`
`(12) Ulllted States Patent
`Zhou et al.
`
`(10) Patent No.:
`(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
`
`7,139,340 B2 * 11/2006 Scarpa ..................... .. 375/344
`
`(75)
`
`Inventors: Yong-Xing Zhou, Yongin-si (KR);
`Jong-Han Kim, Suwon-si (KR)
`
`(Continued)
`FOREIGN PATENT DOCUMENTS
`
`(73) Assignee: Samsung Electronics Co., Ltd.,
`Suwon-si (KR)
`
`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.
`
`.
`(commued)
`OTHER PUBLICATIONS
`
`(21
`
`(22
`
`(65
`
`Appl. NOJ 10/965,087
`
`Filed:
`
`Oct. 14 2004
`3
`Prior Publication Data
`
`US 2005/0084030 A1
`
`Apr. 21, 2005
`
`Foreign Application Priority Data
`(30
`Oct. 16, 2003
`(KR)
`.................... .. 10-2003-0072176
`
`(51
`
`(52
`(58
`
`(56)
`
`Int. Cl.
`(2006.01)
`H04B 7/02
`...................... 375/267; 370/208
`......
`U..S. Cl.
`......
`Field of Classification Search ............... .. 375/299,
`375/260: 340: 316: 347: 355: 267: 262: 344:
`375/148: 357; 370/208: 210: 209: 503: 203:
`370/206: 334: 320
`See appheatien file for eemplete Seareh hi5t01'Y-
`References Cited
`U.S. PATENT DOCUMENTS
`
`6 226 337 B1*
`’
`’
`
`.............. .. 375/367
`
`5/2001 Klank et al.
`at
`6mmm4BH ymm mmmamHm .W3mBm
`7,061,854 B2 '1
`6/2006 Tarokh etal.
`370/206
`7,068,628 B2 *
`6/2006 Li et al.
`370/334
`7,136,410 B2* 11/2006 Choiet 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 V01. 1, Mar.
`16-20, 2003 pp. 38-43 V01. 1.*
`
`.
`(Continued)
`
`Primary Examiner—DaVid C Payne
`Assisllml Examiner—Tanmay K Shah
`74 A
`A
`F' —NS1P L
`(
`) m”"ey’
`gem’ or W’
`ABSTRACT
`
`57
`
`(
`
`)
`
`aw
`
`A method and apparatus for transmitting a preamble for
`frame Synchronization and Channd estimation in a MIMO-
`OFDM communication system are provided. An OFDM
`communication system using Q transmit antennas generates a
`base preamble sequence including a CP and an orthogonal
`sequence. If Qé a predetermined number M, a preamble
`sequence for a kth antenna is S(t—(k—1)T/M). If Q>M and
`k§M the preamble sequence transmitted for the kth antenna
`is S(t—(k—1)T/M). If Q>M and k>M, the preamble sequence
`for the kth antenna is (-1 )(PS‘1)S(t—(k—M—1 )T/M). Here, S(t)
`~
`~
`~
`th
`rth
`1
`T
`th
`d f th
`rth
`1
`2:
`P
`gbl
`Th
`bl
`=
`Sh
`1°
`6 Dream e S.equd"I1}°e' he pream 9? Sequences are a
`east twice transmitte
`ronit e Q transmit antennas.
`
`t
`
`14 Claims, 14 Drawing Sheets
`
`time
`
`
`
`CF
`
`on
`
`cp
`
`CP
`
`S(t)
`
`s(1—1/4)
`A
`
`S(t—T/2)
`
`s(x—aT/4)
`
`P
`
`P
`
`P
`
`P
`
`50)
`
`S11-T/4)
`A
`
`511-1/2)
`
`S(t-ST/4)
`
`P
`
`P
`
`P
`
`P
`
`50)
`
`s(z—1/4)
`A
`
`S(t-T/2)
`
`S(1-3T/4)
`
`CP
`
`op
`
`OP
`
`CP
`
`50)
`
`an-1/4)
`A
`
`s(t—i/2)
`
`S(t-ST/4)
`
`Antenna 0:
`
`Antenna1I
`
`Antenna 2:
`
`Antenna 3:
`
`Antenna 4:
`
`
`
`--
`
`APPLE 1007
`
`
`
`
`
`
`— I
`
`
`II!
`-A
`A
`
`-A
`
`1
`
`APPLE 1007
`
`
`
`US 7,702,028 B2
`Page 2
`
`U.S. PATENT DOCUMENTS
`
`OTHER PUBLICATIONS
`
`Fast burst systems synchronisation technique for OFDM-WLAN by
`KY. Prasetyo, F.Said and AH. Aghvami. Communications, IEE Pro-
`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 Mostofi, Y.; Cox, D.C.;
`Bahai, A.;Wireless Personal Multimedia Communications, 2002.
