(12) Unlted States Patent
`(10) Patent No.:
`US 6,466,568 B1
`
`Raith et al.
`(45) Date of Patent:
`Oct. 15, 2002
`
`U5006466568B1
`
`(54) MULTI-RATE RADIOCOMMUNICATION
`SYSTEMS AND TERMINALS
`
`(75)
`
`Inventors: Alex Krister Raith, Durham; James
`Ragsdale, Raleigh; John Diachina,
`Garner, all of NC (US)
`
`(73) Assignee: Telefonaktiebolagel; LM Ericsson
`(publ), Stockholm (SE)
`
`( * ) Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U'S'C' 154(b) by 0 days.
`
`.
`(21) Appl. No“ 09/399’771
`(22) Filed:
`Sep. 21, 1999
`Related US. Application Data
`
`(62) DiViSion 0f application N0~ 08/725543; filed 011 001~ 157
`1996’ now Pat‘ No’ 599879019
`
`Int. Cl.7 ................................................. G06F 11/00
`(51)
`....................... 370/347; 370/328; 370/471,
`(52) US. Cl.
`455/422
`(58) Field of Search ................................. 370/320 342
`370/335 347 280 294 328 329’ 321’
`330 337’ 479’ 468’ 469’ 470’ 471’ 472?
`’
`’
`’
`’
`455/422, 561 575’
`’
`’
`
`(56)
`
`References Cited
`US. PATENT DOCUMENTS
`
`5,177,740 A
`5,182,753 A
`5,230,003 A
`5,299,235 A
`5,570,467 A
`
`1/1993 Toy et al.
`1/1993 Dahlin et a1.
`7/1993 Dent et 211.
`3/1994 Larsson et a1.
`10/1996 Sawyer
`
`5,603,081 A
`5,757,813 A *
`5,770,927 A
`
`2/1997 Raith et a1.
`5/1998 Raith ......................... 370/468
`6/1998 Abe
`
`7/1999 Raith ......................... 455/422
`5,930,706 A *
`FOREIGN PATENT DOCUMENTS
`399612
`11/1990
`
`605312
`642233
`WO 95/01012
`
`7/1994
`3/1995
`1/1995
`
`EP
`
`EP
`EP
`W0
`
`7/1996
`
`ABSTRACT
`
`WO 96/21998
`W0
`* cited by examiner
`Primary Examiner—Wellington Chin
`Assistant Examiner—Frank Duon
`g
`(74) Attorney, Agent, or Firm—Burns, Doane, Swecker &
`Mathls, LLP
`57
`(
`)
`Variances in bandwidth used by a radiocommunication
`connection are adapted to by changing the type of informa-
`tion being transmitted. For example,
`in a TDMA
`environment, a first downlink time slot associated with a
`doeble- 0r triple-rate connectien may. have a first format,
`whlle a second tlme slot assoc1ated w1th the same connec-
`“on may hm a second form“ different from the fir“
`format. Bandwidth in the second (or third) time slot can be
`used to carry information in a fast out-of-band channel
`(FOC). The FOC may provide information relating to the
`same connection as the payload or data field in that time slot,
`e.g., a service type identifier which informs the mobile or
`base station of the type of information (e.g., voice, video or
`data) being conveyed in the payload. Alternatively, the FCC
`information may be associated with a connection or con-
`nections which are different from that supported by the
`payload or data field containing the FCC.
`
`7 Claims, 8 Drawing Sheets
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`US 6,466,568 B1
`
`1
`MULTI-RATE RADIOCOMMUNICATION
`SYSTEMS AND TERMINALS
`
`RELATED APPLICATION
`
`This application is a divisional, of application Ser. No.
`08/725,643 filed Oct. 15, 1996 now US. Pat. No. 5,987,019.
`This application is related to US. Pat. No. 6,028,854,
`entitled “Radiocommunication Systems and Terminals with
`Increased Payload Bandwidth”.
`BACKGROUND
`
`Applicant’s invention relates generally to radiocommu—
`nication systems, e.g., cellular or satellite systems, that use
`digital traffic channels in a multiple access scheme, e.g., time
`division multiple access (TDMA) or code division multiple
`access (CDMA).
