(19) United States
`(12) Patent Application Publication (10) Pub. No.: US 2007/0270273 A1
`Fukuuaetal
`(m)Pub.Daw:
`:NOV.22,2007
`
`US 20070270273A1
`
`(75)
`
`(54) METHOD AND APPARATUS FOR FAST
`CELL SEARCH
`Inventors:
`Masaya Fukuta, Yokohama-shi
`(JP); Hidenori Akita,
`Higashimurayama-shi (JP); Hiroshi
`Hayashi, Nishitokyo-Shi (JP)
`
`Corres ondence Address:
`P
`MOTOROLA, INC.
`1303 EAST ALGONQUIN ROAD, IL01/3RD
`SCHAUMBURG’ IL 60196
`(73) Assigneez
`MOTOROLA’ INC Schaumburg
`IL
`S
`(U )
`11/333 971
`3
`May 18, 2006
`
`21 A 1. No‘;
`)
`(
`pp
`(22)
`Filed:
`
`Publication Classification
`
`(51)
`
`Int Cl-
`FMH 37/08
`
`(200601)
`
`(52) U.S. Cl.
`
`..................................................... .. 475/206
`
`57
`
`ABSTRACT
`
`,,
`
`“
`.
`.
`Reference sequences are constructed from distinct classes
`of GCL sequences that have an optimal cyclic cross corre-
`lation property. The fast cell search method disclosed detects
`the “class indices” with simple processing. In a system
`deployment that uniquely maps sequences of certain class
`indices along with a circular shift amount in time domain to
`certain cells/cell IDs, the identification of a sequence index,
`and its circular shift will therefore provide an identification
`Of the 0611 ID.
`
`20
`
`Remaining transmission/Data
`
`Reference Sequence
`
`201
`
`202
`
`1
`
`APPLE 1013
`
`APPLE 1013
`
`1
`
`

`
`Patent Application Publication
`
`Nov. 22, 2007 Sheet 1 of 8
`
`US 2007/0270273 A1
`
`Remote Unit P-SCH S-SCH
`
`301
`
`P-SCH : Primary - synchronization channel
`S-SCH : Secondary — synchronization channel
`
`time
`
`FIG. 3
`
`FIG. 2
`E
`
`Reference Sequence
`
`
`
`Remaining transmission/Data
`
`201
`
`202
`
`2
`
`

`
`Patent Application Publication
`
`Nov. 22, 2007 Sheet 2 of 8
`
`US 2007/0270273 A1
`
`Cell-common
`
`Sequence generation
`
`Transmitted
`Signal
`
`
`Sequence index “u”
`Shift amount ”m*Q”
`
`FIG. 4
`E
`
`Received
`SCH
`
`501
`
`P- SCH
`
`
`
`Channel
`response
`
`Sequence
`index “u"
`
`_
`A sequence Index &
`
`E
`
`5
`
`Circular shift index
`detector
`
`109
`
`Identifier
`505
`
`Circular shift
`index “m"
`
`
`
`
`
`Circular
`Shifter
`
`401
`
`Cell-specific
`Sequence generation
`
`
`
`402
`
`3
`
`

`
`Patent Application Publication
`
`Nov. 22, 2007 Sheet 3 of 8
`
`US 2007/0270273 A1
`
`
`
`Select
`
`sequence
`index
`
`
`604
`
`
`
`
`Channel
`Sequence
`response
`nepnca
`
`Generation
`
`Search the
`
`sequence with
`maximum peak
`
`
`
`
`603
`
`Sequenceindex“u”
`
`Circular shift index “m”
`
`4
`
`

`
`Patent Application Publication
`
`Nov. 22, 2007 Sheet 4 of 8
`
`US 2007/0270273 A1
`
`S-SCH
`
`Sequence index
`Detector
`
`Sequence index “u"
`
`Channel
`
`response
`
`Sequence
`replica
`Generation
`
`Circular shift index “m"
`
`
`
`Peak poison
`Search
`
`
`
`
`
`09
`
`Np-points
`
`
`
`
`
`
`EQ gain
`Generation
`
`Np-points
`Multiplication
`
`707
`
`71 1
`
`713
`
`715
`
`FIG. 7
`E
`
`5
`
`

`
`Patent Application Publication
`
`Nov. 22, 2007 Sheet 5 of 8
`
`US 2007/0270273 A1
`
`Received
`SCH
`
`
`
`Sequence
`replica
`Generation
`
`index
`Sequence
`Detector
`
`
`
`Shift index
`
`Detector
`
`_"_fi7— ______ '_
`109
`
`Transmitted
`signa|
`
`905
`
`906
`
`F I G. 9
`E
`
`Cell-specific
`Sequence generation
`/I/
`
`8
`
`801
`
`Sequence index “u”
`
`Shift amount ”m*Q”
`
`FIG. 8
`E
`
`6
`
`

`
`Patent Application Publication
`
`Nov. 22, 2007 Sheet 6 of 8
`
`US 2007/0270273 A1
`
`Generate common sequence
`among cells in frequency domain
`
`/l/1001
`
`l
`Generate cell-specific GCL sequence/V 1003
`in frequency domain
`
`L
`
`Transform common
`sequence and ce||—specific
`.
