US007702028B2
`
`(12) Ulllted States Patent
`Zhou et al.
`
`(10) Patent N0.:
`(45) Date of Patent:
`
`US 7,702,028 B2
`Apr. 20, 2010
`
`(54) METHOD OF TRANSMITTING PREAMBLE
`FOR SYNCHRONIZATION IN A MIMO-OFDM
`COMMUNICATION SYSTEM
`
`(75) Inventors: Yong-Xing Zhou, Yongin-si (KR);
`J (mg-Han Kim, SHWOH-Sl (KR)
`
`(73) Assignee: Samsung Electronics Co., Ltd.,
`SuWon-si (KR)
`
`7,139,340 B2 * 11/2006 Scarpa ..................... .. 375/344
`
`(Continued)
`FOREIGN PATENT DOCUMENTS
`
`EP
`
`1 261 181
`
`11/2002
`
`( * ) Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(1)) by 916 days.
`
`_
`(Contmued)
`OTHER PUBLICATIONS
`
`(21) Appl. N0.: 10/965,087
`
`(22) Filed:
`
`Oct. 14, 2004
`
`(65)
`
`Prior Publication Data
`
`US 2005/0084030 A1
`
`Apr. 21, 2005
`
`(30)
`
`Foreign Application Priority Data
`
`Oct. 16, 2003
`
`(KR) .................... .. 10-2003-0072176
`
`(51) Int. Cl.
`(2006.01)
`H04B 7/02
`...... .................... .. 375/267; 370/208
`(52) U..S. Cl. ......
`(58) Field of Classi?cation Search ............... .. 375/299,
`375/260, 340: 316: 347: 355: 267: 262: 344:
`375/148, 367; 370/208, 210, 209, 503, 203,
`_
`370/296’ 334: 320
`_
`See aPPhCaUOn ?le for Complete Search hlstory-
`R f
`C-t d
`e erences l e
`U.S. PATENT DOCUMENTS
`
`56
`(
`)
`
`375667
`5/2001 Klank et a1‘
`6’226’337 B1 *
`375/299
`6,377,632 B1* 4/2002 Paulraj et al.
`370/320
`6 731 614 B1 *
`5/2004 Ohlson et al
`370/206
`730613854 B2* 6/2006 Tarokh etal.‘
`7,068,628 B2 *
`6/2006 Li et al. .................... .. 370/334
`
`7,136,410 B2 * 11/2006 Choi et al. . . . . . .
`. . . .. 375/148
`7,139,320 B1 * 11/2006 Singh et al. ............... .. 375/260
`
`Training sequence assisted channel estimation for MIMO OFDM by
`Sumei Sun; Wiemer, 1.; Ho, C.K.; Tjhung, T.T.; Wireless Communi
`cations and Networking, 2003. WCNC 2003. 2003 IEEE vol. 1, Mar.
`16-20, 2003 pp. 38-43 V01. 1.*
`
`_
`(Contmued)
`Primary ExamineriDavid C Payne
`ASSiSZanZ ExamineriTanmay K Shah
`74 All
`A 2
`F' iNSIP L
`(
`)
`Omey’ gen ’ or m”
`aw
`
`57
`(
`)
`
`ABSTRACT
`
`A method and apparatus for transmitting a preamble for
`frame Synchronization and Channel estimation in a MIMO
`OFDM communication system are provided. An OFDM
`communication system using Qtransmit antennas generates a
`base preamble sequence including a CP and an orthogonal
`sequence. 1f Q; a predetermined number M, a preamble
`sequence for a kth antenna is S(t—(k—l)T/M). If Q>M and
`kéM the preamble sequence transmitted for the kth antenna
`is S(t—(k—l)T/M). 1f Q>M and k>M, the preamble sequence
`for the kth antenna is (—l)(PS_1)S(t—(k—M—l)T/M). Here, S(t)
`is the orthogonal sequence T is the period ofthe orthogonal
`.
`.
`’
`.
`.
`.
`.
`.
`.
