(12) United States Patent
`Lee et al.
`
`(54) APPARATUS AND METHOD FOR SIGNAL
`CONSTITUTION FOR DOWNLINK OF
`OFDMA-BASED CELLULAR SYSTEM
`(75) Inventors: Sok-Kyu Lee, Daejeon (KR);
`Kwang-Soon Kim, Daejeon (KR);
`Kyung-Hi Chang, Daej eon (KR)
`
`(73) Assignee: Electronics and Telecommunications
`Research Institute, Daejeon (KR)
`
`* ) Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 266 days.
`
`(21) Appi. No.:
`
`10/539,166
`
`(22)
`
`PCT Filed:
`
`Jun. 2, 2003
`
`(86) PCT No.:
`
`PCT/KR03/01083
`
`§ 371 (c)(1),
`(2), (4) Date:
`
` Mar. 3, 2006
`
`(87) PCT Pub. No.: W02004/056022
`
`PCT Pub. Date: Jul. 1, 2004
`
`Prior Publication Data
`
`US 2006/0146867 Al
`
`Jul. 6, 2006
`
`Foreign Application Priority Data
`
`(65)
`
`(30)
`
`Dec. 13, 2002 (KR)
`
` 10-2002-0079598
`
`(51)
`
`Int.Cl.
`H04Q 7/00
`(52) U.S. Cl.
`
`(2006.01)
` 370/208; 370/252; 370/311;
`370/334; 370/465; 455/452.1; 455/461
`(58) Field of Classification Search
` 370/347,
`370/344; 455/562.1
`See application file for complete search history.
`
`1111111111111111111111111111111111111111111111111111111111111111111111111 II 0I II
`
`US007460466B2
`
`(10) Patent No.:
`(45) Date of Patent:
`
`US 7,460,466 B2
`Dec. 2, 2008
`
`(56)
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`6,496,535 132 * 12/2002 Xu
`
` 375/219
`
`(Continued)
`
`FOREIGN PATENT DOCUMENTS
`
`EP
`
`0 938 208 Al
`
`8/1999
`
`(Continued)
`
`OTHER PUBLICATIONS
`
`Magnus Sandell, et a!., "A comparative study of pilot-based channel
`estimators for wireless OFDM", Sep. 1996, Research Report, LuleA
`University, Sweden, 1402-1528.
`
`(Continued)
`
`Primary Examiner Chi H Pham
`Assistant Examiner Shick Hom
`(74) Attorney, Agent, or Firm Blakely, Sokoloff, Taylor &
`Zafman LLP
`
`(57)
`
`ABSTRACT
`
`Disclosed are an adaptive pilot symbol assignment method
`that flexibly controls the number of transmit antennas accord-
`ing to each user's moving speed, channel status, or user
`request, and assigns proper pilot symbols in the downlink of
`an OFDMA (Orthogonal Frequency Division Multiplexing
`Access) based cellular system; and a sub-carrier allocation
`method for high-speed mobile that allocates some sub-carri-
`ers to assign proper pilot symbols for ultrahigh-speed mobile
`users, and the rest of the sub-carriers to the other users to
`assign proper pilot symbols to the users, on the assumption
`that the ultrahigh-speed mobile users have a traffic volume
`almost insignificant to the whole traffic volume.
`
`10 Claims, 9 Drawing Sheets
`
`I
`
`S200
`
`Store t raff ic channel data
`-S21O
`I
`S220
`Determine transmission mode ^TCrhminer'n'Itr'tui;
`affc
`wd nuilber of additional
`el8ot
`antennas Moving speed
`Assign pilot symbols for
`additional antenna
`
`-S23O
`
`Traffic
`channel data
`
`Transmission
`mode
`
`Perform coding,
`interleaving, and
`symbol-mapping
`according to
`transmission mode
`S5O
`
`Assign additional pilot
`symbols for baslcl
`additional antennas
`Traffic channel symbol
`OUT
`
`Moving speed
` S240
`Additional pilot symbol
`
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`US 7,460,466 B2
`Page 2
`
`U.S. PATENT DOCUMENTS
`
`6,545,997 Bi * 4/2003 Bohnke et a!.