`.
`sequence into each time
`domain signal separately
`
`/V 1005
`
`a specific index and circular-shift amount
`
` Receive common signal among cells and signal having
`
`e-multiplex common signal among cells and GCL signal/1/1103
`having a specific index and circular—shift amount
`
`
`
`Perform channel estimation by using
`
`common signal among cells
`
`A/ 1105
`
`
` i
`
`Determine sequence index
`using channel estimation result
`(i.e., Detect sequence index coherently)
`
`/l/ 1107
`
`l
`Determine circular-shift amount
`
`using channel estimation result
`(i.e., Detect circular shift amount coherently )
`
`Identify base station based on
`
`Index and circular—shift
`amour“
`
`Circularly shift cell-specific
`time domain signal in time domain
`
`1007
`
`Mu1t1p|eX Common time domain
`signal among cells and
`cell specific circularly-shifted,
`time domain signal
`
`Transmit multiplexed‘
`time domain signal
`
`1009
`
`1011
`
`|=|G_ 10
`
`FIG. 11
`
`7
`
`

`
`Patent Application Publication
`
`Nov. 22, 2007 Sheet 7 of 8
`
`US 2007/0270273 A1
`
` Receive common signal among cells and signal having
`
`a specific index and circular-shift amount
`
`1201
`
`e—multiplex common signal among cells and GCL signal
`having a specific index and circular-shift amount
`
`Determine sequence index
`without using channel estimation result
`(i.e., Detect sequence index non—coherently)
`
`Determine circu|ar—shift amount
`
`using channel estimation result
`(i.e., Detect circular shift amount coherently )
`
`Identify base station based on
`Index and circu|ar—shift
`
`amount
`
`FIG. 12
`
`8
`
`

`
`Patent Application Publication
`
`Nov. 22, 2007 Sheet 8 of 8
`
`US 2007/0270273 A1
`
`enerate ce||—specific GCL sequence
`in frequency domain
`
`1301
`
`Transform cell-specific
`sequence into ce||—specific time
`domain signal
`
`1303
`
`Receive GCL signal having
`3 Specific index and CiFCU'8F-
`shift amount
`
`Determine sequence index
`(i_e,, Detect sequence index
`non-coherently)
`
`n0“‘C0here”t'Y) ransmit time domain-circularly-shifted,
`
`Circularly shift ce||—specific
`time domain signal in time domain
`
`1305
`
`Determine circu|ar—shift amount
`(i.e., Detect circular shift amount
`
`Ce” Specific signai
`
`1307
`
`'d9“tifY base 5t'_a“0n b33<_9d 0"‘
`Index and circu|ar—shift
`amount
`
`F|G- 13
`
`FIG. 14
`
`1401
`
`1403
`
`1405
`
`1407
`
`9
`
`

`
`US 2007/0270273 A1
`
`Nov. 22, 2007
`
`METHOD AND APPARATUS FOR FAST
`CELL SEARCH
`
`FIELD OF THE INVENTION
`
`[0001] The present invention relates generally to fast cell
`search, and in particular to a method and apparatus for fast
`identification of a service cell or sector during initial or
`periodic access, or handover in a mobile communication
`system.
