as) United States
`a2) Patent Application Publication 0) Pub. No.: US 2002/0032030 Al
`(43) Pub. Date: Mar. 14, 2002
`
`Berglundetal.
`
`US 20020032030A1
`
`(54) COMMUNICATION SYSTEM
`
`(30)
`
`Foreign Application Priority Data
`
`(76)
`
`Inventors: Arne Kristian Berglund, Upplands
`Vasby (SE); David Bladsjo, Stockholm
`(SE); Stefan Eriksson, Stockholm (SE);
`Joakim Sorelius, Uppsala (SE)
`
`Correspondence Address:
`JENKENS & GILCHRIST, PC
`1445 ROSS AVENUE
`SUITE 3200
`
`DALLAS, TX 75202 (US)
`
`(21) Appl. No.:
`
`09/939,083
`
`(22)
`
`Filed:
`
`Aug. 24, 2001
`
`Related U.S. Application Data
`
`(63) Non-provisional of provisional
`60/228,733,filed on Aug. 28, 2000.
`
`application No.
`
`Aug. 28, 2000
`
`(SE) we cecseseesscssceneeeneenneess 0003042.9
`
`Publication Classification
`
`(SL) Ute C07 cacsccsssssssssssnssessesnstsntsnstsevee H04Q 7/20
`(52) US. Ch.
`cecescsssssssscnstssssetnsenssstn 455/434; 455/436
`
`(57)
`
`ABSTRACT
`
`Method in a communication system using a logical control
`channel transmitting a control channel message, including
`handover information, between a mobile station 10 and a
`base station 26, in said system, using a first control channel
`modulation scheme. The method comprises the step of
`re-transmitting the control channel message using a second
`control channel modulation scheme, if the control channel
`message using the first control channel modulation scheme
`is not received or understood by the mobile station. Accord-
`ing to a preferred embodiment the control channelis a Fast
`Associated Control Channel (FACCH).
`
`GELL
`
`MOBILE STATION
`
`DELL EX. 1012
`
`DELL EX. 1012
`
`1
`
`

`

`Patent Application Publication Mar. 14, 2002 Sheet 1 of 2
`
`US 2002/0032030 Al
`
`Cr
`
`CELL
`
`C |
`
`MOBILESTATION
`
`IP NETWORK
`
`BACKBONE
`IP NETWORK
`
`EXTERNAL
`
`2
`
`

`

`Patent Application Publication Mar. 14, 2002 Sheet 2 of 2
`
`US 2002/0032030 Al
`
`TOMA
`£OAME tember
`
`7? SO/ /2/3 fy UF...
`Tee 3 Talele 8
`Lo|fz|Pr2 2 ||LE]LA2|5S|lélé]|
`
`
`
`4l?l2l2lelel-[-[2[s]4[e)TT|
`
`Speecs
`frame iqbbe
`
`FIG. 3a
`
`
`
`FIG. 3b
`
`3
`
`

`

`US 2002/0032030 Al
`
`Mar. 14, 2002
`
`COMMUNICATION SYSTEM
`
`FIELD OF THE INVENTION
`
`invention relates to a method in a
`[0001] The present
`communication system and the communication system in
`accordance with the preambles of the independent claims.
`
`BACKGROUND OF THE INVENTION
`
`[0002] U.S. Pat. No. 5,182,753 discloses a method of
`transmitting signaling messages in a mobile radio commu-
`nication system and gives an overview of the principles
`applicable also in the present application.
`
`[0003] FIG. 1 in the present application illustrates the
`basic configuration of a cellular mobile telecommunication
`system. Such a system comprises a numberofcells, each cell
`comprising a base station for radio communication with a
`numberof mobile stations, of which only one is shownin the
`figure. Basically the mobile station communicates with the
`base station of the cell in which it is presently moving. As
`the mobile movesfrom cell to cell, communication with the
`mobile station is handed over from the base station of one
`cell
`to the base station of another cell. This is called
`
`handover procedure. During handover procedure the old
`base station informs the mobile station to switch commu-
`nication to another channel of the new base station. A
`handover command or message does this. However, since
`one of the reasons for the handover commandin thefirst
`place is typically that the transmission conditions between
`the old base station and the mobile station are no longer
`acceptable, there is a risk that the handover command never
`reaches the mobile station or cannot be interpreted correctly
`by said station. In such a case there is a possibility that the
`call might be lost. One object of the present invention is to
`protect
`the handover command in such a way that
`the
`chances of correctly receiving it by the mobile station are
`significantly increased.
