(12) United States Patent
`Hou et al.
`
`(10) Patent No.:
`
`(45) Date of Patent:
`
`US 8,116,195 B2
`Feb. 14, 2012
`
`IJS008ll6l95B2
`
`8/2002 Taura et al.
`6,438,183 B1 *
`10/2002 Helard et al.
`6,459,744 B1
`8/2007 Joo ............................. .. 370/203
`7,263,058 B2 *
`6/2008 McKoWn
`.. 375/232
`7,388,910 B2 *
`8/2008 Gore et al.
`.. 375/260
`7,418,046 B2 *
`7/2002 Struhsaker et al.
`.. 455/561
`2002/0086707 A1*
`2002/0159544 A1* 10/2002 Karaoguz ..... ..
`.. 375/329
`2003/0152023 A1*
`8/2003 Hosur et al.
`.. 370/208
`2004/0165650 A1*
`8/2004 Miyazaki et al.
`.. 375/141
`2004/0170157 A1*
`9/2004 Kim et al.
`................... .. 370/349
`
`
`
`................. .. 375/343
`
`(Continued)
`
`JP
`
`FOREIGN PATENT DOCUMENTS
`8-228188
`9/1996
`
`(Continued)
`
`OTHER PUBLICATIONS
`
`“Air Interface for Fixed Broadband Wireless Access Systems,” IEEE
`Standard for Local and Metropolitan Area Networks—Part 16, IEEE
`Std 802.16/2004, pp. 551-568.
`
`(54) TRANSMISSION AND RECEPTION OF
`REFERENCE PREAMBLE SIGNALS IN
`OFDMA OR OFDM COMMUNICATION
`SYSTEMS
`
`(75)
`
`Inventors: Jason Hou, 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)
`
`(73)
`
`Assignee: ZTE (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.
`
`(21)
`
`Appl. No.: 11/192,420
`
`(22)
`
`Filed:
`
`Jul. 27, 2005
`
`(65)
`
`(60)
`
`(51)
`
`(52)
`(58)
`
`(56)
`
`Prior Publication Data
`
`US 2006/0050799 A1
`
`Mar. 9, 2006
`
`Related U.S. Application Data
`
`(Continued)
`
`Primary Examiner — Aung S Moe
`Assistant Examiner — Awet Haile
`
`Provisional application No. 60/591,894, filed o11 Jul.
`27, 2004.
`
`(74) Attorney, Agent, or Firm — Perkins Coie LLP
`
`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
`
`U.S. PATENT DOCUMENTS
`9/ 1995 Mueller
`6/ 1998 Yarnauchi et al.
`5/2001 Klank et al.
`................ .. 375/367
`
`5,450,456 A
`5,761,190 A
`6,226,337 B1*
`
`(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 frequency-
`domain and the resulting time-domain waveform is a near-
`CAZAC sequence.
`
`20 Claims, 7 Drawing Sheets
`
`
`
`APPLE1011
`
`APPLE 1011
`
`1
`
`

`
`US 8,116,195 B2
`Page 2
`
`U.S. PATENT DOCUMENTS
`2005/0002361 A1*
`1/2005 Dick et al.
`.................. .. 370/335
`2005/0036541 A1*
`2/2005 McKown .
`. 375/233
`2006/0050624 A1*
`3/2006 Akita ..... ..
`. 370/208
`
`2008/0107211 A1*
`5/2008 Min et al.
`.................... .. 375/326
`
`JP
`JP
`JP
`JP
`JP
`W0
`W0
`W0
`
`FOREIGN PATENT DOCUMENTS
`9-502318
`3/1997
`11-308194
`11/1999
`2000-503494
`3/2000
`2003-283455
`10/2003
`2004-253899
`9/2004
`WO 97/26742
`7/1997
`WO 2006/015108
`2/2006
`WO 2006/129166
`12/2006
`
`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.
`“TP for Downlink Sychror1ization Channel Schemes for E-UTRA”
`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
`05781679.5 (6 pages).
`State Intellectual Property Office of China, Office Action, dated Apr.
`24, 2009 for Chinese Patent Application No. 2005800275431 (5
`pages).
`English language translation of State Intellectual Property Office of
`China, Office Action, dated Apr. 24, 2009 for Chinese Patent Appli-
`cation No. 200580027543.l (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,
`Office Action in Japanese Patent App. No. 2007-523803, mailed Dec.
`1, 2009, 3 pages.
`
`* cited by examiner
`
`2
`
`

