United States Patent [19]
`Tanaka
`
`I lllll llllllll Ill lllll lllll lllll lllll lllll 111111111111111111111111111111111
`US005454026A
`[11] Patent Number:
`[ 45] Date of Patent:
`
`5,454,026
`Sep.26, 1995
`
`[54] MOBILE COMMUNICATION CONGESTION
`CONTROL SYSTEM
`
`[75]
`
`Inventor: Shoji Tanaka, Tokyo, Japan
`
`[73] Assignee: NEC Corporation, Japan
`
`[21] Appl. No.: 43,346
`
`[22] Filed:
`
`Apr. 6, 1993
`
`[30]
`
`Foreign Application Priority Data
`
`Apr. 7, 1992
`
`[JP]
`
`Japan .................................... 4-084296
`
`Int. Cl.6
`.•..•....•.......••..•..••.............•............ H04M 11/00
`[51]
`[52] U.S. CI •................................. 379/60; 379/59; 379/58;
`455/33.2
`[58] Field of Search .................................. 379/58, 59, 60;
`455/33.1, 33.2, 54.1, 54.2, 56.1
`
`[56]
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`4,670,899
`5,042,082
`5,101,501
`
`6/1987 Brody et al. .............................. 379/60
`8/1991 Dahlin ................................... 379/60 X
`3/1992 Gilhousen et al. . .................. 379/60 X
`
`5,111,535
`
`5/1992 Tokunaga ............................... 455/33.2
`
`Primary Examiner-Curtis Kuntz
`Assistant Examiner--G. J. Oehling
`
`[57]
`
`ABSTRACT
`
`A mobile communication system contains a mobile switch(cid:173)
`ing center connected to a plurality of base stations consti(cid:173)
`tuting a radio service zone and a plurality of mobile stations.
`The mobile switching center monitors the usage of a traffic
`channel for each radio service zone and has a usage thresh(cid:173)
`old value for each radio service zone, and informs those to
`the mobile station via the base station. The busy mobile
`station compares the traffic channel usage information and
`the usage threshold value information and the radio zone
`communicated. When the traffic channel usage is greater
`than the usage threshold value, the mobile station selects a
`hand-over radio zone destination based on the received
`signal strength and quality, and the traffic channel usage
`information and the usage threshold value information of the
`adjacent zone, and then transmits a hand-over request des(cid:173)
`ignating the hand-over radio zone destination to the mobile
`switching center.
`
`6 Claims, 4 Drawing Sheets
`
`MOBILE EXCHANGE UNIT 1
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`32
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`LEVEL MONITOR
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`34
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`

`

`U.S. Patent
`
`Sep. 26, 1995
`
`Sheet 3 of 4
`
`5,454,026
`
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`
`S111
`
`AND SEND HAND-OVER REQUEST
`{(THRESHOLD VALUE)-(USAGE) } ,
`BETWEEN USAGE AND THRESHOLD VALUE
`ZONE HAVING LARGEST DIFFERENCE
`DEFINE.AS HAND-OVER DESTINATION,
`
`YES
`
`) <(THRESHOLD VALUE) ?
`
`( USAGE
`ANY ZONE SATISFYING CONDITION
`
`S110
`
`S109
`
`VALUES OF ALL ZONES
`CHECK USAGES AND THRESHOLD
`
`END
`
`HAND-OVER REQUEST
`DESIGNATION AND SEND
`DEFINE ZONE AS HAND-OVER
`
`'1: (THRESHOLD VALUE
`
`OF ZONE TO BE CHECKED) ?
`CHECKED)
`( USAGE OF ZONE TO BE
`
`NO
`
`SAME RECEPTION LEVEL ?
`A PLURALITY OF ZONES HAVING
`
`NO
`
`S108
`
`END
`
`\ YES
`
`S106
`
`TO BE CHECKED
`DEFINE NEXT ZONE AS ZONE
`
`ZONES CHECKED ?
`ALL ADJACENT
`NO
`
`END
`
`IN ORDER FROM
`YES
`
`TO BE CHECKED
`DEFINE THESE ZONES AS ZONES
`HIGHER RECEPTION LEVEL AND
`REARRANGE ZONES
`
`S103
`
`NO
`
`HAND-OVER ENABLE ZONE PRESENT ?
