`(10) Patent N0.:
`US 6,721,569 B1
`
`Hashem et al.
`(45) Date of Patent:
`Apr. 13, 2004
`
`U5006721569B1
`
`(54) DYNAMIC SUB-CARRIER ASSIGNMENT IN
`OFDM SYSTEMS
`
`(75)
`
`Inventors: Bassam M. Hashem, Nepean (CA);
`David G. Steer, Nepean (CA); Shalini
`S. Periyalwar, Ottawa (CA)
`
`(73) Assignee: Nortel Networks Limited, St. Laurent
`(CA)
`.
`.
`.
`.
`Subject. to any disclaimer, the term of this
`patent 15 extended or adjusted under 35
`U'S'C' 154(b) by 419 days.
`
`*
`
`.
`) Notice:
`
`(
`
`6,347,071 B1 *
`6,545,997 B1 *
`6,584,092 B1 *
`
`................. 370/203
`2/2002 Cupo et a1.
`4/2003 Bohnke et a1.
`370/347
`
`6/2003 Sudo .......................... 370/344
`
`FOREIGN PATENT DOCUMENTS
`
`EP
`
`1033853 A2 *
`
`9/2000
`
`........... H04L/27/26
`
`OTHER PUBLICATIONS
`Jack H. Winters, “Smart Antennas for Wireless Systems”,
`IEEE Personal Communications, pp. 23—27, Feb. 1998.
`Agrogyaswami J. Paulraj, et al. “Space—Time Modems for
`Wireless Personal Communication”, IEEE Personal Com-
`munications, pp. 36—47, Feb. 1998.
`
`(21) Appl. No.2 09/672,704
`
`(22)
`
`Filed:
`
`Sep. 29, 2000
`
`* cited by examiner
`
`Primary Examiner—Cong Van Tran
`
`Int. Cl.7 .................................................. H04Q 7/20
`(51)
`(52) U.S. Cl.
`.................... 455/450, 455/509, 455/414.1,
`_
`455/464’ 370/203’ 370/208
`(58) Fleld of Search ................................. 455/450, 455,
`455/464> 509, 511, 515, 154-1, 166.2, 185.1,
`130> 135> 277~2> 403, 408, 407, 4141;
`370/343> 344> 330> 341> 342> 208> 203>
`480; 375/260> 349
`
`(56)
`
`_
`References Clted
`U.S. PATENT DOCUMENTS
`.
`57726978 A :
`3/1998 Fmdlgh 6t a1~
`~~~~~~~~~~~~~ 370/252
`39:23:63; 2 *
`3/1333 gum” “‘i"""""""" 333/33:
`
`’
`’
`/
`aun} et a ‘
`"""
`/
`5,889,759 A *
`3/1999 McGibney ............... 370/207
`
`6,243,424 B1 *
`6/2001 Kroeger et al.
`.
`..... 375/265
`6,298,035 B1 * 10/2001 Heiskala ..................... 370/206
`
`ABSTRACT
`(57)
`A method and apparatus are pI'OVIdCd for selecting and
`signaling the identity of sub-carriers to be used for trans-
`mission of data in a radio communication system, and for
`using other sub-carriers. A remote unit determines which
`sub-carriers are acceptable for use in data transmission by
`comparing the signal to interference ratio of each sub-carrier
`with a threshold. A base station transmits data over the
`acceptable sub-carriers at the optimum Link Mode or Link
`Modes. The base station may use some of the unacceptable
`sub-carriers for transmission of low sensitivity data at the
`optimum Link Mode, and may use some of the unacceptable
`sub-carriers for transmission of data at a lower Link Mode.
`The transmission power any unused unacceptable sub-
`carriers can be diverted to other sub-carriers.
