WORLD lNrEL.‘LECI'UAL_ PROPERTY ORGANIZATION
`
`International Bureau
`
`INTERNATIONAL APPLICATION PUBLISHED UNDER THE PATENT COOPERATION TREATY (PCT)
`
`(51) International Patent Classificafion 5 =
`H04
`7/28
`Q
`
`(11) International Publication Number:
`(43) International Publication Date:
`
`W0 95/10920
`20 April 1995 (20.04.95)
`
`(21) International Application Number:
`
`PCT/JP94/01708
`
`(81) Designated States: CN, JP, US, European patent (AT, BE, CH,
`DE, DK, ES, FR, GB, GR, IE, IT, LU, MC, NL, PT, SE).
`
`(22) International Filing Date:
`
`13 October 1994 (1110.94)
`
`Published
`With intematiomzl search report.
`
`(30) Priority Data:
`5/255808
`6/1 12734
`6/112739
`6/158846
`6!158352
`
`13 October 1993 (13.10.93)
`26 May 1994 (2605.94)
`26 May 1994 (26.05.94)
`11 July 1994 (11..O7.94)
`11 July 1994 (11.07.94)
`
`(71) Applicant (for all designated States except US): NTT MOBILE
`COIVIMUNICATION NETWORK INC. [JP/JP]; 10-1, Tora-
`nomon 2-chome, Minato-ku, Tokyo 105 (JP).
`
`(72) Inventors; and
`(75) Inventors/Applicants (for US only): NAKAMURA, Takehiro
`[JP/JP]; 1-2356-920C, Take, Yokosuka-shi, Kanagawa 238-
`03 (JP). UIVIEDA, Narumi
`[JP/JP]; 1-2356-920C, Take,
`Yokosuka-shi, Kanagawa 238-03 (JP).
`I-IIROIKE, Akira
`[JP/JP]; 1-2356-920C, Take, Yokosuka—shi, Kanagawa 238-
`03 (JP).
`
`(74) Agent: MIYOSI-II, Hidekazu; Toranomon-Daiichi Building,
`3rd floor, 2-3, Toranomon 1-chome, Minato-ku, Tokyo 105
`(JP).
`
`(54) Title:
`
`(METHOD AND APPARATUS FOR swrrcrinlc RADIO LINKS IN MOBILE COMMUNICATION
`
`(57) Abstract
`
`A radio link switching scheme in which a communication between the base station and the mobile station is carried out through the
`radio link given in terms of flames while providing a switching timing information formed by a layer 1 bit data in each flame of the radio
`link such as a unique word, a flag, a housekeeping bit, and a frame number. Then, the switching timing information in at least one of the
`flames of the radio link at the base station is changed to indicate a switching of the radio link whenever a need for switching the radio link
`between the base station and the mobile station arises, and transmitted to the mobile station. At the base station, the radio link is switched
`at a switching timing which is a prescribed period of time after each flame in which the switching timing information is changed, while at
`the mobile station the radio link is switched at a switching timing which is a predetermined period of time after each flame in which the
`changed switching timing information is detected.
`
`

