or at
`least reduced interference with the TDMA system
`signals. Additionally there is the benefit
`that
`the
`same components may be utilised for more than
`one terminal mode and so cost/size savings can be
`made. Both TDMA sharing of receiver functions
`between the systems and also transmitter functions
`can be utilised. By sharing transmitter functions,
`communications with a second system maystart
`Specific embodiments of the invention will now
`before releasingafirst system.
`In a third aspect of the invention there is pro-
`be described, by way of example only, and with
`vided a radio telephone system adapted to co-
`reference to the above described drawings.
`operate with at
`least one other
`radio telephone
`Figure 1
`illustrates a block diagram of a cellular
`cordless telephone system 100 which embodies
`system,
`comprising user
`information exchange
`means respectively associated with each of the at
`the present invention. The system includes a Pub-
`least one other
`radio telephone system for ex-
`lic Switched Telephone Network (PSTN) 117, con-
`changing user information signals between the ra-
`nected by landlines to cordless base stations 114,
`dio telephone system and the at least one other
`116 and 118 having respective landline telephone
`numbers and respectively located in an office
`radio telephone system, monitoring means for
`monitoring user
`information signals of
`the radio
`building 110, domestic residence 120 or some oth-
`telephone systems and selection means for auto-
`er geographical location.
`Cordless base stations 114, 116 and 118 com-
`matically assigning and re-assigning a user to one
`of the radio telephone systems in accordance with
`municate with the cellular cordless telephone (CCT)
`200 via antennas 112, 119 and 122. Antennas 112,
`the user control signals fulfilling at least one pre-
`determined criterion, and in a fourth aspect of the
`119 and 122 may be implemented as any sort of
`suitable antenna such as a whip antenna, helical
`invention there is provided a method for operating
`a radio telephone system adapted to co-operate
`antenna or printed circuit antenna. The cordless
`with at
`least one other
`radio telephone system,
`base stations 114 and 116 may be a conventional
`cordless base station. Cordless base station 118 is
`comprising, exchanging user
`information respec-
`tively associated with each of the at least one other
`radio telephone system between the radio tele-
`phone system and the at
`least one other radio
`telephone system, monitoring user information sig-
`nals exchanged between the radio telephone sys-
`tems, determining whether the user information sig-
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`EP 0 660 626 A2
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`equipment not usually found in conventional cord-
`less base stations for billing calls to a telephone
`number of CCT 200.
`
`Also connected through telephonelandlines to
`the PSTN 117 is
`the Mobile Switching Centre
`(MSC) 138 associated with Base Station Controller-
`(s) (BSC) 136 for controlling Cellular Base Station-
`(s) 130. Cellular base station 130 comprises both a
`receive antenna 132 and a transmit antenna 134 for
`
`communicating with CCTs 200.
`The CCT 200 may be a mobile unit installed in
`a vehicle, a so called transportable unit or a hand
`held portable unit. CCT 200 comprises an antenna
`228 for cordless communication and an antenna
`
`238 for cellular communication. The CCT 200 may
`alternatively comprise a single antenna 238 for
`both cellular and cordless communication and ar-
`
`ranged asillustrated in figure 2 by the dotted line
`272. Typically in the UK cordless telephone sys-
`fems operate in frequency bands at 49 MHz (CT®),
`860 MHz (CT2) and 1880-1900 MHz (DECT) and
`cellular telephone systems in frequency bands 890
`- 905 MHz and 935 - 950 MHz (TACS), 905-915
`MHz and 950-960 MHz (GSM) or 1800 MHz (DCS).
