US007852746 B2
`
`(12) United States Patent
`Jalali
`
`(10) Patent No.:
`(45) Date of Patent:
`
`US 7,852,746 B2
`Dec. 14, 2010
`
`(54) TRANSMISSION OF SIGNALING IN AN
`OFDM-BASED SYSTEM
`
`7,242,958 B2 *
`7.372,909 B2 *
`
`7/2007 Chung et al. ................ 455,522
`5/2008 Miyoshi ..................... 375,260
`
`(75) Inventor: Ahmad Jalali, Rancho Santa Fe, 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
`
`(21) Appl. No.: 10/944,146
`
`22) Filed:
`(22)
`
`Sep. 16, 2004
`p. 10,
`
`(65)
`
`Prior Publication Data
`US 2006/OO45OO1 A1
`Mar. 2, 2006
`
`Related U.S. Application Data
`(60) Provisional application No. 60/604,660, filed on Aug.
`25, 2004.
`(51) Int. Cl.
`(2006.01)
`H04 II/00
`(52) U.S. Cl. ........................ 370/208: 370/329; 455/102
`(58) Field of Classification Search ......... 370,208 210,
`37O/329, 341348; 455A101102 105
`See application file for complete search history. s
`References Cited
`U.S. PATENT DOCUMENTS
`
`(56)
`
`4, 1997 Bi .............................. 370,209
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`6,366,779 B1 * 4/2002 Bender et al. ............... 455,450
`6,393,077 B1 *
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`
`(Continued)
`FOREIGN PATENT DOCUMENTS
`
`DE
`
`36441.75
`
`T 1988
`
`(Continued)
`
`Patent Cooperation Treaty, “International Search Report and Written
`Opinion', dated Jan. 18, 2006, Issued over corresponding PCT Appli
`cation No. PCT/US2005/030229, 15 pages.
`
`(Continued)
`Primary Examiner William Trost, IV
`Assistant Examiner—Roberta A Shand
`(74) Attorney, Agent, or Firm Turocy & Watson, LLP
`(57)
`ABSTRACT
`
`Techniques for efficiently transmitting various types of sig
`naling on the forward and reverse links in an OFDM-based
`system are described. Instead of specifically allocating sub
`bands to individual signaling channels, signaling data for a
`signaling channel on a given link is sent as “underlay to other
`transmissions that may be sent on the same link. Each wire
`less terminal is assigned a different PN code. The signaling
`data for each terminal is spectrally spread overall or a portion
`of the system bandwidth using the assigned PN code. For the
`reverse link, a wireless terminal may transmit signaling on all
`Nusable subbands and may transmit traffic data on L sub
`bands assigned for data transmission, which may be a Subset
`of the Nusable subbands. For the forward link, a base station
`may transmit signaling and traffic data for all terminals on the
`Nusable subbands.
`
`52 Claims, 10 Drawing Sheets
`
`Data
`Pilot
`Data
`Symbolis Symbols Symbols
`---all
`r. 82 XXX
`
`2 s
`is
`
`Freq
`
`3
`2
`
`
`
`|
`
`1
`
`|
`
`|
`
`|
`
`|
`
`2 3 4 5 is
`
`Subband Group for
`Traffic Channex
`
`2
`e
`
`
`
`|
`
`|
`
`|
`
`7 8 9
`to 11 12 13 14 15 16
`Hop Period
`
`Time
`
`VWGoA EX1016
`U.S. Patent No. 10,965,512
`
`

`

`US 7,852,746 B2
`Page 2
`
`U.S. PATENT DOCUMENTS
`7.447,517 B2 * 1 1/2008 Roy et al. ................... 455,522
`7.551,546 B2
`6/2009 Ma et al. ...
`... 370,208
`7,660,275 B2 *
`2/2010 Vijayan et al. .............. 370/312
`2002fO086707 A1* 7, 2002 Struhsaker et al.
`... 455,561
`2005, OOO9476 A1* 1/2005 Wu et al. ..........