`The 5th International Symposium on Vol. 3, Oct. 27-30, 2002 pp.
`1309-1313 Vol. 3.*
`Ye Li, Simplified Channel Estimation for OFDM Systems with Mul-
`tiple Transmit Antennas, IEEE Transactions on Wireless Communi-
`cations, Vol. 1, No. 1, Jan. 2002, pp. 67-75.
`Imad Barhumi et al., Optimal Training Design for MIMO OFDM
`Systems in Mobile Wireless Charmels, 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
`
`......... .. 375/299
`Al-Dhahir et al.
`.......... .. 375/340
`Thomson et al.
`Joo .......................... .. 370/203
`
`............... .. 370/210
`............... .. 370/208
`
`Mody et al.
`Mody et al.
`Li
`Hudson .................... .. 375/144
`Marrecau et al.
`55/282.3
`Li
`............... ..
`375/341
`Walton et al.
`. 370/344
`Sandell et al.
`. 370/210
`Mody et al.
`............... .. 370/210
`
`
`
`7,154,964
`7,184,495
`7,263,058
`7,269,127
`2002/0181390
`2003/0016621
`2003/0043887
`2004/0050022
`2004/0071234
`2004/0081131
`2004/0131011
`2004/0131012
`
`B1*
`B2*
`B2*
`B2*
`A1*
`A1
`A1*
`A1*
`A1*
`A1*
`A1*
`A1*
`
`12/2006
`2/2007
`8/2007
`9/2007
`12/2002
`1/2003
`3/2003
`3/2004
`4/2004
`4/2004
`7/2004
`7/2004
`
`FOREIGN PATENT DOCUMENTS
`
`W0
`
`W0 02/098088
`
`12/2002
`
`2
`
`
`
`U.S. Patent
`
`Apr. 20, 2010
`
`Sheet 1 of 14
`
`US 7,702,028 B2
`
`
`
`I52;5%E2.220E.§_
`
`
`
`T|uI:|I|||I||.l|52$230Iii
`
`SE4moan:H.05
`
`3
`
`
`
`
`
`U.S. Patent
`
`Apr. 20, 2010
`
`Sheet 2 of 14
`
`US 7,702,028 B2
`
`-————————-—-——-->- time
`
`Antenna 21
`
`SWVQ)
`
`s[HQ~1)T/Q!
`
`'.
`
`%
`
`
`
`Antenna Q!
`
`
`
`SDEIGB
`
`T!"
`
`N
`
`FIG.2
`
`(PRIOR ART)
`
`4
`
`4
`
`
`
`U.S. Patent
`
`Apr. 20, 2010
`
`Sheet 3 of 14
`
`US 7,702,028 B2
`
`20
`
`40
`
`60 .
`
`time
`
`80
`
`100
`
`120
`
`|<——->|
`channel size
`
`FIG.3
`
`(PRIOR ART)
`
`5
`
`
`
`U.S. Patent
`
`Apr. 20, 2010
`
`Sheet 4 of 14
`
`US 7,702,028 B2
`
`FIG.4
`
`(PRIOR ART)
`
`6
`
`
`
`U.S. Patent
`
`Apr. 20, 2010
`
`Sheet 5 of 14
`
`US 7,702,028 B2
`
`20
`
`
`
`
`RECEIVER
`
`S
`
`30
`
`FIGS
`
` TRANSMITTER
`
`10
`
`7
`
`
`
`U.S. Patent
`
`Apr. 20, 2010
`
`Sheet 6 of 14
`
`US 7,702,028 B2
`
`\
`
`H2
`
`\
`
`120
`
`FIG.6
`
`8
`
`
`
`U.S. Patent
`
`Apr. 20, 2010
`
`Sheet 7 of 14
`
`US 7,702,028 B2
`
`NOE
`
`9
`
`
`
`U.S. Patent
`
`Apr. 20, 2010
`
`Sheet 8 of 14
`
`US 7,702,028 B2
`
`time
`
`Antenna 13
`
`SH 364]
`
`SH354]
`
`Antenna 2:
`
`S[33I64] SHI32] S[33I64] S[1:32]
`
`SDBCB
`
`FIG.8
`
`10
`
`10
`
`
`
`U.S. Patent
`
`Apr. 20, 2010
`
`Sheet 9 of 14
`
`US 7,702,028 B2
`
`8
`
`am8
`
`8
`
`N
`
`tfim_
`
`tflm
`
`ox
`
`E
`
`11
`
`QUE
`
`11
`
`
`
`U.S. Patent
`
`Apr. 20, 2010
`
`Sheet 10 of 14
`
`US 7,702,028 B2
`
`Coarse Frame Sync
`
`SN R=5dB
`
`Trms=50ns
`
`Ts=25ns
`>
`Freq. offse=0.000044f
`
`SNR=5dB
`
`Trms=50ns
`
`Ts=25ns
`Freq. offse=0.000044 T
`
`FIG.1O
`
`12
`
`12
`
`
`
`U.S. Patent
`
`Apr. 20, 2010
`
`Sheet 11 of 14
`
`US 7,702,028 B2
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`Apr. 20, 2010
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`Sheet 13 of 14
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`US 7,702,028 B2
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`Apr. 20, 2010
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`Sheet 14 of 14
`
`US 7,702,028 B2
`
`FIG.14
`
`16
`
`16
`
`
`
`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 ofTransmitting Preamble for
`Synchronization in a MIMO-OFDM Communication Sys-
`tem” filed in the Korean Intellectual Property Office 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-charmels 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 prefix 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
`configured as to have a relatively low complexity and offer a
`maximum performance in the synchronization and estima-
`tion process.