`Figure 2 illustrates a detailed block diagram of
`an embodiment of a CCT 200 in accordance with
`
`invention. CCT 200 comprises a cel-
`the present
`lular telephone transceiver 230, and antenna 238, a
`cordless telephone transceiver 220 and antenna
`228, a microprocessor 210, keypad 201, display
`205, audio switch 260, microphone 261 and speak-
`er 262. The microphone 261, speaker 262 and
`keypad 201 may alternatively be located in a hand-
`set separate from the rest of
`the CCT 200. An
`alternative embodiment
`is shown by dashed line
`272,
`in which cordless transceiver 220 and cellular
`transceiver 230 may be coupled to a single an-
`fenna 238 by way of band passfilters (BPF) 270
`and (BPF) 271,
`respectively. Cordless telephone
`transceiver 220 may be any conventional cordless
`transceiver. However,
`it would be advantageous if
`the cordless telephone transceiver 220 conformed
`to a common air interface for cordless telephones,
`since this would facilitate roaming of the CCT 200
`between different cordless systems. An example of
`such an interface is the recently introduced com-
`mon air interface CAI
`for CT2 cordless systems.
`The cellular transceiver 230 may likewise be any
`conventional cellular transceiver. The keypad 201,
`microprocessor 210, display 205 and the like can
`be any available type, connected and arranged to
`operate in the CCT 200. The microprocessor 210
`includes a Service Available Register (SAR) 211 for
`storing which radio systems are currently available
`to the CCT 200.
`
`The microprocessor 210 illustrated in Figure 2
`is adapted to operate in accordance with the flow
`chartsillustrated in Figures 3 - 4, for controlling the
`
`CCT 200 as a cordless telephone, a cellular tele-
`phone or a cellular cordless telephone. Thus,
`in
`accordance with the present invention the CCT 200
`may operate, as far as a user is concerned, si-
`multaneously as a cellular telephone and a cord-
`less telephone. For the sake of explanation and
`clarity,
`it should be noted that the CCT 200 can be
`so arranged such that both cellular and cordless
`operations are in progress at the same time. Alter-
`natively, if components are shared between cellular
`and cordless parts, cellular and cordless operations
`can be performed at different times although this
`would be done at a speed sufficient for it
`to be
`undetectable by the user and therefore appear to
`be simultaneous operation.
`When operating as a cordless telephone con-
`trol signals from the microprocessor 210 enable
`cordless receiver 221 and cordless transmitter 222.
`
`The microprocessor 210 also monitors signals from
`the cordless receiver 221 indicating received signal
`strength and for detecting receive data, and from
`the cordless transmitter 222 for sending transmit
`data. Additionally, the microprocessor 210 monitors
`control signals from the cordless transceiver 220
`for detecting incoming calls
`(ringing),
`security
`codes and broadcast
`information relevant
`to the
`
`cordless system, and for sending dialling informa-
`tion.
`
`The microprocessor 210 controls the CCT 200
`in a similar way when operating as a cellular tele-
`phone, but appropriately modified for the signalling
`protocols and data encryption used in the cellular
`system. The signalling protocols, data encryption
`techniques and the like used in respective tele-
`phone systems are well known in the art, and the
`microprocessor can be arranged to operate in a
`known manner to effect control of the signals in
`such systems.
`The audio switch 260 is controlled by the
`microprocessor 210 to link the cordless audio
`channel 240 or the cellular audio channel 250 to
`
`the microphone 261 and loudspeaker 262 as ap-
`propriate.
`Referring to Figure 3, there is illustrated a flow
`diagram showing the steps used by microproces-
`sor 210 for operating the CCT 200 to receive and
`place cellular or cordless telephone calls.
`In this
`example, for simplicity, there is a presumption that
`a cellular service is generally available when a
`cordless service is not, and that a cordless service
`is the preferred service. Entering at block 302, the
`user activates the CCT 200 and the microprocessor
`210 monitors both the cellular and cordless system
`availability.
`If the cellular and/or cordless system
`are available the microprocessor 210 updates the
`display 205 and SAR 211 at step 304. Next af
`block 305 a check is made to see if a call
`is in
`
`progress or not.