`

`FOR THE PURPOSES OF INFORMAITON ONLY
`
`Codes used to identify States party to the PCT on the front pages of pamphlets publishing international
`applications under the PCT.
`-
`
`AT
`
`AU
`
`BB
`BE
`BF
`BG
`BJ
`
`BR
`
`BY
`
`CA
`
`CF
`CG
`CH
`CI
`
`CM
`CN
`
`CS
`CZ
`DE
`DK
`FS
`FI
`
`FR
`GA
`
`Austria
`
`Australia
`
`Barbados
`Belgium
`Burltina Fa-so
`Bulgmia
`Benin
`
`Brazil
`
`Belarus
`
`Canada
`
`Central African Republic
`Congo
`Switzerland
`Cote d'Ivoire
`
`_
`
`Cameroon
`China
`
`Czechoslovakia
`Czech Republic
`Germany
`Denmark
`Spain
`Finland
`
`France
`Gabon
`
`GB
`
`GE
`
`GN
`GR
`EU
`IE
`1'!‘
`
`JP
`
`United Kingdom
`
`Georgia
`
`Guinea
`Greece
`I-Izungary
`Ireland
`Italy
`
`Japan
`
`Q Kenya
`
`KG
`
`KP
`
`KR
`KZ
`
`LI
`LK.
`
`LU
`LY
`MC
`MD
`MG
`ML
`
`MN
`
`Kyrgystan
`
`Democratic People's Republic
`of Korea
`Republic of Korea
`Kazakhstan
`
`Liechtenstein
`Sri Lanka
`
`Luzxembottrg
`Latvia
`Monaco
`Republic of Moldova
`Madagascar
`Mali
`
`Mongolia
`
`'
`
`MR
`
`MW
`
`Mauritania
`
`Malawi
`
`NE
`NL
`N0
`NZ
`PL
`
`P'I‘
`
`R0
`
`RU
`
`SD
`SE
`SI
`SK
`
`SN
`TD
`
`TG
`TJ
`Tl‘
`UA
`US
`UZ
`
`VN
`
`Niger
`Netherlands
`Norway
`New Zealand
`Poland
`
`Portugal
`
`Romania
`
`Russian Federation
`
`Sudan
`Sweden
`Slovenia
`Slovakia
`
`Senegal
`Chad
`
`Togo
`Tajiltistan
`Trinidad and Tobago
`Ukraine
`United sum of America
`Uzbekistan
`
`Yiet Nam
`
`«,2
`
`+-
`
`

`

`W0 95! 10920
`
`PCTIJ'P94I01708
`
`DESCRIPTION
`
`METHOD AND APPARATUS FOR SWITCHING RADIO LINKS
`
`IN MOBILE COMMUNICATION
`
`TECHNICAL FIELD
`
`The present invention relates to a radio link
`
`switching scheme suitable for an intra-base station radio
`
`link switching._
`
`BACKGROUND ART
`
`In recent years,
`
`the mobile communication for carrying
`
`out communications by utilizing radio links between base
`
`stations and mobile stations has come to be widely used.
`
`The service area of such a mobile communication is formed
`
`by a multiplicity of cells (or zones), where a base station
`
`is provided in each cell. Thus, usually,
`
`the mobile station
`
`which moves within a certain cell is going to carry out the
`
`communication through a prescribed radio link with the base
`
`station provided in that certain cell. Here,
`
`the radio link
`
`used is not necessarily fixed to a particular one, and will
`
`be switched as the need arises.
`
`There are two types of the radio link switching
`
`required in the mobile communication,
`
`including the inter-
`
`base station radio link switching and an intra—base station
`
`radio link switching. The inter—base station radio link
`
`switching is executed in a case of changing the base
`
`station in communication with the mobile station, as the
`
`mobile station moves from a cell of the currently
`
`communicating base station to a neighboring cell of another
`
`base station. On the other hand,
`
`the intra-base station
`
`radio link switching is executed in a case the degradation
`
`

`

`W0 95/ 10920
`
`PCTIJP9-“M01708
`
`of the radio link quality is caused by the interference
`
`from the other base station or mobile station. A
`
`conventional scheme for this intra—base station radio link
`
`switching will now be described.
`
`In the digital mobile telephone system currently in
`
`practical use,
`
`the intra—base station radio link switching
`
`within one and the same base station is handled by the
`
`identical scheme as in the inter-base station radio link
`
`switching, which proceeds according to the sequence chart
`
`10
`
`shown in Fig. 1 as follows.
`
`Namely, while the base station and the mobile station
`
`are in communication by using the transceiver circuit of
`
`the base station, when this transceiver circuit detects the
`
`degradation of the quality of the radio link in use,
`
`this
`
`15
`
`fact is notified to a control circuit of the base station
`
`in a form of a link switching request (step S101).
`
`In
`
`response,
`
`the control circuit makes the selection of unused
`
`wire link, frequency,
`
`and time—slot (step S103),
`
`and
`
`notifies the selected wire link to an exchanger station.
`
`20
`
`Then,
`
`the exchange station makes the signal transmission
`
`with respect to the notified wire link in addition to the
`
`wire link currently in use (step S105) while the control
`
`circuit notifies the selected frequency and the time—slot
`
`to the transceiver circuit connected with the selected wire
`
`25
`
`link.
`
`The transceiver circuit which received this notice
`
`then generates the new radio link synchronization signal
`
`according to the notified frequency and the time—slot, and
`
`transmits the generated new radio link synchronization
`
`30
`
`signal to the mobile station (step S107).
`
`In addition,
`
`the
`
`control circuit transmits the new radio link designation
`
`signal to the mobile station via the old radio link so as
`
`to notify the frequency and the time-slot of the new radio
`link (step S109).
`
`35
`
`Then.
`
`the control circuit of the mobile station
`
`