`
`ADJACENT ZONES
`CHECK RECEPTION LEVELS OF
`
`START
`
`S107
`
`

`

`5,454,026
`
`1
`MOBILE COMMUNICATION CONGESTION
`CONTROL SYSTEM
`
`BACKGROUND OF THE INVENTION
`
`2
`FIG. 3 is a table showing a data format of a traffic channel
`usage and a usage threshold value; and
`FIG. 4 is a flow chart of congestion control of a mobile
`station according to the present invention.
`
`DESCRIPTION OF THE PREFERRED
`EMBODIMENTS
`
`5
`
`The present invention relates to a mobile communication
`congestion control system for controlling mobile-assisted
`hand-over between radio zones.
`In a conventional system for performing congestion con(cid:173)
`trol using the usages of traffic channels in a mobile com-
`munication system, a mobile switching center has threshold
`values for the usages of the traffic channels. The mobile
`switching center compares the traffic channel usage with the
`usage threshold value and sends a hand-over instruction to
`a mobile station, as needed.
`In a conventional congestion control system, a mobile
`switching center measures and monitors the channel usage
`for each radio zone, must select a hand-over destination, and
`must send a hand-over instruction to a mobile station.
`Processing in the mobile switching center is complicated 20
`and large. As a result, the processing capacity of the mobile
`switching center is undesirably decreased.
`
`10
`
`A preferred embodiment of the present invention will be
`described below with reference to the accompanying draw(cid:173)
`ings. FIG. 1 shows a mobile communication system for
`performing congestion control of the present invention. This
`system comprises mobile stations 3 used by mobile sub(cid:173)
`scribers, a mobile switching center 1 for communicating
`15 with the mobile station 3 to allow communication between
`the mobile stations 3 and between a mobile station 3 and a
`wire subscriber, and base stations 21, 22, and 23 as repeaters,
`arranged in the respective radio zones, for converting the
`information such as speech or data communication between
`the mobile switching center 1 and the mobile station 3 from
`wire communication to radio communication and vice versa.
`The mobile switching center 1 comprises base station
`controllers 111, 112, and 113 for respectively controlling the
`base stations 21, 22, and 23, a switch controller 12, con-
`25 nected to the base station controllers 111, 112, and 113, for
`controlling a traffic channel between the mobile stations 3 or
`between a mobile station 3 and a wire subscriber, a call
`controller 13, connected to the switch controller 12, for
`controlling a call connection between the mobile stations 3
`or between a mobile station 3 and a wire subscriber, a traffic
`monitor unit 14, connected to the call controller 13, for
`measuring and monitoring a mobile station traffic, and a
`memory 15, connected to the call controller 13, for storing
`station data such as frequencies between the mobile stations
`35 3 and the base station controllers 111, 112, and 113, the
`subscriber number of a mobile station 3, and a usage
`threshold value.
`FIG. 2 shows the arrangement of the mobile station 3. The
`mobile station 3 comprises a mobile station controller 31, a
`transmitter 36, a transmission controller 37, a receiver 32, a
`reception controller 33, a level monitor unit 34, and a
`congestion monitor unit 35. The mobile station controller 31
`controls the overall operation of the mobile station 3. The
`transmitter 36 modulates a carrier wave using control data
`and a speech signal as modulation signals and transmits the
`modulated data in air through an antenna 38. The transmis(cid:173)
`sion controller 37 controls to repeat the control data and the
`speech signal which are sent to the mobile switching center
`1 and to set the frequency of the transmitter 36. The receiver
`32 receives the control data and the speech or data from the
`mobile switching center 1 through the antenna 38 and
`demodulates them. The reception controller 33 controls to
`distribute the control data and the speech or data which are
`received and demodulated by the receiver 32 and controls
`the frequency of the receiver 32. The level monitor unit 34
`measures and monitors the levels of the received signal
`strengths (RSSs) from the base stations 21, 22, and 23 and
`quality levels when the mobile station is set in an idle or
`60 busy state. The congestion monitor unit 35 as the feature of
`the present invention receives traffic channel usage infor(cid:173)
`mation and the usage threshold value information of radio
`zones from the mobile switching center 1 and monitors the
`congestion condition.
`An operation of the mobile communication system will be
`described below. First, an operation for connecting an origi(cid:173)
`nation call from the mobile station 3 and then setting a busy
`
`SUMMARY OF THE INVENTION
`
`It is an object of the present invention to provide a mobile
`communication congestion control system capable of sim(cid:173)
`plifying and reducing the processing in a mobile switching
`center.
