`Hou et al.
`
`TRANSMISSION AND RECEPTION OF
`REFERENCE PREAMBLE SIGNALS IN
`OFDlVlA OR OFDM COMMUNICATION
`SYSTEMS
`
`Inventors: Jason I-Inn, Carlsbad, CA (US); Jing
`Wang, San Diego, CA (US); Sean Cai,
`San Diego, CA (US); Dazi Feng, San
`Diego, CA (US); Yonggang Fang, San
`Diego, CA (US); Yunsong Yang, San
`Diego, CA (US)
`
`Assignee: ZTF. (USA) Inc., Iselin, NJ (US)
`
`Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 1121 days.
`
`Appl. No.2 11/192,420
`
`Filed:
`
`Jul. 27, 2005
`
`Prior Publication Data
`
`US 2006/0050799 A1
`
`Mar. 9, 2006
`
`Related U.S. Application Data
`
`Provisional application No. 60/591,894, filed on Jul.
`27, 2004.
`
`Int. Cl.
`(2006.01)
`H04J 11/00
`U.S. Cl.
`....... .. 370/210; 370/208; 370/335; 375/364
`Field of Classification Search ................ .. 370/208,
`370/210, 335, 342, 441, 349, 385; 375/130,
`375/140, 146, 362—367; 708/40(L405
`See application file for complete search history.
`
`References Cited
`
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`9/1995 Mueller
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`US008116195B2
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`(10) Patent No.:
`(45) Date of Patent:
`
`US 8,116,195 B2
`Feb. 14, 2012
`
`6,438,183
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`Stnihsakcr ct a1.
`7/2002
`A1"‘
`. 375/329
`A1"‘ 10/2002
`Karaoguz
`. 370/208
`..
`llosur et al.
`Al*
`8/2003
`. 375/141
`A1"‘
`8/2004 Miyazalci etal
`A1*
`9/2004 Kimetal.
`................... .. 370/349
`
`(Continued)
`1"O1{1J1GN 1’A1'lJN'1" D()CUMlJN'1'S
`8-228188
`9/1996
`
`(Continued)
`
`OTHER PUBLICATIONS
`
`“Air Interface for rixed Broadband Wireless Access Systems,” 1l:l:'l:'
`Standard for Local and Metropolitan Area NetWorks—1’a1t 16, 1131111’
`Std 802.16/2004, pp. 551-568.
`
`(Continued)
`
`Primary Examiner — Aung S Moc
`Assistant Emminer — Awet 1-laile
`
`(74) Atmrnqx, Agent, or Firm — Perkins Coie LLP
`
`(57)
`
`ABSTRACT
`
`Techniques for generating preamble sequences for OFDM
`and OFDMA communication systems based on CAZAC
`sequences with desired properties of constant amplitudes
`(CA) and zero autocorrelation (ZAC). Such preamble
`sequences may be used for synchronization and identification
`of individual transmitters. For example, the OFDMA symbol
`is constructed using a CAZAC sequence in the freque11cy-
`domain and the resulting time-domain Wavefonn is a near-
`CAZAC sequence.
`
`20 Claims, 7 Drawing Sheets
`
`ZTE/SAMSUNG/HTC 1011-0001
`
`
`
`US 8,116,195 B2
`Page 2
`
`U.S. PATENT DOCUMENTS
`2005/0002361 A1*
`1/2005 Dick et al.
`..
`.
`2005/0036541 A1 *
`2/2005 McKoWn .
`2006/0050624 A1 *
`3/2006 Akita
`2008/0107211 A1"‘
`5/2008 Min et al.
`
`370/335
`.. 375/233
`.. 370/208
`.. 375/326
`
`FOREIGN PATENT DOCUMENTS
`9—502318
`3/1997
`11-308194
`11/1999
`2000-503494
`3/2000
`2003-283455
`10/2003
`2004-253899
`9/2004
`W'O 97/26742
`7/1997
`WO 2006/015108
`2/2006
`WO 2006/129166
`12/2006
`
`JP
`JP
`J1’
`JP
`JP
`\VO
`VVO
`VVO
`
`OTHER PUBLICATIONS
`“Part 16: Air Interface for Fixed and Mobile Broadband Wireless
`Access Systems Amendment for Physical and Medium Access Con-
`trol Layers for Combined Fixed and Mobile Operation in Licensed
`Bands,” Draft Amendment to IEEE Standard for Local and Metro-
`politan Area Networks, p802.16e/D3, May 31, 2004, 161 pages.
`
`“Air Interface for Fixed Broadband Wireless Access Systems,” Local
`and Metropolitan Area Networks —Part 16, IEEE P802.16—REVd/
`D5, May 2004, pp. 379-396.