`An ideal preamble configuration satisfies the following
`requirements:
`
`5
`
`10
`
`15
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`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 charmel estimation.
`
`A first 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:
`
`k =
`¢()
`
`‘l‘(k) =
`
`N—l
`1
`k = 0
`*
`.
`n
`_
`n:05n5q,(+I<)N {O /(#0
`N—l
`n:0
`
`5*,” -sq/7(n+,()N = 0 for all k, £1 i £1’
`
`(1)
`
`(2)
`
`where superscript * denotes a conjugate operator, N denotes
`the length of sequences, q and q' denote indexes of transmit
`antennas, and sM denotes an nth data symbol in a sequence of
`length N transmitted from a qth transmit antenna. A sequence
`that satisfies 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 first preamble transmitting/receiving scheme, a pre-
`amble sequence is designed for frame synchronization by
`copying a predetermined orthogonal sequence designated for
`a first antenna to be used for the other antennas, and is repre-
`sented by
`
`sqy,,:s,, for all q
`
`(3)
`
`A distinctive shortcoming ofthe 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
`
`rj-[n, k] = Z I-I,-J-[n, k]S[n, k] + nj-[n, k]
`
`(4)
`
`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 charmel 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]. 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.
`
`17
`
`17
`
`
`
`US 7,702,028 B2
`
`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-don1ain 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
`
`p2—1
`1
`>I<
`<I>(k) = Z sq,”-sq/W1, 2 5 F for all k, L] ¢ q
`n:0
`”
`
`/
`
`(5)
`
`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 ofthe 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 first 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 first preamble transmitting/
`receiving scheme for MIMO-OFDM charmel estimation.
`Referring to FIG. 2, given Q transmit antennas, a first
`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 thc prcvious antenna a prcdctcrmincd numbcr 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-charmel. Floor( ) is a function of obtaining an integer
`and T is the period of the preamble sequence. T is the product
`ofthe 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
`
`rj-[n, k] = Z H1-J-[n, k]S[n, /c]wf,"° + nJ-[n, k]
`
`(6)
`
`where WN represents an N—point fast Fourier transform
`(FFT). Ifp [n, k]:r[n, k] *S* [n, k], Equation (6) is expressed as
`
`P]-[n, k] = Z H1-J-[n, /c]wg”° + nJ-[n, k] - S* [n, k]
`
`(7)
`
`FIG. 3 illustrates an example of the time—domain channel
`response characteristics of Pj[n, k]. Referring to FIG. 3, ho]. is
`a channel response characteristic from a first transmit antenna
`to a receiver, h1j is a channel response characteristic from a
`second transmit antenna to the receiver, hzj is a channel
`response characteristic from a third transmit antenna to the
`receiver, and h3j is a charmel response characteristic from a
`fourth transmit antenna to the receiver. Preamble sequences
`transmitted from the transmit antennas experience charmels
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`n
`MSE = 3 -0'2
`N
`
`(8)
`
`wherein, on-on indicates a noise power.
`In accordance with the first 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 sufiices. 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
`
`[S1
`S2]
`_S; S:
`
`(9)
`
`The above matrix means that symbols S1 and S2 are
`sequentially transmitted from a first transmit antenna and
`symbols —S2* and S1* are sequentially transmitted from a
`second transmit antenna.
`
`For a 4x4 system, a preamble sequence can be formed by
`
`S1
`S1
`S1
`S1
`S3
`S1 —S4
`—S2
`S4
`S1 —S2
`—S3
`—S4 —S3
`S2
`S1
`
`(10)
`
`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 charmel response character-
`istics h11 to hQL. L OFDM demodulators collect signals R1 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
`
`18
`
`18
`
`
`
`US 7,702,028 B2
`
`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 object of the present invention is o 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 prefix (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 k§M, the pre-
`amble sequence transmitted for the kth antenna is S(t—(k—1)
`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.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`The above and other objects, features and advantages ofthe
`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-
`111itti11g/receivi11g scheme for MIMO-OFDM channel estima-
`tion;
`FIG. 5 is a simplified 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;
`
`10
`
`15
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`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 charmel estimation gain with
`respect to MSE in a multi-charmel 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 first be described below.