`It is another object of the present invention to provide a
`mobile communication congestion control system capable
`of increasing the processing capacity of the mobile switch(cid:173)
`ing center.
`In order to achieve the above objects of the present
`invention, there is provided a mobile communication con(cid:173)
`gestion control system comprising information transmitting
`means, arranged in a mobile switching center connected to
`base stations of a plurality of radio zones constituting a
`service area, for transmitting traffic channel usage informa(cid:173)
`tion and usage threshold value information of radio zones to
`a busy mobile station and an idle mobile station, information
`receiving means, arranged in the mobile station, for receiv(cid:173)
`ing the traffic channel usage information and the usage
`threshold value information of the radio zone communicated
`and adjacent radio zones from the information transmitting
`means, comparing means, arranged in the mobile station, for
`comparing the traffic channel usage information and the
`usage threshold value information of the radio zone com(cid:173)
`municated, which are received by the information receiving
`means, and selecting means, arranged in the mobile station,
`for selecting a hand-over radio zone destination based on the
`level of a received signal strength and quality from the
`adjacent radio zones and the traffic channel usage informa(cid:173)
`tion and the usage threshold value information of the adja(cid:173)
`cent zones which are received by the information receiving
`means and for transmitting a hand-over request designating
`the hand-over radio zone destination to the mobile switching
`center when the traffic channel usage is not less than the
`usage threshold value of the radio zone communicated.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`30
`
`40
`
`45
`
`50
`
`55
`
`FIG. 1 is a block diagram of a mobile communication
`system for performing congestion control according to an
`embodiment of the present invention;
`FIG. 2 is a block diagram showing an arrangement of a 65
`mobile station in the mobile communication system shown
`in FIG. 1;
`
`

`

`5,454,026
`
`10
`
`4
`and at the same time checks how many mobile stations 3 use
`the traffic channel. The call controller 13 informs the events
`that a call setup or disconnection is performed to the traffic
`5 monitor unit 14. The traffic monitor unit 14 calculates the
`traffic channel usage for every radio zone, i.e., base stations
`21 to 23 by the information from the call controller 13, and
`then sends the calculated result to the call controller 13 every
`predetermined period. The call controller 13 sends the traffic
`channel usages for every base station from the traffic moni-
`tor unit 14 and the traffic channel usage threshold values for
`every radio zone from the memory 15 to the base station
`controllers 111, 112, and 113. The base station controllers
`111, 112, and 113 send out the traffic channel usages and
`their threshold values from the call controller 13 to all
`control channels and all busy traffic channels through the
`base stations 21, 22, and 23. These data are sent to the busy
`and idle mobile stations 3 every predetermined period. As a
`result, the busy mobile stations can receive the traffic
`channel usages and usage threshold values of the radio zone
`communicated in addition to the RSS and quality levels of
`the used traffic channel every predetermined period. In
`measurements (both the RSS and quality levels) for the
`control of the adjacent radio zones to perform a hand-over
`operation, the busy mobile station 3 can receive the usages
`and usage threshold values of the adjacent radio zones which
`30 are informed on the control channels of the adjacent radio
`zones. This adjacent radio zone information may be
`informed together with zone information of its own to the
`mobile station 3. FIG. 3 shows a data format of usages and
`35 usage threshold values of 4-bit of traffic channels for sending
`information on traffic channels and control channels under
`the conditions:
`
`25
`
`3
`state will be described below. The receiver 32 in the mobile
`station 3 receives system information sent on a control
`channel from the mobile switching center 1 through the base
`station 21, and the mobile station controller 31 receives this
`information through the reception controller 33 and analyzes
`it. If the received system information is normal, the mobile
`station controller 31 sets the mobile station 3 in an idle state.
`In this idle state, when a mobile subscriber operates to
`request an origination call, the mobile station controller 31
`controls the transmission controller 37 by this request. The
`mobile station unit 31 causes the transmitter 36 to transmit
`an origination call request signal.
`The base station 21 repeats this origination call request
`signal to the mobile switching center 1. The base station
`controller 111 in the mobile switching center 1 receives the 15
`origination call request signal and outputs it to the call
`controller 13. Upon reception of the origination call request
`signal, the call controller 13 analyzes it and hunts an idle
`channel of the traffic channels accommodated in the base
`station 21. The call controller 13 causes the base station 20
`controller 111 to send out a signal for assigning a hunted idle
`channel to the mobile station 3, i.e., a traffic channel
`assignment signal.