`6,836,484 132
`12/2004 Suzuki
`6,907,026 132 * 6/2005 Akiyama
`6,959,052 132 * 10/2005 Harada eta!.
`6,993,092 BI
`1/2006 Murakami eta!.
`2001/0004604 Al * 6/2001 Toshimitsu eta!.
`12/2001 Sawadaetal.
`2001/0055287 Al
`2001/0055296 Al * 12/2001 Akiyama
`
` 370/347
` 370/465
` 370/344
` 375/340
` 375/298
` 455/562
`
` 370/344
`
`FOREIGN PATENT DOCUMENTS
`
` 2001-1030114
`2001-238269
`WO 02/065685
`
`4/2001
`8/2001
`8/2002
`
`WO
`
`7/2004
`WO 2004/056022 A3
`OTHER PUBLICATIONS
`CheongYui Wong, eta!, "A Rea!-time Sub-carrier Allocation Scheme
`for Multiple Access Down!ink OFDM Transmissiom", 0-7803-5435-
`4/99 1999 IEEE VTC '99 pp. 1124-1128.
`Srihari Adireddy, eta!, "Detection with Embedded Known Symbols;
`Optima! Symbol Placement and Equalization"; 0-7803-6293-4 2000
`IEEE; pp. 2541-2544.
`F. C!assen, et a!., "Channel estimation units for an OFDM system
`suitable for mobile communication", in ITG Conference on Mobile
`Radio, Neu-Ulm, germany, Sep. 1995.
`P. Hoeher, et a!., "Pilot-symbol-aided channel estimation in time and
`frequency", Kluwer Academic Publishers, Multi-carrier Spread-
`Spectrum, 1997.
`Ma. J. Fernandez-Getino Garcia, et a!., "Efficient pilot patterns for
`channel estimation in OFDM systems over HF channels", Proc. IEEE
`VTC1999.
`
`* cited by examiner
`
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`

`

`U.S. Patent
`
`Dec. 2, 2008
`
`Sheet 1 of 9
`
`f
`
`III
`
`Fig 1
`
`1
`
`I
`
`NE
`
`MEN 0 M
`U.. U.-__U
`
`N1
`
`data symbol
`
`pilot symbol
`
`Fig. 2
`
`MEMEMENNEWEEN
`MEMEMEMMENNEW
`MEMEMEMMEMEME
`MOMMEMEMEMEME
`VEMEMEMMEMEMEN
`ONERNMENEEMEN
`WEENNEVEMEMEME
`MEMENWOMMEMEME
`MEMEMEMEMMEME
`data symbol
`
`NT
`
`pilot symbol
`
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`U.S. Patent
`
`Dec. 2, 2008
`
`Sheet 2 of 9
`
`US 7,460,466 B2
`
`f4
`
`/
`
`¶
`
`Fig. 3
`
`IA
`
`EMEMEMENIN
`
`nnurniir
`
`MEMEMEMEN
`'Vt
`J data symbol
`pilot symbol
`
`N1
`
`t
`
`7//,
`
`Common/
`control channel data
`
`Traffic channel data
`
`Fig. 4
`
`Assign symbols for common!