`

`W0 95!10920
`
`'
`
`'
`
`. IP94/01708
`
`notifies the notified new frequency and time—slot
`
`to the
`
`transceiver circuit of the mobile station such that this
`
`transceiver circuit switches the frequency and time—slot to
`
`the notified new frequency and time-slot (step S111). Then,
`
`this transceiver circuit receives the new radio link
`
`synchronization signal transmitted from the transceiver
`
`circuit of the base station to carry out the
`
`synchronization establishing processing for the new radio
`
`link (step S113), so as to establish the new radio link
`
`synchronization on the mobile station side and the base
`
`station side (steps S115 and S117).
`
`When this synchronization is established,
`
`the base
`
`station stops the signal transmission to the old wire link,
`
`and releases the old wire link and radio link (step S119).
`
`Then,
`
`the communication through the newly set up radio link
`
`is started.
`
`Now,
`
`in contrast to the intra—base station radio link
`
`switching in the TDMA—FDMA system in which it suffices to
`
`change only the radio section parameters given by the
`
`frequency and the time-slot,
`
`the above described
`
`conventional
`
`intra—base station radio link switching in the
`
`mobile communication system uses two transceiver circuits
`
`on the mobile station side and the base station side,
`
`so
`
`that it also becomes necessary to change the wire link
`
`connected with the transceiver circuit of the base station
`
`side, and for this reason the switching processing is
`
`considerably more complicated.
`
`In order to eliminate the necessity for changing the
`
`wire link, it is necessary to switch the radio section
`
`parameters within one and the same transceiver circuit.
`
`Here,
`
`in a case of switching the radio section parameters
`
`within one and the same transceiver circuit, if the
`
`switching timings are different at the base station side
`
`and the mobile station side,
`
`there is a high probability
`
`for the breaking of the communication over a considerable
`
`

`

`W0 95/ 10920
`
`PCTIJP94/01708
`
`period of time to occur between the completion of the
`
`switching on one side and the completion of the switching
`
`on the other side. For this reason, it is preferable to
`
`carry out
`
`the radio link switching at the same timing on
`
`both the base station side and the mobile station side.
`
`However,
`
`in the above described conventional scheme,
`
`there
`
`is no means for notifying the switching timing among the
`
`base station and the mobile station, so that it has been
`difficult to carry out the radio link switching at the same
`
`10
`
`timing on both the base station side and the mobile station
`
`side.
`
`Moreover, even when the use of two transceiver
`
`circuits as in the above described conventional scheme is
`
`accepted,
`
`there still remains the problem of the breaking
`
`15
`
`of the communication between the base station and the
`
`mobile station over a period of time required since the
`
`switching of the frequency and time—slot at the transceiver
`
`circuit of the mobile station to the new values selected at
`
`the base station until
`
`the establishment of the
`
`20
`
`synchronization for the new radio link.
`
`DISCLOSURE OF THE INVENTION
`
`25
`
`It is therefore an object of the present invention to
`
`provide a radio link switching scheme capable of carrying
`
`out
`
`the radio link switching at the same timing on both the
`
`base station and the mobile station, eliminating the need
`
`for a processing with respect to the wire link, and
`
`30
`
`preventing the breaking of the communication at a time of
`
`the radio link switching.
`
`It is another object of the present invention to
`
`provide a radio link switching scheme capable of reducing a
`
`probability of the failure of the radio link switching,
`
`35
`
`carrying out the radio link switching at appropriate
`
`