`The mobile station controller 31 in the mobile station 3
`receives the traffic channel assignment signal through the
`receiver 32 and the reception controller 33 and sends out an
`instruction to the reception controller 33 so as to cause it to
`control the receiver 32. The receiver 32 is tuned to the traffic
`channel designated by the traffic channel assignment signal.
`The mobile station control unit 31 outputs an instruction to
`the transmission controller 37 so as to cause that the trans(cid:173)
`mission controller 37 orders the transmitter 36 to transmit a
`signal representing that the idle channel is assigned and set
`to the busy state.
`In this mobile switching center 1, when the base station
`controller 111 receives this signal and sends it to the call
`controller 13. As a result, the call controller 13 controls the
`switch controller 12 to establish a traffic channel and set a
`busy state.
`The mobile station 3 set in the busy state checks whether
`hand-over to another radio zone is required due to a dete(cid:173)
`rioration of the signal strength and/or quality level upon
`movement of the mobile station 3. The level monitoring unit
`34 controls the reception controller 33 to monitor the 45
`received signal strength (RSS) and quality levels of the
`currently used traffic channel, and the RSS and quality levels
`of the control channel of the base stations 22 and 23 installed
`in the adjacent radio zones. At the same time, the level
`monitor unit 34 sends the usage and the usage threshold 50
`value from the reception controller 33 to the congestion
`monitor unit 35. Upon monitoring, if the RSS and quality
`levels of the control channel of the base station 22 are kept
`at levels to ensure communication quality and higher than
`those of the traffic channel of the base station 21, the mobile 55
`station controller 31 sends a hand-over request designating
`the base station 22 to the mobile switching center 1 through
`the base station 21. Another case, if the RSS and quality
`levels of the control channels of the base stations 22 and 23
`are kept at levels to ensure communication quality, these 60
`levels, however, are lower than those of the base station 21,
`and the mobile station controller 31 does not send a hand(cid:173)
`over request and keeps communication through the base
`station 21.
`A congestion control operation as the feature of the 65
`present invention will be described below. The call control-
`ler 13 of the mobile switching center 1 performs call control
`
`40
`
`Usage:
`
`0 to 100
`
`Usage
`Threshold
`Value:
`
`0 to 100
`
`(number of traffic
`channels used)/(total
`number of traffic
`channels in a radio
`zone) x 100 [%]
`(number of traffic
`channels used)/(total
`number of traffic
`channels in a radio
`zone) x 100 [%]
`
`This embodiment is based on the GSM radio interface. A
`BCCH (Broadcast Channel) is used as the control channel,
`and an SACCH (Slow Associated Control Channel) for
`traffic channel control is used as the traffic channel.
`An operation of the mobile station 3 will be described
`below.
`FIG. 4 shows congestion control of the mobile station 3
`of the present invention. Assume that the traffic channel
`usages and usage threshold values are informed from the
`base station controllers 111 to 113 to the mobile station 21
`as the following table. Note that the zone of the base station
`21 is defined as a zone communicated with the mobile
`station 3, and that the zones of the base stations 22 and 23
`are defined as adjacent zones, and the received signal quality
`is omitted to simplify the description.
`
`

`

`5,454,026
`
`5
`
`Communi(cid:173)
`cated Zone
`
`Zone of
`Base
`Station
`21
`
`50%
`80%
`
`60 dBµ
`85%
`80%
`
`Adjacent Zone
`
`Zone of
`Base
`Station
`22
`
`Zone of
`Base
`Station
`23
`
`50 dBµ
`60%
`70%
`
`40 dBµ
`65%
`75%
`
`55 dBµ
`
`45 dBµ
`
`50 dBµ
`
`TimeT
`
`TimeT1
`
`Usage
`Usage
`Threshold
`Value
`RSS Level
`Usage
`Usage
`Threshold
`Value
`RSS Level
`
`6
`plurality of zones having the same RSS level in step SlOS,
`the usages of all the zones are compared with the corre(cid:173)
`sponding threshold values (step S109). If a zone having a
`usage smaller than the corresponding threshold value is
`5 present (step SllO), a zone having the largest difference
`between the threshold value and the ((threshold value)(cid:173)
`(usage)) is defined as a hand-over destination, thereby
`sending a hand-over request (step Slll). If no zone having
`a usage smaller than the corresponding threshold value is
`10 present, the flow returns to step S104 through steps S106 and
`S107.