`The growth of commercial radiocommunications and, in
`particular, the explosive growth of cellular radiotelephone
`systems have compelled system designers to search for ways
`to increase system capacity without reducing communica-
`tion quality beyond consumer tolerance thresholds. One way
`to increase capacity is to use digital communication and
`multiple access techniques such as TDMA, in which several
`users are assigned respective time slots on a single radio
`carrier frequency.
`In North America, these features are currently provided by
`a digital cellular radiotelephone system called the digital
`advanced mobile phone service (D-AMPS), some of the
`characteristics of which are specified in the interim standard
`IS-54B, “Dual-Mode Mobile Station-Base Station Compat-
`ibility Standard”, published by the Electronic Industries
`Association and Telecommunications Industry Association
`(EIA/TIA). Because of a large existing consumer base of
`equipment operating only in the analog domain with
`frequency-division multiple access (FDMA), IS-54B is a
`dual—mode (analog and digital) standard, providing for ana—
`log compatibility in tandem with digital communication
`capability. For example, the IS-54B standard provides for
`both FDMA analog voice channels (AVC) and TDMA
`digital traffic channels (DTC), and the system operator can
`dynamically replace one type with the other to accommodate
`fluctuating traffic patterns among analog and digital users.
`The AVCs and DTCs are implemented by frequency modu-
`lating radio carrier signals, which have frequencies near 800
`megahertz (MHZ) such that each radio channel has a spectral
`width of 30 kilohertz (KHz). A subsequent standard, referred
`to as IS-136, adds specifications for digital control channels.
`This standard document, in particular the version identified
`as PN-3474.1, dated Dec. 15, 1995 and published by EIA/
`TIA, is incorporated here by reference.
`In a TDMA cellular radiotelephone system, each radio
`channel is divided into a series of time slots, each of which
`contains a burst of information from a data source, e.g., a
`digitally encoded portion of a voice conversation. The time
`slots are grouped into successive TDMA frames having a
`predetermined duration. According to IS-54B and IS-136,
`each TDMA frame consists of six consecutive time slots and
`
`has a duration of 40 milliseconds (msec). Thus, each frame
`can carry from one to six traffic channels (e.g., one to six
`radio connections). The number of connections which can be
`supported by each TDMA frame depends on the desired
`information transmission rate. For example, if the connec-
`tions are used to support
`the transmission of voice
`information, the number of slots used per channel depends
`on the source rates of the speech coder/decoders (codecs)
`used to digitally encode the conversations. Such speech
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`codecs can operate at either fiill-rate or half-rate, with
`full-rate codecs being expected to be used until half-rate
`codecs that produce acceptable speech quality are devel-
`oped.
`Thus, a full-rate DTC requires twice as many time slots in
`a given time period as a half-rate DTC, and in IS-54B, each
`radio channel can carry up to three full-rate DTCs or up to
`six half-rate DTCs. Each full-rate DTC uses two slots of
`each TDMA frame, i.e., the first and fourth, second and fifth,
`or third and sixth of a TDMA frame’s six slots. Each
`half-rate DTC uses one time slot of each TDMA frame.
`During each DTC time slot, 324 bits are transmitted, of
`which the major portion, 260 bits, is due to the speech output
`of the codec, including bits due to error correction coding of
`the speech output, and the remaining bits are used for guard
`times and overhead signalling for purposes such as synchro-
`nization.
`
`In addition to voice information being transmitted on the
`traffic channels, various other types of data can and will be
`transmitted thereon. For example, facsimile (fax) transmis-
`sions are commonly supported by radiocommunication sys—
`tems. Similarly, packet data transmissions, which divide
`information streams into packets rather than providing dedi-
`cated (i.e., “connection-oriented”) channels for each infor-
`mation stream, will be supported in radiocommunication
`systems. Other types of information transmission, e.g., video
`or hybrid voice, data and video to support
`internet
`connections, will likely be supported in the future.
