US00707955OB2
`
`(12) United States Patent
`Padovani et al.
`
`(10) Patent No.:
`(45) Date of Patent:
`
`US 7,079,550 B2
`*Jul.18, 2006
`
`(54) METHOD AND APPARATUS FOR HIGH
`RATE PACKETDATA TRANSMISSION
`
`(56)
`
`References Cited
`
`(75) Inventors: Roberto Padovani, San Diego, CA
`(US); Paul E. Bender, San Diego, CA
`(US); Peter J. Black, La Jolla, CA
`(US); Matthew S. Grob, La Jolla, CA
`(US); Jurg K. Hinderling, San Diego,
`CA (US); Nagabhushana T.
`Sindhushayana, San Diego, CA (US);
`s
`s
`s
`Charles E. Wheatley, III, Del Mar, CA
`(US)
`(73) Assignee: Qualcomm, Incorporated, San Diego,
`CA (US)
`
`(*) Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`This patent is Subject to a terminal dis-
`claimer.
`
`(21) Appl. No.: 10/318,489
`
`(22) Filed:
`
`Dec. 12, 2002
`O
`O
`Prior Publication Data
`US 2003/O142656 A1
`Jul. 31, 2003
`
`(65)
`
`Related U.S. Application Data
`(63) Continuation of application No. 08/963.386, filed on
`Nov. 3, 1997, now Pat. No. 6,574,211.
`
`(51) Int. Cl.
`H04 3/22
`
`(2006.01)
`
`(52) U.S. Cl. ....................................... 370/468; 370/329
`(58) Field of Classification Search ................ 370328,
`370/329, 331, 332, 335, 342, 458, 442,336,
`370/337,352; 455/525,517,426
`See application file for complete search history.
`
`
`
`U.S. PATENT DOCUMENTS
`370,104
`4.256.925 A
`3, 1981 Good
`4 - W
`OOOle . . . . . . . . . . . . . . . . . . . . . . . .
`... 370, 89
`4.383,315 A
`5/1983 Torng.....
`4,491.947 A
`1/1985 Frank .......................... 370.94
`4,547,880 A 10/1985 De Vita et al. ............... 370.91
`5,003,534 A
`3, 1991 Gerhardt et al. ...
`370.94.1
`5,022,046 A
`6/1991 Morrow, Jr. ................... 37.5/1
`5, 101,501 A
`3/1992 Gilhousen et al. ............ 455,33
`5,115,429 A
`5/1992 Hluchy et al. ............... 370.84
`5,267.262 A 1 1/1993 Wheatley, III ................. 375/1
`5,280,537 A
`1/1994 Sugiyama et al. ............. 375/1
`(Continued)
`
`EP
`
`FOREIGN PATENT DOCUMENTS
`O412583
`2, 1991
`
`(Continued)
`Primary Examiner Bob A. Phunkulh
`(74) Attorney, Agent, or Firm Philip Wadsworth; Sandra L.
`Godsey
`
`(57)
`
`ABSTRACT
`
`In a data communication system capable of variable rate
`transmission, high rate packet data transmission improves
`utilization of the forward link and decreases the transmission
`delay. Data transmission on the forward link is time multi
`plexed and the base station transmits at the highest data rate
`supported by the forward link at each time slot to one mobile
`station. The data rate is determined by the largest C/I
`measurement of the forward link signals as measured at the
`mobile station. Upon determination of a data packet
`received in error, the mobile station transmits a NACK
`message back to the base station. The NACK message
`results in retransmission of the data packet received in error.
`The data packets can be transmitted out of sequence by the
`use of sequence number to identify each data unit within the
`data packets.
`
`51 Claims, 15 Drawing Sheets
`
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`US 7,079,550 B2
`Page 2
`
`U.S. PATENT DOCUMENTS
`370/18
`5,373,502. A 12, 1994 Turban
`5.400.328 A
`3, 1995 Burren et al. 37Of79
`5,416,797 A
`5/1995 Gilhousen et al. .......... 375/705
`5,442,625 A * 8/1995 Gitlin et al. ......
`... 370,342
`5,528,593 A * 6/1996 English et al. .............. 370,391
`5,537,410 A
`7/1996 Li ............................... 370/84
`5,566,175 A 10/1996 Davis .........
`370/84
`5,603,093 A * 2/1997 Yoshimi et al. ............ 455, 63.1
`5,638.412 A
`6/1997 Blakeney, II et al.