`sequence, and PS 1s an mdex 1nd1cat1ng atransm1ss1on period
`10f the Preamble 5.9013106‘ Tile preamble? Sequences are at
`east “we transmme mm‘ 6 Q transm“ antennas'
`
`14 Claims, 14 Drawing Sheets
`
`time
`
`Antenna 0:
`
`Antenna 1:
`
`Antenna 2:
`
`Antenna 3:
`
`Antenna 4:
`
`CPL S(t)
`
`‘
`
`P
`
`—S(l)
`
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`
`CP
`
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`
`Antenna 51 RP SKI-TM)
`spas
`
`P I —Slt—T/4)
`
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`
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`SAMSUNG 1007-0001
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`

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`US 7,702,028 B2
`Page 2
`
`U.S. PATENT DOCUMENTS
`
`OTHER PUBLICATIONS
`
`*
`
`375/299
`
`7 154 964 B1 * 12/2006 A1_Dhahir et al
`’
`’
`' """"" "
`$332; 5; * $88; E‘g’mson et a1‘ """"" " 233332
`’
`’
`*
`""""""" "
`370/210
`7’269’l27 B2 *
`9/2007 Mody et a1‘
`2002/0181390 A1 12/2002 Mody et al. ............... .. 370/208
`2003/0016621 A1
`1/2003 L1
`2003/0043887 Al *
`3/2003 Hudson .................... .. 375/144
`2004/ 0050022 A1 *
`3/2004 Marrecau et al. ......... .. 55/2823
`2004/0071234 A1 *
`4/2004 Li ............................ .. 375/341
`2004/0081131 A1 *
`4/2004 Walton et al. ............. .. 370/344
`2004/0l3l0ll Al *
`7/2004 Sandell et 31
`370/210
`2004/0l3 1012 Al *
`7/2004 Mody et a1 ' """""" " 370/210
`
`FOREIGN PATENT DOCUMENTS
`
`W0 02/098088
`
`12/2002
`
`Fast burst systems synchronisation technique for OFDM-WLAN by
`B.Y. Prasetyo F. Said and AH. Aghvami. Communications IEE Pro
`7
`a
`ceedings- vol. 147, Issue 5, Oct. 2000 pp. 292-298.*
`Effect of frame synchronization errors on pilot-aided channel esti
`mation in OFDM: analysis and solution by Mosto?, Y.; Cox, D.C.;
`Bahai, Aéwireless Personal Multimedia Communications, 2002‘
`The 5th International Symposium on vol. 3, Oct. 27-30, 2002 pp.
`1309-1313 VOl. 3*
`Ye Li, Simpli?ed Channel Estimation for OFDM Systems With Mul
`tipls Transmit Antennas, IEEE Transactions on Wireless Communi
`Canons, V°1~ 1: N°~ 1, Jan~_2002, PR _67-75~ _
`Imad Barhumi et al., Optimal Training Design for MIMO OFDM
`Systems in Mobile Wireless Channels, IEEE Transactions on Signal
`Processing, vol. 51, No. 6, Jun. 2003, pp. 1615-1624.
`Apurva N. Mody et al., Receiver Implementation for a MIMO OFDM
`System, IEEE Global Telecommunications Conference, Nov. 2002,
`pp. 716 -720.
`* cited by examiner
`
`SAMSUNG 1007-0002
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`

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`US. Patent
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`Apr. 20, 2010
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`Apr. 20, 2010
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`US 7,702,028 B2
`
`1
`METHOD OF TRANSMITTING PREAMBLE
`FOR SYNCHRONIZATION IN A MIMO-OFDM
`COMMUNICATION SYSTEM
`
`PRIORITY
`
`This application claims priority under 35 U.S.C. § 119 to
`an application entitled “Method of Transmitting Preamble for
`Synchronization in a MIMO-OFDM Communication Sys
`tem” ?led in the Korean Intellectual Property Of?ce on Oct.
`16, 2003 and assigned Serial No. 2003-72176, the contents of
`Which are incorporated herein by reference.