`‘‘'11’ for Downlink Sychronization Channel Schemes for E-U'1'RA”’
`3GPP TSG—RAN1 WG1 #42bis, R1—051072. Oct. 10-14. 2005, 13
`pages.
`European Examiner Paul Scriven. Supplementary European Search
`Report dated Jun. 15, 2009 for European Patent Application No. EP
`057816795 (6 pages).
`State Intellectual Property Office of China, Oflice Action, dated Apr.
`24, 2009 for Chinese Patent Application No. 2005800275431 (5
`pages).
`English language translation of State Intellectual Property Officc of
`China, O1Iice Action, dated Apr. 24, 2009 for Chinese PatentAppli-
`cation No. 2005800275431 (2 pages).
`Japanese Intellectual Property Office, Office Action in Japanese
`Patent App. No. 2007-523803., mailed Dec. 1, 2009, 3 pages.
`English language translation ofJapanese Intellectual Property Office,
`OfficeAction in Japanese Patent App. No. 2007-523803, mailed Dec.
`1, 2009, 3 pages.
`
`* cited by examiner
`
`ZTE/SAMSUNG/HTC 1011-0002
`
`
`
`U.S. Patent
`
`21024a1b.eF
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`Sheet 1 of 7
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`US 8,116,195 B2
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`ZTE/SAMSUNG/HTC 1011-0003
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`
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`U.S. Patent
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`Feb. 14, 2012
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`7f024|.8BhS
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`US 8,116,195 B2
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`ZTE/SAMSUNG/HTC 1011-0004
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`
`U.S. Patent
`
`Feb. 14, 2012
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`Sheet 3 of7
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`US 8,116,195 B2
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`ZTE/SAMSUNG/HTC 1011-0005
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`U.S. Patent
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`21024a1b.eF
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`Sheet 4 of 7
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`US 8,116,195 B2
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`ZTE/SAMSUNG/HTC 1011-0006
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`Feb. 14, 2012
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`
`1
`TRANSNIISSION AND RECEPTION OF
`REFERENCE PREAMBLE SIGNALS IN
`OFDMA OR OFDM COMMUNICATION
`SYSTEVIS
`
`CROSS REFERENCE TO RELATED
`APPLICATION
`
`This application claims the aenefit of provisional U.S.
`application Ser. No. 60/591,894, entitled “METHOD FOR
`THE TRANSMISSION AND {ECEPTION OF REFER-
`ENCE PREAMBLE SIGNALS IN AN OFDMA SYSTE "
`and filed Jul. 27, 2004. which is incorporated herein by ref-
`erence in its entirety for all purposes.
`
`BACKGROUND
`
`1
`
`This application relates to orthogonal frequency division
`multiplexing (OFDM) and orthogonal frequency division
`multiple access (OFDMA) communication systems, and
`more particularly to generation and transmission ofpreamble
`signals for fast cell searching, time-synchronization, and cor-
`recting initial frequency offset in an OFDM or OFDMA com-
`munication system.
`OFDM and OFDMA systems may be used in various tele-
`communication systems, including wired and wireless com-
`munication systems, to provide various types of communica-
`tion services,
`such as voice and data. A wireless
`communication system covers a certain geographic area by
`dividing the area into a plurality of cells, which can be further
`divided into two or more sectors. The base stations, which
`conceptually locate at the center of respective cells of their
`coverage, transmit infomiation to the mobile subscriber sta-
`tions (MSS) via downlink (DL) radio signals. A mobile sta-
`tion is also known as the mobile station (MS), the subscriber
`station (SS), or thc wirclcss station. Thc mobile stations trans-
`mit information to their serving base stations via uplink HIL)
`radio signals.
`The downlink radio signals from the base stations to
`mobile stations may include voice or data traffic signals or
`both. In addition, the base stations generally need to transmit
`preamble signals in their downlink radio signals to identify to
`the r11obile stations the corresponding cells and correspond-
`ing segments in the cells to which the downlink radio signals
`are directed. Such a prcamblc signal from a base station
`allows a mobile station to synchronize its receiver in both
`time and frequency with the observed downlink signal and to
`acquire the identity, such as IDcell and Segment, of the base
`station that transmits the downlink signal.