`

`W0 95/ 10920
`
`PCT/JP94/01708
`
`timings, and causing no trouble in the frame order control
`
`at the mobile station side.
`
`According to one aspect of the present invention there
`
`is provided a method for switching a radio link in a mobile
`
`communication between a base station and a mobile station,
`
`comprising the steps of: carrying out a communication
`
`between the base station and the mobile station through the
`
`radio link given in terms of frames while providing a
`switching timing information formed by a layer 1 bit data
`
`10
`
`in each frame of the radio link; changing the switching
`timing information in at least one of the frames of the
`
`radio link at the base station to indicate a switching of
`
`the radio link whenever a need for switching the radio link
`
`between the base station and the mobile station arises;
`
`15
`
`transmitting the changed switching timing information from
`
`the base station to the mobile station; switching the radio
`
`link at the base station to a new radio link at a switching
`
`timing which is a prescribed period of time after each
`
`frame in which the switching timing information is changed
`
`20
`
`at the changing step; detecting the changed switching
`
`timing information transmitted from the base station at the
`
`mobile station; and switching the radio link at the mobile
`
`station to the new radio link at a switching timing which
`
`is a predetermined period of time after each frame in which
`
`25
`
`the changed switching timing information is detected at the
`
`detecting step.
`
`According to another aspect of the present invention
`
`there is provided a mobile communication system including a
`
`base station and a mobile station, comprising: means for
`
`30
`
`carrying out a communication between the base station and
`
`the mobile station through the radio link given in terms of
`
`frames while providing a switching timing information
`
`formed by a layer 1 bit data in each frame of the radio
`
`link; means for changing the switching timing information
`
`35
`
`in at least one of the frames of the radio link at the base
`
`

`

`W0 95/10920
`
`-
`
`PCT/J'P94I01708
`
`station to indicate a switching of the radio link whenever
`
`a need for switching the radio link between the base
`
`station and the mobile station arises; means for
`
`transmitting the changed switching timing information from
`
`the base station to the mobile station; means for switching
`
`the radio link at the base station to a new radio link at ai
`
`switching timing which is a prescribed period of time after
`
`each frame in which the switching timing information is
`changed by the changing means; means for detecting the
`
`changed switching timing information transmitted from the
`
`base station at the mobile station; and means for switching
`
`the radio link at the mobile station to the new radio link
`
`at a switching timing which is a predetermined period of
`
`time after each frame in which the changed switching timing
`
`information is detected by the detecting means.
`
`Other features and advantages of the present invention
`
`will become apparent from the following description taken
`
`in conjunction with the accompanying drawings.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`Fig.
`
`1 is a sequence chart for a conventional intra-
`
`base station radio link switching scheme.
`
`Fig. 2A is a schematic block diagram of a base station
`
`of a mobile communication system according to the present
`
`invention.
`
`Fig. 2B is a schematic block diagram of a transceiver
`
`unit in the base station of Fig. 2A.
`
`Fig. 3A is a schematic block diagram of a mobile
`
`station of a mobile communication system according to the
`
`present invention.
`
`Fig. 3B is a schematic block diagram of a transceiver
`
`unit in the mobile station of Fig. 3A.
`
`Fig. 4 is a sequence chart for the first embodiment of
`
`

`

`W0 95/ 10920
`
`PCTIJP94/01708
`
`a radio link switching scheme according to the present
`
`invention.
`
`Fig. 5 is a signal form diagram for a new spread code
`
`designation signal used in the scheme of Fig. 4.
`
`Fig. 6 is a diagram of transmission signals in the
`
`scheme of Fig. 4 for explaining the first case of using a
`
`unique word as the switching timing information.
`
`Fig. 7 is a diagram of transmission signals in the
`
`scheme of Fig. 4 for explaining a modification of the first
`
`case of using a unique word as the switching timing
`
`information.
`
`Fig.
`
`8 is a diagram of a transmission signal in the
`
`scheme of Fig. 4 for explaining the second case of using a
`
`flag in each frame as the switching timing information.
`
`Fig. 9 is a diagram of a transmission signal in the
`
`scheme of Fig. 4 for explaining one modification of the
`
`second case of using a flag in each frame as the switching
`
`timing information.
`F Fig. 10 is a diagram of a transmission signal in the
`scheme of Fig. 4 for explaining another modification of the
`
`second case of using a flag in each frame as the switching
`
`timing information.
`
`Fig. 11 is a signal form diagram of a transmission
`
`signal in the second case of using a flag in each frame as
`
`the switching timing information.
`
`Fig. 12 is a diagram of a transmission signal in the
`
`scheme of Fig. 4 for explaining the third case of using a
`
`flag in a housekeeping bit as the switching timing
`
`information.
`
`Fig. 13 is a diagram of a transmission signal in the
`
`scheme of Fig. 4 for explaining one modification of the
`third case of using a flag in a housekeeping bit as the
`switching timing information.
`4
`Fig. 14 is a diagram of a transmission signal in the
`
`scheme of Fig. 4 for explaining another modification of the
`
`