`... 455,101
`2005, OO3O886 A1* 2, 2005 Wu et al. ...
`... 370,206
`2005, 0096061 A1* 5/2005 Ji et al. ....................... 455,450
`2005.0185725 A1
`8, 2005 Maeda et al.
`2005/0271012 A1* 12/2005 Agrawal et al. ............. 370,331
`2006/0018347 A1* 1/2006 Agrawal ........
`... 370,537
`2006/0135171 A1* 6/2006 Roy et al. ...
`... 455,450
`2006/0171416 A1* 8, 2006 Seidel et al. ................ 370/473
`
`
`
`FOREIGN PATENT DOCUMENTS
`
`EP
`EP
`EP
`EP
`JP
`JP
`JP
`JP
`
`1179.902 A1
`1435 750 A1
`1435750
`1560 359 A1
`2001.268044
`2003-309533. A
`2003.309533
`2005244960
`
`3, 2001
`12/2003
`T 2004
`1, 2005
`9, 2001
`10, 2003
`10, 2003
`9, 2005
`
`WO
`
`WO97,385O1
`10, 1997
`OTHER PUBLICATIONS
`Kishiyama et al. "Experiments on throughput performance above
`100Mbps in forward link for VSF-OFCDM broadband wireless
`access', 2003, pp. 1863-1868.
`Bahai et al. “Multi-carrier Digital Communications Theory and
`Applications of OFDM”, Jan. 1, 1999, p. 210-213.
`Office Action mailed Aug. 6, 2008 for Chilean Patent Application No.
`2159-2005, 6 pages.
`Office Action mailed Jan. 25, 2008 for European Patent Application
`No. 0.5791214.9, 2 pages.
`Office Action mailed Oct. 14, 2009 for European Patent Application
`No. 0.5791214.9, 6 pages.
`Office Action mailed Aug. 20, 2008 for Malaysian Patent Application
`No. PI 20053977, 2 pages.
`Translated Japanese Office Action dated Dec. 24, 2009, mailed Apr.
`7, 2010, for Japanese Application Serial No. 2007-530118, 4 pages.
`Canadian Office Action dated Feb. 24, 2010 for Canadian Patent
`Application Serial No. 2,585,239, 3 pages.
`Kishiyama, et al., “Experiments on throughput performance above
`100-Mbps in forward link for VSF-OFCDM broadband wireless
`access'. Vehicular Technology Conference, 2003. VTC 2003-Fall.
`2003 IEEE 58th, Oct. 6, 2003, pp. 1863-1868.
`* cited by examiner
`
`

`

`U.S. Patent
`U.S. Patent
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`Dec. 14, 2010
`Dec. 14, 2010
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`Sheet 1 of 10
`Sheet 1 of 10
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`US 7,852,746 B2
`US 7,852,746 B2
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`U.S. Patent
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`Dec. 14, 2010
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`Sheet 4 of 10
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`US 7,852,746 B2
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`

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`1.
`TRANSMISSION OF SIGNALNG IN AN
`OFDM-BASED SYSTEM
`
`US 7,852,746 B2
`
`This application claims the benefit of provisional U.S.
`Application Ser. No. 60/604,660 entitled “Transmitting
`Physical Layer Signaling as Underlay in OFDMA Systems.”
`filed Aug. 25, 2004.
`
`BACKGROUND
`
`10
`
`I. Field
`The present invention relates generally to communication,
`and more specifically to transmission of signaling in a wire
`less communication system.
`II. Background
`15
`A multiple-access system can concurrently support com
`munication for multiple terminals on the forward and reverse
`links. The forward link (or downlink) refers to the communi
`cation link from the base stations to the terminals, and the
`reverse link (or uplink) refers to the communication link from
`the terminals to the base stations. An orthogonal frequency
`division multiple access (OFDMA) system is a multiple
`access system that utilizes orthogonal frequency division
`multiplexing (OFDM). OFDM is a multi-carrier modulation
`technique that effectively partitions the overall system band
`width into multiple (N) orthogonal frequency subbands.