`controlchannels
`
`kloo
`
`Assign symbols for traffic channel
`Traffic channel
`Add i t i ona I
`pilot symbol
`symbol
`Constitute traffic channel signal
`
`V
`
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`

`U.S. Patent
`
`Dec. 2, 2008
`
`Sheet 3 of 9
`
`US 7,460,466 B2
`
`Fig. 5
`
`IN
`
`S200
`I)
`
`Store traffic channel data
`
`}S21 0
`
`Determine transmission mode
`and number of additional
`antennas
`
`4,
`
`S220
`cJ
`---Channel status
`..i—Traffjc requirement
` -.---Moving speed
`
`Assign pilot symbols for
`additional antenna
`
`S23O
`
`1
`
`Assign additional pilot
`symbols for basic/
`additional antennas
`
`----- Moving speed
`-S240
`
`Traffic
`channel data
`
`Transmission
`mode
`
`Perform coding,
`interleaving, and
`symbol-mapping
`according to
`transmission mode
`IN
`S250
`
`Traffic channel symbol
`
`OUT
`
`I
`
`Additional pilot symbol
`
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`

`U.S. Patent
`
`Dec. 2, 2008
`
`Sheet 4 of 9
`
`US 7,460,466 B2
`
`Fig. 6
`
`000000000000000
`000 00000000000
`0000000
`0 0 0 0 0 0 0
`00000 00000000000
`
`0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0
`
`A A A A A A A A A A A A A A
`A A A A A A A A A A A A A A A A
`A A A A AA A A
`A A
`A A
`A
`A AA A A A A A A A A
`A A
`A A A
`A A A A A A A A A A A A A A A A
`
`* * * *
`*
`* * * * *=:=** * * I * * *
`
`0000000000000
`00000000000000
`0 0 0 0 00 0 0 0 0 0
`0.0 0 0 0 0
`0000000
`000000 0000
`000
`
`Antenna 0
`Pilot
`
`Antenna 2
`Pilot
`
`V V
`
`Antenna I
`Pilot
`
`I'll'
`
`Antenna 3
`Pilot
`
`I A I
`
`User 1 Data
`High Speed
`Using 1
`Antenna
`
`User 2 Data
`Low Speed
`Using 2
`Antenna
`
`User 3 Data
`Low Speed
`Using 4
`Antenna
`
`User 4 Data
`High Speed
`Using 2
`Antenna
`
`Commom &
`Control
`channeles
`
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`

`U.S. Patent
`
`Dec. 2, 2008
`
`Sheet 5 of 9
`
`US 7,460,466 B2
`
`f
`
`Fig. 7
`
`********* *****
`*****
`
`********** ,2r****
`**
`**
`*
`********** *****
`********** *****
`****T***** *****
`
`********* r****
`
`pt
`
`Antenna 0
`Pilot
`
`Antenna 2
`Pilot
`
`User 3 Data
`Nd Low Speed
`Using 4
`Antenna
`
`11111
`
`Antenna 3
`Pilot
`
`Antenna 1
`Pilot
`
`Commom &
`Control
`channe I es
`
`** **
`
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`

`U.S. Patent
`
`Dec. 2, 2008
`
`Sheet 6 of 9
`
`US 7,460,466 B2
`
`f
`
`Fig. 8
`
`am-t
`
`00*000000000 GO 0
`*000*0000000
`0 0
`0 0 0 0 0 o
`0 0 0
`00000000 •
`0 0 0
`00000000 • 00*0
`0 0
`00000000 • 000*00
`00 00 0 0 0 0 • 0 0 0
`0
`0
`0
`000000 o\f.o 00
`0
`00
`00000000 • 0000
`0
`0
`00000000.0000
`0 0
`00000000 • 00000
`0
`00000000• 000
`0 0
`0
`00000 oo\4$.,o 0
`0 0
`0
`0
`* 0000000 • 00*
`0
`0
`0
`00000000 • 00
`0
`0
`00
`00000000 • 0
`0 0 0
`00000000 • 0
`0
`0 0
`0000*0 o\Jo *0*00
`000000000000000
`000000000000000
`
`0 0 0
`
`0 0
`
`Antenna 0
`Pilot
`
`Antenna 1
`Pilot
`
`Commom &
`Control
`channeles
`
`0
`
`User 4 Data
`High Speed
`Using 2
`Antenna
`
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`

`U.S. Patent
`
`Dec. 2, 2008
`
`Sheet 7 of 9
`
`US 7,460,466 B2
`
`Fig. 9
`
`0
`0
`0
`0
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`Pilot Using 1
`Antenna
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`Antenna
`
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`Pilot
`
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`
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`
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`
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`High Speed
`Using 2
`Antenna
`
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`
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`
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`
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`
`Coffiffiom &
`Con t r of
`channefes
`
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`

`U.S. Patent
`
`Dec. 2, 2008
`
`Sheet 8 of 9
`
`US 7,460,466 B2
`
`0q
`
`CI
`CD
`
`C00C
`
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`
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`

`U.S. Patent
`
`Dec. 2, 2008
`
`Sheet 9 of 9
`
`US 7,460,466 B2
`
`Fig 11
`
`IN
`
`Store traftic channel
`information
`
`'-S410
`
`channel
`Traffic
`status requirement
`
`Channel
`Traffic
`High speed status requirement
`
`Perform coding, interleaving,
`and symbol-mapping according
`to transmission mode,
`and allocate sub-carrier
`t
`Assign pilot symbol
`for low speed
`
`S420
`
`-S430
`
`S440-'
`
`"-8450 S460-
`
`Perform coding, interleaving,
`and symbol-napping according
`to transmission made, and
`allocate sub-carrier
`i$.