`
`If not, then at block 306, a check
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`is made to see if an incoming call is present. If not,
`the NO branch is taken to decision block 312 to
`
`If not,
`see if a call has been initiated by the user.
`NO branch is taken back to block 304 to continue
`
`monitoring for cellular and cordless system avail-
`ability.
`is present then the YES
`If an incoming call
`branch from decision block 306 is taken to decision
`block 308 whereit is determined whether or not the
`
`the YES
`If so,
`is a cordless call.
`incoming call
`branch is taken from decision block 308 to block
`
`310 and the incoming call is connected as a cord-
`less call. Additionally, the display 205 can be ac-
`tivated to show that a cordless telephonecall is in
`progress. If the incoming call is not a cordless call,
`then the NO branch is taken from decision block
`308 to block 316 and the call
`is connected as a
`
`cellular call. The display 205 again can be ac-
`tivated, but this time to indicate that a cellular call
`is In progress.
`if a call has
`Returning to decision block 312,
`been initiated then the YES branch is taken to
`
`decision block 314 to see if the system available
`register indicates that the cordless system is avail-
`able.
`If so, then the YES branch is taken and the
`call is connected as a cordlesscall. If not, then the
`call is connected as a cellular call. Once a call has
`
`been connected then the CCT 200 microprocessor
`210 returns to block 304 and continues monitoring
`for cellular and cordless systems.
`Referring now to Figure 4, once a call has
`been connected the CCT 200 continues to monitor
`
`for the availability of the cellular and cordless sys-
`tems. At decision block 322 a check is made to
`
`If so, then
`see if the cordless system is available.
`the YES branch is
`taken to decision block 328
`
`whereit is determined whether the call in progress
`is a cordless call or not.
`If
`it is a cordless call, then
`the YES branch is taken to block 304 and the CCT
`
`200 continues monitoring for the availability of the
`cellular
`and cordless
`systems.
`If
`the
`call
`in
`progress at decision block 328 is not a cordless
`call
`then the NO branch is taken to block 330
`where handover of the call to the cordless system
`is initiated. Referring back to decision block 322, if
`the cordless
`system is not available,
`the NO
`branch is
`taken to decision block 304 where a
`
`the cellular system is
`check is made to see if
`available.
`If not,
`the NO branch is taken and the
`CCT 200 continues to monitor
`for cellular and
`
`cordless systems. If the cellular system is available
`then the YES branch is taken from decision block
`324 to decision block 325 to check whether the call
`
`then the YES
`If so,
`in progress is a cellular call.
`If not, then the NO
`branch is taken to block 304.
`branch is taken to block 326 where handoverof the
`
`call to the cellular system is initiated. The CCT 200
`then continues to monitor the availability of cellular
`
`and cordless systems at block 304.
`The handover of a call from a cordless system
`to a cellular system and vice versa can be per-
`formed manually or preferably automatically. Ex-
`amples of handover between cellular and cordless
`systems, and vice versa, are disclosed in co-pend-
`ing British Patent applications no 9320814.8 and no
`9320815.8.
`
`As mentioned previously, for simplicity the pre-
`sumption that the cordless system is the preferred
`system, and that the cellular system is generally
`available when the cordless service is not has been
`made. The criteria for the decision blocks in the
`
`figures 3 and 4 have been cor-
`flow charts of
`respondingly chosen. However, the criterion, or cri-
`teria, at each decision block may be far more
`complex. For example, they may be based on the
`quality of the signal from the available systems. In
`particular, the handover of a call in progress would
`typically be preceded by decisions based on the
`quality of system signals and whether signals from
`the current system were deteriorating or not. Such
`criteria would enable a call
`to be handed over
`
`10
`
`15
`
`20
`
`25
`
`before it was lost due to signal breakdown or the
`like.
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`the criteria may be based on a
`Additionally,
`user indicated or preprogrammed preference for
`any particular system, such as a cellular system
`and not a cordless system as presumed in the
`foregoing example.