`

`W0 95/ 10920
`
`p
`
`PCTlJ.P94IO1'708
`
`third case of using a flag in a housekeeping bit as the
`
`switching timing information.
`
`Fig. 15 is a signal form diagram of a transmission
`
`signal in the third case of using a flag in a housekeeping
`
`bit as the switching timing information.
`
`Fig. 16 is a diagram of a transmission signal in the
`
`scheme of Fig. 4 for explaining a frame number.
`
`Fig. 17 is a diagram of a transmission signal in the
`scheme of Fig. 4 for explaining the fourth case of using a
`
`frame number as the switching timing information.
`
`Fig. 18 is a diagram of a transmission signal in the
`
`scheme of Fig. 4 for explaining one modification of the
`
`fourth case of using a frame number as the switching timing
`
`information.
`
`Fig. 19 is a signal form diagram of a transmission
`
`signal in the fourth case of using a frame number as the
`
`switching timing information.
`
`Fig. 20 is a sequence chart for the second embodiment
`
`of a radio link switching scheme according to the present
`
`invention.
`
`Fig. 21 is a diagrammatic illustration of a table used
`
`in the scheme of Fig. 20.
`
`Fig. 22 is a diagram of a transmission signal in the
`scheme of Fig. 20 for explaining one possible relationship
`
`between the switching timing notice frames and the
`
`switching timing.
`Fig. 23 is a diagram offa transmission signal in the
`
`scheme of Fig. 20 for explaining another possible
`
`relationship between the switching timing notice frames and
`
`the switching timing.
`
`Fig. 24 is a diagram of a transmission signal in the
`
`scheme of Fig. 20 for explaining still another possible
`
`relationship between the switching timing notice frames and
`the switching timing.
`
`Fig. 25 is a sequence chart for a modification of the
`
`

`

`wo 9sx1o920
`
`N
`
`PCTIJPSWO1708
`
`second embodiment of a radio link switching scheme
`
`according to the present
`
`invention.
`
`BEST MODE FOR CARRYING OUT THE INVENTION
`
`Now,
`
`the embodiments of the radio link switching
`
`scheme in the mobile communication according to the present
`
`invention will be described.
`
`It is to be noted first that,
`
`in the mobile
`
`communication, a plurality of mobile stations make
`
`communications with one base station, so that the multiple
`
`access scheme is adopted. Here,
`
`the multiple access scheme
`
`includes the frequency division multiple access (FDMA)
`
`scheme,
`
`the time division multiple access (TDMA) scheme,
`
`and a code division multiple access (CDMA) scheme, and each
`
`of these may be realized separately, or in combination with
`
`the others.
`
`In the present invention, it is possible to use
`
`any of these schemes as long as the radio link has a frame
`
`structure capable of containing the layer 1 bit data
`
`representing the switching timing information as will be
`
`described below. The only difference that arises due to the
`
`choice of the multiple access scheme will be the parameter
`
`for specifying the radio link, which is given by the
`
`frequency in the FDMA,
`
`the time-slot in the TDMA, and the
`
`spread code in the CDMA. In the following,
`
`the embodiments
`
`of the present invention will be described for an exemplary
`
`case of using the CDMA scheme for the sake of explanation,
`although the present invention is equally applicable to the
`
`FDMA scheme or the TDMA scheme.
`
`In this case,
`
`the base station suitable for the radio
`
`link switching scheme of the present invention has a
`
`configuration as shown in Fig. 2A, while the mobile station
`
`suitable for the radio link switching scheme of the present
`
`invention has a configuration as shown in Fig. 3A.
`
`In Figs.
`
`