`Assign pilot symbol
`for high speed
`
`II,
`
`OUT
`
`Fig. 12
`
`Sub-Carrier
`
`Al
`
`FIR
`
`I
`
`I
`
`For High speed
`Mobile user
`
`For Middle/Low!
`Fixed speed
`Mobile user
`
`[1 data symbol
`
`I
`
`I
`
`I
`
`Symbol
`
`pilot symbol
`
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`

`US 7,460,466 B2
`
`1
`APPARATUS AND METHOD FOR SIGNAL
`CONSTITUTION FOR DOWNLINK OF
`OFDMA-BASED CELLULAR SYSTEM
`
`The present application is a non-provisional application of 5
`International Application No. PCT/KR2003/001083, filed
`Jun. 2, 2003.
`
`BACKGROUND OF THE INVENTION
`
`10
`
`2
`The symbol time T, during which the maximum pilot
`distance is proportional to the coherent time, is normalized by
`the number of symbols. So, the maximum pilot distance in the
`time domain is proportional to the coherent bandwidth and
`normalized by the sub-carrier bandwidth.
`The balanced design (P. Hoeher et al., "Pilot-symbol-aided
`channel estimation in time and frequency", Multi-carrier
`Spread-Spectrum, accepted for publication in Kluwer Aca-
`demic Publishers, 1997) defines that the estimation uncer-
`tainty in the time domain is equal to that in the frequency
`domain. Here, P. Hoeher et al. suggest a design guide having
`two-fold oversampling as defined by a heuristic formula as
`follows:
`
`2fDTsNfN½
`
`[Formula 3]
`
`(a) Field of the Invention
`The present invention relates to an apparatus and method
`for signal constitution for a downlink of an OFDMA (Or-
`thogonal Frequency Division Multiplexing Access) based
`cellular system. More specifically, the present invention 15
`relates to an apparatus and method for adaptive pilot symbol
`assignment and sub-carrier allocation that reduces transmis-
`sion power consumption and overhead caused by pilot sym-
`bols and increases the total data rate on the downlink of an
`OFDMA-based cellular system.
`(b) Description of the Related Art
`In the design of pilot assignment, it is necessary to use a
`sufficiently large number of pilot symbols for the sake of
`preventing a deterioration of reception performance caused
`by a channel variation, and to prevent an excessive increase of
`a power loss or a bandwidth loss caused by pilot symbols
`above an expected value. The positioning (assignment) of
`pilot symbols is of a great significance to the receiver of an
`OFDMA-based system, which estimates a transfer function
`value of channels in a two-dimensional (time, frequency)
`space. Hence, both the time domain and the frequency
`domain must be taken into consideration in pilot symbol
`assignment so as to transmit the pilot symbols. In case of
`using a plurality of antennas, the pilot symbols of the multiple
`antennas are assigned in consideration of both the time
`domain and the frequency domain.
`The distance between pilot symbols must be quite small in
`designing pilot symbols in the worst environment, or when
`using non-optimal channel estimation filters having a lower
`complexity.
`Let f be a sub-carrier bandwidth, then the maximum pilot
`distance NF in the frequency domain based on the conven-
`tional sampling theory (F. Classen, M. Speth, and H. Meyr,
`"Channel estimation units for an OFDM system suitable for
`mobile communication", in ITG Conference on Mobile
`Radio, Neu-Ulm, Germany, September 1995) is determined
`by the following formula:
`[Formula 1]
`
`25
`
`20
`
`where NF is the pilot distance in the frequency domain. The
`above-mentioned pilot symbol assignment is primarily a rect-
`angular pilot symbol assignment, which is illustrated in FIG.