`These subbands are also referred to as tones, sub-carriers,
`bins, frequency channels, and so on. Each Subband is associ
`ated with a respective sub-carrier that may be modulated with
`data. The OFDMA system may assign a different set of sub
`bands to each terminal, and data for the terminal may be sent
`on the assigned Subbands. By using non-overlapping Subband
`sets for different terminals, interference among the terminals
`may be avoided, and improved performance may beachieved.
`Various signaling channels are typically used by a physical
`layer to Support data transmission on the forward and reverse
`links. These signaling channels may carry requests for certain
`information, the requested information, acknowledgments
`(ACKs), and so on. Some Subbands may be set aside on each
`40
`link and used for the signaling channels for that link. How
`ever, dedicating Subbands specifically for the signaling chan
`nels may represent inefficient use of the available subbands
`since the signaling channels may be intermittently active and
`may carry only a small amount of data when active. Each
`Subband that is dedicated for the signaling channels repre
`sents one less Subband that may be used for data transmission.
`There is therefore a need in the art for techniques to more
`efficiently transmit signaling in an OFDMA system.
`
`25
`
`30
`
`35
`
`45
`
`SUMMARY
`
`50
`
`Techniques for efficiently transmitting various types of
`signaling on the forward and reverse links in an OFDM-based
`system are described herein. Instead of specifically allocating
`Subbands to individual signaling channels, signaling data for
`a given signaling channel on a given (forward or reverse) link
`may be sent as “underlay to other transmissions that may be
`sent on the same link. Each wireless terminal may be assigned
`a different pseudo-random number (PN) code or sequence.
`The signaling data for each terminal may be spectrally spread
`over all or a portion of the system bandwidth using the PN
`code assigned to the terminal. The processing gain from the
`spreading allows the signaling data to be sent at a low power
`level so that the signaling may only marginally impact the
`performance of the other transmissions being sent concur
`rently.
`
`55
`
`60
`
`65
`
`2
`In an embodiment, a transmitting entity (which may be a
`base station or a wireless terminal) includes a signaling
`modulator, a data modulator, and a combiner. The signaling
`modulator spectrally spreads signaling data over M Subbands
`and generates signaling chips. The M Subbands may be all or
`a Subset of N Subbands usable for transmission. The signaling
`modulator may multiply the signaling data with a PN
`sequence and directly generate the signaling chips. Alterna
`tively, the signaling modulator may multiply the signaling
`data with the PN sequence to obtain spread signaling data,
`map the spread signaling data onto the M Subbands, and
`perform OFDM modulation on the mapped and spread sig
`naling data to generate the signaling chips. The data modu
`lator maps data symbols onto L Subbands used for data trans
`mission, where 1.<LsN, and further performs OFDM
`modulation on the mapped data symbols to generate data
`chips. The combiner combines (e.g., Scales and Sums) the
`signaling chips with the data chip and generates output chips.
`For the reverse link, a wireless terminal may transmit sig
`naling on all N usable subbands and may transmit traffic/
`packet data on the L Subbands assigned to the terminal for
`data transmission, which may be a subset of the N usable
`subbands. For the forward link, a base station may transmit
`signaling and traffic data for all terminals on the N usable
`Subbands. Signaling and traffic data may also be transmitted
`in other manners on the forward and reverse links, as
`described below. Various types of signaling may be sent in the
`manner described herein.
`A receiving entity performs the complementary processing
`to recover the transmitted signaling and traffic data. Various
`aspects and embodiments of the invention are described in
`further detail below.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`The features and nature 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 through
`out and wherein:
`FIG. 1 shows an OFDMA system with base stations and
`wireless terminals;
`FIG. 2 illustrates a frequency hopping (FH) scheme:
`FIG.3 shows an incremental redundancy (IR) transmission
`scheme;
`FIGS. 4A through 4D illustrate different signaling trans
`mission schemes;
`FIG. 5 shows a block diagram of a base station and a
`terminal;
`FIG. 6 shows a modulator with a data/pilot modulator, a
`multi-carrier signaling modulator, and a time-domain com
`biner;
`FIG. 7 shows a modulator with a data modulator, a pilot
`modulator, a single-carrier signaling modulator, and a time
`domain combiner;
`FIG. 8 shows a modulator with a data modulator, a pilot
`modulator, a multi-carrier signaling modulator, and a fre
`quency-domain combiner;
`FIG. 9 shows a demodulator for the modulator in FIG. 6;
`and
`FIG. 10 shows a demodulator for the modulator in FIG. 7.