`
`BACKGROUND OF THE INVENTION
`
`1. Field of the Invention
`The present invention relates generally to a multi-input
`multi-output-orthogonal frequency division multiplexing
`(MIMO-OFDM) communication system, and in particular, to
`a method and apparatus for transmitting a preamble for frame
`synchronization.
`2. Description of the Related Art
`OFDM is Widely considered an essential transmission
`scheme for next-generation Wireless communications for its
`simple implementation, robustness against multi-channel
`fading, and its capability of increasing the data rate through
`parallel transmission of data signals at frequencies called
`sub-carriers. The sub-carriers are mutually orthogonal to
`avoid inter-carrier interference. Their spectrums are over
`lapped so that the sub-carriers are spaced from each other
`With a minimum gap.
`An OFDM system is sensitive to errors or offsets including
`a frequency offset, timing errors in a frame or a symbol, and
`non-linearity caused by a high peak-to-average poWer ratio
`(PAPR). Some OFDM systems utilize a coherent detection
`rather than differential modulation and demodulation in order
`to achieve an additional signal-to-noise ratio (SNR) gain of
`about 3 dB. Their performance depends considerably on
`Whether or not channel state information (CSI) is available.
`The use of multiple transmit/receive antennas further
`improves communication quality and throughput in an
`OFDM system. This OFDM system is called a MIMO
`OFDM system Which is distinguished from a single-input
`single-output (SISO)-OFDM system.
`The MIMO-OFDM system can simultaneously transmit
`data on a plurality of sub-channels in the space domain irre
`spective of Whether or not a transmitter requires the CSI. The
`sub-channels refer to radio paths from a plurality of transmit
`antennas to a plurality of receive antennas. Thus, the MIMO
`OFDM system offers a higher data rate than the SISO-OFDM
`system.
`Typically, the MIMO/SISO-OFDM system requires frame
`synchronization in both time and frequency and estimation of
`channel parameters and noise changes. For the synchroniza
`tion and estimation, a preamble sequence (i.e. training sym
`bols or a training sequence) is used.
`FIG. 1 illustrates the structure of an OFDM frame includ
`ing a preamble sequence in a typical OFDM communication
`system. Referring to FIG. 1, the preamble sequence consists
`of special symbols added as a pre?x to the OFDM frame. In
`general, the structure and contents of the preamble are knoWn
`betWeen a transmitter and a receiver. The preamble is so
`con?gured as to have a relatively loW complexity and offer a
`maximum performance in the synchronization and estima
`tion process.
`An ideal preamble con?guration satis?es the folloWing
`requirements:
`
`20
`
`25
`
`30
`
`35
`
`40
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`45
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`50
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`55
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`60
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`65
`
`2
`(1) Excellent compensation for timing synchronization;
`(2) LoW PAPR for high-poWer transmission;
`(3) Feasibility for channel estimation;
`(4) Feasibility for frequency offset estimation over a Wide
`range; and
`(5) LoW computation complexity, loW overhead and high
`accuracy.
`A description Will be made beloW of conventional pre
`amble structures for MIMO-OFDM frame synchronization
`and channel estimation.
`A ?rst knoWn preamble transmitting/receiving scheme for
`MIMO-OFDM frame synchronization transmits the same
`information sequence through all transmit antennas.
`The MIMO-OFDM system must have excellent properties
`in time-domain periodic auto-correlation of sequences as
`Well as in cross-correlation of sequences transmitted from
`different transmit antennas. Ideal auto -correlation and cross
`correlation properties are determined by Equation (1) and
`Equation (2), respectively:
`
`Where superscript * denotes a conjugate operator, N denotes
`the length of sequences, q and q' denote indexes of transmit
`antennas, and s (M denotes an nth data symbol in a sequence of
`length N transmitted from a qth transmit antenna. A sequence
`that satis?es Equation (1) is an orthogonal sequence. Here,
`subscript N denotes the period of the sequence.
`In an ideal situation a space-time matrix for sequences
`transmitted from N transmit antennas is a unit matrix. HoW
`ever, this is impossible in its application because the number
`of the transmit antennas must be equal to the length of the
`sequences.