`5,726,978 A * 3/1998 Frodigh et al. ............. 370/252
`5,822.318 A * 10/1998 Tiedemann et al. ......... 370,391
`5,903,554 A
`5/1999 Saints ........................ 370,342
`5,914,959 A * 6/1999 Marchetto et al.
`... 370/468
`5,946,346 A * 8/1999 Ahmed et al. .............. 375,219
`
`
`
`5,978,657 A * 11/1999 Suzuki ....................... 455,522
`5.991,627 A * 11/1999 Honkasalo et al. ......... 455,437
`6,002.919 A * 12/1999 Posti ....................... 455,554.2
`6,137,991 A * 10/2000 Isaksson .................. 455/67.11
`6,347,217 B1 *
`2/2002 Bengtsson et al. ......... 455/67.7
`6,570,860 B1
`5/2003 Hämäläinen et al.
`6,574,211 B1
`6/2003 Padovani et al.
`
`FOREIGN PATENT DOCUMENTS
`
`EP
`EP
`GB
`* cited b
`cited by examiner
`
`O418865
`0779755
`2293947
`
`3, 1991
`6, 1997
`4f1996
`
`IPR2018-1556
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`
`

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`U.S. Patent
`U.S. Patent
`
`Jul.18, 2006
`Jul. 18, 2006
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`Sheet 1 of 15
`Sheet 1 of 15
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`US 7,079,550 B2
`US 7,079,550 B2
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`U.S. Patent
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`Jul.18, 2006
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`US 7,079,550 B2
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`U.S. Patent
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`Jul.18, 2006
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`US 7,079,550 B2
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`U.S. Patent
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`US 7,079,550 B2
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`U.S. Patent
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`Jul.18, 2006
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`Sheet 14 of 15
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`U.S. Patent
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`Jul.18, 2006
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`US 7,079,550 B2
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`US 7,079,550 B2
`
`1.
`METHOD AND APPARATUS FOR HIGH
`RATE PACKET DATA TRANSMISSION
`
`CLAIM OF PRIORITY UNDER 35 U.S.C. S 120
`
`The present Application for Patent is a Continuation and
`claims priority to patent application Ser. No. 08/963.386
`entitled METHOD AND APPARATUS FOR HIGHRATE
`PACKETDATA TRANSMISSION filed Nov. 3, 1997, now
`U.S. Pat. No. 6,574.211, issued Jun. 3, 2003, and assigned
`to the assignee hereof and hereby expressly incorporated by
`reference herein.
`
`5
`
`10
`
`BACKGROUND OF THE INVENTION
`
`2
`process are disclosed in U.S. Pat. No. 5,267.261, entitled
`MOBILE ASSISTED SOFT HANDOFF IN A CDMA
`CELLULAR TELEPHONE SYSTEM,” assigned to the
`assignee of the present invention and incorporated by ref
`erence herein. Softer handoff is the process whereby the
`communication occurs over multiple sectors which are ser
`viced by the same base station. The process of softer handoff
`is described in detail in U.S. patent application Ser. No.
`08/763,498, entitled “METHOD AND APPARATUS FOR
`PERFORMING HANDOFF BETWEEN SECTORS OF A
`COMMON BASE STATION, filed Dec. 11, 1996, now
`U.S. Pat. No. 5,933,787, issued Aug. 3, 1999, by Klein S.
`Gilhousen et al., assigned to the assignee of the present
`invention and incorporated by reference herein.
`Given the growing demand for wireless data applications,
`the need for very efficient wireless data communication
`systems has become increasingly significant. The IS-95
`standard is capable of transmitting traffic data and Voice data
`over the forward and reverse links. A method for transmit
`ting traffic data in code channel frames of fixed size is
`described in detail in U.S. Pat. No. 5,504,773, entitled
`METHOD AND APPARATUS FOR THE FORMATTING
`OF DATA FOR TRANSMISSION', assigned to the
`assignee of the present invention and incorporated by ref
`erence herein. In accordance with the IS-95 standard, the
`traffic data or voice data is partitioned into code channel
`frames which are 20 msec wide with data rates as high as
`14.4 Kbps.
`A significant difference between Voice services and data
`services is the fact that the former imposes Stringent and
`fixed delay requirements. Typically, the overall one-way
`delay of speech frames must be less than 100 msec. In
`contrast, the data delay can become a variable parameter
`used to optimize the efficiency of the data communication
`system. Specifically, more efficient error correcting coding
`techniques which require significantly larger delays than
`those that can be tolerated by voice services can be utilized.