`

`W0 95! 10920
`
`PCT/JP94I0l'708
`
`2A and 3A,
`
`the solid lines indicate communication lines
`
`while the dashed lines indicate control lines.
`
`In the configuration of Fig. 2A,
`
`the base station
`
`comprises: an antenna 11 for transmitting and receiving
`
`radio signals to and from the mobile station; a common
`
`amplifier 13 for amplifying the transmitting signals and
`
`the receiving signals;
`
`a plurality (N pieces) of
`
`transceiver units 15 (151.
`
`152 I
`
`"""" I
`
`15”) for encoding and
`
`modulating the transmitting signals,
`
`as well as decoding
`
`10
`
`and demodulating the receiving signals; and a base station
`
`control unit 17 for carrying out a control of the operation
`
`in the base station and a generation of a control signal
`
`for the mobile station.
`
`Here, each transceiver unit 15 corresponds to one
`
`15
`
`radio link. As shown in Fig. 2B, each transceiver unit 15
`
`includes a switching timing set up unit 15a, a switching
`
`timing information changing unit 15b, a spread code
`
`switching unit 15c, and a control unit 15d for controlling
`
`the operation in the transceiver unit 15.
`
`In addition, each
`
`20
`
`transceiver unit 15 is connected with a wire link for
`
`transmitting speech or data from the user which is
`
`connected to an exchange station.
`
`On the other hand.
`
`in the configuration of Fig. 3A,
`
`the mobile station comprises:
`
`an antenna 21 for
`
`25
`
`transmitting and receiving radio signals to and from the
`
`base station; an amplifier 23 for amplifying the
`
`transmitting signals and the receiving signals; a
`
`transceiver unit 25 for encoding and modulating the
`
`transmitting signals, as well as decoding and demodulating
`
`30
`
`the receiving signals; a mobile station control unit 27 for
`
`carrying out a control of the operation in the mobile
`
`station and a generation of a control signal for the base
`
`station; an operation unit 29 formed by a man-machine
`
`interface such as dial buttons; and an input/output circuit
`
`35
`
`31 for carrying out input and output processing for the
`
`-10-
`
`

