(12) United States Patent
`Hashem et al.
`
`I IIIII IIIIIIII Ill lllll lllll lllll lllll lllll lllll lllll lllll 111111111111111111
`US006721569Bl
`
`(10) Patent No.:
`(45) Date of Patent:
`
`US 6,721,569 Bl
`Apr. 13, 2004
`
`(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)
`
`EP
`
`(73) Assignee: Nortel Networks Limited, St. Laurent
`(CA)
`
`( *) Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 419 days.
`
`(21) Appl. No.: 09/672,704
`
`(22) Filed:
`
`Sep. 29, 2000
`
`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) Field of Search ................................. 455/450, 455,
`455/464, 509, 511, 515, 154.1, 166.2, 185.1,
`130, 135, 277.2, 403, 408, 407, 414.1;
`370/343, 344, 330, 341, 342, 208, 203,
`480; 375/260, 349
`
`(56)
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`5,726,978 A * 3/1998 Frodigh et al.
`............. 370/252
`5,748,677 A * 5/1998 Kumar ....................... 375/285
`5,867,478 A * 2/1999 Baum et al. ................ 370/203
`5,889,759 A * 3/1999 McGibney .................. 370/207
`6,243,424 Bl * 6/2001 Kroeger et al. ............. 375/265
`6,298,035 Bl * 10/2001 Heiskala ..................... 370/206
`
`6,347,071 Bl * 2/2002 Cupo et al. ................. 370/203
`6,545,997 Bl * 4/2003 Bohnke et al.
`............. 370/347
`6,584,092 Bl * 6/2003 Sudo .......................... 370/344
`
`FOREIGN PATENT DOCUMENTS
`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(cid:173)
`munications, pp. 36-47, Feb. 1998.
`* cited by examiner
`
`Primary Examiner-Cong Van Tran
`
`(57)
`
`ABSTRACT
`
`A method and apparatus are provided for selecting and
`signaling the identity of sub-carriers to be used for trans(cid:173)
`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(cid:173)
`carriers can be diverted to other sub-carriers.
`
`23 Claims, 6 Drawing Sheets
`
`USER
`~
`
`DECODER
`ll
`
`REMOTE UNIT 1§
`
`FFT
`PROCESSOR
`fl!
`
`SUB·CARRIER
`ANALYSIS
`PROCESSOR
`~
`
`BASE STATION .!!l_
`
`14
`
`SIGNAL
`GENERATOR
`I§.
`
`LINK MODE
`EVALUATOR
`M
`
`EXTRACTOR
`TI.
`
`DATA
`
`38
`
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`U.S. Patent
`
`Apr. 13, 2004
`
`Sheet 1 of 6
`
`US 6,721,569 Bl
`
`USER
`24
`
`REMOTE UNIT 1.§.·
`
`FFT
`PROCESSOR
`20
`
`1 - - - DECODER .____ SUB-CARRIER
`22
`ANALYSIS
`PROCESSOR
`26
`
`28
`
`BASE STATION !Q
`
`\V
`
`tv14
`--
`
`SIGNAL
`GENERATOR
`36
`
`I-+--
`
`-
`
`-:: -
`
`LINK MODE
`EVALUATOR
`34
`
`I+-
`
`EXTRACTOR
`33
`
`l
`
`\V
`
`32v-
`
`-
`
`DATA
`_)
`38
`
`FIG. 1
`
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`U.S. Patent
`
`Apr. 13, 2004
`
`Sheet 2 of 6
`
`US 6,721,569 Bl
`
`dB
`....
`14dB -
`
`r,...-42
`
`12dB -
`
`lOdB .- -
`
`-
`
`-
`
`-
`
`-
`
`BdB -
`
`6dB -
`
`4dB
`
`2dB -
`
`OdB
`
`-
`
`-
`
`,-..