`
`DETAILED DESCRIPTION
`
`The word “exemplary' is used herein to mean “serving as
`an example, instance, or illustration.” Any embodiment or
`
`

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`US 7,852,746 B2
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`15
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`25
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`30
`
`35
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`40
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`3
`design described herein as "exemplary' is not necessarily to
`be construed as preferred or advantageous over other embodi
`ments or designs.
`FIG. 1 shows an exemplary OFDMA system 100 with a
`number of base stations 110 that support communication for
`a number of wireless terminals 120. A base station is a fixed
`station used for communicating with the terminals and may
`also be called an access point, a Node B, or some other
`terminology. Terminals 120 are typically dispersed through
`out the system, and each terminal may be fixed or mobile. A
`10
`terminal may also be called a mobile station, a user equipment
`(UE), a wireless communication device, or some other termi
`nology. Each terminal may communicate with one or possibly
`multiple base stations on the forward and reverse links at any
`given moment. A system controller 130 provides coordina
`tion and control for base stations 110 and further controls
`routing of data for the terminals served by these base stations.
`Each base station 110 provides communication coverage
`for a respective geographic area. A base station and/or its
`coverage area may be referred to as a “cell, depending on the
`context in which the term is used. To increase capacity, the
`coverage area of each base station may be partitioned into
`multiple (e.g., three) sectors. Each sector is served by a base
`transceiver subsystem (BTS). For a sectorized cell, the base
`station for that cell typically includes the BTSs for all sectors
`of that cell. For simplicity, in the following description, the
`term “base station' is used generically for both a fixed station
`that serves a cell and a fixed station that serves a sector. The
`terms “user” and “terminal are also used interchangeably
`herein.
`The OFDMA system has N total Subbands, which are
`created by OFDM. Allor a subset of the N total subbands may
`be used to transmit traffic data, pilot, and signaling. Typically,
`Some Subbands are not used for transmission and serve as
`guard Subbands to allow the system to meet spectral mask
`requirements. For simplicity, the following description
`assumes that all N total subbands are usable for transmission,
`i.e., there are no guard Subbands.
`FIG. 2 illustrates a frequency hopping (FH) scheme 200
`that may be used for the forward and/or reverse link in the
`OFDMA system. Frequency hopping can provide frequency
`diversity against deleterious path effects and randomization
`of interference from other cells/sectors. With frequency hop
`ping, each terminal is assigned a traffic channel that is asso
`ciated with an FH sequence that indicates a specific group of
`45
`one or more subbands to use in each “hop' period. The FH
`sequence may also be called a hop pattern or Some other
`terminology. A hop period is the amount of time spent on a
`given subband group and spans R OFDM symbol periods (or
`simply, “symbol period’), where R21. The FH sequence may
`pseudo-randomly select different Subband groups in different
`hop periods. Frequency diversity is achieved by selecting all
`or many of the Nusable subbands over some number of hop
`periods.
`For the embodiment shown in FIG. 2, the Nusable sub
`bands are arranged into G groups. Each group contains S
`subbands, where in general G-1, S21, and G:SsN. The
`Subbands in each group may be contiguous (as shown in FIG.
`2) or non-contiguous (e.g., distributed across the N usable
`Subbands). Each terminal may be assigned one group of S
`subbands in each hop period. Pilot symbols may be time
`division multiplexed (TDM) with data symbols (as shown in
`FIG. 2), frequency division multiplexed (FDM) with data
`symbols (not shown in FIG. 2), or sent in some other manner.