`
`23 Claims, 6 Drawing Sheets
`
`REMOTE UNIT E
`
`
`SUB-CARRIER
`
`FFT
`
`ANALYSIS
`
`
`PROCESSOR
`
`PROCESSOR
`
`
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`gg
`
`
`
`
`
`12
`
`
` BASE STATION fl
`
`LINK MODE
`
`. EXTRABTUR
`
`SIGNAL
`g
`GEN ERATOR
`
`II EVALUATIJFI I
`
`
`
`E
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`Sheet 1 0f 6
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`US 6,721,569 B1
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` REMOTEUMTJQ‘
`
`
`m
`
`PROCESSOR
`29
`
`
`12
`
`DECZDZDEH
`
`_
`
`
`
`SUB-CARRIER
`ANALYSIS
`PROCESSOR
`26
`
`
`3
`
`
`
`LINK MODE
`
`EVALUATDR
`
`EXTRACTOR
`
`‘
`
`Q
`
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`
`BASESTAUONJQ
`
`
` SENAL
`15 u
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`GENERATOR
`
`
`
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`
`38
`
`FIG. 1
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`Sheet 2 0f 6
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`US 6,721,569 B1
`
`14dB
`
`12dB
`
`10dB
`
`SUB-BAND
`
`8dB
`
`BdB
`
`4dB
`
`2dB
`
`UdB
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`Sheet 3 0f 6
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`US 6,721,569 B1
`
`
`RECEIVE
`SIGNAL
`
`
`
` g9
`
`
`
`
`SEPARATE SIGNAL
`
`INTO COMPONENTS
`
`Q
`
`.
`
`
`
`SELECT FIRST
`
`SUB-CARRIER
`
`fl
`
`MEASURE SII
`
`@
`
`
`
`
`
`
`
`
`
`
`MORE
`SELECT NEXT SUB-
`SUB-CARRIERS?
`CARRIER
`
`
`
`9_4
`9_2
`
`
`
`
`
`
`
`
`HIGHER THAN
`
`TOTAL-TOTAL+SII;
`coum=coum+1;
`
`
`ADJUST BITMASK
`THRESHOLD?
`
`@
`
`AVERAGE SIEM-A—L
`fig
`coum
`
`TRANSMIT AVERAGE
`
`SII AND BITMASK
`
`' %
`
`FIG. 3
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`Sheet 4 0f 6
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`US 6,721,569 B1
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`
`
`RECEIVE RETURN
`SIGNAL
`
`
`
`1.22
`
`
`
`
`
`
`
`EXTRACT BITMASK.
`AVERAGE SII
`g
`
`SELECT LINK MODE
`
`m
`
`
`
`
`
`RECEIVE
`DATA
`
`LZ§
`
`
`
`ENCODE LOW SENSITIVITY
`DATA USING LINK MODE
`
`
`
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`
`
`
`
`
`SELECT EIRST
`SUB-CARRIER
`
`
`THIS AN
`ENOODE DATA USINO
`
`
`
`CCEPTABLE SUB- Y
`LINK Mggg
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`
`CARRIER?
`132
`
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`SUB-CARRIERS?
`CARRIER
`
`1%
`
`
`
`
`
`
`m N
`
`TRANSMIT SIGNAL
`
`I_4O
`
`FIG. 4
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`Apr. 13, 2004
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`Sheet 5 0f 6
`
`US 6,721,569 B1
`
`RECEIVE
`
`
`SIGNAL
`
`fl
`
`
`SEPARATE SIGNAL
`
`INTo COMPONENTS
`
`g;
`
`
`
`
`SELECT FIRST
`SUB-CARRIER
`
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`
`
`MEASURE SII
`@
`
`
`
`
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`
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`
`MDDE
`THRESHOLD?
`
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`
`
`
`
`ADJUST SEDUENCE
`0F SUB-CARRIER
`
`
`
`MORE
`SELECT NEXT SUB-
`LINK MDDES
`
`SUB-BARRIERS?
`CARRIER .
`
`
`
`g1
`
`gg
`
`
`9_4
`
`
`
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`
` TRANSMIT
`
`SEDUENcE 0F SUB-
`CARRIER LINK MDDES
`
`gg
`
`
`FIG. 5
`
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`Sheet 6 0f 6
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`US 6,721,569 B1
`
`
`' RECEIVE RETURN
`SIGNAL
`
`m
`
`
`EXTRACT SEDUENCE
`DF LINK MODES
`
`E
`
`
`RECEIVE
`DATA
`
`Eb:
`
`
`
`
`SELECT FIRST
`SUB-CARRIER
`
` E
`
`
`IS
`
`THIS AN
`
`ACCEPTABLE sua-
`CARRIER?