`

`W0 95/10920
`
`PCT/JP94/01708
`
`speech or the user data.
`
`Here, as shown in Fig. 3B,
`the transceiver unit 251
`includes a switching timing information change detection
`
`unit 25a, a switching timing set up unit 25b, a spread code
`
`switching unit 25c, and a control unit 25d for controlling
`
`the operation in the transceiver unit 25.
`
`Next,
`
`the first embodiment of the radio link switching
`
`scheme according to the present invention will be described
`
`with reference to the sequence chart of Fig. 4.
`
`Namely,
`
`in this first embodiment,
`
`the radio link
`
`switching scheme is carried out according to the sequence
`
`chart of Fig. 4 as follows.
`
`Initially, it is assumed that the transceiver unit 15
`
`of the base station and the transceiver unit 25 of the
`
`mobile station is in communication by setting up a certain
`
`radio link.
`
`Then, when the transceiver unit 15 of the base station
`
`detects the quality degradation of that certain radio link
`
`(step S1),
`
`the transceiver unit 15 notifies this quality
`
`degradation to the control unit 17 of the base station
`
`(step S3).
`
`In response,
`
`the control unit 17 selects the unused
`
`spread codes of the switching target for each of the
`
`reverse link (mobile station 9 base station) and the
`
`forward link (base station 9 mobile station)
`
`(step S5), and
`
`transmits the new spread code designation signal containing
`
`the selected spread codes to the mobile station by using
`
`the radio link before the switching (step S7). Here,
`
`the
`
`new spread code designation signal has an exemplary form as
`
`shown in Fig. 5 which comprises a signal type 31 indicating
`
`that this signal is the new spread code designation signal,
`
`a reverse spread code number 32 indicating the selected
`
`spread code for the reverse link, and a forward spread code
`
`number 33 indicating the selected spread code for the
`
`foward link.
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`Then,
`
`the control unit 27 of the mobile station which
`
`received this new spread code designation signal stores the
`
`reverse and forward spread code numbers 32 and 33, and
`
`returns the new spread code designation confirmation signal
`
`as an acknowledge to the base station (step S9).
`
`Next,
`
`the control unit 17 of the base station which
`
`received the new spread code designation confirmation
`
`signal from the control unit 27 of the mobile station then
`
`notifies the spread codes for the reverse and forward links
`
`to be used in the new radio link to the transceiver unit 15
`
`(step S11). The transceiver unit 15 of the base station
`
`which received this notice then stores the notified spread
`
`codes for the reverse and forward links, and returns the
`
`new spread code confirmation to the control unit 17 (step
`
`S13).
`
`Next,
`
`in the transceiver unit 15 of the base station,
`
`an appropriate switching timing is set up at the switching
`
`timing set up unit 15a (step S15), and the switching timing
`
`information contained in a frame in a prescribed relation
`
`with respect to the switching timing set up at the
`
`switching timing set up unit 15a is changed in a manner
`
`described in detail below by the switching timing
`
`information changing unit 15b and the frame containing the
`
`changed switching timing information is transmitted to the
`
`transceiver unit 25 of the mobile station (step S17).
`
`Then,
`
`in the transceiver unit 25 of the mobile
`
`station, whether the switching timing information contained
`
`in the received frame is changed or not is checked by the
`
`switching timing information change detection unit 25a, and
`
`when the switching timing information change is detected
`
`(step S21),
`
`the switching timing is set up at the switching
`
`timing set up unit 25b in a prescribed relation with
`
`respect to the frame containing the changed switching
`
`timing information detected (step S23) such that the
`
`switching timing set up in the mobile station is identical
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`to the switching timing set up in the base station.
`
`Finally,
`
`the spread code switching unit 15c of the
`
`transceiver unit 15 of the base station carries out the
`
`switching of the spread code at the switching timing set up
`
`at the switching timing set up unit 15a, while the spread
`
`code switching unit 25c of the transceiver unit 25 of the
`
`mobile station carries out the switching of the spread code
`
`at the switching timing set up at the switching timing set
`
`up unit 25b, so as to realize the simultaneous switching of
`
`the spread code at the base station and the mobile station
`
`(step S25). After the switching is completed,
`
`the
`
`transceiver unit 15 of the base station and the transceiver
`
`unit 25 of the mobile station notify the completion of the
`
`spread code switching to the respective control units 17
`
`and 27.
`
`It is to be noted that,
`
`in a case the frame timing is
`
`not coinciding in the reverse link and the forward link, it
`
`suffices for the forward link to carry out the spread code
`
`switching on both the base station and the mobile station
`
`at a prescribed number of frames after the frame containing
`
`the switching timing information that has been changed at
`
`the base station. As for the reverse link, it suffices to
`
`carry out the spread code switching on both the base
`
`station and the mobile station at a prescribed number of
`
`frames after the frame which is closest to the frame in the
`
`forward link which contains the switching timing
`
`information that has been changed at the base station.
`
`Also,
`
`in this procedure of Fig. 4,
`
`in switching the
`
`spread codes at the base station and the mobile station,
`