`
`-
`
`(
`44
`
`n
`
`8
`
`9
`
`10 1 1 12 r
`
`40
`
`SUB-BAND
`
`FIG. 2
`
`-
`
`-
`
`1
`
`2
`
`3
`
`4
`
`5
`
`6
`
`7
`
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`U.S. Patent
`
`Apr. 13, 2004
`
`Sheet 3 of 6
`
`US 6,721,569 Bl
`
`RECEIVE
`SIGNAL
`80
`
`SEPARATE SIGNAL
`INTO COMPONENTS
`82
`
`SELECT FIRST
`SUB-CARRIER
`84
`
`MEASURE S/1
`86
`
`SELECT NEXT SUB(cid:173)
`CARRIER
`94
`
`TOTAL- TOTAL+ S/1;
`y
`COUNT =COUNT+ 1;
`>----- ADJUST BITMASK
`90
`
`AVERAGE S/1= TOTAL
`96 COUNT
`
`TRANSMIT AVERAGE
`SIi AND BITMASK
`98
`
`FIG. 3
`
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`U.S. Patent
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`Apr. 13, 2004
`
`Sheet 4 of 6
`
`US 6,721,569 Bl
`
`RECEIVE RETURN
`SIGNAL
`120
`
`EXTRACT BITMASK,
`AVERAGE S/1
`122
`
`SELECT LINK MODE
`124
`
`SELECT FIRST
`SUB-CARRIER
`128
`
`y
`
`ENCODE DATA USING
`LINK MOOE
`132
`
`ENCODE LOW SENSITIVITY
`DATA USING LINK MOOE
`134
`
`SELECT NEXT SUB·
`CARRIER
`138
`
`FIG. 4
`
`TRANSMIT SIGNAL
`140
`
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`U.S. Patent
`
`Apr. 13, 2004
`
`Sheet 5 of 6
`
`US 6,721,569 Bl
`
`RECEIVE
`SIGNAL
`80
`
`SEPARATE SIGNAL
`INTO COMPONENTS
`82
`
`SELECT FIRST
`SUB-CARRIER
`84
`
`MEASURE SIi
`86
`
`SELECT NEXT SUB(cid:173)
`CARRIER .
`94
`
`TRANSMIT
`SEQUENCE OF SUB(cid:173)
`CARRIER LINK MODES
`99
`
`DETERMINE LINK
`MODE
`89
`
`ADJUST SEQUENCE
`OF SUB-CARRIER
`LINK MODES
`91
`
`FIG. 5
`
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`U.S. Patent
`
`Apr. 13, 2004
`
`Sheet 6 of 6
`
`US 6,721,569 Bl
`
`SIGNAL
`120
`
`EXTRACT SEOUENCE
`OF LINK MODES
`123
`
`SELECT FIRST
`SUB-CARRIER
`128
`
`y
`
`DETERMINE LINK
`MODE
`131
`
`ENCODE LOW SENSITIVITY
`DATA USING LOW LINK MODE
`134
`
`SELECT NEXT SUB(cid:173)
`CARRIER
`138
`
`y
`
`ENCODE DATA USING
`LINK MODE
`132 .
`
`TRANSMIT SIGNAL
`140
`
`FIG. 6
`
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`US 6,721,569 Bl
`
`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(cid:173)
`larly to dynamic use of sub-carriers within such systems.
`
`BACKGROUND OF THE INVENTION
`
`5
`
`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
`10 describing the channel quality and a set of optimum trans(cid:173)
`mission parameters for each sub-carrier would require sig(cid:173)
`nificant overhead, reducing the efficiency of the communi(cid:173)
`cation system. In current methods, the signal to interference
`ratio is averaged over all sub-carriers so that only one signal
`15 to interference ratio is reported to the base station and only
`one set of new optimum transmission parameters is trans(cid:173)
`mitted to the remote unit. In this method, the single set of
`new optimum transmission parameters results in an unnec(cid:173)
`essarily low transmission rate for individual sub-carriers
`20 whose signal to interference ratio is higher than the average
`signal to interference ratio reported by the receiver.
`
`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(cid:173)
`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(cid:173)
`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 25
`example, from 16-QAM to QPSK) or the coding rate must
`be improved (for example, from% 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 30
`modulation level or decrease the coding rate in order to
`obtain a higher transmission rate.
`In a communication system that implements adaptive
`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(cid:173)
`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(cid:173)
`mission parameters. The remote unit then decodes subse(cid:173)
`quent frames of data using the new optimum transmission 50
`parameters.
`In communication systems that make use of multiple
`antennae for transmission and reception, the transmission
`parameters may include adaptive antenna and coding param(cid:173)
`eters. For example, some "smart antenna" systems may 55
`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. 60
`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(cid:173)
`sion parameters include both space ( across multiple
`antennae) and time (different time of transmission) aspects 65
`that adapt the transmissions to the multiple propagation
`environment. An outline of such systems may be found in
`
`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
`35 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
`40 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
`45 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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`US 6,721,569 Bl
`
`3
`numbers, each number corresponding to one group of sub(cid:173)
`carriers, and at least one value by which the base station can
`determine at least one Link Mode. The base station deter(cid:173)
`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(cid:173)
`carriers as all groups of sub-carriers for which the corre(cid:173)
`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(cid:173)
`carriers. In one embodiment, the return signal includes an 15
`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 20
`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 25
`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 30
`remaining unused unacceptable sub-carriers to other sub-
`earners.