`An exemplary efficient coding scheme for data is disclosed
`in U.S. patent application Ser. No. 08/743,688, entitled
`SOFT DECISION OUTPUT DECODER FOR DECOD
`ING CONVOLUTIONALLY ENCODED CODEWORDS,
`filed Nov. 6, 1996, now U.S. Pat. No. 5,933.462, issued Aug.
`3, 1999, by Andrew J. Viterbi et al., assigned to the assignee
`of the present invention and incorporated by reference
`herein.
`Another significant difference between voice services and
`data services is that the former requires a fixed and common
`grade of service (GOS) for all users. Typically, for digital
`systems providing Voice services, this translates into a fixed
`and equal transmission rate for all users and a maximum
`tolerable value for the error rates of the speech frames. In
`contrast, for data services, the GOS can be different from
`user to user and can be a parameter optimized to increase the
`overall efficiency of the data communication system. The
`GOS of a data communication system is typically defined as
`the total delay incurred in the transfer of a predetermined
`amount of data, hereinafter referred to as a data packet.
`Yet another significant difference between voice services
`and data services is that the former requires a reliable
`communication link which, in the exemplary CDMA com
`munication system, is provided by soft handoff. Soft handoff
`results in redundant transmissions from two or more base
`stations to improve reliability. However, this additional
`reliability is not required for data transmission because the
`data packets received in error can be retransmitted. For data
`services, the transmit power used to Support Soft handoff can
`be more efficiently used for transmitting additional data.
`
`15
`
`25
`
`I. Field of the Invention
`The present invention relates to data communication.
`More particularly, the present invention relates to a novel
`and improved method and apparatus for high rate packet
`data transmission.
`II. Description of the Related Art
`A modern day communication system is required to
`Support a variety of applications. One such communication
`system is a code division multiple access (CDMA) system
`which conforms to the “TIA/EIA/IS-95 Mobile Station-Base
`Station Compatibility Standard for Dual-Mode Wideband
`Spread Spectrum Cellular System', hereinafter referred to as
`the IS-95 standard. The CDMA system allows for voice and
`data communications between users over a terrestrial link.
`The use of CDMA techniques in a multiple access commu
`30
`nication system is disclosed in U.S. Pat. No. 4,901.307.
`entitled “SPREAD SPECTRUM MULTIPLE ACCESS
`COMMUNICATION SYSTEM USING SATELLITE OR
`TERRESTRIAL REPEATERS, and U.S. Pat. No. 5,103,
`459, entitled “SYSTEMAND METHOD FOR GENERAT
`35
`ING WAVEFORMS IN A CDMA CELLULAR TELE
`PHONE SYSTEM, both assigned to the assignee of the
`present invention and incorporated by reference herein.
`In this specification, base station refers to the hardware
`with which the mobile stations communicate. Cell refers to
`the hardware or the geographic coverage area, depending on
`the context in which the term is used. A sector is a partition
`of a cell. Because a sector of a CDMA system has the
`attributes of a cell, the teachings described in terms of cells
`are readily extended to sectors.
`In the CDMA system, communications between users are
`conducted through one or more base stations. A first user on
`one mobile station communicates to a second user on a
`second mobile station by transmitting data on the reverse
`link to a base station. The base station receives the data and
`can route the data to another base station. The data is
`transmitted on the forward link of the same base station, or
`a second base station, to the second mobile station. The
`forward link refers to transmission from the base station to
`a mobile station and the reverse link refers to transmission
`from the mobile station to a base station. In IS-95 systems,
`the forward link and the reverse link are allocated separate
`frequencies.
`The mobile station communicates with at least one base
`station during a communication. CDMA mobile stations are
`capable of communicating with multiple base stations simul
`taneously during soft handoff. Soft handoff is the process of
`establishing a link with a new base station before breaking
`the link with the previous base station. Soft handoff mini
`mizes the probability of dropped calls. The method and
`system for providing a communication with a mobile station
`through more than one base station during the soft handoff
`
`40
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`3
`The parameters which measure the quality and effective
`ness of a data communication system are the transmission
`delay required to transfer a data packet and the average
`throughput rate of the system. Transmission delay does not
`have the same impact in data communication as it does for
`Voice communication, but it is an important metric for
`measuring the quality of the data communication system.