`In the ?rst preamble transmitting/receiving scheme, a pre
`amble sequence is designed for frame synchronization by
`copying a predetermined orthogonal sequence designated for
`a ?rst antenna to be used for the other antennas, and is repre
`sented by
`
`sq7n:s,, for all q
`
`(3)
`
`A distinctive shortcoming of the above scheme is that SNR
`may be very loW in the case of a correlated channel. For a 2x2
`MIMO system using tWo transmit antennas and tWo receive
`antennas, for instance, a received signal is expressed as
`
`Where rj[n, k] denotes a frequency-domain signal received at
`a jth receive antenna, nj[n, k] denotes White Gaussian noise,
`Hi]- denotes a channel response from an ith transmit antenna to
`a jth receive antenna, and S [n, k] denotes an nth symbol in a
`k-th sub-carrier. As noted from Equation (4), if H 1 j is approxi
`mately equal to —H2]., the SNR of the received signal is very
`loW.
`Another conventional preamble transmitting/receiving
`scheme for MIMO-OFDM frame synchronization utilizes a
`direct modulated orthogonal poly-phase sequence.
`
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`US 7,702,028 B2
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`3
`A direct modulated orthogonal poly-phase sequence is a
`chirp-like sequence used to form a preamble sequence. If P is
`a prime number, the direct modulated orthogonal poly-phase
`sequence is comprised of (P-l) orthogonal sequences. Its
`excellent cross-correlation property is given as
`
`4
`having different characteristics. The time-domain siZe T/ Q of
`the channels varies With the number of the transmit antennas
`Q.
`A mean square error (MSE) in the single-symbol optimal
`training technique is calculated by
`
`According to the second preamble transmitting/receiving
`scheme, the transmit antennas transmit the same preamble
`sequence having (P-l) orthogonal sequences. This scheme
`faces the folloWing problems:
`(1 ) Although the length of the direct modulated orthogonal
`poly-phase sequence is the square of a prime number, the
`length of an OFDM frame must generally be a poWer of 2, for
`example, 64, 128,256, .
`.
`.
`; and
`(2) While an ideal frame must be acquired at each point, it
`is impossible to reduce the complex multiplications required
`and thus considerably greater computation is required.
`NoW, knoWn preamble transmitting/ receiving schemes for
`MIMO-OFDM channel estimation Will be described beloW.
`A ?rst preamble transmitting/receiving scheme for
`MIMO-OFDM channel estimation is Geoffrey Li’s single
`symbol optimal training technique. FIG. 2 illustrates a pre
`amble structure according to the ?rst preamble transmitting/
`receiving scheme for MIMO-OFDM channel estimation.
`Referring to FIG. 2, given Q transmit antennas, a ?rst
`antenna transmits a preamble sequence S(t), and each of the
`other antenna transmits a preamble sequence S(t-T/Q), .
`.
`. ,
`or S {t- (Q- l )T/Q} produced by rotating a preamble sequence
`for the previous antenna a predetermined number of symbols,
`that is, T/ Q symbols. Q:Floor(N/LO) in Which N is the num
`ber of sub-carriers and L0 is the maximum time delay spread
`of a sub-channel. Floor( ) is a function of obtaining an integer
`and T is the period of the preamble sequence. T is the product
`of the number of symbols included in the preamble sequence,
`N, and a symbol period TS.
`A received signal at the jth receive antenna is determined
`by
`
`Where WN represents an N-point fast Fourier transform
`(FFT). If p [n, k]:r[n, k] *S* [n, k], Equation (6) is expressed as
`
`FIG. 3 illustrates an example of the time-domain channel
`response characteristics of Pj[n, k]. Referring to FIG. 3, hoj is
`a channel response characteristic from a ?rst transmit antenna
`to a receiver, hlj is a channel response characteristic from a
`second transmit antenna to the receiver, h2j is a channel
`response characteristic from a third transmit antenna to the
`receiver, and h3j is a channel response characteristic from a
`fourth transmit antenna to the receiver. Preamble sequences
`transmitted from the transmit antennas experience channels
`
`20
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`25
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`40
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`45
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`50
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`55
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`60
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`MSE: -0'
`
`(3)
`
`Wherein, (LT-on indicates a noise poWer.