`These various types of information communication (also
`referred to herein as different “services”) will likely have
`different optimal transmission characteristics. For example,
`services between a remote user and the internet may benefit
`by providing a greater bandwidth in the downlink (i.e., from
`the internet to the remote station) than in the uplink, since
`many users spend a significant portion of their connection
`time downloading information from the internet rather than
`uploading thereto. Thus, it may be desirable in such cases to
`allocate a triple rate connection in the downlink (e.g., all six
`time slots of an IS-136 TDMA frame) but only a full rate
`connection in the uplink (e.g., two time slots of an IS-136
`frame). This inequality between uplink and downlink band-
`width is referred to herein as an “asymmetrical” connection.
`In addition to bandwidth considerations, other transmission
`characteristics may also be impacted. For example, different
`services may require different degrees of error protection.
`Thus,
`for example, an optimal channel coding for the
`transmission of voice information might be rate 1/2 since
`voice information transmission is typically not provided
`with a procedure for retransmission, while optimal channel
`coding for the transmission of data, e.g, facsimile, might be
`rate % since retransmission procedures are typically pro-
`vided. Other transmission characteristics, for example, the
`ability to tolerate delay in the reception of information, may
`also vary between services. All of these differences in
`transmission characteristics should be considered together
`when determining an optimal specification for the air inter-
`face.
`
`Accordingly, it would be desirable to provide techniques
`for transmitting information between remote stations and the
`system in radiocommunication networks that provide suffi-
`cient flexibility for the anticipated variety of information
`communication services described above, while also pro-
`viding sufficient compatibility with existing technology so
`that equipment used by the existing consumer base will not
`become obsolete.
`
`SUMMARY
`
`According to exemplary embodiments of the present
`invention, the type of information transmitted in the uplink
`
`

`

`US 6,466,568 B1
`
`3
`or downlink may vary depending upon the transmission rate.
`For example, in a TDMA environment, a first downlink time
`slot associated with a double- or triple-rate connection may
`have a first format, while a second time slot associated with
`the same connection may have a second format different
`from the first format. The different formats take into account
`the need to transmit certain types of information at only full
`rate, and not double- or triple-rate.
`According to some exemplary embodiments, bandwidth
`in the second (or third) time slot can be used to carry
`information in a fast out-of-band channel (FOC). The FOC
`may provide information relating to the same connection as
`the payload or data field in that time slot, e.g., a service type
`identifier which informs the mobile or base station of the
`
`type of information (e.g., voice, video or data) being con-
`veyed in the payload. This information can be used by the
`receiving equipment to aid in processing the information
`conveyed in the payload, e.g., by knowing the channel
`coding rate. These exemplary embodiments find particular
`application to multimedia communications where the type of
`payload may vary rapidly, e.g., on a slot-by-slot basis, or
`even within each slot.
`
`Various exemplary mapping techniques for associating
`the FOC information with each time slot or each block of
`data which may be interleaved over two or more time slots
`are also described herein. These exemplary mapping tech-
`niques also account for the fact that there may not be FOC
`information provided in each time slot.
`According to other exemplary embodiments of the present
`invention, the FOC information may be associated with a
`connection or connection which is different from that sup-
`ported by the payload or data field containing the FOC. For
`example, in asymmetrical connections, e.g., where a mobile
`station transmits in a different number of slots per frame than
`it receives, a downlink channel may carry payload to a first
`mobile station in the data fields in several time slots of a
`
`frame but the FOC may provide control information to one
`or more other mobile stations which are not interested in the
`
`payload. All of these mobile stations may share the same
`frequency on the uplink, e.g., the one or more other mobile
`stations may transmit packet data and use the FOC to receive
`retransmission requests.