`
`
`m
`
`
`
`
`
`ENCDDE LOW SENSITIVITY
`
`DATA USING LDW LINK MDDE
`HA
`
`
`
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`
`MDDE
`
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`
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`
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`MDRE
`ENCDDE DATA USING
`SELECT NEXT sua-
`LINK MDDE
`-
`
`SUB-CARRIERS?
`CARRIER
`
`
`
`
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`
`BE
`
`
`
`TRANSMIT SICNAL'
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`
`m
`
`FIG. 6
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`US 6,721,569 B1
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`1
`DYNAMIC SUB-CARRIER ASSIGNMENT IN
`OFDM SYSTEMS
`
`FIELD OF THE INVENTION
`
`This invention relates to digital radio communication
`systems employing multiple sub-carriers, and more particu-
`larly to dynamic use of sub-carriers within such systems.
`BACKGROUND OF THE INVENTION
`
`In a digital radio communication system a base station
`transmits a signal at a transmission rate to a remote unit
`through a radio channel having channel characteristics, such
`as an attenuation. The signal is transmitted using transmis-
`sion parameters, such as a modulation level and a coding
`rate. The transmission rate depends on the transmission
`parameters. The transmission parameters are constrained by
`an acceptable bit error rate and by a signal to interference
`ratio of the signal, the latter varying in time with the channel
`characteristics. The communication system can use adaptive
`modulation to adjust the transmission parameters to accom-
`modate changes in channel characteristics over time. If a
`change in channel characteristics results in a lower signal to
`interference ratio, the modulation level must be reduced (for
`example, from 16-QAM to QPSK) or the coding rate must
`be improved (for example, from 3A1 to 2/3) in order to maintain
`the acceptable bit error rate, albeit at a lower transmission
`rate. If a change in channel characteristics results in a higher
`signal to interference ratio, the base station can increase the
`modulation level or decrease the coding rate in order to
`obtain a higher transmission rate.
`implements adaptive
`In a communication system that
`modulation, the base station and the remote unit must be
`synchronized with respect to the transmission parameters. In
`current communication systems the remote unit determines
`a channel quality when the remote unit receives a frame of
`data. The remote unit may estimate, for example, the signal
`to interference ratio of the channel. The remote unit sends a
`
`signal back to the base station reporting the channel quality.
`Using the channel quality report received from the remote
`unit, the base station calculates a set of optimum transmis-
`sion parameters which the base station will use in its next
`transmission of data. However, the base station must first
`send the set of new optimum transmission parameters to the
`remote unit using the previous transmission parameters. The
`remote unit receives the set of new optimum transmission
`parameters, interpreting the signal using the previous trans-
`mission parameters. The remote unit then decodes subse-
`quent frames of data using the new optimum transmission
`parameters.
`In communication systems that make use of multiple
`antennae for transmission and reception, the transmission
`parameters may include adaptive antenna and coding param-
`eters. For example, some “smart antenna” systems may
`adaptively adjust
`their directional patterns towards the
`remote units. An outline of such systems may be found in the
`paper by J. H. Winters, “Smart Antennas for Wireless
`Systems”, IEEE Pers. Commun., vol. 5, no. 1, February
`1998, pp. 23—27, which is incorporated by reference herein.
`Similarly, the radio system may make use of the multiple
`communication channels that exist between transmitters and
`
`receivers with multiple antennae. In this case, the transmis-
`sion parameters include both space (across multiple
`antennae) and time (different time of transmission) aspects
`that adapt the transmissions to the multiple propagation
`environment. An outline of such systems may be found in
`
`10
`
`15
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`20
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`25
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`30
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`35
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`40
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`45
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`50
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`55
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`60
`
`65
`
`2
`the paper by A. J. Paulraj and B. C. Ng, “Space-time
`Modems for Wireless Personal Communications”, IEEE
`Pers. Commun., vol. 5, no. 1, February 1998, pp. 36—48,
`which is incorporated by reference herein.