`
`U.S. Patent
`
`Feb. 14, 2012
`
`Sheet 1 017
`
`US 8,116,195 B2
`
`9:
`
`\om_.
`
`3
`
`
`

`
`U.S. Patent
`
`1b.eF
`
`2102
`
`.m2tee
`
`US 8,116,195 B2
`
`mm.0:
`
`<N.0_n_
`
`4,at
`
`
`
`~..\n__uwE_.rEmEmcoaEooBEm0:265.:
`
`SN
`
`>ocm:cm._n_c_mEmcoaEooB«Em2.96EE
`
`wE_.rEmEw:oaEoO
`
`7BEm2.05EooomSN
`
`
`
`>2§a9n_:_mEm:oaEooSEm2.9628%
`
`oN—
`
`EN
`
`EN
`
`4
`
`
`

`
`U.S. Patent
`
`Feb. 14, 2012
`
`Sheet 3 017
`
`US 8,116,195 B2
`
`FIG.3
`
`5
`
`

`
`U.S. Patent
`
`Feb.14,2012
`
`Sheet40f7
`
`US 8,116,195 B2
`
`.O_n_
`
`
`
`90Emonam>o:m:Um._u_
`
`oov
`
`>IseIN Iemeds
`
`6
`
`

`
`U.S. Patent
`
`Feb. 14, 2012
`
`Sheet 5 017
`
`US 8,116,195 B2
`
`m.O_n_
`
`oomw
`
`ooov
`
`oow
`
`oom
`
`OOV
`
`DON
`
`
`
`
`
`AmumcHw_:U>ZVm_QEmw
`
`7
`
`

`
`U.S. Patent
`
`Feb. 14, 2012
`
`Sheet 6 017
`
`US 8,116,195 B2
`
`
`
`
`
`A99.,w_:U>Zvmm_QEmw
`
`8983ex:om?om?29
`
`89
`
`(D
`
`(0
`
`VI’
`
`(D\-
`
`VI‘1-
`
`(V1-
`
`O1-
`
`W\
`
`-
`
`ON
`
`141519113 U°99|9JJ0°'3901O OVZVO
`
`m.O_u_
`
`8
`
`
`

`
`U.S. Patent
`
`2
`
`
`u........................
`
`
`
` .....--3ma.___M.........--__.........-._................1Sm__F1........Im.........I”........................................I9W........--M................--8_M
`
`
`7.........................
`
`
`
`......-1mM__7__W...T.................4.......................................--2«MH_
`.........me
`
`mm_QEmwH,common_.N8Eomom88Ba_.M-.....................-- O1_M........--".........NUu_
`flA99..w_:_o>Zv
`
`
`........V
`
`........0
`
`l415U9-I15 U0!1l3|9JJ00'59°JO Ovzvo
`
`N.O_u_
`
`9
`
`