`
`BACKGROUND OF THE INVENTION
`
`the geographical
`In a mobile cellular network,
`[0002]
`coverage area is divided into many cells, each of which is
`served by a base station (BS). Each cell can also be further
`divided into a number of sectors. When a mobile station
`
`(MS) is powered up, it needs to search for a BS to register
`with. Also, when the MS finds out that the signal from the
`current serving cell becomes weak, it should prepare for a
`handover to another cell/sector. Because of this, the MS is
`required to search for a good BS for communication. The
`ability to quickly identify a BS for initial registration or
`handover is important for reducing the processing complex-
`ity and thus lowering the power consumption.
`[0003] The cell search function is often performed based
`on a cell-specific reference signal (or preamble) transmitted
`periodically on a synchronization charmel (SCH). A straight-
`forward method is to perform an exhaustive search by trying
`to detect each reference signal and then determine the best
`BS. There are two important criteria when determining
`reference sequences for cells or sectors. First, the reference
`sequences should allow good channel estimation to all the
`users within its service area, which is often obtained through
`a correlation process with the reference of the desired cell.
`In addition, since a mobile will receive signals sent from
`other sectors or cells, a good cross correlation between
`reference signals is important to minimize the interference
`effect on channel estimation to the desired cell.
`
`the cross-correlation
`like auto-correlation,
`Just
`[0004]
`between two sequences is a sequence itself corresponding to
`different relative shifts. Precisely, the cross-correlation at
`shift-d is defined as the result of summing over all entries
`after an element-wise multiplication between a sequence and
`another sequence that is conjugated and shifted by d entries
`with respect to the first sequence. “Good” cross correlation
`means that the cross correlation values at all shifts are as
`
`even as possible so that after correlating with the desired
`reference sequence, the interference can be evenly distrib-
`uted and thus the desired charmel can be estimated more
`
`reliably. Minimization of the maximal cross-correlation val-
`ues at all shifts, which is reached when they are all equal, is
`refer to as “optimal” cross correlation.
`[0005]
`Prior-art techniques, such as those described in US
`Patent Application Publication No. 2006/0039451 A1,
`(which is incorporated by reference herein) describe the use
`of reference sequences that are constructed from distinct
`“classes” of a Generalized Chirp—Like (GCL) sequence. By
`assigning a base station a particular index of a GCL
`sequence, the identification of a sequence index will there-
`fore provide the identification of the base station.
`[0006] While using GCL sequences does provide for supe-
`rior reference signals, there can only exist Ng—1 sequences
`to utilize in a communication system when the length of the
`GCL sequences being used is Ng. Typical communication
`
`systems are required to provide more than 512 cell identi-
`fications. This
`requirement would require large GCL
`sequences to accommodate 512 unique GCL sequences.
`This would greatly increase system overhead. Therefore, a
`need exists for a method and apparatus for fast cell search in
`a communication system that utilizes GCL sequences, and
`yet has lower overhead for communication systems with
`large numbers of cell identifications.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is a block diagram of a communication
`
`[0007]
`system.
`FIG. 2 illustrates reference signal transmission for
`[0008]
`the communication system of FIG. 1.
`[0009]
`FIG. 3 illustrates a primary synchronization chan-
`nel and a secondary synchronization channel for the com-
`munication system of FIG. 1.
`[0010]
`FIG. 4 is a block diagram of a transmitter trans-
`mitting a primary synchronization charmel and a secondary
`synchronization charmel.
`[0011]
`FIG. 5 is a block diagram of receiver designed to
`identify a sequence index (u) and a circular shift index (m).
`[0012]
`FIG. 6 is a block diagram ofa sequence index (u)
`& a circular shift index (m) detector.
`[0013]
`FIG. 7 is a block diagram ofa sequence index (u)
`& a circular shift index (m) detector.
`[0014]
`FIG. 8 is a block diagram of a transmitter.
`[0015]
`FIG. 9 is a block diagram of a receiver.
`[0016]
`FIG. 10 is a flow chart showing operation of a
`transmitter.
`
`[0017]
`receiver.
`
`[0018]
`receiver.
`
`FIG. 11 is a flow chart showing operation of a
`
`FIG. 12 is a flow chart showing operation of a
`
`FIG. 13 is a flow chart showing operation of a
`[0019]
`transmitter.