`
`[0004] Although the detailed description of preferred
`embodimentsof the present invention will concentrate of the
`handover command, it is to be understood that the same
`principles can also be used for other signaling messages, and
`that
`the invention is therefore not
`limited to handover
`commandsonly.
`
`[0005] A Fast Associated Control Channel, FACCH,is a
`logical signaling channel for the transmission of control and
`supervision messages between the base station and the
`mobile station during an ongoing call. The FACCH by
`definition replacesor steals the speech or user data frames on
`the traffic channel whenever system considerations deem it
`appropriate to do so. Thus, the FACCH is a blank and burst
`channel.
`
`In the above-mentioned U-S. Pat. No. 5,182,753it
`[0006]
`is described how messages on the FACCHcan beprotected
`by meansoferror correcting and/or channel coding schemes.
`Further protection of critical messages, for instance the
`handover order, can be obtained by retransmission of the
`coded message if acknowledgementis not received within a
`specified time period, by sending the message repeatedly in
`one sequence until acknowledgementis received, by send-
`ing the message repeatedly a fixed numberof times without
`requiring acknowledgement, or by sending the message
`repeatedly a fixed number of times or until acknowledge-
`
`ment has been received. The radio link quality is often
`fluctuating and it is hoped that the mobile then will by able
`to receive the message properly at least once.
`
`[0007] General packet radio service (GPRS)is a standard
`from the European Telecommunications Standards Institute
`(ETSDon packet data in GSM systems. GPRShasalso been
`accepted by the Telecommunications Industry Association
`(TIA) as the packet-data standard for TDMA/136 systems.
`By adding GPRS functionality to the public land mobile
`network (PLMN), operators can give their subscribers
`resource-efficient access to external Internet protocol-base
`(IP) networks.
`
`GPRSoffers air-interface transfer rates up to 115
`[0008]
`kbit/s subject to mobile terminal capabilities and carrier
`interference. Even higher transfer rates are provided by the
`use of so called EDGE technology,i.e. Enhanced Data Rates
`for Global Evolution. EDGE technology can increase end
`user data rates up to 384 kbit/s, and potentially higher in high
`quality radio environments. Moreover, GPRSallowsseveral
`users to share the same air-interface resources and enables
`
`operators to charge customers for wireless services based on
`the amountof transferred data instead of on connection time.
`
`[0009] The present inventionis in particular applicable in
`GSM/EDGERadio Access Network (GERAN).
`
`[0010] When introducing an 8-PSK modulation for radio
`transmission of speech in GERAN,the associated control
`channels have to be specified. An especially important
`control channel
`is the above-mentioned Fast Associated
`Control Channel (FACCH).
`
`[0011] Due to the importance of the FACCH messages,
`this control channel is designed so that it is transmitted
`instead of speech frames. Because the speech has very high
`delay requirements, there is no meaning in transmitting the
`blanked speech frames at a later time.
`
`It is important to design the FACCH so that a
`[0012]
`minimal number of speech frames are blanked in order to
`avoid bad speech quality. This is achieved when the FACCH
`has the same type of interleaving as the speech. However,it
`is also important to make the FACCHas robust as possible
`when the radio connection has been deteriorated, as a
`handoveris likely to be necessary at such conditions.
`
`[0013] Different modulation methods for the FACCH for
`the 8-PSK speech bearer may be used: e.g. 8-PSK modu-
`lated FACCH and GMSK modulated FACCH. 8-PSK modu-
`lated FACCH blanks a minimum numberof speech frames
`as it can be interleaved in the same way as speech data.