`As used herein, a “data' symbol is a modulation symbol for
`traffic data, a “pilot' symbol is a modulation symbol for pilot,
`and a modulation symbol is a complex value for a point in a
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`signal constellation for a modulation scheme. A pilot is typi
`cally composed of known modulation symbols that are pro
`cessed and transmitted in a known manner.
`The traffic channels for different terminals in communica
`tion with the same base station are typically orthogonal to one
`another so that no two terminals use the same Subband in any
`given hop period. This avoids intra-cell/sector interference
`among the terminals communicating with the same base sta
`tion. The traffic channels for each base station may be pseudo
`random with respect to the traffic channels for nearby base
`stations. Interference between two terminals communicating
`with two different base stations occurs whenever their traffic
`channels use the same Subband in the same hop period. How
`ever, this inter-cell/sector interference is randomized due to
`the pseudo-random nature of the FH sequences used for the
`traffic channels.
`FIG. 2 shows an exemplary data and pilot transmission
`scheme with frequency hopping. Traffic data and pilot may
`also be transmitted in other manners, with or without fre
`quency hopping.
`The OFDMA system may utilize various signaling chan
`nels at the physical layer to Support data transmission on the
`forward and reverse links. The signaling channels may also be
`called control channels, overhead channels, and so on. The
`signaling channels are often used to send (typically) Small
`amounts of signaling for the physical layer and may be pro
`cessed and transmitted by the physical layer with small
`amount of delay. The signaling channels needed for each link
`are typically dependent on various factors such as, e.g., the
`manner in which traffic data is transmitted, the manner in
`which signaling is transmitted, the design of the traffic and
`signaling channels, and so on. Some exemplary signaling
`channels are described below. For clarity, each signaling
`channel sent on the forward link (FL) is labeled as “channel
`name-FL, and each signaling channel sent on the reverse
`link (RL) is labeled as “channel name-RL'.
`FIG. 3 shows an exemplary incremental redundancy (IR)
`transmission scheme for the forward link, which is also com
`monly called a hybrid Automatic Repeat ReOuest (H-ARQ)
`transmission scheme. If a base station has data to send to a
`terminal, the base station transmits a data rate control (DRC)
`request on a DRCReq-FL channel to the terminal. This DRC
`request asks for the received signal quality at the terminal So
`that data may be sent at an appropriate data rate to the termi
`nal. The terminal receives the DRC request, estimates the
`received signal quality for the forward link from the base
`station, and sends a DRC value on a DRC-RL channel to the
`base station. The received signal quality may be quantified by
`a signal-to-interference-and-noise ratio (SINR), an energy
`per-chip-to-total-noise ratio (E/N), an energy-per-chip-to
`noise ratio (E/N), a carrier-to-interference ratio (C/I), or
`some other signal quality metric. The DRC value may be a
`quantized version of the SINR measured by the terminal, a
`data rate deemed to be supported by the measured SINR, or
`Some other information.
`The base station receives the DRC value from the terminal
`and selects a data rate to use for data transmission to the
`terminal. The base station then processes (e.g., encodes and
`modulates) a data packet at the selected data rate and parti
`tions the coded packet into multiple data blocks. The first data
`block may contain sufficient information to allow the termi
`nal to recover the data packet under good channel condition.
`Each remaining data block contains additional redundancy
`information for the data packet.
`The base station transmits the first data block on a traffic
`channel to the terminal. The terminal receives the transmitted
`data block, processes (e.g., demodulates and decodes) the
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`US 7,852,746 B2
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`received block, and determines whether the data packet was
`decoded correctly. If the packet was not decoded correctly,
`the terminal sends a negative acknowledgment (NAK) on an
`ACK-RL channel to the base station. The base station then
`transmits the second data block upon receiving the NAK. The
`terminal receives the transmitted data block, combines soft
`decision symbols for the first and second data blocks, and
`decodes the packet based on the soft-decision symbols. The
`terminal sends another NAK on the ACK-RL channel if the
`packet is not decoded correctly. The block transmission and
`decoding continue in this manner until the packet is decoded
`correctly by the terminal or all data blocks for the packet have
`been transmitted by the base station. The terminal may send
`new DRC values periodically on the DRC-RL channel, when
`ever requested by the base station, after successfully decod
`ing data packets, and so on.
