USOO8462611B2
`
`(12) United States Patent
`Ma et al.
`
`(10) Patent No.:
`(45) Date of Patent:
`
`US 8.462,611 B2
`Jun. 11, 2013
`
`(54) PILOT DESIGN FOR OFDM SYSTEMS WITH
`FOUR TRANSMITANTENNAS
`
`(75) Inventors: Jianglei Ma, Kanata (CA); Ming Jia,
`Ottawa (CA); Wen Tong, Ottawa (CA);
`Peiying Zhu, Kanata (CA); Claude
`Royer, Gatineau (CA)
`(73) Assignee: Apple Inc., Cupertino, CA (US)
`
`(*) Notice:
`
`(21) Appl. No.:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 1888 days.
`10/593,053
`
`Mar. 15, 2005
`PCT/CA2005/000387
`
`(22) PCT Filed:
`(86). PCT No.:
`S371 (c)(1),
`Sep. 15, 2006
`(2), (4) Date:
`(87) PCT Pub. No.: WO2005/088882
`PCT Pub. Date: Sep. 22, 2005
`
`(65)
`
`Prior Publication Data
`US 2008/O253.279 A1
`Oct. 16, 2008
`
`Related U.S. Application Data
`(60) Provisional application No. 60/553,161, filed on Mar.
`15, 2004, provisional application No. 60/558,566,
`filed on Apr. 2, 2004, provisional application No.
`60/566,009, filed on Apr. 28, 2004.
`(51) Int. Cl.
`H04 II/00
`(52) U.S. Cl.
`USPC ........... 370/208: 370/328; 370/334; 370/437;
`375/260; 375/267; 455/562.1; 455/101
`
`(2006.01)
`
`
`
`(58) Field of Classification Search
`USPC .................. 370/328,334, 208, 437; 375/260,
`375/267; 455/101,562.1
`See application file for complete search history.
`
`(56)
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`5,867.478 A
`2f1999 Baum et al.
`6,298,035 B1
`10/2001 Heiskala
`6,359,938 B1
`3/2002 Keevillet al.
`6,473,393 B1
`10/2002 Ariyavisitakul et al.
`6,473.467 B1
`10/2002 Wallace et al.
`6,654.429 B1 1 1/2003 Li
`(Continued)
`FOREIGN PATENT DOCUMENTS
`03/034642
`4/2003
`03/034644
`4/2003
`2004/077730
`9, 2004
`
`WO
`WO
`WO
`
`OTHER PUBLICATIONS
`
`Fernández-Getino Garcia, Ma Julia et al; Efficient Pilot Patterns for
`Channel Estimation in OFDM Systems Over HF Channels; IEEE
`Sep. 19, 1999: pp. 2193-2 197; XP-000896002.
`(Continued)
`Primary Examiner — Christopher Grey
`(74) Attorney, Agent, or Firm — Meyertons, Hood, Kivlin,
`Kowert & Goetzel, P.C.
`
`ABSTRACT
`(57)
`Pilot, preamble and midamble patterns are provided that are
`particularly suited for four transmit antenna OFDM systems.
`Pilots are inserted in a scattered manner for each of the four
`antennas, either uncoded, space-time coded in pairs, space
`time frequency coded in pairs, or space-time-frequency
`coded.
`
`30 Claims, 28 Drawing Sheets
`
`LAIR
`
`OAR
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`AR
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`DAR
`26
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`US 8,462,611 B2
`Page 2
`
`U.S. PATENT DOCUMENTS
`7,248,559 B2
`7, 2007 Ma et al.
`7.460,466 B2 * 12/2008 Lee et al. ...................... 370,208
`7,545,734 B2
`6, 2009 Ma et al.
`7,848,438 B2 * 12/2010 Baum et al. ................... 375,260
`8,111,763 B2 * 2/2012 Ma et al. ....................... 375,260
`1/2002 Wang et al.
`2002fOOO3774 A1
`3/2002 Wang et al.
`2002.0034213 A1
`6/2002 Jeong et al.