`1. FIGS. 2 and 3 show a straight pilot symbol assignment and
`a hexagonal pilot symbol assignment, respectively. Gener-
`ally, the hexagonal pilot symbol assignment allows more
`efficient sampling, compared with two-dimensional signals,
`and exhibits excellent performance relative to other assign-
`ments. An example of the pilot symbol assignment is dis-
`closed in "Efficient pilot patterns for channel estimation in
`OFDM systems over HE channels" (M. J. Fernandez-Getino
`Garcia et al., in Proc IEEE VTC1 999).
`As the pilot symbol assignment becomes denser, the chan-
`30 nel estimation performance becomes more excellent but the
`data rate is decreased. Hence, a trade-off lies between the data
`rate and the channel estimation performance (i.e., pilot sym-
`bol distance).
`There exits a pilot symbol distance that optimizes the trade-
`35 off between the improved channel estimation and the signal-
`to-noise ratio (SNR) reduced by data symbols. By varying the
`pilot symbol distances NF and NT, the values approximate to
`the optimum with reference to the performance of bit error
`rate (BER) can be determined. In FIG. 1, for example, NF=4
`40 and NT=3 in optimum means that one twelfth (about 8%) of
`the consumed transmission power and bandwidth are used for
`pilot symbols.
`In this optimal assignment of pilot symbols, the channel
`environment and the moving speed of the mobile users are of
`45 a great importance as parameters to be considered.
`
`NE :^
`
`where
`is the maximum exceedance delay time of a chan-
`nel. The maximum pilot distance N, inthe frequency domain
`is determined by the following formula:
`[Formula 2]
`
`NT :^
`
`2fDT,
`
`where ID is the maximum Doppler frequency; and T is the
`symbol time.
`
`50
`
`55
`
`60
`
`65
`
`SUMMARY OF THE INVENTION
`
`It is an advantage of the present invention to provide an
`apparatus and method for adaptive pilot symbol assignment
`and sub-carrier allocation that reduces transmission power
`and overhead caused by pilot symbols and increases the total
`data rate on a downlink in an OFDMA-based cellular system.
`In one aspect of the present invention, there is provided a
`downlink signal constitution method, which is for a downlink
`of a cellular system using an orthogonal frequency division
`multiplexing access method, the downlink signal constitution
`method including: (a) coding, interleaving, and symbol-map-
`ping data of a common channel and a control channel, and
`assigning fundamental pilot symbols, necessary for a
`demodulation of the common channel and the control chan-
`nel, to time, frequency, and antenna; (b) receiving data to be
`transmitted through a traffic channel of each user, and deter-
`mining a transmission mode of each user according to the
`user's moving speed, channel information, and traffic
`requirement; (c) determining additional pilot symbols, addi-
`tionally necessary for a demodulation of the traffic channel,
`
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`US 7,460,466 B2
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`3
`according to the transmission mode and moving speed by
`users; and (d) coding, interleaving and symbol-mapping the
`data of the traffic channel according to the transmission mode
`by users, and assigning the mapped symbols and the addi-
`tional pilot symbols according to time, frequency and
`antenna.
`In another aspect of the present invention, there is provided
`a downlink signal constitution method, which is for a cellular
`system using an orthogonal frequency division multiplexing
`access method, the downlink signal constitution method
`including: (a) dividing users into a first user group including
`high-speed mobile users and a second user group including
`the rest of the users, in consideration of each user's moving
`speed and traffic volume; (b) allocating a first sub-carrier
`band for the first user group, and a second sub-carrier band for
`the second user group; and (c) assigning pilot symbols to the
`first and second sub-carrier bands, the pilot symbols assigned
`to the first sub-carrier band being different in assignment
`density from the pilot symbols assigned to the second sub-
`carrier.
`In a further aspect of the present invention, there is pro-
`vided a downlink signal constitution apparatus, which is for a
`cellular system using an orthogonal frequency division mul-
`tiplexing access method, the downlink signal constitution
`apparatus including: a first memory for storing traffic channel
`information of each user; a second memory for storing chan-
`nel information, traffic requirement, and moving speed infor-
`mation of each user; a transmission user and transmission
`mode determiner for determining a transmission user and a
`transmission mode according to a defined method using the
`information stored in the second memory; a traffic channel
`processor for reading the traffic channel information stored in
`the first memory according to the transmission mode deter-
`mined by the transmission user and transmission mode deter-
`miner, and performing coding, interleaving, and symbol-
`mapping of the traffic channel; an additional pilot symbol
`generator for generating additional pilot symbols necessary
`for a demodulation of the traffic channel, using the transmis-
`sion mode determined by the transmission user and transmis-
`sion mode determiner and the moving speed information
`stored in the second memory; and a time/sub-carrier/antenna
`mapper for multiplying the traffic channel symbols output
`from the traffic channel processor and the additional pilot
`symbols output from the additional pilot symbol generator by
`a channel gain by channels/users, and mapping the resulting
`symbols to time, sub-carrier, and antenna by a defined
`method.