`However, one potential problem of 8-PSK modulation is its
`limited robustness in bad channel conditions. GMSK modu-
`
`lated FACCHis more robust to the propagation channel than
`8-PSK modulated FACCH and can be transmitted at a higher
`power, which is well-known in the art, but results in the
`blanking of more speech frames due to the different inter-
`leaving pattern.
`
`Thereis currently no possibility to make a trade-off
`[0014]
`between the requirements of minimal blanking of speech
`frames and robustness.
`
`[0015] U.S. Pat. No. 5,909,469, assigned to the same
`assignee as the present application, discloses a link adapta-
`tion method and system for communication links using
`modulation schemes that have different symbol rates. The
`
`4
`
`

`

`US 2002/0032030 Al
`
`Mar. 14, 2002
`
`system supports at least two modulation schemesthat do not
`have the same symbol rate. The system measures the per-
`formance of the communication. This measurement may be
`performed by determining a numberof parameters, includ-
`ing received signal strength at the mobile station, bit error
`rate at the mobilestation, the multipath propagation property
`of the uplink RF channel, for example, time dispersion, or a
`combination thereof. A higher (less robust) modulation
`scheme is maintained if the performance of the communi-
`cation is acceptable, i.e. above some predetermined thresh-
`old. If the performance is not acceptable the modulation
`scheme is changed to a more robust one. In above-refer-
`enced U.S. Pat. No. 5,909,469 the change of modulation
`scheme for the communication is performed in response to
`a continuously measured quality measure of the traffic
`channel radio-link. U.S. Pat. No. 5,909,469 is not concerned
`with transmission on logical control channels specifically.
`
`[0016] The object of the present invention is to achieve a
`system where an optimal performance of the system is
`achieved,i.e. to achieve a robust logical control channel so
`that a minimum of speech frames are blanked.
`
`SUMMARYOF THE INVENTION
`
`is achieved by a
`[0017] The above-mentioned object
`method and a system according to the characterizing parts of
`the independent claims.
`
`[0018] Preferred embodiments are set forth in the depen-
`dent claims.
`
`[0019] Thus, by using a control channelthat is modulation
`adaptive, the above-mentioned objects are achieved.
`
`the used
`[0020] According to a preferred embodiment
`control channel is a FACCH. However,
`the invention is
`applicable on any control channel transmitting signaling
`messages during ongoing calls, e.g. handover information,
`in a communication system.
`
`[0021] According to the preferred embodiment of the
`invention the network first transmits the FACCH message
`using the 8-PSK modulation, thus blanking a minimal num-
`ber of speech frames. If this transmission fails, i.e. if the
`mobile station does not properly receive the FACCH mes-
`sage (detected by the network, e.g. that no acknowledge-
`mentis transmitted to the base station or if no handoveris
`performed), the FACCH is retransmitted using the GMSK
`modulation. Then more speech framesare blanked, but it can
`be expected that the channel quality is so low that the speech
`is close to unintelligible anyway.
`
`[0022] The system according to the preferred embodiment
`of the invention combines the minimal speech quality
`impact that comes from using the 8-PSK modulation, with
`the robustness of the GMSK modulation.
`
`SHORT DESCRIPTION OF THE APPENDED
`DRAWINGS
`
`[0023] FIG. 1 illustrates the basic configuration of a
`cellular mobile telecommunication system,
`
`[0024] FIG. 2 illustrates an exemplary communication
`system in which the present invention may be implemented.
`
`[0025] FIGS. 3a and 35 illustrate interleaving patterns
`when the FACCH messageis transmitted with 8-PSK modu-
`lation and GMSK modulation, respectively.
`
`DETAILED DESCRIPTION OF PREFERRED
`EMBODIMENTS OF THE INVENTION
`
`[0026] FIG. 2 illustrates an exemplary communication
`system 2 in which the present invention may be imple-
`mented.
`In particular the system 2 depicted in FIG. 2
`conforms to the GSM specifications and supports GPRSand
`Enhanced GPRS (EGPRS)(e.g. GERAN) technology. The
`mobile telecommunications system 2 includes a circuit-
`switched network 4, a packet-switched network 6, and a
`radio network 8 that is shared by the circuit-switched an
`packet-switched networks 4 and 6. Generally, the circuit-
`switched network 4 is primarily used for voice applications,
`while the packet-switched network 6 is primarily used for
`data applications.