`2002fO080887 A1
`2002/O122383 A1
`9, 2002 Wu et al.
`2002fO144294 A1
`10, 2002 Rabinowitz et al.
`12/2002 Mody et al.
`2002fO181390 A1
`2003, OO16621 A1
`1, 2003 Li
`2003/0072254 A1* 4/2003 Ma et al. ....................... 370,208
`2003/0072255 A1
`4/2003 Ma et al. ....................... 370,208
`2003/OO72395 A1
`4/2003 Jia et al.
`2004/0001429 A1
`1/2004 Ma et al.
`3/2007 Tong et al.
`2007/0053282 A1
`2009, OO67534 A1*
`3/2009 Kwak et al. ................... 375,267
`2009/02383O3 A1*
`9/2009 Mondal et al. ................ 375,295
`
`
`
`OTHER PUBLICATIONS
`Jones, V.K.; Raleigh, Gregory G.; Channel Estimation for Wireless
`OFDM Systems: IEEE Nov. 8, 1998; pp.980-985; XP-000825895.
`
`Specification of U.S. Appl. No. 12/468,628, filed May 12, 2009.
`Mincai, Qiu; Wenyi, Guo: The Theories of W-CDMA and cdma2000
`in the Third Mobile Communications Systemand the Draft for Imple
`mentation (II); Modern Science & Technology of Telecommunica
`tions, No. 11, pp. 24-26, Nov. 2000.
`EN 300 744 V1.1.2. Digital Video Broadcasting (DVB); Framing
`structure, channel coding and modulation for digital terrestrial tele
`vision, Aug. 1997.
`Tong, et al.: “Enhancing MIMO features for OFDMA PHY layer”
`IEEE 802.16d-04/65, Apr. 15, 2004.
`Qinghua, et al: “Corrected pilot allocation for 4 BS transmit anten
`nas' IEEE C802.16e-04/5311r2, Nov. 12, 2004.
`Siew, et al.: “A Channel Estimation Method for MIMO-OFDM Sys
`tems' Jul. 25, 2002, University of Bristol, UK.
`International Search Report dated Jul. 5, 2005 from International
`Patent Application No. PCT/CA2005/000387.
`“Extension of collaborative spatial multiplexing in OFDMA;
`C80216e-04/286r2, IEEE Draft; C802.16E-04/286R2, IEEE-SA,
`Piscataway, NJ. USA. vol. 802.16e, Aug. 29, 2004, pp. 1-6,
`XPO 17624418, retrieved on Sep. 23, 2004).
`Communication in EP Application No. 05714626.8-2415, issued
`Nov. 14, 2012, pp. 1-6.
`* cited by examiner
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`U.S. Patent
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`US 8.462,611 B2
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`F.
`OATR
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`OAOR
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`AR
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`ODATOR
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`US 8.462,611 B2
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`OFDM
`MODULATOR
`20
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`OFDM
`MODULATOR
`22
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`OFDM
`MODULATOR
`24
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`OFDM
`MODULATOR
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`US 8.462,611 B2
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`OFDM
`MODULATOR
`20
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`OFDM
`MODULATOR
`22
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`OFDM
`MODULATOR
`24
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`OFDM
`MODULATOR
`26
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`FIG. 3
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`US 8.462,611 B2
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`US 8.462,611 B2
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`US 8.462,611 B2
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`US 8.462,611 B2
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`

`1.
`PILOT DESIGN FOR OFDM SYSTEMIS WITH
`FOUR TRANSMITANTENNAS
`
`US 8,462,611 B2
`
`This application claims the benefit of and is a National
`Phase Entry of International Application Number PCT/
`CA2005/000387 filed Mar. 15, 2005, which claims the ben
`efit of U.S. provisional applications 60/553,161, filed Mar.
`15, 2004, 60/558,566, filed Apr. 2, 2004, and 60/566,009,
`filed Apr. 28, 2004, all of which are incorporated herein by
`reference.