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`The features and advantages of Applicants’ invention will
`be understood by reading this description in conjunction
`with the drawings, in which:
`FIG. 1 is a block diagram of an exemplary cellular radio
`telephone system in which the present invention may be
`applied;
`FIG. 2 illustrates an exemplary TDMA frame structure;
`FIG. 3 illustrates a conventional downlink traffic channel
`time slot format;
`FIG. 4A illustrates triple rate downlink frame usage;
`FIG. 4B illustrates full rate uplink frame usage;
`FIG. 5 illustrates a base station and three mobile stations
`
`communicating therewith;
`FIG. 6 illustrates downlink time slot formats according to
`a first exemplary embodiment of the present invention;
`FIG. 7A illustrates downlink time slot formats according
`to a second exemplary embodiment of the present invention;
`FIG. 7B illustrates downlink time slot formats according
`to a third exemplary embodiment of the present invention;
`FIGS. 8A78C illustrate exemplary mappings of FOC
`information to payload according to various exemplary
`embodiments of the present invention;
`
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`FIG. 9 is a flowchart illustrating an exemplary, alternative
`usage of an FOC field by a mobile station according to the
`present invention;
`FIG. 10 is a conventional format for all uplink traffic
`channel time slots; and
`FIG. 11 is an exemplary format for two or more uplink
`traffic channel time slots according to an exemplary embodi—
`ment of the present invention.
`DETAILED DESCRIPTION
`
`The following description is scripted in terms of a cellular
`radiotelephone system, but it will be understood that Appli-
`cant’s invention is not limited to that environment. Also, the
`following description is in the context of TDMA cellular
`communication systems, but it will be understood by those
`skilled in the art that the present invention may apply to
`hybrid access methodologies, e.g,. those including TDMA
`and Code Division Multiple Access (CDMA).
`FIG. 1 represents a block diagram of an exemplary
`cellular mobile radiotelephone system, including an exem-
`plary base station 110 and mobile station 120. The base
`station includes a control and processing unit 130 which is
`connected to the MSC 140 which in turn is connected to the
`
`PSTN (not shown). General aspects of such cellular radio-
`telephone systems are known in the art, as described by the
`above-cited US. patent applications and by US. Pat. No.
`5,175,867 to Wejke et al., entitled “Neighbor-Assisted
`Handoff in a Cellular Communication System,” and US.
`patent application Ser. No. 07/967,027 entitled “Multi-Mode
`Signal Processing,” which was filed on Oct. 27, 1992, both
`of which are incorporated in this application by reference.
`The base station 110 handles a plurality of traffic channels
`through a traffic channel transceiver 150, which is controlled
`by the control and processing unit 130. Also, each base
`station includes a control channel transceiver 160, which
`may be capable of handling more than one control channel.
`The control channel transceiver 160 is controlled by the
`control and processing unit 130. The control channel trans-
`ceiver 160 broadcasts control information over the control
`channel of the base station or cell to mobiles locked to that
`control channel. It will be understood that the transceivers
`
`150 and 160 can be implemented as a single device, like the
`traffic and control transceiver 170 in the mobile station, for
`use with control channels and traffic channels that share the
`same radio carrier frequency.
`The traffic channels can be used in a dedicated,
`connection-oriented manner to transmit information, e.g.,
`for a voice connection, where each channel is used continu-
`ously for a period of time to support transmission of a single
`stream of information or in a packet-oriented manner where
`each channel can be used to send independent units of
`information associated with different information streams.
`When used in the former sense, control channels and traffic
`channels will be referred to herein as DCCHs and DTCs,
`respectively. When used in the latter sense, control channels
`and traffic channels win be referred to herein as PCCHs or
`PDTCs,
`respectively. For more information regarding
`packet data radiocommunication systems generally,
`the
`interested reader is referred to US. patent application Ser.
`No. 08/544,836, entitled “Packet Channel Feedback”, filed
`on Oct. 18, 1995,
`the disclosure of which is expressly
`incorporated here by reference.
`After an idle mobile station 120 has located a control
`channel, e.g., by using digital control channel
`location
`information found on a traffic channel, it can then read the
`control information transmitted on that control channel, e. g.,
`
`

`

`US 6,466,568 B1
`
`5
`paging messages, using its traffic and control channel trans-
`ceiver 170. Then,
`the processing unit 180 evaluates the
`received control channel information, which may include,
`for example, paging messages or requests to measure signals
`strengths on identified channels. When a connection
`between the mobile station 120 and the system is desired, the
`transceiver 170 will tune to an appropriate traffic channel as
`described below.
`
`An exemplary organization of the information transmitted
`on each radio channel, i.e., the channel bursts, or time slots,
`in accordance with Applicant’s invention is shown in FIG.