`In accordance with third generation
`mobile telecommunications standard, however, the circuit-
`switched network 4 can also support data communications,
`and the packet-switched network 6 can also support voice
`communications.
`
`[0027] The circuit-switched network 4 includes a number
`of mobile services switching center/visitor location registers
`(MSC/VLRs) 12. For purposes of simplifying the illustra-
`tion, however, only on MSC/VLR 12 is shown. Each MSC/
`VLR 12 servesa particular geographic region and is used for
`controlling communications in the served region and for
`routing communications to other MSC/VLRs 12. The VLR
`portion of the MSC/VLR 12 stores subscriber information
`relating to mobilestations 10 that are currently located in the
`served region. The circuit-switched network 4 further
`includes at least one gateway mobile services switching
`center (GMSC) 14 that serves to interconnect the circuit-
`switched network 4 withc external networks, such as a
`public switched telephone network (PSTN) 16.
`[0028] The packet-switched network 6 includes a number
`of serving GPRS support nodes (SGSN)18, which are used
`for routing and controlling packet data communications, and
`a backboneIP network 20. A gateway GPRS support node
`(GGSN) 22 interconnects the packet-switched network 6
`with an external IP network 24 or other external data
`networks.
`
`[0029] The radio network 8 includes a plurality of cells.
`Each cell in the mobile telecommunications system 2 is
`served by a base station 26 that communicates with mobile
`stations 10 in the cell via an air interface 28. The radio
`network 8 comprises a plurality of base stations 26 and a
`base station controller (BSC) 27,alternatively referred to as
`a Radio Network Controller (RNC), controlling said plural-
`ity of base stations 26. For circuit-switched communica-
`tions, signals are routed from the MSC/VLR12,to the base
`station controller 27 via an interface 34, to the base station
`26 for the cell in which the target mobile station 10 is
`currently located, and overthe air interface 28 to the mobile
`station 10. For packet data transmissions, on the other hand,
`signals are routed from the SGSN 18, to the base station
`controller 27 via an interface 35, to the base station 26 for
`the cell in which the target mobile station 10 is currently
`located, and over the air interface 28 to the mobile station
`10.
`
`[0030] Each mobile station 10 is associated with a home
`location register (HLR) 30. The HLR 30 stores subscriber
`data for the mobile station 10 that is used in connection with
`
`circuit-switch communications and can be accessed by the
`MSC/VLRs12 to retrieve subscriber data relating to circuit-
`switched services.
`
`5
`
`

`

`US 2002/0032030 Al
`
`Mar. 14, 2002
`
`[0031] Each mobile station 10 is also associated with a
`GPRSregister 32. The GPRS register 32 stores subscriber
`data for the mobile station 10 that is used in connection with
`packet-switched communications and can be accessed by the
`SGSNs18 to retrieve subscriber data relating to packet-
`switched services.
`
`[0032] The handover procedure between a mobile station
`and a base station in a communication system was generally
`described above in the general part of the description. In
`order to fully understand the present invention, a description
`of an exemplary handover procedure which may be used in
`GSMsystemsand using a FACCHasthe control channel is
`given below.
`
`[0033] When a mobile station is moving around in a
`network, it eventually leaves the coverage area of one cell
`and enters another one. If there is no call in progress, the
`mobilestation will tell the network if it has left one location
`area and in that case update the registration aboutits location
`area in the network. If there is a call going on, the network
`decides whenit is time to changecells and to which new cell
`(to perform a handover). The network takes the decision
`mainly based on the measurement reports from the mobile
`station. The performance of the handover procedure on the
`air interface will be described, e.g. how messages and timers
`are involved in the air interface for a handover. After the
`
`network has decided that a handover will be performed it
`sends a Handover Command message on the downlink
`FACCH. This message contains information on which new
`cell the mobile shall access and which frequency and time
`slot
`to use for the new Traffic Channel (TCH). It also
`specifies whether a synchronized handover(including type)
`shall be performed by the mobile station. In the case of
`frequency hopping,a list of hopping frequencies to be used
`is also added.