`
`10
`
`FIELD OF THE INVENTION
`
`The invention relates to pilot designs for OFDM (orthogo
`nal frequency division multiplexing) systems with four trans
`mit antennas.
`
`15
`
`BACKGROUND OF THE INVENTION
`
`30
`
`35
`
`40
`
`New applications of mobile communications demand
`high-speed and high-quality, bandwidth-efficient wireless
`access solutions. The application of MIMO (multiple anten
`nas both in the transmitter and in the receiver) has been
`demonstrated to drastically improve channel capacity com
`pared to single-antenna systems. On the other hand, OFDM
`25
`has demonstrated its high spectral efficiency and ability to
`deal with frequency selective fading and narrow band inter
`ference. Therefore the combination of OFDM with spectrally
`efficient multiple antenna techniques opens the door to high
`data-rate wireless communication.
`Compared with the single input single output (SISO) sys
`tems, two kinds of gains are provided by the MIMO wireless
`systems, namely diversity gain and multiplexing gain. With
`diversity gain more reliable reception can be realized. With
`multiplexing gain the capacity of MIMO systems increases
`linearly with the number of transmit and receive antennas.
`This is due to the fact that a rich scattering environment can
`provide multiple data pipes within the same frequency band
`by using techniques such as space-time coding and space
`time layering. Since the capacity can be potentially increased
`by the application of multiple antennas, the use of up to four
`antennas at the transmitter and/or receiver has been consid
`ered to achieve an increased data rate for a given link perfor
`mance criterion, or to improve link performance for a given
`data rate.
`45
`For wireless propagation environments, the inherent tem
`poral and spatial variations of wireless channels impose more
`challenges on the design of a reliable communication system.
`For noise and interference limited systems, coherent demodu
`lation can achieve 2.5-3 dB SNR gain compared to the dif
`ferential demodulation. When coherent detection is per
`formed in a receiver, reliable channel estimation is very
`important to the system performance. Channel estimation in
`MIMO systems is more complicated because multiple chan
`nels should be obtained individually. As the number of trans
`mit antennas increases, the sensitivity to any channel estima
`tion error becomes more pronounced.
`OFDM modulation has been adopted by several standards,
`such as DVT-T, IEEE802.11a/g and IEEE802.16a/d. Differ
`ent training schemes have been employed in these standards,
`including preamble, fixed-location pilot and variable-loca
`tion pilot. However MIMO is not mandatory and is only
`adopted by IEEE802.16a as optional, and only two transmit
`antennas on the base station side and one receive antennas on
`the SS (subscriber station) side are employed. Since
`IEEE802.16a is designed for fixed and portable applications,
`the channel varies slowly. For the Wireless MAN (metropoli
`
`50
`
`55
`
`60
`
`65
`
`2
`tan area network) OFDM air-interface, the channel estima
`tion is obtained from the preambles. For the Wireless MAN
`OFDMA air-interface, although variable location pilot sym
`bols are introduced, they are only used to update the channel
`slowly.
`
`SUMMARY OF THE INVENTION
`
`According to one broad aspect, the invention provides a
`method of transmitting over four transmit antennas compris
`ing: for each antenna, generating a respective sequence of
`OFDM symbols, each OFDM symbol having a plurality of
`Sub-carriers carrying at data or pilots, and transmitting the
`sequence of OFDM symbols; wherein pilots are inserted for
`the four antennas collectively in blocks of two sub-carriers by
`two OFDM symbols scattered in time and frequency.
`In some embodiments, pilots are inserted for the four
`antennas collectively in blocks of two sub-carriers by two
`OFDM symbols scattered in time and frequency by: inserting
`such blocks of two sub-carriers by two OFDM symbols scat
`tered in a first regularly spaced pattern in even pairs of OFDM
`symbols; inserting such blocks of two sub-carriers by two
`OFDM symbols scattered in a second regularly spaced pat
`tern offset from said first regularly spaced pattern in odd pairs
`of OFDM symbols.