`FIG. 5 is a simplified block diagram of a typical MIMO
`system. Referring to FIG. 5, Q><L sub-charmels 30 are defined
`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 hq, 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 ofmodulation 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-transforrn (FfFT) 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 frarne-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 charmel 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-
`
`19
`
`19
`
`
`
`US 7,702,028 B2
`
`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
`
`1,1,1,1,1,j,-1,-j,1,-1,1,-1,1,-j,-1,j
`
`(13)
`
`By inserting three zeroes between every adjacent pair of
`elements in the base CAZAC sequence,
`the following
`sequence is generated
`
`(14)
`
`1 -
`
`1, 0, 0, 0, -j, 0, 0, 0, 1, 0, 0, 0, -1, 0, 0, 0,
`,0, 0, -1, 0, 0, 0, 1, 0, 0, 0, -j, 0, 0, 0, -1, 0,
`J
`
`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.
`
`TABLE 1
`
`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 MIMO-OFDM system having the above configura-
`tion, a preamble sequence consists of special symbols gener-
`ated by an OFDM modulator and attached to an OFDM frame
`to i11dicate 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/charmel 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 prefix (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 fine 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
`
`10
`
`15
`
`25
`
`30
`
`¢n=
`
`G—l
`k:0
`Z (rEn+k "'j,n+k+N)
`
`2
`
`35
`
`(11)
`
`CPO
`CP1
`
`S54[1:64]
`S64[33:64]
`s54[1:32]
`
`S54[1:64]
`S64[33:64]
`s54[1:32]
`
`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 <1)”.
`The coarse frame synchronization reduces the range of fine
`frame synchronization. The computation range of Equation
`(12) is narrow compared to that of Equation (2), in calculating
`the cross-correlation property for the fine frame synchroni-
`zation
`
`2L
`Ho
`
`n
`
`¢<k> =
`
`s;,..
`
`-sq/Y(n+,(,N = 0 for all k E Kcmh, £1 ¢ £1’
`
`(12)
`
`where sq, M denotes an nth data symbol in a sequence trans-
`mitted from a qth transmit antenna and Kmtch denotes the
`range of the fine frame synchronization. Thus, the frame start
`point is a time index k that makes the fine frame synchroni-
`zation metric (21 (k) zero.
`An embodiment ofa preamble sequence structure design in
`a multi-charmel 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
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`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 first antenna (antenna 1) transmits a
`sequence of 64 bits, S[1:64] for a first 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 first 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 fine frame synchronization by
`
`Q
`n _
`,1) _ Z I¢q,.,|2
`(P,’,)2
`q:l
`N—l
`» Ho
`
`where gbqvn =
`
`(sgvk -rj-,,,+k)
`
`(15)
`
`20
`
`20
`
`
`
`US 7,702,028 B2
`
`9
`
`-continued
`
`lrj-7,11,. |2 = constant
`
`P’:rt
`
`2
`Ho
`
`k
`
`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>,,: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 suffice.
`With the sequence rotation of the present invention, a
`received signal is determined by
`
`V,-(k):Ho,-(k)S(k)+H1,(k)'(-1)kS(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 transmissionperiods to allow more stable frame synchro-
`nization and more accurate channel estimation.
`
`FIG. 11 illustrates an embodiment of a preamble structure
`for a 4x4 MIMO system according to the present invention.
`Referring to FIG. 11, for a first transmission period, a first
`antenna (antenna 0) transmits an extended CAZAC sequence
`S(t) and a second antenna (antenna 1) transmits a T/4-symbol
`rotated version of S(t), S(t—T/4). T denotes the period of the
`sequence. In the same manner, third and fourth antennas
`(antenna 2 and antenna 3) transmit T/2-symbol and 3T/4-
`symbol rotated versions of S(t), S(t—T/2) and S(t—3T/4),
`respectively. Each transmit antenna repeatedly transmits the
`same sequence for two or more transmission periods.
`In general, a preamble structure for Q transmit antennas is
`given as illustrated in Table 2. In Table 2, PS denotes the index
`of a transmission period for the preamble sequence.
`
`TABLE 2
`
`PS
`
`1
`
`Antenna 1
`Antenna 2
`
`S(t)
`S(t — T/Q)
`
`2
`
`S(t)
`S(t — T/Q)
`
`Antenna k
`
`skt — (k — 1)T/Q)
`
`skt — (k — 1)T/Q)
`
`Antenna Q
`
`s(t — (Q — 1)T/Q)
`
`s(t — (Q — 1)T/Q)
`
`Meanwhile, if Q is greater than a predetermined number
`M, an (M+1)th to the last antenna cyclically tran