`In a still further aspect of the present invention, there is
`provided a recording medium with a built-in program, which
`implements a downlink signal constitution method for a cel-
`lular system using an orthogonal frequency division multi-
`plexing access method, the program including: a function of
`coding, interleaving, and symbol-mapping data of a common
`channel and a control channel, and assigning fundamental
`pilot symbols, necessary for a demodulation of the common
`channel and the control channel, to time, frequency, and
`antenna; a function of receiving data to be transmitted
`through a traffic channel of each user, and determining a
`transmission mode of each user according to the user's mov-
`ing speed, channel information, and traffic requirement; a
`function of determining additional pilot symbols, addition-
`ally necessary for a demodulation of the traffic channel,
`according to the transmission mode and moving speed by
`users; and a function of coding, interleaving and symbol-
`mapping the data of the traffic channel according to the trans-
`
`4
`mission mode by users, and assigning the mapped symbols
`and the additional pilot symbols according to time, frequency,
`and antenna.
`In a still further aspect of the present invention, there is
`5 provided a recording medium with a built-in program, which
`implements a downlink signal constitution method for a cel-
`lular system using an orthogonal frequency division multi-
`plexing access method, the program including: a function of
`dividing users into a first user group including high-speed
`10 mobile users and a second user group including the rest of the
`users, in consideration of each user's moving speed and traffic
`volume; a function of allocating a first sub-carrier band for the
`first user group, and a second sub-carrier band for the second
`user group; and a function of assigning pilot symbols to the
`15 first and second sub-carrier bands, the pilot symbols assigned
`to the first sub-carrier band being different in assignment
`density from the pilot symbols assigned to the second sub-
`carrier.
`
`20
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`30
`
`The accompanying drawings, which are incorporated in
`and constitute a part of the specification, illustrate an embodi-
`ment of the invention, and, together with the description,
`25 serve to explain the principles of the invention:
`FIG. 1 is an exemplary diagram of a rectangular pilot
`symbol assignment;
`FIG. 2 is an exemplary diagram of a straight pilot symbol
`assignment;
`FIG. 3 is an exemplary diagram of a hexagonal pilot sym-
`bol assignment;
`FIG. 4 is a flow chart showing a symbol assignment method
`for a downlink of an OFDMA-based cellular system accord-
`ing to an embodiment of the present invention;
`FIG. S is a detailed diagram showing a symbol assignment
`method for the traffic channel of FIG. 4;
`FIG. 6 is a diagram showing a downlink signal constitution
`method according to the embodiment of the present inven-
`tion;
`FIG. 7 is an exemplary diagram of a pilot symbol assign-
`ment for low-speed mobile users using four antennas;
`FIG. 8 is an exemplary diagram of a pilot symbol assign-
`ment for high-speed mobile users using two antennas;
`FIG. 9 is an exemplary diagram showing a downlink signal
`constitution method when using additional antennas only in a
`part of the whole band in an FDD system;
`FIG. 10 is a diagram of a downlink signal constitution
`apparatus for an OFDMA-based cellular system according to
`50 the embodiment of the present invention;
`FIG. 11 is a detailed flow chart showing a pilot symbol
`assignment according to sub-carrier allocation; and
`FIG. 12 is an exemplary diagram showing a pilot symbol
`assignment according to a sub-carrier allocation for high-
`55 speed mobile users and a moving speed.
`
`40
`
`DETAILED DESCRIPTION OF THE PREFERRED
`EMBODIMENTS
`
`60
`
`In the following detailed description, only the preferred
`embodiment of the invention has been shown and described,
`simply by way of illustration of the best mode contemplated
`by the inventor(s) of carrying out the invention. As will be
`realized, the invention is capable of modification in various
`65 obvious respects, all without departing from the invention.