`In some embodiments, the first regularly spaced pattern
`comprises a repeating pattern of two pilot Sub-carriers, ten
`data Sub-carriers and the second regularly spaced pattern
`comprises six data Sub-carriers followed by a repeating pat
`tern of two pilot sub-carriers and ten data Sub-carriers.
`In some embodiments, each block of two sub-carriers by
`two OFDM symbols comprises a single pilot for each of the
`four antennas in a respective position within the block.
`In some embodiments, the single pilot for each of the four
`antennas takes the same position in every block of two Sub
`carriers by two OFDM symbols.
`In some embodiments, each block of two sub-carriers by
`two OFDM symbols is divided into pilot pairs, each pilot pair
`being transmitted by a respective pair of antennas.
`In some embodiments, each pilot pairs is arranged sequen
`tially in time.
`In some embodiments, each pilot pair is arranged sequen
`tially in frequency.
`In some embodiments, pilots are inserted for the four
`antennas collectively in blocks of two sub-carriers by two
`OFDM symbols scattered in time and frequency in a repeat
`ing pattern of six OFDM symbols comprising each compris
`ing a first, second and third pair of OFDM symbols, the
`method comprising: inserting Such blocks of two Sub-carriers
`by two OFDM symbols scattered in a first regularly spaced
`pattern in each first pair of OFDM symbols; inserting such
`blocks of two sub-carriers by two OFDM symbols scattered
`in a second regularly spaced pattern offset from said first
`regularly spaced pattern in each second pair of OFDM sym
`bols; and inserting such blocks of two sub-carriers by two
`OFDM symbols scattered in a third regularly spaced pattern
`offset from said second regularly spaced pattern in each third
`pair of OFDM symbols.
`In some embodiments, pilots are inserted for the four
`antennas collectively in blocks of two sub-carriers by two
`OFDM symbols scattered in time and frequency in a repeat
`ing pattern of OFDM symbols that is a multiple of two OFDM
`symbols in length.
`According to another broad aspect, the invention provides
`a method of transmitting over four transmit antennas com
`prising: for antenna, generating a respective sequence of
`OFDM symbols, each OFDM symbol having a plurality of
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`
`10
`
`15
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`25
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`30
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`3
`Sub-carriers carrying at data or pilots, and transmitting the
`sequence of OFDM symbols; wherein for a first pair of the
`four antennas, pairs of pilots are inserted scattered in time and
`frequency; wherein for a second pair of the four antennas,
`pairs of pilots are inserted scattered in time and frequency in
`locations different from pilots for the first pair of antennas.
`In some embodiments, for each pair of two pilots, the two
`pilots are not consecutive in time or frequency.
`In some embodiments, for each pair of two pilots, the two
`pilots are arranged consecutively in time.
`In some embodiments, pilots are inserted in a repeating
`pattern of six OFDM symbols comprising each comprising a
`first, second and third pair of OFDM symbols, wherein each
`pair of pilots is arranged sequentially in time: inserting pilot
`pairs for the first pair of antennas Scattered in a first regularly
`spaced pattern in each first pair of OFDM symbols; inserting
`pilot pairs for the first pair of antennas Scattered in a second
`regularly spaced pattern offset from said first regularly spaced
`pattern in each second pair of OFDM symbols; inserting pilot
`pairs for the first pair of antennas Scattered in a third regularly
`spaced pattern offset from said second regularly spaced pat
`tern in each third pair of OFDM symbols; inserting pilot pairs
`for the second pair of antennas Scattered in a fourth regularly
`spaced pattern in each first pair of OFDM symbols offset from
`said first pattern; inserting pilot pairs for the second pair of
`antennas Scattered in a fifth regularly spaced pattern offset
`from said fourth regularly spaced pattern and said second
`regularly spaced pattern in each second pair of OFDM sym
`bols; inserting pilot pairs for the second pair of antennas
`scattered in a sixth regularly spaced pattern offset from said
`fifth regularly spaced pattern and said third regularly spaced
`pattern in each third pair of OFDM symbols.