`As has been described above, according to the mobile
`communication congestion control system arranged in the
`mobile station, mainly a hand-over request is sent to a
`15 mobile switching center, and the mobile switching center(cid:173)
`need not monitor congestion. The processing in the mobile
`switching center can be simplified and small. The processing
`capacity of the mobile switching center can thus be
`increased.
`Although the present invention has been described in
`relation to particular embodiments thereof, many other
`variations and modifications and other uses will become
`apparent to those skilled in the art. It is preferred, therefore,
`that the present invention be limited not by the specific
`25 disclosure herein, but only by the appended claims.
`What is claimed is:
`1. A mobile communication congestion control system
`comprising:
`information transmitting means located in a mobile
`switching center connected to respective base stations
`of a plurality of radio zones constituting a service area,
`the
`information
`transmitting means
`transmitting
`through said base stations traffic channel usage infor(cid:173)
`mation and usage threshold value information for each
`of said plurality of radio zones to busy and idle mobile
`stations located in said plurality of radio zones;
`each said mobile station including:
`information receiving means for receiving from said
`information transmitting means the traffic channel
`usage information and the usage threshold value
`information of a first radio zone in which said mobile
`station is located and of adjacent radio zones;
`comparing means for comparing the traffic channel
`usage information and the usage threshold value
`information of the first radio zone to monitor a
`congestion state of the first radio zone;
`level monitoring means for measuring and monitoring
`a level of a received signal strength in a control
`channel from each of said adjacent radio zones; and
`selecting and transmitting means for:
`(a) selecting one of the adjacent radio zones as a
`second radio zone to which said mobile station is
`to be handed-over based on:
`(A) the level of the received signal strength in the
`control channel from the second radio zone being
`not less than a predetermined hand-over level, and
`(B) the traffic channel usage of the second radio
`zone being less than the usage threshold value of
`the second radio zone, and
`(b) transmitting a hand-over request designating the
`second radio zone as a hand-over destination to
`said mobile switching center when the traffic
`channel usage of the first radio zone is not less
`than the usage threshold value of the first radio
`zone,
`wherein said selecting and transmitting means selects, as
`
`35
`
`When the mobile station 3 is started to communicate with
`the base station 21 (time T), the usage is less than the 20
`threshold value, and the RSS level is greater than those of
`the adjacent zones. In this case, neither congestion control
`nor hand-over are performed.
`At time T 1, the RSS level of the zone of the base station
`21 is higher than those of the adjacent zones of the base
`stations 22 and 23, and hand-over is not performed. How(cid:173)
`ever, the usage is more than the threshold value, the con(cid:173)
`gestion monitor unit 35 sends the usages and threshold
`values of the adjacent zones received and sends them to the 30
`mobile station controller 31. The mobile station controller
`31 gets the RSS level measurement results of the adjacent
`zones from the level monitor unit 34. The congestion control
`operation of the mobile station controller 31 will be contin(cid:173)
`ued with reference to a flow chart in FIG. 4 below.
`The mobile station controller 31 checks the RSS levels of
`the adjacent zones (the zones of the base stations 22 and 23
`in this case) (step SlOl) to determine whether a hand-over
`enable zone is present (step S102). If a condition of a
`hand-over enable RSS level is defined as 40 dBµ, the zones 40
`of the base stations 22 and 23 are hand-over enable zones.
`All the hand-over enable zones are arranged in an order from
`a higher RSS level to check a best zone at first (step S103).
`At first, it is determined whether a plurality of zones having
`the same RSS level are present (step S104). If NO in step 45
`Sl04, the traffic channel usage of this zone is compared with
`its usage threshold value to determine whether the usage is
`equal to or larger than the threshold value (step SlOS). The
`RSS levels of the zones to be checked are decreased in an
`order of the zones of the base stations 23 and 22 (step S103), 50
`and the RSS levels of the base stations 22 and 23 are not
`equal to each other (step S104). So, the zone to be checked
`is defined as the zone of the base station 23, and its usage
`and threshold value are checked (step S104). The usage of
`the zone of the base station 23 is smaller than the threshold 55
`value (step SlOS), so that the zone of the base station 23 is
`determined as a hand-over zone (step SlOS). The mobile
`station controller 31 sends information representing that the
`zone of the base station 23 is a hand-over destination zone
`to the transmission controller 37. The transmission control- 60
`ler 37 sends a hand-over request to the mobile switching
`center 1 through the transmitter 36 and the base station 21.