`2. The consecutive time slots on a radio channel are orga-
`nized in TDMA frames of, for example, six slots each so that
`a plurality of distinct channels can be supported by a single
`radio carrier frequency. Each TDMA frame in this example
`has a duration of 40 msec and supports six half-rate logical
`channels, three full-rate logical channels, or greater band-
`width channels as indicated in the following table. Each slot
`can, for example, have a duration of 6.67 msec and carry 324
`bits (162 symbols), which have positions in each slot that are
`conventionally consecutively numbered 1—324.
`
`Number of Slots
`
`Used Slots
`
`1
`2
`4
`6
`
`1
`1, 4
`1, 4, 2, 5
`1, 4, 2, 5, 3, 6
`
`Rate
`
`half
`full
`double
`triple
`
`Currently, IS-136 defines a downlink DTC slot format as
`illustrated in FIG. 3. Therein, the numbers above each field
`denote the number of bits associated therewith. For example,
`the SYNC field is used for synchronization equalizer train-
`ing and time slot identification. The SACCH (Slow Asso-
`ciated Control Channel) is a signalling channel used, for
`example, for transmission of control and supervision mes-
`sages between the mobile station and the base station. The
`two DATA fields are used to transmit the “payload” of the
`slot, e.g., user information or control channel information as
`part of the FACCH (Fast Associated Control Channel). The
`CDVCC (Coded Digital Verification Color Code) is a cell
`identifier that identifies the base station which is transmitting
`to the mobile station. The CDL (Coded Digital Control
`Channel Locator) is a pointer which can be used to indicate
`on which frequency, or set of frequencies, a digital control
`channel is likely to be found. Conventionally, this downlink
`format is used for each time slot in a TDMA frame, i.e., all
`six time slots for systems operating according to IS-136.
`According to the present invention, however, it may be
`desirable to provide alternative slot formats to accommodate
`the diiferent communication services provided above.
`Consider again the situation where it
`is desirable to
`provide a triple rate connection in the downlink (i.e., base-
`to-mobile direction) and a full rate connection in the uplink
`(i.e., mobile—to—base direction). This situation is shown in
`FIGS. 4A and 4B. Therein, FIG. 4A illustrates a downlink
`frame wherein all six time slots are allocated to a particular
`mobile, as denoted by the cross—hatching of each of time
`slots 1—6. FIG. 4B illustrates a corresponding uplink frame.
`Note that only slots 1 and 4 are allocated to the particular
`mobile station which is using all of the time slots of FIG. 4A.
`Thus, the remaining time slots 2, 3, 5 and 6 are unallocated
`and, conventionally, would go unused.
`Bandwidth being a precious commodity, exemplary
`embodiments of the present invention provide techniques for
`using unallocated bandwidth in a single link without
`
`6
`adversely impacting compatibility with existing air interface
`specifications, e.g., IS-136. According to a first exemplary
`embodiment described below, the unused uplink time slots
`can be used to send packet data. Packet data communica-
`tions support independent usage of uplink and downlink
`frequencies. Accordingly, packet data can be sent on the
`unused time slots in the uplink from one or more other
`mobile stations to the base station. Consider FIG. 5. Therein,
`mobile station 500 is allocated the downlink and uplink time
`slots illustrated in FIGS. 4A and 4B for communicating with
`base station 505. To fully utilize the bandwidth resources
`according to the present invention, another mobile station
`510 transmits packet data to base station 505 in a PDTC
`comprising time slots 2 and 5 of FIG. 4B, while a third
`mobile station 520 transmits packet data to base station 505
`on a PDTC comprising time slots 3 and 6.