`According to another broad aspect, the invention provides
`a method of transmitting over four transmit antennas com
`prising: for each antenna, generating a respective sequence of
`35
`OFDM symbols, each OFDM symbol having a plurality of
`Sub-carriers carrying at data or pilots, and transmitting the
`sequence of OFDM symbols; wherein pilots are arranged in
`groups of four consecutive pilots in time, each group of four
`consecutive pilots containing pilots for the four antennas.
`In some embodiments. Such groups of four consecutive
`pilots are inserted in each set of four consecutive OFDM
`symbols, and in each of a plurality of spaced Sub-carriers.
`In some embodiments, each group of four consecutive
`pilots comprises a pair of space time coded pilots for a first
`pair of antennas of the four antennas, and a pair of space time
`coded pilots for a second pair of antennas of the four antennas.
`In some embodiments, each group of four consecutive
`pilots comprises a single pilot for each of the four antennas.
`In some embodiments, the location of the single pilot for
`each antenna varies across groups of four consecutive pilots.
`In some embodiments, the method further comprises:
`using different pilot patterns for respective four antenna
`transmitters to reduce interference between pilots of different
`four antenna transmitters.
`In some embodiments, the method further comprises:
`transmitting pilots with a power higher than average power.
`In some embodiments, data and pilots are transmitted using
`QPSK, with the pilots being transmitted with a relative power
`boost.
`60
`In some embodiments, data is transmitted using a QAM
`constellation, and pilots are transmitted using QPSK with
`signal constellation points at corners of the QAM constella
`tion.
`In some embodiments, the method further comprises trans
`mitting at least one fixed pilot for each of at least one of the
`four antennas.
`
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`4
`In Some embodiments, the method further comprises trans
`mitting at least one fixed pilot for each of two pairs of anten
`nas within said four antennas.
`In Some embodiments, the method further comprises trans
`mitting at least one fixed signalling channel for each of two
`pairs of antennas within said four antennas.
`In some embodiments, the method further comprises:
`transmitting relatively reliable signalling channel informa
`tion proximal in time and frequency to locations of pilots.
`In some embodiments, transmitting relatively reliable sig
`nalling channel information proximal in time and frequency
`to locations of pilots comprises: for pairs of antennas of the
`four antennas, transmitting space time coded signalling chan
`nel information pairs adjacent in time to pairs of pilots.
`In some embodiments, for a given antenna, a spacing
`between pilots in the time direction is determined with con
`sideration to the maximum Doppler frequency, while a spac
`ing between pilot in the frequency direction is determined
`with consideration to a delay spread of multi-path fading.
`According to another broad aspect, the invention provides
`a method of transmitting over four transmit antennas com
`prising: for each antenna, generating a respective sequence of
`OFDM symbols, each OFDM symbol having a plurality of
`Sub-carriers carrying at data or pilots, and transmitting the
`sequence of OFDM symbols; wherein the OFDM symbols
`include at least one preamble OFDM symbol or midamble
`OFDM symbol comprising a repeating pattern of four pilot
`Sub-carriers for the four antennas.
`In some embodiments, the repeating pattern of four pilot
`sub-carriers comprises a first space-frequency coded pilot
`pair for a first pair of the four antennas, and a second space
`frequency coded pilot pair for a second pair of the four anten
`aS.
`In some embodiments, the repeating pattern of four pilot
`sub-carriers comprises a respective pilot for each of the four
`antennas without overlapping.
`In some embodiments, the preamble comprises two iden
`tical OFDM symbols.
`In Some embodiments, the method further comprises trans
`mitting the pair of identical OFDM symbols by: transmitting
`a prefix; transmitting a first OFDM symbol having first and
`second portions in time, the second portion being identical to
`the prefix, such that the prefix functions as a cyclic prefix for
`the first OFDM symbol; transmitting a second OFDM symbol
`identical to the first OFDM symbol, such that the second
`portion of the first OFDM symbol functions as a prefix for the
`second OFDM symbol.
`In some embodiments, the prefix and pair of identical
`symbols are transmitted with a total time duration equal to a
`time for transmitting a prefix and a single OFDM symbol that
`is not part of the preamble or midamble.