`In accordance With the ?rst preamble transmitting/receiv
`ing scheme for MIMO-OFDM channel estimation, although
`a preamble sequence is transmitted on all sub-carriers, only
`one training sequence structure su?ices. HoWever, due to the
`rotation of a training sequence by a predetermined number of
`symbols for each transmit antenna, the number of transmit
`antennas is limited by the number of the rotated symbols and
`the length of the training sequence.
`A second preamble transmitting/receiving scheme for
`MIMO-OFDM channel estimation utiliZes Cordon L. Stuber
`and Apurva N. Mody’s space-time coding. In this scheme,
`knoWn symbols are orthogonally transmitted in the space
`domain through inversion and conjugation according to time
`and space, namely according to transmit antennas. A pre
`amble sequence for a 2x2 system using tWo transmit antennas
`and tWo receive antennas is formed by
`
`The above matrix means that symbols S1 and S2 are
`sequentially transmitted from a ?rst transmit antenna and
`symbols —S2* and 81* are sequentially transmitted from a
`second transmit antenna.
`For a 4x4 system, a preamble sequence can be formed by
`
`FIG. 4 illustrates transmission/reception of a preamble
`sequence according to the second preamble transmitting/re
`ceiving scheme for MIMO-OFDM channel estimation.
`Referring to FIG. 4, Q preamble sequences, each having Q
`symbols are provided to Q transmit antennas from time t to
`time t+(Q—l)TS through Q OFDM modulators. TS is a symbol
`duration. The preamble sequences arrive at L receive anten
`nas on Q><L sub-channels having channel response character
`istics hl 1 to hQL. L OFDM demodulators collect signals Rl to
`RQL received at the L receive antennas from time t to time
`t+(T—l)TS and form a Q><L received signal matrix.
`In the second preamble transmitting/receiving scheme, the
`minimum number of training symbols needed for each trans
`mit antenna is equal to the number of transmit antennas. As
`
`SAMSUNG 1007-0018
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`

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`US 7,702,028 B2
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`5
`more training symbols are used, the preamble sequences are
`longer. This is not feasible for burst or high-mobility commu
`nications.
`
`SUMMARY OF THE INVENTION
`
`An obj ect of the present invention is to substantially solve
`at least the above problems and/or disadvantages and to pro
`vide at least the advantages beloW. Accordingly, an object of
`the present invention is to provide an effective preamble
`sequence structure and an effective preamble sequence trans
`mitting method in a MIMO-OFDM system.
`Another object of the present invention is to provide a
`method and apparatus for generating a preamble of a multi
`symbol space-time structure in a MIMO-OFDM system.
`The above objects are achieved by a method and apparatus
`for transmitting a preamble for frame synchronization and
`channel estimation in a MIMO-OFDM communication sys
`tem. An OFDM communication system using Q transmit
`antennas generates a base preamble sequence including a
`cyclic pre?x (CP) and an orthogonal sequence, generates a
`preamble sequence for each of the Q transmit antennas by
`rotating the orthogonal sequence by a predetermined number
`of symbols, and at least tWice transmits the generated pre
`amble sequences from the Q transmit antennas.
`If Qéa predetermined number M, a preamble sequence for
`a kth antenna is S(t—(k—l)T/M). If Q>M and kéM, the pre
`amble sequence transmitted for the kth antenna is S(t-(k-l)
`T/M). If Q>M and k>M, the preamble sequence for the kth
`antenna is (—l)(PS_1)S(t—(k—M—l)T/M). Here, S(t) is the
`orthogonal sequence, T is the period of the orthogonal
`sequence, and PS is an index indicating a transmission period
`of the preamble sequence.
`
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`BRIEF DESCRIPTION OF THE DRAWINGS
`
`35
`
`6
`FIG. 12 illustrates an embodiment of a preamble structure
`for a 6x6 MIMO system according to the present invention;
`FIG. 13 illustrates preambles illustrated in FIG. 12 in
`matrix blocks; and
`FIG. 14 is a graph illustrating channel estimation gain With
`respect to MSE in a multi-channel WLAN (Wireless Local
`Access Network) system using the preamble structure of the
`present invention.