`I E E 4 802.16 OFDMA has been developed to provide wire— I
`less communications based on an orthogonal frequency divi-
`sion multiple access (OFDMA) modulation technique. In the
`DL preambles currently defined in IEEE 802.1 6 (:)F[)MA, the
`MSSs storc prcdcfincd and handcrafted pscudo-noisc (PN)
`like sequences for identifying IDcell numbers and segment
`numbers of the adjacent cells. In operation, a MSS captures
`the preamble symbols in received downlink signals and cor-
`rclatc thc prcamblc in cach rcccivcd downlink signal with the
`stored pseudo—noise (PN) like sequences to determine IDcell
`and Segment of a specific sector for that received downlink
`signal. These prea111ble sequences are handcrafted in advance
`and are processed by the MSS one at a time. There are more
`than 100 such sequences in some implementations of the
`current IEEE 802.16 OFDMA. Performing the cross—corre—
`lation with such a large number ofpreamble sequences can be
`time constuning and increase the hardware costs. In addition,
`
`2
`MSSs store the cntirc sct of prcamblc scqucnccs and this
`storage further increases the hardware cost.
`One important perfomiance parameter of the preambles is
`the peak—to—average—power—ratio (PAPR). To reduce the sys-
`tem cost, the PAPR for the preamble should be as small as
`possible. It is well known that OFDM usually has a relative
`higher PAPR ratio than other modulations. This is especially
`important for a preamble because the preamble is transmitted
`in every frame.
`
`SUMMARY
`
`This application providcs, among others, tcchniqucs for
`gcncrating prcamblc scqucnccs for OFDM and OFDMA
`communication systems based on CAZAC sequences with
`desired properties ofconstant amplitudes (CA) and zero auto-
`correlation (ZAC).
`In one implementation, a method for connnunications
`» based on OFDM or OFDMA is described to include selecting
`an initial CAZAC sequence; modifying the initial CAZAC
`sequence to generate a modified sequence which has fre-
`quency guard bands; and using the modified sequence as part
`of a preamble of a downlink signal from a base station to a
`mobile station.
`In another in1plen1e11tatio11, a method for conmiunications
`based on OFDM or (:)F[)MA is described to include selecting
`a CAZAC sequence ofa length L in frequency which includes
`spectral components in first, second and third sequential por-
`tions ir1 frequency, and modifying the CAZAC sequence to
`produce a first modified sequence. The modification includes
`setting amplitudes of spectral components in the first portion
`ofthe CAZAC sequence to zeros and adding a first phase shift
`on spectral components of the second portion of the CAZAC
`sequence, without changing the third portion. The CAZAC
`sequence is then modified to produce a second modified
`scqucncc by sctting amplitudes of spectral components in the
`third portion of the CAZAC sequence to zeros and adding a
`second phase shift spectral components of the second portion
`of the CAZAC sequence, without changing the first portion.
`The first and second modified sequences are then combined to
`form a combined sequence in frequency of a length 2L. The
`first portion from the first modified sequence is positioned
`next to the third portion fror11 tl1e second modified sequence in
`the combined sequence. An inverse fa st Fourier transform is
`thcn pcrformcd on thc combincd scqucncc to gcncratc a first
`preamble sequence in time for OFDM and OFDMA commu-
`nication.
`In another implementation, a method for communications
`based on OFDM or OFDMA is disclosed to include sub
`sampling a preamble signal in a downlink signal received at a
`mobile station receiver to create a frequency overlap and to
`minimize a variation ir1 amplitude, extracting an order of
`signal components in the preamble signal to identify at least
`a base station at which thc downlink signal is gcncratcd. Thc
`preamble signal
`is generated from an initial CAZAC
`sequence to preserve properties of the initial CAZAC
`sequence and has frequency guard bands.
`In somc applications, the tcclmiqucs dcscribcd hcrc may be
`used to provide the downlink (DL) preamble design to allow
`for a structural generation ofpreamble sequences to facilitate
`fast cell searching, si111ple time-synchronization and correc-
`tion of initial frequency offset. The new DI, preamble design
`is based on CAZAC sequences. The IDcell and Segment
`parameters are encoded as the code phase of the CAZAC
`sequence ir1 the frequency domain or the code phase of the
`near-CAZAC sequence in the time domain.
`
`,
`
`ZTE/SAMSUNG/HTC 1011-0010
`
`
`
`3
`These and other implementations and thcir variations,
`enhancements are described in greater detail in the attached
`drawings, the detailed description and the claims.
`
`4
`whcrc ck is a standard basis vcctor of lcngth L. For cxamplc,
`ek can be an all zero vector except the k—th element of unity.
`Define the circulant matrix C of the CAZAC sequence as
`
`US 8,116,195 B2
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1A illustrates the processing steps of one exemplary
`method of construction of a preamble sequence.
`FIG. 1B illustrates the resulting sequence of each process-
`ing step oftl1e exemplary method shown ir1 FIG. 1A.