`The average throughput rate is a measure of the efficiency of
`the data transmission capability of the communication sys
`tem.
`It is well known that in cellular systems the signal-to
`noise-and- interference ratio C/I of any given user is a
`function of the location of the user within the coverage area.
`In order to maintain a given level of service, TDMA and
`FDMA systems resort to frequency reuse techniques, i.e.,
`not all frequency channels and/or time slots are used in each
`base station. In a CDMA system, the same frequency
`allocation is reused in every cell of the system, thereby
`improving the overall efficiency. The C/I that any given
`user's mobile station achieves determines the information
`rate that can be supported for this particular link from the
`base station to the user's mobile station. Given the specific
`modulation and error correction method used for the trans
`mission, which the present invention seek to optimize for
`data transmissions, a given level of performance is achieved
`at a corresponding level of C/I. For idealized cellular system
`with hexagonal cell layouts and utilizing a common fre
`quency in every cell, the distribution of C/I achieved within
`the idealized cells can be calculated.
`The C/I achieved by any given user is a function of the
`path loss, which for terrestrial cellular systems increases as
`r to r, where r is the distance to the radiating source.
`Furthermore, the path loss is Subject to random variations
`due to man-made or natural obstructions within the path of
`the radio wave. These random variations are typically mod
`eled as a lognormal shadowing random process with a
`standard deviation of 8 dB. The resulting C/I distribution
`40
`achieved for an ideal hexagonal cellular layout with omni
`directional base station antennas, r' propagation law, and
`shadowing process with 8 dB standard deviation is shown in
`FIG 10.
`The obtained C/I distribution can only be achieved if, at
`any instant in time and at any location, the mobile station is
`served by the best base station which is defined as that
`achieving the largest C/I value, regardless of the physical
`distance to each base station. Because of the random nature
`of the path loss as described above, the signal with the
`largest C/I value can be one, which is other than the
`minimum physical distance from the mobile station. In
`contrast, if a mobile station was to communicate only via the
`base station of minimum distance, the C/I can be substan
`tially degraded. It is therefore beneficial for mobile stations
`to communicate to and from the best serving base station at
`all times, thereby achieving the optimum C/I value. It can
`also be observed that the range of values of the achieved C/I,
`in the above idealized model and as shown in FIG. 10, is
`such that the difference between the highest and lowest value
`can be as large as 10,000. In practical implementation the
`range is typically limited to approximately 1:100 or 20 dB.
`It is therefore possible for a CDMA base station to serve
`mobile stations with information bit rates that can vary by as
`much as a factor of 100, since the following relationship
`holds:
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`(CfI)
`= W
`b - "E. I.).
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`(1)
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`where R, represents the information rate to a particular
`mobile station, W is the total bandwidth occupied by the
`spread spectrum signal, and E/I is the energy per bit over
`interference density required to achieve a given level of
`performance. For instance, if the spread spectrum signal
`occupies a bandwidth W of 1.2288 MHZ and reliable com
`munication requires an average E/I equal to 3 dB, then a
`mobile station which achieves a CII value of 3 dB to the best
`base station can communicate at a data rate as high as 1.2288
`Mbps. On the other hand, if a mobile station is subject to
`Substantial interference from adjacent base stations and can
`only achieve a C/I of-7 dB, reliable communication cannot
`be supported at a rate greater than 122.88 Kbps. A commu
`nication system designed to optimize the average throughput
`will therefore attempts to serve each remote user from the
`best serving base station and at the highest data rate R,
`which the remote user can reliably support. The data com
`munication system of the present invention exploits the
`characteristic cited above and optimizes the data throughput
`from the CDMA base stations to the mobile stations.
`
`SUMMARY
`
`The present invention is a novel and improved method
`and apparatus for high rate packet data transmission in a
`CDMA system. The present invention improves the effi
`ciency of a CDMA system by providing for means for
`transmitting data on the forward and reverse links. Each
`mobile station communicates with one or more base stations
`and monitors the control channels for the duration of the
`communication with the base stations. The control channels
`can be used by the base stations to transmit Small amounts
`of data, paging messages addressed to a specific mobile
`station, and broadcast messages to all mobile stations. The
`paging message informs the mobile station that the base
`station has a large amount of data to transmit to the mobile
`station.