`Accordingly, the drawings and description are to be regarded
`as illustrative in nature, and not restrictive.
`
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`US 7,460,466 B2
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`5
`FIG. 4 is a diagram showing a downlink symbol assign-
`ment method for an OFDMA-based cellular system accord-
`ing to an embodiment of the present invention.
`The symbol assignment method according to the embodi-
`ment of the present invention comprises, as shown in FIG. 4,
`a symbol assignment step S100 for common/control chan-
`nels, a symbol assignment step S200 for traffic channels, and
`a traffic channel signal constitution step S300.
`More specifically, the symbol assignment step S100 for
`common/control channels performs coding, interleaving, and
`symbol mapping on data of common and control channels,
`and assigns the mapped symbols to time, frequency, and
`antennas. Also, fundamental pilot symbols necessary for
`demodulation of the common and control channels are
`assigned to time, frequency, and antennas.
`The symbol assignment step S200 for traffic channels
`receives data to be transferred through the traffic channel of
`each user; determines each user's transmission mode accord-
`ing to the user's moving speed, channel information, and
`traffic requirement; performs coding, interleaving, and sym-
`bol-mapping according to the transmission mode of the user;
`and assigns the traffic channel symbols of each user to time,
`frequency, and antennas. Also, pilot symbols additionally
`necessary for a demodulation of the traffic channel are gen-
`erated according to the transmission mode by users, and
`assigned to time, frequency, and antennas.
`The traffic channel signal constitution step S300 consti-
`tutes the signal of the traffic channel using the traffic channel
`symbols of each user and the additional pilot symbols output
`from the step S200.
`FIG. 5 is a detailed diagram ofthe symbol assignment S200
`for traffic channels shown in FIG. 4.
`When the base station has information about the moving
`speed and channel status of each user, a required number of
`pilot symbols are inserted, reducing transmission power and
`overhead caused by pilot symbols.
`According to the embodiment of the present invention, the
`transmitter antennas are divided into basic antennas and addi-
`tional antennas. The basic antenna refers to an antenna used
`for transmitting common and control channels, while the
`additional antenna refers to an antenna additionally used to
`enhance the transmission rate or performance of the traffic
`channel of the user.
`In the OFDMA system, one frequency band is divided into
`a plurality of sub-carrier bands to transmit the traffic channel
`of each user through the allocated sub-carriers. Namely, the
`OFDMA system properly allocates a sub-carrier band
`according to the user's moving speed, channel environment,
`and traffic requirement, or selects a defined sub-carrier band,
`determines the number of transmitter antennas according to
`the user's moving speed, channel environment, and traffic
`requirement, and then assigns additionally necessary pilot
`symbols to the allocated sub-carrier band.
`More specifically, as illustrated in FIG. 5, the OFDMA
`system stores data to be transmitted through a traffic channel,
`in step S210.
`The transmission mode and the number of additional
`antennas are determined in consideration of the user's chan-
`nel information (i e , channel status), traffic requirement, and
`moving speed, in step S220.
`In step S230, the system assigns pilot symbols for addi-
`tional antennas, when the additional antennas are needed
`according to the transmission mode determined in the step
`S220.
`
`2 5
`
`6
`The additional pilot symbols according to the moving
`speed of the basic antennas and the additional antennas are
`then assigned in consideration of the user's moving speed, in
`step S240.
`5 The system performs coding, interleaving, and symbol
`mapping using the transmission mode determined in the step
`S220 and the traffic channel data stored in the step S210 to
`generate coded, interleaved, and symbol-mapped traffic
`channel symbols, in step S250.
`10 In the step S220, the transmission mode for each user is
`determined independently, or the transmission mode for mul-
`tiple users is determined by optimization in consideration of
`the total transmission rate, the quality of service, or the total
`transmission power.
`15 FIG. 6 is an exemplary diagram showing a downlink signal
`constitution method according to the embodiment of the
`present invention.
`In FIG. 6, when using one basic antenna and at most three
`additional antennas, the pilot symbols are assigned to the
`20 sub-carrier band, which is allocated to a user 1 moving at high
`speed with one basic antenna, a user 2 moving at low speed
`with one additional antenna, a user 3 moving at low speed
`with three additional antennas, and a user 4 moving at high
`speed with one additional antenna.