`In communication systems employing many sub-carriers,
`such as those that employ Orthogonal Frequency Division
`Multiplexing (OFDM systems),
`the channel quality may
`vary with the frequency of each sub-carrier. OFDM systems
`can use 1000 sub-carriers, and transmission of information
`describing the channel quality and a set of optimum trans-
`mission parameters for each sub-carrier would require sig-
`nificant overhead, reducing the efficiency of the communi-
`cation system. In current methods, the signal to interference
`ratio is averaged over all sub-carriers so that only one signal
`to interference ratio is reported to the base station and only
`one set of new optimum transmission parameters is trans-
`mitted to the remote unit. In this method, the single set of
`new optimum transmission parameters results in an unnec-
`essarily low transmission rate for individual sub-carriers
`whose signal to interference ratio is higher than the average
`signal to interference ratio reported by the receiver.
`
`SUMMARY OF THE INVENTION
`
`The present invention provides a method of selecting and
`signalling the identity of acceptable groups of sub-carriers in
`a radio communication system. A remote unit receives a
`signal as more than one sub-carrier signal from a base
`station. The remote unit determines a channel quality (such
`as a signal to interference ratio or a reciprocal of an error
`rate) of each group of sub-carrier signals, and compares the
`channel quality of each group of sub-carrier signals with a
`threshold. A sequence of numbers is generated, there being
`one number for each group of sub-carrier signals. Each
`number has a value belonging to a first set of values if the
`channel quality of the corresponding group of sub-carriers is
`above the threshold, and has a value belonging to a second
`set of values if the channel quality of the corresponding
`group of sub-carriers is not above the threshold, the two sets
`of values having no values in common. The first set of values
`may consist of the value one and the second set of values
`may consist of the value zero, in which case each number in
`the sequence has a length of one bit. The remote unit
`generates at least one value by which the base station can
`determine one or more Link Modes, a Link Mode being a set
`of transmission parameters. The remote unit transmits the
`sequence of numbers and the values by which the base
`station can determine the Link Mode or Link Modes.
`
`The remote unit may calculate the average channel quality
`of groups of sub-carriers whose channel quality is above the
`threshold,
`in which case the average channel quality is
`transmitted to the base station. The remote unit may also
`determine a Link Mode using the average channel quality, in
`which case the Link Mode is transmitted to the base station.
`
`The remote unit may alternatively determine a Link Mode
`for each group of sub-carriers whose channel quality is
`above the threshold, in which case the sequence of numbers
`and the values by which the base station can determine the
`Link Mode of each sub-carrier whose channel quality is
`above the threshold is transmitted. In the latter case, the
`sequence of numbers and the values by which the base
`station can determine the Link Modes can be combined into
`
`a single sequence of numbers.
`The present invention also provides a method of assigning
`transmission tasks to at least one sub-carrier in a radio
`
`communication system. A base station receives a return
`signal, and extracts from the return signal a sequence of
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`3
`numbers, each number corresponding to one group of sub-
`carriers, and at least one value by which the base station can
`determine at least one Link Mode. The base station deter-
`mines at least one Link Mode based on the at least one value.
`
`The base station defines a set of acceptable groups of
`sub-carriers as all groups of sub-carriers for which the
`corresponding number has a value belonging to a first set of
`values, and defines a set of unacceptable groups of sub-
`carriers as all groups of sub-carriers for which the corre-
`sponding number has a value belonging to a second set of
`values, the two sets of values having no values in common.
`The base station allocates for data transmission at one of the
`
`Link Modes the sub-carriers which belong to the groups of
`sub-carriers within the set of acceptable groups of sub-
`carriers. In one embodiment, the return signal includes an
`average channel quality and the base station determines a
`single Link Mode based on the average channel quality. In
`another embodiment, the return signal includes a reference
`to a Link Mode and the base station determines a single Link
`Mode based on the reference to the Link Mode. In yet
`another embodiment, the return signal includes references to
`one Link Mode for each acceptable group of sub-carriers,
`possibly within the sequence of numbers, and the base
`station determines a Link Mode for each acceptable group of
`sub-carriers based on the corresponding reference. The base
`station may allocate for low sensitivity data transmission
`sub-carriers within some of the unacceptable sub-carriers,
`may allocate for data transmission at a low transmission rate
`sub-carriers within some of the remaining unacceptable
`sub-carriers, and may divert transmission power from the
`remaining unused unacceptable sub-carriers to other sub-
`carriers.