`
`[0020]
`receiver.
`
`FIG. 14 is a flow chart showing operation of a
`
`DETAILED DESCRIPTION OF THE DRAWINGS
`
`To address the above-mentioned need, a method
`[0021]
`and apparatus for fast cell search based on a chirp reference
`signal transmission is disclosed herein. In particular, refer-
`ence sequences are constructed from distinct “classes” of
`GCL sequences that have an optimal cyclic cross correlation
`property. The fast cell search method disclosed detects the
`“class indices” with simple processing. In a system deploy-
`ment that uniquely maps sequences of certain class indices
`along with a circular shift amount in time domain to certain
`cells/cell IDs, the identification of a sequence index, and its
`circular shift will therefore provide an identification of the
`cell ID (transmitter).
`[0022] The present invention encompasses a method for
`fast cell search. The method comprises the steps of receiving
`a Generalized Chirp—Like (GCL) sequence from a transmit-
`ter, determining a GCL index from the GCL sequence, and
`determining a circular shift of a GCL sequence. Atransmitter
`identification is then determined based on the GCL index
`
`and the circular shift of the GCL sequence.
`[0023] The present invention additionally encompasses an
`apparatus comprising a receiver receiving a Generalized
`Chirp—Like (GCL) sequence from a transmitter, a sequence
`index and circular shift detector determining a GCL index
`
`10
`
`10
`
`

`
`US 2007/0270273 A1
`
`Nov. 22, 2007
`
`and a circular shift of the GCL sequence, and base identi-
`fication circuitry determining a transmitter identification
`based on the GCL index and the circular shift of the GCL
`S€C]l1€I1C€.
`[0024] The present invention additionally encompasses a
`method comprising the steps of circularly shifting a GCL
`sequence having a specific index and transmitting the cir-
`cularly-shifted GCL sequence with the specific index,
`wherein a unique combination of the index and the circular
`shift uniquely identifies a transmitter.
`[0025] The present invention additionally encompasses an
`apparatus comprising a circular shifter circularly shifting a
`GCL sequence having a specific index, and a transmitter
`transmitting the circularly-shifted GCL sequence with the
`specific index, wherein a unique combination of the index
`and the circular shift uniquely identifies a transmitter.
`[0026] The present invention additionally encompasses a
`method for fast cell search. The method comprises the steps
`of receiving a Generalized Chirp-Like (GCL) sequence from
`a transmitter, determining a GCL index from the GCL
`sequence, and determining a circular shift of a GCL
`sequence. Information such as system bandwidth, broadcast
`channel bandwidth, a number of transmission antennas, and
`mobile unit patterns is determined based on the GCL index
`and the circular shift of the GCL sequence.
`[0027]
`Turr1ir1g now to the drawings, where like numerals
`designate like components, FIG. 1 is a block diagram of
`communication system 100 that utilizes reference transmis-
`sions. Communication system utilizes an Orthogonal Fre-
`quency Division Multiplexing (OFDM) protocol; however
`in alternate embodiments communication system 100 may
`utilize other digital cellular communication system proto-
`cols such as a Code Division Multiple Access (CDMA)
`system protocol, a Frequency Division Multiple Access
`(FDMA) system protocol, a Spatial Division Multiple
`Access (SDMA) system protocol or a Time Division Mul-
`tiple Access (TDMA) system protocol, or various combina-
`tions thereof.
`
`[0028] As shown, communication system 100 includes
`base unit 101 and 102, and remote unit 103. A base unit or
`a remote unit may also be referred to more generally as a
`communication unit. The remote units may also be referred
`to as mobile units. A base unit comprises a transmit and
`receive unit that serves a number of remote units within a
`
`sector. As known in the art, the entire physical area served
`by the communication network may be divided into cells,
`and each cell may comprise one or more sectors.