`
`[0034] The numberof bits necessary for the message will
`today not exceed 184 in GSM,but morebits are needed for
`a frequency hopping DCS 1800 system. For GSM one
`message frame is needed while two frames are needed for
`DCS 1800 if frequency hopping is used. If necessary even
`more than two frames may be used. The maximum number
`of frames that a message can contain is limited to 13 for
`FACCH.
`
`[0035] A synchronized handover can be made in three
`different ways although the messages transmitted after the
`Handover Commandare the same. Thedifferencelies in the
`waythe time alignment is calculated from the information in
`the Handover Command message.
`
`[0036] After reception of a Handover Command message
`the mobile station suspends the acknowledge mode,tunes to
`the new channel, transmits four Handover Access bursts and
`then initiates an establishment of acknowledge mode. After
`establishment, the mobile station transmits the Handover
`Complete message to finish the handover.
`
`Inthe non synchronized handovercase, after recep-
`[0037]
`tion of a Handover Command andsuspension of acknowl-
`edge mode, the mobile sends Handover Access bursts in
`consecutive FACCHtime slots. The Handover Access bursts
`
`are transmitted until a reception of a Physical Information
`message from the network, or until a timer expires. The
`timer is set when the first Handover Access burst is trans-
`mitted.
`
`[0038] The network must correctly receive two of three
`Handover Access bursts to continue and transmit a Physical
`Information message. The physical
`information message
`contains the calculated time alignment from the reception of
`the access bursts and is transmitted in an unnumbered
`information frame, thus the mobile station does not need to
`acknowledge this message. Therefore the network sets a
`timer when transmitting the Physical Information message.
`If this timer expires before receiving a correct speech or
`message frame, the network retransmits the Physical Infor-
`mation message frame. Thefirst time the Physical Informa-
`tion message is transmitted,
`it is transmitted in two con-
`secutive frames. A constant set to a default value limits the
`number of retransmissions.
`
`[0039] After reception of a Physical Information the Han-
`dover Complete message is transmitted from the mobile
`station.
`
`[0040] The Handover Complete messagetells the network
`that the handover has been successful. After receiving a
`Handover Complete message the system erases the alloca-
`tion of the old channel, thus the old channel hereafter is free
`to be used by another user.
`
`[0041] Below are described situations that might occur
`when, for some reason, the FACCH Handover Command
`message is not properly received by the mobile station,
`including the case that the message is not received atall.
`
`If a preset timer expires or the establishment of
`[0042]
`acknowledge mode on the new channel fails, the mobile
`station returns to the old channel, initiates an establishment
`of acknowledge mode and then transmits a Handover Failure
`message containing the failure reason.
`
`If Handover Complete, Handover Failure or the
`[0043]
`establishment on the old channelis unsuccessful, the mobile
`station will consideritself as lost and a specified procedure
`(not within the scope of the present application) is followed.
`
`invention is applicable during the
`[0044] The present
`above-described handover procedure whenever the network
`detects that the mobile station has failed to properly receive
`the various signaling messages transmitted from the base
`station. In practice, this may basically be detected in three
`different ways:
`
`1. The mobile station continues to transmit
`[0045]
`speech frames on the old channel
`
`2. The mobile station sends the Handover
`[0046]
`Failure message
`
`3. No acknowledge message, e.g., Handover
`[0047]
`Complete,
`is received from the mobile station as
`stipulated.
`
`[0048] Referring back to FIG. 2 the present invention is
`primarily implementedin the radio network 8 that comprises
`a plurality of base stations 26 and a base station controller
`(BSC) 27 controlling said plurality of base stations 26.