`FIG. 2A shows an example of using the cyclic shift of
`initial CAZAC sequence in the frequency domain to generate
`two new initial CAZAC scqucnccs in thc frequency domain.
`FIG. 2B shows an example of using the cyclic shift of the
`preamble sequence in the time domain to generate two new
`preamble sequences in the time domain.
`FIG. 3 shows an example of a 3-ticr ccll dcsign uscd in
`various OFDM or OFDMA systems.
`FIG. 4 shows an example of the subcarrier allocations in
`the frequency domain ofthe preamble sequence in segment 0.
`FIG. 5 shows the corresponding amplitude of the wave-
`form in the time domain that corresponds to thc cxamplc in
`FIG. 4.
`FIG. 6 shows the time waveform of the result of matched '
`filtering of the CAZAC sequence (spaced by symbols) with-
`out charmel distortion.
`
`FIG. 7 shows the result of matched filtering ofthe CA7.AC
`sequence in a mu tipath fading environment.
`
`DETAILED DESCRIPTION
`
`Designing a set of preambles with good correlation prop-
`erty and low PAPR is a difficult task because these two
`requirements tenc to be conflicting. A family of mathe1nati—
`cally well studied sequences known as CAZAC sequences
`l1as the desired properties of a constant amplitude (CA) (rep-
`resenting the lowest PAPR) and zero autocorrelation (ZAC).
`Wcll-known cxarnplcs ofCAZAC scqucnccs includc Chu and
`Frank—Zadoff sequences.
`A Chu sequence is defined as
`
`C(H):eXP(/9ph,.(7Ws 71:0, 1.. -
`
`- ,L-1
`
`where the phase in Chu sequences is
`
`ITIIZ
`Ochu 1”) 2 T
`
`and L is the length of the sequence and can be any positive
`integer. The Frank-Zadoff sequences are also defined ir1 (1)
`but the phase is defined as
`
`9}-m,,k(IL : 17 +
`
`Z7rpq
`x/I ,
`
`:
`
`. ,\/L—l, andL is thc
`.
`. ,\/L—l and q:0, 1, .
`.
`.
`whcrc p:0, 1,
`length of the sequence and can be the square of any positive
`integer.
`, cO]T be a CAZAC sequence and
`.
`.
`.
`Let c”[cL_1, cL_2,
`define the cyclic shift operator matrix M as
`
`—M’[“1 52---"'1_—19ol>
`
`C : CirL'{L'}
`
`:[c MC
`ML'lc]
`FL 1
`171
`470
`CL-2
`C0
`CL-1
`= CL-3 CL-2 CL-1
`
`_
`
`a)(I.—l )(I.—l) LXL
`
`where u):exp(—j 2511/L). It can be shown that a circulant matrix
`ca11 be uniquely expressed as
`C:FLHACFL>
`
`(7)
`
`. gL_1} is the eigen matrix of the
`.
`0 where AC:diag{gU, gl, .
`circulant matrix and (C)H denote the Hermitian transpose.
`A 7ero-autocorrelation sequence is characterized by its
`identity autocorrelation matrix, or
`<DC:CCH:1” :FLHACACHFL.
`
`(8)
`
`Equation (8) can be used to derive the following:
`’\cAcH:di3gilg0l2: grlzr -
`-
`-
`: lgL-ll2}:FLFLH:ILxL
`
`(9)
`
`40
`
`In other words, eigenvalues ofa circulant matrix have equal
`amplitudes, or I gkl:const, k:0, .
`.
`. L—1. Furthermore, these
`eigenvalues constitute tl1e frequency spectral components of
`the 7.AC sequence as is evident in the following equation,
`
`1
`c : C50 : Ff’/\gFLeo : —Ffg.
`«/I
`
`'10
`l
`
`)
`
`where em is the last colun1n vector of M, defined in Equation
`(4), and g:[gO, gl, .
`.
`.
`, gL_1]Tis the column vector formed by
`thc cigcnvalucs of C.
`Property 1: If c is a CAZAC sequence, then its frequency
`domain spectral components also form a CAZAC sequence
`(necessary condition).
`Proof:
`Let AM be the eigen matrix of the cyclic shift operator
`matrix M defined in Equation (4). It can be proved that
`AM—diag{1, ou, U32,
`.
`.
`.
`, u)L‘1}, uJ—e‘72“’z. Because M is a real
`matrix, the following expression ca11 be obtained:
`M=FLHAMFL:F,_AMHFLH.
`
`(11)
`
`50
`
`60
`
`For k:(), .
`
`.
`
`. L—1, the following can be written:
`
`gH (M*' g) = LCH Ff’ M‘ F,_c
`
`= 11:” (A’,‘,, )H r?