`
`US 8,116,195 B2
`
`1
`TRANSMISSION AND RECEPTION OF
`REFERENCE PREAMBLE SIGNALS IN
`OFDMA OR OFDM COMMUNICATION
`SYSTEMS
`
`CROSS REFERENCE TO RELATED
`APPLICATION
`
`This application claims the benefit of provisional U.S.
`application Ser. No. 60/591,894, entitled “METHOD FOR
`THE TRANSMISSION AND RECEPTION OF REFER-
`ENCE PREAMBLE SIGNALS IN AN OFDMA SYSTEM”
`
`10
`
`and filed Jul. 27, 2004, which is incorporated herein by ref-
`erence in its entirety for all purposes.
`
`15
`
`BACKGROUND
`
`2
`
`MSSs store the entire set of preamble sequences and this
`storage further increases the hardware cost.
`One important performance 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 provides, among others, techniques for
`generating preamble sequences for OFDM and OFDMA
`communication systems based on CAZAC sequences with
`desired properties of constant amplitudes (CA) and zero auto-
`correlation (ZAC).
`In one implementation, a method for communications
`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 implementation, a method for communications
`based on OFDM or OFDMA 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 in 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
`sequence by setting amplitudes of spectral components in the
`third portion of the CAZAC sequence to zeros and adding a
`second phase shift spectral components ofthe 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 from the second modified sequence in
`the combined sequence. An inverse fast Fourier transform is
`then performed on the combined sequence to generate a first
`preamble sequence in time for OFDM and OFDMA commu-
`nication.
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`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 information 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 the wireless station. The mobile stations trans-
`mit information to their serving base stations via uplink (UL)
`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 mobile stations the corresponding cells and correspond-
`ing segments in the cells to which the downlink radio signals
`are directed. Such a preamble signal from a base station
`allows a mobile station to synchronize its receiver in both
`time and frequency with the ob served downlink signal and to
`acquire the identity, such as IDcell and Segment, of the base
`station that transmits the downlink signal.
`IEEE 802 . 1 6 OFDMA has been developed to provide wire-
`less communications based on an orthogonal frequency divi-
`sion multiple access (OFDMA) modulation technique. In the
`DL preambles currently defined in IEEE 802.16 OFDMA, the
`MSSs store predefined and handcrafted pseudo-noise (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-
`relate the preamble in each received 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 preamble 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 consuming and increase the hardware costs. In addition,
`
`50
`
`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 in amplitude, extracting an order of
`signal components in the preamble signal to identify at least
`a base station at which the downlink signal is generated. The
`preamble signal
`is generated from an initial CAZAC
`sequence to preserve properties of the initial CAZAC
`sequence and has frequency guard bands.
`In some applications, the techniques described here may be
`used to provide the downlink (DL) preamble design to allow
`for a structural generation ofpreamble sequences to facilitate
`fast cell searching, simple time-synchronization and correc-
`tion of initial frequency offset. The new DL preamble design
`is based on CAZAC sequences. The IDcell and Segment
`parameters are encoded as the code phase of the CAZAC
`sequence in the frequency domain or the code phase of the
`near-CAZAC sequence in the time domain.
`
`55
`
`60
`
`65
`
`10
`
`10
`
`