`It is an object of the present invention to improve utili
`zation of the forward and reverse link capacity in the data
`communication system. Upon receipt of the paging mes
`sages from one or more base stations, the mobile station
`measures the signal-to-noise-and-interference ratio (C/I) of
`the forward link signals (e.g. the forward link pilot signals)
`at every time slots and selects the best base station using a
`set of parameters which can comprise the present and
`previous C/I measurements. In the exemplary embodiment,
`at every time slot, the mobile station transmits to the selected
`base station on a dedicated data request (DRC) channel a
`request for transmission at the highest data rate which the
`measured C/I can reliably support. The selected base station
`transmits data, in data packets, at a data rate not exceeding
`the data rate received from the mobile station on the DRC
`channel. By transmitting from the best base station at every
`time slot, improved throughput and transmission delay are
`achieved.
`It is another object of the present invention to improve
`performance by transmitting from the selected base station
`at the peak transmit power for the duration of one or more
`time slots to a mobile station at the data rate requested by the
`mobile station. In the exemplary CDMA communication
`system, the base stations operate at a predetermined back-off
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`(e.g., 3 dB) from the available transmit power to account for
`variations in usage. Thus, the average transmit power is half
`of the peak power. However, in the present invention, since
`high speed data transmissions are scheduled and power is
`typically not shared (e.g., among transmissions), it is not
`necessary to back-off from the available peak transmit
`power.
`It is yet another object of the present invention to enhance
`efficiency by allowing the base stations to transmit data
`packets to each mobile station for a variable number of time
`slots. The ability to transmit from different base stations
`from time slot to time slot allows the data communication
`system of the present invention to quickly adapt to changes
`in the operating environment. In addition, the ability to
`transmit a data packet over non-contiguous time slots is
`possible in the present invention because of the use of
`sequence number to identify the data units within a data
`packet.
`It is yet another object of the present invention to increase
`flexibility by forwarding the data packets addressed to a
`specific mobile station from a central controller to all base
`stations which are members of the active set of the mobile
`station. In the present invention, data transmission can occur
`from any base station in the active set of the mobile station
`at each time slot. Since each base station comprises a queue
`which contains the data to be transmitted to the mobile
`station, efficient forward link transmission can occur with
`minimal processing delay.
`It is yet another object of the present invention to provide
`a retransmission mechanism for data units received in error.
`In the exemplary embodiment, each data packet comprises
`a predetermined number of data units, with each data unit
`identified by a sequence number. Upon incorrect reception
`of one or more data units, the mobile station sends a negative
`acknowledgment (NACK) on the reverse link data channel
`indicating the sequence numbers of the missing data units
`for retransmission from the base station. The base station
`receives the NACK message and can retransmit the data
`units received in error.
`It is yet another object of the present invention for the
`mobile station to select the best base station candidates for
`communication based on the procedure described in U.S.
`patent application Ser. No. 08/790,497, entitled “METHOD
`AND APPARATUS FOR PERFORMING SOFT HAND
`OFF IN A WIRELESS COMMUNICATIONS SYSTEM,
`45
`filed Jan. 29, 1997, now U.S. Pat. No. 6,151,502, issued
`Nov. 21, 2000, by Roberto Padovani et al., assigned to the
`assignee of the present invention and incorporated by ref
`erence herein. In the exemplary embodiment, the base
`station can be added to the active set of the mobile station
`if the received pilot signal is above a predetermined add
`threshold and dropped from the active set if the pilot signal
`is below a predetermined drop threshold. In the alternative
`embodiment, the base station can be added to the active set
`if the additional energy of the base station (e.g., as measured
`by the pilot signal) and the energy of the base stations
`already in the active set exceeds a predetermined threshold.
`Using this alternative embodiment, a base station which
`transmitted energy comprises an insubstantial amount of the
`total received energy at the mobile station is not added to the
`active set.
`It is yet another object of the present invention for the
`mobile stations to transmit the data rate requests on the DRC
`channel in a manner Such that only the selected base station
`among the base stations in communication with the mobile
`station is able to distinguish the DRC messages, therefore
`assuring that the forward link transmission at any given time
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`slot is from the selected base station. In the exemplary
`embodiment, each base station in communication with the
`mobile station is assigned a unique Walsh code. The mobile
`station covers the DRC message with the Walsh code
`corresponding to the selected base station. Other codes can
`be used to cover the DRC messages, although orthogonal
`codes are typically utilized and Walsh codes are preferred.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`The features, objects, and advantages of the present
`invention will become more apparent from the detailed
`description set forth below when taken in conjunction with
`the drawings in which like reference characters identify
`correspondingly throughout and wherein:
`FIG. 1 is a diagram of a data communication system of the
`present invention comprising a plurality of cells, a plurality
`of base stations and a plurality of mobile stations.