`The method provides improved efficiency of a commu(cid:173)
`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(cid:173)
`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(cid:173)
`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(cid:173)
`carriers;
`FIG. 5 is a flow chart of an alternative method by which
`a remote unit determines and conveys information concern(cid:173)
`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(cid:173)
`able sub-carriers.
`
`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
`5 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. A signal generator 36 within the base
`station 10 generates a signal 12. The signal is transmitted
`10 through a base station transmitting antenna 14. Each sub(cid:173)
`carrier carries data encoded with a Link Mode. A Link Mode
`is a set of at least one transmission parameter, such as a
`modulation level, a coding rate, a symbol rate, a transmis(cid:173)
`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
`(S/I) of each sub-carrier signal. The sub-carrier analysis
`processor 26 compares the S/I 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 S/1 of the corresponding sub-carrier
`signal is higher than the threshold. Sub-carriers for which
`35 the measured S/I 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
`S/1 of the S/Is of the acceptable sub-carriers. Referring to
`FIG. 2, an example set of S/ls is shown. A horizontal axis 40
`40 indicates a sub-band number of each of twelve sub-carriers.
`A vertical axis 42 indicates the S/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
`45 12) as being acceptable sub-carriers. The sub-carrier analy(cid:173)
`sis processor 26 calculates the average S/1 of the acceptable
`sub-carriers in FIG. 2 as having a value of 9.1 dB, whereas
`the average S/1 of all sub-carriers in FIG. 2 would be 6.8 dB.
`Returning to FIG. 1, the remote unit 16 transmits a return
`50 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 S/1 of acceptable
`sub-carriers and a sequence of numbers identifying the
`55 acceptable sub-carriers. If a value of "1" is used to identify
`acceptable sub-carriers and a value of "O" 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
`60 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/1 having a value of 9.1 dB and a bitmask having
`65 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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`US 6,721,569 Bl
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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(cid:173)
`carriers. The new signal only codes the data 38 over the 20
`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(cid:173)
`able sub-carrier can be allocated for one of several uses. 25
`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
`the unused unacceptable sub-carriers is assigned to other
`sub-carriers.
`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(cid:173)
`carriers, then at step 94 the next sub-carrier is selected and 55
`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 60
`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 65
`measuring the S/I of each sub-carrier signal could be used.
`Steps 90 and 96 calculate the average S/I, although any
`
`6
`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
`5 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
`10 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
`15 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 "O" since that is the value of the sixth bit,
`and the selected sub-carrier would therefore be an unaccept(cid:173)
`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
`30 130 the base station determines that the selected sub-carrier
`is an unacceptable sub-carrier, then at step 134 low sensi(cid:173)
`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
`35 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
`40 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
`45 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.
`50 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.
`
`IPR2018-1555
`HTC EX1005, Page 10
`
`

`

`US 6,721,569 Bl
`
`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 U.S. 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 S/I, as described
`above. Alternatively, if the remote unit determines the
`optimum Link Mode itself, then the value may be a refer(cid:173)
`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 S/I of acceptable sub-carriers. The extractor 33 20
`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 25
`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 30
`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 35
`each acceptable sub-carrier and one or more distinct values
`identifying unacceptable sub-carriers. If in the example of
`FIG. 2 a S/I of 10 dB or higher (for example) allows use of
`a seventh Link Mode within the set of allowed Link Modes,
`a S/I of 7 dB or higher allows use of a sixth Link Mode 40
`within the set of allowed Link Modes, and a S/I 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 45
`"777660000567" to the base station along the reverse link.
`A number having a value of "O" indicates that the corre(cid:173)
`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 50
`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
`in FIG. 3, except that the average S/I is not calculated
`( eliminating steps 90 and 96), a sub-carrier Link Mode is
`evaluated for each sub-carrier (adding steps 89 and 91), and
`the average S/I 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 S/I 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.
`
`8
`The method carried out by the base station in the embodi(cid:173)
`ment in which the Link Mode of each acceptable sub-carrier
`is transmitted to the base station i