`the synchronization state before the switching is
`
`maintained, and the signal transmission is carried out in
`
`the same syschronization state even after the switching.
`
`In
`
`the i3;ra—ba;r station radio link switching,
`
`the distance
`
`beta an the ba:e station and the mobile station is hardly
`
`changed imm fig Lely before and immediately after the
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`switching so that the synchronization timing is also hardly
`
`changed. Consequently,
`
`the synchronization timing can be
`
`maintained at a time of switching without any problem.
`
`Here,
`
`in a case the switching timing information
`
`detection processing result satisfies the out of
`
`synchronization condition during the radio link switching
`
`operation,
`
`the radio link switching operation is cleared
`
`and the synchronization establishment processing in the old
`
`radio link is carried out. However, even when it fails to
`
`10
`
`detect the switching timing information in a certain frame
`
`during the radio link switching operation, as long as the
`
`out of synchronization condition is not satisfied by the
`
`time immediately before the spread code switching timing,
`
`it is judged that it is not out of the synchronization, and
`
`15
`
`the switching of the spread codes is carried out.
`
`Thus,
`
`according to this first embodiment, it is
`
`possible to complete the radio link switching within the
`
`same transceiver unit 15 of the base station,
`
`so that there
`
`is no need for the switching of the wire link connected to
`
`20
`
`the transceiver unit 15 at the exchange station side. Also,
`
`by carrying out
`
`the switching at the same timing on both of
`
`the mobile station and the base station, it is possible to
`
`prevent the occurrence of the period in which the
`
`communication is impossible, and further by maintaining the
`
`25
`
`synchronization state at a time of the switching, it is
`
`also possible to eliminate the synchronization
`
`establishment processing for the new radio link,
`
`so that
`
`the breaking of the communication due to the
`
`synchronization establishment processing can be prevented.
`
`30
`
`Here,
`
`the maintaining the same synchronization state can be
`
`done very easily as only one transceiver unit of the base
`
`station is involved in the radio link switching operation
`
`of this first embodiment.
`Here,
`in order to prevent the breaking of the
`
`communication, it is also necessary for the time required
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`for the radio link switching, i.e., the time required in
`
`changing the radio link parameter within the transceiver
`
`unit,
`
`to be sufficiently shorter than the signal
`
`transmission rate.
`
`In other words,
`
`the upper limit of the
`
`signal transmission rate is determined by the time required
`
`in changing the radio link parameter.
`
`In this regard,
`
`the
`
`CDMA is suitable for the high speed signal transmission
`
`rate because the change of the radio link parameter in the
`
`CDMA is the change of the spread code set up in the
`
`correlator which can be realized at a high speed very
`
`easily. Consequently,
`
`the use of the CDMA is advantageous
`
`in this first embodiment.
`
`Now,
`
`in this first embodiment,
`
`the switching timing
`
`information to be provided in each frame used in the above
`
`procedure can be given by the layer 1 bit data in general,
`
`and there are several possible choices for this layer 1 bit
`
`data including the following cases.
`
`As a first case,
`
`the switching timing information can
`
`be given by the so called unique word (UW) which is
`
`provided for the purpose of the maintaining the frame
`
`synchronization in the synchronization establishment
`
`processing, and includes two types of UWO and UW1. Here,
`
`UWO and UW1 have a sufficient Hamming distance from each
`
`other such that the probability for confusing these two
`
`types is very low.
`
`In this case, as shown in Fig. 6,
`
`the transceiver unit
`
`15 of the base station normally uses UWO as the switching
`
`timing information for the forward link as in the frame 61.
`
`Then, at the step S17 of Fig. 4,
`
`the transceiver unit 15
`
`changes the switching timing information to UW1 as in the
`
`frame 62. Thereafter,
`
`the transceiver unit 15 uses UWO
`
`again as in the subsequent frames 63 to 66.
`
`Similarly,
`
`the transceiver unit 25 of the mobile
`
`station normally uses UWO as the switching timing
`
`information in the reverse link as in the frame 71.
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`Meanwhile,
`
`the transceiver unit 25 of the mobile
`
`station regularly monitors the switching timing information
`
`in the forward link to detect UWO normally. Then, whenever
`
`UWO cannot be detected,
`
`the transceiver unit 25 carries out
`
`the detection of UW1, and when UW1 is detected,
`
`the
`
`transceiver unit 25 recognizes the start of the radio link
`
`switching operation.
`
`In response, as an acknowledgement of
`
`the start of the radio link switching operation,
`transceiver unit 25 changes the switching timing
`
`the
`
`information in the forward link to UW1 as in the frame 73.
`
`Here,
`
`the frame 73 is the first frame in the reverse link
`
`for which the switching timing information can be changed
`
`since the detection of UW1 in the forward link in the frame
`
`62. Thereafter the transceiver unit 25 continues to use UWl
`
`until the spread code switching timing as in the frames 74
`
`and 75.
`
`On the other hand,
`
`the transceiver unit 15 of the base
`
`station regularly monitors the switching timing information
`
`in the reverse link to detect UWO normally. However, at the
`frames 73 to 75 for which UW1 is expected to be used in the
`
`reverse li