`If either of the mobile stations 510 and 520 require
`downlink bandwidth,
`then a downlink channel may be
`assigned on some other frequency, since mobile station 500
`is using all of the time slots of the frequency represented by
`FIG. 4A. Alternatively,
`it may be the case that, for a
`particular time period during a packet data connection which
`is referred to herein as an “activity burst”, one or both
`mobile stations 510 and 520 only need to transmit packet
`data and, therefore, do not require downlink bandwidth for
`the purposes of receiving packet data. Nonetheless, mobile
`stations 510 and 520 will still need to receive overhead
`information from base station 505, e.g., relating to which
`packets were not received and whether each mobile station
`is allowed to transmit in a particular frame. Assigning a
`downlink PDTC purely for the transmission of such over-
`head information is spectrally inefficient. One solution
`would be to provide this overhead information to mobiles
`510 and 520 on a PCCH and require the mobile stations to
`return to the PCCH periodically, e.g., after transmitting
`packets on the PDTC during the activity burst which lasts,
`for example, one second.
`However, according to exemplary embodiments of the
`present invention, another technique for providing overhead
`information to mobile stations 510 and 520 using one or
`more downlink time slots whose data or “payload” fields are
`being used to transmit information to mobile station 500.
`Specifically,
`the downlink time slot format illustrated in
`FIG. 3 can be altered to (1) provide overhead information
`regarding packet data communications to mobile stations
`510 and 520, without (2) significantly altering mobile station
`500’s ability to receive triple rate downlink information.
`FIG. 6 illustrates downlink time slot formats according to
`this exemplary embodiment of the present invention.
`Therein, three downlink slot formats are illustrated for an
`exemplary traffic channel according to the present invention.
`These three slot formats might correspond, for example, to
`slots 1, 2 and 3 of FIG. 4A. Slots 4, 5 and 6 would have the
`same format as slots 1, 2 and 3, respectively for this
`exemplary embodiment. Unlike conventional systems, e.g.,
`those currently specified by IS-136, the downlink formats
`illustrated in FIG. 6 differ within the frame. Specifically,
`while time slot 1 has the same slot format as conventional
`
`downlink traffic time slots (see, e.g., FIG. 3), time slots 2 and
`3 differ in that the SACCH, CDVCC and CDL fields of slot
`1 have each been replaced by an FOC (fast out-of-band
`channel) field. It will be noted that, for the purposes of
`simplicity,
`the RSVD bit illustrated in FIG. 3 has been
`omitted. However,
`this bit may also be reserved and
`included in downlink slot formats according to the present
`invention.
`
`In the example described above, mobile station 500 is
`using a triple rate downlink connection, i.e., it is reading the
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`US 6,466,568 B1
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`7
`data fields of each of time slots 1, 2 and 3 in FIG. 6.
`However, some of the other fields provided in the conven-
`tional downlink time slot format of FIG. 3 need not be
`transmitted in each time slot under these circumstances. For
`example, the type of overhead signalling that occurs on the
`SACCH is such that mobile station 500 need not receive the
`SACCH at triple rate. That is, mobile station 500 may only
`need to receive one SACCH burst every three time slots.
`Thus the field that is normally used for SACCH information
`in slots 2 and 3 can be replaced by FOC information
`according to the present
`invention. The CDVCC field
`includes information that aids in the identification of the
`radio link and is conventionally used for radio link control,
`e.g., tearing down of a connection. However, this informa-
`tion can be provided to the mobile station over the control
`channel at call-setup and, accordingly, need not be trans-
`mitted by the base station in each downlink time slot.
`Various techniques are described below to avoid problems
`caused by omitting the CDL information from some down-
`link time slots.
`
`Omitting these fields in time slots 2 and 3 (as well as 5 and
`6) provides an opportunity to inform the other mobile
`stations, e.g., mobile stations 510 and 520, of information
`pertaining to their uplink connections, without assigning a
`new PDTC or forcing mobile stations 510 and 520 to revert
`periodically to listening to the PCCH. For example, the FOC
`fields can be used to inform mobile station 510 or mobile
`station 520 that a previously transmitted packet was not
`properly received and should be retransmitted. Note that
`since the FOC information is “out-of—band” (i.e.,
`is not
`encoded as part of the data), mobile stations 510 and 520
`advantageously need not be aware of the channel coding and
`interleaving needed to read the data fields in time slots 2 and
`3.