`[0029] When multiple antennas are used to serve each
`sector to provide various advanced communication modes
`(e.g., adaptive bearnforrning,
`transmit diversity,
`transmit
`SDMA, and multiple stream transmission, etc.), multiple
`base units can be deployed. These base units within a sector
`may be highly integrated and may sl1are various hardware
`and software components. For example, all base units co-
`located together to serve a cell can constitute what
`is
`traditionally known as a base station. Base units 101 and 102
`transmit downlink communication signals 104 and 105 to
`serving remote units on at
`least a portion of the same
`resources (time, frequency, or both). Remote unit 103 com-
`municates with one or more base units 101 and 102 via
`
`uplink communication signal 106. A communication unit
`that is transmitting may be referred to as a source commu-
`nication unit. A communication Lmit that is receiving may be
`referred to as a destination or target communication unit.
`
`It should be noted that while only two base units
`[0030]
`and a single remote unit are illustrated in FIG. 1, one of
`ordinary skill in the art will recognize that typical commu-
`nication systems comprise many base units in simultaneous
`communication with many remote units. It should also be
`noted that while the present invention is described primarily
`for the case of downlink transmission from multiple base
`units to multiple remote units for simplicity, the invention is
`also applicable to uplink transmissions from multiple remote
`units to multiple base units. It is contemplated that network
`elements within communication system 100 are configured
`in well known manners with processors, memories, instruc-
`tion sets, and the like, which function in any suitable manner
`to perform the function set forth herein.
`[0031] As discussed above, reference assisted modulation
`is commonly used to aid in many functions such as channel
`estimation and cell identification. With this in mind, base
`units 101 and 102 transmit reference sequences at known
`time intervals as part of their downlink transmissions.
`Remote unit 103, knowing the set of sequences that different
`cells can use and the time interval, utilizes this information
`in cell search and charmel estimation. Such a reference
`transmission scheme is illustrated in FIG. 2. As shown,
`downlink transmissions 200 from base units 101 and 102
`
`typically comprise reference sequence 201 followed by
`remaining transmission 202. The same or a different
`sequence can show up one or multiple times during the
`remaining transmission 202. Thus, each base unit within
`communication system 100 comprises a transmitter 107 that
`transmits one or more reference sequences along with data
`channel circuitry 108 transmitting data. In a similar manner,
`each remote unit 103 within communication system 100
`comprises sequence index detector and circular shift detec-
`tor 109.
`
`It should be noted that although FIG. 2 shows
`[0032]
`reference sequence 201 existing at
`the beginning of a
`transmission, in various embodiments of the present inven-
`tion, the reference charmel circuitry may include reference
`sequence 201 anywhere within downlink transmission 200,
`and additionally may be transmitted on a separate charmel.
`Remaining transmission 202 typically comprises transmis-
`sions such as, but not limited to, sending information that the
`receiver needs to know before performing demodulation/
`decoding (so called control information) and actual infor-
`mation targeted to the user (user data).
`[0033] As discussed above, it is important for any refer-
`ence sequence to have optimal cross-correlation. With this in
`mind,
`communication system 100
`utilizes
`reference
`sequences constructed from distinct “classes” of chirp
`sequences with optimal cyclic cross-correlation. The con-
`struction of such reference sequences is described below. In
`order to increase the amount of unique base unit (cell/sector)
`identifications, a unique circular shift of a GCL sequence is
`utilized to identify the base unit. Thus, a first base unit may
`be utilizing a GCL sequence having a first circular shift
`amount for identification, while a second base unit may be
`utilizing the same GCL sequence having a second circular
`shift amount for identification.
`
`the time domain reference
`In one embodiment,
`[0034]
`signal is an Orthogonal Frequency Division Multiplexing
`(OFDM) symbol that is based on N-point FFT. A set of
`length-Np sequences are assigned to base units in commu-
`nication system 100 as the frequency-domain reference
`sequence (i.e., the entries of the sequence will be assigned
`
`11
`
`11
`
`

`
`US 2007/0270273 A1
`
`Nov. 22, 2007
`
`onto a set of NP (NP<:N) reference subcarriers in the
`frequency domain). The spacing of these reference subcar-
`riers is preferably equal (e.g., 0, l, 2, etc. in subcarrier(s)).