`
`DETAILED DESCRIPTION OF THE PREFERRED
`EMBODIMENTS
`
`Preferred embodiments of the present invention Will be
`described herein beloW With reference to the accompanying
`draWings. In the folloWing description, Well-known functions
`or constructions are not described in detail since they Would
`obscure the invention in unnecessary detail.
`A MIMO-OFDM system to Which the present invention is
`applied Will ?rst be described beloW.
`FIG. 5 is a simpli?ed block diagram of a typical MIMO
`system. Referring to FIG. 5, Q><L sub-channels 30 are de?ned
`betWeen a transmitter 10 having Q transmit antennas and a
`receiver 20 having L receive antennas. The sub-channels 30
`each have a unique channel response characteristic hql and
`these characteristics are expressed as a Q><L channel matrix
`H L.
`QFIG. 6 is a block diagram of a transmitter in a MIMO
`OFDM system to Which the present invention is applied. The
`transmitter transmits the same user information through a
`plurality of transmit antennas to achieve an antenna diversity
`gain.
`Referring to FIG. 6, an encoder (ENC) 102 generates a
`coded sequence by encoding an information sequence S(t) at
`a predetermined code rate. A demultiplexer (DEMUX) 104
`distributes the coded sequence to a plurality of interleavers
`(INTs) 106 to 114 corresponding to transmit antennas 112 to
`120. The interleavers 106 to 114 each interleave the input bits.
`Mappers (MAPs) 108 to 116 each map the interleaved bits to
`modulation symbols according to a mapping rule, for
`example, PSK (Phase Shift Keying) or QAM (Quadrature
`Amplitude Modulation).
`OFDM modulators (MODs) 110 to 118 each generate an
`OFDM symbol by inserting a pilot symbol for every prede
`termined number of modulation symbols, generate an OFDM
`frame by adding a preamble sequence having knoWn symbols
`at the start of a predetermined number of OFDM symbols,
`and inverse-fast-Fourier-transform (FfF T) the OFDM frame.
`The IFFT OFDM frames are transmitted through their corre
`sponding transmit antennas 112 to 120 through an RF (Radio
`Frequency) module (not shoWn).
`FIG. 7 is a block diagram of a receiver in the MIMO
`OFDM system to Which the present invention is applied. The
`receiver is a counterpart to the transmitter illustrated in FIG.
`6.
`
`Referring to FIG. 7, signals received at receive antennas
`202 to 216 are applied to the inputs of OFDM demodulators
`(DEMODs) 204 to 218 through an RF module (not shoWn).
`The OFDM demodulators 204 to 218 each distinguish a pre
`amble from OFDM symbols on a frame-by-frame basis, accu
`rately acquire frame synchronization by detecting the pre
`amble, and generate a plurality of modulation symbols by
`fast-Fourier-transforming the signal. While not shoWn, the
`detected preamble is used in a channel estimator that esti
`mates channel response characteristics from the transmitter to
`the receiver.