`
`The method provides improved efficiency of a commu-
`nication system by allowing sub-carriers having a high
`signal to interference ratio to use a higher transmission rate.
`Sub-carriers having a low signal to interference ratio can be
`used for less sensitive traffic, or their transmission power can
`be diverted to sub-carriers having a high signal to interfer-
`ence ratio.
`
`Other aspects and features of the present invention will
`become apparent to those ordinarily skilled in the art upon
`review of the following description of specific embodiments
`of the invention in conjunction with the accompanying
`figures.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`The invention will now be described in greater detail with
`reference to the accompanying diagrams, in which:
`FIG. 1 is a block diagram of a portion of a radio
`communication system in which the invention is imple-
`mented;
`FIG. 2 is a chart of example signal to interference ratios
`in several sub-carriers;
`FIG. 3 is a flow chart of a method by which a remote unit
`determines and conveys information concerning acceptable
`sub-carriers to a base station;
`FIG. 4 is a flow chart of a method by which a base station
`makes use of information concerning acceptable sub-
`carriers;
`FIG. 5 is a flow chart of an alternative method by which
`a remote unit determines and conveys information concern-
`ing acceptable sub-carriers to a base station; and
`FIG. 6 is a flow chart of an alternative method by which
`a base station makes use of information concerning accept-
`able sub-carriers.
`
`10
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`4
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENTS
`
`Referring to FIG. 1, a portion of a radio communication
`system is shown. The radio communication system employs
`a plurality of sub-carriers to transmit traffic from a base
`station 10 to a remote unit 16. For example,
`the radio
`communication system may employ Orthogonal Frequency
`Division Multiplexing. Asignal generator 36 within the base
`station 10 generates a signal 12. The signal is transmitted
`through a base station transmitting antenna 14. Each sub-
`carrier carries data encoded with a Link Mode. ALink Mode
`is a set of at least one transmission parameter, such as a
`modulation level, a coding rate, a symbol rate, a transmis-
`sion power level, antenna directional parameters, or space-
`time coding parameters. The signal 12 propagates along a
`forward link to the remote unit 16, where it is received at a
`remote unit receiving antenna 18 as a received signal. A Fast
`Fourier Transform (FFT) processor 20 within the remote
`unit 16 separates the received signal into a plurality of
`sub-carrier signals, there being one sub-carrier signal for
`each sub-carrier. A decoder 22 within the remote unit
`
`decodes the sub-carrier signals to produce received data.
`Each sub-carrier signal
`is decoded using a Link Mode
`appropriate to the sub-carrier of the sub-carrier signal. The
`decoded data is then passed to a user 24.
`The sub-carrier signals are also passed to a sub-carrier
`analysis processor 26 within the remote unit. The sub-carrier
`analysis processor 26 measures a signal to interference ratio
`(8/1) of each sub-carrier signal. The sub-carrier analysis
`processor 26 compares the 8/1 of each sub-carrier signal
`with a threshold to determine which sub-carriers are accept-
`able sub-carriers. Acceptable sub-carriers are sub-carriers
`for which the measured 8/1 of the corresponding sub-carrier
`signal is higher than the threshold. Sub-carriers for which
`the measured 8/1 of the corresponding sub-carrier signal is
`not higher than the threshold are unacceptable sub-carriers.
`The sub-carrier analysis processor 26 calculates an average
`8/1 of the S/Is of the acceptable sub-carriers. Referring to
`FIG. 2, an example set of S/Is is shown. Ahorizontal axis 40
`indicates a sub-band number of each of twelve sub-carriers.
`A vertical axis 42 indicates the 8/1 in dB of the sub-carrier
`signals. If a threshold 44 having a value of 4 dB is used, then
`the sub-carrier analysis processor 26 will identify eight of
`the sub-carriers (sub-band numbers 1, 2, 3, 4, 5, 10, 11, and
`12) as being acceptable sub-carriers. The sub-carrier analy-
`sis processor 26 calculates the average S/I of the acceptable
`sub-carriers in FIG. 2 as having a value of 9.1 dB, whereas
`the average 8/1 of all sub-carriers in FIG. 2 would be 6.8 dB.