`The final reference sequences transmitted in the time domain
`can be cyclically extended where the cyclic extension is
`typically longer than the expected maximum delay spread of
`the channel (LD). In this case, the final sequence sent has a
`length equal to the sum of N and the cyclic extension length
`LCP. The cyclic extension can comprise a prefix, postfix, or
`a combination of a prefix and a postfix. The cyclic extension
`is an inherent part ofthe OFDM communication system. The
`inserted cyclic prefix makes the ordinary auto- or cross-
`correlation appear as a cyclic correlation at any shift that
`ranges from 0 to LCP. If no cyclic prefix is inserted, the
`ordinary correlation is approximately equal to the cyclic
`correlation if the shift is much smaller than the reference
`
`sequence length.
`[0035] The construction of the frequency domain refer-
`ence sequences depends on at least three factors, namely, a
`desired number of reference sequences needed in a network
`(K), a number of circular-shift indices (M), and a desired
`reference length (NP).
`In fact,
`the number of reference
`sequences available that has the optimal cyclic cross-corre-
`lation of P—l where P is the smallest prime factor of NP other
`than “I” after factoring NP ir1to the product of two or r11ore
`prime numbers including “I”. For example, the maximum
`value that P can be is NP—l when NP is a prime number. But
`when NP is not a prime number, the number of reference
`sequences often will be smaller than the desired number K.
`In order to obtain a maximum number of sequences, the
`reference sequence will be constructed by starting with a
`sequence whose length NG is a prime number and then
`performing modifications. In the preferred embodiment, one
`of the following two modifications is used:
`[0036]
`1. Choose NG to be the smallest prime number
`that is greater than NP and generate the sequence set.
`Truncatc the sequences in the set to NP; or
`[0037]
`2. Choose NG to be the largest prime number that
`is smaller than NP and generate the sequence set.
`Repeat the beginning elements of each sequence in the
`set to append at the end to reach the desired length NP.
`[0038] The above design of requiring NG to be a prime
`number will give a set of NG—l sequences that has ideal auto
`correlation and optimal cross correlation. However, if only
`a smaller number of sequences are needed, NG does not need
`to be a prime number as long as the smallest prime factor of
`NG excluding “l” is larger than K.
`[0039] When a modification such as truncating or inserting
`is used, the cross-correlation will not be precisely optimal
`anymore. However, the auto- and cross-correlation proper-
`ties are still acceptable. Further modifications to the trun-
`cated/extended sequences may also be applied, such as
`applying a unitary transform to them.
`[0040]
`It should also be noted that while only sequence
`truncation and cyclic extension were described above, in
`alternate embodiments of the present invention there exist
`other ways to modify the GCL sequences to obtain the final
`sequences of the desired length. Such modifications include,
`but are not limited to extending with arbitrary symbols,
`shortening by puncturing, etc. Again, further modifications
`to the extended/punctured sequences may also be applied,
`such as applying a unitary transform to them.
`
`[0041] As discussed above, in the preferred embodiment
`of the present
`invention Generalized Chirp-Like (GCL)
`sequences are utilized for constructing reference sequences.
`There are a number of “classes” of GCL sequences and if the
`classes are chosen carefully (see GCL property below);
`sequences with those chosen classes will have optimal
`cross-correlation and ideal autocorrelation. Class-u GCL
`
`sequence (S) of length NG are defined as:
`Su:(au(0)b,au(1)b, -
`-
`- ,au(No-1)b),
`
`(1)
`
`where b can be any complex scalar of unit amplitude and
`
`au(/C) = eXp[_j27mk(/c +1)/2 + qk )5
`NC
`
`(2)
`
`where,
`. NG—l is known as the “class” ofthe GCL sequence,
`u:l, .
`.
`k:0, l, .
`.
`. NG—l are the indices of the entries in a sequence,
`q:any integer.
`[0042] Each class of GCL sequence can have infinite
`number of sequences depending on the particular choice of
`q and b, but only one sequence out of each class is used to
`construct one reference sequence. Notice that each class
`index “u” produces a different phase ramp characteristic
`over the elements of the sequence (i.e., over the “k” values).