`
`ZTE/SAMSUNG/HTC 1011-0011
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`US 8,116,195 B2
`
`5
`—continued
`
`L—l
`
`n:u
`: L2 nFk"|L'(rI.)l2
`= L<5(/t),
`
`Therefore, the column vec tor g is a ZAC sequence. The eigen-
`values of tl1e cireula11t matrix C of a CAZAC sequence l1ave
`equal amplitudes. Vv’ith Equation (12) it is proven that the
`g”[g0, gl, .
`.
`.
`, gL_1]T sequence is a CAZAC sequence.
`Property 2: Ifg:[g0, gl, .
`.
`.
`, gL,1]Tis a CAZAC sequence
`in the frequency domain, then its corresponding time-domain
`sequence is also a CAZAC sequence (sufficient condition).
`Proof:
`Equations (10) and (11) can be used to derive the follow-
`1ng:
`
`‘
`
`This shows that the time—domain sequence possesses ZAC
`property.
`From Equation (10), g can be written as
`
`g:\/TFLC
`
`(14)
`
`Because g is a CAZAC sequence, the following can be
`derived:
`
`a(/<3 =g“M*g
`
`: 1;” FE FL(AE )" Ff FLC
`L—1
`
`n:U
`=L:1|c,._|2ur"",k = 0, 1,
`
`,L— 1.
`
`Rewriting Equation (15) in matrix form yields the follow-
`
`_ w—(L—1t(L—1)
`
`lCI.—Il2
`
`Solving Equation (16) leads to the following:
`
`1
`|q|2: Z,/«:0, 1,... ,L—1.
`
`Therefore, the corresponding sequence in the time domain is
`also a CAZAC sequence.
`From Property 1 and Property 2, the desired properties of 65
`the consta11t-arnplitude and zero -autoeorrelation ofa CAZAC
`sequence are preserved in both time and frequency domain.
`
`6
`Therefore, a CAZAC sequence can be used for time and
`frequency synchronization and charmel estimation by the
`mobile station receiver. However. due to guard bands and
`channel selective filtering in the IEEE 802.16 OFDMA sys-
`tem, a CAZAC sequence may not be directly usedto construct
`a preamble, because such a CAZAC sequence does not have
`proper breaks and voids in frequency to meeting the transmit
`frequency spectrum mask for the guard bands and channel
`selective filtering.
`In several exemplary implementations described below, a
`CAZAC sequence,
`such as
`the Chu or Frar1k—Zadoff
`sequence, can be modified in the frequency domain to gener-
`ate a modified CAZAC sequence in the frequency domain
`that satisfies the IEEE 802.16 transmit frequency spectrum
`mask for the guard bands and charmel selective filtering. The
`modified CAZAC sequence is no longer a mathematically
`perfect CAZAC sequence but is a near-CAZAC sequence
`whose amplitudes are nearly constant and the autocorrelation
`is nearly a delta function. This modified CAZAC sequence is
`transformed into the time domain under an inverse FFT to
`produce the desired preamble sequences for an OFDM or
`OFDMA based communication system. Similarly, a CAZAC
`sequence in the time domain may also be used to produce a
`modified CAZAC sequence in the frequency domain that
`satisfies the IEEE 802.16 transmit frequency spectrum mask
`for the guard bands a11d channel selective filtering.
`FIGS. 1A and 1B illustrate one exemplary method of con-
`struction of a preamble sequence 170 with a length of 2L in
`the time domain from a CAZAC sequence 120 with a length
`of L in the frequency domain. FIG. 1A shows the processing
`steps according to an exemplary operation flow and FIG. 1B
`shows the resulting sequence of each processing step in FIG.
`IA.
`Initially at step 102 in FIG. 1A. a CAZAC sequence of a
`length L is selected as the basis for construction of the pre-
`amble sequence. An example of such a CAZAC sequence 120
`in the frequency domain is shown in FIG. 1B, where the
`sequence 120 is partitioned into a left or first portion C1. a
`center or second portion C2, and a right or third portion C3.
`The sizes of C1, C2 and C3 may vary depending on the
`specific requirements of the left guard band size, the right
`guard band size, and the length L. Next, the CAZAC sequence
`120 in the frequency domain is transformed into a first modi-
`fied CAZAC sequence 130 and a second modified CAZAC
`sequence 140, still in the frequency domain, as shown in FIG.
`1B through the processing steps 104 and 106, respectively.
`The first and second modified CAZAC sequences 130 and
`140 may be carried out in any order or simultaneously.