`
`3
`These and other implementations and their variations,
`enhancements are described in greater detail in the attached
`drawings, the detailed description and the claims.
`
`4
`where ek is a standard basis vector of length L. For example,
`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
`
`5
`
`10
`
`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 of the exemplary method shown in 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 sequences in the 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-tier cell design used in
`various OFDM or OFDMA systems.
`FIG. 4 shows an example of the subcarrier allocations in 20
`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 the example in
`FIG. 4.
`FIG. 6 shows the time waveform of the result of matched 25
`
`15
`
`C = Circ{c}
`
`=[c MC
`
`Mblc]
`
`C1
`CH C0
`CH CH C0
`= CH CH CH
`'
`'
`'
`C0
`01
`c2
`
`CH
`CH
`CH
`'
`cL_1
`
`'
`
`1
`
`1
`
`1
`L—1
`
`FL 2 L 1
`‘(I '
`1
`
`w
`L I
`0’ T
`
`in
`L I L 1
`‘"1 T 1 T ’ LXL
`
`(5)
`
`(6)
`
`where u):exp(—j 231/L). It can be shown that a circulant matrix
`can be uniquely expressed as
`
`filtering of the CAZAC sequence (spaced by symbols) with-
`out charmel distortion.
`
`FIG. 7 shows the result of matched filtering of the CAZAC
`sequence in a multipath fading environment.
`
`DETAILED DESCRIPTION
`
`(7)
`C:FLHACFL>
`. gbl} is the eigett matrix of the
`t
`30 Where AC:dia.g{gO, gl, _
`circulant matrix and (O)H denote the Hermitian transpose.
`A zero-autocorrelation sequence is characterized by its
`identity autocorrelation matrix, or
`
`Designing a set of preambles with good correlation prop-
`erty and low PAPR is a difiicult task because these two 35
`requirements tend to be conflicting. A family of mathemati-
`cally well studied sequences known as CAZAC sequences
`has the desired properties of a constant amplitude (CA) (rep-
`resenting the lowest PAPR) and zero autocorrelation (ZAC).
`Well-known examples ofCAZAC sequences include Chu and
`Frank-Zadoff sequences.
`A Chu sequence is defined as
`
`40
`
`¢C:CCH:ILXL:FLHACACHFL_
`.
`t
`t
`Equauen (8) ea“ be used to derwe the feuewmgi
`AC/\CH:djag{lg0l2, g1l2, .
`. ., \gL,1\2}:FLFLH:ILxL
`
`(8)
`
`(9)
`
`In other words, eigenvalues ofa circulant matrix have equal
`amplitudes, or I gk|:const, k:0, .
`.
`. L—1. Furthermore, these
`eigenvalues constitute the frequency spectral components of
`the ZAC sequence as is evident in the following equation,
`
`- ,L-1
`-
`1, -
`C(n):€XP(/9c;.u(n)),n:0,
`.
`.
`where the phase in Chu sequences is
`
`(1)
`
`45
`
`C=Ce0=F£]/\CFL90= ‘F58,
`
`(10)
`
`M2
`0chu(”) = T
`
`(2)
`
`and L is the length of the sequence and can be any positive
`integer. The Frank-Zadoff sequences are also defined in (1)
`but the phase is defined as
`
`t/— 37W‘!
`0f,ank(n=p+q L)=T,
`VI
`
`(3)
`
`, \/L—l, and L is the
`.
`.
`, \/L—1 and q:0, 1, .
`.
`.
`where 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:[9192---9L—19ol>
`
`(4)
`
`where eo is the last column vector of M, defined in Equation
`(4), and g:[gO, gt, .
`.
`.
`, gL_1]Tis the column vector formed by
`50 the eigenvalues 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
`AA/[:diag{1s 00, 002, ~
`~
`~
`, UJL_l}, (n:e_j2"/L. Because M is a real
`.
`.
`.
`.
`.
`matrix, the following expression can be obtained.
`
`55
`
`60
`
`M:FLHAMFL :FLAMHFLH.
`
`For k:0, ,
`
`,
`
`, L—1, the following can be written:
`
`gH(M"g)=1£HFfM"FLc
`:1; (A’‘ ) cM
`
`H
`H
`
`65
`
`11
`
`(1 1)
`
`(13)
`
`11
`
`