`Returning to FIG. 1, the remote unit 16 transmits a return
`signal 30 along a reverse link to the base station 10 through
`a remote unit transmitting antenna 28, which may or may not
`be the same antenna as the remote unit receiving antenna 18.
`The return signal 30 includes the average 8/1 of acceptable
`sub-carriers and a sequence of numbers identifying the
`acceptable sub-carriers. If a value of “1” is used to identify
`acceptable sub-carriers and a value of “0” is used to identify
`unacceptable sub-carriers, or the reverse, then the sequence
`of numbers can be a bitmask, using one bit to indicate the
`acceptability of each sub-carrier. Of course other values can
`be used to indicate which sub-carriers are acceptable and
`which sub-carriers are unacceptable, but then more bits are
`required in the sequence of numbers for each sub-carrier. In
`the example of FIG. 2, the remote unit 16 would transmit an
`average S/I having a value of 9.1 dB and a bitmask having
`a value of “111110000111”.
`
`The return signal 30 is received by the base station 10 at
`a base station receiving antenna 32, which may or may not
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`5
`be the same antenna as the base station transmitting antenna
`14. An extractor 33 within the base station extracts the
`average S/I from the return signal 30 and passes it to a Link
`Mode evaluator 34 within the base station. The extractor 33
`also extracts the bitmask from the return signal 30 and
`passes it to the signal generator 36. The extractor 33 is a
`processor. The Link Mode evaluator 34 selects an optimum
`Link Mode based on the average S/I received from the
`remote unit 16 and on requirements of the communication
`system, such as an acceptable bit error rate. The Link Mode
`evaluator 34 is a processor, and may be combined with the
`extractor 33. The optimum Link Mode is passed to the signal
`generator 36 within the base station, along with the bitmask
`from the extractor 33 and along with data 38 which is to be
`transmitted.
`
`The signal generator 36 encodes the data 38 using the
`optimum Link Mode, and generates a new signal. The signal
`generator 36 differentiates the sub-carriers into a set of
`acceptable sub-carriers and a set of unacceptable sub-
`carriers. The new signal only codes the data 38 over the
`acceptable sub-carriers. Whether a particular sub-carrier is
`acceptable or unacceptable is determined from the bitmask,
`or more generally from the sequence of numbers, most
`recently received from the remote unit 16. Each unaccept-
`able sub-carrier can be allocated for one of several uses.
`
`Zero or more of the unacceptable sub-carriers are used to
`transmit low sensitivity traffic encoded using the optimum
`Link Mode. Low sensitivity traffic can include inessential
`but potentially useful information, such as control bits or
`parity bits. Although unacceptable sub-carriers have a low
`S/I and may have a high bit error rate when received by the
`remote unit, the low sensitivity of the traffic transmitted over
`these sub-carriers means that some errors are tolerable. Zero
`
`or more of the remaining unacceptable sub-carriers are used
`to transmit low rate traffic using a second Link Mode having
`a lower transmission rate than the optimum Link Mode. Any
`remaining unacceptable sub-carriers are unused, and power
`that would otherwise be used to transmit information over
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`the unused unacceptable sub-carriers is assigned to other
`sub-carriers.
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`Referring to FIG. 3, a method by which the remote unit 16
`determines the acceptable sub-carriers and the average S/I is
`shown. The remote unit receives a signal at step 80 at the
`remote unit receiving antenna 18. At step 82 the signal is
`separated into its sub-carrier signals by the FFT processor
`20. The first sub-carrier is selected at step 84. At step 86 the
`S/I of the sub-carrier signal of the selected sub-carrier is
`measured. If at step 88 the measured S/I is higher than a
`threshold,
`then the selected sub-carrier is an acceptable
`sub-carrier. At step 90 a running total is increased by the
`measured S/I, a count of acceptable sub-carriers is increased
`by one, and the sequence of numbers, preferably a bitmask,
`is adjusted to identify the selected sub-carrier as being an
`acceptable sub-carrier. If at step 92 there are further sub-
`carriers, then at step 94 the next sub-carrier is selected and
`the algorithm returns to step 86 to measure the S/I of
`sub-carrier signal of the next sub-carrier. If at step 92 there
`are no further sub-carriers, then at step 96 the average S/I of
`acceptable sub-carriers is calculated as the running total
`divided by the count of acceptable sub-carriers. Steps 82 to
`96 are carried out by the sub-carrier analysis processor 26.