`In some embodiments, antennas can be turned off and pilot
`groups assigned to the turned off antennas re-assigned to the
`remaining two transmit antennas to improve the channel esti
`mation performance for fast frequency selective fading chan
`nel.
`In some embodiments, the four transmit antennas form part
`of a single base station transceiver.
`In some embodiments, the four transmit antennas form part
`of multiple base station transceivers.
`In some embodiments, the four transmit antennas form part
`of multiple mobile stations.
`In some embodiments, the pilots are space-time coded.
`In Some embodiments, the pilots are space-frequency
`coded.
`
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`In Some embodiments, the pilots are space-time-frequency
`coded.
`In some embodiments, the pilots are uncoded.
`According to another broad aspect, the invention provides
`a method of transmitting over at least two transmit antennas 5
`comprising: for each antenna, generating a respective
`sequence of OFDM symbols, each OFDM symbol having a
`plurality of Sub-carriers carrying at data or pilots, and trans
`mitting the sequence of OFDM symbols; wherein pilots are
`inserted for the antennas collectively in blocks of two sub
`carriers by two OFDM symbols scattered in time and fre
`quency.
`According to another broad aspect, the invention provides
`a method of transmitting a pair of identical OFDM symbols
`15
`comprising: transmitting a prefix; transmitting a first OFDM
`symbol having first and second portions in time, the second
`portion being identical to the prefix, such that the prefix
`functions as a cyclic prefix for the first OFDM symbol; trans
`mitting a second OFDM symbol identical to the first OFDM 20
`symbol, such that the second portion of the first OFDM sym
`bol functions as a prefix for the second OFDM symbol.
`In another embodiment, a transmitter comprising four
`transmit antennas is provided, the transmitter is adapted to
`implement one of the methods Summarized above.
`In another embodiment, at least two base station transceiv
`ers collectively comprising four transmit antennas are pro
`vided, the at least base station transceivers are adapted to
`implement one of the methods as Summarized above.
`In another embodiment, at least two mobile stations col- 30
`lectively comprising four transmit antennas are provided, the
`at least two mobile stations are adapted to implement one of
`the methods as Summarized above.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`35
`
`Preferred embodiments of the invention will now be
`described with reference to the attached drawings in which:
`FIG. 1 is a block diagram of a four antenna OFDM trans
`mitter in which data and pilot are modulated onto each OFDM 40
`signal;
`FIG. 2 is a block diagram of a four antenna OFDM trans
`mitter in which data, TPS and pilot are modulated onto each
`OFDM signal;
`FIG. 3 is a block diagram of a four antenna OFDM system 45
`in which the four antennas are located on respective mobile
`stations each transmitting data and pilot:
`FIG. 4 through 16B are time-frequency diagrams of scat
`tered pilot patterns for use with four antenna OFDM systems
`provided by embodiments of the invention;
`FIG. 17 is an example of how power boost may be applied
`to pilot symbols;
`FIGS. 18 and 19 are time-frequency diagrams of pilot
`patterns that are suitable for use as a preamble or midamble;
`and
`FIG. 20 is a detailed example of how two OFDM symbols
`for use in a preamble or midamble can be transmitted during
`the nominal single OFDM symbol duration.
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`DETAILED DESCRIPTION OF THE PREFERRED
`EMBODIMENTS
`
`60
`
`Problems and disadvantages of the existing solutions
`which included in 802.11a/g and 802.16a are that they doe not
`support MIMO transmission with four transmit antennas, do 65
`not support high speed mobility and are not optimal for TDD
`employment.
`
`6
`A system block diagram is shown in FIG. 1. A MIMO
`transmitter 10 is shown having four transmit antennas 12.14.