`
`US 8,116,195 B2
`
`5
`-continued
`
`L—1
`
`n:0
`= L2 cf""|c(n)|2
`
`= L6(/c),
`
`Therefore, the column vector g is a ZAC sequence. The eigen-
`values of the circula11t matrix C of a CAZAC sequence have
`equal amplitudes. With Equation (12) it is proven that the
`g:[gO, gl, .
`.
`.
`, gL_1]Tsequence is a CAZAC sequence.
`Property 2: Ifg:[gO,g1, .
`.
`.
`, 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-
`ing:
`
`10
`
`15
`
`(13)
`
`20
`
`= 50:)
`
`This shows that the time-domain sequence possesses ZAC
`property.
`From Equation (10), g can be written as
`
`g:‘/ZFLC
`
`(14)
`
`Because g is a CA7.AC sequence, the following can be
`derived:
`
`25
`
`30
`
`35
`
`6(k) = g”M*g
`
`= Le” Ff FL(Az )* Ff no
`L—1
`
`n:0
`= L2 |c,,|2of"", k = 0, 1,
`
`,L— 1.
`
`(15)
`
`40
`
`45
`
`Rewriting Equation (15) in matrix form yields the follow-
`ing:
`
`1
`0
`
`Q
`
`1
`1
`1 w“
`= L .
`.
`
`1
`w““”
`
`|c0|2
`lC1l2
`_
`
`1
`
`w—(L 1)
`
`_ w—(L—1)(L—l)
`
`lcbllz
`
`Solving Equation (16) leads to the following:
`
`1
`
`|ck|2 = Z,/(=0, 1,
`
`,L—1.
`
`50
`
`(16)
`
`55
`
`(17)
`
`60
`
`Therefore, the corresponding sequence in the time domain is
`also a CAZAC sequence.
`From Property 1 and Property 2, the desired properties of
`the constant-amplitude and zero-autocorrelation of a CAZAC
`sequence are preserved in both time and frequency domain.
`
`65
`
`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 used to 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 channel 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 and 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.
`1A.
`
`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‘je 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 spectrum under the Nyquist sampling
`rate and the Right Buffer is at the right side of the 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
`
`12
`
`12
`
`

`
`US 8,116,195 B2
`
`7
`Step 108, the amplitude ofthe DC component is set to zero, if
`the DC subcarrier is not used, for example, as in the IEEE
`802.16 OFDMA system.
`Next in step 110, the first and second modified CAZAC
`sequences 150 and 140 are joined together in the frequency
`domain to construct a new sequence 160 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 in 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 performing 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.
`Then 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 can 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 an 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 2L 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 can
`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 114 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 Phase Shift Keying (PSK)
`modulation with a CAZAC sequence cyclically shifted with a
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`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 formula:
`
`Preamb1eCarrierSetm:m+4*k
`
`(13)
`
`where PreambleCarrierSetm 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 seg111e11t 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 MHz at the Nyquist rate. The basic preamble
`time-domain symbol rate is 10 MHz. 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.
`
`, h2L_1]T be a time-domain waveform of
`.
`.
`Let h:[h0, hl, .
`length 2L at the Nyquist rate. Its spectral components can be
`computed using Equation (14) as follows:
`
`gh = \/iF2Lh = [ gm]
`gHU
`
`(19)
`
`is the Fourier transform matrix of dimension
`where F2,,
`2L><2L and gHL and gHU are lower and upper portions of the
`frequency spectrum. When subsarnpling (i.e., down sam-
`pling) the waveform at the mobile station receiver at the
`symbol rate which is one half of the Nyquist rate, a spectrum
`folding in the frequency domain is introduced in the sampled
`signal at the mobile station. Let hE:[h0, h2, h4, .
`.
`.
`, hz,,_2]T be
`the subsampled sequence of the even-numbered samples and
`hO:[h1, h3, h5, .
`.
`. , h2L_ 1]Tthe odd-numbered samples. Define
`to be the matrix operation that rearranges matrix columns into
`even and odd columns:
`
`S:[e0e2...e2L_25e1e3...e2L_1].
`
`Therefore,
`
`its
`ha
`
`_ S_lh =
`
`1
`V 2L
`
`SAFE“ gHL
`gHU
`
`When simplified, the following can be derived:
`
`1
`+
`1
`hE = EFHEHL 281111) = fiFEgHE
`1
`—
`1
`ha = WF/7/\s(gHL Zglw) = WFITEHO
`
`(20)
`
`(21)
`
`(22)
`
`(23)
`
`where gHE and gHO are spectral components of the even and
`odd sample sequences, and AE:diag{1, e, E2,
`.
`.
`. eL‘1},
`e:exp(ja'c/L).
`
`13
`
`13
`
`