`As illustrated, the first modified CAZAC sequence 130 is
`the right buffer and is formed by setting the amplitude of each
`component in C3 to zero and by adding a phase shift factor e76
`for each component in C2. The frequency components in the
`left portion C1 are not changed. The second modified
`CAZAC sequence 140 is the left buffer and is formed by
`setting the amplitude of each component in C1 to zero and by
`adding a phase shift factor e‘’‘’ for each component in C2.
`This phase shift is opposite to the phase shift in the first
`modified CAZAC sequence 130. The right portion C3 is not
`changed. These processing steps set the amplitudes of the
`guard bands of the OFDMA spectral components to zeros. In
`FIG. 1A, the Left Buffer is at the left side of the DC compo-
`nent in the frequency spectrmn under the Nyquist sampling
`rate and the Right Buffer is at the right side ofthe DC com-
`ponent. The DC component is the first frequency component
`in the first modified CAZAC sequence and is represented by
`the index “ 1 ” in FIG. 1B. Hence, the name designations do not
`reflect whether they appear on the left or right in FIG. 1B. In
`
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`Step 108, the amplitude ofthe DC component is set to zero, if
`the DC subcarrier is not used, for example, as in the IE4 4
`802.16 OFDMA system.
`Next i11 step 110, the first and second modified CAZAC
`sequences 150 and 140 are joined together in the frequency
`domain to construct a new sequence 1 60 ofa length 2L, where
`the C3 of the first modified CAZAC sequence 150 is con-
`nected to the C1 of the second modified CAZAC sequence
`140 in the frequency domain. In step 112, an inverse FFT is
`then performed on the new sequence 160 in the frequency
`domain to form the near—CAZAC sequence 170 as the pre-
`amble sequence iii the time domain.
`The above process forms one preamble sequence for iden-
`tifying a particular cell sector or segment in a particular cell
`among many segments of adjacent cells within the radio
`ranges of the base stations in these adjacent cells. Different
`preamble sequences for different IDcells and different seg-
`ments may be generated in different ways. As one exemplary
`implementation, a new preamble sequence may be generated
`by first perfomiing a cyclic shift of components of the initial
`CAZAC sequence 120 in the frequency domain to produce a
`new initial CAZAC sequence. FIG. 2A illustrates this cyclic
`shift of the frequency components to generate two new
`CAZAC sequences 210 and 220 from the initial CAZAC
`sequence 120 of L components in the frequency domain.
`Tl1en the two new initial CAZAC sequences 210 and 220 are
`processed according to step 104 to step 112 in FIG. 1A,
`respectively,
`to produce two corresponding near—CAZAC
`sequences in the time domain. Under this approach, a total of
`L different preamble sequences car1 be generated from the
`cyclic shift of the L components.
`FIG. 2B shows another way of generating different pre-
`amble sequences based on a cyclic shift of CAZAC sequence
`components in the time domain. The components ofthe near-
`CAZAC preamble sequence 170 generated from ar1 initial
`CAZAC sequence 120 can be shifted in time to produce
`different near—CAZAC preamble sequences in time. As illus-
`trated. the cyclic shift of preamble sequence 170 is used to
`generate two new preamble sequences 230 and 240. A total of
`2L different preamble sequences can be generated from the
`cyclic shift of the 21, components. These sequences are suf-
`ficient to represent all IDcell and cell sectors/’segments.
`As an example, FIG. 3 shows a 3-tier cell design used in
`various OFDM or OFDMA systems where a base station car1
`reach three layers of cells and each cell may have up to 6 cell
`segments and 6 adjacent cells. Hence, under this specific
`3—tier cell design, the maximum number of cell segments in
`the total of 19 reachable cells from one base station is
`19><6:114. Therefore, a CAZAC sequence of a length of at
`least 1 14 can have sufficient number of sequences for carry ,
`IDcell and segment numbers based on the above described
`implementation.
`For illustration purpose, an exemplary OFDMA system
`with a 1024-FFT (Fast Fourier Transform) size, a left guard
`band of 87 FFT bins, commonly referred to as subcarriers, a
`right guard band of 86 subcarriers, and a configuration of four
`preamble carrier-sets is described here. For those skilled in
`the art, different values for the FFT size, the left and right
`guard band sizes, or the number ofpreamble carrier-sets may
`be used.