`In FIG. 6, seventeen OFDM symbols constitute one slot.
`FIG. 6 shows the case where a demodulation can be enabled
`with one pilot symbol in one slot in the time domain because
`the moving speed is low.
`30 Referring to FIG. 6, the common and control channels are
`used to transmit OFDM symbols such as pilot symbols of the
`basic antenna, and demodulate them irrespective of the mov-
`ing speed of the users. The traffic channel is used to transmit
`the additional pilot symbols necessary according to the mov-
`ing speed of the users and the number of antennas in the
`allocated sub-carrier band by users.
`FIG. 7 is an exemplary diagram showing a pilot symbol
`assignment in the sub-carrier band allocated to a low-speed
`mobile user using one basic antenna and three additional
`40 antennas according to the embodiment of the present inven-
`tion.
`The pilot symbols (N, 5) of the basic antenna (antenna 0)
`and the common and control channels are transmitted for the
`first OFDMA symbol, and the traffic channel is transmitted
`for the other OFDMA symbols. The pilot symbols of the
`additional antennas (antenna 1, antenna 2, antenna 3) are
`additionally transmitted. In the meantime, the symbols of the
`traffic channel can be generated by any one of the following
`methods: (1) a first method of generating traffic channel sym-
`50 bols previously in consideration of the number of additional
`pilots; (2) a second method of generating the maximum num-
`ber of traffic channel symbols and then puncturing at posi-
`tions to transmit additional pilot symbols; and (3) a third
`method of generating traffic channel symbols previously in
`consideration of the number of a part of additional pilot
`symbols, and then puncturing at positions to transmit the rest
`of the additional pilot symbols.
`FIG. 8 is an exemplary diagram showing a pilot symbol
`assignment in the sub-carrier band allocated to a high-speed
`60 mobile user using one basic antenna and one additional
`antenna according to the embodiment of the present inven-
`tion.
`The pilot symbols (N, 5) of the basic antenna (antenna 0)
`and the common and control channels are transmitted for the
`65 first OFDMA symbol, and the traffic channel is transmitted
`for the other OFDMA symbols. The pilot symbols of the
`additional antenna (antenna 1) are additionally transmitted.
`
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`US 7,460,466 B2
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`7
`In the meantime, the symbols of the traffic channel can be
`generated by one of the following methods: (1) a first method
`of generating traffic channel symbols previously in consider-
`ation of the number of additional pilots; (2) a second method
`of generating the maximum number of traffic channel sym-
`bols and then puncturing at positions to transmit additional
`pilot symbols; and (3) a third method of generating traffic
`channel symbols previously in consideration of the number of
`a part of additional pilot symbols, and then puncturing at
`positions to transmit the rest of the additional pilot symbols.
`In summary, there are four cases of pilot symbol assign-
`ment in relation to the number of antennas of the traffic
`channel
`(1) moving at a low speed with one basic antenna using
`no additional pilot symbol;
`(2) moving at low speed with additional antennas assign-
`ing pilot symbols for additional antennas;
`(3) moving at high speed with one basic antenna addi-
`tionally inserting pilot symbols for basic antenna in confor-
`mity to the high-speed environment; and
`(4) moving at high speed with additional antennas addi-
`tionally inserting pilot symbols for basic and additional
`antennas in consideration of the moving speed.
`To use the methods illustrated in FIGS. 4 to 8, the base
`station must have information about the channel information,
`moving speed, and traffic requirement of each user. The mov-
`ing speed is measured at the base station, or is measured at the
`mobile station and then reported to the base station. The
`traffic requirement is reported to the base station by the
`mobile station, or is detected by the base station from the
`amount or characteristic of data to be transmitted. The chan-
`nel information is measured at the base station, or is measured
`at the mobile station and then reported to the base station. The
`former case is primarily for the TDD (Time Division Duplex)
`based system, and the latter one is for the FDD (Frequency
`Division Duplex) based system.
`In the former case, the mobile station sends a signal (e.g.,
`preamble, pilot, etc.) for channel measurement, and then the
`base station measures the channel information of the uplink
`