`For clarity, FIG. 3 shows transmission of both NAKs and
`ACKs on the ACK-RL channel. For an ACK-based scheme,
`the terminal transmits an ACK only if a packet is decoded
`correctly and does not transmit any NAKs. The absence of an
`ACK is presumed to be a NAK.
`As shown in FIG. 3, some delays are incurred for the
`terminal to decode a packet and send feedback on the ACK
`RL channel and for the base station to detect the ACK-RL
`channel and determine whether another block needs to be sent
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`for the packet. The transmission time line may be partitioned
`into frames. Each frame may be further partitioned into mul
`tiple (Q) slots that may be assigned slot indices of 1 through
`Q, where Q-1 (e.g., Q=4). One data block may be sent in each
`slot, and the Q slots in each frame may be used to send data
`blocks for up to Q different packets to the same terminal or to
`different terminals. The data blocks for each packet may be
`sent in consecutive frames and on slots with the same slot
`index.
`Table 1 lists exemplary signaling channels for the forward
`and reverse links. Each of these signaling channels is
`described below.
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`6
`for forward link transmissions, partial loading on the forward
`link (e.g., transmitting on only a Subset of the Nusable Sub
`bands), and so on. Consequently, the SINR estimate obtained
`by the terminal for a given slot may be a poor prediction of the
`SINR for a future slot.
`The base station may select an aggressive data rate for each
`packet and rely on the IR transmission to correct for predic
`tion error and to ensure robust reception of the packet. The IR
`transmission allows for less accurate SINR estimates and
`lower update rate for the DRC values. In one embodiment, the
`terminal sends DRC values at a low rate. In another embodi
`ment, the base station prompts the terminal to send a DRC
`value whenever a packet is scheduled to be sent to the termi
`nal. For this embodiment, only terminals that are scheduled to
`receive packets will send DRC values. The average number of
`DRC values sent on the reverse link is then equal to the
`average number of packets sent on the forward link. In yet
`another embodiment, the base station determines if a new
`SINR estimate is needed for the terminal, e.g., based on the
`age of the last DRC value received from the terminal. If the
`base station determines that the SINR estimate needs updat
`ing, then the base station sends a DRC request on the
`DRCReq-FL channel.
`The terminal sends ACKs on the ACK-RL channel for
`packets received from the base station. The number of ACKs
`sent per second on the ACK-RL channel is approximately
`equal to the number of packets sent per second on the forward
`link. The number of packets sent on the forward link is a
`function of the applications being carried on the forward link.
`The ACK rate for the ACK-RL channel may be estimated as
`the throughput persector divided by the average packet length
`on the forward link. The average packet length may be com
`puted based on an assumption on the mix of applications
`being Supported on the forward link.
`The base station sends ACKs on the ACK-FL channel for
`packets received from the terminals. The ACK-FL channel
`may be operated in the same manner described above for the
`ACK-RL channel.
`The terminal uses the ResReq-RL channel to send requests
`for air-link resources (e.g., subbands) on the reverse link. The
`terminal may send a resource request whenever it has data to
`send on the reverse link. The resource request may include
`any number of bits. In an embodiment, to minimize overhead
`for the ResReq-RL channel, the resource request consists of
`one bit and informs the base station that the terminal has data
`to send. The base station may assign a predetermined amount
`of reverse link resources to the terminal, e.g., a certain num
`ber of Subbands, a traffic channel for a certain data rate (e.g.,
`9.6 Kbps), and so on. In another embodiment, the resource
`request indicates a specific data rate that the terminal has
`selected from among multiple data rates Supported by the
`OFDMA system. The base station may assign the terminal
`with reverse link resources for the requested data rate or some
`other data rate. In yet another embodiment, the resource
`request indicates the amount of data (or buffer size) to be sent
`by the terminal. The base station may allocate reverse link
`resources to the terminal based on the buffer size.