`
`In the method according to the invention, a com-
`[0049]
`munication system transmits a control signaling message,
`including e.g. handover information, during an ongoing call
`connection between a mobile station and a base station on a
`
`logical control channel using a first control channel modu-
`lation scheme. The logical control channel is used during an
`ongoing call, e.g. when the quality of the radio communi-
`
`6
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`

`

`US 2002/0032030 Al
`
`Mar. 14, 2002
`
`cation, e.g. data or voice communication, is low. This is
`determined in accordance with well established techniques
`and is therefore not further described herein. The method
`comprises the step of re-transmitting the control channel
`message using a second control channel modulation scheme,
`if the control channel message usingthefirst control channel
`modulation scheme is not received or understood by the
`mobile station. According to a preferred embodiment the
`re-transmission of the control channel messageis triggered
`by a failed handover,
`i.e.
`the control channel message
`indicates that a handover should be performed but no
`handover is performed. This may be detected as described
`above.
`
`In the general case the control channel modulation
`[0050]
`scheme may thus be changed for re-transmission if no
`acknowledgement message is transmitted from the mobile
`station to the base station confirming proper receipt of the
`control channel message.
`[0051] According to another preferred embodimentof the
`invention the control channel is a Fast Associated Control
`Channel (FACCH). The FACCHis described in detail in the
`above-referenced U.S. Pat. No. 5,182,753.
`
`[0052] Accordingtostill another preferred embodiment of
`the invention the first control channel modulation scheme is
`an 8-PSK modulation and the second control channel modu-
`lation scheme is a GMSK modulation. However, the inven-
`tion is equally applicable when using any known control
`channel modulation schemes, such as: GMSK, 4-PSK,
`8-PSK, 16 QAM and 64 QAM.
`
`[0053] As mentioned above the first control channel
`modulation scheme may involve a lower numberof blanked
`speech frames than the second control channel modulation
`scheme. The reason is that an 8-PSK modulated control
`
`message, in the embodiment using 8-PSK and GMSKasthe
`first and second control channel modulation scheme, uses
`the sameinterleaving pattern as the 8-PSK modulated voice
`bearer. Therefore, the control message will fit exactly into
`the interleaving sequence without having to transmit empty
`frames as will be explained below.
`[0054] FIG. 3a illustrates the interleaving pattern when
`the FACCH message(F) is transmitted with 8-PSK modu-
`lation. During speech, each TDMA (TimeDivision Multiple
`Access) frame burst contains bits whichare interleaved from
`two speech frames, e.g., the bursts in TDMAframes 1 and
`2 contain bits from both speech frames 0 and 1, TDMA
`frames 3 and 4 contain bits from both speech frames 1 and
`2 and so forth. When the FACCH messageis transmitted, the
`FACCH frames F can replace speech frames 3 and 4 in
`TDMAframes 5-10 since the same modulation enablesthat
`
`FACCH and speech can share TDMAframes.
`[0055] FIG. 3d illustrates the interleaving pattern when
`the FACCH message (F) is transmitted with GMSK modu-
`lation. In this case, the modulation changes from 8-PSK to
`GMSK in TDMAframe7 and goes back again to 8-PSK in
`TDMAframe 11. The modulation change can not occur in
`the middle of a speech frame but must wait until speech
`frame 2 in this case is transmitted, i.e. after TDMAframe 6.
`The FACCHis then transmitted during TDMAframes 7-10,
`but three speech frames 3, 4 and 5 are blanked instead of
`only two speech frames 3 and 4 as in FIG.3a.
`[0056] According to still another embodiment of the
`invention the method comprises the step of re-transmitting
`
`the control channel message using a third control channel
`modulation scheme,if the control channel message using the
`second control channel modulation schemeis not received
`or understood by the mobilestation. Thisstep is triggered in
`the same way as when changing from the first to the second
`modulation scheme.
`
`[0057] The present invention is not limited to the above-
`described preferred embodiments. Various alternatives,
`modifications and equivalents may be used. Therefore, the
`above embodiments should not be taken as limiting the
`scope of the invention, which is defined by the appendant
`claims.
`
`1. Method in a radio communication system using a
`logical control channel for transmitting control signaling
`messages from a base station to a mobile station during an
`ongoing call, characterized in that the method comprises the
`steps of:
`
`transmitting from the base station at least one control
`signaling message using a first modulation scheme,
`
`detecting that the mobile station has not properly received
`and understood said message, and
`
`re-transmitting said control signaling message using a
`second modulation scheme in response to said detect-
`ing step.