`At step 98 the average S/I and the sequence of numbers are
`transmitted to the base station through the remote unit
`transmitting antenna 28. Steps 84, 92, and 94 form a loop
`that cycles through each sub-carrier, although any method of
`measuring the S/I of each sub-carrier signal could be used.
`Steps 90 and 96 calculate the average S/I, although any
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`method of calculating the average S/I could be used. For
`example, the count of the acceptable sub-carriers could be
`determined at step 96 from the sequence of numbers.
`Referring to FIG. 4, a method by which the base station
`uses the sub-carriers is shown. At step 120 the base station
`receives a return signal at the base station receiving antenna
`32. At step 122 the sequence of numbers, preferably a
`bitmask, and the average S/I are extracted from the return
`signal by the extractor 33. At step 124 the Link Mode
`evaluator 34 selects an optimum Link Mode based on the
`average S/I. At step 126 the base station receives data to be
`transmitted. The first sub-carrier is selected at step 128. At
`step 130 the base station determines whether the selected
`sub-carrier is an acceptable sub-carrier by comparing the
`sub-band number of the sub-carrier with the sequence of
`numbers. For example, if the sequence of numbers is a
`bitmask with a value of “111110001111” and the selected
`
`sub-carrier was the first sub-carrier, then a comparison of the
`sub-band number and the bitmask would produce a value of
`“1” since that is the value of the first bit, and the selected
`sub-carrier would therefore be an acceptable sub-carrier. If
`the selected sub-carrier was the sixth sub-carrier, then a
`comparison of the sub-band number and the bitmask would
`produce a value of “0” since that is the value of the sixth bit,
`and the selected sub-carrier would therefore be an unaccept-
`able sub-carrier. If at step 130 the base station determines
`that the selected sub-carrier is an acceptable sub-carrier, then
`at step 132 the data is encoded for transmission over the
`selected sub-carrier using the optimum Link Mode. If at step
`130 the base station determines that the selected sub-carrier
`
`is an unacceptable sub-carrier, then at step 134 low sensi-
`tivity data is encoded for transmission over the selected
`sub-carrier using the optimum Link Mode. Alternatively, at
`step 134 regular data could be encoded for transmission over
`the selected sub-carrier at a second Link Mode having a
`lower transmission rate than the optimum Link Mode, or no
`data could be encoded for transmission over the selected
`
`sub-carrier and the transmission power of the selected
`sub-carrier diverted to other sub-carriers. If at step 136 there
`are further sub-carriers, then at step 138 the next sub-carrier
`is selected and the algorithm returns to step 130 to determine
`whether the next sub-carrier is an acceptable sub-carrier. If
`at step 136 there are no further sub-carriers, then at step 140
`the signal is transmitted to the receiver. Steps 128 to 138 are
`carried out by the signal generator 36. Steps 128, 136, and
`138 form a loop that cycles through each sub-carrier,
`although any method of determining which sub-carriers are
`acceptable sub-carriers could be used.
`Signalling overhead can be reduced in a number of ways.
`Overhead on the forward link from the base station to the
`remote unit can be reduced if the remote unit calculates the
`
`optimum Link Mode itself, using an algorithm similar to that
`used by the Link Mode evaluator 34. After a delay sufficient
`to allow the return signal 30 to reach the base station and to
`allow the signal 12 carrying a frame encoded using the
`optimum Link Mode to reach the remote unit, the decoder 22
`decodes frames using the optimum Link Mode. The base
`station need not transmit the optimum Link Mode to the
`remote unit. If the frames include numbered packets, then
`synchronization of the optimum Link Mode can be achieved
`more precisely if the remote unit includes a packet number
`in the return signal rather than estimating the delay. The base
`station begins using the optimum Link Mode when it
`transmits a packet having the packet number identified in the
`return signal, and the remote unit begins using the optimum
`Link Mode when it receives a packet having the packet
`number identified in the return signal.