`
`US 8,116,195 B2
`
`9
`Equations (22) and (23) can be used to derive the following
`spectrum folding relationships:
`
`k
`
`k
`
`gm:
`
`k
`/C —
`gH0(k)=5’<( )
`
`24
`
`25
`
`>
`
`)
`
`<
`
`(
`
`Equations (24) and (25) sum up the spectral folding phe-
`nomenon of the waveform sub sampling of the downlink pre-
`amble signal at the mobile station. Hence, the subsampling is
`likely to introduce frequency folding, or spectrum aliasing. If
`the subsampling frequency is sufiiciently 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-sampling 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 chamiel bandwidth is much smaller than the
`
`sub-sampled signal bandwidth, there is little adverse effect to
`the preamble signals (not true for voice or data signals, how-
`cvcr). As an cxamplc, a ‘/2 sub-sampling can bc used to
`intentionally create a “folded” or “aliased” spectrum that is
`exactly the CAZAC sequence. By virtue of the time-fre-
`quency duality property of a CAZAC sequence, the corre-
`sponding sequence in the time-domain is also a CAZAC
`sequence. Although the sub-sampled sequences maintain the
`desired CA7.AC property, the non-sub-sampled (transmitted)
`sequences do not maintain the CAZAC property. For
`example, the PAPR is about 4.6 dB when the phase rotation
`shown in FIG. 1B is 6:75/3. To achieve lower PAPR, the phase
`0 canbe adjusted to at/4.Although the “folded spectrum” 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-
`element Chu sequence such that, after the spectrum folding
`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.
`Let cdm denote the time-domain 512-element CAZAC
`sequence and its frequency-domain CAZAC sequence be
`denoted as gdm (512 elements) and expressed as
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`gchu(4”+k)={e
`
`jig’:
`0,
`
`’n— ’
`
`0 1
`otherwise
`
`177
`
`“,
`
`(26)
`
`60
`
`where k denotes the fixed preamble carrier-set. cdm and gdm
`form a time-frequency pair and their relationship is expressed
`as
`
`65
`
`Cchu:IFFT512(gchul-
`
`(27)
`
`10
`In IEEE P802.16e/D3, the 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 of assembling gL and gR of
`the left- and right-hand sides 1024-FFT OFDMA preambles
`are
`
`gR(1:86):gChu(1:86)
`
`gR(87:425):e’j"/3gC,,u(87:425)
`
`gR(426:512):0
`
`gL(1:86):0
`
`gL(87:425):e7"/3gC),u(87:425)
`
`gL(426:512):gC,m(426:512)
`
`(28)
`
`(29)
`
`(30)
`
`(31)
`
`(32)
`
`(33)
`
`In addition, if the DC component is not used, for example in
`IEEE 802.16 OFDMA system, then
`812(1):‘)
`
`(34)
`
`The final reconstructed 1024-FFT frequency components of
`the preamble symbol is
`111:1:1024):[gR(l:512):gL(1:512)]
`
`(35)
`
`and its final reconstructed 1024 time-domain preamble
`sequence at Nyquist rate is
`C:IFFT1o24(‘I)-
`
`(3 5)
`
`After spectrum folding due to subsampling at symbol rate
`in the time domain, the resulting folded frequency spectral
`components of even-numbered samples are, based on Equa-
`tion (24),
`g(1:512)~gL(1:512)+gR(1:512)
`
`(37)
`
`The overlapped area has the following relationship
`g(87:425)ac(eW3+e"f’“3j)gC,,u(87:425):gC,,,,(87:425).
`
`(3 8)
`
`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 :425)~(e/"/3—e*f"/3:)gC,,,,(87 :425):;V
`‘/3gC,,u(87:425).
`
`(39)
`
`Therefore, the reconstructed time sequence has the lowest
`PAPR for the even-numbered sampled sequences and very
`low PAPR for the odd-numbered sampled sequences that only
`slightly deviat