`2. Method according to claim 1, characterized in that said
`logical control channel is transmitted on a traffic channel of
`said ongoing call using blank and burst.
`3. Method according to claim 2, characterized in that said
`logical control channelis a Fast Associated Control Channel
`(FACCH).
`4. Method according to claim 1, characterized in that said
`at least one control signaling message includes handover
`information.
`
`5. Method according to any preceding claim, character-
`ized in that said first control channel modulation scheme is
`the 8-PSK modulation.
`
`6. Method according to any preceding claim, character-
`ized in that said second control channel modulation scheme
`is the GMSK modulation.
`
`7. Method according to any preceding claim, character-
`ized in that the first control channel modulation scheme
`
`involves a lower numberof blanked speech frames than the
`second control channel modulation scheme.
`
`8. Method according to any preceding claim, character-
`ized in that the first control channel modulation scheme is
`less robust
`than the second control channel modulation
`scheme.
`
`9. Method according to any preceding claim, character-
`ized in that said detecting step includes detecting that no
`acknowledge message is received by the base station con-
`firming receipt of the control channel message.
`10. Method according to any preceding claim, character-
`ized in that said detecting step includes receiving by the base
`station a message indicating a failed handover.
`11. Method according to any of claims 4-8, characterized
`in that said detecting step includes detecting that the mobile
`station continues to transmit on an old channel.
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`7
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`US 2002/0032030 Al
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`Mar. 14, 2002
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`12. Method according to any preceding claim, character-
`ized in that the method comprises the stepsof:
`
`detecting that the mobilestation has not properly received
`and understood said message transmitted by use of said
`second modulation scheme, and
`
`re-transmitting said control signaling message using a
`third modulation scheme in response to said detecting
`step.
`13. Radio communication system comprising base sta-
`tions (26), mobile stations (10) and a control unit (27)
`controlling at least one of said base stations, wherein the
`control unit controls the base station to transmit at least one
`
`control signaling message to a mobile station during an
`ongoing call using a logical control channel, characterized in
`that the control unit further controls the base station to
`
`re-transmit said at least one control signaling message using
`a second modulation scheme in response to detecting that
`the mobile station has not properly received and understood
`said message as transmitted using said first modulation
`scheme.
`
`14. Radio communication system according to claim 13,
`characterized in that said control unit (27) is a Base Station
`Controller (BSC).
`15. Radio communication system according to any of
`claim 13 or 14, characterized in that said logical control
`channelis a blank and burst channel.
`16. Radio communication system according to any of
`claims 13-15, characterized in that said at least one control
`signaling message includes handover information.
`17. Communication system according to any of claims
`13-16, characterized in that said control channel is a Fast
`Associated Control Channel (FACCH).
`
`18. Communication system according to any of claims
`13-17, characterized in that said first control channel modu-
`lation scheme is the 8-PSK modulation.
`
`19. Communication system according to any of claims
`13-18, characterized in that said second control channel
`modulation scheme is the GMSK modulation.
`
`20. Communication system according to any of claims
`13-19, characterized in that the first control channel modu-
`lation scheme involves a lower number of blanked speech
`frames than the second control channel modulation scheme.
`
`21. Communication system according to any of claims
`13-20, characterized in that the first control channel modu-
`lation schemeis less robust than the second control channel
`modulation scheme.
`
`22. Communication system according to any of claims
`13-21, characterized in that the detection that the mobile
`station has not properly received and understood said mes-
`sage includes that no acknowledge message is received by
`the base station confirming receipt of the control channel
`message.
`23. Communication system according to any of claims
`13-21, characterized in that the detection that the mobile
`station has not properly received and understood said mes-
`sage includes receiving by the base station a message
`indicating a failed handover.
`24. Communication system according to any of claims
`13-21, characterized in that the detection that the mobile
`station has not properly received and understood said mes-
`sage includesthat the mobile station continuesto transmit on
`an old channel.
`
`8
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