`
`Many variations of the foregoing exemplary embodiment
`are possible and contemplated by the present invention. For
`example, although the foregoing example is provided in
`terms of an asymmetrical connection wherein the downlink
`is triple rate and the uplink is full rate, any asymmetrical
`connection lends itself to application of the present inven-
`tion. For example, the downlink may be double rate and the
`uplink full rate, whereupon the FOC fields would replace the
`SACCH, CDVCC and CDL fields in only one of slots 2 and
`3 illustrated in FIG. 6.
`
`Moreover, it may not be desirable to replace all three of
`the SACCH, CDVCC and CDL fields with FOC informa-
`tion. For example, it may be determined that 36 bits of FOC
`information is not needed. Alternatively, for compatibility
`reasons,
`it may be determined that one or more of the
`SACCH, CDVCC and CDL fields should be maintained in
`each downlink slot. Thus, for example, downlink slot for-
`mats for the triple rate downlink/full rate uplink example
`provided above could instead be as illustrated in one of
`FIGS. 7A and 7B. Therein,
`the FOC replaces only the
`SACCH and CDVCC in FIG. 7A and only the SACCH in
`FIG. 7B. Those skilled in the art will appreciate that many
`more variations exist, such as time slot 2 or 3 being the
`“master” channel having the conventional slot format of
`FIG. 3 instead of slot 1.
`
`As mentioned above, exemplary embodiments of the
`present invention wherein the base station 505 only trans-
`mits the CDVCC and/or the CDL in some downlink slots of
`a traffic channel may cause difficulties for mobile stations
`that expect this information in all time slots on a downlink
`traffic channel. For extensive information relating to the
`CDL and mobile functionality relating to locating digital
`traffic channels, the reader is referred to US. patent appli-
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`cation Ser. No. 08/331,711 entitled “Method and Apparatus
`for Locating a Digital Control Channel in a Radiocommu-
`nication System”, filed on Oct. 31, 1994, the disclosure of
`which is incorporated here by reference. In brief, the CDL
`field is used by unconnected mobile stations (e. g., at power-
`up) to locate a control channel if the first channel to which
`it tunes is a traffic channel. According to one exemplary
`technique, a mobile station reads the field corresponding to
`the CDVCC in the time slot to which it first tunes on a
`frequency. This conventional mobile station will identify
`this field as either a CDVCC (implying a traffic channel per
`the format of FIG. 3) or a coded superframe phase (CSFP)
`(implying a control channel per IS—136). If a traffic channel,
`the mobile station will then use the CDL information as a
`pointer to search another channel number, or set of channel
`numbers, for a control channel.
`Thus, if the CDVCC information is replaced by FOC
`information on some downlink time slots, a conventional
`mobile station reading this field for the purpose of identi—
`fying the channel as either a control channel or a traffic
`channel may misidentify a traffic channel as a control
`channel. Alternatively,
`the mobile station might read the
`FOC information as valid CDVCC data (thus correctly
`identifying the channel as a traffic channel) and then look for
`the CDL, which is not present (thus moving to an incorrect
`channel number or set to search).
`Both of these problems can be avoided according to
`exemplary embodiments of the present invention by recog—
`nizing that both the CSFP and the CDVCC according to
`IS-136 are (12,8) encoded data words, i.e., 8 bits of data
`encoded to 12 bits that have particular characteristics.
`Specifically, the CDVCC is a 12,8) code word that remains
`the same in each time slot associated with a particular
`channel and has non-inverted checkbits, while the CSFP is
`a (12,8) code word that has inverted checkbits and acts as an
`upcounter. Since the universe of (12,8) codewords having
`these characteristics is relatively small as compared with the
`number of total number of 12 bit binary words, the FOC
`information can be made distinct from the CDVCC and
`
`CSFP to avoid confusion. Specifically, the base station can
`transmit FOC information in the field which conventionally
`been used in downlink channels as either the CSFP or the
`
`CDVCC (i.e., bits 169—181 in FIG. 3), which is carefully
`tailored to avoid similarity with a (12,8) codeword having
`these characteristics by adding filler bits to distinguish
`therefrom as will be readily appreciated by those skilled in
`the art.
`
`those mobile stations (or other receiving
`Of course,
`equipment) which are designed with the present invention in
`mind will be aware