`Demappers (DEMAPs) 206 to 216 each demap received
`modulation symbols according to a demapping rule corre
`
`The above and other objects, features and advantages of the
`present invention Will become more apparent from the fol
`loWing detailed description When taken in conjunction With
`the accompanying draWings in Which:
`FIG. 1 illustrates the structure of an OFDM frame includ
`ing a preamble sequence in a typical OFDM communication
`system;
`FIG. 2 illustrates the structure of a preamble according to a
`conventional preamble transmitting/receiving scheme for
`MIMO-OFDM channel estimation;
`FIG. 3 illustrates an example of time-domain channel
`response characteristics of Pj[n, k];
`FIG. 4 illustrates transmission/reception of a preamble
`sequence according to another conventional preamble trans
`mitting/receiving scheme for MIMO-OFDM channel estima
`tion;
`FIG. 5 is a simpli?ed block diagram of a typical MIMO
`system;
`FIG. 6 is a block diagram of a transmitter in a MIMO
`OFDM system to Which the present invention is applied;
`FIG. 7 is a block diagram of a receiver in the MIMO
`OFDM system to Which the present invention is applied;
`FIG. 8 illustrates an embodiment of a preamble structure
`according to the present invention;
`FIG. 9 illustrates the transmission of preambles illustrated
`in FIG. 8;
`FIG. 10 illustrates the results of frame synchronization
`according to the present invention;
`FIG. 11 illustrates an embodiment of a preamble structure
`for a 4x4 MIMO system according to the present invention;
`
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`US 7,702,028 B2
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`7
`sponding to the mapping rule used in the transmitter. Deinter
`leavers (DEINTs) 208 to 216 each deinterleave demapped
`bits according to a deinterleaving rule corresponding to the
`interleaving rule used in the transmitter. A multiplexer
`(MUX) 212 multiplexes the deinterleaved bits and a decoder
`210 recovers the information sequence S(t) by decoding the
`multiplexed bits at the code rate used in the transmitter.
`In the MlMO-OFDM system having the above con?gura
`tion, a preamble sequence consists of special symbols gener
`ated by an OFDM modulator and attached to an OFDM frame
`to indicate the start of the OFDM frame. A mobile station
`must synchronize to the start point of the data to receive the
`data. For this purpose, the mobile station acquires a preamble
`sequence commonly used in the entire system before receiv
`ing the data.
`The preamble sequence is used for frame synchronization,
`frequency synchronization (i.e. frequency offset estimation),
`and channel estimation. The OFDM communication system
`estimates time/ frequency/ channel information using the pre
`amble sequence at the start of each frame or data burst, and
`updates the time/frequency/channel information using a
`cyclic pre?x (CP), inserted to avoid inter-symbol interfer
`ence, and pilot symbols inserted betWeen modulation sym
`bols.
`As knoWn, frame synchronization is performed in tWo
`stages: coarse frame synchronization and ?ne frame synchro
`nization.
`The coarse frame synchronization is the process of detect
`ing the start point of an OFDM frame by sampling in an
`approximate range. The correlation peak of a CP is used for
`the coarse frame synchronization. The folloWing equation
`represents a metric for the coarse frame synchronization
`
`8
`length of valid data in the data is 112 points, and the DC
`(Direct Current) and edge components in a signal frequency
`band are nulls. Here, a 2x2 MIMO system using tWo transmit
`antennas and tWo receive antennas is used as an example. A
`point refers to the position of a sub-carrier subject to N-point
`FFT. For example, if a CP is 32 points long, this implies that
`the CP is transmitted on 32 sub-carriers.
`First of all, orthogonal sequences are generated using an
`extended CAZAC (Constant Amplitude Zero Auto-Correla
`tion) sequence.
`For example, a base CAZAC sequence is
`
`By inserting three zeroes betWeen every adjacent pair of
`elements in the base CAZAC sequence, the folloWing
`sequence is generated
`
`1,0, ,0,1,0,0,0,1,0,0,0,1,0,0,0,1,0,0,0,j,0,
`0,—1,0,0,0,—j,0,0,0,1,0,0,0,—1,0,0,0,
`0,0,0,—1,0,0,0,1,0,0,0,—j,0,0,0,—1,0,
`0] a
`
`(14)
`
`The peak-to-average poWer ratio of the above extended
`CAZAC sequence is 6 dB.
`The above orthogonal sequence is converted to the fre
`quency domain, for spectrum shaping. The resulting neW
`sequence is again converted to the time domain, to thereby
`create a preamble sequence.
`Thus, the preamble structure according to the present
`invention is given as illustrated in Table 1 beloW.
`
`20
`
`25
`
`30
`
`TABLE 1
`
`G41
`
`2
`
`(11)
`
`35
`
`CPO
`CH
`
`S64[1:64]
`S64[33:64]
`$64152]
`
`S64[1:64]
`S64[33:64]
`$64152]
`
`Antenna 0
`Antenna 1
`
`Where G denotes the WindoW size of the frame synchroniza
`tion, rj, x denotes an xth signal in a sequence received at a jth
`receive antenna, and N denotes the length of the sequence.