`[0043]
`It should also be noted that if an NG-point DFT
`(Discrete Fourier Transform) or IDFT (inverse DFT) is
`taken on each GCL sequence, the member sequences of the
`new set also have optimal cyclic cross-correlation and ideal
`autocorrelation, regardless of whether or 11ot the new set can
`be represented in the form of (1) and (2). In fact, sequences
`formed by applying a matrix transformation on the GCL
`sequences also have optimal cyclic cross-correlation and
`ideal autocorrelation as long as the matrix transformation is
`unitary. For example, the NG-point DFT/IDFT operation is
`equivalent to a size-NG matrix transfonnation where the
`matrix is ar1 NG by NG unitary matrix. As a result, sequences
`formed based on unitary transformations performed on the
`GCL sequences still fall within the scope of the invention,
`because the final sequences are still constructed from GCL
`sequences. That
`is,
`the final sequences are substantially
`based on (but are not necessarily equal
`to)
`the GCL
`sequences.
`
`the cross-correlation
`If NG is a prime number,
`[0044]
`between any two sequences of distinct “class” is optimal and
`there will be NG—l sequences (“classes”) in the set. When a
`modification such as truncating or inserting is used, the
`modified reference sequence can be referred to as nearly-
`optimal reference sequences that are constructed from GCL
`sequences.
`[0045] The integer “u” is the GCL sequence index. This
`sequence index is assigned to each cell. NG in the equation
`is the length of the GCL sequence. A total of NG—l different
`sequences are available for use in different cells. NG is a
`prime number equal or near the needed sequence length. If
`the needed sequence length is not a prime number,
`the
`next-largest prime number can be used for NG and the
`resulting GCL sequence can be truncated to the desired
`length NP.
`[0046]
`If the OFDM symbol with the GCL sequence in the
`time domain is denoted by:
`{s.<n>}:IDFT<{s.<k>}>
`
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`
`.
`
`where
`. NG—l is known as the “class” of the GCL sequence,
`u:l, .
`.
`. NP—l
`is known as time domain sample, where
`n:0, .
`NP-points IDFT is assumed, and
`k:0, 1,
`.
`.
`. NP—l are the indices of the subcarriers in a
`frequency domain sequence.
`[0047] The GCL symbol circularly shifted by “m*Q” in
`time domain is denoted by the following equation:
`{S/"(")}:{Su("-m><Q)}
`
`. M—l is known as circular shift index, and
`.
`where, m:0, .
`“Q” is circular shift unit amount, “M” is available number
`of circular shift indices.
`
`It should be noted that circular shifting may occur
`[0048]
`by multiplying the GCL sequence by complex exponential
`with a frequency in the frequency domain. In this case, the
`GCL symbol, which a complex exponential with frequency
`“m*Q” is multiplied, is denoted by the following equation:
`
`s;"(n) = 1DFT{s,,(k) - exp[J/Zflm X Q ' k
`Np
`
`Note: GCL sequence is utilized as reference sequence in the
`application, but it is possible to adopt thc othcr scqucncc
`such as M-sequence.
`
`[0049] There are three techniques for sequence index
`detection and circular shift index namely:
`
`(1) Coherent detection for both a sequence index and a
`circular shift index;
`
`(2) Non-coherent detection for a sequence index and coher-
`ent detection for a circular shift index, and
`
`(3) Non-Coherent detection for both a sequence index and a
`circular shift index.
`
`In case of the technique (1), any sequence (such as
`[0050]
`M-sequences) is applicable as a synchronization channel
`sequence (i.e., reference sequence or preamble) while in the
`case of the techniques (2) and (3), GCL sequences are
`preferable due to non-coherent detection of a sequence
`index.
`
`(1) Coherent Detection for Both a Sequence Index and a
`Circular Shift Index
`
`For coherent detection of a sequence index (u) and
`[0051]
`a circular shift index (m), an estimated channel impulse
`response is needed. Therefore, another synchronization
`channel (i.e., another reference sequence or another pre-
`amble) is needed for performing channel estimation. FIG. 3
`shows the example of the preferred synchronization channel
`(i.e., preambles or reference sequences) structure. In FIG. 3
`the primary synchronization channel sequence (i.e., primary
`reference sequence or primary preamble) is common among
`all cells and is used for channel estimation at a receiver. Also
`
`circular shift is not applied to the primary synchronization
`channel. The secondary synchronization channel sequence
`(i.e., secondary reference sequence or secondary preamble)
`is cell-specific GCL sequence with cell specific circular shift
`in the time domain.