`16.18. For each transmit antenna, there is a respective OFDM
`modulator 20,22,24,26. The OFDM modulators 20.22,24,26
`have respective data inputs 28.30.32.34 and pilot inputs
`36,38,40.42. It is noted that while one OFDM modulator is
`shown per antenna, Some efficiencies may be realized in
`combining these functions. Alternatively, since the pilot
`channel inputs are predetermined, this can be determined
`within the OFDM modulators per se. Furthermore, while
`separate data inputs 28.30,32.34 are shown, these may be
`used to transmit data from one or more sources. Encoding
`may or may not be performed. Details of the OFDM modu
`lators are not shown. It is well understood that with OFMD
`modulation, the data and the pilot channel symbols are
`mapped to sub-carriers of an OFDM signal. In order to gen
`erate a particular pilot design, this involves controlling the
`timing of when data symbols versus pilot symbols are applied
`to particular sub-carriers and for particular OFDM symbol
`durations.
`In another system block diagram, shown in FIG. 2 the
`OFDM modulators 20.22.24.26 also receive respective TPS
`(transmit parameter signalling) data 43.44.46.48 which is
`also used in generating the OFDM signals. Once again, the
`TPS symbols are mapped by the OFDM modulator to appro
`priate sub-carrier locations and OFDM symbol durations.
`In yet another embodiment, shown in FIG. 3, four mobile
`stations 60,62,64,66 are shown. Each of these has their own
`OFDM modulator and transmit antenna. These embodiments
`can function similar to the embodiments of FIGS. 1 and 2 but
`with the respective signals being generated in different
`mobile stations rather than in a single MIMO transmitter.
`Collectively, the four mobile stations 60.62,64,66 function as
`a MIMO transmitter.
`Coherent detection is required to achieve high spectrum
`efficiency. Pilot assisted channel estimation is a widely
`applied approach to measure the change of the amplitude and
`phase of the transmitted signals caused by the corruption of
`the radio channel.
`For pilot-assisted channel estimation, known pilot symbols
`are multiplexed into the data stream at certain Sub-channels
`(sub-carriers) and certain times. The receiver interpolates the
`channel information derived from the pilot symbols and
`obtains the channel estimates for the data symbols.
`Embodiments of the invention provide pilot channel
`designs for four antenna OFDM.
`Various pilot channel designs that might, for example, be
`employed within the systems of FIGS. 1, 2 and 3 will now be
`described.
`In designing the new pilot designs, consideration is made
`to the fact that pilot symbols introduce the overhead. As such,
`from a channel utilization standpoint, fewer pilot symbols is
`better. For a channel with both frequency and time dispersion,
`pilot symbols have to be inserted both in frequency and time
`direction. The spacing between pilot symbols in time direc
`tion is determined with consideration to the maximum Dop
`pler frequency, while the spacing between pilot symbol in the
`frequency direction is determined with consideration to the
`delay spread of the multi-path fading. In some embodiments,
`for broadband mobile access application, the channel is
`updated more frequently both in the frequency direction and
`in the time direction in order to obtain the correct channel
`responses across the whole bandwidth during the whole
`transmission period. To deal with high frequency and time
`dispersions, a denser pilot grid is preferably employed.
`Some embodiments feature a TDD (time division duplex)
`deployment. To support slot-based TDD switching, in some
`
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`embodiments, channel estimation processing is performed
`slot by slot, i.e. the channel responses are calculated only
`based on the pilot symbols in the current slot.
`FIG. 4 through 16B are examples of pilot designs provided
`by various embodiments of the invention. In all of these
`drawings, time is shown on the vertical axis and frequency is
`shown on the horizontal axis. The Small circles each represent
`the content of a particular Sub-carrier transmitted at a particu
`lar time. A row of such circles represents the sub-carriers of a
`single OFDM symbol. A vertical column of any of these
`drawings represents the contents transmitted on a given
`OFDM sub-carrier over time. All of the examples show a
`finite number of sub-carriers in the frequency direction. It is to
`be understood that the number of sub-carriers in an OFDM
`symbol is a design parameter and that the drawings are to be
`15
`considered to give only one example of a particular size of
`OFDM symbol.
`Referring now to FIG. 4A, a first example pilot design is
`shown. With this design, there is a rep