`Thus, a coarse frame start point is a time index n that maxi
`mizes q)”.
`The coarse frame synchronization reduces the range of ?ne
`frame synchronization. The computation range of Equation
`(12) is narroW compared to that of Equation (2), in calculating
`the cross-correlation property for the ?ne frame synchroni
`zation
`
`(12)
`
`Where s q, n denotes an nth data symbol in a sequence trans
`mitted from a qth transmit antenna and Kcatch denotes the
`range of the ?ne frame synchronization. Thus, the frame start
`point is a time index k that makes the ?ne frame synchroni
`zation metric o (k) zero.
`An embodiment of a preamble sequence structure design in
`a multi-channel WLAN system according to the present
`invention Will noW be described.
`Let a root mean square (RMS) delay be equal to 50 ns, a
`sampling time be equal to 25 ns, a CP length be equal to 32
`points, and the total length of data be equal to 128 points. The
`
`FIG. 8 illustrates the preamble structure according to an
`embodiment of the present invention, and FIG. 9 illustrates
`transmission of preambles illustrated in FIG. 8. As stated
`earlier, the illustrated preamble structure is for the 2x2 MIMO
`system.
`Referring to FIG. 8, a ?rst antenna (antenna 1) transmits a
`sequence of 64 bits, S[1:64] for a ?rst transmission period and
`a second antenna (antenna 2) transmits a 32-bit rotated ver
`sion ofthe sequence, S[33:64]S[1:32]. 32 bits is the quotient
`of dividing the sequence length, 64, by the number of the
`transmit antennas, 2. These sequences are repeatedly trans
`mitted for a second transmission period. Transmission of a
`64-bit sequence is equivalent to the use of 64 sub-carriers.
`Therefore, as illustrated in FIG. 9, the ?rst antenna transmits
`the input sequence on sub-carriers #0 to #63, While the second
`antenna transmits the input sequence on sub-carriers #32 to
`#31.
`Then, the receiver cross-correlates the extended CAZAC
`sequence With received complex symbols, thereby perform
`ing the ?ne frame synchronization by
`
`(15)
`
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`where ¢.,,.. = 2 (53k Mn)
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`US 7,702,028 B2
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`9
`
`-continued
`
`|rJ-7,11,‘ |2 = constant
`
`P’:n
`
`2-1
`k Ho
`
`where N is the length of the preamble sequence according to
`the present invention, Q is the number of the transmit anten-
`nas, sq, k is a kth symbol in a preamble sequence transmitted
`from a qth transmit antenna, and rj, Mk is an (n+k)th signal in
`a preamble sequence received at a jth receive antenna.
`Similarly, the start point of the frame is determined as a
`time point n where CI>n:0.
`Since time index n in the fine frame synchronization indi-
`cates an FFT point, full complex multiplications will increase
`complexity considerably. However, with the use of the
`CAZAC sequence of a simple structure according to the
`present invention, only addition and switching will sufiice.
`With the sequence rotation of the present invention, a
`received signal is determined by
`
`V,-(k):Ho,-(k)S(k)+H1,-(k)'(-1)k5(k)+n;(k)
`
`(16)
`
`Even if charmels are correlated, it is impossible to reduce
`SNR in the system. Yet, simulation results reveal that the
`present invention is robust compared to the conventional
`technology in which the same sequence is applied to all
`antennas.
`
`FIG. 10 illustrates the results of frame synchronization
`according to the present invention. Changes over time in
`coarse and fine frame synchronization metrics are illustrated.
`In FIG. 10, time points having the highest metric values are
`conspicuous in the fine frame synchronization.
`While each transmit antenna transmits the same preamble
`sequence for two transmission periods as illustrated in FIG. 8
`according to the embodiment of the present invention, it can
`be further contemplated as another embodiment that each
`transmit antenna transmits the same sequence for more than
`two transmissionperiod