`In the case of four-sector configuration in which each cell
`contains four sectors, one way to generate preambles is to
`divide the entire 1024 subcarriers into four equal subset,
`arranged in an interlaced manner. Effectively, there are four
`preamble carrier-sets. The subcarriers are modulated, for
`example, using a level boosted P iase Shift Keying (PSK)
`modulation with a CAZAC sequence cyclically shifted with a
`
`8
`code phase defined by IDcell and Segment, which are the base
`station identity. More specifically, the four preamble carrier-
`sets are defined using the following fonnula:
`
`PreambleCarrierSetm:m+4*k
`
`(18)
`
`where l’reambleCarrierSetm specifies all subcarriers allo-
`cated to the specific preamble, m is the number of the pre-
`amble carrier-set indexed as 0, 1, 2, or 3, and k is a running
`index. Each segment of a cell is assigned one of the four
`possible preamble carrier-sets in this particular example.
`To further illustrate, let the 1024-FFT OFDMA sampling
`rate be 20 MIIz at the Nyquist rate. The basic preamble
`time-domain symbol rate is 10 MI Iz. The frequency—domain
`components are composed of a Chu sequence described in
`Equations (1) and (2) of length 128 that is zero-inserted to
`length 512 by inserting CAZAC symbols one for every four
`frequency bins. In the following, it can be established that a
`time-domain CAZAC sequence at the symbol rate (10 MHz)
`introduces a CAZAC sequence in frequency domain after
`spectrum folding. Its frequency—domain CAZAC sequence
`can be computed using a 512—FFT operation instead of a
`1024—FET operation.
`Let h”[h0, hl, .
`.
`.
`, h2L_l]Tbe a ti1ne—domain waveform of
`length 2L at the Nyquist rate. Its spectral components can be
`computed using Equation (14) as follows:
`
`~
`
`.
`
`gh = V’FF2Lh=
`
`g
`H‘gm;
`
`where FM is the Fourier transform matrix of dimension
`2L><2L and gHL and gHU are lower and upper portions of the
`frequency spectrum. W11en sub sampling (i.e., down sam-
`pling) the waveform at the mobile station receiver at the
`symbol rate which is one halfofthe Nyqui st rate, a spectrum
`folding in the frequency domain is introduced in the sampled
`signal at the mobile station. Let hE:[hO, hz, 114, .
`.
`.
`, h2L_2]Tbe
`the subsampled sequence of the even—numbered samples and
`h0:[h1, h3, h5, .
`.
`. , h2L, 1]Tthe odd—numbered samples. Define
`to be the matrix operation that rearranges matrix columns into
`7 even and odd cohmms:
`
`S:[9o 52- -
`
`- 92L—2E€1 93- -
`
`- e2L-1l-
`
`1:20)
`
`Therefore,
`
`When si111plified, the following can be derived:
`
`’8HL+gHu
`/15 = T1751
`7
`1
`kg = —F['/is
`\/I
`
`/gIlL‘gIIUj
`1‘
`2
`
`1
`Z TFEEHE
`J V;
`_T H
`1
`‘\/ZFLEHD
`
`where gH,,. and gm, are spectral components of the even and
`odd sample sequences, and A;:diag{l, e, E2,
`.
`.
`. eL‘1},
`e:exp(jn:/L).
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`Equations (22) and (23) can be used to dcrivc thc following
`spectrum folding relationships:
`
`3'1-JEUC)
`
`: £’HL(k) 4' .§’HU(/0
`2
`, gHLlkl’gHUl/Cl
`§’Ho(/<) =5’
`
`V
`
`Equations (24) and (25) sum up the spectral folding phe-
`nomenon of the waveform subsampling of the downlink pre-
`amble signal at the mobile station. Hence, the subsampling is
`likcly to introduce frcqucncy folding, or spcctrum aliasing. If
`the sub sampling frequency is sufficiently low when sampling
`a received preamble sequence in time, the spectral compo-
`nents ofthe sampled signal overlap, resulting in the frequency
`folding. In some OFDM/OFDMA applications, this phenom-
`enon is intentionally avoided in order to perfect the signal
`restoration.
`The spectral folding via sub-san1pling at the mobile station
`receiver, however, may be advantageously used as a tech-
`nique to recover the CAZAC property of a unfortunately
`truncated CAZAC sequence due to spectral
`filtering
`described above. This is in part based on the recognition that,
`if the coherent channel bandwidth is much smaller than the
`sub-samplcd signal bandwidth, thcrc is little advcrsc cffcct to
`the preamble signals (not true for voice or data signals, how-
`ever). As an example, a 1/2 sub-sarnpling can be used to
`intentionally create a “folded” or “aliased” spectrum that is
`exactly the CAZAC sequence. By virtue of the ti1ne-fre-
`quency duality property of a CAZAC sequence, the corre-
`sponding sequence in the time-domain is also a CAZAC
`sequence. Although the sub-sar11pled sequences maintain the
`desired CAZAC property, the non-sub-sampled (transmitted)
`scqucnccs do not maintain the CAZAC property. For
`example, the PAPR is about 4.6 dB when the phase rotation
`shown in FIG. 1B is 8:3/3. To achieve lower PAPR, the phase
`6 ca11be adjustedto :rt/4. Althoughthe “folded spectrtun” is no
`longer an exact CAZAC sequence in the frequency domain,
`the resulting time domain waveform has a low PAPR of 3.0
`dB.