`The reverse link allocation may indicate specific param
`eters to use for reverse link transmission (e.g., specific Sub
`bands, code rate, modulation scheme, and transmit power
`level to use for reverse link transmission). The reverse link
`allocation may also allow the terminal some flexibility in the
`reverse link transmission, e.g., to use a higher code rate and/or
`a higher order modulation scheme to send more data, if
`needed. For example, the terminal may be allocated 19.2 KHZ
`of bandwidth on the reverse link and may be allowed to
`transmit at a data rate of 9.6 Kbps or 19.2 kbps on this 19.2
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`TABLE 1.
`
`Signaling
`Description
`channel
`DRCReq-FL Used to send requests for DRC information from the
`terminals.
`ACK-FL Used to send ACKs for packets received from the terminals.
`ResGrant-FL Used to send resource grants, or allocation of air-link
`resources (e.g., Subbands) for the reverse link, to the
`terminals.
`Used to send power control (PC) commands to direct the
`terminals to adjust their transmit power.
`DRC-RL Used to send DRC information to the base station.
`ACK-RL Used to send ACKs for packets received from the base
`station.
`ResReq-RL Used to send requests for air-link resources on the reverse
`link.
`
`PC-FL
`
`The DRCReq-FL, DRC-RL, and ACK-RL channels are
`used for data transmission on the forward link, as shown in
`FIG.3. The base station uses the DRCReq-FL channel to send
`requests for DRC information from the terminal. The termi
`nal uses the DRC-RL channel to send DRC values indicative
`of its received SINR for the forward link. Each DRC value
`may be represented by a predetermined number of bits (e.g.,
`four bits). The base station may select a data rate for each
`packet based on the latest DRC value obtained from the
`terminal.
`In the OFDMA system, the interference level observed by
`the terminal from other cells/sectors may vary considerably
`over time due to various factors such as, e.g., power control
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`KHZ bandwidth. The terminal may use more transmit power
`when transmitting at the higher data rate to ensure reliable
`data reception by the base station. The allocation of a flexible
`rate reduces the number of bits needed for the resource
`request sent on the ResReq-RL channel.
`The base station uses the ResGrant-FL channel to send
`allocation of reverse link resources to the terminals. Each
`terminal in the OFDMA system may be assigned a Medium
`Access Channel (MAC) identifier that unambiguously iden
`tifies that terminal. A grant message sent on the ResGrant-FL
`channel may convey various types of information and may
`have any format. For example, a grant message sent to a given
`terminal may include (1) the MAC identifier (ID) of the
`terminal, (2) a channel ID for the traffic channel assigned to
`the terminal, and (3) possibly some other parameters. The
`number of bits to use for the MAC ID and channel ID are
`dependent on the system design, the MAC design, and pos
`sibly other factors. A 10-bit MAC ID may be adequate to
`cover both active and idle terminals in the OFDMA system,
`although some other MAC ID sizes may also be used. A 6-bit
`channel ID may be used to identify 64 traffic channels. For
`example, if the system bandwidth is 1.2288 MHz, then each
`of the 64 traffic channels may have a bandwidth of 19.2 KHZ.
`Other channel ID sizes may also be used.
`The terminal may be initially assigned one traffic channel.
`The terminal may request additional bandwidth on the reverse
`link, e.g., in a resource request or in a data packet sent on the
`reverse link. The base station may then assign one or more
`additional traffic channels to the terminal and may send the
`channel ID of each assigned traffic channel in a grant mes
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`Sage.
`The base station uses the PC-FL channel to send PC com
`mands to the terminals. The signaling transmission from each
`terminal on the reverse link, if sent as underlay across all N
`usable Subbands, acts as interference to the signaling and data
`35
`transmissions from other terminals on the reverse link. The
`transmit power for the signaling transmission from each ter
`minal may be adjusted to achieve the desired performance
`while reducing the amount of interference to other t