`The operation of CCT 200illustrated in figure 3
`is such that CCT 200initially receives an incoming
`call
`in whatever system the incoming call origi-
`nated.
`If the incoming call
`is from a system which
`does not fulfil the criteria for the choice of system,
`then the call may be transferred to the system
`which doesfulfil the aforesaid criteria. Such a sys-
`tem could utilise a form of call forwarding as dis-
`closed in US patent US 4 989 230. However, such
`call forwarding is instructed by CCT 200 and takes
`place only after the non-preferred part of the CCT
`200 has been paged.
`An alternative would be to have a cellular and
`
`cordless system which were interlinked so as to
`have knowledge of the geographical and/or system
`location of a particular CCT 200. In such aninterlin-
`ked system, a call originating in a non-preferred
`system would be automatically redirected by the
`non-preferred system to the preferred system.
`Such a system would also facilitate inter-system
`handover whilst calls were in progress.
`A radiotelephone suitable for such operation
`would be a combined GSM/DECT CCT 200. In the
`
`GSM system the cellular base stations 130 are
`connected to Mobile Switching Centres (MSCS)
`138 which themselves may be connected to other
`MSCs, the PSTN 117 (as shown in Figure 1) com-
`prising the
`Integrated Services Digital Network
`
`6
`
`6
`
`

`

`11
`
`EP 0 660 626 A2
`
`12
`
`(ISDN), or the like. The MSC 138 is also connected
`to a Visitor Location Register VLR 137, which is a
`data base of GSM subscriber files of GSM sub-
`
`scribers visiting the area of the MSC 138 to which
`the VLR relates. The MSC 138 also has a Home
`
`Location Register HLR 139 which is a database of
`all the subscribers having that particular MSC 138
`as their home MSC. The MSC 138 is also con-
`nected to other elements of the GSM network such
`
`as an Authentication Centre AC and an Equipment
`Identity Register EIR. A more detailed description
`of
`the GSM system may be found in a book
`entitled "The GSM system for mobile communica-
`tions", M. Mouly and M. Pautet, Palaiseau, France,
`ISBN: 2 9507190-0-7.
`
`A cellular telephone or CCT 200 operating in a
`MSC area communicates with the MSC via a base
`
`station in order to register its presence in the area
`assigned to that particular MSC. Such registration
`may be carried out by a dual system GSM/DECT
`CCT 200 whilst
`it monitors the availability of the
`GSM system. Thus,
`the location of a CCT 200
`phone within the GSM system would be known.
`Since the MSC 138 is connected to the PSTN 117,
`the location of a CCT 200 in the GSM system can
`also be known to the DECT system since the
`system is connected to DECT the land line PSTN
`system. DECT phones,
`in common with CT-2 type
`phones, are capable of being paged via community
`base stations as well as domestic base stations.
`
`Thus, knowledgeof the location of a DECT or CT-2
`phone must be possessed by the relevant cordless
`system. An example of such a cordless telephone
`system is disclosed in international application no.
`WO 92/03006.
`In the cordless system disclosed in
`WO 92/03006, a cordless telephone pre-registers
`with a cordless base station indicating to a network
`control centre (connected to the PSTN)the location
`of the cordless telephone. A combined GSM/DECT
`CCT 200 may perform the pre-registering during
`the monitoring for the availability of the GSM and
`DECT systems. Since the network control centre
`(Central Control Fixed Part (CCFP) of the cordless
`system (DECT) is connected to the PSTN, and the
`PSTN in turn is connected to the MSC of the GSM
`
`system, both the network control centre and the
`MSC may have access to the whereabouts of re-
`spective GSM and DECT phone numbers. How-
`ever,
`the signalling between the network control
`centre of the cordless system and the MSC would
`need to be very fast, and need to utilise the ISDN
`of the PSTN.
`
`An alternative GSM/DECT combined system is
`shown schematically in Figure 5.
`In this s