`FIG. 2 is an exemplary block diagram of the subsystems
`of the data communication system of the present invention;
`FIGS. 3A-3B are block diagrams of the exemplary for
`ward link architecture of the present invention;
`FIG. 4A is a diagram of the exemplary forward link frame
`structure of the present invention;
`FIGS. 4B-4C are diagrams of the exemplary forward
`traffic channel and power control channel, respectively;
`FIG. 4D is a diagram of the punctured packet of the
`present invention;
`FIGS. 4E-4G are diagrams of the two exemplary data
`packet formats and the control channel capsule, respec
`tively;
`FIG. 5 is an exemplary timing diagram showing the high
`rate packet transmission on the forward link:
`FIG. 6 is a block diagram of the exemplary reverse link
`architecture of the present invention;
`FIG. 7A is a diagram of the exemplary reverse link frame
`structure of the present invention;
`FIG. 7B is a diagram of the exemplary reverse link access
`channel;
`FIG. 8 is an exemplary timing diagram showing the high
`rate data transmission on the reverse link:
`FIG. 9 is an exemplary state diagram showing the tran
`sitions between the various operating states of the mobile
`station; and
`FIG. 10 is a diagram of the cumulative distribution
`function (CDF) of the C/I distribution in an ideal hexagonal
`cellular layout.
`
`DETAILED DESCRIPTION
`
`In accordance with the exemplary embodiment of the data
`communication system of the present invention, forward
`link data transmission occurs from one base station to one
`mobile station (see FIG. 1) at or near the maximum data rate
`which can be supported by the forward link and the system.
`Reverse link data communication can occur from one
`mobile station to one or more base stations. The calculation
`of the maximum data rate for forward link transmission is
`described in detail below. Data is partitioned into data
`packets, with each data packet being transmitted over one or
`more time slots (or slots). At each time slot, the base station
`can direct data transmission to any mobile station which is
`in communication with the base station.
`Initially, the mobile station establishes communication
`with a base station using a predetermined access procedure.
`In this connected State, the mobile station can receive data
`and control messages from the base station, and is able to
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`transmit data and control messages to the base station. The
`mobile station then monitors the forward link for transmis
`sions from the base stations in the active set of the mobile
`station. The active set contains a list of base stations in
`communication with the mobile station. Specifically, the
`mobile station measures the signal-to-noise-and-interfer
`ence ratio (C/I) of the forward link pilot from the base
`stations in the active set, as received at the mobile station. If
`the received pilot signal is above a predetermined add
`threshold or below a predetermined drop threshold, the
`mobile station reports this to the base station. Subsequent
`messages from the base station direct the mobile station to
`add or delete the base station(s) to or from its active set,
`respectively. The various operating states of the mobile
`station are described below.
`If there is no data to send, the mobile station returns to an
`idle state and discontinues transmission of data rate infor
`mation to the base station(s). While the mobile station is in
`the idle state, the mobile station monitors the control channel
`from one or more base stations in the active set for paging
`messages.
`If there is data to be transmitted to the mobile station, the
`data is sent by a central controller to all base stations in the
`active set and stored in a queue at each base station. A paging
`message is then sent by one or more base stations to the
`mobile station on the respective control channels. The base
`station may transmit all such paging messages at the same
`time across several base stations in order to ensure reception
`even when the mobile station is switching between base
`stations. The mobile station demodulates and decodes the
`signals on one or more control channels to receive the
`paging messages.
`Upon decoding the paging messages, and for each time
`slot until the data transmission is completed, the mobile
`station measures the C/I of the forward link signals from the
`base stations in the active set, as received at the mobile
`station. The C/I of the forward link signals can be obtained
`by measuring the respective pilot signals. The mobile station
`then selects the best base station based on a set of param
`eters. The set of parameters can comprise the present and
`previous C/I measurements and the bit-error-rate or packet
`error-rate. For example, the best base station can be selected
`based on the largest C/I measurement. The mobile station
`then identifies the best base station and transmits to the
`selected base station a data request message (hereinafter
`referred to as the DRC messag