`[0052] Although FIG. 3 shows that the primary synchro-
`nization channel and the secondary synchronization channel
`
`are time-division-multiplexed (TDM), it is possible to apply
`the other multiplexing method such as frequency division
`multiplexing (FDM) of the primary synchronization channel
`and the secondary synchronization charmel. Since a circular
`shift
`index is coherently detected,
`the circular shifted
`sequences are orthogonal for all circular shift indices even if
`“Q” is small (e.g., Q:1 or 2).
`[0053]
`FIG. 4 is a block diagram of a transmitter 107
`which is used to transmit a primary synchronization channel
`and a secondary synchronization charmel
`in the case of
`techniques (1) and (2). As shown, the transmitter comprises
`cell-common sequence generator 401 for generating the
`primary synchronization sequence, cell-specific sequence
`generator 402 for generating the secondary synchronization
`sequence, IFFT circuitry 403 and 404, circular shifter 405
`for
`circular
`shifting
`the
`secondary
`synchronization
`sequence, multiplexer 406, and optional cyclic prefix adder
`407.
`
`[0054] During operation, a cell common sequence is gen-
`erated by generator 401 and is passed to IFFT 403, where the
`sequence is transformed to a time domain signal. Cell
`specific GCL sequence with unique sequence index (u) is
`generated by generator 402 and is passed to IFFT 404, where
`the sequence is transformed to time domain signal. The cell
`specific time domain signal is circularly shifted by shifter
`405. The shift comprises a unique shift amount
`The
`cell-common time domain signal (i.e., P-synchronization
`channel) and the cell-specific time domain signal
`(i.e.,
`S-synchronization chamiel) are passed to multiplexer 406,
`where those signals are multiplexed. An optional cyclic
`prefix is added by adder 407 and the circularly-shifted GCL
`sequence is
`transmitted by transmission circuitry (not
`shown). The unique combination of the sequence index (u)
`and the circular shift
`index (m) uniquely identifies the
`transmitter.
`
`FIG. 5 is a block diagram ofremote unit 103 which
`[0055]
`is designed to identify a sequence index (u) and a unique
`circular shift index (m) via techniques (1) and (2), As shown,
`remote unit 103 comprises standard OFDM demodulator
`501, De-Multiplexer, 502 channel estimator 503, sequence
`index & a circular shift
`index detector 109, and base
`identifier 505.
`
`[0056] During operation of the receiver, the received syn-
`chronization channel signal is passed to standard OFDM
`demodulator 501, where any cyclic prefix is removed and
`then transformed to the received synchronization channel
`signal
`in the frequency domain signal by an FFT (not
`shown). The received synchronization channel in the fre-
`quency domain is passed to de-multiplexer 502 and a
`primary-synchronization channel signal and a secondary
`synchronization channel signal (GCL signal) are obtained in
`the frequency domain. The primary synchronization channel
`signal
`is passed to channel estimator 503 and channel
`impulse response is estimated. The secondary synchroniza-
`tion channel signal in the frequency domain and the esti-
`mated charmel impulse response in the frequency domain
`are passed to sequence index (u) & circular shift index
`detector 109. The sequence index u, and the circular shift
`index m are output to base identifier 505, where base station
`identification takes place.
`[0057]
`FIG. 6 is a block diagram of sequence index (u) &
`a circular shift index (m) detector 109 of FIG. 5 when using
`technique
`Detector 109 comprises Np-points multiplier
`601, equalizing gain generator 602, sequence index selector
`
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`604, sequence replica generator 605, Np-points multiplier
`607, IFFT 609, peak searcher 610, memory 603 to hold a
`peak value and its position, and sequence with maximum
`peak value searcher 608.
`[0058] During operation equalizing gain generator 602
`receives the channel response and generates an equalizing
`gain in the frequency domain based on the estimated channel
`impulse response, where Maximum Ratio Combining
`(MRC), Zero Forcing (ZF) or Minimum Mean Square Error
`(MMSE) can be utilized as equalizing the gain. The received
`secondary synchronization GCL signal
`is passed to Np-
`points multiplier 601 and is multiplied by the equalizing
`gain in the frequency domain. A GCL sequence index is
`selected from all possible indices by selector 604 and is
`passed to sequence replica generator 605. The GCL
`sequence replica with the given index is generated by
`generator 605 and conjugated by circuitry 606. The conju-
`gated sequen