`
`|PR2019-00959
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`Apple Inc. EX1005 Page 10
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`IPR2019-00959
`Apple Inc. EX1005 Page 10
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`US 6,721,569 B1
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`7
`Overhead on the reverse link can be reduced if the remote
`unit calculates the optimum Link Mode itself and transmits
`a reference to the optimum Link Mode to the base station,
`as disclosed in a US. patent application entitled “Receiver
`based adaptive modulation scheme” by Hashem et al., filed
`on Sep. 27, 2000, and assigned to the assignee of the present
`application, and incorporated by reference herein. The return
`signal can be viewed more generally as including a sequence
`of numbers identifying acceptable sub-carriers, and a value
`by which the base station can determine an optimum Link
`Mode. The value by which the base station can determine an
`optimum Link Mode may be the average 8/1, as described
`above. Alternatively,
`if the remote unit determines the
`optimum Link Mode itself, then the value may be a refer-
`ence to the optimum Link Mode. If the optimum Link Mode
`is one of a set of allowed Link Modes agreed upon by the
`base station and the remote unit prior to a transmission, the
`reference may be an index to the optimum Link Mode within
`the set of allowed Link Modes. The return signal 30 contains
`the sequence of numbers and the reference, but not the
`average 8/1 of acceptable sub-carriers. The extractor 33
`simply extracts the reference to the optimum Link Mode
`from the return signal 30 at step 122, and the Link Mode
`evaluator 34 determines the optimum Link Mode at step 124
`from the set of allowed Link Modes using the reference.
`A different Link Mode can be used for each acceptable
`sub-carrier if the Link Modes are calculated by the remote
`unit and transmitted to the base station. In this embodiment,
`the remote unit transmits more than one value by which the
`base station can determine more than one Link Mode,
`preferably as a sequence of references to sub-carrier Link
`Modes. The sequence of numbers identifying the acceptable
`sub-carriers and the values by which the base station can
`determine the Link Modes can be sent along the return
`channel separately, or can be combined by forming the
`sequence of numbers from the sub-carrier Link Mode of
`each acceptable sub-carrier and one or more distinct values
`identifying unacceptable sub-carriers. If in the example of
`FIG. 2 a 8/1 of 10 dB or higher (for example) allows use of
`a seventh Link Mode within the set of allowed Link Modes,
`a 8/1 of 7 dB or higher allows use of a sixth Link Mode
`within the set of allowed Link Modes, and a 8/1 of 4 dB or
`higher allows use of a fifth Link Mode within the set of
`allowed Link Modes (a higher ordinal rank of Link Mode
`having a higher transmission rate),
`then the remote unit
`transmits a sequence of numbers having a value of
`“777660000567” to the base station along the reverse link.
`A number having a value of “0” indicates that the corre-
`sponding sub-carrier is unacceptable, and a number having
`a value other than zero indicates both that the corresponding
`sub-carrier is acceptable and the Link Mode to be used when
`encoding data for transmission over that sub-carrier. The
`method carried out by the remote unit is shown in FIG. 5.
`The method shown in FIG. 5 is similar to the method shown
`
`that the average 8/1 is not calculated
`in FIG. 3, except
`(eliminating steps 90 and 96), a sub-carrier Link Mode is
`evaluated for each sub-carrier (adding steps 89 and 91), and
`the average 8/1 is not included in the return signal (replacing
`step 98 with step 99). At step 89 a Link Mode evaluator (not
`shown in FIG. 1, and which may or may not be a component
`of the sub-carrier analysis processor) in the remote unit
`determines the Link Mode for the selected sub-carrier based
`
`on the 8/1 of the selected sub-carrier measured at step 86, as
`disclosed in patent application entitled “Receiver based
`adaptive modulation scheme”. At step 91 the sequence of
`references to sub-carrier Link Modes is adjusted by setting
`the corresponding number in the sequence of numbers to be
`a reference to the Link Mode determined at step 89.
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`The method carried out by the base station in the embodi-
`ment in which the Link Mode of each acceptable sub-carrier
`is transmitted to the base station is shown in FIG. 6. The
`method shown in FIG. 6 is similar to the method shown in
`FIG. 4, except that the average 8/1 is not included in the
`return signal (replacing step 122 with step 123), and the st