`If the usage of the zone to be checked is equal to or larger
`than the threshold value in step SlOS, the zone to be checked
`is the next zone in the order of RSS levels (step Sl07). The 65
`above operations are repeated until all the adjacent zones are
`checked (step S106). When the zones to be checked are a
`
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`5,454,026
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`7
`the hand-over destination, one of the adjacent radio
`zones in which the level of the received signal strength
`is highest and the traffic channel usage thereof is less
`than the associated usage threshold value, and
`when there are a plurality of adjacent radio zones having 5
`the same highest level of the received signal strength,
`said selecting and transmitting means selects, as the
`hand-over destination, one of the adjacent radio zones
`having a largest difference obtained by subtracting the
`traffic channel usage thereof from the associated usage 10
`threshold value.
`2. A system according to claim 1, wherein said informa(cid:173)
`tion transmitting means transmits at least the traffic channel
`usage information and the usage threshold value information
`of the plurality of radio zones to said mobile stations at every 15
`predetermined period of time.
`3. A system according to clai·m 1, wherein said informa(cid:173)
`tion transmitting means comprises traffic monitor means for
`measuring and monitoring a mobile station traffic for every
`radio zone, memory means for storing the usage threshold 20
`value information, and control means for transmitting the
`traffic channel usage information and the usage threshold
`value information of each radio zone to said mobile station
`through said base stations in accordance with outputs from
`said traffic monitor means and said memory means.
`4. A system according to claim 1, wherein said selecting
`and transmitting means selects one of the adjacent radio
`zones, as the hand-over destination, on the basis of the level
`of the received signal strength and a level of a received
`signal quality.
`S. A system according to claim 1, wherein the level
`monitoring means further monitors a level of the received
`signal strength in a communicating channel of the first radio
`zone, and said selecting and transmitting means selects one
`of the adjacent radio zones when the level of the received 35
`signal strength of said one of the adjacent radio zones is not
`less than the predetermined hand-over level and higher than
`that of the first radio zone.
`6. A mobile communication congestion control system
`comprising:
`information transmitting means located in a mobile
`switching center connected to respective base stations
`of a plurality of radio zones constituting a service area,
`the information transmitting means transmitting traffic
`
`25
`
`30
`
`40
`
`45
`
`8
`channel image information and usage threshold value
`information for each radio zone to busy and idle mobile
`stations located in said plurality of radio zones;
`each said mobile station including:
`information receiving means for receiving from said
`information transmitting means the traffic channel
`usage information and the usage threshold value
`information of a first radio zone in which said mobile
`station is located and of adjacent radio zones;
`level monitoring means for measuring and monitoring
`levels of received signal strengths in a communicat(cid:173)
`ing channel of the first radio zone, and in a control
`channel from each of the adjacent radio zones;
`comparing means for comparing the traffic channel
`usage information and the usage value information
`of the first radio zone to monitor a congestion state
`of the first radio zone; and
`selecting and transmitting means for selecting one of
`the adjacent radio zones as a second radio zone to
`which said mobile station is to be handed-over and
`for transmitting a hand-over request for designating
`the second radio zone as a hand-over destination to
`said mobile switching center when the traffic channel
`usage of the first radio zone is not less than the usage
`threshold value of the first radio zone, and the level
`of the received signal strength from the second radio
`zone is not less than a predetermined hand-over level
`and higher than that of the first radio zone,
`wherein said selecting and transmitting means selects,
`as the hand-over destination, one of the adjacent
`radio zones in which the level of the received signal
`strength is highest and the traffic channel usage
`thereof is less than the associated usage threshold
`value, and
`wherein said are plurality of adjacent radio zones
`having the same highest level of the received signal
`strength, said selecting and transmitting means
`selects, as the hand-over destination, one of the
`adjacent radio zones having a largest difference
`obtained by subtracting the traffic channel usage
`thereof from the associated usage threshold value.
`
`* * * * *
`
`50
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`55
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`60
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`65
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`