`This technique to preserve CAZAC sequence characteris-
`tics of the folded frequency spectrum in both frequency and
`time domains is now further described below.
`Following on the above example, the above described con-
`struction ofthe CAZAC sequence in FIGS. 1A and 1B is used
`to reconstruct the 1024 subcarriers using the 4: 1 zero-inserted
`512-element frequency—domain CAZAC sequence of a 128-
`clcmcnt Chu scqucncc such that, after the spectrum folding 7
`due to the down sampling at the mobile station receiver. the
`folded 512 spectral components form the frequency—domain
`CAZAC sequence of the Chu sequence.
`I,et cc)”, denote the time-domain 5 12-element (7A7.AC
`sequence and its frequency—domain CAZAC sequence be
`denoted as gd“, (512 elements) and expressed as
`
`.7012
`, 127
`am, :1: 0, 1,
`é’ch.(4fl+/0 ={ 0 .
`otherwise
`
`where k denotes the fixed preamble carrier-set. CUM, and gum
`form a time—frequency pair and their relationship is expressed
`as
`
`(‘am
`
`’IFFT512l.gL-)ml-
`
`(27)
`
`10
`In IEEE P802.l 6c/D3, thc 1024-FFT OFDMA has 86
`guard subcarriers on the left—hand side and 87 on the right-
`hand side. The DC (direct current) subcarrier resides on index
`512. The construction procedures ofasse111bling gL a11d gR of
`the left- and right-hand sides 1024-FFT OFDMA preambles
`are
`
`gR(1:86):gC}1ul:1:86)
`
`gR(87 :42 5):e'«"“/—‘gC,,,(s7:425)
`
`gR(426:512):0
`
`g,_(1:86):0
`
`gL(87:425):e’“”gC,m(87:425)
`
`g1_(426:5l2)—gC,m(_'42fi :51 2)
`
`(30)
`
`(31)
`
`(32)
`
`(3 3)
`
`In addition, if the DC component is not used, for example in
`IEEE 802.16 OFDMA system, then
`g;e(_1,)\o
`
`134,)
`
`The final reconstructed 1024-FFT frequency components of
`the preamble symbol is
`q(l:1O24):[gR(1:S12):g,_(1:S12)]
`
`(35)
`
`and its final reconstructed 1024 time-domain preamble
`sequence at Nyquist rate is
`C:IFFT1o24(q)-
`
`(35)
`
`After spectrum folding due to subsampling at symbol rate
`in the time domain, thc rcsulting foldcd frcqucncy spcctral
`components of even—numbered samples are, based on Equa-
`tion (24),
`g(1:512)~gL(l:512)+gR(l:512)
`
`(37)
`
`The overlapped area has the following relationship
`g:j37:425)a<:j¢W3+evW3‘)gC,,,,('87:425‘):gC,,,,('237:425).
`
`('38)
`
`Equations (28)-(33) suggest that the CAZAC property is
`preserved. Note also that overlapped area of odd—numbered
`samples has the following relationship according to Equation
`(25):
`
`g'(87:1E25)-(e"‘”/3—e"l”/3)gC;,,1(87:425)=j\/
`‘/3gC;m(87:425).
`
`(39)
`
`, Therefore, the reconstructed time sequence has the lowest
`PAPR for thc cvcn-numbcrcd sampled sequences and very
`low PAPR for the odd—numbered sampled sequences that only
`slightly deviate from the exact CAZAC sequences due to the
`guard bands requirement. The nominal PAPR of the time-
`domain sub-sampled sequences is less than 3 dB at all differ-
`ent code—phases. The frequency components of the recon-
`structed 1024-FFT in the preamble sequence have constant
`amplitudes ar1d thus may be used to facilitate tl1e channel
`estimation.
`In one implcmcntation, fast ccll scarching can be per-
`formed as follows: "he IDCell and Segment allocation to
`different sector are done via as signing different CAZAC code
`phases of cyclic shif’ of the gym) sequence and forming the
`time-domain sequence in the same manncrs dcscribcd in
`Equations (28)—(36).
`FIG. 4 shows an example of the subcarrier allocations of
`tl1e preamble sequence i11 segment 0.
`FIG. 5 shows the corresponding ampli